,transcriber's ,note      ,in the ,plain
 ,text version of this e,book1          
superscripts are indicated by  1        
subscripts by 1 italics by paired       
underscores1 and boldface  by paired    
equals signs4 ,underlining in the       
advertisement at the end  of the book is
 indicated by paired underscores1 not   
distinguished  from italics4            
,the 8,westminster0 ,series      ,,glass
 ,,manufacture            ,,glass       
,,manufacture      ,,by    ,,walter     
,,rosenhain ,b4,a4 ,b4,c4,e4            
,,superintendent ,,of ,,the ,,department
 ,,of ,,metallurgy ,,and                
,,metallurgical ,,chemistry ,,at ,,the  
,,national    ,,physical ,,laboratory   
,illustration      ,,new ,,york    ,d4  
,,van ,,nostrand ,,company    #bc       
,,murray ,,and #bg ,,warren ,,streets   
#aijh            ,,bradbury1 ,,agnew1   
,,co4 ,,ld41 ,,printers1    ,,london    
,,and ,,tonbridge4          ,,preface   
,the present volume on ,glass           
,manufacture has been written chiefly   

for  the benefit of those who are users 
of glass1 and therefore makes  no claim 
to be an adequate guide or help to those
 engaged in glass  manufacture itself4  
,for this reason the account of         
manufacturing  processes has been kept  
as non-technical as possible2 no        
detailed  drawings of plant or          
appliances have been given1 and only a  
few  illustrative diagrams have been    
introduced for the purpose of avoiding  
lengthy verbal descriptions4 ,in        
describing each process the object  in  
view has been to give an insight into   
the rationale of each  step1 so far as  
it is known or understood1 and thus to  
indicate the  possibilities and         
limitations of the process and of its   
resulting  products rather than to      
provide a detailed guide to the         
technique of  the various operations4   
,the practical aim of the book has      
further been  safeguarded by the fact   
that the processes described in these   
pages  are1 with the exception of those 

described as obsolete1 to the author's  
definite knowledge1 in commercial use at
 the present time4 ,for this  reason    
many apparently ingenious and beautiful 
processes described in  earlier books on
 glass have not been mentioned here1    
since the author  could find no trace of
 their employment beyond the records of 
the  various patents involved4 ,on the  
other hand the reader must be warned  to
 bear in mind that the peculiar         
conditions of the glass manufacturing   
industry have led to the practice on the
 part of manufacturers of  keeping their
 processes as secret as possible1 so    
that the task of the  author who would  
give an accurate account of the best    
modern processes  used in any given     
department of the industry is beset with
 great  difficulties4 ,the author has   
endeavoured to steer the best course    
open  to him under these circumstances1 
and he would appeal to the paucity of   
glass literature in the ,english        
language as evidence of the difficulty  

to which he refers4    ,in addition to  
these difficulties1 which arise largely 
from  considerations of a commercial    
nature1 the writer of a book on glass   
is further confronted with technical    
difficulties of no inconsiderable       
order4 ,as already indicated1 the aim of
 the present author has been to         
describe processes from the point of    
view of principles and methods  rather  
than as mere rule-of-thumb descriptions 
of manufacturing  manipulations1 but in 
doing this he is met at every turn by   
the fact  that from the scientific side 
the greater part of the field of glass  
manufacture is a 8terra incognita40 ,in 
making this statement the  labours of   
many eminent scientific workers are by  
no means forgotten1  but the entire     
field is so large and beset with such   
great experimental  difficulties that   
even the labours of a list of           
investigators that  includes the names  
of ,fraunhofer and ,faraday1 ,stokes1   
,hopkinson1 ,abb  and ,schott1 have     

resulted in little more than an         
accumulation of  empirical data which1  
while they have been productive of great
 direct  practical results1 have left   
the science of glass still in a very    
elementary condition4 ,to take two      
examples in illustration of this  fact  
we may mention the question of the      
connection between chemical  composition
 and any of the physical properties of  
glass1 such as  refraction and          
dispersion of light1 and on the more    
mechanical side the  question why all   
processes1 such as rolling or moulding1 
which involve  the contact of hot glass 
with metal result in a roughening of the
  glass surface4 ,the former question   
has been studied by several of the      
investigators named above1 ,schott and  
,abb having particularly devoted  an    
enormous amount of labour and money to  
the study of the question  with results 
which have proved disappointing from the
 scientific point  of view4 ,by         
prolonged experimenting and the         

employment of a costly  system of trial 
and error an important series of novel  
and useful  glasses has been produced by
 these workers1 but no law by whose aid 
the  optical properties of a glass of   
given chemical composition could be     
predicted has yet been discovered1 and  
as a summary of the known facts  only   
the vaguest general principles are      
available for the guidance  of those who
 wish to produce glasses of definite    
properties4 ,the  same applies in a     
similar degree to most of the other     
properties of  glass1 with the          
exception1 perhaps1 of density and      
thermal expansion2  attempts to         
generalise from the known data of a     
limited number of  glasses generally    
meet with unqualified failure4 ,the     
conclusion which  one is forced to admit
 is that the fundamental principles     
underlying  the nature and constitution 
of glasses have yet to be discovered4   
,a study of the other question mentioned
 above as an example of the  limitations

 of our knowledge leads to the same     
conclusion2 an almost  endless          
succession of inventors have busied     
themselves with devices  for overcoming 
the roughening action of rollers and    
moulds upon  glass1 but without any real
 success4 ,a long list of other examples
  of the same kind could be given1 our  
knowledge of the physical and  chemical 
principles underlying many of the       
phenomena met with in glass  manufacture
 being deplorably deficient4 ,it will   
thus be seen that to  write a truly     
scientific account of glass manufacture 
is at the present  time impossible1 and 
the reader is asked to bear this in mind
 if he  should find the chemical or     
physical explanations given in this book
  less frequent or less adequate than   
could be desired4    ,having dwelt      
somewhat emphatically on the limitations
 of our present  scientific knowledge as
 applied to glass manufacture1 it is    
perhaps  scarcely necessary at the      
present time to emphasise the fact that 

this  state of affairs should act as the
 strongest incentive to further         
investigation of the whole subject4 ,the
 difficulty1 however1 lies  in the fact 
that such investigation can scarcely be 
carried on by  voluntary workers in     
ordinary laboratories1 but must be      
undertaken  with the active help of     
glass manufacturers at their works4     
,glass is  essentially a substance that 
cannot be satisfactorily handled in     
small  quantities1 particularly so far  
as all the phenomena connected with its 
production and manipulation while hot   
are concerned2 the influences  of       
containing vessels1 of furnace gases and
 of rapid cooling are all  enormously   
exaggerated if ounces instead of        
hundredweights or tons of  glass are    
used for experimental purposes1 and     
these influences and  others of the same
 nature vitally affect all the results  
of small-scale  laboratory operations4  
,the progress of our scientific         
knowledge of  glass--and the consequent 

development of the glass industry from  
its  present state where rule-of-thumb  
and 8practical experience0 still hold   
excessive sway--lies in the hands of    
those concerned in the industry  itself4
 ,it must be admitted that to undertake 
such work involves the  expenditure of  
much time and money on the part of a    
manufacturer1 while  the field is so    
large and the problems so complicated   
that any adequate  return cannot be     
promised for the immediate future2 on   
the other  hand the very size of the    
field and the difficulty of the problems
  offers the promise of the greatest    
ultimate reward2 a really important     
scientific discovery in connection with 
glass would be certain to bring  in its 
train industrial developments whose     
limits it is impossible to  foresee4    
,the industrial success of the          
glass-works of ,schott in ,jena  is     
often quoted as a brilliant example of  
commercial success resulting  from      
purely scientific investigations in this

 actual field2 an  example of still     
greater magnitude is furnished by the   
success of the  aniline dye works of    
,germany which are built up on purely   
scientific  achievements4 ,the glass    
industry as a whole1 supplying some of  
the  absolute necessaries of modern     
life1 should be capable of offering the 
greatest rewards to success1 and the    
example of other industries has  shown  
that ultimate success is bound to reward
 properly-conducted and  perseverant    
scientific research4 ,nowhere is this   
more urgently needed  than in the whole 
field of glass manufacture4    ,the     
author is indebted to ,mr4 ,w4 ,c4      
,hancock for valuable assistance in  the
 reading of proofs and various          
suggestions in connection with the      
contents of this book4          ,,table 
,,of ,,contents                         
,,page    ,,preface                     
v        ,,chapter ,i4      ,,the       
,,physical ,,and ,,chemical ,,properties
 ,,of ,,glass4      ,definition of the  

term 8,glass0--,amorphous structure the 
common feature of all vitreous          
bodies--,glass a congealed              
fluid--,glasses not definite chemical   
compounds but        complex            
solutions--,range of chemical           
composition        available for        
glass-making--,considerations governing 
chemical composition--,influence of     
composition on        physical          
properties--,chemical stability of      
glass--        ,permanence of glass     
surfaces--,action of water1 acids1      
and alkalies on glass--,action of light 
on glass             p4 #a              
,,chapter ,,ii4      ,,the ,,physical   
,,properties ,,of ,,glass4              
,mechanical properties3 tensile         
strength1 crushing strength1            
elasticity1 ductility1 and              
hardness--,thermal properties        of 
glass3 thermal endurance1 coefficient of
 expansion1        thermal              
conductivity--,thermometer              
glass--,electrical        properties of 

glass--,transparency and colour of glass
      p4 #ah        ,,chapter ,,iii4    
,,the ,,raw ,,materials ,,of ,,glass    
,,manufacture4      ,general            
considerations--,chemical purity1       
moisture1 and        physical condition1
 constancy of quality--,sources         
of silica1 sand and                     
sandstone--,felspar--,sources of        
alkali3 ,soda ash 7carbonate of soda71  
salt-cake 7sulphate        of soda71    
pearl ash 7carbonate of potash7--,alkali
        nitrates--,natural minerals     
containing alkalies--,sources        of 
other bases3 ,lime1 chalk1 limestone1   
slaked lime--        ,gypsum 7sulphate  
of lime7--,barium compounds--,magnesia  
and zinc--,lead oxide1 red              
lead--,aluminium1 manganese1            
arsenic--,carbon--,coke1 charcoal1      
anthracite coal           p4 #ce        
,,chapter ,,iv4      ,,crucibles ,,and  
,,furnaces ,,for ,,the ,,fusion ,,of    
,,glass4      ,fire-clay and            
silica-brick--,manufacture of           

glass-melting        pots--,drying and  
first heating of pots--,blocks for tank 
and other furnaces--,uses of silica     
brick--,furnaces--        ,coal-fired   
and gas-fired furnaces--,gas producers--
        ,regenerative furnaces1         
principles and construction of          
,siemens' furnaces--,recuperative       
furnaces--,general        arrangements  
of modern tank furnaces--,relative      
advantages of tank and pot furnaces     
p4 #ed        ,,chapter ,v4      ,,the  
,,process ,,of ,,fusion4      ,mixing of
 raw materials by hand and by           
machinery--,the        charging         
operation--,chemical reactions during   
melting of        carbonate mixtures1   
and of sulphate mixtures--,influence    
of carbon on the reactions--,the fining 
process             p4 #gc              
,,chapter ,,vi4      ,,processes ,,used 
,,in ,,the ,,working ,,of ,,glass4      
,ladling1 gathering1 and                
casting--,limitations of ladling--      
,ladling used for rolled glass1         

gathering for blown                     
glass--,rolling of glass--,blowing      
processes and        operations--,use of
 moulds--,pressing--,moulding           
p4 #hd        ,,chapter ,,vii4          
,,bottle ,,glass4      ,raw             
materials--,furnaces--,predominance of  
tank furnaces--        ,process of      
blowing bottles by hand--,gathering1    
marvering1 blowing--,use of fire-clay   
and metal moulds--        ,formation of 
neck--,improved appliances1 moulds and  
tools--,manufacture of bottles by       
machinery--,the        8,boucher0       
bottle-blowing machine--,annealing of   
bottles--        ,large bottles1        
carboys--,aids to the blower--,sievert's
        process--,large shallow vessels1
 bath-tubs                  p4 #ie      
,,chapter ,,viii4      ,,blown ,,and    
,,pressed ,,glass4      ,raw            
materials--,bohemian glass and flint    
glass--,gathering        and            
blowing--,chair work--,hand             
work--,production of        tumblers by 

hand--,application of coloured glass to 
blown        articles--,use of moulds as
 aids to blowing--,roughening           
effect of moulds--,fire-polishing by    
reheating--,use of        compressed    
air--,pressed glass--,moulds and        
presses--        ,capacity and          
limitations of pressing process         
p4 #ajh        ,,chapter ,,ix4          
,,rolled ,,or ,,plate ,,glass4          
,rolled plate glass--,furnaces--,raw    
materials--,process of                  
ladling--,the rolling                   
table--,annealing--,cutting and         
sorting--,patterns on rolled            
plate--8,figured0 rolled                
plate--,machine used for                
double-rolling--,polished               
plate--,raw materials--,casting from    
melting pots--        ,special casting  
pots--,the rolling table--,importance   
of flatness--,annealing kilns--,grinding
 and polishing                          
processes--,machines used for grinding  
and polishing--        ,method of       

holding the glass--,abrasives and       
polishing        materials--,theory of  
the polishing process--,limiting        
sizes of polished plate--,homogeneity of
 polished plate--        ,uses of plate 
glass--,bent polished plate--,mirrors-- 
,bevelling1 process and machines--,wired
 plate glass1        rolled and         
polished--,difficulties and             
limitations--        ,advantages of     
wired glass                             
p4 #abb        ,,chapter ,x4            
,,sheet ,,and ,,crown ,,glass4          
,comparison of sheet with polished      
plate--,raw materials        for        
sheet--,furnaces3 various forms of tank 
furnaces--        ,blowing              
process--,gathering1 forming the        
gathering on        blocks1 forming the 
shoulder of the cylinder1 blowing       
the cylinder1 opening the end of the    
cylinder1 detaching        cylinder from
 pipe--,cutting off the                 
8cap0--,splitting        the            
cylinder--,flattening and               

annealing--,cutting and        sorting  
sheet-glass--,defects of                
sheet-glass--,variations        of the  
process--,attempts to produce 8sheet0   
glass by        rolling--,sievert's     
process--,direct drawing processes--    
,the ,american process for drawing      
cylinders--,fourcault's                 
processes--,difficulties and            
limitations--,crown glass--        ,the 
blowing process--,limitations           
p4 #adi        ,,chapter ,,xi4          
,,coloured ,,glasses4      ,definition  
of coloured glass--,physical causes of  
colour--        ,colouring substances3  
copper1 silver1 gold1 carbon1 tin1      
arsenic1 sulphur1 chromium1 uranium1    
fluorine1 manganese1        iron1       
nickel1 cobalt--,range and depths of    
tints        available--,intensely      
coloured glasses--,the process        of
 8flashing0--,character of 8flashed0    
glass--,colours        produced on glass
 by painting3 use of coloured 8glazes0  
as paints--,ancient stained glass and   

modern glass--        ,technical uses of
 coloured glass1 photography1 railway   
and marine signals                      
p4 #agh        ,,chapter ,,xii4         
,,optical ,,glass4      ,nature and     
properties of optical                   
glass--,homogeneity--        ,formation 
and removal of stri in solutions and in 
glass--,transparency and                
colour--,absorption of light        in  
8decolourised0 glasses--,refraction and 
dispersion--                            
,definitions--,refractive index1        
dispersion1 medium        dispersion1   
the quantity --,specification of optical
        properties in terms of certain  
spectrum lines--,table of        typical
 optical glasses and their optical      
constants--        ,crown and flint     
glasses--,relation between refraction   
and dispersion in the older and newer   
glasses--,work        of ,abb and       
,schott--,applications of the new       
glasses--        ,non-proportionality of
 dispersion in different types        of

 glass--,resulting imperfections of     
achromatism--        ,the relative      
partial dispersions of glasses--,pairs  
of glasses giving perfect achromatism   
not yet fully                           
available--,constants of ,schott's      
telescope crown and                     
flint--,narrow range of optical glasses1
 consequent        limitations in lens  
design--,causes of these narrow         
limits--,possible directions of         
extension--,chemical        stability of
 optical glasses--,double refraction in 
optical glass arising from imperfect    
annealing            p4 #bje            
,,chapter ,,xiii4      ,,optical        
,,glass4      ,the manufacture of       
optical glass--,raw materials--         
,mixing--,furnaces and crucibles--,kilns
 for heating        pots--,transfer of  
pots from kiln to melting furnace--     
,introduction of cullet and raw         
materials--,the fining        process1  
difficulties and limitations--,the      
stirring        process--,the final     

cooling of the glass--,rough sorting    
of the glass fragments--,moulding and   
final annealing        of the moulded   
glass--,grinding and polishing of plates
        and discs for examination2      
smallness of yield obtained--           
,difficulty of obtaining large blocks of
 perfect glass     p4 #bbc              
,,chapter ,,xiv4      ,,miscellaneous   
,,products4      ,glass                 
tubing--,gathering and drawing of       
ordinary tubes--        ,special        
varieties of tube--,combustion          
tubes--,tubes        of vitreous        
silica--,varieties of vitreous silica-- 
,transparent1 glass-like silica         
ware--,great cost of                    
production--,translucent 8milky0 silica 
ware produced                           
electrically--,great thermal endurance  
of vitreous                             
silica--,sensitiveness to chemical      
action of all basic        substances at
 high temperatures--,glass rod and      
fibre--        ,glass wool--,quartz     

fibres--,glass beads--,artificial       
gems--,use of very dense flint glass    
coloured to imitate        precious     
stones--,means of distinguishing        
imitations--        ,precious stones    
produced by artificial means--,chilled  
glass--,great strength and fragility of 
chilled glass--        ,rupert's        
drops--,manufacture of 8tempered0 glass 
by        ,siemens--,de ,la ,bastie's   
process--,massive glass1 used        for
 house construction and paving          
blocks--,water-glass        7silicate of
 soda or potash71 manufacture in tank   
furnaces--,glass for lighthouse lenses  
and searchlight                         
mirrors--,production by casting glass in
 iron moulds--        ,sizes and types  
of lenses and prisms produced           
p4 #bch        ,,appendix--,bibliography
 of ,glass ,manufacture                 
p4 #bec          ,,glass ,,manufacture  
,,chapter ,i4    ,,the ,,physical ,,and 
,,chemical ,,properties ,,of ,,glass4   
,although the term 8glass0 denotes a    

group of bodies which possess in  common
 a number of well-defined and           
characteristic properties1 it is        
difficult to frame a satisfactory       
definition of the term itself4 ,thus    
while the property of transparency is at
 once suggested by the word  8glass10   
there are a number of true glasses which
 are not transparent1  and some of which
 are not even translucent4 ,hardness and
 brittleness  also are properties more  
or less characteristic of glasses1 yet  
very  wide differences are to be found  
in this respect also1 and bodies1 both  
harder and more fragile than glass1 are 
to be found among minerals and  metals4 
,perhaps the only really universal      
property of glasses is that  of         
possessing an amorphous structure1 so   
that vitreous bodies as a  whole may be 
regarded as typical of 8structureless0  
solids4 ,all bodies1  whether liquid or 
solid1 must possess an ultimate         
structure1 be it  atomic1 molecular or  
electronic in character1 but the        

structure here  referred to is not that 
of individual molecules but rather the  
manner  of grouping or aggregation of   
molecules4    ,in the great majority of 
mineral or inorganic bodies the         
molecules  in the solid phase are       
arranged in a definite grouping and the 
body  is said to have a crystalline     
structure2 evidences of this structure  
are generally visible to the unaided eye
 or can be revealed by the  microscope4 
,vitreous bodies on the other hand are  
characterised by  the entire absence of 
such a structure1 and the mechanical1   
optical  and chemical behaviour of such 
bodies is consistent only with the      
assumption that their molecules possess 
the same arrangement1 or rather  lack of
 arrangement1 that is found in liquids4 
,the intimate resemblance between       
vitreous bodies and true liquids  is    
further emphasised when it is realised  
that true liquids can in  many instances
 pass into the vitreous state without   
undergoing any  critical change or      

exhibiting any discontinuity of         
behaviour1 such as  is exhibited during 
the freezing of a crystalline body4 ,in 
the latter  class of substances the     
passage from the liquid to the          
crystalline  state takes place at one   
definite temperature1 and the change is 
accompanied by a considerable evolution 
of heat1 so that the cooling of  the    
mass is temporarily arrested4 ,in the   
case of glasses1 on the other  hand1 the
 passage from the liquid to the         
apparently solid condition is  gradual  
and perfectly continuous1 no evolution  
of heat or retardation  of cooling being
 observed even by the aid of the most   
delicate  instruments4 ,we are thus     
justified in speaking of glasses as     
8congealed  liquids10 the process of    
congealing in this case involving no    
change  of structure1 no re-arrangement 
of the molecules1 but simply implies a  
gradual stiffening of the liquid until  
the viscosity becomes so great  that the
 body behaves like a solid4 ,it is1     

however1 just this power  of becoming   
exceedingly stiff or viscous when cooled
 down to ordinary  temperatures that    
renders the existence of vitreous bodies
 possible4  ,all glasses are capable of 
undergoing the change to the crystalline
  state when kept for a sufficient time 
at a suitable temperature4 ,the  process
 which then takes place is known as     
8devitrification10 and  sometimes gives 
rise to serious manufacturing           
difficulties4    ,molten glass may be   
regarded as a mutual solution of a      
number of  chemical substances--usually 
silicates and borates4 ,when cooled in  
the ordinary way these bodies remain    
mutually dissolved1 and ordinary  glass 
is thus simply a congealed solution4    
,the dissolved substances  have1        
however1 natural freezing-points of     
their own1 and if the molten  mass be   
kept for any length of time at a        
temperature a little below  one of these
 freezing-points1 that particular       
substance will begin to  solidify       

separately in the form of crystals4 ,the
 facility with which  this will occur   
depends upon the properties of the      
ingredients and  upon the proportions in
 which they are present in the glass4   
,in some  cases this devitrification    
sets in so readily that it can scarcely 
be  prevented at all1 while in other    
cases the glass must be maintained at   
the proper temperature for hours before 
crystallisation can be induced  to set  
in4 ,in either of these cases1 provided 
that the glass is cooled  sufficiently  
rapidly to prevent crystallisation1 the 
sequence of events  during the          
subsequent cooling of the mass is this3 
as the temperature  falls further and   
further below the natural freezing-point
 of one or  other of the dissolved      
bodies1 the tendency of that body to    
crystallise  out at first rapidly       
increases2 as the temperature falls1    
however1 the  resistance which the      
liquid presents to the motion of the    
molecules  increases at a still greater 

rate1 so that two opposing forces are at
  work1 one of them an increasing       
tendency towards crystallisation1 the   
other a still more rapidly increasing   
resistance to any change4 ,there  is    
thus for every glass a certain critical 
range of temperature during  which the  
greatest tendency exists for the        
crystallising forces to  overcome the   
internal resistance2 through this range 
the glass must be  cooled at a          
relatively rapid rate if devitrification
 is to be avoided2  at lower            
temperatures the crystallising forces   
require increasingly  longer periods of 
time to produce any sensible effect1    
until1 as the  ordinary temperature is  
approached1 the forces of internal      
resistance  entirely prevent all        
tendency to crystallisation4    ,the    
phenomena just described in reality     
constitute the natural limit  to the    
range of bodies which can be obtained in
 the vitreous state3 as  we approach    
this limit the glass requires more and  

more rapid cooling  through the critical
 range of temperature1 and is thus more 
and more  liable to devitrify during the
 manufacturing processes1 until finally 
the limit is set when no industrially   
feasible rapidity of cooling  suffices  
to retain the mass in the vitreous      
state4    ,while the range of bodies    
that can be obtained in the vitreous    
state  is very large1 only a            
comparatively small number of substances
 are  ordinarily incorporated in        
industrial glasses4 ,with the exception 
of  certain special glasses used for    
scientific purposes1 such as the        
construction of optical lenses1         
thermometers and vessels intended to    
resist unusual treatment1 all industrial
 glasses are of the nature  of mixed    
silicates of a few bases1 viz41 the     
alkalies1 sodium and  potassium1 the    
alkaline earths1 calcium1 magnesium1    
strontium1 and  barium1 the oxides of   
iron and aluminium 7generally present in
 minor  quantities71 and lead oxide4    

,the manner in which these various      
elements  enter into combination and    
solution with one another has been much 
investigated1 and the more general      
conclusions have been anticipated  in   
what has been said above4 ,it is        
abundantly evident that glasses  are not
 definite chemical compounds1 but rather
 solutions1 in varying  proportions1 of 
a series of definite compounds in one   
another4 ,in many  cases the actual     
constitution of industrial glasses is so
 complex as1  for the present at all    
events1 to baffle adequate chemical     
expression4    ,one of the factors that 
limit the range of possible compositions
  of glasses has already been indicated1
 and two others must now be  discussed4 
,for industrial purposes1 the cost and  
rarity of the  ingredients becomes a    
vital bar at a certain stage2 thus the  
use of  such elements as lithium1       
thallium1 etc41 is prohibitively costly4
 ,in  another direction the glass-maker 
is very effectively restrained by  the  

limitations of his furnaces as regards  
temperature4 ,the presence  of excessive
 proportions of silica1 lime1 alumina1  
etc41 tends to  raise the temperature   
required for the free fusion of the     
glass1 and  when this temperature       
seriously exceeds #afjj ,c41 the        
manufacture of  the glass in ordinary   
furnaces becomes impossible4 ,thus pure 
silica  can be converted into a glass   
possessing very valuable properties1 but
  the requisite temperature cannot be   
attained in regenerative gas-fired      
furnaces such as are ordinarily used by 
glass manufacturers4 ,the  production of
 this glass has accordingly been carried
 on upon a small  scale only by means of
 laboratory furnaces heated by          
oxy-acetylene  flames1 while latterly a 
less perfect variety of silica          
glass-ware  has been produced on a large
 scale by the aid of electric furnaces4 
,such methods are1 however1 obviously   
limited to very special products        
commanding special prices4    ,a further

 limitation in the choice of chemical   
components is placed  upon the          
manufacturer by the actual chemical     
behaviour of the glass  both during     
manufacture and in use4 ,as regards     
chemical behaviour  during manufacture1 
it must be borne in mind that1 although 
glasses  are of the nature of solutions 
rather than of compounds1 yet these     
solutions tend towards a state of       
saturation2 thus a glass rich in  silica
 and deficient in bases will readily    
dissolve any basic materials  with which
 it may come in contact1 while1 on the  
other hand1 a glass  rich in bases and  
poor in acid constituents such as       
silica1 boric acid  or alumina1 will    
readily absorb acid bodies from its     
surroundings4  ,during the process of   
melting1 glass is universally contained 
in  fire-clay vessels4 ,these are       
chosen1 as regards their own chemical   
composition1 so as to offer to the      
molten glass a few of those  materials  
in which the glass itself is deficient2 

yet a limit arises in  this respect     
also1 since glasses very rich in bases1 
such as the very  dense lead and barium 
glass made for optical purposes1 rapidly
 attack  any fire-clay with which they  
may come in contact4 ,the finished glass
  also betrays its chemical composition 
by its chemical behaviour towards  the  
atmospheric agents1 such as moisture and
 carbonic acid1 with which  it comes in 
contact2 glasses containing an excessive
 proportion of  alkali1 for example1 are
 found to be seriously hygroscopic and  
to  undergo rapid decomposition1        
especially in a damp atmosphere4        
,within the limits set by these         
considerations1 the glass manufacturer  
chooses the chemical composition of his 
glass according to the purpose  for     
which it is intended2 for most          
industrial products the cheapest and    
most accessible raw materials that will 
yield a glass of the requisite          
appearance are employed1 while for      
special purposes the dependence of      

physical properties upon chemical       
composition is utilised1 as far as      
possible1 in order to attain a glass    
specially suited to the particular      
requirements in question4 ,thus the     
flint and barium glasses used for  table
 and ornamental ware derive from the    
dense and strongly refracting  oxides of
 lead and barium their properties of    
brilliancy and weight4  ,the fusibility 
and softness imparted to the glass by   
the presence  of these bases further    
adapts it to its purpose by facilitating
 the  complicated manipulations to which
 the glass must be subjected in the     
manufacturing processes4    ,taking our 
next example at almost the opposite     
extreme1 the hardest  8combustion       
tubing10 which is intended to resist a  
red heat without  appreciable softening1
 is manufactured by reducing the basic  
contents  of the glass to the lowest    
possible degree1 especially minimising  
the  alkali content1 and using the most 
refractory bases available1 such  as    

lime1 magnesia1 and alumina in the      
highest possible proportions4  ,such    
glass is1 of course1 difficult to melt1 
and special furnaces are  required for  
its production1 but on the other hand   
this material meets  requirements which 
ordinary soda-lime or flint glass tubing
 could never  approach4 ,another        
instance of these refractory glasses is 
to be found  in the ,jena special       
thermometer glasses and in the ,french  
7,tonnelot7  8,verre dur02 the best of  
these glasses show little or no         
plasticity  at temperatures approaching 
#ejj ,c41 and have thus rendered        
possible  a considerable extension of   
the range of the mercury thermometer4   
,further modification of chemical       
composition has resulted in the         
production of glasses which are far less
 subject to those gradual  changes which
 occur in ordinary glass when used for  
the manufacture  of                     
thermometers--changes which vitiated the
 accuracy of most early  thermometers4  

,a still more extensive adaptation of   
chemical composition  to the attainment 
of desired physical properties has been 
reached  primarily as a result of the   
labours of ,schott and ,abb1 in the case
  of optical glasses4 ,the work of these
 men1 and the developments which  have  
followed from it1 both at the works     
founded by them at ,jena and  elsewhere1
 have so profoundly modified our        
knowledge of the range of  possibilities
 embraced by the class of vitreous      
bodies1 that it is  not at all easy at  
the present time to realise the former  
narrow and  restricted meaning of the   
term 8glass40 ,the subject of the       
dependence  of the optical properties of
 glass upon chemical composition will be
  referred to in detail in ,chapter     
,,xii4 on 8,optical ,glass10 but the    
outline of the influence of composition 
on properties here given could  not be  
closed without some reference to this   
pioneer work of the ,german             
investigators4    ,the chemical         

behaviour of glass surfaces1 to which we
 have already  referred1 is of the      
utmost importance to all users of glass4
 ,the  relatively neutral chemical      
behaviour of glass is1 in fact1 one of  
its  most useful properties1 and1 next  
to its transparency1 most frequently    
the governing factor in its employment  
for various purposes4 ,thus the  entire 
use of glass for table-ware depends     
primarily upon the fact  that it does   
not appreciably affect the composition  
and flavour of  edible solids or liquids
 with which it is brought into          
contact--a  property which is only very 
partially shared even by the noble      
metals4  ,again1 the use of glass       
windows in places exposed to the weather
 would  not be feasible if window-glass 
were appreciably attacked by the  action
 of water or of the gases of the        
atmosphere4 ,for these general          
purposes1 it is true1 most ordinary     
glasses are adequately resistant1  but  
this degree of perfection in this       

respect is only the outcome of  the     
centuries of experience which the       
practical glass-maker has behind  him in
 the manufacture and behaviour of such  
glass4 ,when1 however1 a  higher degree 
of chemical resistance is required for  
special purposes1  as for instance when 
glass is called upon to resist exposure 
to hot1  damp climates1 or is intended  
to contain corrosive liquids1 the rules 
which are an adequate guide to the      
glass-maker in meeting ordinary         
requirements are no longer sufficient1  
particularly when the glass  is expected
 to meet other stringent requirements as
 well4 ,it has1 in  fact1 frequently    
happened that a glass-maker1 in striving
 to improve  the colour or quality of   
his glass1 as regards freedom from      
defects1  brilliancy of surface1 etc41  
has spoilt the chemical durability of   
his  products4 ,the reason lies in the  
fact1 long known in general terms1  that
 an increased alkali content reduces the
 chemical resistance  of glass1 while at

 the same time such an increase of      
alkali is the  readiest means whereby   
the glass-maker can improve his glass in
 other  respects by making it more      
fusible and easier to work in every way4
    ,this subject of the chemical       
stability of glass surfaces attracted   
much attention during the later part of 
last century1 and careful               
investigations on the subject were      
carried out1 particularly at the        
,german ,reichsanstalt 7,imperial       
,physical ,laboratory7 at               
,charlottenburg4  ,here also the labours
 of ,schott and ,abb proved helpful1    
until at the  present time such glass as
 that used by the ,jena firm in the     
production  of laboratory ware1 and     
certain other special glasses of that   
kind1 are  fitted to meet the most      
stringent requirements4    ,leaving     
aside the inferior glasses1 containing1 
generally1 more than #ae  per cent4 of  
alkali1 the behaviour of glass surfaces 
to the principal  chemical agents may be

 summed up in the following statements4 
,pure  water attacks all glass to a     
greater or lesser extent2 in the best   
glasses the prolonged action of cold    
water merely extracts a minute  trace of
 alkalies1 but in less perfect kinds the
 extraction of alkali  is considerable  
on prolonged exposure even in the cold1 
and becomes  rapidly more serious if the
 temperature is raised4 ,superheated    
water1  i4e41 water under steam         
pressure1 becomes an active corroding   
agent1  and the best glasses can only   
resist its action for a limited time4   
,for  the gauge-glass tubes of steam    
boilers working at the high pressures1  
which are customary at the present time1
 specially durable glasses  are required
 and can be obtained1 although many of  
the gauge-tubes  ordinarily sold are    
quite unfit for the purpose1 both from  
the present  point of view and from that
 of strength and 8thermal endurance40   
,in certain classes of glass1 the action
 of water1 especially when hot1  is not 

entirely confined to the surface1 some  
water penetrating into  the mass of the 
glass to an appreciable depth4 ,the     
exact mechanism of  this action is not  
known1 but the writer inclines to the   
view that it  arises from a partial     
hydration of some of the silica or      
silicates  present in the glass4 ,if    
such glasses be dried in the ordinary   
way and  subsequently heated1 the       
surface will be riddled with minute     
cracks1  some glass may even flake off1 
and the whole surface will be dulled4   
,as  such penetrating action sometimes  
takes place--in the poorer kinds of     
glass--by the action of atmospheric     
moisture when the glass is merely       
stored in a damp place1 it is often     
mistaken for 8devitrification40  ,this  
latter action1 however1 is not known to 
occur at the ordinary  temperature1     
although glass when heated in a flame   
frequently shows  the phenomenon2 it is1
 however1 entirely distinct from the    
surface  8corrosion0 just described4    

,water containing alkaline substances in
  solution acts upon all glasses in a   
relatively rapid manner2 it acts  by    
first abstracting silica from the glass1
 the alkali and lime being  dissolved or
 mechanically removed at a later stage4 
,water containing  acid bodies in       
solution--i4e41 dilute acid--on the     
other hand acts  upon most varieties of 
glass decidedly less energetically than 
even  pure water1 and much less         
vigorously than alkaline solutions2 this
  peculiar behaviour probably depends   
upon the tendency of acids to  prevent  
the hydration of silica1 this substance 
being thereby enabled  to act as a      
barrier to the solvent action of the    
water upon the  alkaline constituents of
 the glass4 ,the better varieties of    
glass are  also practically impervious  
to the action of strong acids1 although 
certain of these1 such as phosphoric and
 hydrofluoric1 exert a rapid  action on 
all kinds of glass4 ,only certain       
special glasses1 containing  an         

excessive proportion of basic           
constituents and of such substances as  
boric or phosphoric acid1 are capable of
 being completely decomposed by  the    
action of strong acids1 such as         
hydrochloric or nitric1 the bases       
entering into combination with the      
acids1 while the silicic and other      
acids are liberated4    ,in connection  
with the action of acids upon glass1    
mention should be  made of certain      
special actions that are of practical   
importance4 ,the  dissolving action of  
hydrofluoric acid upon glass is1 of     
course1 well  known4 ,it is used in     
practice both in the liquid and gaseous 
form1 and  also in that of compounds    
from which it is readily liberated 7such
  as ammonium or sodium fluoride71 for  
the purpose of 8etching0 glass1  and    
also in decomposing glass for purposes  
of chemical analysis4  ,next in         
importance ranks the action of carbonic 
acid gas upon glass1  especially in the 
presence of moisture4 ,the action in    

question is  probably indirect in       
character2 the moisture of the air1     
condensing  upon the surface of the     
glass1 first exerts its dissolving      
action1 and  thus draws from the glass a
 certain quantity of alkali1 which      
almost  certainly at first goes into    
solution as alkali hydrate 7potassium or
  sodium hydroxide72 this alkaline      
solution1 however1 rapidly absorbs      
carbonic acid from the air1 and the     
carbonate of the alkali is formed4  ,if 
the glass dries1 this carbonate forms a 
coating of minute crystals  on the      
surface of the glass1 giving it a dull1 
dimmed appearance2  this1 however1 only 
occurs ordinarily with soda glasses1    
since the  carbonate of potassium is too
 hygroscopic to remain in the dry solid 
state in any ordinary atmosphere4       
,potash glasses are1 as such1 no more   
stable chemically than soda glasses1 but
 they are for the reason just  given    
less liable to exhibit a dim surface4   
,if the dimming process1  in the case of

 a soda glass1 has not gone too far1 the
 brightness of  the surface of the glass
 may be practically restored by washing 
it  with water1 in which the minute     
crystals of carbonate of soda readily   
dissolve1 while separated silica is     
removed mechanically4 ,an attempt  made 
to clean the same dimmed surface by dry 
wiping would only result  in finally    
ruining the surface1 since the small    
sharp crystals of  carbonate of soda    
would be rubbed about over the surface1 
scratching it  in all directions4       
,the dimming process in the case of the 
less resistant glasses is not  only     
confined to the formation of alkaline   
carbonates2 the films of  alkaline      
solution which are formed on the surface
 of glass form a ready  breeding-ground 
for certain forms of bacteria and fungi1
 whose growth  occurs partly at the     
expense of the glass itself2 the precise
 nature of  these actions has not been  
fully studied1 but there can be little  
doubt  that silicate minerals--and glass

 is to be reckoned among these--are     
subject to bacterial decomposition1 a   
well-known example in another  direction
 being the 8maturing0 of clays by       
storage in the dark1 the  change in the 
clay being accompanied by an evolution  
of ammonia gas4 ,in  the case of glass  
it has been shown that specks of organic
 dust falling  upon a surface give rise 
to local decomposition4 ,in this        
connection it  is interesting to note   
the effect of the presence of a small   
proportion  of boric acid in some       
glasses4 ,the presence of this          
ingredient in  small proportions is     
known to render the glass more resistant
 to  atmospheric agencies1 and more     
especially to render it less sensitive  
to the effects of organic dust particles
 lying upon the surface4 ,it  has been  
suggested--probably rightly--that the   
boric acid1 entering  into solution in  
the film of surface moisture1 exerts its
 well-known  antiseptic properties1 thus
 protecting the glass from bacterial and

  fungoid activity4    ,the durability  
of glass under the action of atmospheric
 agents is a  matter of such importance 
that numerous efforts have been made to 
establish a satisfactory test whereby   
this property of a given glass  may be  
ascertained without actually awaiting   
the results of experience  obtained by  
actual use under unfavourable           
conditions4 ,one of the  earliest of the
 tests proposed consisted in exposing   
surfaces of the  glass to the vapour of 
hydrochloric acid4 ,for this purpose    
some strong  hydrochloric acid is placed
 in a glass or porcelain basin1 and     
strips  of the glass to be tested are   
placed across the top of the basin1 the 
whole being covered with a bell-jar4    
,after several days the glass is        
examined1 and as a rule the less stable 
glasses show a dull1 dimmed  surface as 
compared with the more stable ones4 ,a  
more satisfactory  form of test depends 
upon the fact that aqueous ether        
solutions react  readily with the less  

stable kinds of glass2 if a suitable    
dye1 such as  iod-eosin1 be dissolved in
 the water-ether solution1 then the     
effect  upon the less stable glasses    
when immersed in the solution is the    
formation of a strongly adherent pink   
film4 ,the density or depth of  colour  
of this film may be regarded as         
measuring the stability of the  glass2  
the best kinds of glass remain          
practically free from coloured  film    
even on prolonged exposure4 ,a test of a
 somewhat different kind  is one devised
 in its original form by ,dr4           
,zschimmer1 of the ,jena  glass works2  
this depends upon the fact that the     
disintegrating action  of moist air can 
be very much accelerated if both the    
moisture and  the temperature of the air
 surrounding the glass be considerably  
increased4 ,for this purpose the samples
 of glass are exposed to a  current of  
air saturated with moisture at a        
temperature of about #hj  ,c4 in a      
specially arranged incubator for one or 

more days1 means being  provided for    
securing a constant stream of moist air 
during the whole  time4 ,on examining   
the glass surfaces after this           
exposure--any wiping  or other cleaning 
of the surfaces being avoided--various  
qualities  of glass are found to show   
widely varying appearances4 ,the best   
and  most stable glasses remain entirely
 unaffected2 less stable kinds show     
small specks1 which merge into a        
generally dulled surface in unstable    
kinds4 ,there is no doubt that this test
 gives a sharp classification  of       
glasses1 but it yet remains to be proved
 that this classification  agrees with  
their true relative durability in       
practice2 the writer  is inclined to    
doubt whether this is really the case1  
since certain  glasses that have proved 
very satisfactory in this respect in    
practical  use all over the world were  
classed among the less stable kinds by  
this  test4    ,before leaving the      
subject of the chemical behaviour of    

glass1 a  reference should be made to   
the changes which glass undergoes when  
acted upon by light and other           
radiations4 ,under the influence of     
prolonged exposure to strong light1     
particularly to sunlight1 and  still    
more so to ultra-violet light1 or the   
light of the sun at high  altitudes1    
practically all kinds of glass undergo  
changes which  generally take the form  
of changes of colour4 ,glasses          
containing  manganese especially are apt
 to assume a purple or brown tinge under
  such circumstances1 although the      
powerful action of radium radiations  is
 capable of producing similar           
discoloration in glasses free from      
manganese4 ,apart from these latter     
effects1 of which very little is  known 
as yet1 there can be no doubt that the  
action of light brings  about chemical  
changes within the glass1 but it is by  
no means easy  to ascertain the true    
nature of these changes1 although they  
most  probably consist in a transfer of 

oxygen from one to another of the       
oxides present in the glass4 ,although  
it has not been definitely  proved1 it  
seems very unlikely that the glass      
either loses or gains  in any           
constituent during these changes4 ,good 
examples of the changes  undergone by   
glass under the action of sunlight are  
frequently found  in skylights1 where   
the oldest panes sometimes show a       
decided purple  tint which they did not 
possess when first put in place4 ,the   
glass  spheres of the instruments used  
for obtaining records of the duration   
of sunshine at meteorological stations  
also show signs of the changes  due to  
light--the glass of these spheres when  
new has a light greenish  tint1 but     
after prolonged use the colour changes  
to a decided yellow4  ,the coloured     
glass in stained-glass windows also     
shows signs of having  undergone changes
 of tint in consequence of prolonged    
exposure to  light2 glass removed from  
ancient windows usually shows a deeper  

tint  in those portions which have been 
protected from the direct action of     
light by the leading in which the glass 
was set1 and it is at least an  open    
question whether the beauty of ancient  
glass may not be1 in part1  due to the  
mellowing effect of light upon some of  
the tints of the  design4 ,this         
photo-sensitiveness of glass is also of 
some importance  in connection with the 
manufacture of photographic plates4 ,it 
has been  found that if the glass plate 
of a strongly-developed negative be     
cleaned1 a decided trace of the former  
image is retained by the glass1  and    
this image is apt to re-appear as a     
8ghost0 if the same glass be  again     
coated with sensitive emulsion and again
 exposed and developed4  ,the best      
makers of plates recognise this fact and
 do not re-coat glass  that has once    
been used for the production of a       
negative4          ,,chapter ,,ii4      
,,the ,,physical ,,properties ,,of      
,,glass4      ,the ,mechanical          

,properties of ,glass are of            
considerable importance in  many        
directions4 ,although glass is rarely   
used in such a manner that  it is       
directly called upon to sustain serious 
mechanical stresses1 the  ordinary uses 
of glass in the glazing of large windows
 and skylights  depend upon the strength
 of the material to a very considerable 
extent4 ,thus in the handling of        
plate-glass in the largest sheets1 the  
mechanical strength of the plates must  
be relied upon to a considerable        
extent1 and it is this factor which     
really limits the size of plate  that   
can be safely handled and installed4    
,the same limitation applies  to        
sheet-glass also1 for1 although its     
lighter weight renders it less  liable  
to break under its own weight1 its      
thinner section renders it  much more   
liable to accidental fracture4 ,in      
special cases1 also1 the  mechanical    
strength of glass must be relied upon to
 a considerable  extent4 ,gauge tubes of

 high-pressure boilers1 port-hole       
glasses in  ships1 the glass prisms     
inserted in pavement lights1 and the    
glass  bricks which have found some use 
in ,france1 as well as champagne        
bottles and mineral water bottles and   
syphons1 are all examples of  uses in   
which glass is exposed to direct        
stresses4 ,it is1 therefore1  a little  
surprising that while the mechanical    
properties of metals1  timbers1 and all 
manner of other materials have been     
studied in the  fullest possible manner1
 those of glass have received very      
little  attention1 at all events so far 
as published data go4 ,one reason for   
this state of affairs is probably to be 
found in the fact that it is by  no     
means easy to determine the strength of 
so brittle and hard a body  as glass4   
,as a consequence even the scanty data  
available can only be  regarded as first
 approximations4 ,the following data are
 only intended  to give an idea of the  
general order of strength to be looked  

for in  glass3--      ,tensile strength3
      ,from #a to #d  tons per sq4 in4  
7,trautwine74       0    to #a  0    0  
0    7,henrivaux74       0   #b to #e  0
    0     0    7,winkelmann and         
,schott74       0   #e to #f   0    0   
0    7,kowalski74      ,crushing        
strength3      ,from #i to #af tons per 
sq4 in4 7,trautwine74       0   #c to   
#h  0    0     0    7,winkelmann and    
,schott74       0  #bj to #bg  0    0   
0    7,kowalski74    ,of the above      
figures the experiments of ,winkelmann  
and ,schott are  probably by far the    
most reliable1 but these refer to a     
series of  special ,jena glasses1       
selected with a view to determining the 
influence  of chemical composition on   
mechanical properties1 and1             
unfortunately1  this series does not    
include glasses at all closely          
resembling those  ordinarily used for   
practical purposes4 ,the attempt to     
connect tensile  and crushing strength  
with chemical composition was also only 

very  partially successful2 but the     
results serve to show that the chemical 
composition has a profound influence on 
the mechanical strength of  glass1 so   
that by systematic research it would    
probably be possible  to produce glasses
 of considerably greater mechanical     
strength than  those at present known4  
,it must be noted in this connection    
that the  mechanical properties of glass
 depend to a very considerable extent   
upon the rate of cooling which the      
specimen in question has undergone4  ,it
 is well known that by rapid cooling1 or
 quenching1 the hardness of  glass can  
be considerably increased2 such         
treatment also increases  the strength  
both as against tension and compression1
 and numerous  processes have been put  
forward for the purpose of utilising    
these  effects in practice4             
,unfortunately the 8hardened0 glass thus
 obtained  is extremely sensitive to    
minute scratches1 and flies to pieces as
 soon  as the surface is broken1 and the

 great internal stress which always     
exists in such glass is thereby         
relieved4 ,all these peculiarities are1 
of course1 dependent as to their degree 
upon the rapidity with which  the glass 
has been cooled1 and the aim of         
inventors in this field  has been to    
devise a rapid cooling process which    
should strike the  happy mean between   
the increased strength and the induced  
brittleness  resulting from quenching4  
,thus processes for 8tempering0 glass by
  cooling it in a blast of steam or in a
 bath of hot oil or grease have  been   
brought forward2 but1 although some such
 glass is manufactured1 no  very        
extensive practical application has     
resulted4    ,elasticity and ,ductility 
of ,glass4--,in a series of glasses     
investigated by ,winkelmann and ,schott1
 the modulus of elasticity  7,young's   
,modulus7 varied from #c1ejj to #e1ajj  
tons per sq4 in41 the  value being      
largely dependent upon the chemical     
composition of the  glass4 ,measurable  

ductility has not been observed in glass
 under  ordinary conditions except in   
the case of champagne bottles under     
test by internal hydraulic pressure2 in 
these tests it was found that  a        
permanent increase of volume of a few   
tenths of a cubic centimetre  could be  
obtained by the application of an       
internal pressure just short  of that   
required to burst the bottle--pressure  
of the order of #ah to #cj  atmospheres 
being involved4 ,this small permanent   
set has been ascribed  to incipient     
fissuring of the glass1 and this        
explanation is probably  correct4 ,on   
the other hand1 it is in the writer's   
opinion very probable  that glass is    
capable of decided flow under the       
prolonged action  of relatively small   
forces2 the behaviour of large discs of 
worked  optical glass suggests some such
 action1 but the view as yet lacks full 
experimental confirmation4    ,the      
,hardness of glass is a property of some
 importance in most of  the applications

 of glass4 ,the durability of glass     
objects which are  exposed to handling  
or to periodical cleaning must largely  
depend  upon the power of the glass to  
resist scratching2 this applies to  such
 objects as plate-glass windows and     
mirrors1 spectacle and other  lenses1   
and in a minor degree to table-ware4 ,on
 the other hand1  the exact definition  
and means of measuring hardness are not 
yet  satisfactorily settled4            
,experimenters have found it very       
difficult to  measure the direct        
resistance to scratching1 since it is   
found1 for  example1 that two glasses of
 very different hardness are yet capable
 of  decidedly scratching each other    
under suitable conditions4 ,resort has  
therefore been had to other methods of  
measuring hardness2 the method  which1  
from the experimental point of view1 is1
 perhaps1 the most  satisfactory1       
depends upon principles laid down by    
,hertz and elaborated  experimentally by
 ,auerbach4 ,this depends upon measuring

 the size of  the circular area of      
contact produced when a spherical lens  
is pressed  against a flat plate of the 
same glass with a known pressure4       
,auerbach  himself found some difficulty
 in deciding the exact connection       
between  the 8indentation modulus0 thus 
determined and the actual hardness of   
the glass4 ,this method is1 therefore1  
of theoretical interest rather  than of 
use in testing glasses for hardness4 ,a 
test of a more practical  kind consists 
in exposing specimens of the glasses to 
be tested to  abrasion against a        
revolving disc of cast-iron fed with    
emery or other  abrasive1 and to measure
 the loss of weight which results from a
 given  amount of abrading action under 
a known contact pressure4 ,if a number  
of specimens of different glasses are   
exposed to this test at one time1  a    
very good comparison of their power of  
resisting abrasion can be  obtained4 ,it
 is not quite certain that this test    
measures the actual  8hardness0 of the  

glass1 but it affords some information  
as to its  power of resisting abrasion1 
and for many purposes this power is the 
important factor4    ,hardness being1 as
 indicated above1 a somewhat indefinite 
term1 it  is not possible to give any   
precise statement as to the influence of
  chemical composition upon the hardness
 of glass4 ,in general terms it  may be 
said that glasses rich in silica and    
lime will be found to be  hard1 while   
glasses rich in alkali1 lead or barium1 
are likely to be  soft4 ,it must1       
however1 be borne in mind that rapid    
cooling1 or even  the lack of careful   
annealing1 will produce a very great    
increase of  hardness in even the       
softest glasses4 ,the actual behaviour  
of a given  specimen of glass will1     
therefore1 depend at least as much upon 
the  nature of the processes which it   
has undergone as upon its chemical      
composition4    ,the ,thermal           
,properties of ,glass1 although not of  
such general  importance as the         

mechanical properties1 are yet of       
considerable  interest in a large number
 of the practical uses to which glass is
  constantly applied4 ,perhaps the most 
important of these properties  is that  
known as thermal endurance1 which       
measures the amount of  sudden heating  
or cooling to which glass may be exposed
 without risk  of fracture2 the chimneys
 employed in connection with            
incandescent  gas burners1 boiler gauge 
glasses1 laboratory vessels1 and even   
table  and domestic utensils are all    
exposed at times to sudden changes of   
temperature1 and in many cases the value
 of the glass in question  depends      
principally upon its power of undergoing
 such treatment without  breakage4 ,the 
property of 8thermal endurance0 itself  
depends upon a  considerable number of  
more or less independent factors1 and   
their  influence will be readily        
understood if we follow the manner in   
which  sudden change of temperature     
produces stress and1 sometimes1 fracture

  in glass objects4 ,if we suppose a hot
 liquid to be poured into a cold        
vessel1 the first effect upon the       
material of the vessel will be to  raise
 the temperature of the inner surface4  
,under the influence of  this rise of   
temperature the material of this inner  
layer expands1  or endeavours to expand1
 being restrained by the resistance of  
the  central and outer layers of        
material which are still cold2 the      
result  of this contest is1 that while  
the inner layer is thrown into a state  
of compression1 the outer and central   
layers are thrown into a state  of      
tension4 ,accordingly1 if the tension so
 produced is sufficiently  great1 the   
outer layers fracture under tension and 
the whole vessel is  shattered by the   
propagation of the crack thus initiated4
 ,from this  description of the process 
it will be seen that a high coefficient 
of  expansion and a low modulus of      
elasticity will both favour fracture1   
while high tensile strength will tend to

 prevent it4 ,the thermal  conductivity 
of the glass will also affect the       
result1 because the  intensity of the   
tensile stress set up in the colder     
layers of glass  will depend upon the   
temperature gradient which exists in the
 glass2  thus if glass were a good      
conductor of heat it would never be     
possible  to set up a sufficient        
difference of temperature between       
adjacent  layers to produce fracture2   
for the same reason1 vessels of very    
thin  glass are less apt to break under 
temperature changes than those having   
thick walls1 since the greatest         
difference of temperature that can be   
set up between the inner and outer      
layers of a thin-walled vessel can      
never be very considerable4 ,it also    
follows from these considerations1  that
 if a cold glass vessel be              
simultaneously heated or cooled from    
both sides1 it can be safely exposed to 
a much more sudden change of            
temperature than it could withstand if  

heated from one side alone2 on  the     
other hand1 when very thick masses of   
glass have to be heated1 this  must be  
done very gradually1 as a considerable  
time will necessarily  elapse before an 
increment of temperature applied to the 
outside will  penetrate to the centre of
 the mass4 ,it should also be noted     
here1  that in addition to the thermal  
conductivity of the glass1 its heat     
capacity or specific heat also enters   
into this question1 since heat  will    
obviously penetrate more slowly through 
a glass whose own rise of  temperature  
absorbs a greater quantity of heat4 ,it 
will thus be seen  that 8thermal        
endurance0 is a somewhat complicated    
property1 depending  upon the factors   
named above1 viz43 coefficient of       
expansion1 thermal  conductivity1       
specific heat1 ,young's modulus of      
elasticity1 and tensile  strength4      
,the coefficient of thermal expansion   
varies considerably in different        
glasses1 and we can here only state the 

limiting values between which  these    
coefficients usually lie2 these are #cg 
#aj-g as the lower1  and #abb  #aj-g as 
the upper limit4 ,these figures express 
the cubical  expansion of the glass per 
degree ,centigrade1 the corresponding   
figures  for steel and brass            
respectively being about #cfj  #aj-g and
 #fdh   #aj-g respectively4 ,it should  
be noted that vitreous bodies of        
extremely low expansibility are         
obtainable by the suitable choice of    
ingredients1 but in some cases these    
8glasses0 are white opaque bodies1  and 
in all cases they present great         
difficulty in manufacture1 owing to  the
 fact that alkalies and lime must be    
avoided in their composition4    ,quite 
apart from the question of thermal      
endurance1 the expansive  properties of 
glass are of some importance4 ,thus when
 several kinds of  glass have to be     
united1 as1 for example1 in the process 
of producing  8flashed0 coloured glass1 
it is essential that their coefficients 

of expansion should be as nearly as     
possible the same2 otherwise            
considerable stresses will be set up    
when the glasses1 which have been       
joined at a red heat1 are allowed to    
cool4 ,on the other hand1 this  mutual  
stressing of two glasses owing to       
differences in their thermal  expansion 
has been utilised for the production of 
tubes and other glass  objects          
possessing special strength4 ,if a tube 
be drawn out of glass  consisting of two
 layers1 one considerably more          
expansible than the  other1 and the     
cooling process be rightly conducted1 it
 is possible  to produce a tube in which
 both the inner and outer layers of     
glass  are under a considerable         
compressive stress4 ,not only is glass1 
as  we have seen above1 enormously      
stronger as against compression than  it
 is against tension1 but glass under    
compressive stress behaves as  though it
 were a much tougher material1 being    
less liable to injury by  scratches or  

blows4 ,moreover1 if a tube in this     
condition be heated and  then exposed to
 sudden cooling1 the first effect of the
 application of  cold will be a         
contraction of the surface layers1      
resulting in a relief  of the initial   
condition of compression4 ,these tubes  
are1 therefore1  remarkably indifferent 
to sudden cooling1 although they are    
naturally  more sensitive to sudden     
heating4 ,in this respect they differ   
entirely  from ordinary glass1 which is 
considerably more sensitive to sudden   
cooling than to sudden heating1         
particularly when the heat or cold is   
applied to all the surfaces of the      
object at the same time4 ,the special   
tubes made of two layers of glass above 
referred to are manufactured  by the    
,jena ,glass ,works for special         
purposes1 among which boiler gauge      
glasses are the most important4 ,it     
should be also mentioned here that  the 
remarkable thermal endurance of         
vitrified silica1 which can be  raised  

to a red heat and then immersed in cold 
water without risk of  breakage1 is     
chiefly due to its very low coefficient 
of expansion4    ,in another direction  
the expansive properties of glass are of
  importance wherever glass is rigidly  
attached to metal4 ,at the present  time
 this is done in several industrial     
products1 such as incandescent  electric
 lamps and 8wired0 plate glass4 ,in     
certain varieties of  incandescent      
lamps1 metallic wires are sealed into   
the glass bulbs1  and the only metal    
available for this purpose1 at all      
events until  recently1 has been        
platinum1 whose coefficient of expansion
 is low as  compared with most metals1  
and whose freedom from oxidation when   
heated  to the necessary temperature    
makes it easy to produce a clean joint  
between glass and metal4 ,more recently 
the use of certain varieties of  nickel 
steel has been patented for this        
purpose1 since it is possible  to obtain
 nickel steel alloys of almost any      

desired coefficient of  expansion from  
that of the alloy known as 8invar10     
having a negligibly  small expansion    
compared with that of ordinary steel4   
,by choosing  a suitable member of this 
series a metal could be obtained whose  
coefficient of expansion corresponds    
exactly with that of the glass to  which
 it is to be united4 ,the oxidation of  
the nickel steel when heated  to the    
temperature necessary for effecting its 
union with the glass  presented serious 
difficulties to the production of a     
tight joint1  and several devices for   
avoiding this oxidation have been       
patented4  ,in the incandescent electric
 lamp1 although the joint between glass 
and metal is required to be perfectly   
air-tight1 the two bodies are  only     
attached to one another over a very     
short length4 ,in wired plate  glass1   
however1 an entire layer of wire netting
 is interposed between  two layers of   
glass1 the wire being inserted during   
the process of  rolling4 ,here a certain

 amount of oxidation of the wire is not 
of any  serious importance1 as it only  
appears to give rise to a few bubbles1  
whose presence does not interfere with  
the strength and usefulness of  the     
glass2 but any considerable difference  
of coefficient of expansion  will       
produce the most serious results on     
account of the great lengths  of glass  
and metal that are attached to each     
other4 ,this factor has  been neglected 
by some manufacturers1 with the result  
that much of the  wired glass of        
commerce is liable to crack             
spontaneously some time  after it has   
left the manufacturer's hands1 while    
there is also much  loss by breakage    
during the process of manufacture4      
,thermal expansion is a vital factor in 
yet another of the uses of  glass4 ,our 
ordinary instrument for measuring       
temperature--the mercury                
thermometer--is very considerably       
affected by the expansive behaviour  of 
glass4 ,when a mercury thermometer is   

warmed the mercury column rises  in the 
stem because the mercury expands upon   
warming to a greater  extent than the   
glass vessel1 bulb and stem1 in which it
 is contained4  ,the subject of the     
graduations and corrections of the      
mercury glass  thermometer is a very    
large one and somewhat outside the scope
 of the  present volume2 but attention  
should be drawn in this place to the    
peculiarities of the behaviour of glass 
that have been discovered in  this      
connection4 ,one of these is that when  
first blown the bulb of a  thermometer  
takes a very considerable time to       
acquire its final volume1  the result   
being1 that if a freshly made           
thermometer is graduated1  after some   
time the zero of the instrument will be 
found considerably  changed1 generally  
in a direction which indicates that the 
volume of  the bulb has slightly        
increased4 ,by a special annealing or   
8ageing0  process this change can be    
completed in a comparatively short time 

before the instrument is graduated4     
,there is1 however1 a further           
peculiarity which is prominent in some  
thermometers1 although very  greatly    
reduced in the best modern glasses4     
,this becomes apparent in  a decided    
change of zero whenever the thermometer 
has been exposed for  any length of time
 to a high temperature1 the zero        
gradually returning  more or less to its
 original position in the course of     
time4 ,with  thermometers made of       
glasses liable to these aberrations1 the
 reading  for a given temperature       
depended largely upon the immediate past
  history of the instrument2 but1 thanks
 to the ,jena ,works1 thermometer       
glasses are now available which are     
almost entirely free from this  defect4 
,in this connection the curious fact has
 been observed that  glass containing   
both the alkalies 7potash and soda7     
shows these  thermal effects much more  
markedly than a glass containing one of 
the  alkalies only4    ,the thermal     

conductivity of glass1 except in so far 
as it affects  the thermal endurance1 is
 not a matter of any great direct       
practical  importance1 although the fact
 that glass is always a comparatively   
poor  conductor of heat is utilised in  
many of its applications1 as1 for       
example1 the construction of            
conservatories and hot-houses1 although 
even in that case the opacity of glass  
to thermal radiations of long           
wave-lengths is of more importance than 
its low thermal conductivity4  ,similar 
statements apply1 in a still more marked
 degree1 to the subject  of the specific
 heat of glass4    ,the electrical      
properties of glass are of much greater 
practical  importance1 glass being      
frequently used in electrical appliances
 as  an insulating medium4 ,the         
insulating properties of glass1 as well 
as  the property known as the specific  
inductive capacity1 vary greatly        
according to the chemical composition of
 the material4 ,generally  speaking1 the

 harder glasses1 i4e41 those richest in 
silica and lime1  are the best          
insulators1 while soft glasses1 rich in 
lead or alkali1  are much poorer in this
 respect4 ,in practice1 particularly    
when the  glass insulator is exposed to 
even a moderately damp atmosphere1 the  
nature of the glass affects the         
resulting insulation or absence of      
insulation1 in another way4 ,almost all 
varieties of glass have the  property of
 condensing upon their surfaces a       
decided film or layer  of moisture from 
the atmosphere1 and1 as we have seen    
above1 glasses  differ very considerably
 in the degree to which they display    
this  hygroscopic tendency4 ,the softer 
glasses are much more hygroscopic  than 
the hard ones1 and the resulting film of
 surface moisture serves  to lessen or  
even to break down the insulating power 
of the glass1 the  electricity leaking  
away along the film of moisture4 ,in the
 case of  appliances for static         
electricity1 where very high voltages   

have to  be dealt with1 an endeavour is 
sometimes made to avoid this leakage  by
 varnishing the surface of the glass    
with shellac or other similar           
substance1 and this proves a            
satisfactory remedy up to a certain     
point4  ,quite recently a variety of    
glass has been brought forward which is 
peculiar in having a comparatively low  
electrical resistance1 so that  for     
certain purposes it can be used as an   
electric conductor4 ,although           
interesting in itself1 this glass is not
 very likely to prove useful  even for  
the limited number of applications that 
could be found for an  electrically     
conducting glass1 since it is very rich 
in alkali1 and is1  therefore1 likely to
 be unstable chemically1 even under the 
action of  atmospheric agencies alone4  
,the most valuable and in many ways the 
most interesting of the  properties of  
glass--its transparency--has not been   
dealt with as yet1  and all mention of  
this subject has been postponed to the  

end of the  present chapter1 because the
 whole subject of the optical properties
  of glass will be dealt with more fully
 in the chapter on optical glass        
7,chap4 ,,xii471 so that a very brief   
reference only need be made to the      
matter here4    ,there can be no doubt  
that1 in most of its practical          
applications1  transparency is the      
fundamental and essential property which
 leads to  the employment of glass in   
the place of either stronger or cheaper 
materials4 ,by transparency1 in this    
sense1 we wish to include mere          
translucence also1 since very frequently
 it is as necessary to avoid            
undisturbed visibility as it is to      
secure the admission of light4  ,it is  
indeed hard to find any use to which    
glass is extensively put  into which the
 function of transmitting light does not
 very largely  enter4 ,almost the only  
such example of use is the modern       
application  of opal glass to the       
covering of walls1 and the use--not as  

yet widely  extended--of pressed glass  
blocks as bricks and paving stones2 in  
these  cases it is the hardness and     
smoothness of surface that gives to the 
vitreous body its superiority over other
 materials1 but apart from  these       
special cases1 the fact remains that    
well over #ie per cent4 of  the glass   
used in the world is employed for       
purposes where transmission  of light is
 essential to the attainment of the     
desired result1 either  from the point  
of view of utility or from that of      
beauty4 ,it is  interesting to note that
 the power of transmitting light is not 
shared  by many solid bodies4 ,some     
colloidal organic bodies1 such as       
gelatine  and celluloid1 possess the    
property to a degree comparable with    
glass1  while certain mineral crystals1 
such as quartz and fluor-spar1 may  even
 surpass the finest glass in this       
respect2 while some of the other        
optical properties of glass are greatly 
exceeded by such natural  substances as 

the diamond and the ruby4 ,but the very 
brevity of this  list is in itself      
striking1 because it must be borne in   
mind that  transparency by no means     
constitutes the only common             
characteristic of  vitreous bodies4     
,although the transparency of glass is  
so valuable and indeed so  essential a  
property of that substance1 it must be  
remembered that  no kind of glass is    
perfectly transparent4 ,quite apart from
 the fact  that of the light that falls 
upon a glass surface1 however perfectly 
polished1 a considerable proportion is  
turned back by reflection at  the       
surface of entry and again by reflection
 at the surface of exit  from the glass1
 a certain proportion of light is       
absorbed during  its passage through the
 glass itself1 and the transmitted beam 
is correspondingly weakened4 ,in the    
purest and best glasses this  absorption
 is so small that in any moderate       
thickness very delicate  instruments are
 required to show that there has been   

any loss of light  at all2 but even the 
best glass1 when examined through a     
thickness of  #bj in4 or more1 always   
shows the effects of the absorption of  
light  quite unmistakably4 ,in fact1 not
 only does all glass absorb light1  but 
it does this to a different degree      
according to the colour of the  light1  
so that in passing through the glass a  
beam of white light  becomes weakened in
 one of its constituent colours more    
than in the  others1 with the result    
that the emergent light is slightly     
coloured4  ,thus the purest and whitest 
of glasses1 when examined in very thick 
pieces1 always show a decided blue or   
green tint1 although this tint  is quite
 invisible on looking through a few     
inches of the glass4 ,the  ordinary     
glass of commerce1 however1 is far      
removed from even this  approach to     
perfect transparency4 ,the best plate   
glass shows a slight  greenish-blue     
tint1 which is just perceptible to the  
trained eye when  a single sheet of     

moderate thickness is laid down upon a  
piece of  white paper4 ,when a sheet of 
this glass is viewed edgewise1 in such  
a way that the light reaching the eye   
has traversed a considerable  thickness1
 the greenish-blue tint of the glass    
becomes more apparent4  ,by holding     
strips of various kinds of glass1 cut to
 an equal length1  close together and   
comparing the colour exhibited by their 
ends1 a  means of comparing the colours 
of apparently 8white0 glasses is readily
  obtained4 ,it will be found that      
different specimens of glass differ     
most markedly in this respect4 ,sheet   
glass is1 as a rule1 decidedly  deeper  
in colour than polished plate1 but      
rolled plate is as a rule  much         
greener--the colour of this glass can1  
in fact1 in most cases be  seen quite   
plainly in looking through or at the    
sheets in the ordinary  way4    ,the    
question of how far the colour of glass 
affects the value of the  light which it
 transmits depends for its answer upon  

the purpose to  which the lighted space 
is to be put4 ,where delicate           
comparisons  of colour are to be made1  
or other delicate work involving the use
  of the colour sense is to be carried  
on1 it is essential that all            
colouration of the entering daylight    
should be avoided1 and the use  of the  
most colourless glass obtainable will be
 desirable4 ,again1 in  photographic    
studios it is important to secure a     
glass which shall  absorb as small a    
proportion of the chemically active rays
 contained  in daylight as possible1 and
 special glasses for this purpose are   
available4 ,although for the present the
 price of these special glasses  may    
prove prohibitive for the glazing of    
studio lights1 their use is  found      
highly advantageous where artificial    
light is to be used to the  best        
advantage4 ,on the other hand1 for      
every-day purposes1 the slight  tinge of
 colour introduced into the light by the
 colour of ordinary  sheet and plate    

glass1 or even of greenish rolled plate 
glass1 has no  deleterious effect       
whatever1 the majority of persons being 
entirely  unconscious of its presence4  
,the transmission of light by glass1 its
  absorption1 refraction1 dispersion1   
etc41 are1 however1 best grouped        
together as the 8optical0 properties of 
glass1 and under that heading  they will
 receive a fuller treatment in          
connection with the subject of  the     
manufacture of glass for optical        
purposes4          ,,chapter ,,iii4     
,,the ,,raw ,,materials ,,of ,,glass    
,,manufacture4      ,the choice of raw  
materials for all branches of glass     
manufacture is a  matter of vital       
importance4 ,as a rule all 8fixed0      
bodies that are once  introduced into   
the glass-melting pot or furnace appear 
in the finished  glass1 while volatile  
or combustible bodies are more or less  
completely  eliminated during the       
process of fusion4 ,thus while the      
chemical  manufacturer can purify his   

products by filtration1 crystallisation 
or some other process of separation1 the
 glass-maker must eliminate  all        
undesirable ingredients before they are 
permitted to enter the  furnace1 and the
 stringency of this condition is        
increased by the fact  that the         
transparency of glass makes the         
detection of defects of colour  or      
quality exceedingly easy4 ,for the      
production of the best varieties  of    
glass1 therefore1 an exacting standard  
of purity is applied to  the substances 
used as raw materials4 ,as the quality  
of the product  decreases1 so also do   
the demands upon the purity of raw      
materials1  until finally for the       
manufacture of common green bottles1    
even such  very heterogeneous substances
 as basaltic rock and the miscellaneous 
residues of broken1 defective and       
half-melted glass forming the refuse  of
 other glassworks may be utilised more  
or less satisfactorily4    ,for the best
 kinds of glass the most desirable      

quality in raw materials  is thus as    
near an approach to purity as possible  
under commercial  conditions1 and next  
to that1 as great a constancy of        
composition as  possible4 ,for instance1
 the quantity of moisture contained in a
 ton of  sand appreciably affects the   
resulting composition of the glass1 and 
if  the sand cannot be obtained         
perfectly dry1 it should at least       
contain  a constant proportion of       
moisture1 otherwise it becomes necessary
  to determine1 by chemical tests1 the  
percentage of moisture in the  sand that
 is used from day to day1 and to adjust 
the quantity used  in accordance with   
the results of these tests1 a proceeding
 which1  of course1 materially          
complicates the whole process4 ,in other
 cases1  variable composition is not so 
readily allowed for1 and uncontrollable 
variations in the composition of the    
glass result--at times the quality      
falls off unaccountably1 or the glass   
refuses to melt freely at the  usual    

temperature4 ,the systematic employment 
of chemical analysis in  the supervision
 of both the raw materials and of       
various products  will frequently enable
 the manufacturer to trace the causes of
 such  undesirable occurrences2 but     
however necessary such control          
undoubtedly  is1 it cannot entirely     
compensate for the use of raw materials 
liable  to too great a variation in     
composition or physical character4 ,for 
not  only the chemical composition1 but 
also the physical condition and         
properties of the material are of       
importance in glass manufacture4 ,thus  
it is essential that materials to be    
used for glass-melting should  be       
obtainable in a reasonably fine state of
 division1 and in this  connection it   
must be remembered that both exceedingly
 hard bodies and  soft plastic          
substances can only be ground with very 
great difficulty4  ,further1 where a    
substance occurs naturally as a powder1 
this powder  should be of uniform and   

not too fine a grain1 more especially if
 it  belongs to the class of refractory 
rather than of fluxing ingredients4  ,in
 that case the presence of coarser      
grains will result in their  presence in
 the undissolved state in the finished  
glass1 unless  excessive heat and       
duration of 8founding0 be employed to   
permit of  their dissolution4 ,this     
applies chiefly to siliceous and        
calcareous  ingredients1 but hardened   
nodules of salt-cake may behave in a    
similar  manner4    ,a further          
consideration in the choice of raw      
materials is facility of  storage4 ,thus
 limestone in the shape of large lumps  
of stone which are  only ground to      
powder as required1 is readily stored1  
and undergoes no  deleterious change    
even if exposed to the weather2 on the  
other hand1  sulphate of soda           
7salt-cake71 if stored even in          
moderately dry places1  rapidly         
agglomerates into hard masses1 at the   
same time absorbing  a certain          

percentage of moisture4 ,such properties
 are not always  to be avoided1         
salt-cake for example being at the      
present time an  indispensable          
ingredient in many kinds of             
glass-making1 but the value  of a       
substance is in some cases materially   
lessened by such causes4    ,the raw    
materials ordinarily employed in        
glass-making may be grouped  into the   
following classes3--      7#a7 ,sources 
of silica4    7#b7 ,sources of alkalies4
    7#c7 ,sources of bases other than   
alkalies4    7#a7 ,sources of           
,silica4--,the principal source of      
silica is sand4 ,this  substance occurs 
in nature in geological deposits1 often 
of very  considerable area and depth4   
,these deposits of sand have always been
  formed by the disintegration of a     
siliceous rock1 and the fragments  so   
formed have been sifted and transported 
by the agency of water1  being finally  
deposited by a river either in the sea  
7marine deposits7  or in lakes          

7lacustrine deposits71 while the action 
of the water1  either during transport  
or after deposition1 has frequently worn
 the  individual particles into the     
shape of rounded grains4    ,in         
consequence of this origin1 the chemical
 composition of sand varies  very       
greatly with the nature of the rock     
whose denudation gave rise to  the      
deposit4 ,where rocks very rich in      
silica1 or even consisting of  nearly   
pure silica1 have been thus denuded1 the
 resulting sand is often  very pure1    
deposits containing up to #iii per cent4
 silica being known4  ,more frequently1 
however1 the sand contains fragments of 
more or less  decomposed felspar1 which 
introduce alumina1 iron and alkalies    
into its  composition4 ,finally1 8sands0
 of all ranges of composition from the  
pure varieties just referred to down to 
the clay marls1 very rich in  iron and  
alumina1 are known4    ,for the best    
varieties of glass1 viz41 optical glass1
 flint glass and  the whitest           

sheet-glass1 as well as for the best    
,bohemian glass1 a very  pure variety of
 sand is required1 preferably containing
 less than #jje  per cent4 of iron1 and 
not more than #jje per cent4 of other   
impurities  such as alumina1 lime or    
alkali4 ,as a matter of fact1 sands     
containing  so little iron rarely       
contain any other impurity except       
alumina in  measurable quantities4 ,the 
best-known deposit of such sand in      
,europe  is that at ,fontainebleau near 
,paris1 but equally good sand is found  
at ,lippe in ,germany1 whence sand is   
delivered commercially with a           
guaranteed silica content of #iiih per  
cent4 ,sand of excellent quality1       
although not quite so good as the above1
 is obtained at ,hohenbocka in  ,germany
 7,saxony7 and at a few other places in 
,europe4 ,in ,england no  deposit of    
sand of such purity is at present being 
exploited4    ,next in order of value to
 these exceedingly pure sands1 come the 
glass-making sands of ,belgium1 notably 

of ,epinal4 ,these usually contain  from
 #jb to #jc per cent4 of iron and rather
 more alumina1 but they  are used very  
largely for the manufacture of sheet and
 plate-glass4  ,when the standard of    
quality is further relaxed1 a large     
number of  sand deposits become         
available1 and the manufacturers of each
 district  avail themselves of more or  
less local supplies2 thus in ,england   
the  sands of ,leighton in ,bedfordshire
 and of ,lynn on the ,east ,coast1 are  
largely used4 ,finally1 for the         
manufacture of the cheapest class of    
bottles1 sands containing up to #b per  
cent4 of iron and a considerable        
proportion of other substances are      
employed4    ,silica1 in various states 
of purity1 occurs in nature in a number 
of  other forms than that of sand4 ,by  
far the commonest of these is that  of  
more or less compact sedimentary rock1  
known as 8sandstone40 ,as  far as       
chemical composition is concerned1 some 
of these stones are  admirably suited   

for making the best kinds of glass1     
although as a  rule a stone is not so   
homogeneous as the material of a good   
sand-bed4  ,the stone has the further   
disadvantage that it requires to be     
crushed  to powder before it can be used
 for glass-making1 and the crushed      
product is generally a mixture of grains
 of all sizes ranging from a  fine dust 
to the largest size of grain passed by  
the sieves attached  to the crushing    
machine4 ,the presence of the very fine 
particles is  a distinct objection from 
the glass-maker's point of view1 so that
  it would probably be necessary to wash
 the sand so as to remove this  dust--a 
process that in itself adds to the cost 
of the crushed stone  and at the same   
time leads to the loss of a serious     
percentage of the  material4 ,objections
 of the same kind apply1 but with still 
greater  force1 to the use of powdered  
quartz or flint as sources of silica for
  the glass-maker2 further1 these       
materials are exceedingly hard and      

therefore difficult to crush1 so that   
the price of the materials is           
prohibitive for glass-making purposes4  
,the use of ground quartz and  flint is 
therefore confined to the ceramic       
industries in which these  substances   
serve as sources of silica for both     
bodies and glazes2  in former times1    
however1 ground flint was extensively   
used in the  manufacture of the best    
kinds of glass1 as the still surviving  
name of  8flint glass0 testifies4       
,minerals of the felspar class1         
consisting essentially of silicates  of 
alumina and one or more of the alkalies1
 are extensively used  in glass-making  
and should be mentioned here1 since     
their high  silica-content 7up to #gj   
per cent47 constitutes an effective     
source of  silica4 ,as a source of this 
substance1 however1 most felspars would 
be  far too expensive1 and their use is 
due to their content of alumina and     
alkali4    7#b7 ,sources of             
,alkali4--,originally the alkaline      

constituents of glass  were derived from
 the ashes of plants and of seaweed or  
8kelp02 in  both cases the alkali was   
obtained in the form of carbonate and   
was  ordinarily used in a very impure   
form2 at the present time1 however1  the
 original source of alkali for          
industrial purposes is found in the     
natural deposits and other sources of   
the chlorides of sodium and  potassium4 
,at the present time it is not yet      
industrially possible  to introduce the 
alkalies into glass mixtures in the     
natural form of  chlorides4 ,the        
principal difficulty in doing this      
arises from the fact  that the chlorides
 are volatile at the temperature of     
glass-melting  furnaces and are only    
acted upon by hot silica in the presence
 of water  vapour4 ,introduced into an  
ordinary glass furnace1 therefore1 these
  salts would be driven off as vapour   
before they could combine with the      
other ingredients in the desired form of
 double silicates4    ,alkalies are1    

therefore1 introduced into the glass    
mixture in less  volatile and more      
readily attackable forms4 ,of these the 
carbonate is  historically the earlier1 
while the sulphate is at the present    
time  industrially by far the more      
important4 ,the ,carbonate of ,soda1 or 
soda ash1 which is used in the          
production of some special glasses1 and 
is an ingredient of ,english flint      
glasses1 is produced by either of two   
well-known chemical processes4 ,one of  
these is the 8black ash10 or 8,le       
,blanc0 process1 in which the chloride  
is first converted into sulphate  by the
 direct action of sulphuric acid1 and   
the sulphate thus formed is  converted  
into the carbonate by calcination with a
 mixture of calcium  carbonate and coal4
 ,the sodium carbonate thus formed is   
separated by  solution and subsequent   
evaporation4 ,a purer form of sodium    
carbonate  can be obtained with great   
regularity by the 8ammonia soda0        
process1  in which a solution of sodium 

chloride is acted upon by ammonia and   
carbonic acid under pressure4 ,soda ash 
produced by this process is now         
supplied regularly for glass-making     
purposes in a state of great purity  and
 constancy of composition4 ,it is upon  
these qualities that the great          
advantages of this substance depend1    
since its relatively high cost          
precludes its use except for special    
kinds of glass1 and for these  purposes 
the qualities named are of great value4 
,for most purposes of glass-making1 such
 as the production of sheet  and        
plate-glass of all kinds1 the alkali is 
introduced in the form  of              
salt-cake--i4e41 sulphate of soda4 ,this
 product is obtained  as the result of  
the first step of the ,le ,blanc process
 of alkali  manufacture--i4e41 by the   
action of sulphuric acid on sodium      
chloride2 salt-cake is thus a relatively
 crude product1 and its use  is due to  
the fact that it is by far the cheapest 
source of alkali  available for         

glass-making4 ,there are1 however1      
certain disadvantages  connected with   
its use4 ,the chief of these is the fact
 that silica  cannot decompose salt-cake
 without the aid of a reducing agent2   
such a  reducing agent is partly        
supplied by the flame-gases in the      
atmosphere  of the furnace1 but in      
addition to these a certain proportion  
of  carbon1 in the form of coke1        
charcoal or anthracite coal must be     
added  to all glass mixtures containing 
salt-cake4 ,the use of a slightly       
incorrect quantity of carbon for this   
purpose leads to disastrous  results1   
while even under the best conditions it 
is not easy to remove  all traces of    
sulphur compounds from glass made in    
this way4 ,a further  risk of trouble   
arises in connection with salt-cake from
 the fact that  it is never entirely    
free from more or less deleterious      
impurities4  ,according to the exact    
manner in which it has been prepared1   
the  substance always contains a small  

excess either of undecomposed sodium    
chloride or of free sulphuric acid1 or  
the latter may be present in the  form  
of sulphate of lime4 ,a good salt-cake1 
however1 should contain  at least #ig   
per cent4 of anhydrous sodium sulphate1 
and not more than  #aj per cent4 of     
either sodium chloride or sulphuric     
acid4 ,while pure  sodium sulphate is   
readily soluble in water1 ordinary      
salt-cake always  leaves an insoluble   
residue1 consisting frequently of minute
 particles  of clay or other material   
derived from the lining of the furnace  
in  which it was prepared1 or from the  
tools with which it was handled2  and   
these impurities are liable to become   
deleterious to the glass if  present in 
any quantity4 ,the insoluble residue    
should not exceed #je  per cent4 in     
amount1 and in the best salt-cake is    
generally under #jb  per cent4          
,salt-cake possesses certain other      
properties that make it somewhat        
troublesome to deal with as a           

glass-making material4 ,thus1 on        
prolonged  exposure1 particularly to    
moist air1 the powdered salt-cake       
absorbs  moisture from the atmosphere   
and undergoes partial conversion into   
the  crystalline form of 8,glauber's    
,salt10 a process which results in the  
formation of exceedingly hard masses4   
,ground salt-cake1 therefore1  cannot be
 stored for any length of time without  
incurring the necessity  of regrinding1 
and this accretive action even comes    
into play when  mixtures of glass-making
 materials1 containing salt-cake as one 
ingredient1 are stored4 ,in practice1   
therefore1 salt-cake can only be  ground
 as it is wanted1 and its physical      
properties make it difficult  to grind  
it at all fine1 while the dust arising  
from this process is  peculiarly        
irritating1 although not seriously      
injurious to health4    ,potash is      
utilised in glass-making almost entirely
 in the form of  carbonate1 generally   
called 8pearl-ash40 ,originally derived 

from the  ashes of wood and other land  
plants1 this substance is now           
manufactured  by processes similar to   
those described in the case of soda1 the
 raw  material being potassium chloride 
derived from natural deposits such  as  
those at ,stassfurth4 ,the pearl-ash    
thus commercially obtainable  is a      
fairly pure substance1 but its use is   
complicated by the fact  that it is     
strongly hygroscopic and rapidly absorbs
 water from the  atmosphere4 ,where it  
is desired to produce potash glasses of 
constant  composition1 frequent         
analytical determinations of the        
moisture  contents of the pearl-ash are 
necessary1 and the composition of the   
glass mixture requires adjustment in    
accordance with the results of  these   
determinations4    ,the alkalies are    
also introduced into glass in the form  
of nitrates  7potassium nitrate1 or     
saltpetre1 and sodium nitrate1 or       
nitre72 but  although these substances  
act as sources of alkali in the glass1  

they  are employed essentially for the  
sake of their oxygen contents4 ,such    
oxidising agents are not1 of course1    
added to glass mixtures containing      
sulphates and carbon1 but are employed  
to purify the mixtures  containing      
alkali carbonates1 and more especially  
to oxidise the flint  glasses4 ,since   
these substances are only introduced    
into glass in small  quantities their   
extreme purity is not of such great     
importance to the  glass-maker1 and the 
ordinary 8refined0 qualities of both    
nitrates are  found amply pure enough to
 answer the highest requirements4    ,a 
certain number of natural minerals which
 contain an appreciable  quantity of    
alkali are sometimes utilised as raw    
materials for  glass manufacture4 ,the  
most important of these are the minerals
 of  the felspar class already referred 
to4 ,these1 however1 contain a          
considerable proportion of alumina1     
while all but the purest varieties  also
 contain more or less considerable      

quantities of iron4 ,some  glass-makers 
regard alumina as an undesirable        
constituent1 while others  take the     
opposite view1 and upon this view their 
use of felspathic  minerals will depend4
 ,for the cheaper varieties of glass1   
however1  such as bottle glass1         
felspathic minerals and rocks1 such as  
granite  and basalt1 are freely used as 
raw materials4 ,another mineral in which
  both alkali and alumina are found is  
cryolite4 ,this mineral is a  double    
fluoride of soda and alumina1 whose     
properties are particularly  valuable in
 the production of opal and opalescent  
glasses4 ,as a mere  source of alkali1  
however1 cryolite is much too expensive4
    7#c7 ,sources of ,bases other than  
,alkalies4--,the most important of      
these are lime and lead oxide1 the      
former being required for the           
production of all varieties of plate and
 sheet-glass1 as well as for  bottles   
and a large proportion of pressed and   
blown glass1 while lead  is an essential

 ingredient of all flint glass4 ,the    
only other base  having any considerable
 commercial importance in connection    
with  glass-making is barium oxide1     
while oxide of zinc1 magnesia1 and a    
few other substances are used in the    
manufacture of special glasses  for     
scientific1 optical or technical        
purposes1 where glass of special        
properties is required4 ,the metallic   
oxides which are used for the           
production of coloured glass are1 of    
course1 also basic bodies4 ,these  will 
be treated in connection with coloured  
glasses1 with the exception  of         
manganese dioxide1 which is used in     
large quantities in the  manufacture of 
many ordinary 8white0 glasses4          
,calcium ,oxide 7lime7 is generally     
introduced into glass mixtures  in the  
form of either the carbonate or the     
hydrated oxide 7slaked  lime74 ,the     
carbonate may be derived either from    
natural sources1 or  it may be of       
chemical origin1 while the hydrate is   

always obtained  by the calcination of  
the carbonate1 followed by 8slaking0 the
 lime  thus produced4 ,natural calcium  
carbonate occurs in great quantities  in
 the form of chalk and limestone rocks4 
,both varieties are used for            
glass-making4 ,chalk is a soft friable  
material which is apt to clog  during   
the grinding operations1 particularly as
 the natural product  is generally      
somewhat moist4 ,as regards the greater 
part of its mass1  chalk is often found 
in a state of great purity1 but it is   
frequently  contaminated by the presence
 of scattered masses of flint4          
,chemically  this impurity is not very  
objectionable to the glass-maker1 since 
it  merely introduces a small proportion
 of silica whose presence need  scarcely
 be allowed for in laying down the      
mixture4 ,on the other  hand1 if any    
fragments of flint remain in the mixture
 when put into  the furnace1 they prove 
very refractory1 and are apt to be found
 as  opaque enclosures in the finished  

glass4 ,natural limestone can also be   
obtained in great purity in many parts  
of the world4 ,it is generally  a hard  
and rather brittle rock that can be     
readily ground to powder  of the        
requisite degree of fineness4 ,flint    
concretions are not so  frequently found
 in this material1 but1 on the other    
hand1 it is often  contaminated with    
magnesia and iron4 ,the former          
ingredient1 when  present in small      
quantities1 tends to make the glass hard
 and viscous1  so that limestone of the 
lowest possible magnesia content should 
be  used1 especially for the harder     
kinds of glass1 such as plate and       
sheet-glass1 etc4 ,the iron contents of 
the limestone used must also  be low    
where a white glass is required2 but    
since a smaller quantity  of limestone  
is used for a given weight of glass     
produced than the  quantity of sand used
 for the same purpose1 the presence of a
 somewhat  higher percentage of iron is 
permissible in the limestone as compared

  with the sand2 for the better         
varieties of glass1 however1 the iron   
should not exceed #jc per cent4 of the  
limestone4    ,slaked lime is sometimes 
used as the source of lime for special  
glasses  where the process of           
manufacture renders it desirable to     
avoid the  evolution of carbonic acid   
gas which takes place when the carbonate
 is  heated and attacked by silica4     
,when slaked lime is used only the water
  vapour of the hydrate is driven off1  
and this occurs at a much lower         
temperature4 ,for the production of     
slaked lime1 an adequately pure form  of
 limestone1 preferably in the form of   
large lumps1 is burnt in a kiln  until  
the carbonic acid is entirely driven    
off2 after cooling1 the  lime so formed 
is slaked by hand4 ,the product so      
obtained is1 however1  apt to vary both 
as regards contents of moisture and     
carbonic acid1  which latter is readily 
absorbed from the atmosphere2 the use of
 this  material1 therefore1 requires    

frequent analytical determinations of   
the  lime contents and corresponding    
adjustments of the mixture if constant  
results are required4    ,it is possible
 to introduce lime into glass mixtures  
in the form of  gypsum or calcium       
sulphate1 but the decomposition of this 
compound1  like that of sodium sulphate1
 requires the intervention of a reducing
  agent such as carbon1 and the         
difficulties arising from this source in
  connection with the use of salt-cake  
are still further increased in the  case
 of the calcium compound4 ,since        
limestones of considerable purity  are  
more or less plentiful in many          
districts1 the commercial value of      
calcium sulphate for glass-making is    
probably slight4    ,the ,compounds of  
,barium may best be dealt with at this  
stage1 since  they are chemically so    
closely allied to the compounds of lime 
just  described4 ,barium occurs in      
nature in considerable quantities in the
  minerals known as barytes 7heavy spar7

 and witherite respectively4  ,the      
former is essentially sulphate of       
barium1 while the latter is  a carbonate
 of barium4 ,the use of the sulphate    
meets with the same  objection here as  
in the case of calcium sulphate         
discussed above1  except that the barium
 compound is much more easily reduced   
and  decomposed than the lime compound4 
,the natural mineral witherite is  used 
to a considerable extent in the         
production of barium glasses1 and  these
 have been found capable of replacing   
lead glasses for certain  purposes4 ,on 
the other hand1 for the best kinds of   
barium glasses1  viz41 those required   
for optical purposes1 the element is    
introduced in  the form of artificially 
prepared salts4 ,of these the most      
important  is the carbonate1            
commercially described as 8precipitated 
carbonate  of barium02 this precipitated
 compound1 however1 does not ordinarily 
correspond to the chemically pure       
substance1 but contains more or less    

considerable quantities of sulphur      
compounds4 ,the question whether  these 
impurities are or are not objectionable 
can only be determined  for each        
particular case1 since much depends upon
 the special character  of the glass to 
be produced4 ,both the nitrate and the  
hydrate of barium  are commercially     
available1 but they are very costly     
ingredients for  use in the production  
of even the most expensive kinds of     
glass2 these  substances are1 however1  
obtainable in a state of considerable   
purity1  although the hydrate has the   
inconvenient property of rapidly        
absorbing  carbonic acid from the       
atmosphere1 thus becoming converted into
 the  carbonate4    ,magnesia is another
 glass-forming base that is closely     
related1  chemically1 to calcium and    
barium4 ,this element is usually        
introduced  into glass mixtures in the  
form of either the carbonate or the     
oxide4  ,the carbonate occurs in nature 
in a more or less pure state in the form

  of magnesite1 and by calcination1 the 
oxide is obtained4 ,the natural  mineral
 and its product are1 of course1 by far 
the cheapest sources  of magnesia1 but  
as the element is only used in          
comparatively small  quantities1 the    
artificial precipitated carbonate or    
calcined magnesia  are frequently       
preferred4 ,magnesia is only introduced 
intentionally in  notable quantities in 
special glasses where the properties it 
confers  are of special value2 in       
ordinary lime glasses this element1 as  
has  already been mentioned1 is to be   
regarded as an undesirable impurity4    
,zinc oxide lies1 chemically1 between   
the bases already discussed  on the one 
hand1 and lead oxide on the other4 ,this
 element is only  introduced into       
special optical glasses1 a special 8zinc
 crown0 having  found some application4 
,chemically prepared zinc oxide is      
almost the  only form in which the      
element is used1 but the very volatile  
character  of this substance must be    

borne in mind when it is introduced into
  glass mixtures4    ,lead is one of the
 most widely-used ingredients of glass2 
the glasses  containing this substance  
in notable quantity are all             
characterised to  a greater or less     
degree by similar properties1 such as   
considerable  density and high          
refractive power1 and are classed       
together under the  name 8flint         
glasses40 ,lead is now almost           
universally introduced into  glass      
mixtures in the form of red lead1       
although the other oxides of  lead might
 be employed almost equally well4 ,red  
lead is a mixture  of two oxides of lead
 7,pb,o and ,pb#b,o#c7 in approximately 
such  proportions as to correspond to   
the formula ,pb#c,o#d4 ,it is prepared  
by the roasting of metallic lead in     
suitable furnaces1 where the molten     
lead is exposed to currents of hot air4 
,the product is obtainable in           
considerable purity1 very small         
proportions of silica1 derived from the 

furnace bed1 and of iron derived from   
the tools with which the lead is        
handled1 being the principal foreign    
substances found in good red lead4      
,silver would be an objectionable       
impurity1 but owing to the modern       
perfect methods of de-silvering lead1   
that element is rarely found in  lead   
products4 ,analytical control of red    
lead as used in the glass  mixtures1 and
 consequent adjustments of the mixture1 
are1 however1  necessary where exact    
constancy in the glass produced is      
desired4 ,the  reason for this necessity
 lies in the fact that the oxygen       
content1 and  therefore the lead-oxide  
7,pb,o7 content1 varies decidedly from  
batch to  batch1 while the material as  
actually delivered and used frequently  
contains notable proportions of         
moisture4    ,a word should perhaps be  
said here as to methods of handling red 
lead  on account of the injurious       
effects which the inhalation of lead    
dust  produces upon the workmen exposed 

to it4 ,for glass-making purposes  it is
 not feasible to adopt the method       
adopted by potters of first  8fritting0 
the lead and thus rendering it          
comparatively insoluble  and innocuous2 
even if this were done1 the difficulty  
would only be  moved one step further   
back1 and would have to be overcome by  
those  who undertook the preparation of 
the frit4 ,the proper solution of the   
problem1 in the writer's opinion1 is to 
be found in properly preventing  the    
formation of lead dust1 or at all events
 in protecting the workmen  from the    
risk of inhaling it4 ,where only small  
quantities of lead  glass are made1 and 
therefore only small quantities of lead 
are handled  and mixed at a time1 it is 
no doubt sufficient to provide the      
workmen  engaged on this task with some 
efficient form of respirator to be worn 
during the whole of the time that they  
are engaged on such work1 and  to take  
the further precautions necessary--by   
way of cleanliness and  the provision of

 proper mess-rooms--to avoid any risk of
 lead dust  either directly or          
indirectly contaminating their food4    
,where1 however1  large quantities of   
flint-glass are made every day1 it is   
possible and  proper to make more       
perfect arrangements for the mechanical 
handling  and mixing of the lead with   
the other ingredients by the provision  
of suitable mixing and transporting     
machinery1 so arranged as to be         
dust-tight4 ,it is only fair to state1  
however1 that partly under their  own   
initiative1 partly under pressure from  
the authorities1 glass  makers in this  
country are complying with these        
requirements in an  adequate manner4    
,aluminium4--,there are several         
varieties of glass into which alumina   
enters in notable quantities1 the       
principal examples being certain        
optical and many opal glasses1 while    
most ordinary glasses contain  this     
substance in greater or less degree4 ,in
 the latter1 the alumina  is derived by 

the inevitable processes of solution1   
from the fire-clay  vessels or walls    
within which the molten glass is        
contained1 while in  some cases the     
element is intentionally introduced in  
small proportions  7about #b per cent4  
to #c per cent4 of ,al#b,o#c7 by the use
 of  felspar as an ingredient of the    
mixture4 ,where larger proportions of   
alumina are required1 the substance is  
introduced in the form of the  hydrate1 
which is obtainable commercially in a   
state of almost chemical  purity1 but of
 course at a correspondingly high cost4 
,in opal glasses  alumina is derived    
partly or wholly from felspars1 or in   
some cases  from the use of the mineral 
cryolite4 ,this is a double fluoride of 
aluminium and sodium which is found in  
great natural masses1 chiefly in        
,greenland4 ,owing to the high price of 
this mineral1 however1 artificial       
substitutes of nearly identical         
composition and properties have been    
introduced and are used successfully in 

the glass and enamelling  industries4   
,manganese4--,although the oxides of    
this element really belong to  the class
 of colouring compounds1 they are so    
widely used in the  manufacture of      
ordinary 8white0 glasses that it is     
desirable to deal  with them here4 ,the 
element manganese is most usually       
introduced into  glass mixtures in the  
form of the per-oxide 7,mn,o#b71        
although the  lower oxide 7,mn#c,o#d7   
can also be used4 ,the material         
ordinarily  used is the natural         
manganese ore1 mined chiefly in ,russia2
 the purest  forms of this ore consist  
almost entirely of the per-oxide1 but   
8brown0  ores1 containing more or less  
of the lower oxide1 are also used with  
success4 ,these ores always contain     
small amounts of iron and silica1  but  
provided the iron is not present in any 
considerable quantity1 the  value of the
 ore is measured by the percentage of   
manganese which it  contains4 ,the      
colouring and 8decolourising0 action of 

manganese will be  discussed in a later 
chapter4 ,certain other substances1     
which have been  suggested as either    
substitutes for1 or improvements upon1  
manganese  for this purpose need only be
 mentioned here1 viz41 nickel1 selenium 
and gold4    ,arsenic is another        
substance frequently introduced into    
8white0  glass mixtures4 ,this element  
is universally introduced in the form of
  the white arsenic of commerce 7i4e41  
arsenious acid1 ,as#b,o#c7  which is    
obtained in a pure form by a process of 
sublimation4 ,owing to  the very        
poisonous nature of this material1      
special precautions must  be taken in   
its use for glass-making purposes to    
avoid all risk of  poisoning4           
,carbon4--,as has already been          
indicated1 an admixture of carbon in    
some suitable form is essential in the  
case of certain glass mixtures4  ,the   
carbon for this purpose may be used in  
the form of either charcoal1  coke1 or  
anthracite coal4 ,of these1 charcoal is 

undoubtedly the purest  form of carbon1 
but it is excessively expensive in this 
country4 ,coke  varies very much in     
quality according to the coal from which
 it has  been produced1 but it always   
contains notable proportions of ash rich
  in iron1 and also some sulphur4       
,anthracite coal can be obtained in a   
very pure form1 containing considerably 
less ash than that found in  most kinds 
of coke1 and this is therefore probably 
the most convenient  form of carbon for 
this purpose4          ,,chapter ,,iv4  
,,crucibles ,,and ,,furnaces ,,for ,,the
 ,,fusion ,,of ,,glass4      ,for the   
successful production of substances     
which are formed by a  process of       
fusion1 the use of refractory materials 
of a proper kind is  of great           
importance4 ,in the production of glass 
the double difficulty  has to be        
overcome of finding substances capable  
of being formed into  furnaces and      
crucibles which shall not only resist   
the softening and  melting action of the

 furnace heat for long periods of time1 
but  shall also resist the dissolving   
action of the molten glass itself4  ,the
 refractory materials employed in       
connection with glass-making  thus fall 
into two distinct groups1 members of one
 group being those  which meet both of  
the above requirements and can therefore
 be used in  positions exposed to direct
 contact with molten glass1 while       
members  of the second group are        
materials which resist the action of the
 heat  and flame gases but cannot resist
 the dissolving effect of the glass     
itself2 these1 of course1 can only be   
placed where molten glass is not  liable
 to touch them4 ,we shall deal with the 
former group first4    ,those portions  
of glass-melting plant which come into  
contact with  molten glass are almost   
universally made of some form of        
fire-clay4  ,to discuss in detail the   
composition and properties of the       
varieties  of fire-clay best suited to  
this purpose would exceed the entire    

limits of this book1 so that only a few 
leading principles can be  stated4      
,taking first the clays intended for the
 production of crucibles  or 8pots10 we 
find that for the purposes of the       
production of such  objects the prepared
 clay must possess a certain degree of  
plasticity  while damp and a            
considerable degree of strength when    
dried4 ,the  dried and burnt material   
must be so refractory as to resist the  
high  temperatures used in glass-melting
 without undergoing fusion or even      
serious softening4 ,clays of various    
composition and physical nature  also   
differ very widely in their power of    
resisting the chemical  attack of molten
 glass2 all clays are more or less      
dissolved under  these circumstances1   
but not only the rate1 but also the     
manner1 of  dissolution is of           
importance1 so that frequently a clay   
which dissolves  rapidly but uniformly  
is preferred to one which dissolves more
  slowly but in such an irregular manner

 as to throw off particles of           
undissolved material which contaminate  
the glass in the form of opaque         
enclosures or 8stones40 ,it is also to  
be noted that the best results  in this 
direction can only be obtained by       
careful adaptation of the  clay employed
 to the particular kind of glass which  
is to be melted in  the crucibles in    
question4 ,in ,england this question has
 not received  the amount of attention  
it deserves1 but in ,germany and        
,america the  available fire-clays of   
the country have been systematically    
studied  and exploited4 ,as a result the
 glass-maker has at his disposal a large
  selection of materials of accurately  
known physical and chemical  properties4
 ,by carefully correlating these with   
the performance of his  8pots0 in the   
furnaces1 the manufacturer is able to   
select the most  suitable material1 and 
is1 moreover1 in a position to know in  
what  direction to look for improvement 
or for replacement if the supply of a   

satisfactory brand should cease4    ,we 
may now follow briefly the process of   
manufacture of a fire-clay  pot or      
crucible4 ,the size and shape of the    
crucible will depend upon  the          
particular purpose for which it is      
intended4 ,crucibles varying in         
capacity from #d cwt4 to #b tons of     
glass are used for various kinds  of    
glass1 but the more usual sizes lie     
between #cj in4 and #ej in4 in          
diameter4 ,for many kinds of glass the  
shape of the pot is simply that  of an  
open basin1 circular or oval in plan and
 larger in diameter at  the brim than at
 the base 7,fig4 #a71 but for the       
production of flint  glass1 and of other
 glasses which are to be protected from 
contact with  the flame and gases of the
 furnace1 so-called 8covered0 pots are  
used4  ,in these the basin--here of a   
more nearly cylindrical shape--is       
covered  over by a dome1 and access is  
allowed only by a relatively small      
hooded  opening 7,fig4 #b74 ,covered    

pots are built up on wooden moulds1     
which are  made collapsible1 and are    
removed before the drying of the pot is 
begun4    ,illustration3 ,,fig4         
#a4--,open 8pot0 or crucible for glass  
melting4    ,illustration3 ,,fig4       
#b4--;,covered pot for glass melting1 as
 used for  flint glass and optical      
glass4    ,the material for pot-making  
is first prepared with great care4 ,the 
proper variety of clay having been      
selected1 it is ground to a fine  powder
 in suitable mills and carefully sieved2
 with this fine clay  powder is mixed1  
in accurately determined proportions1 a 
quantity of  crushed burnt fire-clay4   
,in some works this burnt material is   
obtained  by simply grinding up         
fragments of old used pots1 but the     
better  practice is to burn             
specially-selected fire-clay separately 
for this  purpose4 ,the quantity of such
 burnt material added to the mixture    
depends upon the chemical nature and    
especially on the plasticity of  the    

virgin clay employed2 with so-called    
8fat0 or very plastic clays up  to #ej  
per cent4 of burnt material is added1   
but with the leaner clays1  such as     
those of the ,stourbridge district in   
,england1 very much smaller  proportions
 are used4 ,the object of this addition 
of burnt material is  to facilitate the 
safe drying of the finished pots and to 
diminish--by  dilution--the total amount
 of contraction which takes place both  
when  plastic clay is allowed to dry1   
and further when the dry mass is        
subsequently burnt2 the burnt material  
or 8chamotte10 having already  undergone
 these shrinking processes1 acts both as
 a neutral diluent  and also as a       
skeleton strengthening the whole mass   
and reducing the  tendency to form      
cracks4    ,the virgin clay and chamotte
 having been intimately mixed1 the whole
  mass is 8wet up0 by the addition of a 
proper proportion of water and          
prolonged and vigorous kneading1 usually
 in a suitable pug mill4 ,the  mass     

leaves this mill as a fairly stiff1     
plastic dough1 but the full  toughness  
and plasticity of such clay mixtures can
 only be developed by  prolonged storage
 of the damp mass4 ,in the next stage of
 the process1  the plastic clay is      
passed to the 8pot maker0 in the form of
 thick  rolls1 and with these he        
gradually builds up the pots or         
crucibles from  day to day1 allowing the
 lowest parts to dry sufficiently to    
enable  them to bear the weight of the  
upper parts without giving way4 ,the    
building of large pots in this way      
occupies several weeks1 and during  this
 time the premature drying of any part  
of the pot must be carefully  avoided4  
,after the completion of the pot1 drying
 is allowed to take  place1 slowly at   
first1 but more vigorously after a time 
when the risk  of cracking is smaller2  
when it is taken into use1 the pot is   
usually  many months old and is         
thoroughly air-dry4 ,the clay1 however1 
is still  hydrated1 i4e41 contains      

chemically combined water1 and this is  
only  expelled during the early stages  
of the burning process4 ,this process   
is carried out in smaller furnaces or   
kilns placed near the melting  furnaces4
 ,in these the pot or pots are exposed  
to a very gradually  increasing         
temperature1 until a bright red heat is 
finally attained4  ,this is a delicate  
process in which great care is required 
to secure  gradual and uniform heating1 
especially during the earlier stages1   
otherwise the pots are apt to crack and 
become useless4 ,finally1 when  a bright
 red heat has been maintained for at    
least a day1 the pots are  ready to be  
placed in the furnace1 and this is      
ordinarily done while  both pots and    
furnace are at a red heat1 the pots     
never being allowed  to cool down again 
once they have been burnt4    ,fire-clay
 is also used in the manufacture of     
bricks and blocks of  various sizes     
required for the construction of        
glass-melting furnaces4  ,here fire-clay

 is only used in positions where contact
 with molten  glass is expected1 as in  
the walls of the basin or tank proper in
  8tank0 furnaces1 or at a level below  
that of the pot or crucible in  pot     
furnaces2 in the latter position leakage
 of glass from broken pots  or overflow 
being liable to result in an            
accumulation of molten glass  on the    
floors or walls of the furnace and      
passages4 ,the fire-bricks  used in     
these latter positions are usually of a 
much poorer quality  of fire-clay than  
that used for the manufacture of pots1  
and this  is justified in so far as     
certain of the requirements that apply  
to  crucibles do not apply here--but on 
the other hand the use of more          
refractory bricks would result in a     
longer life for the furnace4 ,such      
bricks1 it should be noted1 are not laid
 in mortar when used for  furnace       
construction1 but are set in a thin     
paste of fire-clay in  water1 and these 
joints are kept as thin as possible4    

,the part of the  furnace known as the  
8siege0 7,french 8sige071 i4e41 the     
floor of the  furnace upon which the    
pots are placed1 is usually built of    
very large  blocks of fire-clay1 made of
 coarse materials calculated to give    
great  strength4 ,at or near the points 
where the flame enters the furnace1     
these blocks rapidly wear away1 partly  
by melting but chiefly by a  process of 
abrasion1 for it seems that a rapidly   
moving flame has an  abrading action of 
a very marked kind4    ,the actual tanks
 or basins which contain the molten     
glass in tank  furnaces are also built  
of large blocks of fire-clay1 but these 
are  made of the best procurable        
materials1 and should receive at least  
as  much care in every respect as       
crucibles2 it is true that their shape  
and size gives them greater strength1   
but on the other hand these  blocks are 
expected to resist the contact of molten
 glass for very  much longer periods of 
time than the average crucible4 ,to     

understand  the requirements for        
tank-blocks it is necessary to          
anticipate the  next section to the     
extent of stating that in tank furnaces 
the glass  is contained1 during melting1
 refining and working1 in a basin built 
up of large blocks4 ,these blocks are   
not cemented together in any way1  but  
are built up 8dry0 and are supported on 
the outside by a system of  iron bars   
and rods4 ,the molten glass penetrates  
between the blocks to a  certain extent1
 but as the outside of all such blocks  
is intentionally  kept as cold as       
possible the glass rapidly stiffens as  
it penetrates  further into these       
interstices1 and this stiffened glass   
effectually  binds the blocks together  
and prevents all leakage4 ,it will thus 
be  seen that the blocks are exposed to 
the full heat of the furnace and  to the
 corroding action of the glass on the   
inner side1 but are kept  cold on the   
outer side4 ,as this state of affairs   
tends to produce  cracks1 these blocks  

are necessarily made of rather coarse   
material4  ,on the other hand1 the      
material of a block never gets so hot as
 the  wall of a crucible1 which is      
heated from both sides1 so that extreme 
refractoriness is not so essential4     
,it is impossible1 within the limits of 
this chapter1 to go into the  details of
 the choice of materials for            
tank-blocks2 it is a subject  upon which
 no finally satisfactory conclusion has 
yet been reached1  and what has been    
said above will suffice to show the     
nature of the  considerations upon which
 such choice must be based4    ,we now  
turn to the second class of refractory  
materials used in the  construction of  
glass-melting furnaces1 viz41 those     
which are so  placed as not to come into
 contact with molten glass4 ,here       
mechanical  strength and refractoriness 
are almost the only considerations1  but
 in the roof-vaults or 8crowns0 of tank 
furnaces and also of  furnaces in which 
glass is melted in open pots1 there is  

the further  consideration that the     
material of the bricks used shall not   
contain  notable quantities of any      
colouring oxide1 since small flakes1    
etc41  are apt to drop down into the    
molten glass1 and would thus be liable  
to  cause serious discolouration4 ,such 
a material as chrome-ore brick is       
therefore excluded4 ,as a matter of     
fact1 some form of 8silica brick0 is  in
 universal use4 ,bricks of this         
material1 otherwise known as 8,dinas    
bricks0 from the place of their first   
origin1 in ,wales1 consist of  about #ih
 per cent4 of silica 7,si,o#b74 ,pure   
silica cannot be baked  or burnt into   
coherent bricks entirely by itself1     
since it possesses  neither plasticity  
when wet nor any binding power when     
burnt1 but an  admixture of about #b per
 cent4 of lime and a little alumina     
makes it  possible first to mould the   
bricks when wet and then to burn them so
  as to form fairly strong1 coherent    
blocks4 ,these are of amply adequate    

refractoriness for the highest          
temperatures that can be attained  in   
industrial gas-fired furnaces1 and their
 mechanical strength is  sufficient to  
make it possible to build vaults of     
considerable span1  but on the other    
hand this material requires very gradual
 heating and  constant watching while   
the temperature is rising or falling to 
any  considerable extent2 the reason for
 this difficulty lies in the fact  that 
silica bricks swell very markedly during
 heating1 so that unless a  vault built 
of this material is given room to spread
 somewhat1 it will  rise seriously and  
may even break up completely4 ,this risk
 is avoided  by gradually slackening the
 tie-bolts that hold the vault together1
  and correspondingly 8taking up the    
slack0 as the vault cools when the      
furnace is let out4 ,sudden local heat  
also has a disastrous effect on  this   
material1 producing serious flaking4    
,for positions where intense  heat is to
 be borne1 and at the same time         

mechanical strength is  required1 silica
 brick is a most valuable material1 but 
owing to its  chemical composition it is
 rapidly attacked by molten glass or by 
any  material containing a notable      
proportion of basic constituents1 so    
that  the silica bricks can only be     
employed out of contact with glass4     
,we now turn to consider1 very briefly1 
the general design and  arrangement of  
some typical glass-melting furnaces4    
,the oldest  and simplest form of       
furnace is1 in effect1 simply a box     
built of  fire-brick1 in the centre of  
which stands the crucible1 while a fire 
of wood or coal is placed upon either   
side4 ,to attain any great degree  of   
heat by such means1 however1 the size of
 the box or chamber and  especially of  
the grates in which the fires are       
maintained must be  properly            
proportioned both to the dimensions of  
the crucible and to  each other4 ,the   
grates are generally wide and deep1     
while draught is  provided by means of a

 tall conical chimney which stands over 
the  entire chamber and communicates    
with it by a number of small openings4  
,in a more refined furnace1 the chamber 
itself is double1 and the flame1  after 
playing around the crucible in the      
inside of the chamber1 is made  to pass 
through the space between the outer and 
inner chamber before  passing to the    
chimney or cone4 ,we need not give any  
greater attention  to these primitive   
furnaces1 since they are practically    
obsolete at  the present time4 ,in      
modern furnaces the process of          
combustion is  carried on in two        
distinct stages2 the first stage takes  
place in a  subsidiary appliance known  
as a 8gas producer10 where part of the  
heat which the fuel is capable of       
generating is utilised for the          
production of a combustible gas2 this   
gas passes into the furnace  proper1    
either direct1 while it is still hot    
from the producer1 or  after being      
conveyed some distance1 when it is again

 heated up by  the waste heat of the    
furnace4 ,in either case the gas is hot 
when it  enters the furnace proper1 and 
there it meets a current of air1 also   
heated by the aid of the waste heat of  
the furnace4 ,hot gas and hot  air burn 
rapidly and completely1 and if properly 
proportioned yield  exceedingly high    
temperatures4 ,seeing that in this      
process a part of  the heat of          
combustion yielded by the fuel is       
generated in a subsidiary  appliance and
 is thus lost to the furnace1 it appears
 at first sight  somewhat surprising    
that this system of firing is very      
considerably  more efficient than the   
old 8direct0 system where the whole of  
the  fuel is burnt in the furnace       
itself4 ,but the advantage arises from  
the fact that in the newer system the   
fuel is handled in the gaseous  form4   
,this has the advantage1 first and most 
important1 that the heat  escaping from 
the furnace in the hot products of      
combustion 7chimney  gases7 can be      

transferred to the incoming unburnt gas 
and air and  can thus be returned to the
 furnace4 ,the manner in which this is  
accomplished will be considered below1  
but it may be noted here that  in some  
furnaces the escaping products of       
combustion are so thoroughly  cooled    
that they are unable to produce an      
effective draught in the  chimney of the
 furnace4 ,another advantage of the use 
of gaseous fuel  is the fact that       
complete combustion can be obtained     
without the use  of so great excess of  
air1 such as is required when solid     
fuels are to  be burnt completely4 ,for 
this reason much higher temperatures can
 be  readily obtained with gaseous fuel1
 while the pre-heating of both gas  and 
air also facilitates the attainment of  
high temperatures2 further1  the great  
facility with which the flow of either  
gas or air can be  regulated by means of
 suitable valves1 makes it possible to  
secure much  greater regularity in the  
working of the furnaces4 ,finally1 in   

modern  gas-producers1 the amount of    
sensible heat generated and therefore   
lost  to the furnace1 is kept very low1 
the greater part of the heat set free   
by the partial combustion of coal in the
 producer being absorbed by the         
decomposition of a corresponding        
quantity of steam into hydrogen and     
carbonic oxide gas4 ,the gas as it      
leaves one of these producers is not    
very hot1 and the percentage of heat    
lost in this way is therefore much      
smaller than in the older forms of      
gas-producer4    ,it is again           
impossible1 within the limits of this   
chapter1 to enter  into the details of  
construction and working of             
gas-producers4 ,we must  content        
ourselves with saying that most modern  
producers are of the  form of a tower in
 which a thick bed of fuel is partially 
burnt and  partly gasified under the    
action of a blast of air mixed with     
steam4  ,the chemical actions that take 
place are complicated1 but the final    

result is the production of a gas       
containing from #b to #h or #aj per     
cent4 of carbonic acid1 #aj to #bj per  
cent4 of hydrogen1 #h to #be per  cent4 
of carbonic oxide 7,,co71 #a to #c per  
cent4 methane 7,,ch#d71 and #de  to #fj 
per cent4 of nitrogen1 with varying     
quantities of moisture1 tarry  matter1  
and ammonia4 ,in good producer gas1 the 
combustible constituents  7hydrogen1    
carbonic oxide and methane7 should total
 from #cj to #dh per  cent4 of the whole
 by volume1 but the exact composition to
 be expected  depends very much on the  
type of producer and the class of fuel  
used4  ,some producers are capable of   
dealing with exceedingly low-grade      
fuels1 and the gas which they yield can 
still be utilised for obtaining  the    
highest temperatures--a proceeding that 
would have been impossible  if it had   
been attempted to burn these fuels      
directly in the furnace4                
,illustration3 ,,fig4 #c4--;,diagram of 
the arrangements of a regenerative      

furnace4    ,the gas on leaving the     
producer passes along fire-brick flues  
or  passages to the furnace proper2 the 
path which it is now caused to  take    
varies somewhat according to the        
arrangement of the furnace in  question4
 ,modern gas-fired furnaces usually     
belong to one of two  distinct types    
according to the manner in which the    
heat of the  escaping products of       
combustion is utilised for heating the  
incoming  gas and air2 these two types  
are known as the 8regenerative0 and the 
8recuperative0 respectively4 ,in        
regenerative furnaces the hot products  
of combustion1 after leaving the furnace
 chamber proper1 and before  reaching   
the chimney1 pass through chambers which
 are loosely stacked  with fire-bricks2 
these chambers absorb the heat of the   
escaping  gases1 and thus rapidly become
 hot4 ,as soon as a sufficiently high   
temperature is attained in these        
chambers or 8regenerators10 the path  of
 the gas-currents is altered2 the       

escaping products of combustion are     
made to pass through1 and thus to heat a
 second set of regenerating  chambers1  
while the incoming gas and air are drawn
 through the heated  regenerator        
chambers before entering the furnace    
chamber proper4 ,the  incoming gas and  
air are thus heated1 absorbing in turn  
the heat stored  in the brickwork of the
 regenerators4 ,it is evident that two  
sets of  such regenerators are          
sufficient1 the one set undergoing the  
heating  process at the hands of the    
escaping products of combustion1 while  
the  other set is giving up its heat to 
the incoming gas and air2 when this     
process has gone far enough1 it is only 
necessary to interchange the  two sets  
of chambers1 by the operation of        
suitable valves1 and this  series of    
alternations may be continued           
indefinitely4 ,the arrangement  is shown
 diagrammatically in ,fig4 #c4    ,in   
recuperative furnaces the same principle
 is utilised in a somewhat  different   

manner2 the outgoing products of        
combustion pass through  tubular        
channels formed in fire-clay blocks1    
while the ingoing gas  and air pass     
around the outside of these same blocks2
 the heat of the  outgoing gases is thus
 transferred to the incoming gases by   
the process  of conduction through the  
fire-clay walls of the recuperator      
tubes4  ,the relative merits of the two 
systems have been hotly contested2  the 
regenerative system has the advantage of
 avoiding all reliance on  the heat     
conductivity of fire-clay1 while it also
 avoids the somewhat  complicated       
special tubular blocks required for the 
other system2 on  the other hand1 the   
recuperative system avoids the necessity
 for all  8reversing0 valves and their  
regular periodical working1 while it    
also  occupies somewhat less space4     
,temperatures sufficiently high for all 
glass-melting purposes can be attained  
by both means4    ,in both systems of   
furnace1 heated gas and heated air are  

admitted  to the furnace by separate    
fire-brick flues or passages1 air and   
gas  being allowed to mix just before   
they enter the furnace chamber proper4  
,the economy and efficiency of the      
furnace depend to a very great  extent  
upon the manner in which this mixing is 
accomplished4 ,rapid and  complete      
mixing of air and gas results in an     
intensely hot1 but short  and local     
flame1 while slower mixing tends to     
lengthen the flame and  spread the heat 
through the entire furnace chamber2 on  
the other hand  if the mixing of gas and
 air is too slow1 combustion may not    
have been  completed in the short time  
occupied by the gases in passing through
  the furnace1 and combustion may either
 continue in the outflow flues  and     
regenerators1 or it may be prevented by 
the narrowness of these  passages1 and  
unburnt gases may pass to the chimney4  
,when the openings  or 8ports0 are      
properly proportioned1 and the draught  
of the chimney  is properly regulated1  

combustion should be just complete as   
the gases  leave the furnace chamber1   
and under these circumstances small     
tongues  of keen flame will escape from 
every opening in the furnace2 large     
smoky flames issuing from a gas-fired   
furnace indicate incomplete  combustion4
    ,illustration3 ,,fig4               
#d4--,sectional diagram of a            
regenerative pot furnace  working with  
covered pots4    ,as has already been   
indicated1 glass is melted either in    
pots or  crucibles of various shapes and
 sizes1 or in open tank furnaces4 ,the  
general arrangement of a pot furnace    
working with closed or 8covered0        
crucibles is shown in ,fig4 #d4 ,in this
 particular furnace1 the 8ports0  or    
apertures by which the gas and air enter
 the furnace chamber1 are  placed in the
 floor of the chamber1 but these        
apertures are often  placed in the side 
or end walls1 or even in a central      
column1 the  object being in all cases  
to heat all the pots as uniformly as    

possible  and to avoid any intense local
 heating1 which would merely endanger   
the particular crucible exposed to it1  
without greatly aiding the  real work of
 the furnace4 ,in pot furnaces1 however1
 in which the more  refractory kinds of 
glass are to be melted1 it is generally 
considered  desirable that the flame    
should be made to play about the pots in
  such a way as to heat the lower parts 
of the pots most strongly4 ,in          
connection with the question of the     
uniformity of heat distribution  in a   
gas-fired furnace it must further be    
borne in mind that in the  case of      
regenerative furnaces the direction of  
the flame is reversed  every time the   
valves are thrown over1 and in practice 
this is done  about once every          
half-hour2 this proceeding1 of course1  
tends very  much to equalise the        
temperature of the two sides of the     
furnace4 ,in  recuperative furnaces1 on 
the other hand1 the direction of the    
flame  is not changed1 and for that     

reason a flame returning upon itself1   
usually called a horse-shoe flame1 is   
often employed2 this is obtained  by    
placing the entry and exit ports side by
 side at one end of the  furnace2 the   
impetus of the flame gases and their    
rapid expansion  during combustion carry
 the flame out across the furnace1 while
 the  chimney draught ultimately sucks  
it back to the exit ports1 the shape  of
 the flame being shown in ,fig4 #e4     
,illustration3 ,,fig4 #e4--;,diagram of 
a furnace with 8horse-shoe0 flame4      
,in general arrangement1 a tank furnace 
for glass melting resembles an          
open-hearth steel furnace4 ,the tank or 
basin1 as already indicated1  is built  
up of a number of large fire-clay       
blocks1 forming a bath  varying in depth
 from #bj in4 to #db in4 according to   
the design of  the furnace and the kind 
of glass to be melted in it4 ,the ports 
for  entry of gas and air and for exit  
of the products of combustion are  in   
most modern furnaces placed in the side 

walls of the furnace just  above the    
level of the glass1 the whole being     
covered by a vault  built of silica     
brick4 ,figs4 #f and #g show the general
 arrangement of  a simple form of       
tank-furnace such as that used in the   
manufacture of  rolled plate glass4 ,the
 furnace indicated in the diagram is    
intended  for regenerative working with 
alternating directions of flame2 in     
recuperative furnaces the horse-shoe    
flame is always used in tanks1  while   
this arrangement of ports is sometimes  
adopted for regenerative  tanks also1   
particularly in the manufacture of      
bottles4 ,for the  production of sheet  
glass1 tank furnaces are generally      
sub-divided  into two compartments and  
are also provided with various          
constrictions  intended to arrest       
impurities and to allow only clear glass
 to pass1  but as regards the           
arrangement of flues and ports there is 
a very  general similarity between      
various furnaces of this type4          

,illustration3 ,,fig4 #f4--,longitudinal
 sectional diagram of tank  furnace4    
,illustration3 ,,fig4 #g4--,transverse  
sectional diagram of tank furnace1      
showing regenerators and gas and air    
passages4    ,it is beyond the scope of 
this book to discuss the relative merits
  of tank and pot melting furnaces2     
wherever the former can be made to      
produce glass of adequate quality for   
the purpose desired1 the great  economy 
of the tank furnace inevitably carries  
all before it1 so that  bottle glass1   
for example1 is now made exclusively in 
tanks1 and  the same applies also to    
rolled plate of the ordinary kind1 and  
to  the great majority of sheet glass4  
,on the other hand1 where special       
qualities of glass are required in      
relatively small quantities1 or  where  
the requirements as to quality are very 
extreme1 the pot furnace  remains       
indispensable4 ,optical glass and       
coloured glasses are examples  of this  
kind1 although some tinted glasses are  

used in sufficient  quantity to justify 
the use of small tank furnaces for their
  production4 ,the causes of the greater
 economy of the tank furnace  are       
numerous1 and complicated by the        
detailed requirements of each           
particular manufacture1 but the most    
important factors in the question  may  
be summed up thus3--    7#a7 ,the tank  
furnace utilises the heat of the flame  
more efficiently1  as the glass is      
exposed to the heat in a basin whose    
surface covers  the entire area of the  
furnace1 while in a pot furnace there is
 much  vacant1 unused space4    7#b7    
,the tank furnace permits of continuous 
working1 the raw materials  being       
introduced at one end while the glass is
 being withdrawn and  worked at the     
other end4 ,there are thus no idle      
periods1 and each part  of the furnace  
remains at or near the same temperature 
during the whole  time that a furnace is
 alight4 ,for a given size of plant1    
therefore1  a tank furnace yields a much

 larger output1 with a relatively       
smaller  fuel consumption4    7#c7 ,the 
tank furnace obviates the need for pots 
or crucibles1 which are  not only costly
 and troublesome to produce1 but are    
liable to premature  failure and require
 periodical renewal1 which involves a   
serious loss  of time for the furnace4  
7#d7 ,finally1 the molten glass in a    
tank furnace can be always  maintained  
at or near one constant level and is1   
therefore1 always  convenient for       
withdrawal by means of the gatherer's   
pipe or the ladle4    ,in pot furnaces1 
on the other hand1 the composition of   
the glass can  be more accurately       
regulated1 and the molten glass itself  
can be more  effectively protected from 
contamination either by matter dropping 
into  it or by the action of the furnace
 gases1 while in pots it is also        
possible to effectually melt together   
materials which1 in the open  basin of a
 tank1 could not be kept together long  
enough to combine4          ,,chapter   

,v4    ,,the ,,process ,,of ,,fusion4   
,it has already been indicated that1 for
 glass-making purposes1 the raw         
materials are required in a state of    
reasonably fine division4 ,the  exact   
degree of fineness required depends very
 much upon the nature  of the ingredient
 in question1 the general rule being    
that the more  refractory and chemically
 resistant materials require to be most 
finely  ground1 while substances which  
melt and react readily1 such as soda ash
  and salt-cake1 do not require very    
fine grinding4    ,assuming that the    
materials are available in a suitable   
state of  fineness1 the first step in   
the process of glass melting consists   
in securing their admixture in the      
proper proportions4 ,this may be  done  
by hand entirely1 by hand aided by some 
machinery1 or entirely  automatically4  
,the process of hand mixing is only     
available for  relatively small         
quantities of material and requires very
 careful  supervision if inadequate     

mixing is to be avoided4 ,in most cases 
the  actual weighing out is done by     
hand1 while the mixing is done by       
machinery4 ,in this process the separate
 ingredients are weighed out  from      
barrows or skips and are tipped into a  
large hopper whence each  batch1 as soon
 as it is completed1 passes into the    
mixing chamber  of the mixing machine4  
,this may consist of nothing more than a
  cylindrical chamber in which steel    
arms revolve and stir up the  contents1 
but more modern appliances take the form
 of rotating barrels  or cylinders1 set 
up on an inclined axis and provided with
 suitable  shelves and baffles2 in these
 the materials are very thoroughly      
shaken  over and mixed4 ,where hand     
mixing is adopted1 the various          
ingredients  of each batch are thrown   
into a large bin and are there turned   
over  several times with shovels1 the   
entire material being ultimately sieved 
through a wire sieve of suitable mesh4  
,in all cases the resulting  mixture    

should be perfectly uniform in colour   
and texture1 and analyses  of different 
samples should show only small          
variations4 ,with the  mixture thus     
prepared the 8cullet0 or broken glass   
which is to be  re-melted is now        
incorporated2 ideally this should also  
be uniformly  distributed1 but this is  
rarely attempted in practice on the     
large  scale4    ,the next step in the  
process is the introduction of the      
mixture into  the furnace4 ,in the case 
of tank furnaces this is a simple       
matter1  since in these the temperature 
is kept as nearly constant as possible1 
and raw materials may1 therefore1 be    
introduced at almost any time1  the     
amount introduced being so regulated as 
to keep the level of the  molten glass  
or 8metal0 as nearly constant as        
possible4 ,the actual  introduction is  
managed by means of a large opening or  
door at what is  known as the 8melting  
end0 of the furnace4 ,normally this     
opening is  covered by a large          

fire-brick block suspended by a chain   
running over  pulleys and               
counterbalanced by a counterpoise       
weight4 ,when charging  is to begin1    
this block is raised and the opening is 
uncovered4 ,the  raw materials are then 
introduced either by hand1 by the aid of
  long-handled shovels1 or they are     
first filled into a long scoop  moved by
 mechanical means forward into the      
furnace1 where it is given  a half-turn1
 which empties the contents out1 and is 
then rapidly  withdrawn4    ,this       
charging process may be repeated every  
half-hour1 or larger  quantities may be 
introduced once every four hours1       
according to the  practice that may be  
adopted at any particular furnace4      
,in the case of pot furnaces the        
charging process is not so simple4      
,here the first charge of raw materials 
has to be introduced into a pot  which  
has been almost entirely emptied during 
the working-out process1  and the       
temperature of the furnace has also     

fallen very considerably  during this   
time4 ,before new material is           
introduced1 the heat of the  furnace    
must first be adequately restored4 ,if  
this is not done1 the  fusion of the    
glass takes an abnormal course and very 
imperfect results  arise4 ,further1 the 
quantity of material introduced at one  
time must  be carefully adjusted to the 
capacity of the pot4 ,during the earlier
  stages of fusion most glass mixtures  
form large masses of foam1 and  if the  
crucible has been too heavily charged   
this foam overflows1  with the result   
that valuable material is lost and the  
floor and  passages of the furnace are  
clogged with glass4 ,a certain amount of
  overflow1 as well as leakage from     
defective crucibles1 is1 however1       
unavoidable1 and for this purpose every 
pot furnace is provided with a  chamber 
so placed that the glass will flow into 
it and so be prevented  from finding its
 way into the regenerators or other     
parts where its  presence would hinder  

the working of the furnace4 ,these      
receptacles or  8pockets0 must1 however1
 be periodically cleared of their       
contents  from outside1 and this        
constitutes one of the most irksome     
operations  connected with glass        
manufacture4 ,owing to the occurrence of
 foaming  and to the fact that the raw  
materials occupy much more space than   
the  glass formed from them1 it is      
necessary to fill the pot with fresh    
batches of raw materials several times1 
the quantity which can be  introduced   
decreasing each time4 ,the number of    
times that this must be  done depends   
upon the particular circumstances1 but  
from four to eight  8fillings0 are      
commonly used for various kinds of glass
 and size of  pot4 ,the precise stage at
 which a fresh batch of raw materials   
should  be introduced is another matter 
requiring careful attention4 ,for some  
purposes it is necessary to wait until  
the previous batch is completely        
melted1 while in other cases raw        

material may be added whilst some of    
the previous batch is still floating on 
the surface of the glass in the  pot4   
,we have now to consider the chemical   
reactions which take place in the       
mixture of raw materials that are       
introduced into the hot furnace4 ,the   
exact course of these reactions is not  
known in very great detail1 as  this    
could only be ascertained by an         
elaborate research on the nature  of the
 intermediate products that result under
 various circumstances4  ,a research of 
this kind would throw much light on the 
whole of the  melting processes but is  
in itself so difficult that it has not  
yet  been carried out at all fully4 ,we 
can therefore only give an account of   
the chemical changes from our knowledge 
of the end-results and of a few         
intermediate products that are known4   
,to take the simplest case1 we may      
consider a mixture consisting of sand1  
carbonate of lime and carbonate  of soda
 mixed in suitable proportions4 ,in such

 a case we know that  the mere action of
 heat alone will produce two            
changes--the carbonate  of soda will    
melt and the carbonate of lime will lose
 its carbonic  acid and be 8burnt0 or   
converted into caustic lime4 ,the first 
stage  of the fusion process thus       
probably results in a mass consisting of
  sand grains and grains of carbonate of
 lime undergoing decomposition1  all    
cemented together by molten carbonate of
 soda4 ,this mass will  be full of      
bubbles1 some derived from the air      
enclosed between the  grains of the     
original mixture and thus trapped by the
 melting mass1  and others formed by the
 carbonic acid which is being driven off
 in  the form of gas by the             
decomposition of the carbonate of lime4 
,at the  temperature of the furnace1    
however1 silica has the properties of a 
strong acid1 and not only attacks the   
carbonate of lime much in the  same     
manner as1 for instance1 hydrochloric   
acid would do in the cold1  but the     

silica also attacks the carbonate of    
soda1 which heat alone  can scarcely    
decompose4 ,the exact order in which    
these reactions take  place will depend 
upon the temperature of the furnace and 
the degree  of mixing attained in the   
preparation of the raw materials4       
,although  in the long run the final    
result will probably be the same as     
regards  purely chemical constitution1  
much of the technical success of the    
process must depend upon the exact      
sequence of the changes involved1 as    
this must govern the number and size of 
the bubbles that are formed in  the     
glass and the fluidity of the mass from 
which these bubbles have  to free       
themselves4 ,in the present state of our
 knowledge1 however1  we can only say   
that the final result is the complete   
expulsion of  all carbonic acid from the
 compounds present 7although it may     
remain  entangled in the glass in the   
form of bubbles7 and the formation of   
silicates of both lime and soda which   

remain in the finished glass in a  state
 partly of mutual chemical combination1 
partly of mutual solution4    ,the      
description of the process of fusion    
just given applies1 with  slight        
modifications1 to the melting of        
ordinary flint-glass mixtures  as well  
as to lime glasses1 with the one        
modification that the  carbonate of lime
 of the lime-soda glass is replaced by  
red-lead1 and  the gas evolved by the   
decomposition of the red-lead is oxygen 
in place  of the carbonic acid evolved  
from the decomposition of the carbonate 
of lime4 ,in the case of both lime and  
flint glasses1 however1 certain  other  
substances besides those mentioned are  
usually introduced in  small quantities4
 ,although these substances do not very 
materially  affect the end-products of  
the chemical reactions1 they very       
materially  affect the intermediate     
stages1 and thus serve the purpose for  
which  they are introduced by affecting 
the course of the chemical changes in  a

 favourable manner4 ,the substances     
usually employed for this purpose  are  
arsenic and nitrate of either soda or   
potash4 ,the manner in which  the       
arsenic acts is very obscure and cannot 
be discussed in detail  here2 the chief 
factors in its action are1 however1 its 
volatility and  its power of either     
absorbing oxygen or parting with it     
according to  circumstances4 ,the action
 of the nitrates is chiefly dependent   
upon  the oxygen which they yield on    
decomposition by heat4 ,this oxygen is  
in some cases stored up by other        
ingredients of the mixture and only     
given off at a much later stage1 when   
the evolution of this gas assists  in   
the removal of the last small bubbles of
 inert air or carbonic acid  gas still  
left in the glass4 ,the oxidising action
 of the nitrates1  however1 serves      
chiefly for the destruction of organic  
matter and  the full oxidation of any   
iron present2 both processes which tend 
to  improve the colour of the glass1    

while in the case of flint glasses the  
presence of these oxidising additions is
 necessary to avoid all risk of         
reduction of lead1 since this would     
result in the complete blackening  of   
the glass4    ,a much more complicated  
set of reactions occur when the alkali  
of a  soda-lime glass is introduced     
either partly or wholly in the form of  
sulphate of soda 7salt-cake74 ,we have  
already pointed out that the  unaided   
action of heat and of silica is not     
sufficient to bring about  the rapid    
decomposition of sulphate of soda which 
is required for  successful glass       
manufacture1 and that the intervention  
of reducing  agents is required4 ,for   
this purpose a certain amount of carbon 
in  the form of coke1 charcoal or       
anthracite coal1 is introduced into all 
salt-cake mixtures1 but the reducing    
gases of the furnace atmosphere  also   
play an important part in the reactions 
that take place4 ,here  again it is not 
possible to give anything but an        

incomplete account  of what takes place4
 ,the rationale of the whole process    
lies1 no  doubt1 in the fact that       
sulphite of soda 7,na#b,,so#c7 is much  
more  readily decomposed by the action  
of hot silica than the sulphate         
7,na#b,,so#d7 itself1 so that the       
essential action of the reducing  agents
 consists in robbing the sulphate of    
part of its oxygen1 thus  reducing it to
 the condition of sulphite and rendering
 it accessible  to the attack of silicic
 acid4 ,but if we attempt to express    
such a  reaction in the usual manner by 
a chemical equation from which the      
quantity of carbon required to effect   
the reduction in question can be        
calculated1 we find that the amount of  
carbon required in practice  is very    
considerably less than that given by    
this theory2 it follows  therefore that 
either this very large amount of        
reducing action must  be ascribed to the
 furnace gases1 or that the actual      
reactions are not  strictly of the kind 

we have described4 ,both explanations   
are probably  partly correct1 and in    
practice the amount of carbon to be used
 in a  given mixture and furnace can    
only be found by actual trial1 in which 
the manufacturer is1 of course1 guided  
by the results obtained with  other     
furnaces of a similar type4 ,the        
end-product of the reactions is  again a
 mixture of silicates1 but a certain    
amount of undecomposed  sulphate is     
always found in such glasses1 while     
gaseous oxides of  sulphur escape from  
these furnaces in considerable quantity4
 ,under  exceptional circumstances the  
glass may even contain sulphides of soda
  or of lime1 and sometimes even        
suspended carbon1 but these are abnormal
  constituents and result in the serious
 discolouration of the glass4    ,it is 
obvious that to a mixture containing    
carbon as a reducing agent  such        
oxidising materials as nitrates cannot  
be added1 but small  quantities of      
arsenic and of manganese dioxide are    

added because  their other properties   
are sufficiently valuable to outweigh   
their  disadvantages as oxidising       
agents4    ,having now briefly          
considered the process of fusion proper1
 we pass  to the second stage in the    
melting of glass4 ,in a properly        
conducted  glass-furnace1 when the last 
trace of undecomposed raw materials has 
disappeared1 we find the glass as a     
transparent mass throughout which  gas  
bubbles are thickly disseminated4 ,for  
the majority of purposes it  is         
necessary to free the glass as perfectly
 as possible from these  bubbles before 
it is worked into its final form4 ,this 
freeing or  8fining0 process is carried 
out by further and more intense heating 
of the molten glass1 which is thereby   
rendered more fluid and allows  the     
bubbles to disengage themselves by      
rising to the surface4 ,this  occurs    
much more readily when the bubbles are  
large2 very minute  bubbles1 in fact1   
show no inclination to rise through the 

fluid mass4  ,the glass-maker           
accordingly compounds his mixtures of   
raw materials in  such a way as to yield
 large bubbles1 or1 failing that1 he    
adds to the  molten mass some substance 
that evolves a great many large bubbles1
 and  these in their upward course      
through the glass sweep the small ones  
away with them4 ,the added substance may
 be an inorganic volatile body1  such as
 arsenic1 or more frequently some       
vegetable substance containing  much    
moisture is introduced into the glass4  
,the most usual method is  to place a   
potato in the crook of a forked iron rod
 and then to dip  the rod with the      
attached potato into the molten glass2  
the heat at  once begins to drive off   
the moisture and to decompose the       
potato1 so  that there is a violent     
ebullition of the whole mass4 ,this     
8boiling  up0 process assists the fining
 considerably and also serves to mix the
  whole contents of the pot very        
thoroughly1 but it has some attendant   

disadvantages1 such as the introduction 
of oxide of iron into the glass  from   
the rod which is used in the operation1 
while the contaminated  material        
adhering to the walls of the pot itself 
is dragged off and  mixed with the rest 
of the glass by the violent stirring    
action that  takes place4 ,it is1 of    
course1 further obvious that this       
process can  only be usefully applied to
 glass melted in pots1 since the bulk of
  the molten glass in a tank furnace    
could not be reached at all in  this    
manner4 ,mixtures that are to be melted 
in tanks must therefore be  capable of  
freeing themselves of their enclosed    
bubbles without such  outside aid4 ,in a
 tank1 in fact1 the whole melting       
process proceeds on  somewhat different 
lines1 since the temperature of the     
furnace is never  intentionally varied1 
while on the other hand the melting     
glass travels  down the furnace into    
regions whose temperature can be        
regulated to  favour the various stages 

of the process that take place in each  
part  of the furnace4 ,on the whole1    
however1 it is an undoubted fact that   
while the running of a pot furnace can  
be varied1 within wide limits1  to suit 
the requirements of whatever mixture it 
is desired to melt1 in  the case of tank
 furnaces the mixture must be closely   
adjusted to the  requirements of the    
furnace1 whose general 8run0 cannot be  
very readily  altered4    ,the          
completion of the 8fining0 process is   
generally determined by  taking samples 
of the glass out of the pot or tank and 
examining them  for enclosed bubbles4   
,such samples may be obtained in a      
variety of  ways1 the most usual method 
being to dip a flat iron rod just below 
the surface of the glass and to lift it 
out vertically upwards1 thus  retaining 
on the flat surface of the rod some of  
the glass that lay  there at the moment 
when the rod was immersed4 ,these test  
samples or  8proofs0 are examined very  
carefully1 and if no trace of bubbles   

can  be observed the glass is generally 
regarded as 8fine10 but it is by  no    
means certain that the absence of       
bubbles from such a small sample  will  
prove that the whole mass is free2 that1
 however1 is a point where  the melter's
 experience enables him to judge how far
 he may rely upon  the indications given
 by the 8proofs40 ,when the glass is    
8fine0 it  frequently happens that the  
surface of the molten mass is           
contaminated  by specks of foreign      
matter floating on the glass2 for the   
purpose of  removing these1 the surface 
of all glass is skimmed before work is  
begun upon it4 ,this is done by removing
 the surface skin of glass by  means of 
suitably shaped iron rods1 upon which   
small masses of molten  glass are first 
8gathered40 ,finally1 it only remains to
 reduce the  temperature of the glass   
from that of the melting and fining     
process to  the much lower temperature  
at which the various methods of working 
the  glass are carried out4 ,in pot     

furnaces this is accomplished by        
lowering  the temperature of the entire 
furnace1 while in tank furnaces the fine
  glass flows into the working chamber  
of the tank which is always kept  at the
 working temperature4          ,,chapter
 ,,vi4    ,,processes ,,used ,,in ,,the 
,,working ,,of ,,glass4      ,in the    
previous chapter we have followed in    
outline the process of  fusion and      
fining of glass1 leaving the molten     
material ready for  working up into the 
final shape4 ,up to that point the      
process is very  similar in all kinds of
 glass1 although the furnaces1 pots and 
utensils  employed vary considerably1 as
 do also the temperatures to which the  
materials are heated at various stages4 
,the working processes1 however1  differ
 entirely from one class of product to  
another1 as obviously the  process      
employed for the production of a sheet  
of plate-glass can have  little in      
common with that used in the manufacture
 of a wine-glass4 ,on  the other hand1  

the modes of working hot glass are not  
so numerous as  the products that are   
produced1 so that we find very similar  
appliances  and manipulation recurring  
in various branches of the industry4    
,for  that reason we propose to deal    
here with the principal methods of      
manipulating glass1 leaving the details 
of each method as applied to  special   
purposes to be discussed in connection  
with the special product  in question4  
,the first stage in the working of all  
glass is the removal of a  suitable     
quantity of molten glass from the       
furnace4 ,practically only  three       
methods are available1 viz41 ladling1   
pouring and gathering4  ,if we think of 
a familiar substance of physical        
properties somewhat  resembling those of
 glass1 we may take thick treacle and   
suppose it  confined in a jar or bottle2
 there are three obvious ways of        
extracting  it from the bottle3 we may  
ladle it out with a spoon1 or we may    
pour it  out by tilting the whole       

bottle1 or we may dip a spoon or fork   
into the  thick liquid1 slowly draw it  
out and turn it round as we do so1 thus 
bringing out on the spoon or fork a     
round adherent mass or 8gathering0  of  
treacle4 ,in the case of molten glass1  
the process of ladling is  by far the   
simplest1 but it has certain very       
decided limitations  and disadvantages4 
,these arise from the fact that a ladle 
cannot be  introduced into molten glass 
without contaminating the whole mass of 
glass1 at any rate with numerous air    
bubbles4 ,the metal of the ladle        
carries with it a considerable amount of
 closely adherent air which is          
partially detached while in contact with
 the hot glass1 so that both  the       
contents of the ladle and the glass     
remaining in the furnace are            
contaminated4 ,these bubbles might      
perhaps be avoided if hot ladles were   
used1 but in that case the glass would  
adhere to the surface of the  metal1 and
 each ladle would require laborious     

cleaning after each time  that it was   
used4 ,in practice1 therefore1 ladling  
is only used for the  production of     
those classes of glass where the        
presence of a certain  number of        
air-bells is not injurious1 and the     
ladles are kept cold by  immersion in   
water after each time of use4 ,the use  
of the cold ladle  has1 however1 the    
further disadvantage that a certain     
quantity of the  glass withdrawn in the 
ladle is very considerably chilled by   
contact  with the cold metal1 and is    
thus too stiff to undergo the further   
processes satisfactorily--this chilled  
glass has1 therefore1 to be  rejected   
from each ladleful2 this not only       
involves loss of glass1 but  also       
necessitates the separation of this     
spoilt glass from the rest4    ,the     
general process of rolling requires     
little treatment here4 ,two  essentially
 different processes are used2 in one   
the glass is thrown  on a flat table and
 rolled out by a moving roller passing  

along the  table2 in the other the glass
 passes between two moving rollers1 and 
the sheet so formed is received on a    
moving table or slab4 ,the former  mode 
of rolling is used for the production of
 the ordinary rolled plate  glass2 if   
the surface of both table and roller is 
smooth1 the glass  also has a           
comparatively smooth surface1 but the   
surface is far from  being level or free
 from irregularities4 ,it has been found
 that it is  quite impossible to prevent
 these irregularities1 which appear to  
arise  from the buckling of the glass   
against the iron surfaces with which  it
 comes into contact2 when rolled1 the   
glass is too stiff to recover  its true1
 smooth surface under the influence of  
surface tension1 so  that it retains all
 the marks of roller and table--nor can 
the roller  be made perfectly smooth1   
since in that case it appears to slip   
over  the glass and does not roll it out
 properly4 ,all efforts1 therefore1  to 
produce a glass having a true and smooth

 surface by direct rolling  have failed1
 and are likely to fail1 so long as     
tables and rollers  are made of         
materials similar to those now in use4  
,the process of  rolling on a stationary
 table is1 however1 used for the        
manufacture of  plate-glass2 but here   
the slab as rolled has still the rough1 
uneven  surface similar to that of      
ordinary 8rolled plate10 and this is    
removed  and replaced by a true polished
 surface by the mechanical processes  of
 grinding and polishing4 ,the second    
mode of rolling1 i4e41 with  two or more
 8stationary0 rollers and a moving      
table1 is used for the  production of   
rolled plate having special surface     
features or patterns2  the variety of   
rolled glass known as 8figured rolled   
plate10 having a  deeply imprinted      
pattern1 is produced in this way4 ,this 
method requires  much more complicated  
mechanical appliances1 some of which are
 still  protected by patent rights4     
,ladling being thus limited to the      

production of inferior kinds of  glass1 
the better varieties are dependent upon 
either gathering or  pouring4 ,the      
former process is limited as regards the
 quantity of  glass that can be dealt   
with in one piece1 although surprisingly
 large  quantities can be gathered upon 
a single pipe2 the great masses of      
glass1 however1 that are required for   
the production of modern polished  plate
 could not be handled in this way1 and  
the method of pouring is  accordingly   
adopted4 ,for this purpose either the   
pots in which the  glass has been       
originally melted1 or others specially  
designed for  this purpose1 and into    
which the molten glass has been         
transferred1  are removed bodily from   
the furnace by the aid of powerful      
mechanical  appliances2 they are then   
carried by overhead cranes to the place 
where  the glass is to be rolled into   
the form of a plate1 and there the pot  
is tilted and the molten glass is       
allowed to run out and to form a  pool  

on the rolling table1 the passage of the
 great roller ultimately  rolling the   
pool out into a sheet much as dough is  
rolled out with  a rolling-pin4 ,this   
process is obviously only possible with 
pots or  crucibles of a suitable size1  
and is1 moreover1 very destructive  to  
these pots1 since they are exposed to   
such great variations of  temperature4  
,in the case of tank furnaces1 numerous 
devices have been  patented for allowing
 the glass to flow out over a sill or   
weir of  suitable size1 ready to be     
rolled or drawn into the form of sheets 
or  slabs2 but none of these devices    
have1 so far as the writer is aware1    
found their way into practice2 the      
reason for this probably lies in the    
fact that it is not easy to find a      
material which will present a smooth    
face to the outflowing glass1 such      
materials as fire-clay leading to       
contamination from detached fragments1  
while chilled metal leads to  local     
chilling of the glass4 ,unless1         

therefore1 the various processes  of    
drawing glass into sheets direct from   
the furnace undergo very  material      
improvement1 the laborious process of   
gathering is likely to  retain its      
importance even in the production of    
such large objects as  sheets of window 
glass4    ,in its essence the process of
 gathering consists in introducing into 
the glass a heated iron rod or tube to  
which a small quantity of glass  is     
allowed to adhere2 rod and glass are    
removed from the furnace  together1 and 
the small adherent ball of glass is     
allowed to cool so  far as to become    
stiff enough to carry its own weight4   
,the rod with its  adherent ball is then
 again dipped into the glass1 where a   
fresh layer  of glass attaches itself to
 the ball already on the rod4 ,the whole
 is  again withdrawn1 allowed to cool   
down1 and then dipped into the molten   
glass again to gather a fresh quantity4 
,this cycle of operations is  repeated  
until the desired quantity of glass is  

attached to the rod  or tube4 ,these    
operations1 particularly when weights of
 thirty or  forty pounds of glass have  
to be gathered1 require the exercise of 
a  great deal of skill and care2 the    
introduction of the gathering into  the 
molten glass is each time liable to     
produce air-bells which would  spoil the
 whole mass of glass or would           
contaminate the contents of the         
crucible1 while subsequently the mass of
 hot glass adhering to the rod  or pipe 
tends to run down and even to drop off  
entirely if not properly  checked by    
suitable rotation of the pipe4 ,further1
 the manual labour  and exposure to heat
 involved for the operator all tend to  
increase  the cost of such work4        
,mechanical aids have been invented1 and
 some of  these are in actual use1 but  
they are chiefly confined to mechanism  
for relieving the operator of the great 
weight of the gathering in its  later   
stages4    ,just as ladling is nearly   
always preliminary to rolling1 so       

gathering  is usually the preliminary to
 some blowing process1 although the     
blowing is often combined with and      
sometimes replaced by the mechanical    
pressing of the glass4 ,where the glass 
is to be blown1 the gathering  is always
 made on a glass-maker's pipe4 ,this is 
an iron tube from #d  to #f ft4 long1   
provided at one end with a wooden casing
 to serve as a  handle1 and with a      
suitably arranged mouthpiece for        
blowing4 ,the shape  of the lower or    
8butt0 end of the pipe depends upon the 
character and  size of the objects to be
 blown2 for small articles the pipe must
 be  narrow and light1 but for heavy    
sheet-glass the butt of the pipe is     
extended into a conical mass whose base 
is from #b to #c in4 in diameter4  ,the 
bore of the pipe at both ends also      
depends upon the class of work  for     
which it is intended4 ,the first stage  
of all blowing processes  consists in   
the formation of a hollow sphere by     
blowing into the pipe1  the pressure of 

the breath being as a rule sufficient to
 cause the  gradual distension of the   
hot mass of glass4 ,from this           
rudimentary  hollow sphere the various  
shapes of blown articles are then       
evolved by  a series of manipulations   
which vary very widely in different     
branches  of manufacture4 ,they         
generally consist1 however1 in gradually
 changing  the shape of the mass of     
glass by the pressure either of hand    
tools or  of specially prepared moulds  
or blocks against which the glass is    
held  or turned1 either with or without 
simultaneous blowing into the pipe4     
,the extent to which the aid of such    
moulds and blocks is invoked varies     
continuously from the production of the 
hand-made vase or glass to  the moulded 
bottle2 in the former1 practically only 
hand tools1 whose  shape bears no direct
 resemblance to that of the finished    
article1 are  employed1 while in the    
latter the elongated hollow mass of     
glass is  placed inside a mould1 and    

internal air-pressure is used to press  
the  glass into contact with the mould  
from which the shape of the finished    
bottle is thus directly derived4    ,the
 art of the blower further takes the    
fullest advantages of the  peculiar     
physical properties of glass while in   
the heated viscous  condition1 the      
material being made to flow under the   
action of gravity  and centrifugal      
forces1 as well as under the pressure of
 the breath1  the glass being held      
aloft1 twirled or swung about to ensure 
the  production of the various shapes   
required4 ,for the great majority of    
such purposes the unaided manipulations 
of the operator are sufficient1  but    
various mechanical aids are used to     
facilitate the more laborious  stages of
 the work1 while for the simpler forms  
that are required in  very great        
numbers1 such as bottles1 the whole of  
the operations are  now carried out by  
automatic machines4 ,of the more usual  
mechanical  aids at the disposal of the 

glass-blower1 we have already mentioned 
hand-tools1 blocks1 and moulds of       
various kinds4 ,next in importance  to  
these is the use of compressed air for  
blowing large or heavy  articles2 the   
pressure available by the human breath  
is very limited1  and the volume of air 
that can be thus delivered is not very  
large1  while the constant use of the   
lungs for such a purpose is trying  for 
the workman4 ,in many works1 therefore1 
air under pressure is  supplied to the  
benches or stages where the blowing is  
done1 and the  blowers' pipes can be    
coupled to this air-supply by means of  
flexible  connections when required4    
,the principal difficulty lies in the   
correct regulation of the air-pressure  
for each special purpose2 but  this     
difficulty has been overcome by the use 
of delicate valves under  the control of
 each blower1 who can thus regulate the 
pressure to  his own exact requirements4
 ,such a system1 of course1 requires    
some  little practice on the part of the

 men using it1 but when they have       
become accustomed to the working of the 
plant the results achieved  are         
decidedly better and more regular than  
those obtained by mouth  blowing4       
,besides the use of compressed air      
supplied in the way just  indicated1    
several other devices are in use to aid 
the blower in  producing the requisite  
pressure in the interior of the hollow  
bodies  he is producing4 ,the simplest  
of all these consists in utilising the  
expansive force of the air enclosed in  
the hollow body when that body  is      
exposed to heat4 ,thus1 for instance1 in
 blowing a cylinder of  sheet-glass1 if 
the blower holds his thumb over the     
aperture of his  pipe1 and brings the   
closed end of the cylinder near the hot 
8blowing  hole10 the heat which softens 
that end of the glass will also act upon
  the enclosed air1 and will very       
rapidly produce such an expansive       
effect as to burst open the softened end
 of the cylinder1 and this  means of    

opening the closed ends of the cylinder 
is frequently employed  in practice4 ,it
 is1 of course1 obvious that any other  
expansive fluid  might be employed in a 
similar manner1 and in some blowing     
processes  it has long been the practice
 to introduce a small quantity of water 
into the interior of the hollow body1   
when the rapid expansion of  the steam  
produced thereby is utilised for the    
purpose of generating  the requisite    
internal pressure4 ,this use of the     
expansive force of  steam generated by  
the heat of the hot glass body has      
received great  development at the hands
 of ,sievert in ,germany1 whose process 
is  described in ,chapter ,,vii4        
,whatever mechanical aids are employed  
to facilitate the various stages  of the
 process1 all glass blowing involves a  
series of operations  requiring         
considerable skill1 while the whole     
manner of dealing with  the glass is    
essentially extravagant of material1    
except perhaps in  the production of    

bottles or flasks having narrow mouths4 
,the reason  for this latter statement  
lies in the fact that by blowing it is  
only possible to produce closed or      
nearly closed hollow bodies or  vessels2
 thus a blown wine-glass or tumbler is  
formed with a hood or  dome closing in  
the open top of the glass1 and this hood
 or dome has  subsequently to be removed
 by subsidiary processes1 such as       
cutting off  by the aid of strong local 
heat or by grinding1 and the cut edge   
has to  be provided with a smooth       
finish4 ,in the case of comparatively   
small  articles like glasses the loss   
involved from this cause is not so very 
great1 but were large flat bowls or     
dishes to be produced by blowing1  the  
loss in the dome or covering would be   
very serious4 ,this difficulty  is      
entirely avoided by the process of      
pressing glass4 ,we have already        
indicated the manner in which moulds are
 used for the production of  the desired
 shape in the case of bottles1 etc41 but

 in these cases1  where the final object
 is to be a hollow vessel1 the glass is 
readily  forced into contact with the   
mould by means of internal air--or      
steam--pressure2 in the process to which
 we are now referring1 however1  the hot
 glass is forced into contact with the  
external mould by means  of an internal 
plunger which is forced downward with   
considerable  force4 ,by this means1    
flat or shallow bodies can be produced  
without  the preliminary formation of a 
completely closed vessel1 while it is   
obvious that by the use of suitable     
moulds1 complicated and elaborate       
shapes can be produced4 ,it is true1 of 
course1 that pressed articles do  not   
show the same smooth and brilliant      
surface which is characteristic  of the 
fire-polish of blown articles1 while the
 facility with which  elaborate surface 
ornamentation can be applied by this    
process has  not tended to artistic     
refinement in design1 but the great     
majority  of cheap and useful glass     

articles of domestic use have been made 
available by the development of the     
pressing industry4    ,in the ordinary  
course1 pressed glass is produced direct
 from the  molten material1 which is    
introduced into the presses either by   
gathering or by means of ladles1 but for
 some special purposes glass is  brought
 into its final shape by mechanical     
pressure after having first  been       
allowed to solidify and having then been
 specially re-heated to  undergo the    
pressing or moulding process4 ,this is  
principally done in  the case of the    
best kinds of optical glass1 where the  
molten glass  is first allowed to cool  
in the actual crucible and is then      
broken up  into lumps of a suitable     
size1 from which the more defective     
portions  can be rejected1 the more     
perfect portions only being heated up   
again  in special kilns and then forced 
to take the desired shape by being      
pressed--sometimes with hand tools only 
and sometimes by the aid of  powerful   

presses--into moulds of the required    
shape4 ,small lenses1  however1 for     
which the requirements of quality are   
not so high are  sometimes pressed      
direct from small gatherings taken from 
the molten  glass in the crucible4      
,,chapter ,,vii4    ,,bottle ,,glass4   
,although bottles are in some respects  
the cheapest and crudest products  that 
are manufactured of glass1 their uses   
are so innumerable and their  numbers so
 enormous that their production         
constitutes a most important  branch of 
the industry4    ,in the choice of raw  
materials for the production of ordinary
 bottles  cheapness is necessarily the  
first consideration4 ,natural minerals1 
bye-products of other industries1 and   
the crudest chemicals are  utilised so  
long as it is possible by compounding   
these ingredients  in suitable          
proportions to obtain a glass whose     
composition meets  the somewhat crude   
requirements which bottles are expected 
to meet4  ,the most essential of these  

requirements are that the bottles shall 
be strong enough to resist the internal 
pressure which may come upon  them when 
used for the storage of fermented or    
effervescent liquors as  well as the    
shock of ordinary use1 while the glass  
itself must possess  sufficient chemical
 resistance to remain unattacked by the 
more or less  corrosive liquids which it
 is called upon to contain4 ,further1   
from  the point of view of the bottle   
manufacturer it is desirable that the   
glass shall be readily fusible1 easily  
worked1 and easily annealed4  ,in other 
branches of glass manufacture increased 
fusibility is often  attained by        
increasing the alkali contents of the   
glass1 but in bottle  making this is    
inadmissible1 both on account of the    
prohibitive cost  of alkali and because 
an increased alkali content renders the 
glass  more liable to chemical attack4  
,on the other hand1 in many varieties   
of bottle the colour of the glass is    
nearly1 or quite1 immaterial  so that   

the introduction of relatively large    
proportions of iron  oxide is           
permissible4 ,this substance acts as a  
flux and assists in  the production of a
 fusible1 workable glass containing     
little alkali4  ,such alkali as bottle  
glass does contain is frequently derived
 from  felspathic minerals1 which       
generally also contain considerable     
proportions of iron4 ,the use of these  
minerals also introduces  notable       
proportions of alumina into the glass4  
,in certain classes  of bottles1 notably
 those used for special wines1 certain  
shades of  colour are required--the     
well-known 8,hock bottle0 colour being  
an  example4 ,the presence of iron in   
the glass tends to the production of  a 
green or greenish-yellow colour         
deepening to a black opacity if the     
quantity of iron be high4 ,the lighter  
shades of this green tint may  be       
8neutralised0 by the introduction of    
manganese into the glass1 the  resulting
 colours ranging from light amber to    

purple2 nickel oxide is  also sometimes 
used as a colouring material in these   
glasses4    ,in the production of       
ordinary bottles the continuous tank    
furnace has  now entirely superseded the
 old pot furnaces1 the character of the 
product being in this case particularly 
suited to this process of  production4  
,the modern bottle-glass tank is        
generally an oblong basin  having one   
semi-circular end4 ,the flame is often  
of the 8horse-shoe0  type1 the gases    
both entering and leaving the furnace at
 the flat  or charging end of the       
furnace4 ,the raw materials are thrown  
into  the furnace at the square end of  
the tank1 and the glass flows           
uninterruptedly down the furnace to the 
colder semi-circular end where  the     
working holes are situated4 ,at these   
points fire-clay rings are  kept        
floating on the glass1 and from within  
these the gatherer takes  his gathering1
 the rings serving to retain the grosser
 impurities  carried down by the glass4 

,the producing power of such a furnace1 
even  when the bottles are blown by     
hand1 is very considerable2 a furnace   
having ten working holes and containing 
normally about #he tons of  molten glass
 will yield some four million bottles   
per annum1 and  furnaces of considerably
 larger capacity are in use4    ,the    
methods of bottle making are at the     
present time passing through  what is   
probably a stage of transition4 ,up to  
the middle of last  century the         
processes in use were little better than
 those of the  middle ages2 the first   
step of a more modern development of the
  industry took the direction of        
improved tools and implements for       
carrying out the old operations4 ,more  
recently a whole series of  inventions  
have been put forward with the aim of   
producing bottles by  entirely different
 and wholly mechanical processes with   
the object of  eliminating the uncertain
 element of skilled labour entirely4    
,while it  must be admitted that some of

 the earlier of these inventions proved 
to be brilliantly ingenious failures1   
there is little doubt that  here1 as in 
other manufacturing processes1 the      
machine-made article  will ultimately   
supersede the hand-made product4 ,even  
now1 mechanical  processes are largely  
in use both in ,america and ,europe1 and
 at some  recent exhibitions            
machine-made bottles have been shown    
which in every  point of quality were   
superior to the best hand-made goods4   
,the first stage in the production of   
bottles by hand1 and also for most  of  
the machine processes1 is that of       
gathering the requisite quantity  of    
glass4 ,the bottle-blower's pipe is     
between #e and #f ft4 long1 and is      
provided with a slightly enlarged end or
 8nose0 upon which the glass is         
gathered4 ,three gatherings are         
generally sufficient for the production 
of ordinary bottles1 but for extra large
 bottles1 and especially for  carboys1  
heavier gatherings are necessary1 and   

for these the gatherer  must go to the  
furnace four1 five1 or even six times4  
,when the  requisite quantity of glass  
has been gathered on the pipe the       
gathering  is worked and rounded by     
rolling it either on a flat metal plate 
or  8marver10 or in a hollowed block    
made of wood or more rarely of metal2   
by this process the glass is formed into
 a well-rounded1 symmetrical            
pear-shaped body4 ,the blower now       
distends the mass gradually by the      
pressure of his breath1 at the same time
 swinging the pipe1 the effect  of these
 movements being to draw the bulk of the
 glass downwards1  leaving a thinner and
 colder portion having the rudimentary  
shape of  the neck of the bottle next to
 the pipe4 ,in the oldest form of the   
process the next stage in the production
 of the bottle is accomplished  by the  
aid of a cylindrical mould of fire-clay1
 whose diameter is that  of the external
 size of the finished bottle4 ,the      
pear-shaped bulb of  glass is for this  

purpose re-heated at the melting        
furnace1 and is  then placed inside the 
fire-clay mould4 ,by vigorous blowing1  
and a  rapid rotation of the pipe and   
glass1 the bulb is forced to assume  the
 cylindrical shape of the mould1 the    
glass forming the neck of the  bottle   
being at this stage of the process too  
cold and stiff to be  further deformed4 
,the next step is the formation of the  
concavity found  in the base of wine and
 beer bottles2 this is produced by      
pushing up  the hot plastic glass that  
forms the bottom of the bottle as it    
leaves  the clay mould4 ,this is done by
 a second workman using an iron rod     
known as the 8pontil10 upon which a     
small mass of glass has previously  been
 gathered4 ,this mass of glass remains  
attached to the bottom of  the bottle1  
which is thus for the moment fastened   
both to the 8pontil0  and to the        
blower's pipe4 ,the blower1 however1    
immediately detaches  the bottle from   
the pipe at the point where the neck of 

the bottle  is intended to end1         
effecting this by locally chilling the  
glass--a  process known by the          
descriptive term of 8wetting off40 ,the 
unfinished  bottle is now attached to   
and handled by means of the 8pontil40   
,the  neck is softened by re-heating it 
over the furnace1 and is then moulded   
into the desired shape by the aid of    
specially-shaped tongs4 ,finally  a     
thread of glass is wound round the end  
of the neck to produce the  thickening  
usually found at that point4 ,the       
finished bottle1 still  attached to the 
8pontil10 is now carried to the         
annealing kiln1 where  it is placed in  
position and detached from the 8pontil0 
by a sharp  blow1 which severs the glass
 that had been gathered on the 8pontil0 
from the bottom of the bottle4    ,the  
process1 in the form described above1   
has been obsolete for  many years1      
improvements1 consisting of appliances  
for facilitating  the various           
operations1 having been gradually       

introduced4 ,the most  important of     
these is the substitution of metal      
moulds for the  fire-clay moulds of     
earlier times4 ,these metallic moulds   
are made to  open and close at will by  
the action of a pedal1 and are designed 
to  give the entire bottle its final    
shape1 except for the indentation  of   
the bottom1 although this is sometimes  
produced by a convex  piece placed on   
the bottom of the mould4 ,in the        
formation of the  neck thickening1 also1
 important mechanical aids have become  
almost  universal4 ,these last consist  
of tongs provided with rollers and      
arranged to rotate about an axis that   
terminates in a tapered spike  which    
enters the neck of the bottle2 by       
pressing the tongs together so  as to   
bring the rollers against the outside of
 the neck and rotating  the whole1 the  
rollers are made to form the neck       
thickening in an  accurate and rapid    
manner4    ,important and valuable as   
these improvements of the ancient       

process  of bottle-blowing undoubtedly  
are1 they do not touch the main         
disadvantages of the                    
process--disadvantages that seriously   
affect the  economy of the process and  
the well-being of the workers employed  
upon  it4 ,it is consequently not       
surprising that a great number of       
inventors  have laboured at the problem 
of the purely mechanical production of  
bottles4 ,a large number of patents have
 accordingly been taken out in          
connection with bottle-making machinery4
 ,the first of these to attain  any     
favour was that devised by ,ashley1 but 
although great claims were  made for it1
 its use has not extended4 ,at the      
present time1 however1  there are a     
number of bottle-works actually at work 
producing bottles  by mechanical means2 
one of the most successful of these     
machines is  that devised by ,boucher1  
of ,cognac4 ,the products of this       
machine1  exhibited in ,paris at the    
exhibition of #aijj1 were equal1 and    

possibly  superior1 to the best         
hand-made bottles4 ,the ,boucher        
machine1 although  by no means entirely 
automatic1 requires no highly-skilled   
labour  beyond that of a workman whose  
duty it is to operate the various       
levers of the machine at the right      
instant and in the proper order4    ,the
 details of the machine1 as set forth in
 the patents and other  published       
descriptions1 are somewhat complicated1 
and vary somewhat  in the different     
models2 the general principle and mode  
of operation  is1 however1 the same in  
all varieties of the machine1 and we    
shall  therefore give a brief account of
 it here4    ,in the ,boucher process1  
the glass is first gathered from the    
furnace1  but as no blowing-pipes are   
required1 the gathering is done on a    
light  iron rod1 thus saving the        
gatherer much of the labour of carrying 
the heavy pipes4 ,the requisite quantity
 of the glass so gathered is  then      
dropped into the first or 8measuring0   

mould of the machine1 the  8thread0     
being cut by hand by the operator4 ,from
 the measuring mould1  the glass is next
 caused to pass into the 8neck0 mould2  
the glass  flows into this mould1 and is
 further pressed into it by the aid of  
compressed air1 applied above the free  
surface of the glass4 ,at this  stage   
the still liquid glass has the external 
shape of the neck of  the bottle1 but   
the mass of glass is solid1 i4e41 no    
cavity has yet  been produced in it4    
,the formation of the cavity is next    
begun by the  action of a plunger which 
is driven into the 8solid0 mass of glass
  filling the neck mould1 this plunger  
thus punching out the passage  through  
the neck of the bottle4 ,as soon as the 
plunger is withdrawn1  compressed air is
 admitted into the cavity so formed1 and
 the mass  of glass is at the same time 
inverted1 and that part occupying the   
position of what is to be the shoulder  
of the bottle is allowed to  descend    
while being blown out by the compressed 

air4 ,this process of  distension is    
limited1 and the desired shape is       
imparted to the mass  by bringing       
towards it a third mould1 by contact    
with which the glass  is considerably   
stiffened--a row of jets of compressed  
air1 impinging  on the outside of the   
glass forming the shoulder of the       
bottle1 being  further used to stiffen  
the glass1 once the requisite extension 
has  been attained4 ,the mass has now a 
shape very similar to that known  as a  
8parason0 in hand bottle-blowing1 and is
 by this time decidedly  stiff4 ,it is  
now introduced into the finishing mould 
and is blown into  perfect contact with 
the mould by powerful air-pressure1 thus
 attaining  the proper shape of barrel  
and base2 the indentation of the base   
is1  however1 sometimes produced on a   
separate machine or press4 ,during  all 
these operations the neck of the bottle1
 which was the first part  to be formed1
 has remained firmly held in the neck   
mould1 and all the  movements that have 

been described are performed by means of
 levers  actuating movements of this    
mould as a whole1 which1 of course1     
carry  the glass with them4 ,the last   
movement of the levers1 which releases  
the bottle from the finishing mould1    
also opens the neck mould1 and  thus    
leaves the bottle finished and entirely 
free4    ,it will be seen that the      
process adopted in this machine follows 
as  closely as possible the various     
stages of hand blowing1 but that the    
mechanical movements of the machine     
replace the laborious and difficult     
technique of the blower4 ,one such      
machine is capable of producing  as many
 as #abj bottles1 each weighing #a lbs41
 per hour1 but this  is accomplished    
only by having some of the moulds in    
duplicate and  so arranged as to come   
into use alternately4 ,the machine      
itself is  attended by one 8moulder10   
who operates the levers1 and by a youth1
  who carries the finished bottles to   
the annealing kiln1 while1 of  course1  

the services of a gatherer are also     
required4 ,the appearance of  a bottle  
works equipped with these machines is in
 striking contrast to  that of a        
hand-blowing works1 where the stages    
around the working-holes  are crowded   
with men doing arduous work under very  
severe conditions of  temperature and   
atmosphere4 ,finally1 it must be pointed
 out that the  use of the ,boucher      
machine is by no means confined to the  
production of  the cheapest kinds of    
bottles1 but that it has shown itself   
especially  well suited to the          
production of champagne and other       
bottles that  are required to withstand 
a high internal pressure1 the           
machine-made  bottles showing excellent 
results under pressure tests4 ,the      
machine  is also used for the production
 of moulded glass-ware of white glass1  
since it can be adapted to the          
production of any kind of glass vessel  
that can be produced by blowing into a  
mould4    ,the annealing of bottles was 

formerly carried out in large chambers  
or  kilns of very simple construction1  
in which the bottles were stacked as    
made1 the kiln being previously heated  
to the requisite temperature3  when     
full1 the kiln was closed up in a rough 
temporary manner and  allowed to cool   
naturally1 thus annealing the bottles   
stacked within  it4 ,in this branch of  
glass-making also1 however1 the         
continuous  annealing kiln has          
superseded the older kinds1 and         
continuous kilns are  now almost        
universal in bottle-making4 ,in these   
kilns1 which consist  of long tunnels1  
kept hot at one end and having a        
gradually decreasing  temperature as the
 other end is approached1 the bottles   
are stacked on  trucks which are slowly 
drawn through the kiln from the hot to  
the cold  end4 ,at the cold end the     
trucks are unloaded and are then        
returned1 by  an outside route1 to the  
charging end1 but of course the bottles 
cannot  be stacked on the truck until it

 has actually entered the hot end of    
the tunnel and acquired the temperature 
there prevailing4 ,in a slightly        
different form of kiln1 the bottles are 
carried down the kiln on a  species of  
conveyer belt formed of iron plates1 but
 the principle of  all these appliances 
is similar even when used for very      
different kinds  of glass4    ,in the   
account of bottle manufacture given     
above we have referred  almost          
exclusively to the mode of production of
 the ordinary bottles  used for the     
storage of such liquids as wine1 beer1  
spirits1 etc41 and  we will now deal    
with some other branches of manufacture 
closely allied  to these4    ,an        
important branch of glass manufacture is
 the production of vessels  of large    
dimensions4 ,those most closely allied  
to ordinary bottles are  the vessels    
known as carboys1 used for the storage  
and transportation  in bulk of chemical 
liquids1 and especially of acids4       
,formerly these  were blown by hand in a

 manner closely resembling that used for
  ordinary bottles1 but the weight of   
the mass of glass to be handled by      
gatherer and blower is very great1 while
 the lung-power of a blower is  not     
sufficient to produce the great         
expansion required4 ,formerly the  only 
aid available to the blower was the     
device of injecting into the  hot1      
hollow glass body1 at an early stage of 
the process1 a quantity  of water or    
alcohol2 this liquid was immediately    
vapourised by the  heat of the glass1   
and if the blower closed the mouthpiece 
end of his  pipe by placing his thumb   
over it1 the expansive force of the     
vapour  so generated served to blow out 
the glass to the desired extent4 ,more  
recently mechanical aids to the         
production of these large vessels  have 
become available1 first in the shape of 
mechanical arrangements  for relieving  
the workmen of the full weight of the   
glass and pipe  by providing suitable   
arms upon which the whole can be        

supported  without interfering with the 
blower's freedom of manipulating the    
pipe  and glass in the desired way2     
further1 a supply of compressed air1    
which can be readily connected with the 
pipe at any desired moment1  facilitates
 the blowing process4    ,a process of  
producing hollow glass vessels of very  
large size by  purely mechanical means  
has1 however1 been introduced during    
recent  years by ,p4 ,sievert1 of       
,dresden4 ,by the methods of this       
inventor1  glass vessels of quite       
unprecedented size--such as bath-tubs   
freely  accommodating full-grown        
men--can be produced4 ,for this purpose 
the  glass is spread out on the surface 
of a large cast-iron plate1 provided    
with numerous small holes through which 
steam or compressed air may be  blown   
when desired4 ,the slab of viscous      
glass1 when properly spread  over this  
plate1 is clamped down against it all   
around the outside  edge by means of a  
suitably-shaped iron collar1 which holds

 the glass  in air-tight contact against
 the plate beneath4 ,the whole iron     
plate1  with the slab of glass clamped  
to it1 is now turned over1 so that the  
glass hangs down under the plate4 ,the  
glass immediately begins to sag  under  
its own weight1 and is assisted in this 
tendency by a suitable  blowing of steam
 or air into the space between the plate
 and the glass4  ,in blowing bath-tubs  
in this way the glass is allowed to     
distend  downwards until the desired    
depth is attained1 when further         
distension  is arrested by bringing a   
flat supporting plate under the glass1  
which  is pressed against this flat     
plate by the pressure of the air1 thus  
forming the flat bottom of the tub4 ,in 
this process the outline of the  object 
is determined by the shape of the       
clamping bars or plate that  fix the    
edges of the hot glass against the iron 
plate described above1  and by this     
means almost any desired shape can be   
given to objects of  simple form4    ,it

 is obvious that this process can also  
be employed for blowing a  hollow body  
into contact with a mould of any desired
 form and forcing  the hot glass to take
 the exact shape of the mould2 for      
smaller  bodies1 however1 the blowing in
 of separately generated steam is not   
required1 the heat of the molten glass  
itself being used to generate  the      
necessary steam4 ,for this purpose the  
requisite quantity of glass  is dropped 
on the surface of a wet slab of         
asbestos4 ,on this surface  the glass   
remains floating upon a layer of steam1 
which is constantly  renewed by the     
intense heating action of the hot glass 
on the water  contained in the asbestos 
below4 ,the moulds used in this process 
are  provided with a sharp edge or lip1 
and as soon as the glass has spread     
into a slab of sufficient size1 the     
inverted mould is brought down  upon the
 glass and pressed against it4 ,the     
sharp lip or edge of the  mould forces  
the glass into close contact with the   

asbestos under  it all around the edge  
of the mould1 thereby enclosing the     
space  existing between the rest of the 
glass and the wet asbestos4 ,the heat   
of the glass continues to generate steam
 at a rapid rate1 but now the  steam can
 no longer escape from under the glass  
around the edges1 and  therefore blows  
the glass upwards into the mould1       
ultimately forcing  the glass into      
intimate contact with the surface of the
 mould2 when  this is accomplished1 the 
pressure of the steam rises rapidly1 and
  ultimately lifts the entire mould and 
glass sufficiently to allow  the excess 
steam to escape--and this is the sign   
that the blowing is  complete4 ,the     
whole process takes only a very few     
seconds1 and is very  successful when   
applied to suitable glass and used with 
moulds of  proper shape4 ,it is1 of     
course1 obvious that ordinary           
narrow-mouthed  bottles could not be    
produced in this way1 but wide-mouthed  
bottles and  jars are made in this      

manner1 although the chief utility of   
the process  lies in the production of  
comparatively shallow articles1 which   
are not  of a shape that lends itself to
 pressing4          ,,chapter ,,viii4   
,,blown ,,and ,,pressed ,,glass4        
,in many ways very similar to the       
processes employed in the production  of
 bottles are those used in the          
manufacture of all hollow glass  vessels
 that are produced by blowing1 either   
with or without the aid  of moulds4     
,apart from the actual shapes of the    
articles themselves1  however1 the      
principal difference between bottles and
 the better  classes of hollow          
glass-ware lies in the composition and  
quality of  the glass itself4 ,in this  
respect all grades of manufacture are to
  be met with1 from the light-coloured  
greenish or bluish glass used  for      
medicine bottles to the most perfectly  
colourless and brilliant  8crystal0 or  
flint glass4 ,this gradation in the     
perfection of the  glass represents a   

corresponding gradation in the care     
bestowed upon  the choice of raw        
materials and the various manipulations 
of melting  the glass4 ,as we have seen1
 for the commonest kinds of bottles1    
where  colour and quality are           
immaterial1 all kinds of fusible        
materials  can be utilised1 loamy or    
ferruginous sands and refuse glass of   
all  kinds being employed4 ,where       
somewhat higher requirements have to be 
met1 rather purer sands have to be used 
as sources of silica1 while  lime and   
alkali must be introduced in purer      
forms1 the alkali in the  shape of the  
cheapest qualities of salt-cake and the 
lime in that  of lime-stones reasonably 
free from iron and magnesia4 ,finally1  
for  the best qualities of glass the    
purest sand obtainable is used1 being   
often specially washed to remove all    
loamy matter1 while the alkali  is      
introduced in the form of carbonate1 a  
chemical product which in  its better   
qualities is practically free from      

injurious impurities4  ,in these        
high-class products two very distinct   
kinds of glass are  met with4 ,one      
class1 of which the ,bohemian 8crystal0 
is the highest  example1 is chemically  
of the nature of an alkali-lime         
silicate1 the  alkali in the case of the
 ,bohemian glass being potash2 the other
  variety of glass contains no lime1 its
 place being taken by lead1  typical of 
this class being ,english flint glass4  
,in some varieties of  glass1 lead is   
also replaced1 partially or entirely1 by
 barium1 but  this material is chiefly  
used for the manufacture of pressed     
glass4    ,the higher grades of quality 
in glass1 which thus require increased  
refinement in the raw materials1 also   
demand increased refinement  in the     
furnaces and appliances employed in     
their melting4 ,the  tank-furnace1 which
 holds the field in bottle manufacture1 
is  scarcely met with in the production 
of the better qualities of hollow       
glass-ware2 medicine bottles and other  

articles of moderate quality  might be  
produced in tanks1 but the quantity of  
glass required for  such purposes is    
seldom large enough to justify such     
large plant4 ,for  the best qualities of
 colourless glass-ware1 however1 the    
tank-furnace  could not be used on      
account of the fact that both as regards
 colour  and freedom from defects1 the  
product of a tank-furnace is never equal
  to the best product of pot-furnaces4  
,for flint-glass1 indeed1 covered  pots 
or crucibles must be used in order to   
adequately protect the  molten glass    
from the reducing action of the furnace 
gases and from  contamination by dust4  
,the materials of which the pots are    
constructed  are also chosen with a view
 to avoiding all risk of introducing    
colouring or otherwise injurious        
impurities from that source4    ,in all 
processes for the production of hollow  
glass-ware1 the glass or  8metal0 is    
taken from the pot by the process of    
gathering which has  already been       

described2 where blown articles are to  
be produced1 as  distinct from pressed  
goods1 the initial stage is always the  
formation  of a small hollow globe or   
bulb at the end of the glass-blower's   
pipe4  ,the subsequent manipulations    
depend upon the nature of the article to
  be produced4 ,the article may either  
be made entirely by hand work1 or       
rather 8chair0 work1 as it is usually   
called1 or the manipulations  may be    
facilitated and the product             
cheapened--while its character is1  of  
course1 also modified--by the aid of    
moulds1 which are used to  bring the    
object to its proper shape and to       
impress upon it certain  decorative     
mouldings or markings4 ,as we have      
already seen1 ordinary  bottles are now 
always blown with the aid of moulds1 and
 the same  applies to medicine bottles1 
lamp chimneys1 and the bulbs for        
electric  light2 in connection with     
lamp-chimneys it should be noted that   
they  are blown in moulds in the form of

 cylindrical bottles with a flat  bottom
 and a domed top1 the ends being        
subsequently cut off4    ,many of the   
cheaper varieties of tumblers and       
glasses are also blown in  moulds1 but  
they can be1 and sometimes are1 produced
 by hand1 and as  their manufacture is  
typical of that of all hand-blown hollow
 ware1 we  shall now describe it in some
 detail as an example of this class of  
work4    ,the implements used by the    
glass-blower and his assistants for this
  work are few and simple4 ,the largest 
item is the glass-blower's bench  or    
chair1 which is simply a rough wooden   
bench provided with two  projecting     
side-rails or arms4 ,when finishing a   
piece of work the  blower sits on this  
bench1 and the pipe lies across the two 
rails  in front of him in such a        
position that by rolling it backwards   
and  forwards along the rails he can    
readily keep the pipe in gentle         
rotation4 ,in addition to the ordinary  
blower's pipe and a 8pontil0  or rod for

 attaching small quantities of glass    
whereby the piece in  hand can be held1 
the only other tools used by the blower 
are a number  of shears and pincers of  
various shapes which serve for cutting  
off1  pressing in1 and distending the   
glass as required1 a flat board and  a  
stone or metal plat or 8marver0 being   
also used for the purpose of  moulding  
the glass4    ,as already indicated1 the
 first step in the production of such an
  object as a tumbler consists in       
gathering a suitable quantity of glass  
on the pipe and blowing it into a small 
bulb4 ,this bulb is blown out  to the   
proper size and is then elongated by    
gently swinging the pipe4  ,the next    
step is the flattening of the lower end 
of the bulb by gently  pressing it on   
the 8marver0 or flat plate provided for 
such purposes2  in this way the flat    
bottom of the glass is formed1 and the  
bulb now  has the shape of the finished 
glass1 but remains attached to the pipe 
by a shoulder and neck4 ,the earliest   

practice was to separate the  tumbler   
from the pipe at such a point as to     
leave the tumbler of the  correct       
length1 the remaining operation         
consisting in holding the  glass1 first 
fixed to a pontil for the purpose1 into 
the furnace so  as to heat the broken   
edge2 this edge was thereby rounded off1
 and the  brim of the glass could be    
widened or otherwise shaped by rotating 
the glass or pressing it in or out by   
the aid of pieces of wood4 ,in  modern  
practice1 however1 this is not usual1   
the glass being separated  from the pipe
 well above the shoulder and annealed in
 this shape4  ,subsequently the glass is
 finished in a trimming room or workshop
 by  being cut off at the desired point 
and having the rough edge rounded  off  
by the aid of a blowpipe flame4 ,the    
cutting-off operation is  carried out in
 a great variety of ways1 the most usual
 being by the  action of heat applied   
locally and suddenly1 either by the aid 
of  specially-shaped flat blowpipe      

flames or by an electrically-heated     
wire4 ,machines for carrying out this   
operation1 as well as the  subsequent   
rounding of the edge automatically1 are 
in use1 but the  latter process is      
sometimes replaced by slightly grinding 
and polishing  the edges4               
,illustration3 ,,fig4 #h4--,sectional   
diagram of the evolution of a  tumbler4 
,the evolution of an ordinary tumbler1  
as just described1 and as  illustrated  
diagrammatically in ,fig4 #h1 is typical
 of the whole process  of hollow-glass  
blowing1 but of course the number of    
operations1 as  well as the care and    
skill involved in each step1 increases  
rapidly as  the form of the vessel      
becomes more complex2 in the highest    
class of  work a very considerable      
element of artistic taste and judgment  
on the  part of the operative also      
becomes essential1 for1 although the    
form  of the object as well as the      
choice of colour and ornamentation are  
chosen by the designer1 the blower has  

to translate the drawing of the         
designer into glass1 and although his   
skill enables him to attain a           
considerable degree of fidelity in his  
rendering1 many details remain  at his  
own option1 and the proper management of
 these is no small  factor in the       
success of the whole work4    ,in this  
connection mention should perhaps be    
made of the application of  colour and  
other decorations to this kind of glass4
 ,a very considerable  range of effects 
of this kind is now available to the    
glass-worker4  ,in the first place the  
body of the glass used for the          
production of  the articles in question 
may be coloured by the addition of      
suitable  colouring materials to the    
molten glass or raw materials1 as       
explained  in ,chapter ,,xi41 but this  
procedure has very obvious limitations2 
where  the article is built up of glass 
from several gatherings--as1 for        
example1 is the case in an ordinary     
wine-glass1 where the bowl1 leg and     

foot are each made of separate          
gatherings--it is possible to use glass 
of different colours for these different
 parts1 and this is commonly  done in   
the production of wine glasses having   
ruby or green bowls and  white legs and 
feet4 ,a further modification in the    
application of  colour is obtainable by 
taking up two or more gatherings on the 
same  pipe and superposing a large      
gathering of white glass on a smaller   
one of coloured glass2 this is analogous
 to the process of 8flashing0  sheet    
glass1 described in ,chapter ,x4 and    
this process lends itself to a  variety 
of manipulations resulting in the       
distribution of the coloured  layer of  
glass in almost any desired manner over 
the object in hand4  ,the principal     
objection to this process1 however1 lies
 in the fact that  pots of molten glass 
of all the colours desired must be kept 
available  to the blower at the same    
time1 and this is not easily arranged   
for in  any reasonably economical       

manner4 ,for this reason1 and also      
because the  manipulations are simpler1 
coloured glass intended for application 
to  blown glass-ware is generally used  
in the form of short rods previously    
prepared2 these rods are suitably       
heated1 and the coloured glass can  then
 be applied to the article in hand at   
any desired place and in  as small or   
large a quantity as required4 ,if the   
two glasses thus  brought into contact  
are properly related to one another as  
regards  chemical composition and       
physical properties1 they blend very    
readily  and perfectly1 and the result  
is quite as good as could be obtained by
  using the coloured glass in the molten
 condition4 ,other decorations1  such as
 gilding or other metallic lustres and  
also various kinds of  iridescence1 are 
produced upon the finished glass4       
,metallic lustres  are obtained by      
placing upon the surface of the glass1  
and slightly  fusing into it a layer of 
particles of the actual metal4 ,in some 

cases  this is done by rolling the glass
 vessel1 while still hot1 in a mass of  
metallic foil of the kind desired1 when 
a sufficient quantity readily  adheres2 
in other cases the metal is applied in  
the form of a flux or  glaze containing 
a large proportion of an easily-reduced 
compound of  the metal1 and this is     
afterwards reduced to the metallic state
 by the  action of heat1 sometimes aided
 by that of smoke or other reducing     
gases4 ,an iridescent surface is        
produced upon certain varieties of      
glass by the corrosive action of acid   
vapours2 in fact1 in localities  where  
the atmosphere is tainted with sulphur  
fumes it is quite usual  to see an      
iridescent lustre on the surface of     
ordinary window glass4  ,there are1 of  
course1 numerous other means of         
decorating blown and  other glass1 such 
as cutting1 engraving1 etching1         
silvering1 etc41 but  it would lie      
beyond the scope of the present volume  
to deal with these1  since they are     

outside the field of actual glass       
manufacture4    ,in the production of   
hollow glass-ware by hand1 the          
glass-blower avails  himself to the full
 of the property so characteristic of   
glass of  assuming a pasty or viscous   
condition when suitably heated2 by      
raising  or lowering the temperature of 
his material1 the blower can at will    
render it stiffer or more fluid2 by     
blowing he can distend it1 draw it  out 
by the aid of gravity or centrifugal    
action1 or he can mould it  with the aid
 of rods and tongs of suitable shape1   
while at times he  allows it to fall or 
festoon under its own weight while held 
aloft4  ,with all these manipulations at
 his disposal1 the skilful operative  is
 able to work the glass to his will and 
to fashion objects of great  variety and
 beauty1 but it should be noted that    
objects produced by  hand in this way   
will bear the mark of the processes     
employed in their  production in the    
fact that they do not possess the       

extreme regularity  of size and shape   
which are associated with machine-made  
articles2  there is a certain natural   
variability in the exact shape of curves
 and  festoons that is foreign to the   
products of mechanical processes4 ,for  
some purposes this variability is a     
disadvantage1 while to some minds  it   
appears as a defect1 and methods have   
been devised for facilitating  the      
production of strictly uniform          
glass-ware by the use of moulds as  an  
aid to the work of the glass-blower4    
,while undoubtedly reducing the  value  
and beauty of the ware from the purely  
artistic standpoint1 these  aids to     
hand-work have rendered possible an     
immense expansion of the  entire        
industry1 since1 with the use of moulds1
 presentable glass-ware  can be produced
 by hands far less skilled than those   
required for pure  hand-work4    ,in the
 description given above of             
bottle-blowing by hand we have  already 
seen an example of the use of moulds in 

aiding the blower to  form his object to
 the desired size and shape4 ,much more 
complicated  and decorative objects can1
 however1 be produced by the use of     
moulds4  ,such objects as globes and    
shades for gas1 oil and electric lamps1 
when of a light substance and suitable  
shape1 are usually produced by  blowing 
bulbs of glass into moulds1 where they  
acquire the general  shape as well as   
the detailed decorated surface          
configuration which  they afterwards    
present4 ,here again the body remains a 
closed vessel1  and is only opened and  
trimmed to the final shape at the end of
 the  operation when all the blowing and
 moulding have been done4 ,articles     
blown in this way very frequently show  
8mould marks10 since the  contact of the
 hot glass with the relatively cold     
surface of the mould  results in a      
certain crinkling or roughening of the  
glass1 much as in  the process of       
rolling4 ,this effect can be minimised  
by dressing the  interior surfaces of   

the moulds with suitable greasy         
dressings1 whose  chief property should 
be that they do not stick to the hot    
glass and  leave little or no residue   
when gradually burnt away in the mould2 
the proper care of the moulds and their 
maintenance is in fact the  first       
essential to successful manufacture in  
this as well as in the  pressed-glass   
industry4 ,even under the most          
favourable conditions1  however1 the    
surface of glass blown into moulds is   
not so good as  that of hand-blown      
articles which have never come into     
contact with  cold materials1 and       
therefore retain undiminished the       
natural 8fire  polish0 which glass      
possesses when allowed to cool freely   
from the  molten state4 ,an effort at   
producing a similar brilliance of       
surface  on moulded and pressed articles
 is often made by exposing them1 after  
they have attained their final form1 to 
the heat of a furnace to such  an extent
 as to soften the surfaces and allow the

 glass to re-solidify  under the        
undisturbed influence of surface-tension
 much as it would do  in solidifying    
freely in the first place4              
,unfortunately this process  cannot be  
carried out without more or less        
softening the entire  article1 so that  
skilful manipulation is required to     
prevent serious  deformation of the     
object1 while a certain amount of       
rounding off in  all sharp corners and  
angles cannot be avoided4    ,the       
air-pressure required to bring the whole
 of the surfaces of a large  and        
possibly complicated piece of glass into
 contact with the surfaces  of the mould
 is sometimes very considerable1 and the
 lung-power of the  blower is often     
insufficient for the purpose2 in many   
works1 therefore1  compressed air is    
supplied for the purpose1 arrangements  
being employed  whereby the operative   
can quickly connect the mouthpiece of   
his pipe  with the air-main1 while he   
can accurately control the pressure by  

means of a suitable valve4 ,the ,sievert
 process of moulding by the aid  of     
steam pressure has already been         
described4    ,although the evolution of
 the industry scarcely followed this    
path1  it is not a large step to pass   
from a process in which air-pressure  is
 used to drive viscous glass into       
contact with a mould to a process  in   
which the pressure of the air is        
replaced by the pressure of a           
suitably-shaped solid plunger1 and this 
is essentially the widely-used  process 
of glass pressing4 ,in the first        
instance this mode of  manufacture is   
obviously applicable to solid or flat   
and shallow  articles which could not be
 conveniently evolved from the spherical
  bulb which stands as embryo of all    
blown glass2 at first sight it  would   
seem in fact as though the process must 
be limited to articles  of such a shape 
that a plunger can readily enter and    
leave the concave  portions4 ,by the    
ingenious device1 however1 of pressing  

two halves of  a closed or nearly closed
 vessel simultaneously in two adjacent  
moulds  and then pressing the two halves
 together while still hot enough to     
unite1 it has been made possible to     
produce by the press alone such  objects
 as water-jugs1 for example1 into which 
a plunger could not  possibly be        
introduced when finished4 ,the process  
of pressing being  a purely mechanical  
one and requiring no very elaborate     
plant and  little skilled labour1 has   
placed upon the market a host of cheap  
and  extremely useful articles1 thus    
serving to widen very considerably  the 
useful applications of glass4 ,on the   
other hand1 the process has  been and is
 still used to some extent for the      
production of articles  intended to     
imitate the products of other processes 
such as hand-blown  and cut glass1 with 
the result that a great deal of glass   
has been  produced which cannot possibly
 be classed as beautiful and much of    
which can lay as little claim to        

utility4    ,the essential feature of   
the process of glass pressing consists1 
as  already indicated1 in forcing a     
layer of glass into contact with a      
mould by the pressure of a mechanically 
actuated plunger4 ,for this  purpose a  
suitable mould and plunger as well as a 
press for holding the  former and       
actuating the latter are required4 ,the 
moulds are generally  made of a special 
quality of close-grained cast-iron1 and 
they are  kept trimmed and dressed in   
much the same manner as the moulds used 
for blowing 7except that the latter are 
sometimes made of wood74 ,for  the      
purpose of facilitating the removal of  
the finished article1 the  moulds are   
generally made in several pieces which  
fit into one another  and can be        
separated by means of hinges4 ,a very   
important point about  these moulds is  
that the various pieces should fit      
accurately into  one another1 since     
otherwise a minute 8fin0 of glass will  
be forced  into every interstice1 and   

the traces of these fins will always    
remain  visible on the finished article2
 the very perfect fit required to       
entirely prevent the formation of such  
fins is1 of course1 scarcely  attainable
 in practice except in the case of new  
moulds1 so that the  traces of fins are 
generally to be found on all pressed    
articles1 and  serve as a ready means of
 identifying these products when an     
attempt  is made to imitate better      
classes of glass-ware by their means4   
,the  presses used in this process are  
generally of the hand-lever type2  power
 presses could no doubt be used1 but it 
is contended that the  hand-press has a 
very great advantage in allowing the    
operator to judge  by touch when        
sufficient pressure has been exerted1   
and this is an  important consideration1
 since an excessive pressure would      
either force  the glass out of the mould
 altogether or would be liable to burst 
or  injure the mould seriously4 ,the    
actual presses consist of vertical      

guides and levers for controlling the   
movement of the plunger and a  table for
 holding the moulds1 and in some cases a
 system of cranks and  levers for       
opening and closing the moulds4 ,the    
process of pressing is  exceedingly     
simple4 ,the proper quantity of glass is
 gathered from the  pot on a solid rod  
and dropped into the mould4 ,the thread 
of glass  which remains between the     
glass in the mould and that remaining on
  the iron is cut off with a pair of    
shears1 and then the plunger is  lowered
 into the mould and allowed to remain   
there until the glass  has stiffened    
sufficiently to retain its shape1 when  
the plunger is  withdrawn4 ,in this     
proceeding it will be seen that the     
glass is forced  into intimate contact  
with the relatively cold surfaces of    
mould and  plunger1 and while undergoing
 this treatment the glass must remain   
sufficiently plastic to readily adapt   
itself to the configuration of  the     
mould4 ,it is therefore not surprising  

to find that the pressing  process can  
only be used successfully with glass of 
a kind specially  adapted for it4       
,certain varieties of flint glass and   
some barium  glasses are used for this  
purpose1 but the greater quantity of    
pressed  glass1 particularly as produced
 on the ,continent1 is made of a        
lime-alkali silicate containing         
considerable quantities of both soda    
and potash and relatively little lime2  
while sufficiently resistant for  most  
purposes1 this glass is particularly    
soft and adaptable while in  the viscous
 condition4    ,the deleterious effect  
produced upon glass surfaces when       
brought into  contact with relatively   
cold metal has already been referred to 
above1  and it only remains to add that 
this is the principal difficulty with   
which the glass-pressing process has to 
contend4 ,it is overcome to  some extent
 by the aid of the reheating process    
described above2 but  this is only a    
partial remedy1 and in the majority of  

pressed glass  products the surface is  
8covered0 as far as possible by the     
application  of relief decorations such 
as grooves1 spirals1 and ribbings4 ,an  
attempt is sometimes made to imitate the
 appearance of cut glass1  but the      
rounding of the angles during the       
reheating process destroys  the         
sharpness of the effect and allows of   
the ready detection of the  imitation1  
while the cheapness of the decoration   
when applied in the  mould has          
frequently led manufacturers to grossly 
over-decorate1 and1  therefore1 destroy 
all claim to beauty in their wares4     
,,chapter ,,ix4    ,,rolled ,,or        
,,plate-,,glass4      ,in the present   
chapter we propose to deal with all     
those processes  of glass manufacture in
 which the first stage consists in      
converting  the glass into a slab or    
plate by some process of rolling4 ,we   
have  already considered the general    
character of the rolling process1 and   
have seen that1 although hot1 viscous   

glass lends itself readily to  being    
rolled into sheets or slabs1 these      
cannot be turned out with  a smooth1    
flat surface4 ,in practice the surface  
of rolled glass is  always more or less 
dimmed by contact with the minute       
irregularities  of table or roller1 and 
larger irregularities of the surface    
arise  from the buckling that occurs at 
a great many places in the sheet4       
,these limitations govern the varieties 
of glass that can be produced by        
processes that involve rolling1 and have
 led to the somewhat curious  result    
that both the cheapest and roughest1 as 
well as the best and  most expensive    
kinds of flat glass1 are produced by    
rolling processes4  ,ordinary rough     
8rolled plate10 such as that used in the
 skylights of  workshops and of railway 
stations1 is the extreme on the one     
hand1  while polished plate-glass       
represents the other end of the scale4  
,the  apparent paradox is1 however1     
solved when it is noted that in the     

production of polished plate-glass the  
character of the surface of  the glass  
as it leaves the rollers is of very     
minor importance1 since  it is entirely 
obliterated by the subsequent processes 
of grinding1  smoothing1 and polishing4 
,intermediate between the rough 8rolled0
 and  the 8polished0 plate-glass we have
 a variety of glasses in which the      
appearance of the rolled surface is     
hidden or disguised to a greater or     
lesser extent by the application of a   
pattern that is impressed upon  the     
glass during the rolling process2 thus  
we have rolled plate having  a ribbed or
 lozenge-patterned surface1 or the      
well-known variety of  8figured rolled0 
plate1 sometimes known as 8,muranese10  
whose elaborate  and deeply-imprinted   
patterns give a very brilliant effect4  
,rolled plate-glass being practically   
the roughest and cheapest form of       
glazing1 is principally employed where  
appearance is not considered1  and its  
chief requirement is1 therefore1        

cheapness1 although both the  colour and
 quality of the glass are of importance 
as affecting the  quantity and character
 of the light which it admits to the    
building  where the glass is used4 ,on  
the ground of cheapness it will be      
obvious  from what we have said above   
7,chapter ,,iv471 that such glass can   
only be produced economically in large  
tank furnaces1 and these are            
universally used for this purpose4 ,the 
requirements as regards freedom  from   
enclosed foreign bodies of small size   
and of enclosed air-bells  are not very 
high in such glass1 and1 therefore1     
tanks of very simple  form are generally
 used4 ,no refinements for regulating   
the temperature  of various parts of the
 furnace in order to ensure perfect     
fining of  the glass are required1 and  
the furnace generally consists simply of
 an  oblong chamber or tank1 at one end 
of which the raw materials are fed  in1 
while the glass is withdrawn by means of
 ladles from one or two  suitable       

apertures at the other end4 ,for        
economical working1 however1  the       
furnace must be capable of working at a 
high temperature1 because  a cheap glass
 mixture is necessarily somewhat        
infusible1 at all events  where colour  
is considered4 ,this will be obvious if 
we remember that  the fusibility of a   
glass depends upon its alkali contents1 
and alkali  is the most expensive       
constituent of such glasses4    ,the    
actual raw materials used in the        
production of rolled plate-glass  are   
sand1 limestone and salt-cake1 with the 
requisite addition of  carbon and of    
fluxing and purifying materials4 ,the   
selection of these  materials is made   
with a view to the greatest purity and  
constancy  of composition which is      
available within the strictly-set limits
 of  price which the low value of the   
finished product entails4 ,these        
materials are handled in very large     
quantities1 outputs of from #fj to  #aej
 tons of finished glass per week from a 

single furnace being by no  means       
uncommon2 mechanical means of handling  
the raw materials and of  charging them 
into the furnace are therefore adopted  
wherever possible4    ,the glass is     
withdrawn from the furnace by means of  
large iron ladles4  ,these ladles are   
used of varying sizes in such a way as  
to contain the  proper amount of glass  
to roll to the various sizes of sheets  
required4  ,the sizes used are sometimes
 very large1 and ladles holding as much 
as #ahj to #bjj lbs4 of glass are used4 
,these ladles1 when filled with  glass1 
are not carried by hand1 but are        
suspended from slings attached  to      
trolleys that run on an overhead rail4  
,the ladler1 whose body is  protected by
 a felt apron and his face by a mask    
having view-holes  glazed with green    
glass1 takes the empty ladle from a     
water-trough1 in  which it has been     
cooled1 carries it to the slightly      
inclined gangway  that leads up to the  
opening in the front of the furnace1 and

 there  introduces the ladle into the   
molten glass1 giving it a half-turn so  
as to fill it with a 8solid0 mass of    
glass4 ,by giving the ladle two or      
three rapid upward jerks1 the operator  
then detaches the glass in the  ladle as
 far as possible from the sheets and    
threads of glass which  would otherwise 
follow its withdrawal2 then the part of 
the handle of  the ladle near the bowl  
is placed in the hook attached to the   
overhead  trolley1 and by bearing his   
weight on the other end of the handle1  
the  workman draws the whole ladle up   
from the molten bath in the furnace  and
 out through the working aperture4 ,this
 operation only takes a  few seconds to 
perform1 but during this time the ladler
 is exposed  to great heat1 as a more or
 less intense flame generally issues    
from  the working aperture1 whence it is
 drawn upward under the hood of the     
furnace4 ,from the furnace opening1 the 
ladler1 generally aided by a  boy1 runs 
the full ladle to the rolling table and 

there empties the  ladle upon the table 
just in front of the roller4 ,in doing  
this1 two  distinctly different methods 
are employed4 ,in one1 only the         
perfectly  fluid portion of the glass is
 poured out of the ladle by gradually   
tilting it1 the chilled glass next to   
the walls of the ladle being  retained  
there and ultimately returned to the    
furnace while still hot4  ,in the other 
method1 the chilling of the glass is    
minimised as far as  possible1 and the  
entire contents of the ladle are emptied
 upon the  rolling table by the ladler1 
who turns the entire ladle over with a  
rapid jerk which is so arranged as to   
throw the coldest part of the  glass    
well away from the rest4 ,when the sheet
 is subsequently rolled  this chilled   
portion is readily recognised by its    
darker colour1 and  since it lies       
entirely at one end of the sheet it is  
detached before  the sheet goes any     
further4 ,neither method appears to     
present any  preponderating advantage4  

,illustration3 ,,fig4 #i4--,rolling     
table for rolled plate-glass4    ,the   
rolling table used in the manufacture of
 rolled plate is  essentially a         
cast-iron slab of sufficient size to    
accommodate the  largest sheet which is 
to be rolled2 over this slab moves a    
massive  iron roller which may be       
actuated either by hand or by mechanical
  power--the latter1 however1 being now 
almost universal4 ,the thickness  of the
 sheet to be rolled is regulated by     
means of slips of iron placed  at the   
sides of the table in such a way as to  
prevent the roller from  descending any 
further towards the surface of the      
table3 so long as  the layer of glass is
 thicker than these slips1 the entire   
weight of  the roller comes upon the    
soft glass and presses it down1 but as  
soon as the required thickness is       
attained1 the weight of the roller  is  
taken by the iron slips and the glass is
 not further reduced in  thickness4 ,the
 width of the sheet is regulated by     

means of a pair of  iron guides1 formed 
to fit the forward face of the roller   
and the  surface of the table1 in the   
manner indicated in ,fig4 #i4 ,the      
roller1  as it moves forward1 pushes    
these guides before it1 and the glass is
  confined between them4 ,when the      
roller has passed over the glass1 the   
sheet is left on the iron table in a    
red-hot1 soft condition1 and it  must be
 allowed to cool and harden to a certain
 extent before it can  be safely moved4 
,in this interval1 the chilled          
portion--if any--is  partially severed  
by an incision made in the sheet by     
means of a long  iron implement somewhat
 like a large knife1 and then the sheet 
is  loosened from the bed of the table  
by passing under it1 with a smooth      
rapid stroke1 a flat-bladed iron tool4  
,the sheet is next removed to  the      
annealing kiln or 8lear10 being first   
drawn on to a stone slab and  thence    
pushed into the mouth of the kiln4 ,at  
this stage the chilled  portion of the  

sheet is completely severed by a blow   
which causes the  glass to break along  
the incision previously made4    ,the   
rolled-plate annealing kiln is          
essentially a long1 low tunnel1  kept   
hot at one end1 where the freshly-rolled
 sheets are introduced1  and cold at the
 other end1 the temperature decreasing  
uniformly down  the length of the       
tunnel4 ,the sheets pass down this      
tunnel at a  slow rate1 and are thus    
gradually cooled and annealed           
sufficiently  to undergo the necessary  
operations of cutting1 etc4 ,although   
thus  simple in principle1 the proper   
design and working of these 8lears0 is  
by no means simple or easy1 since       
success depends upon the correct        
adjustment of temperatures throughout   
the length of the tunnel and a  proper  
rate of movement of the sheets1 while   
the manner of handling and  supporting  
the sheets is vital to their remaining  
flat and unbroken4  ,the actual movement
 of the sheets is effected by a system  

of moving  grids which run              
longitudinally down the tunnel4 ,the    
sheets ordinarily  lie flat upon the    
stone slabs that form the floor of the  
tunnel1 and  the grids are lowered into 
recesses cut to receive them4 ,at       
regular  intervals the iron grid bars   
are raised just sufficiently to lift the
  sheets from the bed of the kiln1 and  
are then moved longitudinally a  short  
distance1 carrying the sheets forward   
with them and immediately  afterwards   
again depositing them on the stone bed4 
,the grids return to  their former      
position while lowered into their       
recesses below the level  of the kiln   
bed4    ,when they emerge from the      
annealing kiln or 8lear0 the sheets of  
rolled  plate-glass are carried to the  
cutting and sorting room4 ,here the     
sheets are trimmed and cut to size4 ,the
 edges of the sheets as they  leave the 
rolling table are somewhat irregular1   
and sometimes a little  8beaded10 while 
the ends are always very irregular4     

,ends and edges are  therefore cut      
square or 8trimmed0 by the aid of the   
cutting diamond4  ,for this purpose the 
sheet is laid upon a flat table1 the    
smoothest  side of the sheet being      
placed upwards1 and long cuts are taken 
with  a diamond--good diamonds of       
adequate size and skilful operators     
being  necessary to ensure good cutting 
on such thick glass over long lengths4  
,strips of glass six or eight feet long 
and half an inch wide are  frequently   
detached in the course of this          
operation1 and the final  separation is 
aided by slight tapping of the underside
 of the glass  just below the cut       
and--if necessary--by breaking the strip
 off by the  aid of suitable tongs4     
,no very elaborate 8sorting0 of rolled  
plate glass is required1 except  perhaps
 that the shade of colour in the glass  
may vary slightly from  time to time1   
and it is generally preferable to keep  
to one shade of  glass in filling any   
particular order4 ,apart from this1 the 

rolled  plate cutter has merely to cut  
out gross defects which would interfere 
too seriously with the usefulness of the
 glass4 ,as we have already  indicated1 
air-bells and minute enclosures of      
opaque matter are not  objectionable in 
this kind of glass1 but large pieces of 
opaque  material must generally be cut  
out and rejected1 not only because  they
 are too unsightly to pass even for     
rough glazing purposes1 but  also       
because they entail a considerable risk 
of spontaneous cracking  of the         
glass--in fact1 visible cracks are      
nearly always seen around  large        
8stones10 as these inclusions are       
called4 ,these may arise from  various  
causes1 such as incomplete melting of   
the raw materials1 or the  contamination
 of the raw materials with infusible    
impurities1 but the  most fruitful      
source of trouble in this direction lies
 in the crumbling  of the furnace       
lining1 which introduces small lumps of 
partially melted  fire-clay into the    

glass4 ,in a rolled plate tank furnace  
which is  properly constructed and      
worked1 the percentage of sheets which  
have to  be cut up on account of such   
enclosures should be very small1 at all 
events until the furnace is old1 when   
the linings naturally show an           
increasing tendency to disintegrate4    
,returning now to the rolling process1  
it is readily seen that a very  slight  
modification will result in the         
production of rolled plate-glass  having
 a pattern impressed upon one surface2  
this modification  consists in engraving
 upon the cast-iron plate of the rolling
 table in  intaglio any pattern that is 
to appear upon the glass in relief4 ,as 
a matter of fact only very simple       
patterns are produced in this way1  such
 as close parallel longitudinal ribbing 
and a lozenge-pattern1 the  reason      
probably being that the cost of cutting 
an elaborate pattern  over the large    
area of the bed-plate of one of these   
tables would be  very considerable4     

,further1 as these tables and their     
bed-plates are so  very heavy1 they are 
not readily interchanged or left        
standing idle1 so  that only patterns   
required in very great quantity could be
 profitably  produced in this way4      
,these disadvantages are1 however1      
largely  overcome by the double-rolling 
machine4 ,in this machine1 into whose   
rather elaborate details we cannot enter
 here1 the glass is rolled  out into a  
sheet of the desired size and thickness 
by being passed  between two rollers    
revolving about stationary axes1 the    
finished sheet  emerging over another   
roller1 and passing on to a stone slab  
that  moves forward at the same rate as 
the sheet is fed down upon it4 ,in  this
 machine a pattern can be readily       
imprinted upon the soft sheet  as it    
passes over the last roller by means of 
a fourth roller1 upon  which the pattern
 is engraved2 this is pressed down upon 
the sheet1  and leaves upon it a clear1 
sharp and deep impress of its pattern4  

,the general arrangement of the rollers 
in this machine is shown in  the diagram
 of ,fig4 #aj1 which represents the     
sectional elevation of  the appliance4  
,after leaving the rolling machine1 the 
course of the  8figured rolled plate0   
produced in this manner is exactly      
similar to  that of ordinary rolled     
plate1 except that as a somewhat softer 
kind of  glass is generally used for    
8figured10 the temperature of the       
annealing  kilns requires somewhat      
different adjustment4 ,the cutting of   
the glass  also requires rather more    
care1 and it should be noted that such  
glass  can only be cut with a diamond on
 the smooth side2 the side upon which   
the pattern has been impressed in relief
 cannot be materially affected  by a    
diamond4 ,this is one reason why it is  
not feasible to produce such  glass with
 a pattern on both sides4               
,illustration3 ,,fig4 #aj4--,sectional  
diagram of machine for rolling  8figured
 rolled0 plate-glass4    ,figured rolled

 glass1 being essentially of an         
ornamental or decorative  nature1 is    
generally produced in either brilliantly
 white glass or in  special tints and   
colours1 and the mixtures used for      
attaining these  are1 of course1 the    
trade property of the various           
manufacturers2 the  whiteness of the    
glass1 however1 is only obtainable by   
the use of very  pure and1 therefore1   
expensive materials4 ,as regards the    
coloured  plate-glasses1 a general      
account of the principles underlying the
  production of coloured glass will be  
found in ,chapter ,,xi4    ,the         
manufacture of polished plate-glass     
really stands somewhat by  itself1      
almost the only feature which it has in 
common with the  branches of manufacture
 just described being the initial       
rolling  process4    ,the raw materials 
for the production of plate-glass are   
chosen with  the greatest possible care 
to ensure purity and regularity2 owing  
to  the very considerable thickness of  

glass which is sometimes employed  in   
plate1 and also to the linear dimensions
 of the sheets which allow  of numerous 
internal reflections1 the colour of the 
glass would become  unpleasantly        
obtrusive if the shade were at all      
pronounced4 ,the actual  raw materials  
used vary somewhat from one works to    
another2 but1 as  a rule1 they consist  
of sand1 limestone1 and salt-cake1 with 
some  soda-ash and the usual additions  
of fluxing and purifying material such  
as arsenic1 manganese1 etc4 ,the glass  
is generally melted in pots1 and        
extreme care is required to ensure      
perfect melting and fining1 since  very 
minute defects are readily visible in   
this glass when finished1  and1 of      
course1 detract most seriously from its 
value4    ,the method of transferring   
the glass from the melting-pot to the   
rolling table differs somewhat in       
different works4 ,in many cases the     
melting-pots themselves are taken bodily
 from the furnace and emptied  upon the 

bed-plate of the rolling machine1 while 
in other cases the  glass is first      
transferred to smaller 8casting0 pots1  
where it has to  be heated again until  
it has freed itself from the bubbles    
enclosed  during the transference1 and  
then these smaller pots are used for    
pouring the glass upon the rolling slab4
 ,the advantage of the latter  more     
complicated method lies1 no doubt1 in   
the fact that the large  melting-pots1  
which have to bear the brunt of the heat
 and chemical  action during the early  
stages of melting1 are not exposed to   
the great  additional strain of being   
taken from the hot furnace and exposed  
for some time to the cold outside air4  
,apart from the mechanical  risks of    
fracture1 this treatment exposes the    
pots to grave risks of  breakage from   
unequal expansion and contraction on    
account of the great  differences of    
temperature involved4 ,where smaller    
special casting-pots  are used1 these   
are not exposed to such prolonged heat  

in the furnace1  and are never exposed  
to the chemical action of the raw       
materials1 so  that these subsidiary    
pots may perhaps be made of a material  
better  adapted to withstand sudden     
changes of temperature than the         
high-class  fire-clay which must be used
 in the construction of melting pots4   
,on  the other hand1 the transference of
 the glass from the melting to  the     
casting-pots involves a laborious       
operation of ladling and the  refining  
of the glass1 with its attendant        
expenditure of time and fuel4  ,finally1
 the production of plate-glass in tank  
furnaces could only be  attempted by the
 aid of such casting-pots in which the  
glass would have  to undergo a second   
fining after being ladled from the tank1
 and this  would materially lessen the  
economy of the tank for this purpose1   
while  it is by no means an easy matter 
to produce in tank furnaces qualities   
of glass equal as regards colour and    
purity to the best products of the  pot 

furnace4    ,the withdrawal of the pots 
containing the molten glass from the    
furnace  is now universally carried out 
by powerful machinery4 ,the pots are    
provided on their outer surface with    
projections by which they can be  held  
in suitably-shaped tongs or cradles4 ,a 
part of the furnace wall1  which is     
constructed each time in a temporary    
manner1 is broken down2  the pot is     
raised from the bed or 8siege0 of the   
furnace by the aid  of levers1 and is   
then bodily lifted out by means of a    
powerful fork4  ,the pot is then lifted 
and carried by means of cranes until it 
is in  position above the rolling table2
 there the pot is tilted and the glass  
poured out in a steady stream upon the  
table1 care being taken to avoid  the   
inclusion of air-bells in the mass      
during the process of pouring4  ,when   
empty1 the pot is returned to the       
furnace as rapidly as possible1  the    
glass being meanwhile rolled out into a 
slab by the machine4  ,except for the   

greater size and weight of both table   
and roller1 the  plate-glass rolling    
table is similar to that already        
described in  connection with rolled    
plate4 ,of course1 since the glass is   
poured  direct from the pot1 there is no
 chilled glass to be removed4 ,further1 
owing to the large size of sheets       
frequently required1 the bed of the     
rolling table cannot be made of a single
 slab of cast-iron1 a number of         
carefully jointed plates being1 in fact1
 preferable1 as they are less  liable to
 warp under the action of the hot glass4
    ,in arranging the whole of the      
rolling plant1 the chief consideration  
to be kept in mind is that it is        
necessary to produce a flat sheet of    
glass of as nearly as possible equal    
thickness all over4 ,the final          
thickness of the whole slab when ground 
and polished into a sheet  of           
plate-glass must necessarily be slightly
 less than that of the  thinnest part of
 the rough rolled sheet4 ,if1 therefore1

 there are any  considerable variations 
of thickness1 the result will be that in
 some  parts of the sheet a considerable
 thickness of glass will have to be     
removed during the grinding process4    
,this will arise to a still more        
serious extent if the sheet as a whole  
should be bent or warped so  as to      
depart materially from flatness4 ,the   
two cases are illustrated               
diagrammatically in ,fig4 #aa1 which    
shows sectional views of the sheets     
before and after grinding on an         
exaggerated scale4    ,illustration3    
,,fig4 #aa4--,sectional diagram         
illustrating waste of glass  in grinding
 curved or irregular plate4    ,while it
 is evident that careful design of the  
rolling table will avoid  all tendency  
to the formation of sheets of such      
undesirable form1 it  is a much more    
difficult matter to avoid all distortion
 of the sheet  during the annealing     
process and while the sheet is being    
moved from  the rolling table to the    

annealing kiln4 ,owing to the great size
 of  the slabs of glass to be dealt     
with1 and still more to the stringent   
requirement of flatness1 the continuous 
annealing kiln1 in which the  glass     
travels slowly down a tunnel from the   
hot to the cold end1 has  not been      
adopted for the annealing of            
plate-glass1 and a form of  annealing   
kiln is still used for that glass which 
is similar in its  mode of operation to 
the old-fashioned kilns that were used  
for other  kinds of glass before the    
continuous kiln was introduced4 ,these  
kilns  simply consist of chambers in    
which the hot glass is sealed up and    
allowed to cool slowly and uniformly    
during a more or less protracted        
period4 ,in the case of plate-glass1 the
 slabs are laid flat on the  stone bed  
of the kiln4 ,this stone bed is built up
 of carefully dressed  stone1 or blocks 
of fire-brick bedded in sand in such a  
way that they  can expand freely        
laterally without causing any tendency  

for the floor  to buckle upwards as it  
would do if the blocks were set firmly  
against  one another4 ,the whole chamber
 is previously heated to the requisite  
temperature at which the glass still    
shows a very slight plasticity4  ,the   
hot glass slabs from the rolling table  
are laid upon the bed  of this kiln1    
several being usually placed side by    
side in the one  chamber1 and the slabs 
in the course of the first few hours    
settle down  to the contour of the bed  
of the kiln1 from which shape and       
position  they are never disturbed until
 they are removed when quite cold4 ,in  
modern practice the cooling of a kiln is
 allowed to occupy from four  to five   
days2 even this rate of cooling is only 
permissible if care  is taken to provide
 for the even cooling of all parts of   
the kiln1  and for this purpose special 
air-passages are built into the walls of
  the chamber and beneath the bed upon  
which the glass rests1 and air          
circulation is admitted to these in such

 a way as to allow the whole of  the    
kiln to cool down at the same rate2 in  
the absence of such special             
arrangements1 the upper parts of the    
kiln would probably cool much more      
rapidly than the base1 so that the glass
 would be much warmer on its  under than
 on its upper surface4    ,when the     
slabs of plate-glass are removed from   
the annealing kilns  they very closely  
resemble sheets of rolled plate in      
appearance1  and they are quite         
sufficiently transparent to allow of    
examination  and the rejection of the   
more grossly defective portions2 the    
more  minute defects1 of course1 can    
only be detected after the sheets have  
been polished1 but this preliminary     
examination saves the laborious         
polishing of much useless glass4    ,the
 process of grinding and polishing      
plate-glass consists of three  principal
 stages4 ,in the first stage the        
surfaces of the glass are  ground so as 
to be as perfectly flat and parallel as 

possible2 in order  to effect this      
object as rapidly as possible1 a coarse 
abrasive is  used which leaves the glass
 with a rough grey surface4 ,in the     
second  stage1 that of smoothing1 these 
rough grey surfaces are ground down     
with several grades of successively     
finer abrasive until finally an         
exceedingly smooth grey surface is left4
 ,in the third and final stage1  the    
smooth grey surface is converted into   
the brilliant polished  surface with    
which we are familiar by the action of a
 polishing medium4    ,originally the   
various stages of the grinding and      
polishing processes  were carried out by
 hand1 but a whole series of ingenious  
machines  has been produced for         
effecting the same purpose more rapidly 
and  more perfectly than hand-labour    
could ever do4 ,we cannot hope to give  
any detailed account of the various     
systems of grinding and polishing       
machines which are even now in use1 but 
must content ourselves with  a survey of

 some of the more important             
considerations governing the  design and
 construction of such machinery4    ,in 
the first place1 before vigorous        
mechanical work can be applied to  the  
surface of a plate of glass1 that plate 
must be firmly fixed in a  definite     
position relatively to the rest of the  
machinery1 and such  firm fixing of a   
plate of glass is by no means readily   
attained1 since  the plate must be      
supported over its whole area if local  
fracture is to  be avoided4 ,while the  
surface of the plate is in the uneven   
condition  in which it leaves the       
rolling table1 such a firm setting of   
the glass  can only be attained by      
bedding it in plaster1 and this must be 
done in  such a manner as to avoid the  
formation of air-bubbles between plaster
  and glass2 if bubbles are allowed to  
form1 they constitute places where  the 
glass is unsupported4 ,during the       
grinding and polishing processes  these 
unsupported places yield to the heavy   

pressure that comes upon  them1 and     
irregularities in the finished polished 
surfaces result4  ,the most perfect     
adhesion between glass and plaster is   
attained by  spreading the paste of     
plaster on the up-turned surface of the 
slab of  glass and lowering the iron    
bed-plate of the grinding table down    
upon  it1 the bed-plate with the        
adhering slab of glass being afterwards 
turned over and brought into position in
 the grinding machine4 ,when one  side  
of the glass has been polished1 it is   
generally found sufficient  to lay the  
slab down on a bed of damp cloth1 to    
which it adheres very  firmly1 although 
sliding is entirely prevented by a few  
blocks fixed to  the table in such a way
 as to abut against the edges of the    
sheet4 ,in  many works1 however1 the    
glass is set in plaster for the grinding
 and  polishing of the second side as   
well as of the first4    ,the process of
 grinding and polishing is still        
regarded in many  plate-glass works as  

consisting of three distinct processes1 
known  as rough grinding1 smoothing and 
polishing respectively4 ,formerly  these
 three stages of the process were       
carried out separately2 at  first by    
hand1 and later by three different      
machines4 ,in the most  modern practice1
 however1 the rough and smooth grinding 
are done on  the same machine1 the only 
change required being the substitution  
of  a finer grade of abrasive at each   
step for the coarser grade used in  the 
previous stage4 ,for the polishing      
process1 however1 the rubbing           
implements themselves must be of a      
different kind1 for while the  grinding 
and smoothing is generally done by means
 of cast-iron rubbers  moving over the  
glass1 the polishing is done with felt  
pads4 ,the table  of the machine1 to    
which the glass under treatment is      
attached1 is  therefore made movable1   
and when the grinding and smoothing     
processes  are complete1 the table with 
its attached glass is moved so as to    

come  beneath a superstructure carrying 
the polishing rubbers1 and the whole  is
 then elevated so as to allow the       
rubbers to bear on the glass4    ,the   
earliest forms of grinding machines gave
 a reciprocal motion to  the table which
 carries the glass1 or the grinding     
rubbers were moved  backward and forward
 over the stationary table4 ,rotary     
machines1  however1 were introduced and 
rapidly asserted their superiority1     
until1  at the present time1 practically
 all plate-glass is ground on rotating  
tables1 some of these attaining a       
diameter of over #cj ft4 ,the grinding  
8rubbers0 consist of heavy iron slabs1  
or of wood boxes shod with iron1  but of
 much smaller diameter than the grinding
 table4 ,the rubbers  themselves are    
rotary1 being caused to rotate either by
 the frictional  drive of the rotating  
table below them1 or by the action of   
independent  driving mechanism1 but the 
design of the motions must be so        
arranged  that the relative motion of   

rubber and glass shall be approximately 
the  same at all parts of the glass     
sheets1 otherwise curved instead of     
plane surfaces would be formed4 ,this   
condition can be met by placing  the    
axes of the rubbers at suitable points  
on the diameter of the  table4 ,the     
abrasive is fed on to the glass in the  
form of a thin paste1  and when each    
grade or 8course0 has done the work     
required of it1 the  whole table is     
washed down thoroughly with water and   
then the next  finer grade is applied4  
,the function of the first or coarsest  
grade is  simply to remove the surface  
irregularities and to form a rough but  
plane surface4 ,the abrasive ordinarily 
employed is sharp sand1 but  only       
comparatively light pressure can be     
applied1 especially at the  beginning of
 this stage1 since at that period the   
weight of the rubber  is at times borne 
by relatively small areas of glass that 
project here  and there above the       
general level of the slab4 ,as these are

 ground  away1 the rubbers take a larger
 and more uniform bearing1 and greater  
pressure can be applied4 ,the subsequent
 courses of finer abrasives  are only   
required to remove the coarse pittings  
left in the surface  by the action of   
the first rough grinding sand2 the finer
 abrasive  replaces the deep pits of the
 former grade by shallower pits1 and    
this  is carried on in a number of steps
 until a very smooth 8grey0 surface  is 
attained and the smoothing process is   
complete4 ,the revolving table  or      
8platform0 is now detached from the     
driving mechanism1 and moved  along     
suitably placed rails on wheels provided
 for that purpose1 until  it stands     
below the polishing mechanism4 ,here it 
is attached to a  fresh driving         
mechanism1 and it is then either raised 
so as to bring  the glass into contact  
with the felt-covered polishing rubbers1
 or  the latter are lowered down upon   
the glass4 ,the polishing rubbers are   
large felt-covered slabs of wood or iron

 which are pressed against  the glass   
with considerable force2 their movement 
is very similar to  that of the grinding
 rubbers1 but in place of an abrasive   
they are  supplied with a thin paste of 
rouge and water4 ,the time required for 
the polishing process depends upon the  
perfection of the smoothing  that has   
been attained2 in favourable cases two  
or three hours are  sufficient to       
convert the 8grey0 surface into a       
perfectly polished one2  where1 however1
 somewhat deeper pits have been left in 
the glass1 the  time required for       
polishing may be much longer1 and the   
polish attained  will not be so perfect4
 ,the mode of action of a polishing     
medium such  as rouge is now recognised 
to be totally different in character    
from  that of even the finest abrasive2 
the grains of the abrasive act by  their
 hardness and the sharpness of their    
edges1 chipping away tiny  particles of 
the glass1 so that the glass steadily   
loses weight during  the grinding and   

smoothing processes4 ,during the        
polishing process1  however1 there is   
little or no further loss of weight1 the
 glass  forming the hills or highest    
parts of the minutely pitted surface    
being  dragged or smeared over the      
surface in such a way as to gradually   
fill  up the pits and hollows4 ,the part
 played by the polishing medium is      
probably partly chemical and partly     
physical1 but it results1 together  with
 the pressure of the rubber1 in giving  
to the surface molecules  of the glass a
 certain amount of freedom of movement1 
similar to that  of the molecules of a  
viscid liquid2 the surface layers of    
glass  are thus enabled to 8flow0 under 
the action of the polisher and to       
smooth out the surface to the beautiful 
level smoothness which is so            
characteristic of the surfaces of       
liquids at rest4 ,this explanation  of  
the polishing process enables us to     
understand why the proper  consistency  
of the polishing paste1 as well as the  

proper adjustment  of the speed and     
pressure of the rubbers1 plays such an  
important part  in successful polishing2
 it also serves to explain the          
well-known fact  that rapid polishing   
only takes place when the glass surface 
has begun  to be perceptibly heated by  
the friction spent upon it4    ,it has  
been estimated that1 on the average1    
slabs of plate-glass lose  one-third of 
their original weight in the grinding   
and polishing  processes1 and it is     
obvious that the erosion of this great  
weight of  glass must absorb a great    
amount of mechanical energy1 while the  
cost  of the plant and upkeep is        
proportionately great4 ,every factor    
that  tends to diminish either the total
 weight of glass to be removed per      
square yard of finished plate1 or       
reduces the cost of removal1 must  be of
 the utmost importance in this          
manufacture4 ,the flatness of the       
plates as they leave the annealing kiln 
has already been referred  to1 and the  

reason why the processes of grinding and
 polishing have  formed the subject for 
innumerable patents will now be         
apparent4 ,the  very large expansion of 
the use of plate-glass in modern        
building  construction1 together with   
the steady reduction in the prices of   
plate1 are evidence of the success that 
has attended the efforts of  inventors  
and manufacturers in this direction4    
,at the present time1 plate-glass is    
manufactured in very large sheets1      
measuring up to #bf ft4 in length by #ad
 ft4 in width1 and in thickness  varying
 from #c_/afth of an inch up to #a in41 
or more1 for special  purposes4 ,at the 
same time the quality of the glass is   
far higher  to-day than it was at       
earlier times4 ,this high quality       
chiefly results  from more careful      
choice of raw materials and greater     
freedom from  the defects arising during
 the melting and refining processes1    
while a  rigid process of inspection is 
applied to the glass as it comes from   

the polishing machines4 ,for this       
purpose the sheets are examined in  a   
darkened room by the aid of a lamp      
placed in such a way that its  oblique  
rays reveal every minute imperfection of
 the glass2 these  imperfections are    
marked with chalk1 and the plate is     
subsequently cut  up so as to avoid the 
defects that have thus been detected4   
,perhaps the most remarkable fact about 
the quality of modern  plate-glass is   
its relatively high degree of           
homogeneity4 ,glass1 as we  have seen in
 ,chapter ,i41 is not a chemically      
homogeneous substance1  but rather a    
mixture of a number of substances of    
different density  and viscosity4       
,wherever this mixture is not           
sufficiently intimate1 the  presence of 
diverse constituents becomes apparent in
 the form of stri1  arising from the    
refraction or bending of light-rays as  
they pass from  one medium into another 
of different density4 ,except in glass  
that has  undergone elaborate stirring  

processes1 such stri are never absent1  
but the skill of the glass-maker        
consists in making them as few and as   
minute as possible1 and causing them to 
assume directions and positions  in     
which they shall be as inconspicuous as 
possible4 ,in plate-glass  this is      
generally secured in a very perfect     
manner1 and to ordinary  observation no 
stri are visible when a piece of        
plate-glass is looked  at in the        
ordinary way1 i4e41 through its smallest
 thickness2 if the  same piece of glass 
be looked at transversely1 the edges    
having first  been polished in such a   
way as to render this possible1 the     
glass will  be seen to be full of stri1 
generally running in fine lines parallel
  with the polished surfaces of the     
glass4 ,this uniform direction of  the  
stri is partly derived from the fact    
that the glass has been  caused to flow 
in this direction by the action of the  
roller when first  formed into a slab1  
but this process would not obliterate   

any serious  inequalities of density    
which might exist in the glass as it    
leaves the  pot1 so that successful     
results are only attainable if great    
care is  taken to secure the greatest   
possible homogeneity in the glass during
  the melting process4    ,at the       
present time probably the greater bulk  
of plate-glass is used  for the purpose 
of glazing windows of various kinds1    
principally the  show windows of shops1 
etc4 ,as used for this purpose the glass
 is  finished when polished and cut to  
size4 ,the only further manipulation    
that is sometimes required is that of   
bending the glass to some  desired      
curvature1 examples of bent plate-glass 
window-panes being  very frequently     
seen4 ,this bending is carried out on   
the finished  glass1 i4e41 after it has 
been polished2 the glass is carefully   
heated in a special furnace until       
softened1 and is then gently made to    
lie against a stone or metal mould which
 has been provided with the  desired    

curvature4 ,it is obvious that during   
this operation there are  great risks of
 spoiling the glass2 roughening of the  
surface by contact  with irregular      
surfaces on either the mould1 the floor 
of the kiln1 or  the implements used in 
handling the glass1 can only be avoided 
by the  exercise of much skill and care1
 while all dust must also be excluded   
since any particles settling on the     
surface of the hot glass would be       
8burnt in10 and could not afterwards be 
detached4 ,small defects can1  of       
course1 be subsequently removed by local
 hand-polishing1 and this  operation is 
nearly always resorted to where polished
 glass has to  undergo fire-treatment   
for the purpose of bending4    ,in      
addition to its use for glazing in the  
ordinary sense1 plate-glass  is employed
 for a number of purposes2 the most     
important and frequent  of these is in  
the construction of the better varieties
 of mirrors4  ,for this purpose the     
glass is frequently bevelled at the     

edges1 and  sometimes a certain amount  
of cutting is also introduced on the    
face of  the mirror4 ,bevelling is      
carried out on special grinding and     
polishing  machines1 and a great variety
 of these are in use at the present     
time4  ,the process consists in grinding
 off the corners of the sheet of glass  
and replacing the rough perpendicular   
edge left by the cutting diamond  by a  
smooth polished slope running down from 
the front surface to the  lower edge at 
an angle of from #de to #fj degrees4    
,since only relatively  small quantities
 of glass have to be removed1 small     
grinding rubbers  only are used1 and in 
some of the latest machines these take  
the form  of rapidly-revolving emery or 
carborundum wheels4 ,these grinding     
wheels  have proved so successful in    
grinding even the hardest metals that it
  is surprising to find their use in the
 glass industry almost entirely         
restricted to the 8cutting0 of the      
better kinds of flint and 8crystal0     

glass for table ware or other ornamental
 purposes4 ,the reason probably  lies in
 the fact that the use of such grinding 
wheels results in the  generation of a  
very considerable amount of local heat1 
this effect  being intensified on       
account of the low heat-conducting power
 of glass4  ,if a piece of glass be held
 even lightly against a                 
rapidly-revolving  emery wheel it will  
be seen that the part in contact with   
the wheel  is visibly red-hot4 ,this    
local heating is liable to lead to      
chipping  and cracking of the glass1 and
 these troubles are those actually      
experienced when emery or carborundum   
grinding is attempted on larger  pieces 
of glass4 ,in the case of at least one  
modern bevel-grinding  machine1 however1
 it is claimed that the injurious       
effects of local  heating are avoided by
 carrying out the entire operation under
 water4    ,for the purpose of use in   
mirrors1 plate-glass is frequently      
silvered1  and this process is carried  

on so extensively that it has come to   
constitute an entire industry which has 
no essential connection  with glass     
manufacture itself2 for that reason we  
do not propose to  enter on the subject 
here1 only adding that the nature and   
quality  of the glass itself            
considerably affects the ease and       
success of the  various silvering       
processes4 ,ordinary plate-glass1 of    
course1 takes  the various silvering    
coatings very easily and uniformly1 but 
there  are numerous kinds of glass to   
which this does not apply1 although     
there are probably few varieties of     
glass which are sufficiently  stable for
 practical use1 and to which a silvering
 coating cannot be  satisfactorily      
applied1 provided that the most suitable
 process be  chosen in each case4       
,while there is little if any use for   
coloured glass in the form of  polished 
plate1 entirely opaque plate-glass1     
coloured both black  and white1 is used 
for certain purposes4 ,thus1 glass      

fascias over  shop-fronts1 the counters 
and shelves of some shops1 and even     
tombstones are sometimes made of black  
or white polished plate4 ,from  the     
point of view of glass manufacture1     
however1 these varieties only  differ   
from ordinary plate-glass in respect of 
certain additions to  the raw materials1
 resulting in the production of the     
white or black  opacity4 ,the subsequent
 treatment of the glass is identical    
with that  of ordinary plate-glass1     
except that these opaque varieties are  
rarely  required to be polished on both 
sides1 so that the operations are       
simplified to that extent4    ,certain  
limitations to the use of all kinds of  
plate-glass1 whether  rough-rolled1     
figured or polished1 were formerly set  
by the fact  that under the influence of
 fire1 partitions of glass were liable  
to  crack1 splinter and fall to pieces1 
thus causing damage beyond their  own   
destruction and leaving a free passage  
for the propagation of  the fire4 ,to   

overcome these disadvantages1 glass     
manufacturers have  been led to         
introduce a network or meshing of wire  
into the body of  such glass4 ,provided 
that the glass and wire can be made so  
as to  unite properly1 then the         
properties of such reinforced or 8wired0
  glass should be extremely valuable4   
,in the event of breakage from any      
cause1 such as fire or a violent blow1  
while the glass would still  crack1 the 
fragments would be held together by the 
wire network1  and the plates of glass  
as a whole would remain in place1       
neither  causing destruction through    
flying fragments nor allowing fire or1  
for the matter of that1 burglar a free  
passage4 ,the utility of such  a        
material has been readily recognised1   
but the difficulty lies in  its         
production4 ,these difficulties arise   
from two causes4 ,the most  serious of  
these is the considerable difference    
between the thermal  expansion of the   
glass and of the wire to be embedded in 

it4 ,the wire  is necessarily introduced
 into red-hot glass while the latter is 
being  rolled or cast1 and therefore    
glass and wire have to cool down from a 
red heat together4 ,during this cooling 
process the wire contracts much  more   
than the glass1 and breakage either     
results immediately1 or the  glass is   
left in a condition of severe strain and
 is liable to crack  spontaneously      
afterwards4 ,an attempt has been made to
 overcome this  difficulty by using wire
 made of a nickel steel alloy1 whose    
thermal  expansion is very similar to   
that of glass2 but1 as a matter of fact1
  this similarity of thermal expansion  
is only known to hold for a short  range
 of moderate temperatures1 and probably 
does not hold when the  steel alloy is  
heated to redness4 ,in another          
direction1 greater success  is to be    
attained by the use of wire of a very   
ductile metal which  should yield to the
 stress that comes upon it during       
cooling2 probably  copper wire would    

answer the purpose1 but the great cost  
of copper is a  deterrent from its use4 
,a second difficulty is met with in     
introducing  wire netting into glass    
during the rolling operation1 and this  
lies in  effecting a clean join between 
glass and wire4 ,most metals when heated
  give off a considerable quantity of   
gas1 and when this gas is evolved  after
 the wire has been embedded in glass1   
numerous bubbles are formed1  and these 
not only render the glass very unsightly
 but also lessen the  adhesion between  
the wire and the glass4 ,this           
difficulty1 however1 can  be overcome   
more readily than the first1 since the  
surface of the metal  can be kept clean 
and the gas expelled from the interior  
of the wire  by preliminary heating4 ,on
 the whole1 however1 wired glass is     
perhaps  still to be regarded as a      
product whose evolution is not yet      
complete1  and there can be no doubt    
that there are great possibilities open 
to  the material when its manufacture   

has been more fully developed4          
,,chapter ,x4    ,,sheet ,,and ,,crown  
,,glass4      ,in the preceding chapter 
we have dealt with the processes of     
manufacture employed in the production  
of both the crudest and the  most       
perfect forms of flat glass as used for 
such purposes as the  glazing of window 
openings4 ,the products now to be dealt 
with are  of an intermediate character1 
sheet-glass possessing many of the      
properties of polished plate1 but       
lacking some very important ones2  thus 
sheet-glass is sufficiently transparent 
to allow an observer to  see through it 
with little or no disturbance--in the   
best varieties  of sheet-glass the      
optical distortion caused by its        
irregularities  is so small that the    
glass appears nearly as perfect as      
polished  plate--but in the cheap glass 
that is used for the glazing of ordinary
  windows1 sheets are often employed    
which produce the most disturbing1  and 
sometimes the most ludicrous1           

distortions of objects seen through     
them4 ,it is a curious fact that even in
 good houses the use of such  inferior  
glass is tolerated without comment1 the 
general public being1  apparently1      
remarkably nonobservant in this respect4
 ,in another  direction sheet-glass has 
the great advantage over plate-glass    
that  it is very much lighter1 or can at
 least be produced of much smaller      
weight and thickness1 although this     
advantage entails the consequent        
disadvantage that sheet-glass is usually
 much weaker than plate1 and  can only  
be used in much smaller sizes4 ,in      
recent times the production  of         
relatively thin plate-glass has1        
however1 made such strides that  it is  
now possible to obtain polished         
plate-glass thin enough and  light      
enough for almost every architectural   
purpose4 ,finally1 the most  important  
advantage of sheet-glass1 and the one   
which alone secures its  use in a great 
number of cases in preference to        

plate-glass1 is its  cheapness1 the     
price of ordinary sheet-glass being     
about one-fourth  that of plate-glass of
 the same size4    ,the raw materials   
for the manufacture of sheet-glass are  
sand1  limestone1 salt-cake1 and a few  
accessory substances1 such as arsenic1  
oxide of manganese1 anthracite coal or  
coke1 which differ considerably         
according to the practice of each       
particular works4 ,in a general way     
these materials have already been dealt 
with in ,chapter ,,iii41 and we  need   
only add here that the sheet-glass      
manufacturer must keep in  view two     
decidedly conflicting considerations4   
,on the one hand the  requirements made 
in the case of sheet-glass as regards   
colour and  purity render a rigorous    
choice of raw material and the exclusion
 of  anything at all doubtful very      
desirable2 but on the other hand the    
chief commercial consideration in       
connection with this product is its     
cheapness1 and in order to maintain a   

low selling price at a profit  to       
himself the manufacturer must rigorously
 exclude all expensive raw  materials4  
,for this reason sheet-glass1 works such
 as those of ,belgium  and some parts of
 ,germany1 which have large deposits of 
pure sand  close at hand1 possess a very
 considerable advantage over those in   
less favoured situations1 since sand in 
particular forms so large a  proportion 
of the glass1 and the cost of carriage  
frequently exceeds1  and in many cases  
very greatly exceeds1 the actual price  
of the sand  itself4 ,the same          
considerations will apply1 although in  
somewhat lesser  degree1 to the other   
bulky materials1 such as limestone and  
salt-cake2  but both these are more     
generally obtainable at moderate prices 
than  are glass-making sands of adequate
 quality for sheet manufacture4         
,ordinary 8white0 sheet-glass is now    
almost universally produced in  tank    
furnaces1 and a very great variety of   
these furnaces are used or  advocated   

for the purpose4 ,it would be beyond the
 scope of the present  book to enter in 
detail into the construction of these   
various types  of furnace or to discuss 
their relative merits at length4 ,only a
 brief  outline of the chief            
characteristics of the most important   
forms of  sheet-tank furnaces will      
therefore be given here4    ,sheet tanks
 differ from each other in several      
important respects2 these  relate to the
 sub-division of the tank into one1 two1
 or even three  more or less separate   
chambers1 to the depth of the bath of   
molten  glass and the height of the     
8crown0 or vault of the furnace chamber1
  to the shape and position of the      
apertures by which the gas and air  are 
admitted into the furnace1 and the      
resultant shape and disposition  of the 
flame1 and finally to the position and  
arrangement of the  regenerative        
appliances by which some of the heat of 
the waste gases is  returned into the   
furnace4    ,taking these principal     

points in order1 we find that in some   
sheet tank  furnaces the whole furnace  
constitutes a single large chamber4 ,in 
this  type of furnace the whole process 
of fusion and fining of the glass  goes 
on in this single chamber1 and an       
endeavour is made to graduate  the      
temperature of the furnace in a suitable
 manner from the hot end  where the raw 
materials have to be melted down to the 
colder end where  the glass must be     
sufficiently viscous to be gathered on  
the pipes4 ,it  is obvious that this    
control of the temperature cannot be so 
perfect  in a furnace of the single     
chamber type as in one that is          
sub-divided4  ,such sub-divided furnaces
 are1 as a matter of fact1 much more    
frequent  in sheet-glass practice2 but  
this practice differs widely as to the  
manner and degree of the sub-division   
introduced4 ,in the extreme form  the   
glass practically passes through three  
independent furnaces merely  connected  
with one another by suitable openings of

 relatively small  area through which   
the glass flows from one to the other4  
,if it were  possible to build furnaces 
of materials that could resist the      
action of  heat and of molten glass to  
an indefinite extent1 it is probable    
that  this extreme type would prove the 
best1 since it gives the operator  of   
the furnace the means of controlling the
 flow of glass in such a  way that no   
unmelted material can leave the melting 
chamber and enter  the fining chamber1  
and that no insufficiently fined glass  
can leave  the fining chamber and find  
its way into the working chamber4 ,but  
in  practice the fact that this extreme 
sub-division introduces a great  deal of
 extra furnace wall1 exposed both to    
heat and to contact with  the glass1    
involves very serious compensating      
disadvantages--the cost  of             
construction1 maintenance and renewal of
 the furnace is greatly  increased1     
while there is also an increased source 
of contamination of  the glass from the 

erosion of the furnace walls4 ,it is1   
therefore1 in  accordance with          
expectations to find that the most      
successful furnaces  for the production 
of sheet-glass are intermediate in this 
respect  between the simple open furnace
 and the completely sub-divided one4    
,in some cases the working chamber is   
separated from the melting and  fining  
chamber by a transverse wall above the  
level of the glass1 while  fire-clay    
blocks floating in the glass just below 
this cross wall serve  to complete the  
separation and to retain any surface    
impurities that  may float down the     
furnace4    ,as regards the depth of    
glass in the tank1 practice also varies 
very  much4 ,the advantages claimed for 
a deep bath are that the fire-clay      
bottom of the furnace is thereby kept   
colder and is consequently less         
attacked1 so that this portion of the   
furnace will last for many  years4 ,on  
the other hand the existence of a great 
mass of glass at a  moderate heat may   

easily prove the source of contamination
 arising from  crystallisation or       
8devitrification0 occurring there and   
spreading  into the hotter glass above4 
,also1 if for any reason it should      
become  necessary to remove part or all 
of the contents of the tank1 the        
greater mass of glass in those with deep
 baths becomes a formidable  obstacle4  
,on the whole1 however1 modern practice 
appears to favour the  use of deeper    
baths1 depths of #b ft4 #f in4 or even  
#c ft4 being very  usual1 while depths  
up to #d ft4 have been used4    ,the    
question of the proper height of the    
8crown0 or vault of the  furnace is of  
considerable importance to the proper   
working of the  tank4 ,for the purpose  
of producing the most perfect           
combustion1 it is  now contended that a 
large free flame-space is required4 ,the
 earlier  glass-melting tanks1 like the 
earlier steel furnaces1 were built with 
very low crowns1 forcing the flame into 
contact with the surface  of the molten 

glass1 the object being to promote      
direct heating by  immediate contact of 
flame and glass2 the modern tendency1   
however1 is  strongly in the direction  
of higher crowns1 leaving the heating of
 the  glass to be accomplished by       
radiation rather than direct conduction 
of heat4 ,there can be little doubt that
 up to a certain point the  enlargement 
of the flame-space tends towards greater
 cleanliness of  working and a certain  
economy of fuel1 but if the height of a 
furnace  crown be excessive there is a  
decided loss of economy4 ,flame-spaces  
as  high as #f ft4 from the level of the
 glass to the highest part of the  crown
 have been used1 but the more usual     
heights range from #b ft4 to #e  ft4    
,the 8ports0 or apertures by which      
pre-heated gas and air enter the        
furnace chamber differ very widely in   
various furnaces4 ,in some cases  the   
gas and air are allowed to meet in a    
small combustion chamber just  before   
entering the furnace itself1 while in   

other cases the gas and  air enter the  
furnace by entirely separate openings1  
only meeting in  the furnace chamber4   
,the latter arrangement tends to the    
formation of  a highly reducing flame1  
which is advantageous for the reduction 
of  salt-cake1 but is by no means       
economical as regards fuel consumption4 
,on the other hand1 by producing a      
perfect mixing of the entering gas  and 
air in suitable proportions1 the other  
type of ports can be made to  give      
almost any kind of flame desired1       
although their tendency is to  form a   
more oxidising atmosphere within the    
furnace4 ,the latter type  of ports1    
although widely varied in detail1 are   
now almost universally  adopted in sheet
 tank furnaces4    ,all modern tank     
furnaces work on the principle of the   
recovery of heat  from the heated       
products of combustion as they leave the
 furnace1 and  the return of this heat  
to the furnace by utilising it to       
pre-heat the  incoming gas and air2 but 

the means employed to effect the        
application  of this 8regenerative0     
principle differ considerably in various
 types  of plant4 ,perhaps the most     
widely-used form of furnace is the      
direct  descendant of the original      
,siemens regenerative furnace1 in which 
four  regenerator chambers are provided 
with means for reversing the flow of    
gas and air in such a way that each pair
 of chambers serves alternately  to     
absorb the heat of the outgoing gases   
and subsequently to return  this heat to
 the incoming air that passes through   
one1 and the incoming  gas that passes  
through the other of these chambers4 ,in
 these furnaces1  the regenerator       
chambers themselves are generally placed
 underneath  the melting furnace1 and   
they are built of fire-brick and filled 
with  loosely-stacked fire-bricks1 whose
 function it is to absorb or deliver    
the heat4 ,in the most modern type of   
furnaces of this class1 the             
gas-regenerators are omitted entirely1  

the air only being pre-heated  by means 
of regenerators1 while the gas enters   
the furnace direct  from the producer1  
thus carrying with it the heat generated
 in the  producer during the            
gasification of the fuel4 ,while this   
arrangement  is undoubtedly economical1 
it has the serious disadvantage1        
especially  in the manufacture of       
sheet-glass1 that the gas1 rushing      
direct from  the producer into the      
furnace1 carries with it a great deal of
 dust and  ash1 which it has no         
opportunity of depositing1 as in the    
older types  of furnace1 in long flues4 
,the most serious disadvantages of the  
ordinary types of regenerative  furnaces
 are due to the considerable dimensions 
of the regenerative  apparatus1         
necessitating a costly form of          
construction and occupying  a large     
space1 while the necessity of           
periodically reversing the  valves so as
 to secure the alternation in the flow  
of outgoing and  incoming gases requires

 special attention on the part of the   
men  engaged in operating the furnace1  
as well as the construction and         
maintenance of valves under conditions  
of heat and dirt that are not           
favourable to the life of mechanical    
appliances4 ,it is claimed that  all    
these disadvantages are overcome to a   
considerable extent in one  or other of 
the various forms of furnace known as   
8recuperative40 ,in  these furnaces     
there is no alternation of flow1 and the
 regenerator  chambers are replaced by  
the 8recuperators40 ,these consist of a 
large  number of small flues or pipes   
passing through a built-up mass of      
fire-brick in two directions at         
right-angles to one another2 through    
the pipes running in one direction the  
waste gases pass out to the  chimney1   
while the incoming gas and air pass     
through the other set of  pipes4 ,a     
transference of heat between the two    
currents of gas takes  place by the     
conductivity of the fire-brick1 and thus

 the outgoing  gases are continuously   
cooled while the ingoing gases are      
heated--the  transference of heat being 
somewhat similar to that which takes    
place  in the surface condenser of a    
steam engine4 ,theoretically this is a  
much simpler arrangement than that of   
separate regenerator chambers1  and to  
some extent it is found preferable in   
practice1 but there  are certain        
disadvantages associated with the system
 which arise  principally from the      
peculiar nature of the                  
material--fire-brick--of  which the     
recuperators must be constructed4 ,in   
the first place1 the  heat-conductivity 
of fire-brick is not very high1 so that1
 in order  to secure efficiency1 the    
recuperators must be large1 and while   
the  individual pipes must be of small  
diameter1 their area as a whole must  be
 large enough to allow the gases to pass
 through somewhat slowly4  ,next1 owing 
to the tendency of fire-brick to warp1  
shrink and crack  under the prolonged   

effects of high temperatures1 it becomes
 difficult  to prevent leakage of gases 
from one set of pipes into the other4   
,if  this occurs to a moderate extent   
its only effect will be to allow some   
of the combustible gas to pass direct to
 the chimney1 and at the same  time a   
dilution of the gases entering the      
furnace by an addition of  products of  
combustion from the waste-gas flues4    
,this1 of course1 will  materially      
reduce the efficiency of the furnace and
 require a higher  fuel consumption if  
the temperature of the furnace is to be 
maintained  at its proper level4 ,if1   
however1 the leakage should become more 
serious1 a disastrous explosion might   
easily result1 particularly if  the     
nature of the leakage were such as to   
allow the incoming gas and  air to mix  
in the flues4 ,it follows from these    
considerations that1  although the      
recuperative furnace is somewhat simpler
 and cheaper to  construct1 it requires1
 if anything1 more careful maintenance  

than the  older forms of regenerative   
furnace4    ,tank furnaces for the      
production of sheet-glass in this       
country are  generally worked from early
 on ,monday morning until late on       
,saturday  night1 glass-blowing         
operations being suspended during       
,sunday1 although  the heat of the      
furnace must be maintained4 ,on the     
,continent1 and  especially in ,belgium1
 the work in connection with these      
furnaces goes  on without any           
intermission on ,sunday--a difference   
which1 however  desirable the ,english  
practice may be1 has the effect of      
handicapping  the output of a ,british  
furnace of equal capacity by about #aj  
per  cent4 without materially lessening 
the working cost4    ,the process of    
blowing sheet-glass in an ,english      
glassworks  is generally carried out by 
groups of three workmen1 viz41 a        
8pipe-warmer10 a 8gatherer0 and a       
8blower10 although the precise  division
 of the work varies according to        

circumstances4 ,the  pipe-warmer's work 
consists in the first place in fetching 
the  blowing-pipe from a small          
subsidiary furnace in which he has      
previously  placed it for the purpose of
 warming up the thick 8nose0 end upon   
which the glass is subsequently         
gathered4 ,the sheet-blower's pipe      
itself is an iron tube about #d ft4 #f  
in4 long1 provided at the one  end with 
a wooden sleeve or handle1 and a        
mouthpiece1 while the other  end is     
thickened up into a substantial cone1   
having a round end4  ,before introducing
 the pipe into the opening of the tank  
furnace1 the  pipe-warmer must see that 
the hot end of the pipe is free from    
scale  or dirt and must test1 by blowing
 through it1 whether the pipe is free   
from internal obstructions4 ,he then    
places the butt of the pipe in the      
opening of the furnace and allows it to 
acquire as nearly as possible  the      
temperature of the molten glass4 ,when  
this is the case the pipe is  either    

handed on to the gatherer1 or the       
pipe-warmer1 who is usually  only a     
youth1 may take the process one step    
further before handing  it on to the    
more highly skilled workman4 ,this next 
step consists in  taking up the first   
gathering of glass on the pipe4 ,for    
this purpose  the hot nose of the pipe  
is dipped into the molten glass1 turned 
slowly  round once or twice and then    
removed1 the thread of viscous glass    
that  comes up with the pipe being cut  
off against the fire-clay ring that     
floats in the glass in front of the     
working opening4 ,a small quantity  of  
glass is thus left adhering to the nose 
of the pipe1 and this is now  allowed to
 cool down until it is fairly stiff1 the
 whole pipe being  meanwhile rotated so 
as to keep this first gathering nicely  
rounded1  while a slight application of 
air-pressure1 by blowing down the pipe1 
forms a very small hollow space in the  
mass of glass and secures the  freedom  
of the opening of the pipe4 ,when the   

glass forming the first  gathering has  
cooled sufficiently1 the gatherer       
proceeds to take up the  second         
gathering upon it4 ,the pipe is again   
introduced into the furnace  and        
gradually dipped into the molten glass1 
but this must be done with  great care  
so as to avoid the inclusion of         
air-bells between the glass  already on 
the pipe and the new layer of hotter    
glass that is now taken  up4 ,this      
freedom from air-bells is secured by a  
skilful gatherer by  a gradual rotation 
of the pipe as it is lowered into the   
glass1 thus  allowing the two layers of 
glass to come into contact with a sort  
of  rolling motion that allows the air  
time to escape4 ,when completely        
immersed1 the pipe is rotated a few     
times and is then withdrawn and the     
8thread0 again cut off4 ,the mass of    
glass on the end of the pipe is now     
considerably larger than before and     
requires more careful manipulation  to  
cause it to retain the proper1 nearly   

spherical1 shape4 ,during the  cooling  
process which now follows the pipe is   
laid across an iron  trough1 kept       
brimful of water2 this serves to cool   
the pipe itself1 and  also allows the   
pipe to be readily rotated backwards and
 forwards by  rolling it a little way   
along the trough4 ,when the whole mass  
of glass  has again cooled sufficiently 
to be manipulated without risk of rapid 
deformation1 a third gathering of glass 
is taken up1 in precisely the  same     
manner as that already described for the
 second gathering1 and  if the quantity 
of glass required is large1 or the glass
 itself is so  hot and fluid that only a
 comparatively small weight adheres at  
each  time of gathering1 the process may
 be repeated a fourth or even a fifth   
time1 but as the weight of pipe and     
adhering glass increases with each      
gathering1 each step becomes more       
laborious1 while the hot glass1 being   
now held on a much larger sphere1 tends 
to flow off more readily1 so  that      

greater skill is required to avoid      
8losing0 the gathering4    ,the care and
 skill with which these operations of   
gathering are carried  out determine1 to
 a large extent1 the quality of the     
resulting sheet  of glass2 any want of  
regularity in the shape of the gathering
 leads  inevitably to variations of     
thickness in different parts of the     
sheet1  while careless gathering will   
introduce bubbles or 8blisters0 and     
other  markings4 ,during the            
intermediate cooling stages the glass   
must be  protected from dust and dirt of
 all kinds1 since minute specks falling 
upon the hot glass give rise to an      
evolution of minute gas bubbles  which  
become painfully evident in the sorting 
room4    ,when the last gathering has   
been taken up and the mass cooled so far
  as to allow of its being carried about
 without fear of loss1 the glass  forms 
an approximately spherical mass1 with   
the nose-end of the pipe  at or near the
 centre of the sphere4 ,the next stages 

of the process  consist in the          
preliminary shaping of this mass in such
 a way as to  bring the bulk of the     
glass beyond the end of the pipe1 and   
then in  forming just beyond the end of 
the pipe a widened shoulder of thinner  
and therefore colder glass1 of the      
diameter required for the cylinder  into
 which the glass is to be blown4 ,this  
is done by bringing the  glass into the 
successive shapes shown in ,fig4 #ab1   
the forming of the  glass being effected
 by the aid of specially shaped blocks  
and other  shaping instruments in which 
the glass is turned and blown4 ,the     
final  shape attained at this stage is a
 squat cylinder containing the bulk of  
the glass at its lower end1 and         
connected to the pipe by the thinner    
and colder neck and shoulder already    
mentioned4    ,illustration3 ,,fig4     
#ab4--;,early stages in the formation of
 cylinders for  sheet glass4    ,at this
 point of the process the pipe with its 
adherent glass is handed  over to the   

blower proper4 ,this operator works on a
 special stage  erected in front of     
small furnaces1 called 8blowing holes10 
although  in some works these are       
dispensed with1 and the stages are      
erected in  front of the melting furnace
 itself4 ,the sheet-blower's stage is   
simply  a platform placed over or at the
 side of a suitable excavation which    
gives the blower the necessary space to 
swing the pipe and cylinder  freely at  
arm's length4 ,the blowing process      
itself involves very little  actual     
blowing1 but depends rather upon the    
action of gravitation  and on           
centrifugal effects for the formation of
 the large1 elongated  cylinder from the
 squat cylinder with which the blower   
commences4 ,the  process consists in    
holding the thick1 lower end of the     
cylinder in  the heating-furnace1 and   
when sufficiently hot1 withdrawing it   
and  swinging the pipe with a pendulum  
movement in the blower's pit4 ,the      
cylinder thus elongates itself under its

 own weight1 and any tendency  to       
collapse is counteracted by the         
application of air-pressure by the      
mouth1 the pipe being also1 at times1   
rotated rapidly about its own  axis4    
,the re-heating of the lower end of the 
cylinder is repeated  several times1    
until finally the glass has assumed the 
form of a  cylinder of equal thickness  
all over1 but closed with a rounded dome
 at  the lower end 7,fig4 #ac74 ,this   
rounded end is now opened4 ,in the case 
of fairly thin and light cylinders this 
is done by holding the thumb  over the  
mouthpiece of the pipe in such a way as 
to make an air-tight  seal1 and then    
heating the end of the cylinder in the  
blowing-hole4 ,the  heat both softens   
the glass at the end and at the same    
time causes  considerable expansion of  
the air enclosed in the cylinder1 with  
the  result that the end of the cylinder
 is burst open4 ,after a little  further
 heating1 during which the glass at the 
end of the cylinder  becomes very soft1 

and takes a wavy1 curly shape1 the      
blower withdraws  the cylinder from the 
furnace1 and holding it vertically      
downwards in  his pit1 spins it rapidly 
about its longitudinal axis4 ,the soft  
glass  at the lower end immediately     
opens out under the centrifugal action1 
and the blower increases the speed of   
rotation until the soft glass has       
opened out far enough to form a true    
continuation of the rest of the         
cylinder1 and in this position it is    
allowed to solidify4 ,with thick1  heavy
 cylinders1 the first opening of the end
 is done in a different  way4 ,a small  
quantity of hot glass is taken up by an 
assistant on  an iron rod1 and is laid  
upon the centre of the closed end of the
  cylinder4 ,the heat of this mass of   
hot glass softens the glass of the      
cylinder1 and the operator1 with the aid
 of a special pair of shears1  cuts out 
a small circle of this softened glass1  
thus opening the end of  the cylinder4  
,the final operation of straightening   

out the opened end  is carried out in   
the same way as described above for     
lighter cylinders4    ,illustration3    
,,fig4 #ac4--,later stage in sheet glass
 blowing4    ,the completed cylinder1   
still attached to the pipe1 is now      
carried away  from the blowing-stage and
 laid upon a wooden rack2 then the      
blower  takes up a piece of cold iron1  
and placing it against the neck of glass
  attaching the cylinder to the pipe1   
produces a crack2 a short jerk then     
serves to completely sever the pipe from
 the cylinder4 ,a boy now takes  the    
pipe to a stand where it is allowed to  
cool and where the adhering  glass      
cracks off from it prior to passing it  
back to the pipe-warmer  for fresh use4 
,on the wooden rack the cylinder of     
glass is allowed to cool to a  certain  
extent1 and then the remaining portion  
of the neck and shoulder  7see ,fig4    
#ac7 are removed4 ,this is done by a boy
 who passes a thread  of soft1 hot glass
 around the cylinder at the point where 

it is to  be cut off2 the thread of hot 
glass merely serves to produce intense  
local heating1 for as soon as it has    
become stiff1 the thread of glass  is   
pushed off and a cold or moist iron is  
applied to the cylinder at  the point   
where it had been heated by the thread4 
,as a rule a crack  immediately runs    
completely round the cylinder along the 
line of the  thread1 and the 8cap0 is   
thus removed4 ,the glass is now in the  
form of  a uniform cylinder open at both
 ends1 but it must be opened out into a 
flat sheet before it can assume the     
familiar form of sheet-glass4    ,the   
first stage in the opening-out process  
is that of splitting4 ,for  this purpose
 the cylinders are carried to a special 
stand1 upon which  they are laid in a   
horizontal position1 and here a crack or
 cut is  made along one of the          
generating-lines of the cylinder4 ,this 
may be  done either by the application  
of a hot iron1 followed1 if necessary1  
by slight moistening1 or by the aid of a

 cut from a heavy diamond  drawn        
skilfully down the inside of the        
cylinder4 ,it will be seen  from the    
account of the process so far given1    
that the glass has as  yet undergone no 
real annealing1 although the blower is  
expected to  8anneal0 his cylinder      
during the blowing process1 as far as   
possible1  by never allowing it to cool 
too suddenly1 and this degree of        
annealing  is usually sufficient to save
 the cylinder from breaking under its   
internal stresses when left to cool on  
the racks4 ,the surface of the  glass1  
however1 is left in a decidedly hardened
 condition1 especially  on the outside1 
which has necessarily been most rapidly 
cooled4 ,for  this reason--among        
others--the splitting cut is always made
 on the  inside of the cylinder4 ,the   
difference between the rates of cooling 
of the outside and inside of the        
cylinder has a further effect1 which    
becomes evident as soon as the cylinder 
is split4 ,the outside having  become   

hard while the inside was still         
relatively soft1 the outer  layers of   
glass are in a state of compression and 
the inner layers in  a state of tension 
in the cold cylinder4 ,as soon as the   
cylinder is  split1 however1 these      
stresses are to some extent relieved1   
the inner  layers being then free to    
contract and the outer layers to expand2
 the  result is an increase in the      
curvature of the cylinder1 which        
slightly  decreases in diameter1 the cut
 edges overlapping4 ,if the cylinder has
  been cooled rather too quickly1 or if 
the glass itself has a high  coefficient
 of expansion1 this release of internal 
stresses at the  moment of splitting    
becomes very marked1 and each cylinder  
splits with  the sound of a small       
explosion1 while if the internal        
stresses are  still more severe1 the    
cylinders may even fly to pieces as soon
 as they  are cut4    ,the next stage in
 the manufacture of a sheet of glass is 
the flattening  and annealing process4  

,for this purpose the split cylinders   
are taken  to a special kiln1 generally 
known as a 8lear10 or 8lehr10 where they
  are first of all raised to a dull     
red-heat2 they are then lifted1 one  at 
a time1 on to a smooth stone or slab    
placed in a chamber of the  kiln where  
the heat is great enough to soften the  
glass4 ,here the  cylinder is laid down 
with the split edges upwards1 and by    
means of a  wooden tool the glass is    
slowly spread out1 being finally rubbed 
down  into perfect contact with the slab
 or 8lagre40 ,from the flattening  slab1
 the sheet as it now is passes into the 
annealing kiln1 which  communicates with
 the flattening chamber4 ,this consists1
 similarly to  other continuous         
annealing kilns already described in    
connection with  other varieties of     
glass1 of a long tunnel1 heated to the  
temperature  of the flattening kiln at  
one end and nearly cold at the other4   
,the  sheets are moved down this tunnel 
at a uniform slow rate by the action  of

 a system of grids which1 at intervals1 
lift the sheets from the  bottom of the 
kiln1 move them forward by a short      
distance1 and again  deposit them on the
 bottom1 the grids themselves returning 
to their  former position by a          
retrograde movement made below the level
 of the  kiln-bottom1 and therefore not 
affecting the glass4    ,on leaving the 
annealing kiln the sheets of glass are  
sometimes covered  with a white deposit 
arising from the products of combustion 
in the  kiln and their interaction with 
the glass itself4 ,this deposit can be  
removed by simple mechanical rubbing1   
but it is usual to dip the glass  into a
 weak acid bath1 which dissolves the    
white film and leaves the  glass clear  
and bright1 ready for use4    ,from the 
annealing kiln the finished sheets of   
glass are taken to the  sorting room1   
where they are examined in a good light 
against a black  background1 and are    
sorted according to their quality for   
different  purposes4    ,the defects    

which are found in sheet-glass are of a 
very varied  nature1 as would be        
anticipated from the long and           
complicated process  of manufacture     
which the material undergoes in the     
course of its  transformation from the  
raw materials into the finished sheet of
 glass4  ,a full enumeration of all     
possible defects1 with their technical  
names1  need not be given here1 but a   
description of the more important and   
frequent ones will be useful4 ,the      
defects may be conveniently grouped     
according to the stage of the process   
from which they originate4    ,the first
 class of defects accordingly embraces  
those that arise from  the condition of 
the glass as it exists in the           
working-end of the  furnace4 ,chief of  
these are white opaque enclosures1 known
 as 8stones40  ,these may arise from a  
variety of causes within the furnace1   
such as an  admixture of infusible      
impurities with the raw materials1      
insufficient  heat or duration of       

melting1 leading to a residue of        
unmelted raw  material in the finished  
glass1 or from defective condition of   
the  interior of the furnace1 leading to
 contamination of the glass with  small 
particles of fire-brick4 ,further1 if   
any part of the furnace has  been       
allowed to remain at too low a          
temperature1 or if the composition  of  
the glass is unsuitable1 crystallisation
 may occur in the glass1  and white     
patches of crystalline material may find
 their way into the  finished sheets4   
,another defect that may arise from the 
condition of  the glass in the furnace  
is the presence of numerous small       
bubbles1  known as 8seed0 in the glass4 
,by the blowing process these are  drawn
 out into pointed ovals1 and they are   
rarely quite absent from  sheet-glass4  
,they arise from either incomplete      
fining of the glass in  the furnace or  
from allowing the glass to come into    
contact with minute  particles of dust  
during the gathering process4 ,another  

possible defect  to the glass itself may
 be found at times in too deep a colour4
 ,this  is only seen readily when a     
sheet of some size is examined edgewise1
  as most varieties of ordinary         
sheet-glass are too free from colour to 
allow this to be judged by looking      
through the sheet in the ordinary  way4 
,it follows from this fact that for     
practical purposes1 where  the light    
always traverses one thickness of the   
glass only1 a slight  difference of     
colour should be regarded as a very     
minor consideration1  at all events as  
compared with freedom from other        
defects4    ,the gathering process in   
its turn is responsible for further     
defects of  sheet-glass4 ,some of these1
 such as defects arising from the use of
 a  dirty pipe1 are never allowed to    
pass beyond the sorting-room1 and are   
therefore of no interest to the user of 
glass4 ,of those whose traces  are seen 
in the glass that passes into use1      
8blisters0 and 8string0 are  the most   

important4 8,blisters0 are somewhat     
larger1 flat air-bells1  arising from   
the inclusion of air between successive 
layers of the  gathering4 8,string0 is a
 very common defect in all sheet-glass4 
,to some  extent it may arise from want 
of homogeneity in the glass itself4 ,if 
this consists of layers of different    
densities and viscosities1 the  gatherer
 will take these up on his gathering1   
and ultimately they will  form thickened
 ridges of glass running around the     
cylinders and across  the sheets4 ,such 
stri1 due to want of homogeneity in the 
glass1 are  much more common in flint   
glass than in the soda-lime glasses used
 for  sheet manufacture1 but are not    
unknown in the latter4 ,on the other    
hand1 even if the glass be as           
homogeneous as possible1 the gatherer   
can produce these stri if he takes up   
his glass from a place close  to the    
side of the fire-clay ring that floats  
in the furnace in front  of his working 
opening4 ,glass always acts chemically  

upon fire-clay1  gradually forming a    
layer of glass next to the fire-clay    
that contains  much more alumina than   
the rest of the contents of the furnace4
 ,such  a layer is formed on the surface
 of each ring in a sheet tank1 but if   
the gathering is taken from the centre  
of the ring1 this thick viscous  layer  
of aluminiferous glass remains          
undisturbed4 ,if1 however1 the  gatherer
 brings his pipe too near the side of   
the ring1 the glass  will draw some of  
this different layer on to the          
gathering1 and this  glass will form    
thick ridges and stri running across the
 sheet in  all directions4 ,another     
defect for which the gatherer is        
generally  responsible is that of       
variation of thickness within the same  
sheet4  ,the blower1 however1 can also  
produce this defect4    ,during the     
blowing proper1 a further series of     
defects may be  introduced1 principally 
by allowing particles of glass derived  
from  certain stages of the process to  

fall upon the hot glass of the  cylinder
 and there become attached permanently4 
,more serious1 and also  more frequent1 
is the greater or less malformation of  
the cylinder4 ,if  the glass as it      
leaves the blower is of any shape other 
than that of  a true cylinder1 it       
becomes impossible to spread it into a  
truly flat  sheet in the flattening     
kiln4 ,sometimes1 in practice1 the      
8cylinder0 is  wider at one end than at 
the other1 or1 worse still1 it is of    
uneven  diameter1 showing expanded and  
contracted areas alternately4 ,when such
  a cylinder comes to be spread out on  
the slab it cannot be flattened         
completely1 and various hollows and     
hillocks are left1 which mar the        
flatness of the sheet and interfere with
 the regular passage of light  through  
it when in use4    ,finally1 the process
 of flattening is apt to introduce      
defects of its  own4 ,the most common of
 these are scratches arising from marks 
left by  the flattening tool2 indeed1 in

 all sheet glass it is quite possible   
to see1 by careful examination of the   
surfaces1 upon which side the           
flattening tool was used4 ,sheet-glass  
thus has one side decidedly  brighter   
and better in surface than the other1   
the better side being  that which rested
 upon the 8lagre0 during the flattening 
process4 ,on  the other hand1 if the    
slab itself be not quite perfect1 or if 
any  foreign body be allowed to rest    
upon it1 that side of the glass will be 
marked in a corresponding manner4    ,in
 the account of the manufacture of      
sheet-glass given above1 we have        
outlined one typical form of the        
process1 but nearly every stage is      
subject to modifications according to   
the practice and particular             
circumstances of each works4 ,we will   
now describe one or two special         
modifications that are of more general  
importance4    ,first1 as regards the   
melting process1 although the           
tank-furnace  has almost entirely       

superseded the pot furnace for the      
production  of ordinary sheet-glass1    
there are still some special            
circumstances  under which the pot      
furnace is capable of holding its own4  
,thus1  where for special purposes it is
 desired to produce a variety of        
sheet-glass which1 as regards all       
defects arising out of the glass        
itself1 and especially as regards       
colour1 is required to be as perfect  as
 possible1 melting in pots is found     
advantageous1 and for some very  special
 purposes even covered 7hooded7 pots are
 used4 ,for such special  purposes1 too1
 sulphate of soda is eliminated from the
 raw materials  and carbonate of soda   
7soda ash7 substituted4 ,for the        
production of  tinted glasses also1     
whether they are tinted throughout their
 mass1  or merely covered with a thin   
layer of tinted glass 78flashed071      
manufacture in pot rather than tank     
furnaces is generally adopted1  the     
exact nature and composition of the     

glass being far better under  control in
 the case of pots4    ,the blowing      
process is also subject to wide         
variations of practice4 ,the  most      
important of these variations concerns  
the shape and dimensions  of the        
cylinders4 ,in ,english and ,belgian    
works the dimensions of the  cylinders  
are so chosen that the length of the    
cylinder constitutes the  longest       
dimension of the finished sheet1 the    
diameter of the cylinder  forming the   
shorter dimension4 ,in some parts of    
,germany1 however1 the  practice is the 
reverse of this1 the cylinders being    
blown shorter  and much wider1 so that  
the circumference of the cylinder       
constitutes  the longest dimension of   
the finished sheet4 ,it is1 however1    
pretty  generally recognised that the   
latter method has very serious          
disadvantages1 although it is claimed   
that somewhat more perfect  glass can be
 obtained by its means4 ,for the        
production of a special  variety of     

glass1 known as 8blown plate glass10    
this method of blowing  short wide      
cylinders is still adhered to4 ,this is 
a very pure form of  sheet-glass1 blown 
into thick1 small sheets which are      
subsequently  ground and polished in the
 same manner as plate-glass4 ,here the  
great  thickness of glass required seems
 to render the blowing of long          
cylinders very difficult1 and the other 
form is therefore adopted4 ,on  the     
other hand1 ,english patent plate-glass1
 which is made by grinding  and         
polishing the best quality of ordinary  
sheet-glass1 is made from  glass blown  
into long narrow cylinders in the manner
 described in  detail above4    ,the    
process of blowing described above is   
capable1 with slight  modifications1 of 
yielding glass with surfaces other than 
the plain  smooth face of ordinary      
sheet-glass4 ,thus fluted and 8muffled0 
glass  are produced in a very similar   
manner to that described above for      
ordinary sheet1 except that the fluting 

or the irregular surface  markings which
 constitute the peculiarities of these  
two varieties of  glass1 are impressed  
upon the surface of the cylinder at an  
early stage  in the process4    ,from   
the outline description given above of  
the usual method of  manufacture of     
sheet-glass1 it will readily be seen    
that this is a  long1 complicated1 and  
laborious process1 involving the        
employment  of much skilled labour1 and 
involving the production of a relatively
  complicated form1 viz41 the closed    
cylinder1 as a preliminary to the       
production of a very simple form1 viz41 
the flat sheet4 ,it is therefore  by no 
means surprising to find that a great   
many inventors have worked  and are     
still working at the problem of a direct
 mechanical method of  producing flat   
glass possessing a natural 8fire polish0
 at least equal  to that of ordinary    
sheet-glass4 ,the earlier inventors have
 almost  uniformly endeavoured to attain
 this object by attempting to improve   

the process of rolling glass1 with a    
view to obtaining rolled sheets  having 
a satisfactory surface4 ,we have already
 indicated why these  efforts have never
 met with success and what reasons there
 are for  believing that they are never 
likely to attain their object4 ,a       
totally  different line is that taken by
 ,sievert1 to whose inventions we have  
already referred in connection with the 
mechanical production of blown          
articles4 ,this inventor has endeavoured
 to utilise his process for  blowing    
large articles of glass for the direct  
production of sheets  of flat glass4    
,his method is to blow1 by the steam    
process described  in another chapter1 a
 large cubical vessel1 having flat      
sides1 the  flatness of these sides     
being ensured by blowing the vessel into
 or  against a mould having flat sides4 
,this flat-sided vessel is ultimately   
to be cut up into five large sheets4    
,this process also appears to  involve  
some of the main difficulties of rolling

 as regards the  means of transferring  
the glass from the furnace to the plate 
of the  blowing machine1 and in practice
 the inventor has not yet succeeded in  
producing glass of sufficiently good    
surface for the purposes of sheet       
glass4    ,another class of processes   
entirely avoid all means of transferring
  molten glass from the furnace to any  
machine1 by working on glass  direct    
from the molten bath itself4 ,some of   
these processes are in  actual use in   
,america1 and others are being          
experimented with in  ,europe1 but their
 complete technical and commercial      
success has yet to  be proved2 there    
can1 however1 be little doubt that they 
have overcome  the greatest of the many 
difficulties that stood in the way of   
the  mechanical production of           
sheet-glass1 and that they are therefore
  destined very shortly to solve the    
problem completely1 in which case  they 
would1 of course1 rapidly supersede the 
hand process4    ,one of the earliest of

 these direct processes proposed to     
allow the  molten glass to flow out from
 the furnace1 downward1 through a narrow
  slit formed in the side or bottom of  
the tank4 ,the impossibility of  keeping
 such a narrow orifice open and at the  
same time regulating the  flow of glass 
made this proposal impracticable1       
although the use of  drawing orifices   
has been revived in one of the latest   
processes4    ,the ,american process1   
which is said to be at work under       
commercial  conditions1 is not entirely 
satisfactory in this respect--that it is
  a mechanical process for the          
production of cylinders and not of flat 
sheets1 so that the subsidiary processes
 of splitting and flattening  still     
remain to be carried out as before4 ,in 
this process an iron  ring is lowered   
into the bath of molten glass through an
 aperture  from above2 the glass is     
allowed to adhere to the ring which is  
then  slowly raised by mechanical means1
 drawing a cylinder of glass with  it4  

,if left to itself1 such a cylinder1    
owing to the effects of  surface tension
 in the glass1 would soon contract and  
break off1 but  the ,american invention 
avoids this action by chilling each bit 
of the  cylinder as soon as it is       
formed4 ,this is done by the aid of air 
blasts  delivered upon both sides of the
 glass as it emerges from the bath1  and
 it is claimed that by this means       
cylinders of any desired length  and    
diameter may be drawn direct from the   
bath4 ,the obviously great  mechanical  
difficulties connected with these       
operations have probably  been overcome1
 but not without sacrificing much of the
 simplicity of  the arrangement1 and the
 relative economy of this process as a  
whole1  compared with the hand process1 
has yet to assert itself4    ,the       
inventions of ,fourcault1 which are at  
present being developed on  the         
,continent by a syndicate of glass      
manufacturers1 aim at a much more       
direct process4 ,here also the glass is 

drawn direct from the molten  bath by   
the aid of a drawing-iron that is       
immersed in the glass and  then slowly  
raised1 but in this case the piece      
immersed is simply a  straight bar1 and 
the aim is to draw out a flat sheet4 ,in
 this case  the tendency1 under surface 
tension1 is to contract the sheet into  
a thread1 and apparently the simple     
device of chilling the emerging  glass  
is not adequate to prevent this in a    
satisfactory manner1 and  subsidiary    
devices have been added4 ,those that    
have been patented  include a mechanism 
of linked metal rods so arranged as to  
be immersed  and drawn out of the glass 
continuously with the emerging sheet1 in
  such a manner as to support the       
vertical edges of the glass and so  aid 
in resisting the tendency of the glass  
to contract laterally4  ,another device 
consists in the use of a slit or orifice
 formed in a  large fire-brick that     
floats on the surface of the glass4     
,through this  orifice the glass is     

drawn1 of the desired thickness and     
width4 ,the  use of this orifice1       
however1 interferes markedly with the   
perfection  of the product1 and in fact 
all the glass produced in this way shows
  quite plainly a set of longitudinal   
striations due to the inevitable        
irregularities in the lips of the       
drawing slot4 ,further1 it appears  to  
be impracticable to draw thin glass in  
this way1 a thickness of  from #b to #c 
millimetres 7about #a_/h inch7 being the
 least that is  practicable1 on account 
of the large amount of breakage that    
occurs  with weaker sheets4 ,this       
process1 in its present stage of        
development1  however promising1 does   
not appear to have solved the problem of
  mechanical manufacture of sheet-glass1
 since it is just in the  thinner1      
lighter kinds of glass that the         
advantages of sheet are most            
pronounced4 ,on the other hand1 it is   
quite possible that this drawing        
process1 or some development arising    

from it1 may shortly supplant the       
casting process in the production of    
polished plate-glass1 although for  the 
largest sizes of this product also1 the 
difficulty and danger of  handling the  
weights involved may prove a serious    
obstacle4    ,crown ,glass4--,although  
this is a branch of manufacture that is 
nearly obsolete1 it deserves brief      
notice here1 partly because it is  still
 used for the production of special     
articles1 and also because it           
illustrates some interesting            
possibilities in the use and            
manipulation  of glass4    ,the process 
of blowing crown glass may be briefly   
described as that of  first blowing an  
approximately spherical hollow ball1    
then opening this  at one side and      
expanding the glass into a flat disc by 
the action of  centrifugal forces       
produced by a rapid rotation of the     
glass in front  of a large opening in a 
special heating furnace4 ,the actual    
process  involves1 of course1 the       

preliminary of gathering the proper     
quantity  of glass1 much in the manner  
already described in connection with    
sheet-glass manufacture4 ,this gathering
 is then blown out into a hollow        
spherical vessel4 ,this vessel is now   
attached to a subsidiary iron  rod by   
means of a small gathering of hot glass1
 applied at the point  opposite the pipe
 itself1 the glass being thus1 for a    
moment1 attached  to both the pipe and  
the 8pontil0 or 8punty0 7as the rod is  
called74  ,the pipe is1 however1        
detached by cracking off the neck of the
 original  glass1 which now remains     
attached to the pontil in the shape of  
an open  bowl4 ,this bowl is now        
re-heated very strongly in front of a   
special  furnace1 the open side of the  
bowl being presented to the fire4 ,the  
pontil is meanwhile held in a horizontal
 position and rotated4 ,as the  glass   
softens the rotation spreads it out1    
until finally the entire  mass of glass 
is formed into a simple flat disc       

spinning rapidly before  the mouth of   
the furnace4 ,this flat disc or 8table0 
of crown glass is  allowed to cool      
somewhat1 is detached from the pontil by
 a sharp jerk1  and is then annealed in 
a simple kiln in which the glass is     
stacked1  sealed up1 and allowed to cool
 naturally4    ,it is obvious that by   
this process no very large sheets of    
glass can  be produced2 tables #d ft4 in
 diameter are already on the large side1
  and these can only be cut up into much
 smaller sheets on account of the  lump 
of glass by which the table was         
originally attached to the pontil1  and 
which remains fixed in the centre of the
 finished disc4 ,for certain  ornamental
 purposes1 where an 8antique0 appearance
 is desired1 these  bullions are valued1
 but for practical purposes they        
interfere very  seriously with the use  
of the glass4 ,as a matter of fact1 even
 several  inches away from the central  
bullion itself1 crown glass is generally
  marked with circular wavings1 which   

render it readily recognisable in  the  
windows of older buildings1 but which   
decidedly detract from the  perfection  
of the glass4 ,on the other hand1 crown 
glass is still valued  for certain      
purposes1 such as microscope slides and 
cover glasses1  where entire freedom    
from surface markings1 such as those    
found in  sheet glass as a result of the
 flattening operations1 is desirable4   
,while1 therefore1 the process has      
merely an historical interest so far  as
 ordinary sheet-glass purposes are      
concerned1 it is still used in  special 
cases4          ,,chapter ,,xi4         
,,coloured ,,glasses4      ,in various  
chapters throughout the foregoing       
portions of this book  we have had      
occasion to refer to the colour of glass
 and the causes  affecting it1 but these
 references have chiefly been made from 
the  point of view of the production of 
glasses as nearly colourless as         
possible under the circumstances4 ,while
 it is obvious that for the  great      

majority of the purposes for which it is
 used the absence of  all visible       
coloration is desirable or even         
essential in the glass  employed1 there 
are numerous other uses where a definite
 coloration is  required4 ,thus we have1
 as industrial and technical uses of    
coloured  glass1 the employment of ruby1
 green and purple glasses for signalling
  purposes1 as in the signal lamps of   
our railways1 the red tail-lights  of   
motor-cars1 or even the red or green    
sectors of certain harbour  lights and  
lighthouses2 again1 coloured glasses1   
ruby1 green1 and  yellow1 are           
extensively employed in connection with 
photography4 ,rather  less exacting in  
their demands upon the correctness of   
the colour  employed are the            
architectural and ornamental uses to    
which coloured  glass is so extensively 
put in both public and domestic         
buildings1  while1 finally1 coloured    
glass is largely the foundation upon    
which  the stained-glass worker builds  

up his artistic achievements2 in        
another direction1 coloured glass is    
also utilised in the production of      
ornamental articles and of some         
table-ware4 ,while it must be admitted  
that in a great many cases the          
colour-resources of the glass maker are 
hopelessly misapplied1 yet in really    
artistic hands few other materials  are 
capable of yielding results of equal    
beauty4    ,by the 8colour0 of a glass  
is generally understood the tint or     
colour  which is observed when it is    
viewed1 in comparatively thin slices1 by
  transmitted light2 the actual colour  
is thus a property1 not so much of  the 
kind or variety of glass as of each     
individual piece1 since thick  pieces   
out of the same melting will show a     
different tint from that  seen in       
thinner pieces4 ,as we have already     
pointed out1 such glasses as  sheet or  
plate1 which appear practically         
colourless when viewed in the  ordinary 
way1 show a very decided green colour   

when viewed through a  considerable     
thickness4 ,in the same way1 a very thin
 layer of the glass  known as 8flashing 
ruby0 shows a brilliant red tint1 but a 
thickness of  one-sixteenth of an inch  
is sufficient to render the glass       
practically  opaque1 giving it a black  
appearance by both transmitted and      
reflected  light4 ,again1 cobalt blue   
glass1 when examined with a spectroscope
 in  thin layers1 is found to transmit a
 notable proportion of red rays1  but   
thicker pieces entirely suppress these  
rays4 ,these phenomena will  be readily 
understood when we recollect that colour
 in a transparent  medium arises from   
the fact that the medium has different  
absorbing  powers for light of different
 colours4 ,all transparent substances1  
and certainly glass1 are only partially 
transparent3 all light  waves passing   
through such a substance are gradually  
absorbed1 and  the extent to which they 
are absorbed differs according to the   
length  of these waves4 ,it always      

happens that for some special           
wave-lengths  the substance has the     
power of absorbing the energy of the    
entering  waves and converting it into  
heat-vibrations of its own molecules or 
atoms4 ,in the most transparent and     
colourless glasses this process1 so  far
 as the waves of ordinary light are     
concerned1 only goes on to a  negligibly
 slight extent2 if1 however1 we extend  
our view beyond the  range of ordinary  
visible light1 and consider the region  
of shorter  waves that lies in the      
spectrum beyond the violet1 we find that
  ordinary colourless glass becomes     
strongly absorbent2 thus to waves  of   
about half the length of those which    
produce upon our eyes the  impression of
 yellow light1 ordinary glass is as     
opaque as is a piece  of metal to white 
light4 ,in this wider sense1 then1 we   
may fairly say  that all glasses are    
coloured--i4e41 all have a power of     
selective  absorption2 but in the case  
of those which are nearly colourless in 

the ordinary sense1 this absorption     
takes place only for waves which  are   
either decidedly shorter or decidedly   
longer than those to which  our eyes are
 sensitive4 ,those glasses which appear 
coloured in the  ordinary sense1 on the 
other hand1 owe this property to the    
fact that  the power of absorption for  
light-waves extends into the region of  
the  visible spectrum2 thus a blue or   
violet glass is practically opaque to   
red rays1 while a red glass is opaque to
 blue1 green or violet rays4  ,this     
statement may be verified in a striking 
manner by holding over one  another a   
piece of deep blue or green glass and a 
similar piece of ruby  glass--the       
combination will be found to be very    
nearly opaque even when  each glass by  
itself is practically transparent4      
,the question which now naturally       
presents itself to us is1 what is  the  
essential difference between1 for       
instance1 a piece of red glass  and a   
piece of 8white0 glass that confers upon

 the former the power  of absorbing blue
 light8 ,a perfectly complete and       
satisfactory answer  to this question is
 not1 in the writer's opinion1 available
 in the  present state of our knowledge1
 but to a certain extent the difference 
between the two kinds of glass can be   
explained4 ,the difference is  produced1
 in the first instance by introducing   
into the colourless  glass some         
additional chemical element or elements1
 the substances  in question being      
generally known as 8colouring oxides10  
although  they are by no means always   
introduced in the form of oxides1 and   
are  frequently present in the glass in 
entirely different forms4 ,to a  certain
 extent the colour of the glass may be  
ascribed to a definite  8colouring0     
property of the chemical elements       
concerned2 thus most  of the chemical   
compounds of such elements as nickel1   
cobalt1 iron1  manganese and copper are 
more or less deeply coloured substances1
  and it would seem as if the atoms or  

8ions0 of these elements had  the       
specific power of absorbing certain     
varieties of light-waves  while not     
materially affecting others4 ,but this  
specific 8colouring0  property is not so
 easily explained when we recollect that
 the colours  of iron compounds1 for    
example1 may be green or red according  
to the  state of combination in which   
that element is present1 and that iron  
has also the power of imparting either a
 green or a yellow colour to  glass     
according to circumstances4 ,the        
detailed discussion of these  questions1
 however1 lies outside our present      
scope1 and we must  confine ourselves to
 the broad statement that colouring     
substance in  glass may be roughly      
divided into two kinds or groups2 the   
first  and probably the largest group   
are those bodies which occur in glass   
in true solution1 the element itself    
being present in the combined  state as 
a silicate or other such compound       
7borate1 phosphate1 etc47  which is     

soluble in the glass4 ,in this class1   
the colouring effect  upon the glass is 
specifically that of the element        
introduced1 and is  brought about in the
 same way as the colouring of water when
 a coloured  salt--such as copper       
sulphate--is dissolved in it4 ,the      
second class of  colouring substances1  
however1 behave in a different manner2  
they are  probably present in the glass 
in a state of extremely fine division1  
and held not in true solution1 but      
really in a sort of mechanical          
suspension that approximates to the     
condition of what is known as a         
8colloidal solution40 ,the point which  
is known beyond doubt1 thanks  to the   
researches of ,siedentopf and ,szigmondi
 on ultra-microscopical  particles1 is  
that in certain coloured glasses1 of    
which ruby glass is  the best example1  
the colouring substance1 be it gold or  
cuprous oxide1  is present in the form  
of minute but by no means atomic or     
molecular  particles suspended in the   

glass4 ,the presence of these particles 
has  been made optically evident1       
although it can hardly be said that they
  have been rendered visible1 and it is 
at all events probable that these       
suspended particles act each as a whole 
in absorbing the light-waves            
characteristic of the colour which they 
produce in glass4 ,this being  the case1
 it is easy to understand how readily   
the colour of such  glasses is altered  
or spoilt by manipulations which involve
 heating  and cooling at different      
rates--too rapid a rate of cooling      
producing  a different grouping of the  
minute particles1 altering their size or
  shape1 or even obliterating them      
entirely by allowing the element in     
question to go into or to remain in     
solution in the glass4    ,while it     
would be entirely foreign to the purpose
 of this volume to  give in this place a
 series of recipes for the production of
 various  kinds of coloured glass1 it   
will be desirable to state in general   

terms  the colours or range of colours  
which can be produced in various kinds  
of glass by the introduction of those   
chemical elements which are  ordinarily 
used in this way4 ,in general terms it  
may be said that the  lighter elements  
do not as a rule tend to the production 
of coloured  glasses1 while the heavier 
elements1 so far as they can be retained
 in  the glass in either solution or    
suspension1 tend to produce an intense  
colouring effect4 ,the element lead     
appears to form a striking exception  to
 this rule1 but this is due to the fact 
that while the silicates  of most of the
 other heavy elements are more or less  
unstable1 the  silicate of lead is very 
stable1 and can only be decomposed by   
the  action of reducing agents4 ,when   
lead silicates are decomposed in this   
way1 however1 the resulting glass       
immediately receives an exceedingly     
deep colour1 being turned a deep opaque 
black1 although in very thin  layers the
 colour is decidedly brown4 ,on the     

other hand1 glasses  very rich in lead  
are always decidedly yellow in colour1  
and it has  been shown that this        
coloration is due to the natural colour 
of lead  silicates and not to the       
presence of impurities4 ,what has just  
been  said of lead applies1 with only   
very slight modification1 also to the   
rare metal thallium and its compounds1  
which have been introduced into  glass  
for special purposes4 ,leaving these two
 exceptional bodies on  one side1 we now
 pass to a consideration of the elements
 in the order  of their chemical        
grouping4 ,the rare elements will not be
 considered  except in certain cases    
where their presence in traces is liable
 to  affect results attained in         
practice4    ,the ,alkali ,metals1      
sodium1 potassium1 lithium1 etc41 and   
their  compounds1 have no specific      
colouring effect1 although the presence 
of soda or of potash in a glass affects 
the colours produced by such  substances
 as manganese1 nickel1 selenium1 etc4   

,copper1 as would be anticipated from   
the deep colour of most of  its         
compounds1 produces powerful colouring  
effects on glass4 ,cupric  silicates    
produce intense green1 to greenish-blue 
tints4 ,copper1  either as metal or     
oxide1 added to glass in the ordinary   
way1 always  produces the green colour2 
but when the full oxidation of the      
copper is  prevented by the presence of 
a reducing body1 and the glass is cooled
  slowly1 or is exposed to repeated     
heating followed by slow cooling1  an   
intense ruby coloration is produced4 ,in
 practice this colour is  produced by   
introducing tin as well as copper into  
the mixture1 and  so regulating the     
conditions of melting as to favour      
reduction rather  than oxidation of the 
copper4 ,under these circumstances the  
copper is  left in the glass in a finely
 divided and evenly suspended state2 if 
exactly the right state of division and 
suspension is arrived at1 a  beautiful  
red tint is the result1 although the    

coloration of the glass  is so intense  
that it can only be employed in very    
thin sheets1 being  8flashed0 upon the  
surface of colourless glass to give it  
the necessary  strength and thickness   
for practical use4 ,it is further very  
easy to  slightly alter the arrangement 
of the copper in the glass1 with the    
result of producing an opaque1 streaky  
substance resembling sealing-wax  in    
colour and appearance1 this product     
being1 of course1 useless from  the     
glass-maker's point of view4 ,finally1  
by exceedingly slow cooling1  and under 
other favouring conditions which are not
 really understood1  the particles of   
suspended colouring-material--be it     
metallic copper or  cuprous oxide--grow 
in size and attain visible dimensions1  
appearing as  minute shimmering flakes1 
thus producing the beautiful substance  
known  as 8aventurine40    ,silver is   
never introduced into glass mixtures1   
the reason being that  it is so readily 
reduced to the metallic state from all  

its compounds  that it cannot be        
retained in the glass except in a       
finely-divided  form1 causing the glass 
to assume a black1 metallic appearance  
resembling the stains produced by the   
reduction of lead in flint  glasses4 ,on
 the other hand1 silver yields a        
beautiful yellow colour  when applied to
 glass as a surface stain1 and it is    
widely used for  that purpose4    ,gold 
is introduced into glass for the        
production of brilliant ruby  tints2 its
 behaviour is very similar to that of   
copper1 except that  the noble metal has
 a great tendency to return to the      
metallic state  without the aid of      
reducing agents4 ,no addition of tin is 
therefore  required1 but the rate of    
cooling1 etc41 must be properly         
regulated1  since rapidly cooled glass  
containing gold shows no special colour1
  the rich ruby tint being only         
developed when the glass is re-heated   
and cooled slowly4 ,the colouring effect
 of gold is undoubtedly more  regular   

and uniform than that of copper1 and it 
is accordingly possible  to obtain much 
lighter shades of red with the aid of   
the noble metal4  8,gold ruby0 can      
therefore be obtained of a tint light   
enough to be used  in sheets of ordinary
 thickness1 and the process of          
8flashing0 is not  essential4    ,the   
elements of the second group1 such as   
magnesium1 calcium1  strontium1 barium1 
zinc and cadmium1 exert no strong       
specific colouring  action on glass1    
with perhaps the exception of cadmium1  
and that  element only does so to any   
considerable extent in combination with 
sulphur1 sulphide of cadmium having the 
power of producing rich yellow  colours 
in glass4 ,the sulphur compounds of     
barium also readily produce  deep green 
and yellow colours1 and the formation of
 these tints is1  indeed1 very difficult
 to avoid in the case of glasses        
containing much  barium4 ,a colouring   
effect has sometimes been ascribed to   
zinc1 but  this is not in accordance    

with facts4    ,of the elements of the  
third group1 only boron and aluminium   
are ever  found in glass in any notable 
quantity4 ,boron is present in the form 
of  boric acid or borates1 and as such  
produces no colouring effect1 nor  does 
there seem to be any tendency for the   
separation of free boron4  ,the         
compounds of aluminium also possess no  
colouring effect1 although  certain     
compounds of this element are utilised  
for imparting a white  opacity to glass 
for certain purposes--such glass being  
known as 8opal40    ,the elements of the
 fourth group are of greater importance 
in  connection with glass4 ,carbon is   
capable of exerting powerful colouring  
effects when introduced into glass4     
,these effects are of two kinds1  viz41 
indirect in consequence of the reducing 
action of carbon on other  substances   
present1 and direct from the presence of
 finely-divided  carbon or carbides in  
the glass4 ,the latter are similar in   
kind to  those produced by the presence 

of other finely-divided elementary      
bodies 7copper1 gold1 lead1 etc47 except
 that the lightness of the  carbon      
particles tends to the production of    
yellow and brown colours  rather than of
 red and black1 while the chemical      
nature of carbon  renders the glass in  
which it is suspended indifferent to    
rapid  cooling1 so far as the carbon    
tint is concerned4 ,the indirect effects
  of carbon1 in reducing other          
substances that may be present in the   
glass1 become evident with much smaller 
proportions of carbon than are  required
 to produce visible direct effects4 ,as 
we have seen above1  carbon1 in the form
 of coke1 charcoal or anthracite coal1  
is regularly  introduced1 as a reducing 
medium1 into glass mixtures containing  
sulphate of soda4 ,if even a slight     
excess of carbon be used for this       
purpose1 the formation of sulphides and 
poly-sulphides of sodium and  of calcium
 results1 and these bodies1 like all    
sulphides1 impart a  greenish-yellow    

tint to the glass1 at the same time     
bringing other  undesirable results in  
their train4    ,silicon1 in the form of
 silicic acid and its compounds1 is a   
fundamental constituent of all varieties
 of glass1 and in this form  is in no   
sense a colouring substance2 on the     
other hand1 there is no  doubt that     
under some conditions silicon may be    
reduced to the metallic  state at       
temperatures which normally occur in    
glass-furnaces1 and it  is practically  
certain1 that if present in glass in    
this condition1  silicon would colour   
the glass4 ,it is just possible that    
some of the  colouring effects produced 
in ordinary glass by powerful reducing  
agents1 such as carbon1 either in the   
solid form or as a constituent of       
furnace gases1 may be due to the        
reduction of silicon in the glass4      
,tin by itself does not appear to have  
any colouring effect upon  glass1 except
 that its oxide1 in a finely suspended  
state1 produces  opalescence and1 in    

large quantities1 white opacity4 ,tin1  
however1 is  used in conjunction with   
copper in the production of copper-ruby1
 to  which reference has already been   
made4    ,lead and ,thallium have       
already been dealt with1 and it only    
remains  to add that their presence in  
the glass1 although not in itself       
producing any intense colouring action1 
increases the colouring effects  of     
other substances4 ,this is probably     
merely a particular case of the  fact   
that dense glasses1 of high refractive  
index1 are more sensitive  to colouring 
agencies than the lighter glasses of low
 refractive index2  this applies to     
barium as well as to lead and thallium  
glasses4    ,phosphorus occurs in some  
few glasses in the form of phosphoric   
acid1 and this substance1 as such1 has  
no colouring effect4 ,calcium           
phosphate1 however1 is sometimes added  
to glasses for the purpose of  producing
 opalescence4 ,its action in this       
respect is probably similar  to that of 

tin oxide and aluminium fluoride1 these 
substances all  remaining undissolved in
 the glass in the form of minute        
particles in a  finely divided and      
suspended state4    ,arsenic does not   
exert a colouring effect on glass1 and  
owing to its  volatile nature it can    
only be retained in glass in small      
quantities  and under special           
conditions4 ,a 8decolourising0 action is
 sometimes  ascribed to arsenic1 but if 
this action really exists it can only be
  ascribed to the fact that arsenic     
compounds are capable of acting as      
carriers of oxygen1 and their presence  
thus tends to facilitate the  oxidation 
of impurities contained in the glass4 ,a
 further reference  to this subject will
 be found below in reference to the     
compounds of  manganese4    ,antimony1  
although frequently added to special    
glass mixtures1 does  not appear to     
produce any very powerful effects1      
except possibly  in the direction of    
producing white opacity if present in   

large  proportions4 ,the sulphide of    
antimony1 however1 exerts a colouring   
influence1 although its volatile and    
unstable character renders the  effects 
uncertain4    ,vanadium1 owing to its   
rarity1 is probably never added to glass
  mixtures for colouring purposes1      
although it is capable of producing     
vivid yellow and greenish tints when    
present even in minute  proportions4 ,on
 the other hand1 vanadium occurs in     
small proportions  in a number of       
fire-clays1 including some of those of  
the ,stourbridge  district1 and glass   
melted in pots containing this element  
is liable to  have its colour spoilt by 
taking up the vanadium from the clay4   
,sulphur is an element whose presence in
 various forms is liable  to affect the 
colour of glass in a variety of ways4   
,the colouring  effects of sodium-1     
calcium-1 cadmium-1 and                 
antimony-sulphides have  already been   
referred to4 ,sulphur probably never    
exists in glass in  the uncombined state

 at all1 but sulphur and its oxides1    
which are  often contained in furnace   
gases1 sometimes exert a very marked    
action  upon hot glass4 ,the presence of
 sulphur gases in the atmospheres of    
blowing-holes and annealing kilns is    
liable to produce in the glass  a       
peculiar yellowish milkiness which      
penetrates for a considerable  depth    
into the mass of the glass and cannot be
 removed by subsequent  treatment4      
,glass vessels1 particularly if made of 
glass produced from  raw materials among
 which salt-cake has figured1 are also  
affected by  contact with fused sulphur 
or its vapour1 the effect being a       
gradual  disintegration of the glass4   
,the precise mechanism of these actions 
is not known at present1 but they       
probably consist in the formation  of   
sulphur compounds within the glass1     
possibly giving rise to an  evolution of
 minute bubbles of gas4    ,selenium1   
which is chemically so closely related  
to sulphur1 is  a relatively rare       

element1 which is1 however1 finding some
 use  in glass-manufacture as a         
colouring and a decolouring agent4 ,the 
introduction of selenium or of its      
compounds under suitable conditions     
into a glass mixture produces or tends  
to produce a peculiar  yellowish-pink   
coloration1 the intensity of the colour 
produced being  dependent upon the      
chemical nature of the glass as a whole 
and1 of  course1 upon the amount of     
selenium left in the glass at the end of
 the  melting process1 this latter in   
turn depending upon the duration and    
temperature of the process in question4 
,the pink colour of selenium  glass is  
best developed in those containing      
barium as a base1 but it is  also       
developed in lead glasses1 while        
soda-lime glasses do not show the       
colour so well4 ,as a 8decolouriser0 the
 action of selenium is entirely  that of
 producing a complementary colour which 
is intended to 8cover0  the green or    
blue tint of the glass2 where the depth 

of the tint to  be 8covered0 is small1  
selenium can be used very successfully  
in this  way1 although it is a          
relatively costly substance for such a  
purpose4  ,no oxidising or 8cleansing0  
action can be ascribed to selenium or   
its  compounds4    ,chromium is one of  
the most intensely active colouring     
substances  that are available for the  
glass-maker1 and it is accordingly used 
very extensively4 ,it has the advantage 
of relative cheapness1 and can  be      
conveniently obtained and introduced    
into glass in the form of pure          
compounds whose colouring effect can be 
accurately anticipated2 the  colours    
produced by the aid of chromium have the
 further advantage of  being very       
constant in character1 being little     
affected by oxidising  or reducing      
conditions1 and only very slightly by   
the length or  temperature of the       
melting process4 ,the rate of cooling1  
in fact1  appears to be the only factor 
that materially affects the colours     

produced by compounds of chromium4 ,the 
colours produced by chromium  alone are 
various depths of a bright green1 the   
depth varying1 of  course1 with the     
proportion of chromium that is present  
in the glass  and with the purity of the
 glass itself4 ,very frequently1        
chromium  is used in conjunction with   
either iron or copper to produce various
  tints of 8cold blue0 and 8celadon     
green0 respectively4 ,this element  is  
most usually introduced into the glass  
mixture in the form of  potassium       
bichromate2 although other compounds    
might be employed1  this substance      
presents several advantages to the glass
 maker4 ,in  the first place1 since the 
colouring effect of chromium is very    
intense1 it must be used in very small  
quantities1 and if chromic  oxide itself
 were used1 the weighing would have to  
be carried out with  extreme care2      
potassium bichromate1 however1 contains 
a much smaller  proportion of the       
effective colouring substance1 so that  

much larger  weights can be employed1   
and the accuracy of weighing required is
  proportionately reduced4 ,a further   
consideration arises from the fact  that
 chromic oxide is itself an extremely   
refractory body1 and is  therefore      
comparatively difficult to incorporate  
with glass1 while its  presence tends to
 make the glass itself more viscid and  
refractory2 the  simultaneous           
introduction of the alkali1 as provided 
by the use of the  bichromate1 is thus  
an advantage in restoring the fluidity  
and softness  of the glass when         
finished1 while also facilitating the   
solution of  the chromium in the glass  
during the fusion process2 this process 
of  solution1 however1 takes some time1 
chromium glasses being liable to  appear
 patchy if insufficient time is given to
 the 8founding40    ,uranium is one of  
the rarer and more costly elements1 but 
is  nevertheless used in glass-making   
for special purposes on account of  the 
very beautiful fluorescent yellow colour

 which it imparts when  added in small  
proportions4 ,this yellow is quite      
characteristic and  unmistakable1 so    
that none of the other varieties of     
yellow glass  can ever be used as a     
substitute for uranium glass1 but the   
great  cost of the latter prevents its  
extended use4 ,uranium is usually       
introduced into glass mixtures in the   
form of a chemical compound1  such as   
uranyl-acetate or uranyl-nitrate1 both  
these substances being  obtainable in   
the form of small1 intensely bright     
yellow crystals4    ,fluorine occurs in 
a number of glasses in the form of      
dissolved or  suspended fluorides1      
principally fluoride of aluminium4 ,the 
element  is not essentially a colouring 
substance1 and is only mentioned here   
because the fluoride named is the most  
frequently used means of  producing     
8opal0 glass4 ,the fluoride is most     
frequently introduced  into the glass   
mixtures as calcium fluoride1 used in   
conjunction with  felspar1 or as        

cryolite1 a natural mineral which       
consists of a double  fluoride of sodium
 and aluminium4    ,manganese is one of 
the most important colouring elements   
used by  the glass-maker4 ,when         
introduced into glass in the absence of 
other  colouring ingredients1 compounds 
of manganese produce a range of  colours
 lying in the region of pinkish-purple  
to violet1 according  to the chemical   
nature of the glass4 ,the exact colour  
produced varies  according as the glass 
has lead1 lime or barium as its base1   
and it  also depends upon the presence  
of soda or potash as the alkaline       
constituent4 ,the nature and intensity  
of the colour1 however1 which  the      
addition of a given percentage of       
manganese will produce depends  upon    
other factors besides the chemical      
composition of the bases used  in the   
mixture4 ,the heat and duration of the  
8found0 and the reducing  or oxidising  
conditions of the furnace in which it   
has been carried  on very materially    

affect the result4 ,thus1 a glass having
 a slight  tinge of pink or purple      
derived from manganese can be rendered  
entirely  colourless by the action of   
reducing gases or by introducing into   
the  glass a reducing substance1 such as
 a piece of wood4 ,it will thus be  seen
 that while manganese is a most useful  
element for the glass-maker1  its       
employment requires much skill and care1
 and generally involves  some           
troublesome manipulations before the    
desired result is attained4    ,in      
practice1 manganese is most frequently  
used with other colouring  ingredients  
for the production of what may be called
 8compound0  colours1 the function of   
the manganese being to provide the      
8warm0  element1 i4e41 the pink or      
purple component1 required4 ,one of the 
most important uses of manganese coming 
under this head is its use as  a        
8decolouriser40 ,by a 8decolouriser0 the
 glass-maker understands a  substance   
which can be used to improve the colour 

of a glass which1  from the nature of   
its raw materials and conditions of     
melting1 would  have a greener colour   
than is thought desirable for the       
product  in question4 ,it may be said at
 once that the most perfect and         
satisfactory method of obtaining the    
better colour required is to  adopt the 
use of purer raw materials and methods  
of melting less liable  to lead to      
contamination of the glass4 ,on the     
other hand1 this radical  course is     
often impossible on the ground of       
expense1 and the less  satisfactory     
course must be adopted of covering one  
undesirable colour  by another          
complementary colour which would1 in    
itself1 be equally  undesirable4 ,the   
rationale of this procedure depends upon
 the fact  that a slight amount of      
absorption of light is not readily      
detected by  the human eye if it be     
uniformly or nearly uniformly           
distributed over  the whole range of the
 visible spectrum1 i4e41 if the colour  

of the  resulting light is nearly       
neutral1 while an equally slight        
absorption  in one region of the        
spectrum1 while actually allowing more  
light  to pass through the glass1 is at 
once detected by the eye owing to  the  
colour of the transmitted light4 ,now it
 has been found that the  colour        
produced in glass by the addition of    
very small proportions of  manganese is 
approximately complementary to the      
greenish-blue tinge of  the less pure   
varieties of ordinary glass2 the        
addition of manganese in  suitable      
proportions to such glass therefore     
results in the production  of a glass   
which transmits light of approximately  
neutral1 usually  slightly yellow1      
colour1 the increased total absorption  
only becoming  noticeable in large      
pieces4 ,this 8covering0 of the greenish
 tinge is  generally most completely    
successful in the case of soda-flint    
glasses1  but the method is also used to
 a certain extent in the case of the    

soda-lime glasses used for sheet and    
plate-glass manufacture4 ,manganese     
added to glass for this purpose is      
generally introduced into the  mixture  
in the form of the powdered black oxide 
7manganese dioxide71  which is available
 as a natural ore in a condition of     
sufficient  purity4 ,added in this form1
 the manganese compound exerts a double 
action1 the decomposition of the dioxide
 resulting in the liberation of  oxygen 
within the mass of melting glass1 and   
this oxygen itself exerts  a favourable 
influence on the resulting colour of the
 glass1 since it  removes organic       
materials whose subsequent reducing     
action would be  deleterious1 and it    
also converts all iron compounds present
 into the  more highly-oxidised 7ferric7
 state in which their colouring effects 
are less intense4 ,the actual colouring 
effect of the manganese itself  is1 of  
course1 afterwards developed1 and       
produces the effects discussed  above4  
,the 8covering0 of the greenish tints   

due to iron and other compounds  is only
 possible when these are present in very
 small proportions4  ,when larger       
quantities of these substances have been
 introduced into  the glass the addition
 of manganese modifies the resulting    
colour1 but  is no longer able to       
neutralise it4 ,a very large range of   
colours can  be obtained by using       
various proportions of iron and         
manganese1 the  best-known of these     
being the warm brown tint known as      
8hock-bottle10  while all shades between
 this and the bright green of iron and  
the  purple of manganese can be obtained
 by suitable mixtures4 ,what has been   
said above as to the sensitiveness of   
manganese colours applies with  even    
greater force to these mixed tints1     
since here both the iron and  the       
manganese compounds are liable to       
undergo changes of oxidation4           
,copper-manganese and chromium-manganese
 colours are also used1 as  indeed      
almost any number of colouring          

ingredients may be simultaneously       
introduced into a glass mixture1 the    
resulting colour being1 as a rule1      
purely additive4    ,iron is so widely  
distributed among the materials of the  
earth's  crust that it is exceedingly   
difficult to exclude it entirely from   
any kind of glass1 although the purest  
varieties of glass contain the  merest  
traces of this element4 ,cheaper        
varieties of glass1 however1  always    
contain iron in measurable quantity1    
while the cheapest kinds of  glass      
contain considerable proportions of this
 element4 ,the colouring  effects of    
iron have already been alluded to at    
various points in the  earlier chapters 
as well as in the section on manganese  
just preceding4  ,little further remains
 to be said here4 ,just as the less     
highly  oxidised compounds of           
iron--i4e41 the 8ferrous0               
compounds--always  show a decided green 
tint1 so glasses containing iron when   
melted under  the usually prevalent     

reducing conditions of a glass-making   
furnace1  show a decided green tint     
whose depth depends upon the amount of  
iron present1 provided no manganese or  
other 8decolouriser0 has been           
introduced4 8,ferrous0 compounds are1   
however1 readily converted into  the    
more highly oxidised or 8ferric0 state  
by the action of oxidising  agents1 and 
this change can also be brought about in
 molten glass by  the action of such    
substances as nitrates or other sources 
of oxygen4  ,the ferric compounds1      
however1 show characteristic yellow     
tints which  are much less intense and  
vivid than the corresponding green      
colours  of the 8ferrous0 series1 and a 
similar result is brought about by the  
oxidation of iron compounds contained in
 glass2 hence the 8washing0 or          
cleansing effects ascribed to oxidising 
agents introduced in the fusion  of     
glass4 ,it should1 however1 be borne in 
mind that the oxidation of  other       
substances besides iron compounds1 viz41

 organic matter1 carbon  and sulphur    
compounds1 may1 and probably does1 play 
a most important  part in this process  
in the case of most varieties of glass4 
,nickel exerts a powerful colouring     
influence on glass1 in accordance  with 
the fact that most of the other         
compounds of this element are  also     
deeply coloured4 ,the exact colour      
produced in glass depends  upon the     
nature of the glass and on the condition
 of oxidation in  which the nickel is   
present4 ,the colours1 however1 are     
usually of  a greenish-brown tint1      
although brighter colours can be        
produced  by nickel under special       
conditions4 ,this element is not1       
however1  much used as a colouring agent
 in practice1 although it has been      
advocated as a 8decolouriser40 ,the     
writer is not1 however1 aware  that it  
has ever been successfully used for this
 purpose1 and1 in  fact1 the colours to 
which it gives rise do not appear to be 
even  approximately complementary to the

 ordinary green and blue tints which    
8decolourisers0 are intended to cover4  
,cobalt is one of the most powerful     
colouring agents in glass1 and is  very 
largely used in the production of all   
varieties of blue glass4 ,the  blue     
colour produced by cobalt is1 in fact1  
probably the most 8certain0  of the     
colours available to the glass-maker1   
this tint being least  affected by all  
those circumstances that lead to        
variations in other  tints4 ,almost the 
only difficulty involved in the use of  
cobalt is the  great colouring power of 
this element1 which requires that for   
most  purposes only very small          
quantities may be added to the glass    
mixture4  ,formerly cobalt was added to 
glass mixtures in the form of 8zaffre10 
which was a very impure form of cobalt  
oxide4 ,at the present time1  however1  
the more expensive but much more        
satisfactory pure oxide of  cobalt is in
 almost universal use4 ,this substance  
shows a perfectly  constant composition 

and1 by means of accurate weighing1     
enables the  glass-maker to introduce   
precisely the right amount of cobalt    
into his  batch4    ,the range of       
colours which are available to the      
modern glass  manufacturer are1 as will 
be seen from a consideration of the list
 of  colouring elements given above1    
practically unlimited1 particularly as  
these substances can be used in almost  
any combination to produce mixed  or    
intermediate tints4 ,this practically   
infinite variety of possible  tints1    
indeed1 involves the principal          
difficulty encountered by the           
manufacturer of coloured glass1 i4e41   
that of matching his tints1 or  of      
keeping the colour of any particular    
variety of glass so constant  that      
pieces produced at various times can be 
used indiscriminately  together4 ,this  
ideal is1 perhaps1 never entirely       
realised1 but in the  case of glasses   
intended for special technical uses the 
ideal degree of  constancy is very      

closely approached4    ,in addition to  
being called upon to produce a large    
variety of  different tints1 the        
glass-maker is also called upon to      
produce  various depths of the same     
tint4 ,in many cases this can be readily
  done by the simple means of varying   
the amount of colouring material  added 
to the glass4 ,where the colouring      
effect of small quantities of  these    
substances is not excessively powerful  
there is no very great  difficulty in   
doing this1 but in certain cases this   
mode of regulating  the intensity of the
 colour is not available4 ,thus         
copper-ruby glass  cannot readily be    
made of so light a tint as to appear of 
reasonable  depth when used in sheets of
 the thickness of ordinary sheet-glass4 
,as has already been indicated1 the     
desired tint is obtained by the  process
 of 8flashing10 i4e41 of placing a very 
thin layer of deep  ruby-coloured glass 
upon the surface of a sheet of ordinary 
more  or less colourless glass of the   

usual thickness4 ,this is generally     
accomplished by having a pot of molten  
ruby glass available close to a  pot    
from which colourless glass is being    
gathered4 ,a small gathering  of ruby   
glass is first taken up on the pipe1 and
 the remaining  gatherings required for 
the production of the sheet are taken   
from  the pot of colourless glass4 ,when
 such a composite gathering is blown    
into a cylinder in the manner described 
in the previous chapter1 the  ruby glass
 lies as a thin layer over the inner    
face of the cylinder1  but special care 
and skill on the part of the gatherer   
and blower is  required to ensure that  
this layer shall be evenly distributed  
and  of the right thickness to produce  
just the tint of ruby required4  ,since 
the whole layer of red glass is so thin1
 a very slight want of  uniformity in   
its distribution leads to wide          
variations of tint1 and in  practice    
these are often seen in the less        
successful cylinders of such  glass4    

,the chemical composition of the ruby   
and the colourless glass which  are to  
be employed for this purpose must also  
be properly adapted  to one another in  
order to produce two glasses which shall
 have as  nearly the same coefficient of
 thermal expansion as possible4 ,if this
  requirement is not met1 the resulting 
glass is subjected to internal  strains 
which may lead to fracture1 while1 if   
the ruby glass has the  higher          
coefficient of expansion1 the sheet     
after flattening tends to  draw itself  
up on the 8flashed0 side and cannot be  
passed out of the  annealing kiln in a  
properly flat condition4    ,although   
most usually applied to copper-ruby     
glass1 the flashing  process is often   
used with other colours also4 ,coloured 
glass of  this kind is at once          
recognised when looked at through the   
edges4  ,thus examined the glass simply 
shows the greenish tint of ordinary     
sheet-glass which constitutes           
practically the entire thickness of the 

sheet4 ,in the same way1 if such        
8flashed0 glass be cut or etched in     
such a way that the layer of coloured   
glass is removed in places1 the         
resulting pattern appears in white on   
the coloured ground--a feature  which is
 utilised for certain decorative        
purposes4 ,the flashing  process just   
described1 it should be noted1 is       
applicable to any form  of glass-ware   
which is blown from a gathering1 and the
 coloured layer  can be applied either  
upon the inside or outside of any object
 thus  produced4    ,in addition to the 
palette of colours which the glass-maker
 is able to  supply1 the artist in      
stained glass has a further range of    
colours at  his disposal in the form of 
stains and transparent colours which can
 be  applied to the surface of glass and
 developed and rendered more or less    
permanent by being properly 8fired40    
,the colours produced in this way  are  
also1 in one sense1 coloured glasses1 or
 rather glazes1 whose raw  materials are

 put upon the glass by the brush of the 
painter1 and only  subsequently caused  
to combine and melt by suitable heating4
 ,the degree  of heat applicable under  
these circumstances is1 however1 very   
limited  by the necessity of avoiding   
any great softening of the substratum of
  glass1 while many of the colours      
themselves are composed of materials    
which could not resist very high        
temperatures4 ,the fluxes used in  the  
composition of these colours must for   
this reason be of a very  fusible kind1 
with the inevitable result of a greatly 
reduced chemical  stability as compared 
with the glass itself4    ,the whole    
subject of painting on glass1 even from 
the purely technical  as apart from the 
sthetic point of view1 is a very wide   
one1 and lies  outside the scope of the 
present volume4 ,only one further       
technical  point in connection with     
glass-painting and stained glass work   
will  therefore be touched upon here4   
,this is an example of the fact that    

the more technically 8perfect0 modern   
product is not always preferable  for   
special purposes which have been well   
served by older and far  less 8perfect0 
products4 ,the production of technically
 excellent  coloured glass in modern    
times was1 somewhat surprisingly at     
first1  accompanied by a very marked    
decline in the artistic beauty of       
stained  glass windows produced with    
this modern material2 the ancient art of
  stained glass was1 therefore1 for a   
time regarded as a 8lost art10 and      
glass-makers were blamed for being      
unable to produce the brilliant and     
beautiful tints which had been formerly 
available4 ,more careful study1         
however1 revealed the fact that while   
the actual colour of modern  glass was  
at least as brilliant and varied as that
 of ancient glass1  the difference lay  
in the fact that the modern glass was   
practically  entirely free from such    
imperfections as air-bubbles1 stri1 and 
other defects which improved appliances 

and methods had enabled the  glass-maker
 to eliminate from his products4        
,finding the beauty of his  wares       
greatly improved by this increased      
purity of the glass in the  case of     
window glass and table ware1 it was     
natural for the glass-maker  to         
endeavour to produce the same           
8improvement0 in the coloured glasses   
intended for artistic purposes and1     
indeed1 it is more than likely that  the
 stained-glass workers themselves       
pressed this line of improvement  upon  
him by a demand for 8better0 glass4 ,it 
turned out1 however1 on  close          
examination1 that this very perfection  
of modern glass rendered  it less       
adapted for these artistic purposes4 ,a 
perfect piece of glass1  having smooth  
surfaces and no internal regularities1  
allows the rays  of light falling upon  
it to pass through undeflected in       
direction1  and merely changed in       
colour1 according to the tint of the    
glass in  question4 ,on looking at the  

glass1 external objects can be quite    
clearly seen1 and much of the interest  
and mystery of the glass itself  is     
lost4 ,on the other hand1 when falling  
upon a piece of glass having  an        
irregular surface1 and containing all   
manner of irregularities  such as stri1 
air-bells1 and even pieces of enclosed  
solid matter1  the light is scattered1  
refracted1 and deflected into all manner
 of  directions until it almost appears 
to emanate from the body of the  glass  
itself1 which thus appears almost to    
shine with an internal  light of its    
own2 the eye can hardly perceive the    
presence of external  objects1 and the  
whole window appears as a brilliant     
self-luminous  object4    ,once their   
attention had been drawn to these facts1
 modern glass-makers  endeavoured1 and  
with much success1 to reproduce the     
desirable  qualities of the ancient     
glass1 while still availing themselves  
of  modern methods to produce more      
stable glasses and a wider range of     

colours4 ,the irregular surface of the  
old glass is imitated by using  rolled  
or 8muffed0 instead of ordinary blown   
glass1 while the internal  texture is   
rendered non-homogeneous by the         
deliberate introduction of  solid and   
gaseous impurities and by manipulations 
so arranged as to  leave the glass in   
layers of different density1 which      
appear in the  finished glass as 8stri40
 ,as a consequence1 it is probably not  
too  much to claim that the modern      
workers in coloured glass have materials
  at their disposal which are at least  
as suitable for the purpose as  those   
that were available in the best days of 
the ancient art4    ,some reference has 
already been made to the technical uses 
of coloured  glass1 but one or two      
further points in that connection remain
 to be  discussed4 ,for such technical  
purposes as railway and marine signals1 
the consensus of practical experience   
has decided in favour of certain        
colours of glass1 such as red and green 

of particular tints4 ,on the  other     
hand1 for various purposes in connection
 with photography1  the glass-maker does
 not appear to have been able to meet   
the new  requirements1 with the result  
that flimsy and otherwise unsatisfactory
  screens made of gelatine or celluloid 
stained with organic dyes are  employed 
in place of coloured glass in such      
cases1 for example1 as the  covering of 
lamps for use in photographers' 8dark0  
rooms1 and for the  light-filters used  
for orthochromatic and tri-chromatic    
photography4  ,in all these cases it is 
necessary to use a transparent coloured 
medium which transmits only light of a  
certain very definite range of          
wave-lengths1 and there is no doubt that
 for the glass-maker1 who is  confined  
to the use of a number of elementary    
bodies for his colouring  media1 it is  
by no means easy to comply with these   
requirements of  exact transmission and 
absorption4 ,on the other hand1 the     
field of  available coloured glasses has

 not been fully explored from this point
  of view1 the only extensive work on   
the subject having been done in         
connection with the ,jena firm of       
,schott1 who have put upon the market  a
 series of coloured glasses of          
accurately-known absorbing power4       
,there is1 however1 little doubt that a 
much greater extension of this  field is
 possible1 and that it will be opened up
 by a glass-maker who  undertakes the   
exhaustive study of coloured glasses    
from this point of  view1 although it   
must be admitted that there is          
considerable doubt  whether the results 
obtainable by the aid of aniline and    
other dyes as  applied to gelatine can  
ever be equalled by coloured glasses4   
,,chapter ,,xii4    ,,optical ,,glass4  
,optical glass differs so widely from   
all other varieties of glass that  its  
manufacture may almost be regarded as a 
separate industry1 to  which1 indeed1 a 
separate volume could well be devoted4  
,in the present  chapter we propose to  

give an outline of the most important   
properties  of optical glass1 and in the
 next chapter to describe the more      
important features of the processes used
 in its production4    ,the properties  
which affect the value of optical glass 
may roughly be  divided into two groups4
 ,the first group comprises the         
specifically  8optical0                 
properties--i4e41 those directly        
influencing the behaviour  of light in  
its passage through the glass1 while the
 second group  covers those properties  
of a more general nature1 which are of  
special  importance in glass that is to 
be used for optical purposes4           
,optical ,properties of ,glass4--,the   
most essential property of glass  in    
this respect is homogeneity4 ,we have   
already indicated that glass  can never 
be regarded as a definite chemical      
substance or compound1  but that it     
usually consists of mutual solutions of 
various complex  silicates1 borates1    
etc4 ,solutions being of the very nature

 of mixtures  of two or more different  
substances1 it follows that they can    
only  become homogeneous when complete  
mixing has taken place4 ,we have  a     
familiar example of the formation of    
such a solution when sugar is  dissolved
 in water4 ,the water near the sugar    
becomes saturated with  sugar and of    
different density from the remaining    
water2 if the liquid  is slightly       
stirred a very characteristic phenomenon
 makes its  appearance--the pure water  
and the dense sugar solution do not at  
once  mix completely1 the denser liquid 
remaining for a time disseminated       
throughout the whole fluid mass in the  
form of more or less fine lines1        
sheets1 or eddies1 and these are visible
 because the imperfectly mixed  liquids 
have different effects on the light     
passing through them4 ,in  the case of  
sugar-water we are1 however1 dealing    
with a very mobile  liquid1 and a few   
turns of a tea-spoon suffice to render  
the mixture  complete1 and the liquid1  

which for a few moments had appeared    
turbid1  becomes homogeneous and        
transparent4 ,in the case of glass1 when
 the  raw materials are melted together1
 a mixture is formed of liquids of      
differing densities similar to that     
which was temporarily formed in  the    
sugar-water solution4 ,molten glass1    
however1 is never so mobile  a liquid as
 ordinary water1 nor is it in the       
ordinary course of  manufacture         
subjected to any such thorough mixing   
action as that which  is produced by a  
spoon in a glass of water4 ,in glass as 
ordinarily  manufactured1 therefore1 it 
is not surprising to find that the lack 
of  homogeneity which originates during 
the melting persists to the end4  ,its  
effects can be traced whenever a thick  
piece of ordinary glass is  carefully   
examined1 when the threads or layers of 
differing densities  can be recognised  
in the form of minute internal          
irregularities in the  glass4 ,these    
defects are known as stri or veins1 and 

their presence  in glass intended for   
the better kind of optical work renders 
the  glass useless4 ,as will be seen    
below in the production of optical      
glass1 special means are adopted for the
 purpose of rendering it  as homogeneous
 as possible2 in fact1 the early history
 of optical  glass manufacture is simply
 the history of attempts to overcome    
this  very defect4 ,the problem is1     
however1 beset by chemical and physical 
difficulties of no mean order1 and even 
in the best modern practice  only a     
small proportion of each melting or     
crucible full of glass  is entirely free
 from veins or stri4 ,in many cases     
these defects  are very minute1 and     
sometimes escape observation until the  
stage of  the finished lens is reached4 
,at that stage1 however1 their presence 
becomes painfully evident from the fact 
that they interfere seriously  with the 
sharp definition of the images formed by
 the lens in question4  ,it will be seen
 that in such a case time and money has 

been wasted  by grinding and polishing  
what turns out to be a useless piece of 
glass4 ,methods are1 therefore1 used for
 examining the glass before it  is      
worked1 whereby the existence of the    
smallest stri can scarcely  escape      
detection4 ,these methods depend upon   
the principle that a beam  of parallel  
light passing through a plate of glass  
will meet with no  disturbance so long  
as the glass is homogeneous1 but if stri
 are  present1 they will cause the light
 to deviate from parallelism wherever   
it falls upon them4 ,under such         
illumination1 therefore1 the stri  will 
appear as either dark or bright lines1  
when they can be readily  detected4 ,one
 form of apparatus used for this purpose
 is illustrated in  ,fig4 #ad4          
,illustration3 ,,fig4 #ad4--;,diagram of
 stri-testing apparatus4    ,l1 source  
of light2 ,s1 slit2 ,a and ,b1 simple   
convex lenses2  ,g1 glass under test2   
,e1 eye of observer4 ,the arrows        
indicate the  paths of light-rays4      

,transparency and colour are obviously  
fundamentally important  properties of  
glass4 ,in one sense homogeneity is     
essential to  transparency1 but the     
aspect of the subject which we are now  
considering is that of the absorption of
 light in the course of regular         
transmission through glass4 ,it may be  
said at once that no glass is  either   
perfectly transparent or1 what comes to 
nearly the same thing1  perfectly free  
from colour4 ,in the case of the best   
optical glasses it  is true that the    
absorption of light is very slight1 but 
even these1  when considerable          
thicknesses are viewed1 show a          
greenish-yellow or  bluish colouring4   
,on the other hand1 certain optical     
glasses which are  used at the present  
time for many of our best lenses absorb 
light so  strongly or are so deeply     
coloured that a thickness of a few      
inches  is sufficient to reveal this    
defect4 ,to some extent public taste or 
opinion which objects to the use of even

 a slightly greenish glass  in optical  
instruments of good quality is to blame 
for the tint of  these glasses4 ,in many
 cases glass-makers could produce a very
 slightly  greenish glass1 but in order 
to overcome this colour they            
deliberately  add to the glass a        
colouring oxide imparting to the glass a
 colour more  or less complementary to  
the natural green tint4 ,the result is a
 more  or less neutral-tinted glass     
which1 however1 absorbs much more light 
than the naturally green glass would    
have done4 ,since such glass is         
frequently used for photographic lenses1
 it is interesting to note that  the    
light rays whose transmission is        
sacrificed in order to avoid the  green 
tint are those lying at or near the blue
 end of the spectrum1 so  that the      
photographic rapidity of the resulting  
lenses is decidedly  reduced by the use 
of such glass4    ,refraction and       
,dispersion4--,the quantitative         
properties of glass1  governing its     

effect upon incident and transmitted    
light1 are1 of  course1 of fundamental  
importance in all its optical uses4 ,the
  fundamental optical constant of each  
variety of optical glass is known  as   
its refractive index2 this number really
 represents the ratio of  the velocity  
with which light waves are propagated   
through the glass  to the velocity with 
which they travel through free space4   
,not only  does this ratio vary with    
every change in the chemical composition
 and  physical condition of the glass1  
but it also varies according to the     
length of the light waves themselves4   
,in other words1 the short waves  of    
blue light are transmitted through glass
 with a different velocity  from that   
with which the longer waves of red light
 are transmitted4  ,the consequence is  
that when a beam of white light is      
passed through a  prism it is split up  
and spread out into a number of beams   
representing  all the colours of the    
spectrum in their proper order1 the blue

 light  suffering the greatest          
deflection from its original path1 while
 the  red light suffers least           
deflection4 ,both the actual and        
relative  amount by which light rays of 
various colours are deflected under     
such circumstances depends upon the     
nature of the glass in question2        
therefore1 to fully characterise the    
optical properties of a given  kind of  
glass it is necessary to state not only 
its refractive index  but to specify the
 refractive indices for a sufficient    
number of  different wave-lengths of    
light1 suitably distributed through the 
spectrum4 ,for this purpose a number of 
well-marked spectrum lines have  been   
chosen1 the systematic use of the       
particular set of lines which is  now   
usually employed being due to the       
initiative of ,abb and ,schott at       
,jena1 who initiated the system of      
specifying the optical properties of    
glass in this way4 ,the actual lines    
chosen are the line known as ,a1        

corresponding to a wave-length of #jgfgg
 micro-millimetres1 and the  lines known
 as ,c1 ,d1 ,f1 and ,g1 whose           
wave-lengths1 in the same units1  are   
#jfefc1 #jehic1 #jdhfb1 and #jdcda      
respectively4 ,the ,a line1  however1   
lies so near the extreme red end of the 
spectrum that the data  concerning it   
are seldom required4    ,as a matter of 
fact1 the actual refractive index is    
only stated in most  tables of optical  
glasses for sodium light 7,d line71 the 
dispersive  properties of the glass     
being indicated by tabulating the       
differences  between the refractive     
indices for the various lines1 the table
 thus  containing columns marked ,c-,d1 
,d-,f1 ,f-,g4 ,these figures are usually
  described as the 8dispersion0 of the  
glass from ,c to ,d1 ,d to ,f1 etc4  ,in
 addition to these figures it is usual  
to tabulate what is called  the 8mean   
dispersion0 of the glass1 which is      
simply the difference  between the      
refractive indices for ,c and ,f lines2 

this interval is  usually taken as      
representing that part of the spectrum  
which is of  the greatest importance for
 visual purposes4 ,a further constant   
which  is of great importance in the    
calculations for achromatic lenses is   
obtained by dividing the mean dispersion
 into the refractive index for  the ,d  
line minus one 7usually written         
7,c-,f7_/7n,d-#a774 ,this term1  for    
which no satisfactory name has yet been 
suggested1 characterises  the ratio of  
the dispersive power of the glass to its
 total refracting  power4 ,it is usually
 denoted by the ,greek letter 4 ,the    
following table  7taken from the        
,catalogue of the ,optical ,convention1 
#aije7 gives a list  of optical glasses 
produced by ,messrs4 ,chance1 of        
,birmingham4 ,this  list1 although it is
 not nearly so long as that issued by   
the ,french  and ,german firms who      
manufacture optical glass1 contains     
examples of  the most important types of
 optical glass which are available at   

the  present time4 ,those1 however1 who 
wish to use the data for the purpose  of
 lens calculation are advised to consult
 the latest issues of the  optical      
glass-makers' catalogues1 since the     
range of types available1  and even the 
actual figures for some of the glasses1 
are liable to  variation from time to   
time4      ,,table ,,of ,,optical       
,,properties4                           
----------------------------------------
-------------------------               
,partial and                            
,medium   ,relative ,partial            
,dispersions4                           
,dis- ------------------------------    
,name4     n,d4 4  per-       ,c-,d     
,d-,f       ,f-,g                       
sion4 ,c-,d4 ---  ,d-,f4 --- ,f-,g4 --- 
,c-,f4       ,c-,f4      ,c-,f4         
,c-,f4                                  
----------------------------------------
--------------------    ,extra ,hard    
,crown                                  
#adieifddjjggjjjbbhbifjjedbgjdjjdcaefj  

,boro-silicate                          
,crown                                  
#aejiffccjjhjcjjbcfbidjjefbgjjjjddfeee  
,hard ,crown                            
#aeagefjejjhefjjbebbidjjfjdgjfjjdhdeed  
99,medium ,barium                       
,crown                                  
#aegchegijjiijjjbicbifjjfiggjdjjeebeeg  
99,densest                              
,barium                                 
,crown                                  
#afjfeegijajdfjjcjhbidjjgchgjejjehiefc  
,soft ,crown                            
#aeaebefijjijfjjbfdbiajjfdbgjhjjeagegj  
99,medium ,barium                       
,crown                                  
#aeffjefcjajjfjjbigbiejjgjigjdjjegfegb  
,barium ,light                          
,flint                                  
#aedebecejajbjjjbihbibjjgbbgjajjehbegj  
,extra ,light                           
,flint                                  
#aecafdijjajhejjcacbhhjjggbgaajjfcjehj  
,extra ,light                           
,flint                                  

#aecccdhejajiijjcbbbicjjggggjgjjfdjehb  
,boro-silicate                          
,flint                                  
#aefbcdgdjaahgjjcdcbhijjhddgaajjficehd  
99,barium ,light                        
,flint                                  
#aehccdffjabeajjcfbbhhjjhhigaajjgbaegf  
,soda ,flint                            
#aedhbdehjaaiejjcdcbhgjjhebgacjjfijegg  
,light ,flint                           
#aedgbdehjaaifjjcdhbiajjhdhgjijjgjgeia  
,light ,flint                           
#aefajdcbjabiijjcgbbhgjjibggacjjggjeic  
,light ,flint                           
#aegfjdajjadjdjjdjbbhfjajjbgacjjhdjeih  
,light ,flint                           
#aeghgdjgjadbjjjdjdbhdjajafgaejjhdjeia  
,dense ,flint                           
#afaahcfijafegjjdgjbhdjaahggafjajjdfjf  
,dense ,flint                           
#afbadcfajagbbjjdiabhejabcagaejajdffjh  
,dense ,flint                           
#afbbecfjjagbijjdicbhfjabcfgaejajedfji  
,extra ,dense                           
,flint                                  

#afdficcgjaiagjjedabhejacgfgbjjaagjfee  
,densest ,flint                         
#agabibiijbchdjjfgjbhajagadghijaffafgh  
----------------------------------------
--------------------    ,in the table on
 p4 #bab the first column contains the  
ordinary trade  names by which the      
various types of glass are known4 ,these
 names1  while somewhat arbitrary1      
indicate in a rough way the chemical    
nature  of the glass concerned4 ,thus   
the word 8flint0 always implies a glass 
containing lead and therefore having a  
comparatively high refractive  index and
 low value of 1 while the word 8crown10 
originally applied  only to             
lime-silicate glasses1 is now used for  
all glass having a high  value of 4 ,in 
the next column of the table are given  
the refractive  indices of the glasses1 
while the third column contains the     
values of  4 ,it will be seen that the  
glasses are arranged in descending order
  of magnitude in respect of this       
constant4 ,an inspection of the figures 

in these two columns will reveal the    
fact that for the majority of  the      
glasses contained in this table the     
value of  decreases as the  refractive  
index increases4 ,the glasses which are 
an exception to this  rule are indicated
 by an 994 ,as a matter of fact this    
rule applied  to all glasses that were  
known or were at all events commercially
  available prior to the modern advances
 in optical glass manufacture  which    
were initiated by ,abb and ,schott of   
,jena4 ,it was ,abb's insight  into the 
requirements of optical instrument      
design that led him to  realise the     
importance of overcoming this limitation
 in the ratio  between the dispersive   
and refractive powers of glass4 ,with   
the  collaboration of ,schott he        
succeeded in producing a whole series of
  previously unknown varieties of       
optical glass in which the relation     
between n,d and  is not that of         
approximately simple inverse            
proportionality which holds for the     

older crown and flint-glasses4  ,most   
valuable and in many ways most typical  
of these new glasses are  those known as
 the 8barium crown0 glasses1 which      
combine the high  refractive index of a 
light flint or even a dense flint-glass 
with the  high  value of an ordinary    
crown glass4 ,it would lead too far into
  the subject of lens construction to   
explain in detail the possibility       
opened up to the optician by the use of 
these newer varieties of glass4  ,we    
must content ourselves with pointing out
 that the great forward  strides marked 
by the production of apochromatic       
microscope objectives1  of anastigmatic 
photographic lenses1 and the modern     
telescope  objectives are all based upon
 the employment of these new optical    
media2 and although optical glasses of  
these newer types are at the  present   
time produced in the optical glass      
manufactories of ,france and  ,england1 
in quality and quantity at least equal  
to the output of the  ,jena works       

themselves1 these great optical         
achievements stand as a  lasting        
monument to the pioneer work of ,abb and
 ,schott in this field4    ,the last six
 columns of the table of optical glasses
 given above  contain figures which     
define the manner in which each of the  
glasses  named distributes the various  
sections of the spectrum4 ,the columns  
,c-,d1 ,d-,f1 and ,f to ,g give as      
already indicated the differences       
between  the refractive indices for the 
,c1 ,d1 ,f and ,g lines respectively2   
the  smaller figures in the intermediate
 columns indicate the ratio of  each of 
these differences to the mean dispersion
 of the glass4 ,if all  kinds of glass  
distributed the various portions of the 
spectrum in the  same proportionate     
manner1 merely differing in the total   
amount of  dispersion produced1 these   
figures would be identically the same   
for  all glasses4 ,in actual fact it    
will be seen that the figures differ    
very widely from one type of glass to   

another4 ,a moment's consideration  will
 show that when two glasses are used in 
a lens for the purpose  of achromatising
 one another1 i4e41 when one is used to 
neutralise  the dispersion of the other1
 such achromatisation can only be       
perfect  if these ratios 7the relative  
partial dispersions7 are the same for   
both glasses4 ,to put the same statement
 in more concrete terms1 if  the        
spectrum produced by one glass is       
comparatively long-drawn out  at the red
 end1 relatively compressed at the blue 
end1 while in the  other glass the      
opposite relation holds between the two 
ends of the  dispersion spectrum1 it is 
evident that the two spectra can never  
be  superposed in such a way as to      
entirely neutralise one another--the    
spectrum produced by the one glass will 
predominate and leave a  residual colour
 at the blue end1 while the other will  
predominate  at the other end4 ,in the  
case of lenses achromatised by the use  
of  such glasses1 there will always be a

 slight fringe of colour around  the    
borders of the images which they        
produce4 ,one of the aims which  ,abb   
and ,schott set themselves in the       
production of new varieties of  optical 
glass was to obtain one or more pairs of
 glasses in which the  relative partial 
dispersions should be as nearly alike as
 possible  while the actual values of   
should differ as widely as possible4    
,some  success in this direction was at 
first claimed by the ,jena workers1  but
 unfortunately some of the most         
promising glasses in this respect  were 
found to be too unstable for practical  
use and had ultimately to  be abandoned4
 ,at the present time the only pair of  
really perfectly  achromatic glasses    
offered by the ,jena firm is that       
tabulated below1  and it will be seen   
that although the relative partial      
dispersions  are very closely alike1 the
  values of the two glasses only differ 
by #aj1 and at least one of these       
glasses is not readily obtainable in    

really satisfactory optical quality4 ,on
 the other hand1 practically  perfectly 
achromatised lenses 7generally known as 
8apochromatic07 have  been produced1    
especially by ,zeiss of ,jena1 for      
microscopic purposes1  by the careful   
selection of glasses suited to each     
other in this  respect4 ,such a solution
 of the problem is further facilitated  
by the  fact that in these lenses more  
than two varieties of glass can be  used
 to neutralise one another1 while a     
natural mineral 7fluorite7  is also     
employed4 ,from the glass-maker's point 
of view1 however1 the  problem of       
producing a satisfactory pair of glasses
 capable of entirely  achromatising one 
another has yet to be solved4           
----------------------------------------
---------------------                   
c-d         d-f         f-g      ,name4 
n,d     ,c-,f4  ,c-,d4  ---   ,d-,f4    
---   ,f-,g4 ---                        
c-f4        c-f4        c-f4            
----------------------------------------

---------------------    ,telescope     
,crown  #aebedfagjjheb #jjbej #bib      
#jjfjb #gjg #jjdhd #efh    ,telescope   
,flint  #aebaaeahjjajjgjjbig #bid #jjgaj
 #gje #jjegg #egc                       
----------------------------------------
---------------------    ,the table of  
optical glasses given above1 although   
brief as compared  with the lists issued
 by ,french and ,german optical         
glass-makers1  fairly covers the range  
of practically available glasses1 and a 
rapid  inspection will at once show how 
extremely limited this range really  is4
 ,thus the refractive index varies only 
between the limits #adi  and #aga1 and  
even if we admit as practical glasses   
such extreme  types--offered by some    
makers--as would extend this range to   
#adj  in one direction and to #ahj in   
the other1 this does not affect the     
present argument4 ,of course1 a glass of
 a refractive index as low  as #aj1 or  
even #aaj1 is not theoretically         
possible1 since the mere  density of any

 substance enters into the factors that 
affect its  refractive index1 and a     
glass having a density lower than that  
of  water 7whose refractive index is    
about #ac7 is scarcely conceivable4  ,in
 the other direction1 however1 the      
limits met with in the case of  glass   
are considerably exceeded by certain    
natural mineral substances4  ,thus the  
diamond has a refractive index of #bdb1 
while the garnets  show refractive      
indices from #age to #aha4 ,the values  
of  found in  the table of optical      
glasses are still more narrowly         
restricted1  lying between #fg and #bi1 
while such a mineral as fluorite shows a
  value of #ied4 ,these facts show that 
it is physically possible to  obtain    
transparent substances having optical   
properties lying far  beyond the limited
 range covered by our present optical   
glasses1 and  it scarcely needs showing 
that if such an extended range of       
materials  were available greatly       
increased possibilities would be opened 

up to  the designer of optical          
instruments4 ,it is consequently        
interesting  to inquire as to the actual
 causes which limit the range of optical
  glasses at present available4 ,it will
 be found that these limits are  set by 
the properties of glass itself4 ,while  
the more ordinary kinds  of glass1      
having average optical properties and   
showing dispersive  powers roughly      
conforming to the law of inverse        
proportionality with  refractive index  
which governs the older varieties of    
optical glass1  are chemically stable   
substances1 showing little tendency to  
undergo  either chemical changes or to  
crystallise during cooling1 the more    
extreme glasses exhibit these           
undesirable features to an increasing   
extent the more nearly the limit of our 
present range is approached4  ,as the   
chemical composition of a glass is      
8forced0 by the addition  of special    
substances intended to affect its       
optical properties in  an abnormal      

direction1 so the chemical and physical 
stability of the  glass is rapidly      
lessened4 ,the more extreme glasses1 in 
fact1 behave as  active chemical agents 
readily entering into reaction or       
combination  even with relatively inert 
substances in their environment--they   
act  vigorously upon the fire-clay      
vessels in which they are melted1 and   
they are readily attacked by acids1     
moisture or even warm air1 when in  the 
finished condition1 while many of them  
can only be prevented from  assuming the
 condition of a crystalline 7and opaque7
 agglomerate by  being rapidly cooled   
through certain critical ranges of      
temperature4  ,a limit to the           
possibility of production is set by     
these tendencies  when they exceed a    
certain amount--a point being reached   
where it  ceases to be practicable to   
overcome the tendency of the glass to   
self-destruction4 ,on the lines of our  
present glasses1 therefore1 it  does not
 appear hopeful to look for any         

considerable extension of the  range of 
our optical media4 ,on the other hand1  
as the known optical  properties of     
transparent crystalline minerals show1 a
 much greater  range of optical         
constants would become available if it  
were possible  to manufacture artificial
 mineral crystals of sufficient size and
  purity for optical purposes1 and the  
author believes that in this  direction 
progress in optical materials is        
ultimately bound to lie#a4        #a    
,see a ,paper by the present author on  
8,possible ,directions          of      
,progress in ,optical                   
,glass0--,proceedings of the ,optical   
,convention1 ,london1 #aije4    ,in     
addition to possessing the requisite    
optical constants1 a good  colour and   
perfect homogeneity1 certain other      
properties are essential  in good       
optical glass4 ,these are the general   
physical and chemical  qualities which  
are essential in all good glass1 but    
especially  emphasised by the fact that 

the requirements for optical glass are  
more  stringent than for any other      
variety of the material4 ,thus chemical 
stability is of the greatest importance1
 for the best lenses would  soon become 
useless if the action of atmospheric    
moisture were to  affect them           
appreciably--the polished surfaces would
 rapidly become  dull and the whole lens
 would soon be rendered useless4 ,the   
conditions  governing the chemical      
stability of glass and the methods of   
testing  this quality have already been 
indicated 7,chapters ,i4 and ,,ii474    
,the  harder varieties of optical glass1
 such as the glasses quoted in the      
above table under the names of 8,hard   
,crown0 and ,boro-,silicate ,crown1  are
 probably among the most durable and    
chemically resistant of all  varieties  
of glass1 but as we have already        
indicated1 when extreme  optical        
properties are required1 the necessary  
chemical composition of  the glass      
always entails a sacrifice of this great

 chemical stability1  until a limit is  
reached where valuable optical          
properties no longer  counterbalance the
 serious disadvantage of a chemical     
composition  which renders the glass    
liable to rapid disintegration4 ,in     
certain  special cases it is1 perhaps1  
possible to protect lenses made of such 
unstable glass by covering them with    
cemented-on lenses of stable  glass1 but
 this device entails concomitant        
limitations in the design  of the       
optical system and is1 therefore1 rarely
 used4 ,in any case1  however1 it is    
well for the lens-designer to consider  
the relative  stability of the glasses  
employed when arranging the order in    
which  they are to be used1 since it is 
obviously preferable to put a hard1     
durable glass on the outside of his     
system1 where it is most directly       
exposed to atmospheric moisture1 and is 
also subject to handling and  8cleaning0
 by inexpert hands4 ,this latter factor 
is a very important  one for the life of

 any lens4 ,in the first place1 a glass 
surface is  very seriously affected by  
the minute film of organic matter which 
is  left upon it when it has been       
touched with even a clean finger2 unless
  the glass is of the best quality in   
this respect1 such fingermarks  readily 
develop into iridescent spots and may   
even turn into black  stains4 ,particles
 of dust allowed to settle on the       
surface of the glass  will affect it in 
the same way1 so that the protection    
afforded by mere  mechanical enclosure  
in the tube of an instrument is of      
decided value  in preserving a glass    
surface4 ,it should1 however1 be noted  
that in  some instances the interior    
metal surfaces of optical instruments   
are  varnished with substances that give
 off vapours for a long time after  the 
instrument is completed1 and in that    
case the inside lenses are apt  to be   
tarnished in consequence4 ,on the other 
hand1 outside lenses are  also exposed  
to direct mechanical injury from        

handling and 8cleaning40  ,as far as the
 latter operation is concerned1 it      
frequently happens1  particularly in    
glasses containing soda1 that a slight  
surface dimming  is formed on the glass 
when it has been left in a more or less 
damp  place for a long time4 ,this      
dimming is chiefly due to the formation 
on  the surface of a great number of    
very minute crystals of carbonate of    
soda1 which are hard and sharp enough to
 scratch the glass itself if  rubbed    
about over it4 ,if such a lens be wiped 
with a dry cloth1 however  clean and    
soft1 the effect is a permanent injury  
to the polished  surface1 which could   
readily be avoided by first washing the 
lens with  clean water1 or even by using
 a wet cloth instead of a dry one for   
the  first wiping4    ,the mechanical   
hardness of the glass is an important   
factor in  determining its resistance to
 such injurious treatment or to the     
effects of accidental contact with hard1
 sharp bodies4 ,the subject of  the     

hardness of glass has already been      
discussed in a general way in  ,chapter 
,,ii41 and little remains to be added   
here4 ,broadly speaking1 a  high degree 
of hardness and a low refractive index  
are found together4  ,this statement is 
certainly true where any considerable   
difference of  hardness is considered1  
as1 for example1 in comparing a hard    
crown  glass with a dense flint2 but    
where the difference of refractive index
  or of density is small1 it is not at  
all certain that the lighter  glass will
 also be the harder4    ,the properties 
involved in the quality known as        
8hardness0 also affect  in a very marked
 manner the behaviour of glass when     
subjected to the  grinding and polishing
 processes4 ,the ease with which a good 
polish  can be obtained varies very much
 in different kinds of glass1 both the  
hardest and the softest glasses showing 
themselves difficult in this  respect4  
,the harder glasses are certainly less  
liable to accidental  scratching during 

the polishing operations1 and generally 
work in a  cleaner manner2 but the time 
required to produce a satisfactory      
polish  is much greater owing to the    
resistance to displacement offered by   
the  molecules4 ,both the speed of      
working and the pressure exerted during 
the polishing operation have1 in fact1  
to be carefully adapted to the  quality 
of the glass in this respect if the best
 possible results are  to be obtained4  
,another property which is essential in 
optical glass of the highest  quality is
 that of freedom from internal strains4 
,this subject will be  again referred to
 later in connection with the annealing 
processes used  in the manufacture of   
optical glass1 and it need only be      
mentioned here  that the presence of    
internal strain is readily recognised in
 glass1  by the aid of the polariscope4 
,perfectly annealed glass1 entirely free
  from internal strains1 produces no    
effect upon a beam of polarised  light  
passing through it1 while even slightly 

strained glass becomes  markedly        
doubly-refracting4 ,for many purposes of
 optics this double  refraction becomes 
undesirable or even inadmissible1       
especially as it  is accompanied by     
small variations in the effective index 
of refraction  of various portions of   
the mass of glass4 ,further1 if the     
amount of  double refraction observed is
 at all serious it indicates a state  of
 strain which may easily lead to the    
fracture of the whole piece1            
particularly when undergoing the earlier
 stages of the grinding process  or if  
exposed to shocks of any sort4 ,as will 
be seen below1 perfectly  annealed glass
 is obtainable1 but very special means  
are required for  its production1 and   
the optician should for that reason     
avoid making  unnecessarily extreme     
demands in this direction4 ,the very    
small  amount of double refraction      
frequently found in the better class of 
optical glass is entirely harmless for  
most purposes4          ,,chapter       

,,xiii4    ,,optical ,,glass4      ,the 
process of manufacturing the best       
qualities of optical glass may  be      
briefly described as consisting in      
obtaining a crucible full of  the purest
 and most homogeneous glass1 and then   
allowing it to cool  slowly and to      
solidify in situ4 ,from the resulting   
mass of glass  the best pieces are      
picked and moulded into the desired     
shape for  optical use4 ,it will be seen
 at once that in this process there is  
an essential difference from all others 
that have been described in  this       
book--viz41 that the glass is never     
removed from the melting-pot  while     
molten1 and that none of the operations 
of gathering1 pouring1  rolling1        
pressing1 or blowing are applied to it4 
,the reason for this  apparently        
irrational mode of procedure lies in the
 fact that the  perfect homogeneity     
essential for optical purposes can only 
be attained  by laborious means1 and can
 then only be retained if the glass is  

left  to solidify undisturbed2 any      
movement by the introduction of pipes or
  ladles would result in the            
contamination of the glass by stri and  
other objectionable defects4    ,the    
choice and proportion of raw materials  
used in the production  of any given    
quality of optical glass is governed by 
the chemical  composition which         
experiment has shown to be necessary to 
yield the  desired optical properties4  
,the composition of optical glass       
mixtures  cannot therefore be varied to 
suit the conditions of the furnace or   
to facilitate ready melting and fining1 
so that many of the usual  resources of 
the glass-maker cease to be available in
 the very case  where their aid would be
 most welcome to facilitate the         
production of  technically perfect      
glass4 ,on the other hand1 the          
manufacturer has a  certain amount of   
choice as to the precise form in which  
the various  chemical ingredients are to
 be introduced into the mixture1 and he 

makes his choice among oxides1          
carbonates1 nitrates1 and hydrates1     
according to the behaviour that it is   
desired to impart to the mass  during   
the earlier stages of fusion4 ,the state
 of purity in which the  various        
substances are commercially obtainable  
also enters largely into  the question1 
since the greatest possible degree of   
purity in the raw  materials is         
essential to the production of glass of 
good colour1 or  rather freedom from    
colour4    ,since homogeneity is so     
essential in the finished product1 very 
thorough mixing of the raw materials is 
necessary in the case of  optical glass1
 and the ingredients are for this       
purpose generally used  in a state of   
finer division than is necessary with   
other varieties  of glass4 ,as a rule   
the quantities of mixture of any one    
kind that  are required are not large   
enough to justify the use of mechanical 
appliances1 and very careful hand-mixing
 is carried out4    ,although it is     

quite possible to obtain successful     
meltings from  raw materials alone1 it  
is preferable to mix with these a       
certain  proportion of 8cullet0 or      
broken glass derived from a previous    
melting  of the same sort4 ,the broken  
glass used for this purpose is first    
carefully picked over for the purpose of
 rejecting pieces that contain  visible 
impurities1 although pieces showing stri
 are not usually  rejected4 ,the greater
 part of this cullet is generally mixed 
as evenly  as possible with the raw     
materials1 but a certain proportion is  
reserved for another purpose1 as        
explained below4    ,the furnaces used  
for the production of optical glass vary
 very  much in type in different works4 
,in some the old-fashioned conical  coal
 furnaces are still used1 the           
disadvantages attached to their         
employment being outweighed--in the     
opinion of the manufacturers--by  their 
simplicity and ease of regulation4 ,in  
other works gas-fired  regenerative     

furnaces of the most recent type are    
installed1 and in  these also optical   
glass of the highest quality can be     
produced4 ,as a  rule1 however1 optical 
glass furnaces differ from other        
pot-furnaces  found in glass-works in   
this respect--that the former are       
usually  constructed to receive one pot 
or crucible only1 while in other glass  
furnaces from four to twelve or even    
twenty pots are heated at the same      
time4 ,the reason for this restriction  
in the capacity of the furnaces  lies in
 the fact that since the mixtures used  
for optical glass cannot  be adjusted to
 suit the furnace1 the latter must be   
worked as far as  possible in such a way
 as to suit the mixture to be melted in 
it1 and  this implies that every pot    
will require its own adjustment of times
  and temperatures1 and this it would be
 difficult1 if not impossible1  to      
secure if more than one pot were heated 
in the same furnace4 ,it is  further to 
be remembered that the amount of care   

and attention required  during the      
melting of a pot of optical glass is out
 of all proportion  to that needed with 
other varieties1 so that little would be
 gained  by having a number of pots in  
one furnace1 since several sets of men  
would be required to tend them4    ,in  
addition to the single-pot melting      
furnace1 a very important part of  the  
equipment of the optical glass works is 
formed by a number of kilns  or ovens   
which are used for the preliminary      
heating1 and sometimes for  the final   
cooling of the various crucibles or     
pots4 ,similar kilns are  used in other 
branches of the industry1 but in those  
cases the pots1  once introduced into   
the furnace1 are expected to last for a 
number of  weeks1 or even months4 ,in   
optical glass manufacture1 on the other 
hand1  a pot is used once only1 so that 
fresh pots are required for every  new  
melting4 ,the kilns in which these pots 
are heated up before being  placed in   
the melting furnace are thus in very    

frequent use4 ,as a rule  they are      
simply fire-brick chambers provided with
 sufficient grate-room  and flue-space  
to be gradually raised to a red heat in 
the course of  four or five days1 while 
for the purpose of gradual cooling they 
can be  sealed up like the annealing    
kilns used for polished plate-glass4    
,the pots or crucibles in which optical 
glass is melted are usually of  the same
 shape as the covered pots used for     
flint-glass as illustrated  in ,fig4 #b4
 ,the optical glass pots1 however1 are  
made considerably  thinner in the wall1 
since they are not required to withstand
 the  prolonged action of molten glass  
in the same way as pots used for        
flint-glass manufacture4 ,on the other  
hand1 the fire-clays used for  this     
purpose must be chosen with special care
 so as to avoid any  contamination of   
the glass by iron or other impurities   
which might  reach the glass from the   
pot4 ,for the production of certain     
special  glasses1 in fact1 pots made of 

special materials are required1 since   
these glasses1 when molten1 produce a   
rapid chemical attack upon  ordinary    
fire-clays4 ,a certain amount of the    
aluminiferous material  of the pot is1  
in fact1 always introduced into the     
glass by the  gradual dissolving action 
of glass on fire-clay which we have     
already  described4 ,the glass          
contaminated with these aluminiferous   
substances  is generally more viscous   
than the rest of the contents of the    
pot1 and  therefore ordinarily remains  
more or less adherent to the walls of   
the  crucible1 but the inevitable       
disturbances which accompany the        
processes  of melting and fining lead to
 the dissemination of some of this      
viscous  glass through the entire pot in
 the form of veins or stri1 which are   
only removed during the stirring        
process4 ,on the other hand1 more of    
this viscous glass is constantly being  
formed so long as the glass  remains    
molten1 and if disturbances are not     

sufficiently avoided during  the later  
stages of the process fresh veins may   
easily be formed4    ,the actual        
operations of producing a melting of    
optical glass begin by  the gradual     
heating-up of the pot in the kiln just  
described4 ,when the  pot has reached a 
full red heat the doors of the kiln are 
opened and  the pot drawn out by means  
of a long heavy iron fork running on    
wheels2  this implement is run into the 
mouth of the kiln and the tines of the  
fork are pushed under the pot1 and the  
latter is then readily lifted up  and   
withdrawn from the kiln4 ,meanwhile the 
temperature of the furnace  has been    
regulated in such a manner as to be     
approximately equal to  that attained by
 the heating kiln1 so that the pot1 when
 transferred  as rapidly as possible    
from the kiln to the furnace1 is not    
subjected  to any very sudden heating2  
were it attempted to place the new pot  
in  a furnace at full melting heat the  
fire-clay would shrink rapidly and  the 

entire vessel would fall to pieces4     
,even under the best conditions  it is  
not possible to avoid the occasional    
failure of a pot by cracking  either at 
this or a slightly later stage of the   
process4 ,the latter  occurrence is apt 
to be particularly disastrous1 as the   
pot may then be  full of molten glass1  
which runs out and is lost4    ,as soon 
as the empty pot has been put into      
place1 the melting furnace  is carefully
 sealed up by means of temporary work   
built of large  fire-bricks1 the whole  
being so arranged that the mouth of the 
hood of  the pot is left accessible by  
means of an aperture in the temporary   
furnace wall4 ,this aperture can be     
closed by one or more slabs of          
fire-clay1 and when these are removed an
 opening is left by which the  raw      
materials are introduced1 and through   
which the other manipulations  are      
carried out4    ,when this stage of the 
process is reached1 the wagons          
containing the  mixed raw materials are 

usually wheeled into place in front of  
the  furnace1 but the introduction of   
the materials themselves into the  pot  
is not begun until several hours later1 
when the furnace has been  vigorously   
heated and an approach to the melting   
heat has been attained4    ,when the    
furnace and pot have attained the       
necessary temperature1  but before the  
raw materials are introduced1 a small   
quantity of the  cullet1 which has been 
reserved for this purpose1 is thrown    
into the  pot and allowed time to melt1 
and then only is the first charge of    
mixture put into the pot4 ,the object of
 this proceeding is to coat the  bottom 
and part of the walls of the pot with a 
layer of molten glass  which serves to  
protect it from the chemical and        
physical attack of  the raw materials   
during the violent action which takes   
place when they  are first exposed to   
the furnace heat4    ,the gradual       
filling of the pot with molten glass is 
now carried out by  the introduction of 

successive charges of raw material2 as  
the mixture  not only occupies more     
space than the glass it forms1 but also 
froths  up a good deal during melting1  
the quantities introduced each time     
must be carefully adjusted so as to     
avoid an overflow of half-melted  glass 
through the mouth of the pot4 ,as the   
pot is more and more nearly  filled1 the
 space left for the raw materials is    
proportionately  diminished1 and the    
later charges are therefore much smaller
 than the  first few4    ,when1 finally1
 sufficient material has been introduced
 to fill the pot  completely1 the next  
stage of the process commences4 ,when   
the last  charge of raw materials has   
melted1 the glass in the pot is left in 
the  state of a more or less viscous    
liquid full of bubbles of all sizes2  it
 is essential that these bubbles should 
escape and leave the glass  pure and    
8fine10 and this result can only be     
achieved by raising the  temperature of 
the furnace and allowing the glass to   

become more fluid1  while the rise of   
temperature also causes the bubbles to  
expand owing  to the expansion of the   
gas contained in them4 ,in both ways1   
rise of  temperature facilitates the    
escape of the bubbles1 and the furnace  
is therefore heated to the full1 and    
this extreme heat is maintained  until  
the glass is free from bubbles4 ,in the 
case of the more fusible  glasses the   
temperature required for this purpose is
 not excessively  high1 and1 indeed1 in 
the case of these glasses care is taken 
to avoid  too high a temperature1 as it 
entails other disadvantages4 ,in the    
case  of the harder crown glasses1      
however1 the difficulty lies in         
producing  an adequately high           
temperature without at the same time    
endangering  the life of furnace and    
crucible4 ,the difficulty of freeing the
 molten  glass from bubbles constitutes 
one of the causes that limit the range  
of our optical glasses in one           
direction--still harder glasses could be

  melted1 but it would not be feasible  
to maintain a temperature high  enough  
to render them fluid enough to 8fine40  
,in the case of other kinds of glass1   
again1 it becomes impossible to         
entirely remove the bubbles from the    
molten mass even when very hot and  very
 fluid4 ,the exact cause is not known1  
but in some kinds of glass  the bubbles 
formed are so minute that even when the 
glass is perfectly  mobile the bubbles  
show no tendency to escape1 while in    
other kinds of  glass there appears to  
be a steady evolution of minute bubbles 
as soon  as the temperature is raised   
with a view to removing those already in
  the glass4 ,as this property attaches 
to some of the most valuable of  the    
newer varieties of optical glass1       
opticians and the public have  learnt to
 put up with the presence of minute     
bubbles in the lenses  and prisms made  
of these glasses4 ,these bubbles are1   
however1 very  minute and do not        
interfere with the optical performance  

of the lenses1  c41 except to the extent
 of arresting and scattering the very   
small  proportion of light that falls   
upon them2 their presence is therefore  
to be regarded as a small but           
unavoidable drawback to the use of      
glasses which offer advantages that     
completely outweigh this defect4        
,returning to the melting process1 we   
find that the extreme heating  required 
for the purpose of 8fining0 the glass is
 continued for a  considerable period of
 time1 as long as thirty hours in some  
cases1  the glass being examined from   
time to time to test its condition as   
regards freedom from bubbles4 ,this is  
done by taking a small sample  of glass 
out of the pot and examining it to see  
if it still contains  bubbles4 ,in some 
works this test is made by taking up a  
very small  gathering of glass on the   
end of a small pipe and blowing it into 
a  spherical flask2 on looking at such a
 flask in a suitable light the  presence
 of even minute bubbles is readily      

detected4 ,in other works a  simpler    
process is adopted1 a small quantity of 
glass being ladled out  of the pot on   
the surface of a flat iron rod4 ,it is  
allowed to cool  on the rod1 and when   
pushed off forms a small bar of glass   
some eight  or ten inches long and about
 an inch wide2 in this also the presence
  of bubbles is easily detected4 ,these 
test pieces are known among             
glass-makers as 8proofs40    ,when      
proofs1 taken as just described1 have   
shown that the glass is free  from      
bubbles1 the extreme heat of the furnace
 is allowed to abate1  and the fire-clay
 slabs in front of the mouth of the pot 
are removed4  ,the next step is that of 
skimming the surface of the glass4      
,since  most of the materials liable to 
contaminate the contents of a pot are   
specifically lighter than the molten    
glass1 they will be found floating  on  
the surface1 and the surface glass is   
therefore removed with a  view to       
ridding the glass of anything that may  

have been accidentally  introduced and  
that has not melted and become          
incorporated with the  molten mass4     
,the next steps in the process are those
 of stirring the molten glass  with a   
view to rendering it homogeneous and    
free from stri4 ,the  stirrer used for  
this purpose is usually a cylinder of   
fire-clay1  previously burnt and heated4
 ,this is provided with a deep square   
hole  in one end1 and it is held at     
first by means of a small iron bar      
passed into this hole4 ,by this means   
the red-hot cylinder of fire-clay  is   
introduced into the open mouth of the   
pot1 and when it has attained           
approximately the temperature of the    
molten glass it is dipped into  the     
glass itself1 in which it ultimately    
floats4 ,when stirring is to  begin1 the
 square1 down-turned end of a long iron 
bar is introduced  into the             
corresponding square hole in the upper  
end of the stirrer1 and  by this means  
the fire-clay cylinder is held in a     

vertical position in  the glass and     
given the steady rotatory movement which
 constitutes the  stirring process4 ,for
 this purpose the long iron bar just    
mentioned is  made to pass over a       
swivel-wheel1 while a workman moves it  
steadily by  the aid of a large wooden  
handle4 ,this operation is always       
laborious  and trying2 the workman is   
necessarily exposed to the intense heat 
radiated from the open mouth of the     
crucible1 so that men have to  relieve  
each other at frequent intervals4       
,during the earlier stages of the       
stirring process the glass is very  hot 
and mobile1 but the stirring is         
continued1 with short intervals1  until 
the glass is so cold and stiff that the 
stirrer can scarcely be  moved in it at 
all1 so that the work of moving the     
stirrer becomes  heavy towards the end  
of the operation4 ,the actual amount of 
stirring  required varies according to  
the nature of the glass1 and the size   
of the pot or crucible in question4     

,some meltings are found to be          
satisfactory after as little as four    
hours' stirring1 while for others  as   
much as #bj hours are required4    ,when
 the glass has stiffened to such an     
extent that it is no longer  possible to
 continue the stirring1 preparations are
 made for the final  cooling-down of the
 pot of glass4 ,the fire-clay stirrer is
 sometimes  withdrawn from the glass1   
but this is laborious1 and entails      
dragging  a considerable quantity of    
glass out of the pot with the clay      
cylinder2  more usually1 therefore1 the 
stirrer is simply left embedded in the  
glass4    ,the next object to be        
accomplished is that of cooling the     
glass as  rapidly as safety will permit 
until it has become definitely          
8set0--the  purpose being to prevent the
 recrudescence of stri as a result of   
convection currents or other causes     
which might disturb the homogeneity  of 
the glass4 ,this rapid cooling is       
obtained in various ways2 in one  mode  

of procedure the furnace is so arranged 
that by opening a number  of apertures  
provided for the purpose cold air is    
drawn in and the pot  and its contents  
chilled thereby without being moved4    
,this method has  the advantage that the
 pot containing the viscous glass is    
never moved  or disturbed in any way1   
but on the other hand the cooling which 
can be  effected within the furnace     
itself is never very rapid1 and the     
furnace  as well as the pot is chilled4 
,further when the glass has been chilled
  down to a certain point this rapid    
rate of cooling must be arrested1 as    
otherwise the whole contents of the pot 
would crack and splinter into  minute   
fragments4 ,where the pot has been left 
in the furnace this can  only be done by
 sealing up the whole furnace with      
temporary brickwork  and lutings of     
fire-clay1 leaving it to act as an      
annealing kiln until  the glass has     
cooled down approximately to the        
ordinary temperature1 a  process that   

occupies a period of from one to two    
weeks according to  the size of the     
melting4 ,such enforced idleness of a   
melting furnace  is of course very      
undesirable from an economical point of 
view1 and it  is generally avoided by   
adopting the alternative method of      
drawing  the pot bodily out of the      
furnace as soon as the stirring         
operation is  ended4 ,for this purpose  
the temporary brickwork forming the     
front of  the furnace is broken down1   
and with the aid of a long crow-bar the 
bottom of the pot is levered up from the
 bed or siege of the furnace  to which  
it adheres strongly1 being bound down by
 the sticky viscous  mass of molten     
glass and half-molten fire-clay which   
always accumulates  on the bed of the   
furnace4 ,the pot being temporarily held
 up by the  insertion of a piece of     
fire-brick1 the tines of a long and     
heavy  iron fork running on a massive   
iron truck are introduced beneath the   
pot2 an iron band provided with long    

handles is then passed around  the pot1 
and the latter is then drawn forward by 
the aid of suitable  pulley blocks4 ,the
 tines of the fork are then raised1 and 
the pot is  wheeled out of the furnace  
and deposited upon a suitable support4  
,here  it is allowed to cool to the     
requisite extent1 when it is again      
picked  up on the tines of the fork and 
deposited in an annealing kiln which    
has been previously warmed to a suitable
 temperature4 ,it will be seen  that    
this handling of a heavy mass of        
intensely hot material involves  much   
labour1 while there is also a risk of   
losing the glass if the pot  should     
break before the glass has set          
sufficiently4 ,every care is taken  to  
prevent such an accident1 the pot being 
wrapped round with chains or  otherwise 
supported in such a way that a small    
crack could not readily  develop into a 
large gap4    ,when such a melting of   
glass has cooled sufficiently1 either in
 the  furnace or in the annealing kiln1 

to be safely handled1 the whole  pot is 
drawn out1 and the fire-clay shell1     
which is generally found  cracked into  
many pieces1 is broken away by the aid  
of a hammer4 ,under  favourable         
circumstances the whole of the glass may
 have cooled intact  as one solid lump  
sometimes weighing over half a ton4     
,unless special  care is taken1 however1
 it is more usual to find the glass more
 or less  fissured1 a number of large   
lumps being accompanied by a great mass 
of  small fragments4 ,these are now     
picked over1 and all those which are    
free from visible imperfections or which
 can be readily detached from  such     
imperfections by the aid of a chipping  
hammer are put upon one  side for       
further treatment4    ,the next step of 
this treatment consists in moulding the 
rough broken  lump into the shape of    
plates1 blocks1 or discs according to   
the  purpose for which the glass may be 
required by the optician4 ,the  plant   
used for the moulding process varies    

widely1 but in all cases  the operation 
consists in gradually heating the glass 
in a suitable  kiln until it is soft    
enough to adapt itself to the shape of  
the  mould provided for the purpose4 ,in
 some cases these moulds are made  of   
fire-clay1 and the glass is simply      
allowed to settle into them by  its own 
weight2 in other cases iron moulds are  
used1 and the glass  is worked into them
 by the aid of gentle pressure from wood
 or metal  moulding tools4 ,in yet other
 cases1 particularly where the glass is 
required in the form of small thin discs
 or where it is to be formed  into the  
approximate shape of concave or convex  
lenses1 the aid of a  press is sometimes
 invoked4    ,in all cases the moulding 
process is followed by the final        
annealing1  which consists in cooling   
the glass very gradually from the red   
heat at  which it has been moulded1 down
 to the ordinary temperature4 ,the      
length  of time occupied by such cooling
 depends very much upon the size of the 

object and also upon the degree of      
refinement to which it is necessary  to 
carry the removal of small internal     
strains in the glass4 ,for many         
purposes it is sufficient to allow it to
 cool down naturally in a  large kiln in
 the course of six or eight days4 ,for  
special purposes1  however1 where       
perfect freedom from double refraction  
is demanded1 much  greater refinements  
are required1 and special annealing     
kilns1 whose  temperature can be        
accurately regulated and maintained1 are
 employed4  ,in these the annealing     
operation can be carried out so         
gradually that a  rate of cooling in    
which a fall of #a ,c4 occupies several 
hours can be  maintained1 so that very  
perfectly annealed glass can be produced
 even  in discs or blocks of large size4
    ,when removed from the annealing    
kiln the plates or discs of optical     
glass are taken to a grinding or        
polishing workshop1 where certain of    
their faces or edges are ground and     

polished in such a way as to permit  of 
the examination of the glass for        
bubbles1 stri and other defects  in the 
manner indicated in the previous        
chapter4 ,as the amount of  sorting that
 can be done while the glass is still in
 rough fragments  is necessarily very   
limited1 it follows that a considerable 
proportion  of the glass which has been 
moulded and annealed must be rejected as
  useless when thus finally examined4 ,a
 yield of perfect optical glass1        
amounting to #aj or at most #bj per     
cent4 of the total contents of each     
pot1 is therefore all that can be       
expected1 and smaller yields are by  no 
means infrequent--a consideration that  
will serve to explain the  relatively   
high price of optical as compared with  
other varieties of  glass4    ,a        
consideration of the various factors    
that are involved in the  production of 
a piece of perfect optical glass will   
make it apparent  that the cost and     
difficulty of its production increases  

rapidly with  the weight of the piece to
 be produced1 so that it is not         
surprising to  find that the price of   
very large discs of perfect optical     
glass such  as those required for large 
astronomical telescopes1 reaches figures
  which become prohibitive when very    
large sizes are considered4 ,thus1      
while it is quite possible to obtain say
 #ajj pounds of good glass from  a      
single melting if the glass is to be    
used in the form of pieces not  weighing
 more than five or six pounds each1 it  
is only rarely that a  single block of  
perfect glass can be found weighing #ajj
 pounds4 ,in the  former case the best  
pieces can be picked1 the worst defects 
can be  eliminated by chipping the rough
 fragments1 and at a later stage other  
defective pieces can be cut off or      
ground away2 not so where a large       
single block is required4 ,a single fine
 vein1 perhaps too small to be  visible 
to the unaided eye1 may be found to run 
through a whole block  in such a way    

that it cannot be removed without       
breaking or cutting up  the whole piece1
 and it will be seen that the frequency 
with which this  is liable to occur     
increases with the volume of the piece  
required4  ,the difficulties of         
re-heating and moulding are also        
increased  enormously with the size of  
the individual pieces of glass that have
  to be dealt with1 and where very large
 pieces have to be heated and  cooled   
accidental breakage becomes a serious   
risk4 ,in view of these  difficulties it
 is not surprising to find that the     
dimensions of our  astronomical         
refractors appear to have approached    
their limit1 but  rather are we led to  
admiration of the skill and enterprise  
that has  pushed this limit so far as to
 produce discs of optical glass         
measuring  as much as one metre in      
diameter4          ,,chapter ,,xiv4     
,,miscellaneous ,,products4      ,the   
field of glass-manufacture is so wide   
and the number and variety of  its      

products so great1 that in the limited  
compass of this volume it is  impossible
 to fully enumerate them all2 there are1
 however1 a certain  number of these    
products which1 while of considerable   
importance in  themselves1 yet do not   
fall readily under any of the headings  
of the  preceding chapters4 ,a short    
space will therefore be devoted to some 
of  these in this place4    ,glass      
,tubing4--,a widely-useful form of glass
 is that of tubes of  all sizes and     
shapes1 ranging from the fine capillary 
tubes used in  the construction of      
thermometers to the heavy drawn or      
pressed pipes  that have been employed  
for drainage and other purposes4 ,the   
process  of manufacture employed varies 
according to the size and nature of  the
 tube that is required4 ,thus           
lamp-chimneys are really a variety of   
tube1 used in short lengths and made of 
relatively wide diameter and  thin      
walls4 ,these are not1 however1         
ordinarily made in the form of long     

tubes cut into short sections1 but--as  
has already been mentioned--they  are   
blown into moulds in the form of a      
thin-walled cylindrical bottle1  whose  
neck and bottom are subsequently        
removed4 ,by this process the  various  
forms of chimneys for oil-lamps1 having 
contractions at  certain parts of their 
length1 can be readily produced4    ,the
 articles more strictly described as    
glass tubes are1 however1  produced by a
 process in which actual blowing plays  
only a very minor  part4 ,a gathering of
 suitable size is taken up on a pipe1 a 
very small  interior hollow space is    
produced by blowing into the pipe1 and  
then  the gathering is elongated by     
swinging the pipe in a suitable manner4 
,the end of the elongated gathering     
furthest from the pipe is then  attached
 to a rod or 8pontil0 held by a second  
workman1 and the two men  then proceed  
to move apart1 drawing out the gathering
 of glass between  them4 ,according to  
the bore and thickness of wall required 

in the tube1  the men regulate the speed
 at which they move apart2 the thinner  
the  tube is to be the more rapidly they
 move1 in order to draw the glass  out  
to a sufficient extent before it hardens
 too much4 ,the rate of  drawing must1  
of course1 also be adapted to the nature
 of the glass in  question1 and this    
will vary very widely4 ,for the         
production of the  smaller bored tubes  
the men find it necessary to separate at
 a smart  trot1 while heavy tubes such  
as are used for gauge-glasses1 are drawn
  of hard glass by a very gradual       
movement4 ,in some cases1 the setting   
of the glass1 when the tube has attained
 the desired thickness1 is  hastened by 
the aid of an air-blast1 or--in more    
primitive fashion--by  boys waving fans 
over the hot glass4 ,in any case1       
suitable troughs are  provided for      
receiving the tube when drawn1 and from 
these the tube is  taken to an annealing
 kiln to undergo this necessary         
operation4    ,the glass used for the   

production of tubing varies very widely 
according to the purpose for which the  
product is intended4 ,almost  any of the
 more usual varieties of glass can be   
readily drawn out  into tubes1 and the  
choice of the kind of glass to be       
employed is  therefore left to other    
considerations4 ,tubing required for the
 use of  the lamp-worker1 i4e41 for the 
production of instruments or other      
articles by the aid of the              
glass-blower's blow-pipe1 must have the 
capacity of undergoing repeated cooling 
and heating without showing  signs of   
crystallisation 7devitrification71 while
 reasonable softness  in the flame is   
also required4 ,for this purpose1 also1 
glass containing  lead is not           
admissible1 since this would blacken    
under the influence  of the blow-pipe   
flame4 ,soda-lime glasses rather rich in
 alkali are  most frequently used for   
these purposes2 one consequence of their
  chemical composition1 however1 is that
 such glass tends to undergo            

decomposition when stored for any length
 of time1 more especially in  damp      
places4 ,frequently this decomposition  
only manifests itself on  heating the   
glass in a flame1 when it either flies  
to pieces or turns  dull and rough on   
the surface4 ,such glass is sometimes   
said to have  8devitrified10 but this is
 not really the case2 what has actually 
happened is that the atmospheric        
moisture has penetrated for some  little
 distance into the thickness of the     
glass1 probably hydrating  some of the  
silica2 on heating1 this moisture is    
driven off1 with the  result that either
 a few large cracks1 or innumerable fine
 ones1 are  formed4 ,in the latter case 
these do not readily disappear when the 
glass is softened and the dull1 rough   
surface is left at the end of the       
operation4    ,for purposes where the   
glass is to be exposed to high          
temperatures1  tubing made of so-called 
8hard glass0 is employed4 ,this is      
practically  a form of ,bohemian crystal

 glass1 the chemical composition being  
that of a potash-lime glass rather rich 
in lime4 ,to some extent this  ,bohemian
 hard glass has been superseded by the  
special 8combustion  tube0 glass        
manufactured by ,schott1 of ,jena4 ,this
 is a very refractory  borosilicate     
glass containing some magnesia2 it      
certainly withstands  higher            
temperatures than hard ,bohemian glass1 
and is rather less  sensitive to changes
 of temperature2 on the other hand1 it  
has the  inconvenient property of       
showing a white opalescence when it has 
once  been heated1 and this1 after a    
time1 renders the glass completely      
opaque4    ,for many purposes1 where    
heat-resisting qualities are chiefly    
required1  ordinary glass has now a     
formidable rival in the shape of        
vitrified  silica1 which is now         
available as a satisfactory commercial  
product4  ,this substance offers the    
great advantage that for most ordinary  
purposes it may be regarded as entirely 

infusible1 since the intense  heat of an
 oxygen-fed flame is required to soften 
or melt the silica4  ,further1 vitreous 
silica has an extremely low coefficient 
of expansion1  and appears also to have 
a rather high coefficient of thermal    
conductivity4 ,the result is that tubes 
and other articles made of  this        
material possess an astonishing amount  
of thermal endurance 7see  ,chapter     
,,ii474    ,a white-hot tube or rod of  
this material can be plunged into cold  
water  with impunity1 and no special    
care need be exercised in heating or    
cooling articles made of this substance1
 unless articles of great size  and     
thickness are involved1 and even with   
these only little caution  is needed4   
,the only disadvantages which must be   
balanced against the  great advantages  
just named lie in the relatively high   
cost of the  articles and in their      
somewhat sensitive behaviour to certain 
chemical  influences4 ,as regards cost1 
vitreous silica is at present available 

in  two different forms2 in the first   
form it resembles ordinary glass very   
closely in appearance1 the shape and    
finish of the tubes and vessels  of this
 kind having undergone very great       
improvements quite recently4  ,this     
silica glass has1 in fact1 been worked  
from molten silica in a way  more or    
less analogous to that in which ordinary
 glass is worked1 the  great extra cost 
of the silica ware being due1 in part1  
at all events1  to the extremely high   
temperature required for melting and    
working this  material2 ordinarily1 in  
the production of the class of silica   
ware  now referred to1 this heat is     
generated by the liberal--and therefore 
expensive--use of oxygen gas4 ,in great 
contrast to this glass-like1            
transparent silica ware is the other    
form in which this material is          
available4 ,this is a series of products
 obtained from the fusion of  silica in 
special forms of electric furnace2 in   
this ware the minute  bubbles so readily

 formed in the fusion of all forms of   
quartz are not  even partially          
eliminated1 and by their presence--often
 in the form of  long-drawn-out1        
capillary hollows--they impart to this  
ware its very  characteristic milky     
appearance4 ,the price of this product1 
which is  mostly used in the form of    
tubes1 although such articles as basins1
  crucibles1 and even muffles of        
considerable size are available1 is much
  lower than that of the transparent    
variety1 being in fact decidedly  lower 
than that of the best porcelain2 on the 
other hand1 even this  price is         
considerably above that of the best     
glass tubing4    ,apart from the        
question of cost1 the use of silica ware
 is further  limited by its             
sensitiveness to all forms of basic     
materials4 ,thus  alkaline solutions    
cannot be allowed to come into contact  
with this  substance1 since they attack 
it vigorously1 especially when warm4 ,at
  high temperatures all basic materials 

produce a rapid attack on silica  ware1 
the silica1 in fact1 behaving as a      
strongly acid body at and  above a red  
heat4 ,the attack which occurs when such
 a substance as iron  or copper oxide is
 allowed to come into contact with      
heated vitrified  silica is1 in fact1 so
 rapid that a tube is completely        
destroyed in  a few minutes1 the        
formation of silicates resulting in the 
cracking  and disintegration of the     
whole piece4 ,while1 therefore1 silica  
ware1  especially in its cheaper forms1 
undoubtedly possesses great advantages  
and possibilities1 its use must be      
carried on with careful reference to    
its chemical nature4    ,vitreous       
silica1 in addition to the uses and     
advantages just named1  has also an     
interest from the optical point of view2
 this arises from  the fact that it is  
transparent to short 7ultra-violet7     
light waves  to which all ordinary      
varieties of glass are completely       
opaque4 ,quite  recently1 the ,jena     

works have produced special glasses     
which are more  transparent to these    
ultra-violet rays than ordinary glass1  
but even  these fall far short of silica
 in this respect4 ,this property of     
transparence to ultra-violet light is   
utilised in two widely different        
directions4 ,one of these is in the     
production of ultra-violet light  when  
required for medical or other special   
purposes2 a most energetic  source of   
such rays is available by the use of    
tubes of vitrified  silica within which 
the mercury-vapour arc is produced4 ,in 
another  direction the employment of    
quartz lenses makes it possible to take 
advantage of the optical properties of  
ultra-violet light in connection  with  
microscopy2 for the purpose of          
constructing a perfect optical  system1 
crystalline quartz would be useless1    
since its property of  double refraction
 would interfere hopelessly with the    
performance of  the lenses4 ,this is now
 overcome by the use of vitreous silica 

lenses1  in the case of the             
8ultra-violet microscope10 as made by   
,carl ,zeiss1  of ,jena4 ,so far1       
however1 it has only been possible to   
produce quite  small pieces of vitreous 
silica sufficiently free from bubbles to
 be  used for optical purposes4 ,the    
great difficulty lies not so much in    
merely melting the quartz down as in    
freeing it from the air-bubbles         
enclosed within it2 the course usually  
adopted with glass1 of raising  the     
temperature and allowing the bubbles to 
rise to the surface1  becomes impossible
 in this case1 because the silica itself
 begins to  vapourise and even to boil  
vigorously at temperatures not very far 
above its melting point4 ,quite         
recently1 however1 two ,american workers
  have claimed to be able to overcome   
this difficulty by the use of both      
vacuum and high pressure applied at the 
earlier and later stages of  the fusion 
process respectively1 so that it may    
shortly be possible to  produce vitreous

 silica in large and perfectly clear    
blocks4    ,we have already indicated   
that glass tubing and rod form the basis
  upon which the glass-worker1 with the 
aid of the blow-pipe or 8lamp10         
fashions his productions1 which1 of     
course1 include a great number of       
scientific instruments and appliances   
used more especially in the field  of   
chemistry4 ,in another direction also   
glass tubing serves as a basis  for a   
branch of the glass industry2 this is   
the manufacture of certain  classes of  
glass beads1 which are formed by cutting
 up a heated glass  tube of suitable    
diameter and colour into short1 more or 
less spherical  sections4 ,in some cases
 the colour of the beads is secured by  
using  glass of the desired tint1 but in
 other cases the beads are made of      
colourless glass1 and a colouring       
substance is placed in the interior  of 
the bead4    ,solid glass rods are also 
employed for a variety of purposes2     
their  mode of manufacture is exactly   

analogous to that of tubing1 except     
that the gathering is drawn out without 
having first had a hollow  space        
produced at its centre by the blower4   
,in its most attenuated  form glass rod 
becomes glass thread or fibre2 this is  
produced by  drawing hot glass very     
rapidly1 the resulting thread being     
wound on a  large wheel4 ,at one time   
this material found considerable use1   
since  it was found possible to spin and
 weave the thinnest glass fibres into   
fabrics which could be used for dress   
purposes4 ,it is not1 however1  to be   
regretted that this fashion has neither 
extended nor survived1  since it was    
certainly liable to produce serious     
injury to health4  ,it is a well-known  
fact that there are few more injurious  
or even  dangerous substances to be     
inhaled into the human throat and lungs 
than  finely-divided glass2 glass fibre1
 moreover1 when subjected to constant   
bending and wear1 is bound to undergo   
frequent fracture1 and the  atmosphere  

of a ball-room1 for example1 in which   
several such dresses  were worn would   
soon be contaminated with innumerable   
fine1 sharp  particles of glass which   
would produce an injurious effect on    
those  inhaling them4 ,at the present   
time glass fibre is used for little else
  than the 8glass wool0 required for    
certain special purposes in chemical    
laboratories4    ,fused quartz or silica
 fibres1 of extreme tenuity1 but of     
relatively  very great strength1 are    
employed in many scientific instruments1
 where  their extreme lightness and     
perfect elasticity and freedom from what
  is known as 8elastic fatigue0 renders 
them of very great value4 ,these  fibres
 are not drawn from a mass of molten    
silica1 as is done with  glass1 but are 
produced by attaching a nail or bolt to 
a small bead of  fused silica produced  
by the aid of an oxygen-fed blowpipe2   
the nail or  bolt is then suddenly shot 
away down a long passage or similar     
space by  means of a cross-bow1 drawing 

a very fine fibre of silica with it2 the
  most difficult part of this operation1
 however1 consists in finding and       
handling the fibres thus produced4      
,artificial ,gems4--,the fact that      
pieces of suitably-coloured glass can   
be made to show a superficial1 but      
sometimes more or less deceptive1       
resemblance to precious stones1 has led 
to the manufacture of imitation  jewels 
of all descriptions4 ,the glass used for
 this purpose is usually  a very dense  
flint-glass whose high refractive index 
facilitates the  imitation which is     
aimed at4 ,the external shapes of gems  
are1 of  course1 readily imitated by    
cutting and grinding the glass1 while   
the  requisite colours are attainable by
 means of the colouring materials       
described in ,chapter ,,xi4 ,to a casual
 observer the difference in sparkle  and
 brilliance which arises from the       
difference between the refractive  index
 of the heavy flint-glass 7about #ah7   
and that of minerals  7which ranges from

 #ag to #bb7 is not readily apparent1   
but closer  examination will at once    
reveal the difference4 ,the             
determination of  the optical constants 
by means of a refractometer would at    
once reveal  the true character of the  
imitation1 but an even readier test is  
that  of hardness4 ,the dense           
flint-glass is naturally soft1 and is   
readily  scratched by most of the harder
 minerals1 while the precious stones1   
more particularly garnets1 rubies and   
diamonds1 are very hard4 ,if an  attempt
 is made to scratch an ordinary sheet of
 window-glass1 it will  be found that   
most real precious stones will do so    
readily1 while  flint-glass imitations  
will fail to make more than a slight    
mark1  which is more smear than scratch4
 ,the test by determining the specific  
gravity is also obviously applicable1   
since the flint-glass will  readily     
betray its presence by its high density 
7over #d74    ,in quite a different     
class from the imitation gems made of   

cut  flint-glass are the artificial     
gems1 which in nature and composition   
are exact reproductions of natural gems1
 but which have been produced  by       
artificial processes4 ,as far as the    
writer is aware these are  only found in
 any large numbers in the case of the   
ruby1 but in that  case1 at all events1 
it is said that the production of the   
artificial  crystals is at least as     
costly as the purchase of the natural   
stones4  ,there can1 however1 be very   
little doubt that as the processes of   
fusion and crystallisation become better
 known and understood1 and the          
chemistry of silicate minerals is       
developed1 the artificial production  of
 mineral crystals in1 at all events1    
moderate sizes will become  increasingly
 possible2 it is even to be hoped that  
their production  will be so far        
perfected as to place their really      
valuable properties  at the service of  
man4    ,chilled ,glass4--,in all the   
processes of glass manufacture described

  in the present book1 annealing has    
always played an important part4 ,the   
glass1 after it has undergone its last  
treatment under the influence  of heat1 
is subjected to a gradual cooling       
process with the object of  freeing it  
from the internal strains which it would
 otherwise retain1  and which would1    
ordinarily1 endanger its existence and  
interfere with  its use4 ,it is1        
however1 well known that surfaces of    
glass subjected  to such internal       
strains as result in a compressive      
stress on the  glass near the surface1  
are less liable to injury1 and are      
apparently  stronger than when the glass
 is annealed and the stresses are       
removed4  ,on the other hand1 glass     
surfaces under tension are extremely    
delicate  and fragile4 ,in some         
respects1 therefore1 glass which has not
 been  annealed may appear to be        
stronger than the annealed product4 ,the
  well-known case of the ,rupert's drop 
is an example of this kind4  ,rupert's  

drops are produced by dropping molten   
glass into water2 they  generally take  
the form of a more or less spherical    
body having a long  tail1 tapering off  
into a thread1 attached to it4 ,such a  
,rupert's drop  may be struck with a    
heavy hammer1 and will safely resist a  
blow that  would splinter a similar body
 made of annealed glass4 ,if1 however1  
the  surface be scratched1 or the tip of
 the tail be broken off1 the entire     
8drop0 breaks up1 sometimes with a      
violent explosion1 into minute          
fragments4 ,numerous inventors1 among   
whom ,de la ,bastie and ,siemens  figure
 most conspicuously1 have endeavoured to
 utilise these properties  of chilled   
glass1 not exactly by endeavouring to   
produce that extreme  degree of internal
 strain which is characteristic of the  
,rupert's drop1  but by producing what  
they describe as 8tempered0 glass1 in   
which the  internal strains have been   
reduced by less violent cooling to such 
an  extent as to retain some of the     

advantages of the hardened1 internally  
strained condition while approximating  
more or less to the safer state  of     
annealed glass4 ,at one time articles of
 this kind were frequently  seen as     
curiosities1 such as tumblers that could
 be dropped on the  floor without       
breaking1 etc41 but these articles      
generally ended by  receiving a slight  
scratch or chip and promptly falling    
into fragments4  ,as a matter of fact1  
however1 some tempered glass is actually
  manufactured by the firm of ,siemens  
at the present time for special         
purposes4 ,de la ,bastie's process was  
tried in ,england1 and some success  was
 claimed for it2 but it is not in       
commercial operation at the  present    
time1 and never appears to have attained
 any great importance4    ,massive      
,glass4--,enthusiasts for the extension 
of the use of glass  have endeavoured to
 apply it to a great variety of         
purposes1 including  the construction of
 buildings and the paving of streets4   

,in the  former case1 which was         
exemplified at the ,paris ,exhibition of
 #aijj1  advantage was taken of the     
light-transmitting power of the         
material1  but although the buildings   
erected with large blocks of cast glass 
were  not displeasing in effect1 this   
use has not found any considerable      
extension4 ,for paving purposes1 the    
hardness and durability of glass  are   
the only useful qualities1 and here     
also--although several trials  have been
 made in ,france--no signs of any       
considerable application of  the new    
products are as yet visible4 ,what has  
been said above with  reference to the  
injurious character of glass dust       
applies1 further1 to  glass pavements1  
since their natural wear would result in
 the formation  of considerable         
quantities of this dust4 ,the advocates 
of glass paving1  however1 suggest that 
the hardness of glass would greatly     
reduce the  actual amount of wear1 and  
that consequently the dust would be     

reduced  considerably4 ,this is a matter
 which prolonged experience alone can   
decide1 but it does not seem obvious    
that glass blocks should wear more      
slowly than stone setts made of good    
granite1 for example4 ,on the other     
hand1 the glass blocks could probably be
 produced more cheaply1 since  the      
labour of cutting to size would be      
obviated by casting the blocks  to the  
desired dimensions4    ,water-glass1 or 
silicate of soda or potash is perhaps   
scarcely to be  classed under the       
heading of 8,glass ,manufacture0 at all1
 but it bears a  certain relationship to
 glass in several ways4 ,thus one of the
 modes of  manufacturing water-glass is 
by the fusion of sand and alkali in tank
  furnaces somewhat resembling those    
used for glass production2 the fused    
silicate1 moreover1 solidifies as a     
vitreous mass1 in which respect  it also
 resembles such substances as borax1    
etc4 ,the uses of silicate  of soda and 
potash are1 however1 so far removed from

 the field of  glass-manufacture that we
 cannot enter into them here4    ,in    
concluding this chapter1 we wish to     
describe one more product of the        
glassworks1 and this includes some of   
the most impressive and splendid        
examples of the glass-maker's art4      
,these are the great mirrors and  lenses
 by whose aid our lighthouses and       
searchlights send forth their  powerful 
beams of light4 ,although these objects 
are called 8mirrors0  and 8lenses10     
since they fulfil the functions of such 
optical organs1  yet in their nature and
 mode of manufacture they are so far    
removed  from the glass used for the    
production of other kinds of lenses that
  they could not be included under the  
heading of 8optical glass40    ,the     
characteristic feature in the           
manufacture of optical glass is the     
manner in which each separate pot or    
melting is allowed to cool down  and to 
break up into irregular fragments which 
are subsequently moulded  to the desired

 shape4 ,were it attempted to           
manufacture the large glass  bodies     
required for lighthouse purposes in this
 manner1 the cost would  approximate to 
that of the large discs used for        
telescope objectives1  and this would of
 course be entirely prohibitive4 ,the   
requirements as  regards colour1        
homogeneity and freedom from other      
defects1 which must  be met in          
lighthouse lenses1 are further not      
nearly so stringent as  those which are 
essential in ordinary optical work of   
good quality4  ,the reason for this     
difference arises from the fact that    
lighthouse  lenses and searchlight      
mirrors are used merely to impart a     
desired  direction to a beam of light1  
and not for the purpose of producing    
sharply-defined images2 slight          
irregularities in the glass are         
therefore not of such serious           
importance4    ,lighthouse glass can    
therefore be produced by rather less    
elaborate  means2 although every care is

 taken to make the glass as perfect as  
possible1 it is brought into            
approximately the desired form by       
casting  the molten glass in iron moulds
 of the proper shape4 ,when removed     
from these moulds and annealed1 the     
glass is fixed on large revolving       
tables and ground and polished to the   
final shape of lenses and annular       
lens-segments as required for the       
various types of ,fresnel lighthouse    
lenses4 ,in this way complete rings1    
forming annular lenses1 are  produced up
 to #dh inches diameter4 ,rings of      
larger size are usually  built up of a  
number of segments1 and these built-up  
rings sometimes  have a radius as large 
as #g feet4 ,for the majority of        
lighthouse  lenses1 it should be added1 
a hard soda-lime glass having a         
refractive  index of #aej to #aeb is    
used1 but for special purposes a dense  
flint-glass having a refractive index of
 #afc is employed4    ,mirrors for      
searchlight purposes are of very varied 

forms and sizes1  the shape depending   
largely upon the particular form of beam
 which  they are designed to project4   
,for many purposes a parabolic form  is 
required1 while in others1 where a flat1
 fan-shaped beam is to  be produced1 a  
form having an elliptical section in a  
horizontal  plane and a parabolic       
section in the vertical plane is        
required4 ,in  most cases these mirrors 
are produced by bending plates of glass1
  previously raised to the necessary    
degree of heat1 over suitably  shaped   
moulds1 the surface being subsequently  
re-polished to remove  any roughness    
resulting from the bending process4     
,another type of  mirrors is that known 
as 8,mangin10 which has two spherical   
surfaces  placed eccentrically in such a
 way that the centre of the mirror is   
considerably thinner than the periphery2
 in this type of mirror the  reflecting 
action of the back surface is modified  
by the refracting  action of the front  
surface1 but both are spherical1 and can

 therefore  be accurately ground and    
polished by the usual mechanical means4 
,such  mirrors are manufactured of      
single pieces of glass up to #f feet in 
diameter4          ,,appendix           
,,bibliography4      ,the existing      
literature of glass manufacture is so   
limited that a  complete bibliography   
could almost be given on a single page2 
in the  ,english language1 in           
particular1 there are exceedingly few   
books  and papers on the subject4 ,the  
,french and ,german literature of the   
subject is a little more extensive4 ,in 
giving a list of the works1  and more   
particularly in referring to those which
 he has consulted in  the preparation of
 the present volume1 the author thinks  
it will be  an advantage to indicate    
their scope1 and1 to some extent1 what  
he  believes to be their value1 in order
 to save the student the trouble  of    
seeking out comparatively inaccessible  
works only to find that they  contain   
little that is of value for his purpose4

      ,english ,books and ,papers on    
,glass ,manufacture4    ,the ,principles
 of ,glass ,making 7,george ,bell       
,sons74 ,by ,powell   ,chance4 ,an      
elementary book giving a clear and      
concise account of  the older processes1
 more especially in connection with     
flint and  plate-glass4    ,glass4      
,articles in #ith ,edition of           
,encyclopdia ,britannica4 ,a detailed   
account of processes1 more or less      
covering the entire subject1 but the    
processes described are mostly obsolete 
at the present time4    ,glass4 ,article
 in ,supplement to #ith ,edition of     
,encyclopdia ,britannica4  ,by ,harry   
,j4 ,powell4 ,a brief summary of more   
recent developments4  ,particularly     
valuable in reference to artistic       
,english flint-glass4    ,jena ,glass4  
,by ,hovestadt1 translated by ,j4 ,d4   
and ,a4 ,everett4  ,contains a full     
account of the scientific work on glass 
and its  practical application1 done in 
connection with the ,jena ,works of     

,schott4 ,particularly interesting in   
connection with the subjects of         
,chapters ,i41 ,,ii41 ,,xii41 and       
,,xiii4 ,as the title indicates1 the    
book is  written from the ,jena point of
 view1 and scarcely does justice to work
  done elsewhere4 ,the book has gained  
considerably at the hands of the        
translators4    ,some ,properties of    
,glass4 ,by ,w4 ,rosenhain4             
7,transactions of the ,optical  ,society
 of ,london1 #aijc47 ,gives a brief     
account of the properties of  glass as  
affecting its optical uses4    ,possible
 ,directions of ,progress in ,optical   
,glass4 ,by ,w4 ,rosenhain4             
7,proceedings of the ,optical           
,convention1 ,london1 #aije47 ,has been 
referred to in the text of this book    
7,chapter ,,xii474    ,catalogue of the 
,optical ,convention ,exhibition1       
,london1 #aije4 ,contains  historical   
and general notices of optical and      
lighthouse glass1  glass-working        
machinery1 etc4    ,glass for ,optical  

,instruments4 ,by ,r4 ,t4 ,glazebrook4  
7,cantor ,lectures  to the ,society of  
,arts47 ,gives an account of modern     
optical glass  manufacture4    ,old     
,english ,glasses4 ,by ,albert          
,hartshorne4 ,gives an account of the   
history of glass-making in ,england4    
,the ,methods of ,glass ,blowing4 ,by   
,w4 ,shenstone4 ,describes the          
manipulation of glass-blowing for       
experimental purposes1 i4e41 lamp  work4
      ,french ,books on ,glass          
,manufacture4    ,guide du ,verrier4 ,by
 ,g4 ,bontemps4 ,a classical work by one
 of the  greatest experts of his day4   
,much of the contents of the book is1   
however1 entirely out of date at the    
present time4 ,the book is  interesting 
as being the work of the man who        
introduced optical glass  manufacture   
into ,england4    ,verres et ,emaux4 ,by
 ,l4 ,coffignal4 ,chiefly of interest in
 connection  with the subjects of       
,chapter ,,viii4    ,le ,verre et le    
,crystal4 ,by ,j4 ,henrivaux4 7,p4 ,vicq

 ,dunod et ,cie41  ,paris47 ,a lengthy  
book profusely illustrated and giving a 
great wealth  of detailed information4  
,the writer was for some time the       
general  manager of one of the largest  
plate-glass manufactories in ,europe2   
his  account of plate-glass manufacture 
is1 therefore1 especially valuable4     
,much space in this book is devoted to  
historical and sthetical matter4    ,la 
,verrerie au ,,xxieme ,sicle4 ,by ,j4   
,henrivaux4 7,paris1 ,r4 ,bernard  et   
,cie41 #aijc47 ,practically a supplement
 to the preceding2 some of  the         
processes and products described are1   
however1 not of a practical  nature4    
,chiefly valuable for recent            
developments in plate-glass and         
bottle-glass manufacture4      ,german  
,books on ,glass ,manufacture4    ,die  
,glasfabrikation4 ,by ,r4 ,gerner4 7,a4 
,hartleben's ,verlag1 ,vienna  and      
,leipzig1 #ahig47 ,a concise and clear  
account of most of the more  important  
processes of glass manufacture4 ,very   

practical in character4  ,the           
information given appears to be         
reliable1 although far from  complete4  
,die ,herstellung ,grosser ,glaskoerper 
and ,die ,bearbeitung ,grosser          
,glaskoerper4 ,by ,c4 ,wetzel4          
7,hartleben's ,verlag1 ,vienna and      
,leipzig1  #aijj and #aija              
respectively47 ,describes numerous      
special processes and  appliances       
devised for use in connection with large
 glass objects4  ,some of these         
descriptions1 however1 appear to be     
little more than  transcripts from      
patent specifications4    ,glasfabriken 
und ,hohlglasfabrikation4 ,by ,r4       
,dralle4 7,leipzig1  ,baumgaertner1     
#ahhf47 ,looked upon as a classic in    
,germany4 ,gives  detailed plans and    
drawings of entire bottle-works1        
including furnaces  and all accessories4
 ,deals principally with bottle         
manufacture4    ,die ,glasfabrikation4  
,by ,dr4 ,e4 ,tscheuschner4 7,weimar1   
,b4 ,h4 ,voigt1  #ahhh47 ,a full        

detailed account of all processes known 
at the time4 ,the  rapid progress of    
modern practice has1 however1 already   
rendered this  book to some extent      
obsolete4    ,jenaer ,glas4 ,by         
,hovestadt4 ,already referred to in     
respect of the  ,english translation4   
,der ,sprechsaal4 7,schmidt1 ,weimar47  
,a trade journal devoted to the         
discussion of technical matters relating
 to the glass and ceramic  industries4  
,occasionally contains articles and     
abstracts of technical  or scientific   
interest in connection with glass       
manufacture4           99       99      
99       99       99    ,in addition to 
the books and papers named in the above 
list1 a great  number of scientific     
papers1 notes1 etc41 are to be found    
scattered  throughout the technical and 
scientific publications of the world2   
those that have proved of real interest 
and importance have1 however1  left     
their mark on the industry1 and will be 
found described or  referred to in      

connection with the various branches of 
manufacture  described in the present   
volume or in the books named above4     
,,index        ,a4      ,abb1 #h1 #aj1  
#baj1 #bah      ,absorption of light in 
glass1 #cb1 #agi      ,acid1 action of1 
on glass1 #aa      boric1 action of1 on 
glass1 #aa1 #ahf      carbonic1 action  
of1 on glass1 #ab      hydrofluoric1    
action of1 on glass1 #ab                
phosphoric1 action of1 on glass1 #aa    
,air1 compressed1 #ia1 #aje1 #aag       
,alkali chlorides1 use of1 in glass     
manufacture1 #da      content of        
hygroscopic glass1 #f      metals1 #ahd 
nitrates1 #dd1 #gh      sources of1 #dj 
,alkaline liquids1 action of1 on glass1 
#aa      ,aluminium1 #ea1 #ahf          
,ammonia soda1 #da      ,anastigmatic   
photographic lenses1 #bac      ,ancient 
windows1 colours of1 #af1 #bjb          
,annealing bottles1 #ajc      kiln1 #ajc
        for optical glass1 #bce         
for plate glass1 #ace        for rolled 
plate glass1 #abg      ,anthracite coal1

 #db1 #ec1 #gi      ,antimony1 #ahh     
,apochromatic objectives1 #bac          
,arsenic1 #eb1 #gh1 #aje1 #aag1 #ahh    
,artificial gems1 #bdf      ,auerbach1  
#bb      ,aventurine1 #ahe        ,b4   
,bacteria1 action of1 on glass1 #ac     
,barium compounds1 #dg1 #ahf      crown 
glass1 #bab      glass1 #g              
,barytes1 #dh      ,bases other than    
alkalies1 sources of1 #de      ,beads1  
#bdd      ,behaviour1 chemical1 of      
glass1 #f      ,bending plate glass1    
#add      ,bevelling1 #ade      ,black  
ash1 #da      ,blisters in sheet glass1 
#afj1 #afh      ,blocks1 fire-clay1 #eh 
tank1 #ei      ,blower1 sheet glass1    
#aeh      ,blower's chair1 #aaa         
,blowing glass1 #hi      holes1 #ia1    
#afa1 #ahi      sheet glass1 #afa       
,blown glass1 decoration of1 #aad       
plate glass1 #aga      ,bohemian glass1 
#aji1 #bdj      ,boiling up1 #ha        
,bottles1 annealing of1 #ajc            
blowing1 improvements in1 #ii           
machines1 #ajj      colour of1 #if      

manufacture1 furnace for1 #ig           
moulds for blowing1 #ih      production 
of1 by hand1 #ih      raw materials for1
 #ie      strength of1 #ah      ,boric  
acid1 #aa      ,boron1 #ahf             
,boro-silicate crown1 #bab              
,boucher's bottle-blowing machine1 #aja 
,bricks1 fire-clay1 #eh      silica1 #fj
      ,bubbles in optical glass1 #bcj   
removal of1 #ha      ,burning1 pots1 #eh
        ,c4      ,cadmium1 #ahf         
,calcium carbonate1 #df      oxide1 #de1
 #ahf      sulphate1 #dg      ,carbon1  
#ec1 #gi1 #ahf      ,carbonate of soda1 
#da      ,carbonic acid1 action of1 on  
glass1 #ab      ,carboys1 blowing of1   
#ajd      ,casting plate glass1 #acb    
,chair1 glass-blower's1 #aaa            
,chalk1 #df      ,chamotte1 #eg         
,chance1 #baa      ,charcoal1 #db1 #eh1 
#gi      ,charging furnaces1 #ge        
,chemical behaviour of glass1 #f        
composition of glass1 #e        of      
optical glass1 #bag      reactions      
during fusion1 #gf      ,chilled glass1 

#bdg      ,chimneys1 gaslight1 #bc      
lamp1 #bch      ,chromium1 colouring    
effect of1 #aij      ,cleaning of       
lenses1 #bbj      ,coal1 anthracite1    
#db1 #ec1 #gi      ,cobalt1 colouring   
effect of1 #aig      ,coke1 #db1 #ec1   
#gi      ,colour of ancient windows1 #af
      glass1 #cb        theory of1 #aha 
optical glass1 #bjh      sheet glass1   
#afg      ,coloured blown glass1 #aac   
glass1 #agh        technical uses of1   
#bjc      ,combustion tubing1 #g1 #bda  
,compressed air for glass blowing1 #ia1 
#aje1 #aag      ,conductivity1          
electrical1 of glass1 #cj      thermal1 
of glass1 #bd1 #bi      ,copper1        
colouring effect of1 #ahd      ruby1    
#ahd1 #ahh1 #aih      ,corrosion of     
glass1 #aa      ,covered pots1 #ef1 #aji
      ,crown1 boro-silicate1 #bai       
glass1 #age1 #baa      hard1 #bab1 #bai 
soft1 #bab      telescope1 #bae         
,crowns1 furnace1 #fj      ,crucibles1  
manufacture of1 #ef      for glass      
melting1 #ed      ,crushing strength of 

glass1 #ai      ,cryolite1 #eb          
,crystallisation of glass1 #c           
,crystals1 mineral1 #bah      ,cullet1  
#gd      for optical glass1 #bbd        
,cutting rolled plate glass1 #abh       
,cylinders1 sheet glass1 #afa1 #aga     
,d4      ,decolourisation of glass1 #eb1
 #ahh1 #aij1 #aic1 #aig      ,decoration
 of blown glass1 #aad      ,defects in  
rolled plate glass1 #abi      sheet     
glass1 #aff      ,definition of glass1  
#a      ,de la ,bastie1 #bdh            
,devitrification1 #c1 #aa      ,diamond1
 refractive index of1 #baf      ,dimming
 of glass surfaces1 #ab      ,dinas     
bricks1 #fa      ,dipping of sheet      
glass1 #aff      ,dispersion of optical 
glass1 #bji      partial1 #bad          
,double refraction in optical glass1    
#bba      rolling machine1 #acj         
,drawing tubes1 #bci      ,ductility of 
glass1 #bj      ,durability of glass1   
tests for1 #ad      ,dust1 action of1 on
 lenses1 #bbj      glass1 #bde          
,e4      ,elasticity of glass1 #bj1 #bd 

,electrical properties of glass1 #bi    
,epinal1 #ci      ,etching of glass1 #ab
      ,expansion1 coefficient of        
thermal1 #bd1 #be        ,f4            
,felspar1 #dj1 #dd      ,fibres1 glass1 
#bde      silica1 #bde      ,figured    
rolled plate glass1 #hg1 #acj           
cutting of1 #aca      ,finger-marks on  
lenses1 #bai      ,fining of glass1 #ha 
optical glass1 #bbi      ,fire-clay1    
action of1 on glass1 #f      for pots1  
#ee      wetting up1 #eg                
,fire-polish1 #aag      ,flashed glass1 
#be1 #aii      ,flint1 #dj              
boro-silicate1 #bab      dense1 #bab1   
#bdf      densest1 #bab      extra      
dense1 #bab      glass1 #g1 #di1 #gh1   
#ajh1 #baa      light1 #bab      soda1  
#bab      telescope1 #bae               
,fluorite1 refractive index of1 #baf    
,fontainebleau1 #ch      ,founding of   
optical glass1 #bbg      ,fourcault     
process1 #agd      ,fresnel1 #bea       
,furnace crowns1 #fj      gas1 #fc      
,furnaces for bottle manufacture1 #ig   

glass melting1 #ed1 #fb        optical  
glass1 #bbe        plate glass1 #acc    
rolled plate glass1 #abb        sheet   
glass1 #aea1 #agj      ports1 #fg       
recuperative1 #ff1 #aef                 
regenerative1 #ff1 #aee      tank1 #ei1 
#fi        economy of1 #gb      ,fusion1
 process of1 #gc      temperature of    
glass1 #e      ,freezing of glass1 #b   
,g4      ,gaslight1 chimneys for1 #bc   
,gas producers1 #fb1 #fd      ,gatherer1
 #aeh      ,gathering of glass1 #he1    
#hh1 #aeh      ,gauge tubes1 #aj1 #ah1  
#bc1 #bf      ,gems1 artificial1 #bdf   
,ghosts1 photographic1 #af              
,glauber's salt1 #dc      ,gold1        
colouring effect of1 #ahe      ,grinding
 plate glass1 #acg      ,gypsum1 #dg    
,h4      ,hardened glass1 #bj           
,hardness of glass1 #ba      tests for1 
#bb      ,heavy spar1 #dh               
,henrivaux1 #ai      ,hertz1 #bb        
,hock-bottle colour1 #aie               
,hohenbocka1 #ch      ,hollow           
glass-ware1 #ajh      ,horse-shoe flame1

 #fi      ,hydrofluoric acid1 action of1
 on glass1 #ab      ,hygroscopic glass1 
alkali content of1 #f        ,i4        
,indentation modulus1 #bb      ,index1  
refractive1 #baf      ,insulating       
properties of glass1 #bi      ,iron1 #if
      colouring effect of1 #aif         
oxidation of1 in glass1 #aie            
,irregularities caused by rolling1 #hf  
,j4      ,jena1 #g1 #aj1 #ad1 #bf1 #bi1 
#bjc1 #baj1 #bac1 #bda        ,k4       
,kelp1 #dj      ,kowalski1 #ai          
,l4      ,laboratory ware1 #aj1 #bc     
,ladling glass1 #he      rolled plate   
glass1 #abd      ,lagre1 #aff           
,lamp-chimneys1 #aaj1 #bch              
,lamp-work1 #bdj1 #bdd      ,large      
vessels1 production of1 #aje      ,lead1
 #di1 #ahc1 #ahh      ,lear for rolled  
plate glass1 #abg      sheet glass1 #afe
      ,leighton1 #ci      ,lenses1      
cleaning of1 #bbj      finger-marks on1 
#bbj      pressing small1 #id           
,light1 action of1 on glass1 #ae        
,lighthouse glass1 #agh1 #bej           

,lime1 slaked1 #de      ,lime-stone1 #df
      ,limited range of vitreous bodies1
 #d      ,lippe1 #ch      ,lynn1 #ci    
,m4      ,machines1 bevelling1 #ade     
double rolling1 #acj      grinding1 #aci
      polishing1 #ada      ,magnesia1   
#dh1 #ahf      ,manganese1 #ae1 #eb1 #hj
      ,mangin mirrors1 #beb             
,marver1 #aaa      ,massive glass1 #bdi 
,mechanical properties of glass1 #ah    
,metal1 attachment of1 to glass1 #bf    
,minerals1 crystalline1 #bag            
,mirrors1 #ade      searchlight1 #bea   
,mixing of materials1 #gc      ,moulds  
for glass-blowing1 #ij1 #aaj1 #aaf      
pressed glass1 #aai      ,muffled glass1
 #agb      ,muranese glass1 #abc        
,n4      ,nickel1 #if      colouring    
effect of1 #aig      steel1 #bg1 #adh   
,nitrates1 alkali1 #dd1 #gh        ,o4  
,objectives1 apochromatic1 #bac         
telescope1 #bah      ,opal glass1 #de1  
#eb1 #ahf      ,opaque plate glass1 #adf
      ,open pots1 #ef      ,optical     
glass1 annealing1 #bce        chemical  

composition of1 #bag        cooling of1 
#bcc        cost of1 #bcg        fining1
 #bbi        founding1 #bbg             
furnaces for1 #bbe        hardness of1  
#bbj        moulding1 #bce              
pressing1 #ic        range of1 #baf     
raw materials for1 #bbc        sorting1 
#bce        stability of1 #bai          
strain in1 #bba        stirring1 #bca   
yield of1 #bcf      properties of glass1
 #bje        ,p4      ,painting on      
glass1 #bja      ,parason1 #ajb         
,patent plate glass1 #aga      ,paving  
stones1 glass1 #bdi      ,pearl ash1 #dc
      ,phosphoric acid1 #aa             
,phosphorus1 #ahh      ,photographic    
ghosts1 #af      lenses1 anastigmatic1  
#bac        colour of1 #bji      ,pipe1 
glass-maker's1 #hi      sheet-blower's1 
#aeh      warmer1 #aeh      ,plate      
glass1 annealing kiln for1 #ace         
bending of1 #add      blown1 #aga       
casting1 #acb      colour of1 #cc       
figured rolled1 #hg      flatness of1   
#acd      furnaces for1 #acc            

grinding machines1 #aci        of1 #acg 
mirrors1 #ade      opaque1 #adf         
polishing machines1 #ada      polishing 
of1 #acg      raw materials for1 #acb   
rolled1 #hf1 #abc      silvering1 #adf  
sizes of1 #adc      strength of1 #ae    
stri in1 #adc      wired1 #bg1 #adg     
,platinum1 #bg      ,polishing1 theory  
of1 #ada      ,pontil1 #ih1 #agf1 #bci  
,potash1 #dc      ,potato1 use of1 in   
glass melting1 #ha      ,ports1 furnace1
 #fg      ,pots1 burning of1 #eh        
covered1 #ef      drying of1 #eh        
for flint glass1 #aji        optical    
glass1 #bbf      manufacture of1 #ef    
open1 #ef      ,pouring of glass1 #he1  
#hg      ,pressed glass1 #ib1 #aah      
composition of1 #abj      ,presses for  
glass1 #aai      ,proofs1 #hb1 #bca     
,purity of materials1 #cf        ,q4    
,quartz1 #dj        ,r4      ,range1    
limited1 of vitreous bodies1 #d         
,recuperative furnaces1 #ff1 #aef       
,red lead1 #di      ,refraction1 double1
 in optical glass1 #bba      of light in

 optical glass1 #bji      ,refractive   
index1 #baf      ,regenerative furnace1 
#ff1 #aee      ,reichsanstalt1 #aj      
,resistance to crystallisation of glass1
 #d      ,rings for lighthouse lenses1  
#bea      ,rod1 glass1 #bde      ,rolled
 plate glass1 #hf1 #abc      annealing1 
#abg      cutting1 #abh      defects of1
 #abi      figured1 #acj      furnaces1 
#abc      ladling1 #abd      raw        
materials for1 #abd      rolling1 #abf  
sorting1 #abi      surface of1 #abb     
,rolling of glass1 #hf      ,rubies1    
artificial1 #bdg      ,ruby1 copper1    
#ahd1 #ahh1 #aih      flashed1 #ahd     
gold1 #ahe      ,rupert's drops1 #bdh   
,s4      ,salt-cake1 #cg1 #db1 #gi1 #ahi
      ,sand1 #ch      ,sandstone1 #ci   
,schott1 #h1 #ai1 #bjc1 #bac1 #bda      
,scratches on sheet glass1 #afi         
,searchlights1 #bej      ,seed in sheet 
glass1 #afg      ,selenium1 colouring   
effect of1 #aij      ,sheet glass1 #gj  
blisters in1 #afj1 #afh      blowing1   
#afa      colour of1 #cc1 #afg          

compared with plate1 #adi               
cylinders1 #afa1 #aga      defects of1  
#aff      dipping1 #aff      flattening1
 #afe      furnaces1 #aea1 #agj         
lear1 #afe      mechanical production   
of1 #agc      raw materials for1 #aej   
sorting1 #aff      splitting1 #afd      
strength of1 #ah      ,siedentopf1 #ahb 
,siege blocks1 #ei      ,siemens1 #bdh  
,sievert1 #ib1 #aje1 #aag1 #agb         
processes1 #aje1 #aag      ,signal      
glasses1 #bjc      ,silica bricks1 #fa  
glass1 #e1 #bf1 #bda      sources of1   
#cg      ,silicon1 colouring effect of1 
#ahg      ,silver1 colouring effect of1 
#ahe      ,silvering plate glass1 #adf  
,sizes of plate glass1 #adb      ,soda  
ash1 #da      carbonate1 #da            
sulphate1 #cg1 #db1 #gi      sulphide1  
#hj      sulphite1 #gi                  
,solidification of glass1 #a            
,solutions1 analogy of1 with glass1 #bjf
      ,sorting rolled plate glass1 #abi 
,specific heat of glass1 #be1 #bi       
inductive capacity of glass1 #bi        

,stains1 coloured1 #bjj                 
,stassfurth1 #dd      ,stones in rolled 
plate glass1 #abi      sheet glass1 #afg
      ,storage of materials1 #cg        
,strain in optical glass1 #bba          
,strength of glass1 #ai      ,stri in   
coloured glass1 #bjc        optical     
glass1 #bjf1 #bbg        plate glass1   
#adc      testing apparatus1 #bjg       
,string in sheet glass1 #afh            
,strontium1 #hf      ,structure of      
glass1 #a      ,sulphur1 colouring      
effect of1 #ahi      ,surfaces1 chemical
 behaviour of glass1 #h1 #aj            
,szigmondi1 #ahb        ,t4      ,table1
 rolling1 #abf      ,tank blocks1 #ei   
furnaces1 #ei1 #fi        economy of1   
#gb        for sheet glass1 #aeb        
,telescope objectives1 #bac             
,temperature of fusion of glass1 #e     
,tempered glass1 #bj1 #bdh      ,tensile
 strength of glass1 #ai      ,thallium1 
#ahc1 #ahh      ,theory of colours in   
glass1 #aha      polishing1 #ada        
,thermal endurance of glass1 #bc        

properties of glass1 #bc                
,thermometer glass1 #g1 #h1 #bh         
,tin1 colouring effect of1 #ahg         
,tonnelot1 #g      ,transparency of     
glass1 #ca      optical glass1 #bjh     
,trautwine1 #ai      ,tubing1 #bch      
combustion1 #g      drawing of1 #bci    
,tumblers1 #aaa        ,u4              
,ultra-violet microscope1 #bdc          
,v4      ,vanadium1 colouring effect of1
 #ahi      ,veins in optical glass1     
#bjf1 #bbg        ,w4      ,water1      
action of1 on glass1 #aj     glass1 #bej
      ,wetting up clay1 #eg             
,winkelmann1 #ai      ,wired plate      
glass1 #bg1 #adg      ,witherite1 #dh   
,wool1 glass1 #bde        ,y4           
,young's modulus1 #bj        ,z4        
,zaffre1 #aig      ,zeiss1 #bac1 #bdd   
,zinc1 colouring effect of1 #di1 #ahf   
,zschimmer1 #ad      ,,bradbury1        
,,agnew1  ,,co4 ,,ld41 ,,printers1      
,,london ,,and ,,tonbridge4          ,d4
 ,,van ,,nostrand ,,company    ,publish 
the following ,catalogues which will be 

sent gratis on  ,application4444        
,,chemical4    #add ,page ,alphabetical 
and ,classified ,list of ,books on      
,chemistry1  ,chemical ,technology and  
,physics4 ,arranged by subjects and     
authors4      ,,electrical4    #aab     
,page ,classified ,catalogue of         
,electrical ,books4 ,indexed4 #h page   
list of ,electrical ,books issued in    
#aijg4      ,,technical4    #abb ,page  
,list arranged by ,authors4    #dh ,page
 ,list of ,scientific ,books recently   
issued and in preparation4    #h ,page  
,supplemental ,list of ,books on        
,manufactures and ,industries4    #h    
,page ,list of ,books just issued4      
,,monthly ,,record4    #af ,page        
,monthly ,record of ,scientific         
,literature ,sent regularly to any      
address1 on request4          #bc       
,murray and #bg ,warren ,streets1 ,new  
,york4          ,d4 ,van ,nostrand      
,company    ,announce the early issue of
 the    ,,westminster ,,series4         
#hvo1 cloth1 fully illustrated1         

,each1 #b4jj net4          ,much of the 
information forthcoming in these volumes
 is either      not available to the    
general public1 or if available1 is     
widely      scattered4 ,these volumes   
will be issued at short intervals4      
,,now ,,ready4      ,,india-,,rubber    
,,and ,,its ,,manufacture1 with         
,chapters on ,gutta-,percha        and  
,balata4 ,by ,h4 ,l4 ,terry1 ,f4,i4,c41 
,assoc4 ,inst4 ,m4,m4      ,,electric   
,,power ,,and ,,traction4 ,by ,f4 ,h4   
,davies1 ,a4,m4,i4,e4,e4      ,,liquid  
,,and ,,gaseous ,,fuels1 ,,and ,,the    
,,part ,,they play in ,modern ,power    
,production4 ,by ,professor ,vivian ,b4 
,lewes1 ,f4,i4,c41 ,f4,c4,s4            
,,coal4 ,by ,james ,tonge1 ,m4,i4,m4,e41
 ,f4,g4,s41 etc4 7,lecturer on ,mining  
at ,victoria ,university1 ,manchester74 
,,the ,,book3 ,its ,history and         
,development4 ,by ,cyril ,davenport1    
,v4,d41        ,f4,s4,a4      ,,iron    
,,and ,,steel4 ,by ,j4 ,h4 ,stansbie1   
,b4,sc41 ,f4,i4,c4      ,,town ,,gas    

,,for ,,lighting ,,and ,,heating4 ,by   
,w4 ,h4 ,y4 ,webber1 ,c4 ,e4            
,,glass4 ,by ,walter ,rosenhain1        
,superintendent of the ,department of   
,metallurgy in the ,national ,physical  
,laboratory1 late ,scientific           
,adviser in the ,glass ,works of        
,messrs4 ,chance ,bros4  ,co4      ,,in 
,,preparation4      ,,patents4 ,trade   
,marks and ,designs4 ,by ,kenneth ,r4   
,swan 7,oxon471 of        the ,inner    
,temple1 ,barrister-at-,law4      ,,the 
,,manufacture ,,of ,,paper4 ,by ,r4 ,w4 
,sindall1 ,f4,c4,s4      ,,wood ,,pulp  
,,and ,,its ,,applications4 ,by ,c4 ,f4 
,cross1 ,e4 ,j4 ,bevan1 and        ,r4  
,w4 ,sindall4      ,,steam ,,engines4   
,by ,j4 ,t4 ,rossiter1 ,m4,i4,e4,e41    
,a4,m4,i4,m4,e4      ,,precious         
,,stones4 ,with a ,chapter on           
,artificial ,stones4 ,by ,w4            
,goodchild1 ,m4,b41 ,b4,ch4             
,,electric ,,lamps4 ,by ,maurice        
,solomon1 ,a4,c4,g4,i41 ,a4,m4,i4,e4,e4 
,,steam ,,locomotives4 ,by ,vaughan     

,pendred1 ,m4,i4,mech4,e4      ,,gold   
,,and ,,precious ,,metals4 ,by ,thomas  
,k4 ,rose1 ,d4,sc41 of the ,royal       
,mint4      ,,electro-,,metallurgy4 ,by 
,j4 ,b4 ,c4 ,kershaw1 ,f4,i4,c4         
,,photography4 ,by ,alfred ,watkins1    
,president-,elect of the                
,photographic ,convention1 #aijg4       
,,commercial ,,paints ,,and ,,painting4 
,by ,a ,s4 ,jennings1 ,hon4 ,consulting 
,examiner1 ,city and ,guilds of ,london 
,institute4      ,,brewing ,,and        
,,distilling4 ,by ,james ,grant1        
,f4,s4,c4          ,publishers and      
,booksellers1        #bc ,murray and #bg
 ,warren ,streets1 ,,new ,,york4        
,transcriber's ,notes      ,punctuation1
 hyphenation1 and spelling were made    
consistent when a  predominant          
preference was found in this book2      
otherwise they were not  changed4       
,simple typographical errors were       
corrected2 occasional unbalanced        
quotation marks retained4    ,ambiguous 
hyphens at the ends of lines were       

retained4    ,the appearance of the     
,table of ,contents has been altered    
slightly4    ,index not checked for     
proper alphabetization or correct page  
references4    ,in the ,index1 it was   
unclear to the ,transcriber whether some
 of the  indentations indicated         
sub-entries or implied ditto marks1 so  
the  indentations have been treated here
 as multiple levels of sub-entries4