,trancriber's ,note ,text emphasis is displayed as ,italic and ,bold4 ,whole and fractional parts are displayed as #h-a_/b4 ,although in general1 subscripts are denoted 4 ,the formulae for the minerals are presented as ,k#b,o,al#b,o#cf,si,o#b 7,orthoclase7 where the number following the element would normally be subscripted4 ,the ,cambridge ,manuals of ,science and ,literature ,,the ,,natural ,,history ,,of ,,clay ,,cambridge ,,university ,,press ,london3 ,,fetter ,,lane1 ,e4,c4 ,c4 ,f4 ,,clay1 ,manager ,illustration ,edinburgh3 #ajj1 ,,princes ,,street ,london3 ,,william ,,wesley ,,and ,,son1 #bh1 ,,essex ,,street1 ,,strand ,berlin3 ,a4 ,,asher ,,and ,,co4 ,leipzig3 ,f4 ,a4 ,,brockhaus ,new ,york3 ,g4 ,p4 ,,putnam',s ,,sons ,bombay and ,calcutta3 ,,macmillan ,,and ,,co41 ,ltd4 ,all rights reserved ,illustration ,,the ,,natural ,,history ,,of ,,clay ,,by ,,alfred ,b4 ,,searle ,cantor ,lecturer on ,brickmaking1 ,author of ,british ,clays1 ,shales and ,sands2 ,the ,clayworker's ,handbook1 etc41 etc4 ,cambridge3 at the ,university ,press ,new ,york3 ,g4 ,p4 ,putnam's ,sons #aiab ,cambridge3 ,,printed ,,by ,,john ,,clay1 ,m4,a4 ,,at ,,the ,,university ,,press ,with the exception of the coat of arms at the foot1 the design on the title page is a reproduction of one used by the earliest known ,cambridge printer1 ,john ,siberch1 #aeba ,,preface ,both as raw materials and in the form of pottery1 bricks1 tiles1 terra-cotta and many other articles of use and ornament1 clays are amongst the most important rock products4 ,yet the origin of the substances we know as 'clay1' the processes occurring in its formation and the causes of some of the most important of its characteristics are of such a nature that it is remarkable that its use should have become so extended in the arts and sciences1 while we know so little of its properties when in a pure state4 ,in the following pages an attempt has been made to state in a simple form an outline of our present knowledge of the subject and to indicate the problems which still lie before us4 ,the experimental solution of these problems is rendered peculiarly difficult by the inertness of the materials at ordinary temperatures and the ease with which the clay molecule appears to break down into its constituent oxides at temperatures approaching red heat or as soon as it begins to react with alkaline or basic materials4 ,another serious difficulty is the highly complex nature of that property known as 'plasticity' to which many clays owe their chief value4 ,for many years this has been regarded as an elementary property such as hardness1 cohesion or colour1 but it is now known to be of so elusive a nature as almost to defy measurement with any degree of accuracy4 ,the thoroughness with which the methods of physical chemistry have been applied to geological and mineralogical problems during recent years has been of very great assistance to the student of clay problems1 as will be seen on studying some of the works mentioned in the short bibliography at the end of the present volume4 ,when the principles of hydrolysis1 ionization1 mass reaction and reactional velocity have been applied in still further detail to the study of clays1 our knowledge of their natural history will increase even more rapidly than it has done during the past few years4 ,no industry exercises so great a fascination over those engaged in it as do the various branches of clayworking2 no other substance offers so many problems of such absorbing interest to the artist1 the craftsman1 the geologist1 the chemist and the general student of nature1 whilst the differences in legal opinion as to the nature of clay could themselves occupy a volume far larger than the present one4 ,a4 ,b4 ,s4 ,the ,white ,building1 ,sheffield4 ,november #aiaa4 ,,contents ,,chap4 ,,page ,table of clay rocks viii ,i ,introduction4 ,the chemical and physical properties of clays #a ,,ii ,clay and associated rocks #dh ,,iii ,the origins of clays #gj ,,iv ,the modes of accumulation of clays #hd ,v ,some clays of commercial importance #ajc ,,vi ,clay-substance3 theoretical and actual #ace ,bibliography #afh ,index #agj ,,list ,,of ,,illustrations ,,fig4 #a ,quartz crystals #i #b ,pyrite #ad #c ,marcasite #ad #d ,illustrating the structure of a 'clay crumb' #bd #e ,chart showing rates of drying #bg #f ,seger ,cones indicating a temperature of #abej ,c4 #cd #g ,ludwig's ,chart #cf #h ,coal ,measures sequence in ,north ,staffordshire #ee #i ,lias clay being worked for the manufacture of hand-made sand-faced roofing tiles #eh #aj ,oxford clay near ,peterborough #fj #aa ,cliffs of ,boulder clay at ,filey lying on ,calcareous ,crag #ff #ab ,china clay pit belonging to the ,north ,cornwall ,china ,clay ,co4 #gb #ac ,orthoclase ,felspar #ge #ad ,illustrating the successive deposition of different strata #ij #ae ,lacustrine clay at ,skipsea #ib #af ,clay at ,nostel1 showing ,marine ,band #id #ag ,kaolinite and ,mica #aje #ah ,mining best ,potter's clay in ,devonshire #aaa ,,the ,,chief ,,clay ,,rocks 7arranged geologically7 ---------------------------------------- ---------------- ,recent 7alluvial clay1 silt1 brick earths1 boulder clay7 ---------------------------------------- ---------------- ,tertiary ,pliocene ,miocene 7brick earths1 ball clays1 ,oligocene coarse pottery clays7 ,eocene ---------------------------------------- ---------------- ,cretaceous 7cement clays1 brick clays7 ---------------------------------------- ---------------- ,secondary ,oolitic 7brick and tile clays7 ---------------------------------------- ---------------- ,triassic 7brick1 tile and terra-cotta clays7 ---------------------------------------- ---------------- ,permian 7brick1 tile and flower-pot clays7 ---------------------------------------- ---------------- ,carboniferous 7brick clays1 fireclays1 ganister7 ---------------------------------------- ---------------- ,primary ,devonian ,silurian ,ordovician 7clay schists1 slates and clay shales7 ,cambrian ,pre-,cambrian ---------------------------------------- ---------------- ,igneous ,rocks occur on several horizons 7china clays and kaolins7 ---------------------------------------- ---------------- 7,in the above ,table only the clay-bearing strata are mentioned4 ,the formations named consist chiefly of other rocks in which the clays form strata of variable thickness47 ,,chapter ,i ,,introduction4 ,,the ,,chemical ,,and ,,physical ,,properties ,,of ,,clay ,the chief uses of clay have been recognized since the earliest periods of civilization2 the ancient ,assyrian and ,egyptian records contain numerous references to the employment of clay for the manufacture of bricks and for fulling or whitening cloth4 ,clays are distributed so widely and in many cases are so readily accessible that their existence and some of their characteristics are known in entirely uncivilized regions4 ,the use of certain white clays as a food1 or at any rate as a means of staving off hunger1 is common among some tribes of very primitive peoples4 ,the more important uses of clays for building and other purposes are naturally confined to the more civilized nations4 ,the term clay 7,a4,s4 cloeg2 ,welsh clai2 ,dutch kley7 although used in a scientific sense to include a variety of argillaceous earths 7,fr4 argile clay7 used in the manufacture of bricks1 tiles1 pottery and ceramic products 7,gr4 keramos potter's earth7 generally1 is really a word of popular origin and use4 ,consequently1 it is necessary to bear in mind1 when considering geological or other problems of a scientific nature1 that this term has been incorporated into scientific terminology and that its use in this connection not infrequently leads to confusion4 ,in short1 whilst almost every dictionary includes one or more definitions of clay1 and most text-books on geology1 mineralogy1 and allied sciences either attempt a definition or assume the reader's knowledge of one1 there is no entirely satisfactory limitation in regard to the substances which may or may not be included under the term4 ,clay is a popular term for a variety of substances of very varied origins1 of great dissimilarity in their composition and in many of their chemical and physical properties1 and differing greatly in almost every conceivable respect4 ,it is commonly supposed that all clays are plastic1 but some of the purest china clays are almost devoid of this property and some of the most impure earths used for brickmaking possess it in a striking degree4 ,shales1 on the one hand--whilst clearly a variety of clay--are hard and rock-like1 requiring to be reduced to powder and very thoroughly mixed with water before they become plastic2 many impure surface deposits1 on the other hand1 are so highly plastic as to necessitate the addition of other 7sandy7 materials before they can be used for the manufacture of bricks and tiles4 ,attempts have been made to include in the term clay 'all minerals capable of becoming plastic when moistened or mixed with a suitable quantity of water1' but this definition is so wide as to be almost impracticable1 and leads to the inclusion of many substances which have no real connection with clays4 ,the limitation of the use of the word 'clay' to the plastic or potentially plastic materials of any single geological epoch is also impracticable1 for clays appear to have been deposited in almost every geological period1 though there is some difference of opinion as to the time of the formation of certain clays known as kaolins4 ,clay is not infrequently termed a mineral1 but this does not apply at all accurately to the many varieties of earths known as 'common clays1' which1 together with the 'boulder clays1' contain many minerals and so cannot1 as a whole1 be included under this term4 ,whatever may be the legal significance of the term 'mineral'--which has an important economic bearing on account of minerals being taxed or 'reserved' in some instances where non-minerals 7including brick clay7 are exempt--there can be no doubt that1 scientifically1 clay is not a mineral but a rock4 ,whatever mineral 7if any7 may give the chief characteristic property to the clays as a class must be designated by a special title1 for the general term 'clay' will not serve for this purpose4 ,geologically1 the clays are sedimentary rocks1 some being unaltered1 whilst others--the slates--are notably metamorphosed and can seldom be used for the purposes for which clays are employed4 ,most clays may be regarded as a mixture of quartz grains1 undecomposed rock dbris and various decomposition products of rocks2 if the last-named consists chiefly of certain hydrous alumino-silicates1 they may be termed 'clay substance' 7see ,chapter ,,vi4 ,the imperfections of this statement as a definition are obvious when it is remembered that it may include a mixture of fine sand and clay containing only #cj per cent4 of the latter substance4 ,it is1 at the present time1 quite impossible to construct an accurate definition of the term 'clay4' ,the most satisfactory hitherto published defines 'clay' as 'a solid rock composed mainly of hydro-alumino-silicates or alumino-silicic acids1 but often containing large proportions of other materials2 the whole possessing the property of becoming plastic when treated with water1 and of hardening to a stone-like mass when heated to redness4' ,from what has already been written1 it will be understood that there is no such entity as a standard clay1 for the varieties are almost endless1 and the differences between them are sometimes so slight as to be scarcely distinguishable4 ,a further consideration of this branch of the subject may1 however1 conveniently be deferred to a subsequent chapter4 ,the best-known clays are the surface clays1 loams and marls1 the shales and other sub-surface clays1 and the pottery and china clays4 ,the values of these different materials vary enormously1 some being almost worthless whilst others are highly valued4 ,the surface clays are chiefly used for the manufacture of bricks and tiles 7though some are quite unsuitable for this purpose7 and form the soil employed in agriculture in many districts4 ,the sub-surface clays and shales are harder1 and usually require mechanical treatment before they can be used for brick and terra-cotta manufacture1 or for the production of refractory and sanitary articles4 ,the pottery and china clays are usually more free from accessory constituents1 and are regarded as the 'purest' clays on the market1 though a considerable amount of latitude must be allowed in interpreting the term 'pure4' ,china clays are by no means pure in the state in which they occur1 and require careful treatment before they can be sold4 ,further information with regard to the characteristics of certain clays will be found in ,chapter ,v4 ,the ,chemical ,properties of ,clay4 ,the chief constituents of all clays are alumina and silica1 the latter being always in excess of the former4 ,these two oxides are1 apparently1 combined to form a hydro-alumino-silicate or alumino-silicic acid corresponding to the formula ,h#d;,al#b,si#b,o#ia1 but many clays contain a much larger proportion of silica than is required to form this compound1 and other alumino-silicates also occur in them in varying proportions 7see ,chapters ,v and ,,vi74 ,all clays may1 apparently1 be regarded as consisting of a mixture of one or more hydrous alumino-silicates with free silica and other non-plastic minerals or rock granules1 and their chemical properties are largely dependent on the nature and proportion of these accessory ingredients4 ,the purest forms of clay 7china clays and ball clays7 approximate to the formula above-mentioned1 but others differ widely from it1 as will be seen from the analyses on p4 #af4 ,the chemical properties of pure clay are described more fully in ,chapter ,,vi4 ,footnote #a3 ,this formula is commonly written ,al#b,o#ccb,si,o#bb;,h#b,o1 but although this is a convenient arrangement1 it must not be understood to mean that clays contain water in a state of combination similar to that in such substances as washing soda--,na#b;,,co#cbd;,h#b,o1 or zinc sulphate crystals--,zn,s,o#dg;,h#b,o 7see ,chapter ,,vi74 ,taking china clay1 which has been carefully purified by levigation1 as representative of the composition of a 'pure' clay1 it will be found that the chief impurities in clays are 7a7 stones1 gravel and sand--removable by washing or sifting2 7b7 felspar1 mica and other silicates and free silica--which cannot be completely removed without affecting the clay and 7c7 lime1 magnesia1 iron1 potash and soda compounds1 together with minute quantities of other oxides1 all of which appear to be so closely connected with the clay as to be incapable of removal from it by any mechanical methods of purification4 ,to give a detailed description of the effect of each of the impurities just referred to would necessitate a much larger volume than the present1 but a few brief notes on the more important ones are essential to a further consideration of the natural history of clay4 ,stones1 gravel and sand are most noticeable in the boulder clays1 but they occur in clays of most geological ages1 though in very varying proportions4 ,sometimes the stones are so large that they may be readily picked out by hand2 in any case the stones1 gravel and most of the sand may be removed by mixing the material with a sufficient quantity of water and passing the 'slip' through a fine sieve1 or by allowing it to remain stationary for a few moments and then allowing the supernatant liquid to run off into a settling tank4 ,some clays contain sand grains which are so fine that they cannot be removed in this manner and the clay must then be washed out by a stream of water with a velocity not exceeding #b ft4 per hour4 ,even then1 the clay so removed may be found to contain minute grains of silt1 much of which may be removed by a series of sedimentations for various periods1 though a material perfectly free from non-plastic granules may be unattainable4 ,most of the sand found associated with clays is in the form of fragments of quartz crystals 7fig4 #a71 though it may be composed of irregular particles of other minerals or of amorphous silica4 ,felspar1 mica and other adventitious silicates occur in many natural clays in so fine a state of division that their removal would be unremunerative4 ,in addition to this they act as fluxes when the clays are heated in kilns1 binding the less fusible particles together and forming a far stronger mass than would otherwise be produced4 ,consequently1 they are valuable constituents in clays used for the manufacture of articles in which strength or imperviousness is important4 ,if these minerals are present in the form of particles which are sufficiently large to be removed by elutriation in the manner described on the previous page1 the purification of the clay is not difficult4 ,usually1 however1 the most careful treatment fails to remove all these minerals2 their presence may then be detected by microscopical examination and by chemical analysis4 ,for most of the purposes for which clays are used1 small proportions of these silicates are unimportant1 but where clays of a highly refractory nature are required2 and for most of the purposes for which china clays 7kaolins7 are employed1 they must not be present to the extent of more than #e per cent41 smaller proportions being preferable4 ,illustration3 ,fig4 #a4 ,quartz crystals1 natural size4 7,from ,miers' ,mineralogy by permission of ,macmillan ,co47 ,oxides1 sulphides1 sulphates and carbonates of various metals form the third class of impurities in clays4 ,of these1 the most important are calcium oxide 7lime71 calcium carbonate 7chalk and limestone71 calcium sulphate 7gypsum and selenite71 the corresponding magnesia1 magnesium carbonate1 and sulphate1 the various iron oxides1 ferrous carbonate and iron sulphides 7pyrite and marcasite7 7p4 #ac74 ,potash and soda compounds are commonly present as constituents of the felspar1 mica1 or other silicates present1 and need no further description1 though small proportions of soluble salts--chiefly sodium1 potassium1 calcium and magnesium sulphates--occur in most clays and may cause a white scum on bricks and terra-cotta made from them4 ,lime and magnesia compounds may occur as silicates 7varieties of felspar1 mica1 etc471 but their most important occurrence is as chalk or limestone4 ,chalk is a constant constituent of malms#b and of many marls1 but the latter may contain limestone particles4 ,limestone occurs in many marls and to a smaller extent in other clays4 ,in the boulder clays it frequently forms a large portion of the stony material4 ,if the grains are very small 7as in chalk71 the lime compounds act as a flux1 reducing the heat-resisting power of the clay and increasing the amount of vitrification2 they produce in extreme cases a slag-like mass when the clay is intensely heated4 ,if1 on the contrary1 the grains are larger 7as frequently occurs with limestone71 they are converted into lime or magnesia when the clay is 'burned' in a kiln1 and the lime1 on exposure to weather1 absorbs moisture 7i4e4 slakes71 swells1 and may disintegrate the articles made from the clay4 ,limestone 7except when in a very finely divided state7 is almost invariably objectionable in clays1 but chalk is frequently a valuable constituent4 ,footnote #b3 ,a malm is a natural mixture of clay and chalk 7p4 #fh74 ,chalk is added to clay in the manufacture of malm-bricks to produce a more pleasing colour than would be obtained from the clay alone1 to reduce the shrinkage of the clay to convenient limits and1 less frequently1 to form a more vitrifiable material4 ,chalk1 on heating1 combines with iron oxide and clay1 forming a white silicate1 so that some clays which would1 alone1 form a red brick1 will1 if mixed with chalk1 form a white one4 ,lime compounds have the serious objection of acting as very rapid and powerful fluxes1 so that when clays containing them are heated sufficiently to start partial fusion1 a very slight additional rise in temperature may easily reduce the whole to a shapeless1 slag-like mass4 ,magnesia compounds act much more slowly in this respect and so are less harmful4 ,gypsum--a calcium sulphate--occurs naturally in many sub-surface clays1 often in well-defined crystalline masses4 ,it reduces the heat-resisting power of the clays containing it and may1 under some conditions1 rise to the surface of the articles made from the clay1 in the form of a white efflorescence or scum1 such as is seen on some brick walls4 ,iron compounds are highly important because they exercise a powerful influence on the colour of the burned clays4 ,the red oxide 7ferric oxide7 is the most useful form in burned clay1 but in the raw material ferrous oxide and ferrous carbonate may also occur1 though they are converted into the red oxide on heating4 ,the red iron oxide1 which is closely related to 'iron rust1' occurs in so finely divided a state that its particles appear to be almost as small as those of the finest clays4 ,hence attempts to improve the colour of terra-cotta and bricks by the addition of commercial 'iron oxide' are seldom satisfactory1 the finest material obtainable being far coarser than that occurring in clays4 ,it is a curious fact that red iron oxide does not appear to form any compound with the other constituents of clay under ordinary conditions of firing1 and although a 'base' and capable of reducing the heat-resisting power of clays1 it does not appear to do so as long as the conditions in the kiln are sufficiently oxidizing4 ,it is this which enables red bricks and other articles to be obtained with remarkable uniformity of colour combined with great physical strength4 ,in a reducing atmosphere1 on the contrary1 ferrous oxide readily forms and attacks the clay1 forming a dark grey vitreous mass4 ,if the iron particles are separated from each other they will1 on reduction1 form small slag-like spots1 but if they are in an extremely fine state of division and well distributed1 the brick or other article will become slightly glossy and of an uniform black-grey tint4 ,the famous ,staffordshire 'blue' bricks owe their colour to this characteristic2 they are not really 'blue' in colour4 ,the effect of chalk on the colour of red-burning clays has already been mentioned4 ,iron pyrite 7fig4 #b7 and marcasite 7fig4 #c7--;both of which are forms of iron sulphide--occur in many clays1 particularly those of the ,coal ,measures4 ,mundic is another form of pyrites which resembles roots or twigs1 but when broken show a brassy fracture4 ,when in pieces of observable size the pyrite may be readily distinguished by its resemblance to polished brass and the marcasite by its tin-white metallic lustre and both by their characteristic cubic1 root-like and spherical forms2 the latter only show a brass-like sheen when broken4 ,even when only a small proportion of mundic1 pyrite or marcasite is present1 it is highly objectionable for several reasons4 ,in the first place1 half the sulphur present is given off at a dark red heat and is liable to cause troublesome defects on the goods4 ,secondly1 because the remaining sulphur and iron are not readily oxidized1 so that there is a great tendency to form slag-spots of ferrous silicate1 owing to the iron attacking the clay at the same moment as it parts with its remaining sulphur4 ,for this reason1 clays containing any iron sulphide seldom burn red1 but form products of a buff colour with black spots scattered irregularly over their surface and throughout the mass--an appearance readily observable on most hard-fired firebricks4 ,if chalcopyrite 7copper-iron sulphide7 is present the spots may be bright green in colour4 ,illustration3 ,fig4 #b4 ,pyrite4 ,illustration3 ,fig4 #c4 ,marcasite4 ,slightly magnified4 7,from ,miers' ,mineralogy by permission of ,macmillan ,co47 ,carbon1 either free or as hydrocarbons 7chiefly vegetable matter7 or in other forms1 is a constituent of most clays1 though seldom reported in analyses4 ,its presence exercises an important influence in several respects4 ,on heating the clay1 with an ample supply of air1 the carbonaceous matter may distil off 7as shale oil71 but more usually it decomposes and burns out leaving pores in the material4 ,if the air-supply is insufficient and the heating is so rapid and intense that vitrification commences before the carbon is all burned away1 the pores become filled with the fused ingredients of the clay1 air can no longer reach the carbon particles and a black 'core' or heart is produced4 ,under peculiarly disadvantageous conditions the material may also swell greatly4 ,this is a serious defect in many classes of clay used for brickmaking1 and its causes and prevention have been exhaustively studied by ,orton and ,griffiths 7#a7c but1 beyond the brief summary given above1 these are beyond the scope of the present work4 ,water is an essential constituent of all unburned clays1 though the proportion in which it occurs varies within such wide limits that no definite standard can be stated4 ,this water is found in two conditions3 7a7 as moisture or mechanically mixed with the clay particles and 7b7 in a state of chemical combination4 ,footnote #c3 ,references to original papers1 etc4 will be found in the appendix4 ,,analyses ,,of ,,typical ,,clays ,the samples were all dried at #aje ,c4 ---------------------------------------- ------------------------- ,clay ,china ,ball ,fireclay ,brick ,boulder ,marl ,clay ,clay ,clay ,clay ---------------------------------------- ------------------------- ,locality ,cornwall ,dorset,yorkshire,midlands ,lancs4,suffolk ---------------------------------------- ------------------------- ,ultimate ,analysis3 ,silica #dga #dia #fhi #egg #fcg #dcg ,alumina #cia #ccg #aic #bdc #bjd #aee ,ferric oxide #f #ab #aj #ej #cj #eb ,titanium oxide -- #b #ah #a #b -- ,lime #d #h #i #cg #dc #afc ,magnesia #b #c #c #be #bg #ba ,potash and ,soda #c #be #i #bh #bi #g ,carbon #bf #dc #ah #af #d #af ,water #ic #gg #dh #bj #bb #bd ,other ,matter #d #b #c #c #b #abe ---------------------------------------- ------------------------- ,total #ajjj #ajjj #ajjj #ajjj #ajjj #ajjj ---------------------------------------- ------------------------- ,proximate ,analysis3 ,gravel and ,sand -- #hd #df #bba #bca #ib ,silt -- #dh #ij #ca #hd #afj ,felspar- and mica- dust #eb #aed #ajc #bdc #ahe #hi ,silica-dust #ca #dj #chj #ca #abf #bj ,free calcium carbonate -- -- -- #ba #b #bhd ,free iron oxide and pyrites #d #i #g #db #af #ci ',true clay' #iac #ffe #cgd #daa #cef #caf ---------------------------------------- ------------------------- ,total #ajjj #ajjj #ajjj #ajjj #ajjj #ajjj ---------------------------------------- ------------------------- ,for other analyses the books in the ,bibliography at the end of the present volume should be consulted1 particularly ,no4 #b1 i4e4 ,british ,clays1 ,shales and ,sands4 ,the amount of mechanically mixed water will naturally vary with the conditions to which the clay has been subjected2 it will be greatest in wet situations and will diminish as the clay is allowed to dry4 ,the 'combined water1' on the contrary1 appears to be a function of the true clay present in the material1 and reaches its highest proportions in the china clays and kaolins1 which contain approximately #ac per cent4 ,on heating a clay to #aje ,c4 the moisture or mechanically mixed water is evaporated1 but the combined water remains unaffected#d until the temperature is raised to more than #fjj ,c41 when it is driven off and the clay is converted into a hard stone-like mass with properties entirely different from those it previously possessed 7see ,chapter ,,vi74 ,footnote #d3 ,strictly1 there is a slight loss at lower temperatures1 but it is too small to be important4 ,the ,physical ,characters of ,clays4 ,the physical characters of clays are of far more interest and importance than their chemical ones1 though the two are naturally connected in many ways1 and just as the chemical composition of clays is a subject of extreme complexity so is a study of many of their physical properties4 ,hence only a few of the more important characteristics can be mentioned here3 for further details the reader must consult a larger treatise 7#b74 ,clays are moderately soft1 solid bodies1 particularly when moistened1 and can usually be cut with a knife1 though some indurated clays and shales are almost as hard as felspar4 ,their apparent specific gravity varies greatly1 some clays being much more porous than others1 but the true specific gravity is usually between #be and #bfe2 it is similar to that of quartz and slightly lower than that of felspar and mica4 ,many clays appear to be devoid of structure1 but those obtained from a considerable depth below the surface are frequently laminated and have a structure not unlike that of mica4 ,this will be discussed later4 ,examined under a microscope1 clays are seen to consist of grains of a variety of sizes1 the largest of which will usually be found to be composed of adventitious materials such as sand1 quartz1 felspar1 mica1 chalk and limestone4 ,the smallest particles--to which clays owe their chief characteristics--are so minute as to make any examination of their shape very difficult1 but they are usually composed of minute crystalline plates together with a much larger proportion of apparently amorphous material4 ,the exact nature of both the crystals and the amorphous material is still unknown in spite of many investigations2 in the purer clays both forms of substance appear to have the same chemical composition1 viz4 that of kaolinite 7,h#d;,al#b,si#b,o#i71 which the crystalline portion closely resembles4 ,clays emit a characteristic yet indefinable odour when moist2 the cause of this is very imperfectly understood1 though it is not improbably due to decomposing organic matter1 as this occurs in most clays4 ,the colours of freshly-dug clays are extremely varied and range from an almost pure white through all shades of yellow1 red and brown to black4 ,the predominating colours are grey or greyish brown and a peculiar yellow characteristic of some surface clays4 ,the natural colour of a clay is no criterion as to its purity1 for some of the darkest ball clays produce perfectly white ware on burning1 whilst some of the paler clays are useless to the potter on account of the intensity of their colour when they come out of the kiln4 ,the colour of raw clays is largely due to the carbonaceous matter they contain1 and as this burns away in the kiln1 the final colour of the ware bears no relation whatever to that of the original clay4 ,the colour of burned ware depends upon the iron compounds in the clay--these producing buff1 red1 brown or black 7usually termed 'blue'7 articles--on the presence of finely divided calcium carbonate 7chalk7 which can destroy the colouring power of iron compounds and produce white ware1 and on the treatment the clay has received in the kiln4 ,a clay which is white when underfired will usually darken in colour if heated to vitrification1 and one which burns red in an oxidizing atmosphere may turn blue-grey or black under reducing conditions4 ,the extent to which the carbonaceous matter is burned out also determines the colour of the fired ware4 ,the presence of adventitious minerals in the clay may also affect its colour1 particularly when fired4 ,the most obvious feature in a piece of moist clay is its plasticity#e or ability to alter its shape when kneaded or put under slight pressure and to retain its new shape after the pressure has been removed4 ,it is this property which enables the production of ornaments1 vessels of various kinds1 and the many other articles which are the result of the application of modelling tools1 of moulding or of the action of a potter's wheel4 ,so long as clay contains a suitable proportion of moisture it is plastic and may be made into articles of any desired shape1 but if the amount of moisture in it is reduced or removed completely1 the material is no longer plastic4 ,it may become so1 however1 on adding a further suitable quantity of water and mixing1 provided that it has not been excessively heated4 ,if1 in the removal of the moisture1 the clay has been heated to #fjj ,c4 or more1 it loses its power of becoming plastic and is converted into a material more closely resembling stone4 ,footnote #e3 ,a plastic substance is one with the characteristics of 'a fluid of so great a viscosity that it does not lose its shape under the influence of gravitation4' ,the causes of plasticity appear to be somewhat numerous1 though there is no generally accepted explanation of this remarkable quality which distinguishes clays from most other substances4 ,it is true that wet sand1 soap1 wax1 lead and some other materials possess a certain amount of plasticity1 but not to anything like the same extent as clay4 ,so far as clays are concerned1 their plasticity appears to be connected with the presence of combined water as well as of mechanically mixed water1 for if either of these are removed1 plasticity--both actual and potential--is destroyed4 ,the part played by water is not1 however1 completely known1 for the many theories which have been advanced only cover some of the conditions and facts4 ,a number of observers agree that the molecular constitution of clay is peculiar and that it is to this that plasticity is due4 ,yet the curious fact that the purest clays--the kaolins--are remarkably deficient in plasticity shows that molecular constitution is not1 alone1 sufficient4 ,others hold that the remarkably small size of clay particles enables them to pack together more closely than do particles of other materials and to retain around them a film of water which acts partly as a lubricant1 facilitating the change of shape of the mass when under pressure1 and partly as an adhesive1 causing the particles to adhere to each other when the pressure is removed4 ,zschokke has laid much emphasis on the importance of molecular attraction between clay and water as a cause of plasticity1 and has suggested that the absorption of the water effects a change in the surfaces of the clay particles1 giving them a gelatinous nature and enabling them to change their form and yet keep in close contact4 ,the fact that mica1 fluorspar and quartz1 when in a sufficiently finely divided state1 are also slightly plastic1 appears to be opposed to the molecular constitution theory4 ,smallness of grain undoubtedly has an influence on the plasticity of clay1 coarse-grained clays being notably less plastic than others4 ,daubre pointed out that felspar1 when ground with water1 develops plasticity to a small extent1 and ,olschewsky carried this observation further and has suggested that clays owe their plasticity to prolonged contact with water during their removal from their place of formation and previous to or during their deposition4 ,a further confirmation of this theory is due to ,mellor 7#c7 who showed that on heating china clay with water under very considerable pressure its plasticity was increased and that felspar and some other non-plastic materials developed plasticity under these conditions4 ,johnson and ,blake 7#ba7 supposed that plasticity is due to the clay being composed of extremely minute plates 'bunched together1' a view which was also held by ,biedermann and ,herzfield1 ,le ,chatelier and others4 ,olschewsky enlarged this theory by suggesting that the plasticity of certain clays is dependent on the large surface and the interlocking of irregular particles with the plates just mentioned4 ,these theories of interlocking are1 however1 incomplete1 because the tensile strength of clays should accurately represent the plasticity if interlocking were the sole cause4 ,zschokke has shown that tensile strength is only one factor which must be determined in any attempt to measure plasticity4 ,e4 ,h4 ,l4 ,schwarz 7#ce7 has suggested that many clays are composed of small globular masses of plates so arranged as to form an open network 7fig4 #d7 which is sufficiently strong not to be destroyed by pressure4 ,in the presence of water and much rubbing the plates are separated and are made to lie flat on each other1 thereby giving a plastic and impermeable mass4 ,if this is really the case it would explain the porosity and large surface of some clays and might account for their adsorptive power4 ,illustration3 ,fig4 #d4 ,illustrating the structure of a 'clay crumb4' 7,after ,schwarz47 ,a theory which was first promulgated in #ahej by ,way 7#d71 but which has only received detailed attention during the last few years1 attributes plasticity to the presence of colloid substances in clay or to the fact that clay particles possess physical characters analogous to those of glue and other colloids4 ,these colloid substances have a submicroscopic or micellian structure2 they are web-like1 porous and absorb water eagerly4 ,this water may be removed by drying1 only to be re-absorbed on cooling1 but if the heating temperature is excessive the structure of the colloids is destroyed4 ,this colloid theory explains many of the facts noted by earlier investigators such as ,aron1 ,bischof1 ,seger1 ,olschewsky1 etc41 but it is not entirely satisfactory1 though ,rohland 7#e7--to whom the present prominence of this theory in ,europe is largely due--persistently maintains the contrary4 ,one great objection is the fact that no characteristic inorganic colloid substance has been isolated from pure clay4 ,it is possible that some of the so-called 'colloidal' properties of clay may be due to the smallness of its particles and to their great porosity1 as suggested by ,olschewsky4 ,despite the present impossibility of producing a plastic material from artificially prepared colloidal hydro-alumino-silicates of the same ultimate composition as clay1 and the fact that the addition of colloidal substances does not necessarily increase the true plasticity of clay1 it cannot be denied that the presence of colloids has an important influence on it4 ,the addition of starches1 glue1 gums and similar substances whilst apparently increasing the plasticity of clay does not do so in reality4 ,the addition of #a per cent4 of tannin1 on the contrary1 has been found by ,ries 7#f7 to increase both plasticity and binding power4 ,plasticity appears to be composed of a number of characteristics so that it is scarcely likely that any single cause can be assigned to it4 ,on the contrary1 a study of the binding power1 tensile strength1 extensibility1 adsorption1 texture and molecular constitution of clays suggests very strongly that all these properties are involved in the production of plasticity and that it is due to the chemical as well as the physical nature of clay4 ,no clay is entirely colloidal--or it would be elastic and not plastic--but all appear to contain both colloidal and non-colloidal 7including plate-like7 particles1 and it is not improbable that materials in both these states are required1 the colloidal matter acting as a cement4 ,ries 7#f7 has1 in fact1 pointed out that colloids alone lack cohesiveness and solidity1 and a fine mineral aggregate is necessary to change them into a plastic mass resembling clay4 ,the relative proportions of the colloidal material and the sizes of the non-plastic grains will exercise an important influence on all the physical characteristics mentioned above1 and therefore on the plasticity4 ,the manner in which slightly plastic clays become highly plastic in nature is by no means certainly known4 ,it has long been understood that the increase of plasticity is due to changes undergone by the clay during transportation4 ,the most illuminating suggestion is that made by ,acheson in #aijb1 who concluded that it is due to impurities in the water used in transporting the clay or remaining in contact with it during and after its deposition4 ,these impurities may be considered as derived from the washings of forests1 and after many experiments with plant extracts ,acheson believed the most important substance in this connection to be tannin or gallo-tannic acid1 a dilute solution of which he found increased the plasticity of china clay by #cjj per cent4 ,from this he further argued that the use of chopped straw by the ,israelites in ,egypt in the manufacture of bricks was unconsciously based on the tannin content of the straw increasing the plasticity of the material4 ,illustration3 ,fig4 #e4 ,chart showing rates of drying4 7,after ,bleininger47 ,beadle has stated that #b per cent4 of dissolved cellulose will increase the plasticity of china clay and make it equal to that of ordinary clay4 ,plasticity is diminished by heating clays1 and whilst much of it may be recovered if the temperature has not risen above #djj ,c4 it cannot be completely restored4 ,moreover1 a clay which has once been heated to a temperature above #ajj ,c4 dries in a somewhat different manner to a raw clay4 ,this is well shown in fig4 #e in which are summarized the results obtained by ,a4 ,v4 ,bleininger on a sample of ball clay from ,dorset before heating and after portions of it had been heated for #af hours to #bjj1 #bej1 #cjj1 #cej and #djj ,c4 respectively4 ,it is not impossible that if subjected to the influence of water for a sufficiently long time the whole of the plasticity of a heated clay may be restored1 providing that the temperature has not been sufficient to cause a destruction of the clay molecule1 but as this resumption requires a certain amount of time1 ,bleininger has proposed to use the reduction in plasticity effected by the heating to enable excessively plastic clays to be worked without the necessity of adding non-plastic material to them4 ,if any destruction of the clay-molecules has occurred1 the plasticity of that portion of the clay can never be restored4 ,the binding power of clays is a characteristic closely connected with plasticity and occasionally confused with it4 ,all plastic clays have the power of remaining plastic when mixed with materials such as sand1 brick-dust 7'grog'7 and other materials which are quite devoid of plasticity4 ,the extent to which a clay can thus bind other materials together into a plastic mass depends1 apparently1 on the plasticity of the clay itself and on the size and nature of the particles of the added material2 the more plastic the clay the larger will be the amount of material it can thus 'bind1' and the finer the latter the more easily will it form a strong material when mixed with a plastic clay4 ,rohland 7#e7 has shown that the binding power of clay is not alone due to its cohesion1 but that it is closely associated with the colloidal nature of plastic clays3 'fat' clays being those which are highly colloidal1 highly plastic and possessing great binding power1 whilst 'lean' clays are those deficient in these characteristics4 ,the fact that1 as a general rule1 the dark coloured clays possess the most binding power1 confirms this suggestion1 as the dark colour is largely due to organic materials1 probably in a colloidal state4 ,the shrinkage which all clays undergo on drying and when heated is another important characteristic4 ,it is due to the fact that as water is removed the solid particles approach closer to each other1 the volume of the whole mass being thereby reduced4 ,in a wet piece of clay each particle is surrounded by a film of water1 the thickness of which depends on the nature of the clay4 ,as this water evaporates from the surface of the clay its place is taken by water from the interior which rises to the surface by capillary attraction4 ,so long as there is any water between the particles of clay there will be shrinkage when this water is removed1 but a stage is eventually reached when the particles of clay are in contact with each other and no more shrinkage can occur4 ,that this cessation of shrinkage may take place before all the water has been removed from the clay is easily understood when it is remembered that whilst the clay particles may be in contact1 yet there are still places 7pores7 where the contact is incomplete1 and in these pores water may be retained4 ,the amount of shrinkage clays undergo on drying depends partly on the proportion of water added to them and partly on the sizes of the different particles of clay1 sand1 etc4 present4 ,an average reduction in volume of #ab to #ch per cent4 may be regarded as normal1 but coarse loams may shrink only #a per cent4 and very finely ground1 highly plastic ball clays may shrink as much as #ej per cent41 though this is unusual4 ,as all coagulated colloids1 which have absorbed water1 shrink on drying1 this behaviour of clay appears to confirm the view as to its partially colloidal nature held by some investigators4 ,when a piece of dry clay is heated sufficiently a further shrinkage 7technically known as kiln shrinkage7 occurs4 ,this begins somewhat below a red heat and increases in rough proportion to the temperature and the duration of the heating4 ,prolonged heating at a lower temperature will effect the same amount of shrinkage as a short exposure to a higher temperature1 but though the greater part of the shrinkage occurs in a comparatively short time1 continued heating will be accompanied by a further reduction in volume4 ,this is due to the fact that clays have no definite melting point1 but undergo partial fusion at all temperatures above #iej ,c4 or1 in some cases1 at even lower ones4 ,as a portion of the material fuses1 it fills up the pores in the mass and attacks the unfused material1 this process being continued until either the heating is stopped or the whole material is reduced to a viscous slag4 ,the reduction in the volume of commercial articles made of clay and placed in kilns varies greatly4 ,with bricks1 terra-cotta and pottery it must not1 usually1 exceed #dj per cent4 or the warping and cracking which occur will be so great as to make the articles useless4 ,the fineness of the particles exercises an important influence on the kiln shrinkage of a clay1 and the latter is frequently reduced in commercial clayworking by adding burned clay ground to a coarse powder to the plastic clay before it is used4 ,sand is sometimes added for the same purpose1 though its more frequent use is to reduce the shrinkage in drying4 ,quartz and other forms of free silica expand on heating1 so that clays containing them in large quantities shrink very slightly or may even expand4 ,as clays shrink equally in all directions it is usual to state the contraction in linear instead of volume form4 ,thus instead of stating that a certain clay when moulded into bricks1 dried and burned1 shrinks #ah per cent4 by volume1 it is customary to state that it shrinks #c_/d in4 per 7linear7 foot4 ,for many purposes1 it is sufficient to regard the linear shrinkage as one-third the volume-shrinkage1 but this is not strictly accurate4 ,the fusibility of clays is a characteristic which has been very imperfectly studied4 ,most clayworkers and investigators employ the term 'fusibility' in a special sense which is apt to be misleading4 ,owing to the extremely high temperatures to which refractory clays can be heated without even losing their shape1 it is almost impossible to fuse them completely4 ,in addition to this1 clays are not perfectly homogeneous materials and some of their constituents melt at lower temperatures than others4 ,for this reason a clay may show signs of fusion at #aajj ,c41 but it may be heated for some hours at #ahjj ,c4 and yet not be completely melted6 ,consequently no single 'fusing point' can be stated4 ,in practice1 a suggestion made many years ago by ,seger 7#g7 is used2 the clay to be tested is made into a small tetrahedron 7fig4 #f71 heated slowly until it bends over and the point of the test-piece is almost on a level with the base4 ,the temperature at which this occurs is termed the 'fusing point' though it really only indicates the heat-treatment which is sufficient to soften the material sufficiently to cause it to bend in the manner described4 ,in spite of the apparent crudeness of the test this 'softening point' appears to be fairly constant for most refractory clays4 ,the bending of a test-piece in this manner is the result of the action of all fluxes#f in the clay1 and as this depends on the size of grain and the duration of the heating above incipient fusion and does not give a direct measure of temperature1 nor is the softening effect under one rate of rise in temperature the same as that at another rate4 ,nevertheless a study of the behaviour of various clays heated simultaneously is valuable and the method forms a convenient means of comparing different materials4 ,footnote #f3 ,for fluxing materials see p4 #h4 ,the temperature may be measured by means of a pyrometer1 but for the reason just stated it is more convenient and in some respects more accurate to use standard mixtures known as ,seger ,cones 7fig4 #f71 and to state the softening point in terms of the 'cone' which behaves like the clay being tested4 ,a medium fireclay will not soften below ,seger ,cone #bf 7#afej ,c47 and a really good one will have a softening point of cone #cd or #ce 7#agej to #ahjj ,c474 ,illustration3 ,fig4 #f4 ,seger ,cones indicating a temperature of #abej ,c4 ,the refractoriness of a clay1 or its resistance to high temperatures1 is an important requirement in bricks required for furnace linings1 in crucibles1 gas retorts and other articles used in the metallurgical and other industries4 ,the term is much abused and is frequently understood to mean resistance to the cutting action of flue gases and flame1 the corrosive action of slags1 and the strains set up by the repeated changes in temperature4 ,this is unfortunate1 for the term refractoriness has a perfectly definite meaning and should be employed exclusively to denote that a given clay is capable of retaining its shape at a given temperature or under given conditions when heated alone and without being subjected to any pressure4 ,in ,great ,britain there is no officially recognized standard of minimum refractoriness#g1 but where one is required the suggested minimum of ,seger ,cone #bf 7#afej ,c47 made by ,e4 ,cramer 7#h7 is usually employed4 ,this is the recognized minimum in ,germany for fireclays1 and though objections may be urged against the use of ,seger ,cones as a standard1 equally forcible ones may be brought against making a temperature-scale the basis of measurement4 ,under present circumstances1 however1 it is necessary to adopt one or other of these4 ,footnote #g3 ,see ,refractory ,clays1 ,chapter ,v4 ,various attempts have been made to ascertain the relationship 7if any7 between the refractoriness of clays and their chemical composition4 ,if attention is confined strictly to the more refractory clays1 some kind of relationship does appear to exist4 ,thus ,richter found that the refractoriness of clay is influenced by certain oxides in the following order3 magnesia1 lime1 ferrous oxide1 soda and potash1 but this only applies to clays containing less than #c per cent4 of all these oxides4 ,cramer1 in #ahie1 found that free silica also interfered with the action of these oxides and more recently ,ludwig 7#i7 has devised a chart 7fig4 #g71 on the upright sides of which are plotted the equivalents of the lime1 magnesia and alkalies1 whilst the silica equivalents are plotted on the horizontal base4 ,in each case the 'molecular formula' of the clay is calculated from its percentage composition1 and this 'formula' is reduced so as to have one 'molecule' of alumina1 thereby fixing the alumina as a constant and reducing the number of variables to two--the metallic oxides and the silica4 ,unfortunately ,ludwig's chart is only applicable to the more refractory clays and cannot be relied upon even for these1 though it is extremely useful for comparing clays from identical or similar geological formations4 ,illustration3 ,fig4 #g4 ,ludwig's ,chart4 ,attempts to express the refractoriness of clays by means of formulae proving abortive1 there only remains the direct test of heating a clay under definite conditions in the manner previously described4 ,vitrification is closely connected with the fusibility and refractoriness of clays1 and1 as a term1 indicates the amount of fusion which has occurred under certain conditions of heating4 ,as already mentioned1 all clays1 on being subjected to a high temperature1 undergo partial fusion1 the more powerful bases attacking the finest particles of clay and silica1 forming molten silicates1 and then slowly attacking the more refractory portion2 this slow fusion and solution continues until the whole of the material is melted4 ,if the heating is stopped before the fusion has begun1 the clay will be porous and comparatively soft1 but as more and more material fuses1 the mass 7on cooling7 becomes harder and less porous1 as the fused material occupies the pores and sets to a dense1 firm glassy mass4 ,the amount of vitrification1 or partial fusion1 which occurs is1 therefore1 of great importance in some industries1 as by stopping it at an appropriate stage articles of any desired degree of porosity1 translucency or strength may be obtained4 ,thus for common bricks1 only sufficient vitrification is permitted to bind the particles firmly together1 but in engineering bricks--where much greater strength is required--the vitrification is more complete4 ,porcelain and earthenware may be similarly distinguished4 ,the extent to which a given clay will vitrify depends on the amount of fluxing material 7metallic compounds1 and oxides other than ferric oxide and alumina7 it contains1 on the smallness of its particles and on the duration and intensity of the heating4 ,clays containing alkalies and lime compounds vitrify with great rapidity when once the necessary temperature has been reached1 so that unless great care is exercised the action will proceed too far and the goods will be warped and twisted or may even form a rough slag4 ,refractory clays1 on the contrary1 vitrify more slowly and at much higher temperatures so that accidental overheatings of them are far less common4 ,the difference between the temperature at which sintering or vitrification occurs and that at which the clay melts completely--usually termed the 'vitrification range'--varies with the nature of the clay4 ,in some cases the clay melts as soon as vitrification becomes noticeable1 in others the vitrification occurs at a dull red heat1 but the material does not lose its shape until after a prolonged heating at the highest temperature of a firebrick kiln or testing furnace4 ,calcareous clays have the melting and sintering points close together1 so that it is almost impossible to produce vitrified and impervious ware from them1 as they lose their shape too readily4 ,if1 however1 the difference between the sintering and fusing temperatures can be enlarged--that is1 if the vitrification range can be extended--more impervious ware can be made4 ,the easiest means of extending the vitrification range consists in regulating the proportion of large and small particles4 ,the former increase and the latter diminish the range4 ,basic compounds and fluxes cause a lowering of the melting-point and a shortening of the vitrification range4 ,the porosity of raw clay is usually of small importance1 but the porosity of fired clay or ware is often a serious factor in determining the suitability of certain articles for their intended purposes4 ,in its natural state1 clay does not readily absorb much water2 on the contrary it becomes pasty and impervious unless it is disturbed and its texture destroyed1 when it may be mixed with water to form a paste or1 with more water1 a thin 'cream' or 'slurry4' ,when heated moderately1 clay forms a porous material and1 unless the heating is excessive1 it will absorb about one-eighth of its weight of water4 ,further heating at a higher temperature reduces its porosity--the more easily fused material filling some of the pores--until a stage is reached when the material is completely vitrified and is no longer porous4 ,porosity may thus be regarded as the opposite of vitrification2 porous goods being relatively light and soft whilst vitrified ones are dense and hard4 ,for some purposes1 porosity is an important characteristic3 for example1 building bricks which are moderately porous are preferable to those which are vitrified4 ,the manufacture of porous blocks for the construction of light1 sound-proof partitions1 etc4 has increased rapidly of late4 ,they are made by adding sawdust or other combustible material to the clay4 ,the added substances burn out on firing the goods in a kiln4 ,clays which are porous can be dried more readily and with less risk of cracking than those which are more dense4 ,for this reason1 some clayworkers mix non-plastic material such as sand or burned clay with their raw material4 ,the impermeability of plastic clay to water is a characteristic which is important for many purposes4 ,the absorptive power of clays is closely related to their porosity so far as pure water is concerned1 but if the water contains certain salts in solution a selective absorption occurs1 the bases being retained by the clay in such a manner that they cannot be removed by washing4 ,the selective action is known as adsorption and is most noticeable in highly plastic clays4 ,bourry 7#aj7 has shown that the slightly plastic china clays only exercise a small power of adsorbing calcium carbonate from solution1 but highly plastic clays may adsorb #bj per cent4 of it4 ,the alkaline chlorides and sulphates do not appear to be adsorbed in this manner1 but the carbonates are readily removed from solution4 ,all calcium and magnesium compounds appear to be adsorbed1 though in variable quantities1 the reaction being complicated when several soluble salts are present4 ,ries 7#f7 has found that gallo-tannic acid is adsorbed readily and increases the plasticity of clay4 ,ashley 7#aa7 has endeavoured to measure the plasticity of clays by determining their adsorption capacity for various aniline dyes1 but his untimely decease prevented the investigation being completed4 ,there is reason to suppose that the relation between adsorption and plasticity is extremely close in many clays and that the former may1 to an important extent1 be used as a measure of the latter4 ,in some clays1 however1 this relationship does not exist4 ,sand and burned clay only show faint adsorption phenomena2 felspar shows them to a slight and almost negligible extent and most of the other non-plastic ingredients of clays are non-adsorptive4 ,selective adsorption being an important characteristic of colloidal substances1 the possession of this power by plastic clays supports the claim that plasticity is due1 at least in part1 to the presence of colloids4 ,the addition of small quantities of a solution of certain substances to a stiff clay paste usually reduces its stiffness1 and in some cases turns it into a liquid4 ,the alkalies are particularly powerful in this respect and their action may be strikingly illustrated by mixing a few drops of caustic soda with a stiff clay paste4 ,in a few moments the mixture will be sufficiently liquid to pour readily1 but it may be rendered quite stiff again by adding sufficient acid to neutralize the alkali previously used4 ,weber 7#ab7 has utilized this characteristic to great advantage in the production of sanitary ware and crucibles for glass-making by a process of casting which he has patented4 ,the effect of adding water to a dry clay is curious4 ,at first the particles in contact with the water become sticky and plastic1 and if the proportion of water added is suitable and the mixing is sufficiently thorough a plastic mass will be produced1 the characteristics of which will depend on the nature of the clay used4 ,this process of mixing clay with a limited amount of water is known as 'tempering4' ,the proportion of water required to make a paste of suitable consistency for modelling appears to be constant for each clay4 ,if1 however1 a larger proportion of water is added the particles of clay will be separated so widely from each other that they lose their cohesion1 and instead of a plastic mass1 the material will form a liquid of cream-like consistency4 ,if a piece of stiff clay paste is suspended in a large volume of water without stirring1 disintegration will still occur 7though a much longer time will be required7 and the clay will be deposited as a sediment at the bottom of the vessel4 ,the leaner the clay or the larger the proportion of non-plastic material it contains1 the more rapidly will this disintegration take place4 ,a highly plastic clay will become almost impervious and will retain its shape indefinitely4 ,if a mixture of clay and water in the form of a cream or slurry be allowed to rest1 the larger and less plastic particles will settle1 but many of the particles of true clay will remain suspended for several hours and some of them for several days4 ,some particles of clay are so small that it is doubtful if they would ever settle completely unless some coagulant were added1 and as they readily pass through all ordinary filtering media it is extremely difficult to collect them in a pure state4 ,these turbid suspensions of clay may be rapidly cleared by the addition of sodium chloride which increases the surface tension of the solution4 ,the fine particles behave in the same way as colloidal substances1 i4e4 as if they possessed an electrostatic charge4 ,hence the addition of a salt 7electrolyte71 whose ions annul the opposite charges of the electric double layer assumed by ,helmholtz to be present1 enables the particles to coagulate in accordance with the ordinary laws of surface tension 7#ad74 ,exposure to the action of air and frost has a marked effect on many clays4 ,when freshly dug these may be hard and difficult to crush1 but after exposure they break up readily into small fragments4 ,clays differ greatly in the extent to which they are affected by exposure2 some are completely disintegrated by standing #dh hours in the open air1 whilst others are scarcely affected by exposure in bleak places through several years of storm1 sunshine and frost4 ,usually1 however1 the effect of a couple of nights exposure to hard frost will produce a marked disintegration of the material4 ,this process of exposure is known as 'weathering' and its effects are so important that it is employed whenever possible for clays requiring to be crushed before use4 ,all clays are rendered more workable by exposure1 but some of them are damaged by the oxidation of some impurities 7e4g4 pyrites7 in them1 though in other clays this very oxidation1 if followed by the leaching action of rain1 effects an important purification of the material4 ,weathering appears to have no effect on the chemical composition of the particles of true clay in the material1 though it may decompose the impurities present4 ,on the clay itself its action is largely physical and consists chiefly in separating the particles slightly from each other1 thereby enabling water to penetrate the material more readily and facilitating the production of a plastic paste4 ,the disintegrating action of the weather on some 'clays' is so complete that they require no crushing but can be converted into a homogeneous paste by simply kneading them with a suitable proportion of water4 ,it is possible that on exposure to the heat of the sun's rays--particularly in tropical climates--some chemical decomposition of the clay may occur1 but compared with the purely physical action of weathering the amount of such chemical decomposition must be relatively unimportant in most cases4 ,it may1 however1 account for the presence of free silica and free alumina in some clays4 ,the action of the weather on rocks1 resulting in the formation of clays1 is described in ,chapter ,,iii4 ,heat effects remarkable changes in the physical character of clays2 the most important of these have already been noted4 ,at a gentle heat1 the clay is dried and retains most of its power of becoming plastic when moistened2 very little1 if any1 decomposition occurs4 ,at a higher temperature it loses its 'combined water1' the clay molecule apparently dissociating1 and a hard stony mass--consisting of particles of free silica and free alumina cemented together by the more easily fusible impurities present--is formed4 ,if the heating is continued the hardness of the material is increased owing to more molten silicate having been produced from the impurities present1 and on cooling1 its tensile strength and resistance to crushing will be found to be enormously greater than those of the original clay4 ,all potential plasticity is destroyed by heating to #gjj ,c4 and no method of restoring it has yet been devised4 ,as clays are abundant1 this is not a serious disadvantage for the specially desired characteristics of bricks1 terra-cotta1 pottery and porcelain are all such as to be incompatible with plasticity4 ,the latter is extremely valuable in the shaping of the wares mentioned1 but after the manufacture is completed1 the destruction of the plasticity is an essential feature of their usefulness4 ,if the heating is very prolonged or is repeated several times1 clays change other of their physical characters and become brittle and liable to crack under sudden changes of temperature4 ,this is partly due to the further fusion 7vitrification7 which occurs and partly to the formation of crystalline silicates1 notably ,sillimanite 7#ac74 ,the extent to which clays are ordinarily heated and the conditions under which they are cooled do not usually induce the formation of crystals2 the object of the clayworker being to produce a homogeneous mass1 the particles of which are securely held together4 ,the result is that burned clay products are usually composed of amorphous particles cemented by a glass-like material formed by the fusion of some of the mineral ingredients of the original substance4 ,the silicates formed are1 therefore1 in a condition of solid1 super-cooled solution in which the tendency to crystallize is restrained by viscosity4 ,on raising the temperature of firing or on prolonging the heating at the previous maximum temperature the viscosity of the fused portion is diminished and crystallization may then occur4 ,the facility with which crystallization occurs varies greatly with the composition of the fused material1 those silicates which are rich in lime and magnesia crystallizing more readily than those containing potash or soda4 ,vogt has stated that small quantities of alumina promote the formation of a glassy structure1 and ,morozewicz has shown that a large excess of this substance must be present if crystallization is to occur4 ,the study of the reactions which occur when clays are heated is1 however1 extremely complex1 not only on account of the variety of substances present1 but also on account of the high temperatures at which it is necessary to work1 so that for a further consideration of it the reader should consult special treatises on the fusion of silicates4 ,this subject has now become an important branch of physical chemistry4 ,,chapter ,,ii ,,clay ,,and ,,associated ,,rocks ,clay1 as already mentioned1 is geologically a rock and not a mineral1 and belongs to the important group of sedimentary rocks which have been derived from the igneous or primary ones by processes of weathering1 suspension in water and subsequent deposition or sedimentation4 ,whatever may be the primary origin of clay1 its chief occurrence is in geological formations which have undoubtedly been formed by aqueous action4 ,the materials resulting from the exposure of primary rocks to the action of the elements have been carried away by water--often for long distances--and after undergoing various purifications have been deposited where the speed of the water has been sufficiently reduced4 ,in some cases they have again been transported and re-deposited and not infrequently clay deposits are found which show signs of subsequent immersion at considerable depths and have every appearance of having been subjected to enormous pressures and possibly to high temperatures4 ,some clays have only been carried by small streams and for short distances2 these are seldom highly plastic and resemble the lean china clays and kaolins4 ,others have been carried by rapidly moving rivers and have been discharged into lakes or into the sea2 they have thus undergone a process of gradual purification by elutriation1 the sand and other heavier particles being first deposited and the far smaller particles of clay being carried a greater distance towards the centre of the lake or the quieter portions of the ocean4 ,the nature of such deposits will1 naturally1 differ greatly from each other1 the materials at first associated with the clay1 or becoming mixed with it at a later stage1 exercising an important influence on its texture1 composition and properties4 ,if the transporting stream flows through valleys whose sides are formed of limestone1 chalk1 sandstone or other materials1 these will become mixed with the clay1 and to so great an extent has the mixing occurred that very few clays occur in a state even approximating to purity4 ,the majority of clays are contaminated with iron oxide1 lime compounds and free silica in such a fine state of division that it is impossible to purify them completely without destroying the nature of the clay4 ,in addition to this it must be remembered that the land is continually rising or sinking owing to internal changes in the interior of the earth1 and that these subterranean changes bring about tilting1 folding1 overturning and other secondary changes1 which1 later1 cause a fresh set of materials to be mixed with the clays4 ,further than this1 the action of the weather1 of rivers and of the sea never ceases1 so that a process of re-mixing and re-sorting of materials is continuously taking place1 and has been doing so for countless ages4 ,it is1 therefore1 a legitimate cause for wonder that such enormous deposits of clays of so uniform a character should occur throughout the length and breadth of ,europe1 and practically throughout the world4 ,for although the composition of many of these beds is of a most highly complex nature1 the general properties such as plasticity1 behaviour on heating1 etc41 remain remarkably constant over large areas of country1 and the clays of each geological formation are so much alike in different parts of the world as to be readily recognized by anyone familiar with the material of the same formations in this country4 ,considerable differences undoubtedly exist1 but these are insignificant in comparison with the vastly different circumstances under which the deposits were accumulated4 ,leaving the consideration of the modes of formation of the various clay deposits to later chapters 7,,iii and ,,iv71 it is convenient here to enumerate some of the chief characteristics of the different clay deposits and their associated rocks4 ,in this connection it is not proposed to enter into minute details1 but rather to indicate in broad outline the chief characteristics of the clays from the different deposits4 ,this general view is the more necessary as clay occurs in each main geological division of the sedimentary rocks and in almost every sub-division in various parts of the world4 ,the ,precambrian1 ,cambrian1 ,silurian and ,devonian 'clays' are chiefly in the form of shales or slates1 the latter being clays which have undergone a metamorphic change2 the latter resulted in the production of a hard and partially crystalline material with but little potential plasticity and therefore of small importance for the ordinary purposes of clay working4 ,slates are distinguished from shales by their splitting into thin leaves which are not in the plane of original deposition1 but are due to the deposited material being subjected to great lateral pressure4 ,the re-arrangement of the particles thus produced has imparted to the material a cleavage quite independent of the original lamination4 ,the shales in these formations are occasionally soft and friable and are then termed marls1 but this name is misleading as they contain no appreciable proportion of finely divided calcium carbonate as do the true marls#h4 ,footnote #h3 ,readers desiring more detailed information on the occurrence of the clays mentioned in this chapter should refer to the author's ,british ,clays 7,no4 #b in ,bibliography74 ,the clays in the ,carboniferous ,limestone are not1 as a whole1 of much importance1 but the occurrence in this formation of pockets of white refractory clays in ,staffordshire1 ,north ,wales 7,mold7 and ,derbyshire is interesting1 especially as these are used for the manufacture of firebricks and furnace linings4 ,these clays are highly silicious and in composition are intermediate between the ,yorkshire fireclays and ganister4 ,their origin is uncertain1 but it is generally considered that they have been produced by the action of the weather and streams on the shales and grits of the ,coal ,measures which formerly occupied the higher ground around them1 though ,maw 7#af7 states that 'it is scarcely open to question that they are the remnants of the subaerial dissolution of the limestone' 7see ',fireclays1' ,chapter ,v74 ,in the ,upper ,carboniferous ,system the clays are highly important because of their general refractory nature1 though they differ greatly in this respect1 some red-burning shales of this formation having no greater power to resist heat than have some of the surface clays4 ,those of the ,coal ,measures are of two main kinds--shales1 or laminated rocks which readily split along the planes of deposition1 and unstratified underclays4 ,the shales usually occur above the seams of coal and are either of lacustrine or marine origin1 differences in their fossils and lithological character supporting one origin for some deposits and the other for the remainder4 ,some of them are fairly uniform in composition1 but others vary so greatly in their physical characters1 that they are divided by miners into 'binds' or relatively pure shales1 'rock-binds1' or sandy shales1 and sandstones4 ,they also vary greatly in thickness in different localities1 and whilst they form the main feature in some districts1 in others they are replaced by sandstones4 ,the underclays are so called from their usually lying beneath the coal seams4 ,they are not noticeably stratified and vary greatly in character from soft unctuous materials to hard1 sandy rocks4 ,in composition they vary enormously1 the percentage of silica ranging from #ej per cent41 or less1 to as high as #ig per cent4 ,the mode of formation of the underclays is not certainly known4 ,they do not appear to be soils or of terrestrial origin1 but according to ,arber 7#bd7 correspond closely to the black oozes of marine and semi-marine estuarine deposits of tropical swamps1 or to the muds surrounding the stumps of trees in the buried forests of our coast-lines4 ,they thus appear to be quite distinct from the shales above them1 both in origin and physical characters4 ,the more silicious portions1 known as ,ganister#i1 possess comparatively few of the characteristics of clay though used1 like all the more refractory clays of the ,coal ,measures1 for all purposes for which fireclay is employed4 ,the term fireclay is1 in fact1 frequently applied to all the refractory deposits in the ,coal ,measures1 without much regard to their composition 7see ,chapter ,v74 ,footnote #i3 ,the ,dinas rock used in the ,vale of ,neath 7,wales7 is an even more silicious material found in the ,millstone ,grit immediately below the ,coal ,measures4 ,it is largely employed for firebricks4 ,valuable ,coal ,measure clays occur in enormous quantities in ,northumberland1 ,durham1 ,yorkshire1 ,nottinghamshire1 ,derbyshire1 ,staffordshire1 near ,stourbridge1 in ,warwickshire1 ,shropshire1 ,north and ,south ,wales and ,south ,west ,scotland4 ,in ,ireland1 on the contrary1 the ,coal ,measure clays are of little value except in the neighbourhood of ,coal ,island1 co4 ,tyrone4 ,the position of the ',sagger ,marls' of ,north ,staffordshire 7,keele ,series and ,etruria ,marls71 relative to the ',farewell ,rock' or ,millstone ,grit1 is shown in fig4 #h in which the horizontal lines represent coal-seams and ironstone veins4 ,illustration3 --------------------- ,keele ,series --------------------- ,newcastle --------------------- ,coal ,etruria ,marls ,top ,red ,mine --------------------- ,gubbin ,ironstone --------------------- --------------------- ,knowles ,coal ,burnwood ,ironstone --------------------- --------------------- ,mossfield ,coal --------------------- #e ft4 ,coal --------------------- ,hard mine ,coal --------------------- ,cockshead ,coal --------------------- ,crabtree ,coal --------------------- ,millstone ,grit --------------------- ,fig4 #h4 ,coal ,measures sequence in ,north ,staffordshire4 ,the dissimilarities in the fossils of the ,coal ,measure clays and shales in the ,northern and ,southern ,hemispheres suggest that there is a considerable difference in their formation1 but the number of clays and shales which have been examined is too small for any accurate conclusion to be drawn4 ,for many industrial purposes1 particularly for the manufacture of refractory goods1 the clays and shales of the ,carboniferous ,system are highly important4 ,the less valuable burn to a reddish colour1 often spoiled with many grey spots of ferrous silicate derived from the pyrites in the clay1 but the purer varieties burn to a delicate primrose or pale buff tint and are amongst the most heat-resisting materials known4 ,the ,coal ,measure clays of ,yorkshire are particularly esteemed for their refractory properties2 for the manufacture of glazed bricks and for blocks for architectural purposes somewhat ambiguously termed 'glazed terra-cotta4' ,the inferior qualities are largely used for the manufacture of red engineering bricks1 some of them competing successfully with the more widely known 'blue bricks' of ,staffordshire4 ,the ,coal ,measure clays of ,shropshire are noted for the manufacture of red roofing tiles1 especially in the neighbourhood of ,broseley4 ,agriculturally1 the ,coal ,measure clays are usually poor1 but are occasionally of good quality4 ,the shales produce heavy1 cold clays and the yellow subsoil produces soils of a light1 hungry character so that the two should1 if possible1 be mixed together4 ,permian clays are of little value except for the manufacture of red building bricks4 ,the ,nottinghamshire ,permian clays make excellent roofing tiles1 flower pots and red bricks4 ,agriculturally1 the ,permian clays are a free working loam yielding large crops of most of the ordinary farm products4 ,triassic clays are of great importance in the ,midlands1 those upper portions of them known as the ,keuper ,marls being much used for the manufacture of bricks4 ,they are specially known amongst clayworkers as the material from which the ,midland red bricks of ,nottinghamshire and ,leicestershire and the ,somersetshire tiles are prepared4 ,jurassic clays are an important group1 of marine origin1 occurring in close association with limestone4 ,for this reason they form a valuable source of material for the manufacture of ,portland cement1 but are of less value to the brick and tile manufacturer4 ,the ,jurassic ,system contains so large a variety of clays1 of such widely different ages and characteristics1 that no general description of them can be given in the present volume4 ,illustration3 ,fig4 #i4 ,lias clay being worked for the manufacture of hand-made sand-faced roofing tiles4 7,by courtesy of ,messrs ,webb ,bros4 ,ltd41 ,cheltenham47 ,the ',lias clays'--the lowest of the ,jurassic formation--are chiefly dark1 bituminous shales1 including the 'alum shales1' and are often seriously contaminated with pyrites and ironstone4 ,when carefully selected they may be used to advantage in the production of most red articles such as bricks1 tiles1 chimney pots1 etc4 ,they shrink less in the kiln than do most clays1 and are easily fusible on account of the lime they contain1 but on the whole this formation is of great value for the manufacture of the articles just mentioned4 ,agriculturally1 the ,lias clays are laid down for grass1 but the lighter soils are useful for arable purposes4 ,the ',oolitic clays1' which are also ,jurassic1 usually contain limestone in the form of nodules1 but are nevertheless important4 ,they form a broad belt above the ,lias from ,dorset to ,yorkshire1 and include the blue clays of the ,purbeck beds1 stiff blue bituminous ,kimeridge clays1 the irregular1 sandy ,coral ,rag clays1 the famous ,oxford clay 7from which the ,peterborough and ,fletton bricks are made71 the ,kellaways blue clay1 and the ,fuller's ,earth deposits4 ,the ',kimeridge clays' are dark1 stiff laminated clays1 closely resembling gault1 and are much used in the ,west and ,midlands for brickmaking4 ,a well-known deposit of this character has long been used at ,pickering in ,yorkshire1 but the most typical deposits are in ,huntingdonshire4 ,the ,kimeridge clays contain a bituminous shale1 or ,sapropelic ,coal1 which evolves a characteristic odour on burning4 ,agriculturally1 the ,kimeridge clays resemble gault and are difficult to work as arable land1 though they form first-rate pasturage4 ,illustration3 ,fig4 #aj4 ,oxford clay near ,peterborough4 7,by courtesy of ,messrs ,ruston1 ,proctor ,co4 ,ltd47 ,the ',oxford clays' are valuable for brickmaking when their use is understood1 but to the uninitiated they are very troublesome4 ,their colour is dark blue or grey and they are usually stiff or somewhat shaly in texture with layers of variable composition4 ,the closely associated ,cornbrash 7limestone7 is a source of trouble unless great care is taken in the selection of the material4 ',oxford clays' are not infrequently traversed by seams of poor coal or by oil-shales4 ,agriculturally1 ,oxford clay is difficult to work and1 while much of it is valuable1 large portions are poor and cold4 ,when well exposed to frost it is made much lighter1 but even then is not very suitable for wheat and autumn sown crops4 ,the ',kellaway blue clays' are often included in the ,oxford clays1 though they form irregular bands above them and are of fresh-water origin1 whilst the ,oxford clays are marine deposits4 ,they are chiefly used commercially for domestic firebricks near ,oundle and ,stamford4 ,cretaceous clays occur1 as their name implies1 in association with chalk4 ,the chief clay in this ,system is the gault1 a stiff1 black1 calcareous clay of marine origin chiefly used for brickmaking4 ,when used alone1 gault burns to a reddish colour1 due to the iron present1 but if1 as is more usual1 it is mixed with chalk1 it burns perfectly white4 ,some gaults contain sufficient chalk to render the addition of a further quantity unnecessary4 ,agriculturally1 the ,cretaceous clays form good arable soil where they are not too exposed1 but they suffer from drought4 ,the ',wealden clay' is a stiff yellowish grey or blue clay extensively used for brickmaking in ,kent1 ,sussex and ,surrey4 ,it has been subdivided by geologists into a number of other clays1 such as the ,wadhurst1 ,fairlight1 etc41 but the differences between them lie more in the fossils occurring in them than in the characters of the clays themselves4 ,they are usually contaminated with ironstone1 gypsum and some limestone4 ,agriculturally1 the ,wealden clay produces stiff1 yellowish soils of a wet and poor character1 but sometimes loams of a highly productive nature occur4 ,the ,tertiary clays include all those deposited after the ,chalk and previous to the close of the ,glacial period4 ,they are usually mixed with sand and gravel1 and though the deposits are often thin and irregular they are the most generally important of all clays4 ,they vary greatly in character2 some1 like the ,london clay1 being almost useless unless mixed with other materials1 whilst others like the ball clays of ,devonshire and ,dorset are amongst the purest and most valuable of the plastic clays4 ,the ,tertiary clays are divided by geologists into ,pliocene1 ,miocene and ,eocene formations2 of these the first are commercially unimportant and the second do not exist in ,great ,britain4 ,at one time the ,bovey ,tracey clays were considered to be ,miocene1 but they have recently been classed as ,oligocene by ,clement ,reid4 ,agriculturally1 the most important of the ,tertiary formations is the ,eocene1 particularly near ,london1 though it is much covered by sand or gravel4 ,the ,london clay1 which produces a heavy brown soil1 is of slight value1 though when properly drained it produces good crops of wheat1 beans1 and cabbages and other market-garden produce4 ,for this purpose it is greatly improved by the addition of lime and of town manure4 ,the ,south ,hampshire ,eocene beds of clay are cold1 wet and of small agricultural value4 ,the ,eocene clays are composed of a variety of clays1 many of which are only distinguishable by the different fossils they contain4 ,the most important are the ,reading clays1 the ,london clay and the ,bagshot clays4 ,the ,reading clays extend over a considerable area in the ,south of ,england and are most valuable near the town from which they derive their name4 ,the best qualities are mottled in a characteristic manner and are particularly suitable for the manufacture of roofing tiles and small terra-cotta--an industry for which ,reading is famous4 ,the ,london clay is always a treacherous material and is best avoided in the manufacture of bricks and other articles except under highly skilled technical advice4 ,the ,bagshot clays in ,dorsetshire are famous for the ball and pipe clays shipped from ,poole1 whilst at ,bovey ,tracey and in several parts of ,devonshire equally valuable ball clays are found and are shipped from ,teignmouth4 ,these ball clays are of variable composition and colour and require careful selection and testing4 ,they are closely associated with sands1 but the lower beds of clay are remarkably stiff1 plastic and white-burning4 ,the colour of the raw clay varies from a pale yellow to a dark brown or even to black1 but this is little or no criterion of the colour of goods made therefrom1 as the colour is due to carbonaceous matters1 #d per cent4 or more carbon being usually present4 ,the 'blue' and 'black' ball clays are the most valued by potters1 but the quality is usually ascertained by a burning test4 ,the value of these ball clays both in ,devonshire and ,dorset is due to their comparative freedom from iron and alkalies and to their remarkable unctuousness and plasticity4 ,they are1 therefore1 largely used in the manufacture of all kinds of earthenware of which they form the foundation material4 ,in composition1 ball clays appear to consist chiefly of a hydro-alumino-silicate corresponding to the formula ,h#d;,al#b,si#b,o#i1 and in this they very closely resemble the china clays 7kaolins74 ,the latter are1 however1 but slightly plastic whilst the ball clays are amongst the most plastic clays known4 ,the china clays are also much more refractory than the ball clays owing to the somewhat larger proportion of alkalies in the latter4 ,pipe clays are an inferior quality of ball clay2 they contain rather more iron and alkalies and considerably more silica4 ,for this reason they can only be used for cheaper wares where colour is of less importance and where their excessive contraction can be neutralized by the addition of other substances such as flint4 ,the ,boulder clays occur in a blanket-like covering of ,drift which lies over the greater part of ,northern and ,central ,england1 and over a considerable portion of ,scotland and ,ireland4 ,they are a product of the ,ice ,age and1 whilst varying greatly in character1 may usually be distinguished by the occurrence in them of rounded stones and gravel1 some of the former bearing clear indications of glacial action4 ,the boulder clays are largely used for the manufacture of building bricks1 but the strata in which they occur are so irregular that very careful supervision of the digging is necessary4 ,in some localities these clays form beds #ab ft4 or more in thickness and relatively free from gravel2 in other districts the clay is interspersed with lenticular deposits of gravel or sand 7commonly known as 'pockets'71 and if these are mixed with the clay considerable difficulty in manufacture may be experienced4 ,the total thickness of the drift deposits is often very great1 as in the cliffs at ,filey 7fig4 #aa7 which are #bjj ft4 high4 ,illustration3 ,fig4 #aa4 ,cliffs of ,boulder clay at ,filey lying on ,calcareous ,crag4 ,the boulder clays--considered apart from the stones1 gravel and sandy materials occurring with them--are usually red-burning1 stiff and very plastic1 but the gravel1 sand and crushed stones mixed with them in the formation of the material usually render them of medium plasticity4 ,by careful washing1 most boulder clays may be purified sufficiently to enable coarse brown pottery to be made from them4 ,clean deposits of sufficient size to be worked without any purification are occasionally found4 ,usually1 however1 the boulder clay formation is somewhat treacherous as it is difficult to ascertain its nature2 boreholes are apt to be quite misleading as the formation is so irregular in character4 ,agriculturally1 drift or boulder clays are poor soils1 but by judicious management and careful mixing they may be made more fertile4 ,where it contains chalk--as in ,norfolk and ,suffolk--boulder drift forms an excellent arable soil4 ,pleistocene or ,recent clays are amongst the most important brickmaking materials in the ,south of ,england4 ,they are of remarkably varied character1 having been derived from a number of other formations4 ,usually the deposits are somewhat shallow and irregular in form1 but beds of considerable thickness occur in some localities4 ,agriculturally1 they are of considerable importance4 ,most of the brick earths used in the south-east of ,england are of ,recent formation1 those of the ,thames ,valley being of special importance in this connection1 particularly where they are associated with chalk2 thus forming natural marls or enabling artificial marls to be produced4 ,the brick earths--in the sense in which this term is used in the south--comprise three important types of clay3 7a7 ,plastic clays not particularly differentiated from those already described1 7b7 ,loams or sandy clays which are sufficiently plastic for satisfactory use1 have the advantage of shrinking but slightly in drying1 and are largely used in the manufacture of red facing bricks and as light soils1 and 7c7 ,marls or calcareous clays1 used for the production of light coloured or white bricks1 the chalk they contain combining with any iron compounds present and1 at the same time1 reducing the contractility of the clay4 ,on burning1 they form a cement which binds the particles into a strong mass4 ,these are the 'true marls' or 'malms' composed of clay and chalk and must not be confused with the so-called marls of ,staffordshire and elsewhere which are almost free from lime compounds4 ,there is1 at present1 no definition of 'marl' which is quite satisfactory2 a maker of ,london stock bricks understanding by this term a clay containing at least #aj per cent4 of chalk2 a maker of white ,suffolk bricks a material containing at least twice this amount2 an agriculturalist any soil1 not obviously sandy1 which will make his clay land less sticky2 and many geologists any friable argillaceous earths4 ,a general consensus of opinion is1 however1 being gradually reached that the term 'marl' should be limited1 as far as possible1 to clays containing calcium carbonate in a finely divided state4 ,alluvial deposits--which are also of ,recent formation1 though still of sufficient age for skeletons of mammoths to be found in them--are of so variable a nature as to render any brief1 general description impossible4 ,many of them are so contaminated with sand and crushed limestone as to be useless for manufacturing purposes and of small value agriculturally1 but others are important in both these respects4 ,further details of the occurrence of clays in the various formations described will be found in the ,maps and ,memoirs of the ,geological ,survey and in the author's ,british ,clays 7#b74 ,,chapter ,,iii ,,the ,,origins ,,of ,,clays ,the terms 'primary' and 'residual' are applied to those clays which are found overlying or in close association with the rocks from which they have been derived1 and distinguish them from the 'secondary' or 'transported' clays which have been carried some distance away from their place of origin4 ,residual clays may be formed by the simple removal of other materials1 the clay remaining behind1 as in the decomposition of some argillaceous limestones1 in which the calcareous matter has been removed by solution whilst the clay is unaffected4 ,such a clay is not a primary one as it has probably been derived from some distant source and1 having been deposited along with the limestone ooze1 has formed an intimate mixture from which the limestone has1 at a later geological epoch1 been removed in the manner indicated4 ,residual clays are seldom pure1 being often rich in iron compounds1 though the white clays of ,staffordshire and ,derbyshire are highly refractory4 ,it is seldom necessary to distinguish residual clays from other secondary or transported ones 7,chapters ,,ii and ,,iv74 ,primary clays1 on the contrary1 have been derived from rocks which have undergone chemical decomposition1 one of the products being clay4 ,the most important primary clays are the kaolins1 which are derived from the decomposition of felspar1 but other primary clays derived from other minerals are known1 though less frequently mentioned4 ,the kaolins are primary clays#aj formed by the decomposition of felspar and occur in many parts of the world4 ,in ,great ,britain the most important are the china clays found in ,devon and ,cornwall1 which occur in association with the granite from which they have been formed4 ,the kaolins in ,germany are1 apparently1 of similar origin1 though some are derived from porphyry and not from granite2 they are the chief material used in the manufacture of ,dresden1 ,meissen1 ,berlin and other porcelains4 ,the ,french kaolins from ,st ,yrieux and ,limousin are said by ,granger 7#ag7 to be derived from gneiss amphibole4 ,the ,american kaolins have1 according to ,ries 7#f71 been chiefly formed from the weathering of pegmatite veins1 but the origin of some important deposits in ,texas and ,indiana has not yet been fully explained4 ,footnote #aj3 ,some kaolins in central ,europe appear to have been transported and of secondary origin4 ,illustration3 ,fig4 #ab4 ,china clay pit belonging to the ,north ,cornwall ,china ,clay ,co4 7,by courtesy of ,w4 ,h4 ,patchell ,esq47 ,the corresponding material used by the ,chinese for the manufacture of porcelain bears a name which is really that of the place from whence it was originally obtained2 the term ,kao-ling indicates merely a high ridge4 ,according to ,richthofen 7#ah7 the rock from which ,chinese porcelain is made is not a true kaolin1 but is allied to the jades4 ,the term 'kaolin' is therefore a misnomer when applied to white-burning1 primary clays generally1 but its use has become so firmly established as to render it permanent4 ,kaolins are seldom found in a sufficiently pure state to be used direct1 but must be freed from large amounts of undecomposed rock1 quartz1 mica1 etc41 by a process of washing and sedimentation4 ,when purified in this manner1 the best qualities of china clay yield1 on analysis1 alumina1 silica and water in the proportions indicated by the formula ,h#d;,al#b,si#b,o#i together with about #e per cent4 of mica and other impurities4 ,some high class commercial kaolins contain over #cj per cent4 of mica and #aj per cent4 of quartz4 ,the chief constituents of rocks which take part in the production of kaolins appear to be the felspars1 but the natural processes by which these felspars are decomposed are by no means perfectly understood4 ,some kaolins appear to have been formed by weathering and others by subaerial action4 ,thus ,collins 7#ai7 has stated very emphatically that the kaolinization of ,cornish felspar has been chiefly effected by fluorine and other substances rising from below and not by carbonic acid and water acting from above4 ,ries 7#f7 and other ,american observers are equally convinced that certain kaolins they have examined are the result of 'weathering4' ,german and ,french investigators are divided in their opinions1 and ,fuchs has found that the ,passau 7,saxony7 kaolin is derived from a special mineral1 not unlike a soda-lime felspar deficient in silica1 to which he has given the name 'porcelain spar4' ,the felspars form a class of minerals whose chief characteristic is the combination of an alkaline or alkaline-earth base with silica and alumina4 ,orthoclase 7,k#b,o,al#b,o#cf,si,o#b7--the chief potassium felspar--is typical of the whole class4 ,when treated with water under suitable conditions1 the felspar appears to become hydrolysed and some of the water enters into combination1 the potash being removed by solution4 ,attempts to effect this decomposition artificially have proved abortive though several investigators appear to have effected it to a limited extent by electrolysis or by heating under great pressure 7#c74 ,the effect on felspars of waters containing carbon dioxide in solution has been studied by ,forschammer1 ,vogt1 and others1 and they have concluded that kaolinization may occur with this agent though it does not appear to be the chief cause in the formation of ,cornish china clays4 ,illustration3 ,fig4 #ac4 ,orthoclase ,felspar1 natural size4 7,from ,miers' ,mineralogy by permission of ,macmillan ,co47 ,the probable effect of fluoric vapours has been studied by ,collins 7#ai7 who confirmed von ,buch's observation that fluorides 7particularly lepidolite and tourmaline7 are constantly associated with china clay2 he found by direct experiment that felspar is decomposed by hydrofluoric acid at the ordinary temperature without the other constituents of the granite in which it occurs being affected4 ,this theory is confirmed by the great depths of the kaolin deposits in ,cornwall and in ,zettlitz 7,bohemia7 which appear to be too great to render satisfactory any theory of simple weathering though kaolins in other localities1 especially in ,america1 appear to be largely the result of weathering4 ,according to ,hickling 7#cf7 the product of the action of hydrofluoric acid 'has not the remotest resemblance to china clay4' ,kaolin1 when carefully freed from its impurities1 as far as this is possible1 is peculiarly resistant to the action of water4 ,this resistance may be due to its highly complex constitution1 as the simpler hydro-alumino-silicates1 such as collyrite1 show an acid reaction when ground with water4 ,rohland 7#e71 therefore1 suggests that kaolinization is effected by water first hydrolysing the felspar and forming colloidal silica and sodium or potassium hydroxides which are removed whilst the complex alumino-silicate remains in the form of kaolin4 ,hickling 7#cf71 on the contrary1 believes that the action of the weather on felspar produces secondary muscovite--a form of mica--and that this is1 later1 converted into kaolinite or china clay 7fig4 #ag1 p4 #aje74 ,the various theories which have been propounded may be summarized into three main classes1 and whilst it is probable that any one of them1 or any one combination1 may be true for a particular kaolin1 yet the whole process of kaolinization is so complex and the conditions under which it has occurred appear to be so diverse that it is doubtful if any simple theory can be devised which will satisfactorily meet all cases4 7a7 ,the decomposition of the granite1 and particularly of the felspar within it1 may be ascribed to purely chemical reactions in which the chief agents are water and carbon dioxide4 7b7 ,other substances--possibly of an organic nature and derived from the soil--may have played an important part4 7c7 ,wet steam and hot solutions of fluorine1 boron or sulphur compounds may have effected the decomposition4 ,the recent progress made in the application of the laws of physical chemistry to geological problems is continually throwing fresh light on this interesting subject4 ,thus1 studies of the dissociation pressures and transition points between the anhydrous and the hydrous states of various substances and the effect of water as a powerful agent of decomposition 7hydrolysis7 have shown that hydration is a characteristic result of decompositions occurring in the upper portions of the earth's crust and not in the lower ones1 and that it is usually checked1 or even reversed1 when the substance is under great pressure4 ,at great depths kaolins and other complex hydrous silicates give place to anhydrous ones such as muscovite1 andalusite and staurolite4 ,there is1 therefore1 good reason to believe that the kaolinization of ,cornish felspar has occurred at only moderate depths from the surface and that it has been chiefly produced by the action of water containing acid gases in solution4 ,the acid in the water may have been absorbed from the atmosphere1 or it may be due to vapours rising from below through the felspathic material4 ,in ,great ,britain1 china clay occurs in the form of powdery particles apparently amorphous1 but containing some crystals1 scattered through a mass of harder rock1 the whole being known as china clay rock or 'carclazite4' ,the softer portions of this china clay rock are known as 'growan' and the china clay in it represents only a small proportion of the whole material4 ,the finer particles of clay and other materials are removed by treatment with water1 whereby one-third to one-eighth of the material is separated4 ,this small proportion is then subjected to further washing and sedimentation in order to obtain the china clay in a state of commercial purity4 ,it will thus be understood that the ,cornish china clays are not 'deposits' in the usual acceptation of that term1 the soft growan from which they are obtained being almost invariably the result of decomposition in situ of some species of felspar in disintegrated granite4 ,the commercial kaolins of ,france1 ,germany1 ,america and ,china very closely resemble the ,cornish china clays in composition1 but when used in the manufacture of porcelain they create differences in the finished material which are clearly noticeable1 though microscopical examination and chemical analysis1 at present1 fail to distinguish between them in the raw state on account of their great resistance to ordinary chemical and physical forces4 ,in addition to the breaking up of felspathic rocks with the formation of china clay or kaolin 7kaolinization71 other decompositions which occur may result in the formation of clays1 and an examination of a considerable number of clays by ,j4 ,m4 van ,bemmelen 7#bf7 has led him to suppose that several different clay-forming forces have been at work in the production of clays4 ,he classifies these under four heads3 7#a7 ,kaolinization1 or the decomposition of felspathic and similar rocks by the action of telluric water containing active gases in solution4 7#b7 ,ordinary weathering in which the action is largely mechanical1 but is accompanied by some hydrolysis owing to the impurities contained in the water which is an essential factor4 7#c7 ,lateritic action--or simple decomposition by heat--which occurs chiefly under tropical conditions1 but may also occur in temperate climates1 and has for its main product a mixture of free silica and alumina1 the latter being in the form of 7amorphous7 'laterite4' ,it may not improbably be a result of the decomposition of the clay molecule similar to that which occurs when china clay is heated1 as there is no temperature below which it can be said that china clay does not decompose into free silica and alumina 7#bi74 7#d7 ,secondary reactions in which the products of one of the reactions previously described may undergo further changes1 as the conversion of amorphous clayite into crystalline kaolinite1 or amorphous laterite into crystalline hydrargillite4 ,weathering4 ,the action of the forces conveniently classed under the term weathering are of two main kinds3 7a7 ,the mechanical grinding of sandstone1 quartzite1 limestone1 and other rocks1 causes an addition of adventitious material to clay1 the proportions being sometimes so large as to render it necessary to term the material an argillaceous sand1 rather than a sandy clay4 ,some of these grains of mineral matter are so minute and so resistant to the ordinary chemical reagents as to make it extremely difficult to distinguish them from clay4 7b7 ,the chemical decomposition due to the action of very dilute solutions4 ,by this means simple silicates are decomposed with the formation of colloidal silica which may either remain in solution or may be deposited in a coagulated form4 ,at the same time1 some alumino-silicates will be similarly decomposed into colloidal alumino-silicic acids or clays4 ,the ultimate results of the action of ordinary weathering on silicate rocks are1 therefore1 sands and clays1 the latter being in some ways quite distinct in their origin and physical properties from the china clays4 ,according to ,j4 ,m4 van ,bemmelen 7#bf7 such clays also contain an alumino-silicate soluble in boiling hydrochloric acid followed by caustic soda1 whereas pure china clays are unaffected by this treatment4 ,the variety of silicates and other minerals which--in a partially decomposed condition--go to form 'clays' is so great that the complete separation of the smallest particles of them from those of the true clay present has never been accomplished and our knowledge of the mineralogical constitution of many of the best known clays is far from complete4 ,it is highly probable that the action of water does not cease with the formation of clay1 but that it slowly effects an increase in the plasticity of the clay4 ,there thus appear to be at least three kinds of primary clay1 viz43 ,kaolinic or china clays which are chiefly derived from felspar and can be isolated in a relatively pure state4 ,they are highly refractory1 but only slightly plastic4 ,epigenic or colloidal clays derived from kaolinic clays1 as a secondary product1 or directly from felspar1 mica1 augite and other alumino-silicates by 'weathering4' ,they are usually less refractory and much more plastic than the china clays and contain a large percentage of impurities--sometimes in the form of free silica 7sand7 or of metallic oxides1 carbonates1 sulphides1 sulphates1 silicates1 or other compounds4 ,many so-called secondary clays such as pipe clays1 ball clays and fireclays may be of this type1 though their origin is difficult to trace owing to their subsequent transportation and deposition4 ,lateritic or highly aluminous clays1 of a highly refractory character1 but low plasticity4 ,they are usually somewhat rich in iron oxide which materially affects their plasticity4 ,unlike the china clays1 pure lateritic clays are completely decomposed by hydrochloric acid4 ,bauxite and some of the highly aluminous clays of the ,coal ,measures appear to be of this type4 ,unfortunately these different types of clay are extremely difficult to distinguish and in many instances they have become mixed with each other and with other materials during the actions to be described in the next chapter1 that it is often almost impossible to decide whether the true clay in a given specimen possessed its characteristics ab initio or whether it has gained them since the time when it ceased to be a primary clay4 ,secondary clays are those which have been produced by the action of the weather and other natural forces on primary clays1 the changes effected being of a physical rather than a chemical nature 7see ,chapter ,,iv74 ,the essential constituent of secondary clays has not been positively identified4 ,in so far as it has been isolated it differs from the true clay in the primary clays in several important respects1 and until its nature has been more fully investigated great caution must be exercised in assigning a definite name to it4 ,for many purposes the term pelinite 7p4 #adi7 is convenient1 being analogous to the corresponding one used for material in china clays 7clayite1 p4 #ajg74 ,these terms are purely provisional and must be discarded when the true mineralogical identities of the substances they represent have been established4 ,,chapter ,,iv ,,the ,,modes ,,of ,,accumulation ,,of ,,clays ,from whatever sources clays may have been originally derived1 the manner in which they have been accumulated in their present positions is a factor of great importance both in regard to their chemical and physical characters and their suitability for various purposes4 ,as explained in ,chapter ,,iii1 the china clays or kaolins may usually be regarded as primary clays derived from granitic or other felspathic or felsitic rocks by chemical decomposition4 ,such clays are found near to their place of origin1 are usually obtainable in a comparatively pure state and are generally deficient in plasticity4 ,they may occur in beds of small or great depth1 but these are not 'accumulations' in the ordinary meaning of that term4 ,residual clays 7p4 #gj7 also form a distinct class1 as unlike the majority of argillaceous materials they are left behind when other substances are removed1 usually by some process of solution4 ,in many cases1 however1 the residual clays are really secondary in character1 having been transported from their place of origin1 together with limestone or other minerals1 the mixture deposited and subjected to pressure and possibly to heat1 whereby a rock-like mass is formed4 ,this mass has then been subjected to the solvent action of water containing carbon dioxide or other substances which dissolve out the bulk of the associated minerals and leave the residual clay behind4 ,the chief agents in the transport and accumulation of clays are the air1 in the form of wind2 water1 in the form of rain1 streams1 rivers1 floods1 lakes and seas1 or in the form of ice and snow as in glaciers and avalanches2 earth-movements such as the changes wrought by volcanoes1 earthquakes and the less clearly marked rising and falling of various portions of the earth's crust which result in folded1 twisted1 sheared1 cracked1 inclined1 laminated and other strata4 ,these agents have first moved the clay from its original site and have later deposited it with other materials in the form of strata of widely varying area and thickness1 some 'clay' beds being several hundred feet in depth and occupying many square miles in area1 whilst others are in the form of thin 'veins' only a few inches thick or in 'pockets' of small area and depth4 ,these deposits have in many places been displaced by subsequent earth-movements and have been overlain by other deposits so as to render them quite inaccessible4 ,others have been covered by deposits several miles thick2 but the greater part of the covering has since been removed by glacial or other forces1 so that clays of practically all geological ages may be found within the relatively small area of ,great ,britain4 ,the ,transportation of ,clays4 ,by the action of wind or rain1 or of rain following frost1 the finer of particles clay are removed from their primary site and as the rain drops collect into streamlets1 these unite to form streams and rivers and the clay with its associated minerals is carried along by the water4 ,as it travels over other rocks or through valleys composed of sandstone1 limestone and other materials1 some of these substances are dislodged1 broken into fragments of various sizes and with the clay are carried still further4 ,in their journey these materials rub against each other and against the banks and bed of the stream1 thereby undergoing a prolonged process of grinding whereby the softer rocks are reduced to very fine sand and silt which becomes1 in time1 very intimately mixed with the clay4 ,if the velocity of the stream were sufficiently great1 the mixed materials--derived from as many sources as there are rocks of the districts through which they have passed--would be discharged into a lake or into the sea4 ,here the velocity of the water would be so greatly reduced that the materials would gradually settle1 the largest and heaviest fragments being first deposited and the finer ones at a greater distance4 ,with most streams and rivers1 however1 the velocity of the water is very variable1 and a certain amount of deposition therefore occurs along the course1 the heavier particles only travelling a short distance1 whilst the finer ones are readily transported4 ,if the velocity of the stream increases1 these finer particles 7which include the clay7 may become mixed with other particles of various sizes and the materials thus undergo a series of mixings and partial sortings until they are discharged at the river mouth or are left along its sides by a gradual sinking of the water level4 ,the clay will be carried the whole course of the river1 unless it is deposited at some place where the velocity of the water is reduced sufficiently to permit it to settle4 ,if floods arise1 the area affected by the water will be increased4 ,the alluvial clays have1 apparently1 been formed by overflowing streams and rivers1 the material in suspension in the water being deposited as the rate of flow diminished4 ,such alluvial deposits contain a variety of minerals--usually in a very finely divided state--clay1 limestone-dust or chalk1 and sand being those most usually found4 ,river-deposited clays1 i4e41 those which have accumulated along the banks1 are characterized by their irregularity in thickness1 their variable composition and the extent to which various materials are mixed together4 ,this renders them difficult to work and greatly increases the risks of manufacture as the whole character of a fluviatile clay may change completely in the course of a few yards4 ,according to the districts traversed by the water1 the extent to which the materials have been deposited and re-transported and the fresh materials introduced by earth-movements1 river-deposited clays may be 7a7 plastic and sufficiently pure to be classified as 'clays1' 7b7 marls or clays containing limestone-dust or chalk thoroughly mixed with the clay1 and 7c7 loams or clays containing so much sand that they may be distinguished by the touch from the clays in class 7a74 ,intermediate to these well-defined classes there are numerous mixtures bearing compound names such as sandy loams1 sandy marls1 argillaceous limestone1 calcareous sands1 and calcareous arenaceous clays1 to which no definite characteristics can be assigned4 ,to some extent a transportation of clays and associated materials occurs in lakes1 but the chief processes there are of the nature of sedimentation accompanied by some amount of separation4 ,on the shores of lakes1 and to a much larger extent on the sea coasts1 extensive erosion followed by transportation occurs continuously1 enormous quantities of land being annually removed and deposited in some portion of the ocean bed4 ,the erosion of cliffs and the corresponding formation of sand and pebbles are too well known to need further description4 ,it should1 however1 be noticed that the clay particles1 being much finer1 are carried so far away from the shore that only pebbles and sand remain to form the beaches1 the finer particles forming 'ocean ooze4' ,the action of the sea in the transport of rock-materials is more intense than that of rivers1 the coasts being worn away by repeated blows from the waves and the pebbles and sand grains the latter contain4 ,the ocean currents carry the materials dislodged by the waves and transport them1 sometimes to enormous distances1 usually allowing a considerable amount of separation to take place during the transit4 ,in this way they act in a similar manner to rivers and streams4 ,glaciers may be regarded as rivers of ice which erode their banks and bed in a manner similar to1 but more rapidly than1 streams of water4 ,owing to their much greater viscosity1 glaciers are able to carry large boulders as well as gravel1 sand and clay1 so that the materials transported by them are far more complex in composition and size than are those carried by flowing water4 ,illustration3 ,fig4 #ad4 ,illustrating the successive deposition of different strata4 ,separation and ,sedimentation4 ,the clay and other particles having been placed in suspension in water by one or more of the natural forces already mentioned1 they soon undergo a process of sorting or separation1 previous to their deposition4 ,the power of water for carrying matter in suspension depends largely on its velocity1 and when this is reduced1 as when a river discharges into a lake or sea1 the larger and denser particles at once commence to settle1 the smaller ones remaining longer in suspension1 though if the velocity of the water is reduced sufficiently all the particles will be deposited4 ,hence1 the deposits in lakes 7lacustrine7 and at the mouths of rivers 7estuarine7 increase more or less regularly in fineness according to their distance from the point at which the water enters1 the gravel and stones being deposited first1 then the coarse sand1 next the finer sand and finally the silt and clay 7fig4 #ad74 ,if cross-currents are present1 the deposits will1 naturally1 be made more irregular1 and in some cases variations in the flow of the transporting water may cause the coarser particles to be carried further than usual so that they may cover some of the finer deposits previously formed2 but as the clay and silt particles are so much finer than sand and gravel they usually travel so far before settling that their deposits are very uniform if the area over which they are spread is sufficiently large4 ,lake-deposited clays are for this reason more uniform than estuarine beds1 whilst beds deposited at considerable depths in the sea and at a great distance from land are still more uniform4 ,a lacustrine clay is usually more persistent and uniform than fluviatile beds though sometimes difficult to distinguish from the latter4 ,some of the most valuable clay deposits are of lacustrine formation2 their comparative purity and great uniformity enabling ware of excellent colour and texture to be produced without much difficulty4 ,thus the ,reading mottled clays of the ,hampshire basin1 on the outskirts of the ,london basin and in ,northern ,france are well known for the admirable red bricks1 tiles and terra-cotta produced from them4 ,still purer clays deposited at ,bovey ,heathfield in ,devonshire are also of lacustrine origin1 though they differ in many respects from the ordinary lake-deposited clays and are of unusual thickness for deposits formed in this manner4 ,illustration3 ,fig4 #ae4 ,lacustrine clay at ,skipsea4 7,by courtesy of ,t4 ,sheppard ,esq47 ,the greater purity of lacustrine clays1 as compared with fluviatile ones1 is attributed to the much larger area over which the deposit is spread1--enabling variations in the deposits to be much less noticeable than when a smaller area is covered--and to the very small velocity of the water in lakes1 whereby all the coarser particles are deposited a considerable distance away from the clays and silts4 ,ries 7#f7 has pointed out that many 7,american7 lake-clays are of glacial origin1 having been laid in basins or hollows along the margin of the ice-sheet or in valleys which have been dammed by an accumulation of drift across them4 ,such clay beds are usually surface deposits of variable thickness and frequently impure4 ,like all lacustrine deposits they show 7though in a more marked degree than in the older and larger lakes7 alternate layers of sand and clay1 though the former are usually too thin to be noticeable except for their action in enabling the deposited material to be easily split along the lines of bedding4 ,estuarine deposits partake of the nature of both fluviatile and marine beds1 according to their position relative to the river from which they originate4 ,they are usually uncertain in character and are often irregular in composition owing to the variations in the flow of the water4 ,the ,estuarine clays of ,great ,britain--with the possible exception of the ,jurassic deposits in ,northamptonshire and ,lincolnshire--are of minor importance1 but in some countries they form a valuable source of clay4 ,illustration3 ,fig4 #af4 ,clay at ,nostel1 showing ,marine ,band4 7,by courtesy of ,t4 ,sheppard ,esq47 ,marine clays are1 as their name implies1 those deposited from sea water4 ,they are frequently found at a considerable distance from the shores of the ocean in which they were laid down1 and subsequent risings and fallings of the surface of the earth have so altered the areas occupied by sea water1 that a large number of marine deposits now form dry land4 ,though usually of enormous size and of generally persistent character1 marine clay deposits vary considerably in the composition of the material at different depths1 as well as in different areas4 ,this is only to be expected from the manner of their deposition1 from the varied sources of the material and from the numerous river- and ocean-currents taking part in their formation4 ,for this reason it is generally necessary to mix together portions of the deposit drawn from various depths in order to secure a greater uniformity than would be obtained if a larger area were to be worked to a smaller depth4 ,the ,oxford clay which extends from the centre of ,england to the centre of ,france is a typical marine clay4 ,at the bottom of all oceans at the present day is a deposit1 of unknown thickness1 of red calcareous clay or ooze which is steadily increasing in amount and is thereby forming a fresh marine deposit1 though at present its inaccessibility deprives it of all economic value4 ,it is important not to overlook the enormous part played by variations in the level of the land relative to that of the ocean in past ages4 ,for instance1 there is abundant evidence to show that practically the whole of ,great ,britain has been repeatedly submerged to great depths and has been raised to heights far greater than its present average4 ,these oft-repeated risings and settlings have caused great changes in the nature of the deposited materials so that in the ,coal ,measures1 for example1 there are deposits of obviously fresh-water origin sandwiched in between others undoubtedly marine4 ,it can readily be understood1 as stated by ,arber 7#bd71 that if1 at a given period1 the dry land during the formation of the ,coal ,measures gradually subsided1 it would first be covered with clear water1 whilst from those portions of the area which occupied the higher ground the rivers and streams continued to pour into their estuary a large amount of fresh-water material4 ,later1 a stage would be reached when mud of marine origin invaded the area and covered the previous deposits4 ,when1 after an indefinitely long period1 the ground again rose1 fresh-water deposits might again form1 and this alternation of marine and fluviatile deposits appears to have been repeated with great frequency during the ,carboniferous period4 ,in the ,lower ,coal ,measures of ,yorkshire and ,lancashire1 ,stopes and ,watson 7#bc7 have shown that the shales forming the roof of the ,upper ,foot ,coal were derived from drifted sediments of marine origin4 ,precipitated ,clays4 ,if the plasticity of some clays is really due to the colloidal nature of their particles1 it is obvious that they must have been formed by a process of coagulation or precipitation at a distance from the site of the minerals from which they have been derived4 ,according to the 'colloid theory1' felspar and other alumino-silicates are decomposed by 'weathering1' the chief effect of which is the formation 7by hydrolysis7 of a colloidal solution of 'clay4' ,this apparently clear solution flows along in the form of a small streamlet1 joins other streamlets and continues its journey4 ,so long as it is quite neutral or contains free alkali the solution will remain practically clear1 but as soon as acids enter the stream1 or are formed in it by the decomposition of organic matter1 a coagulation of the colloidal matter will commence and the amount of 'clay' thus thrown out of solution will depend on the amount of such free acid4 ,if the coagulation or precipitation occurs in still water1 the 'clay' will be deposited almost immediately1 otherwise it will be carried forward until it reaches a place where it can be deposited in the manner already described4 ,such precipitated clays need not necessarily be pure1 as other substances may be present in colloidal form and may be coagulated at the same time as the clay4 ,in addition to these1 the admixture of sand and other minerals present in suspension in the solution may become mixed with the particles during coagulation and be deposited with them4 ,clays formed in this manner are extremely difficult to identify on account of the highly complex nature of the reactions occurring in their vicinity both during and subsequent to their formation4 ,re-,deposited ,clays4 ,although many clays and other materials have been transported and accumulated in the manner described1 the majority of those now available have been subjected to repeated transportation and deposition1 owing to the frequent and enormous changes in the relative levels of land and water during the various geological epochs4 ,so far as can be ascertained1 it is during these changes of position and the recurrent exposure to air and to water containing various substances in solution1 together with the almost incessant grinding which took place during the transportation and deposition1 that most secondary clays became plastic4 ,if this is the case1 it explains the impossibility of increasing the plasticity of clay by artificial means1 at any rate on a large scale4 ,the simplest of the agents of re-deposition are rain-storms and floods which1 forming suddenly1 may cause the water of a stream or river to flow with unwonted velocity and so carry away previously formed deposits of various kinds4 ,clays transported in this way are termed by ,ries 7#f7 colluvial clays1 the term 'diluvial' is generally employed in this country4 ,if these are derived from a primary clay which has not travelled far since it left the original granite from which it was formed1 they will usually be white-burning and of only slight plasticity1 but if the flood affects materials which have already been re-deposited several times1 the colluvial clays may be of almost any imaginable composition4 ,floods of a different character--due to the subsidence of the land so that it is partially covered with lake- or sea-water1 which beats on its shores and erodes it in the manner already described--are also important factors in the transportation of clays4 ,so far as clays are concerned1 the action of the sea is both erosive and depository1 though the sedimentation in it being that of the pelagic ooze at great depths the clayey material is quite inaccessible4 ,under certain conditions1 however1 the sea may erode land in one area and may return the transported material to the land in another area4 ,the diluvial clay-silt known as warp in the valley of the ,humber is of this character4 ,quite apart from the action of water1 however1 much denudation1 transportation and re-deposition of clays and associated materials has been due to the action of ice in the form of glaciers1 though these do not appear to have had much effect in increasing the plasticity of the clays concerned4 ,glacially deposited clays are characterised by their heterogeneous composition1 some of them containing far more sand than true clay1 whilst yet retaining a sufficient amount of plasticity to enable them to be used for rendering embankments impervious and for the manufacture of common bricks1 and1 occasionally1 of coarse pottery2 others contain so much sand as to be useless for these purposes4 ,most glacial deposits contain a considerable proportion of stones and gravel which must be removed before the clay can be used4 ,the large proportion of adventitious matter is due in great part to the much greater erosive force and carrying power of ice as compared with water1 resulting in much larger pieces of material being carried1 and as the whole of the ice-borne material is deposited almost simultaneously when the glacier melts1 only a very small amount of separation of the material into different grades takes place4 ,the comparative freedom from coarse sand of some glacial clays shows that some sorting does occur1 but it is very limited in extent as compared with that wrought in materials which have been exclusively transported by water4 ,for the manufacture of bricks1 tiles and coarse pottery in ,yorkshire1 ,lancashire and some of the more northern counties of ,great ,britain1 glacially deposited clays are of great importance in spite of their irregular composition4 ,they are frequently termed 'boulder clays' or 'drift clays' 7p4 #fe71 but in using these or any other terms for clays transported by glacial action it is important that they should not be understood to refer to the whole of the deposited matter4 ,large 'pockets' of coarse sand and gravel frequently occur in deposits of this character and veins of the same materials are by no means uncommon4 ,the custom of some geologists of referring to the whole of a glacial deposit as 'boulder clay' has1 in a number of cases1 led to serious financial loss to clayworkers who have erroneously assumed that1 because some 'boulder clays' are used for brick and tile manufacture1 all deposits bearing a similar title would be equally suitable4 ,this difficulty would largely be avoided if1 as is now increasingly the case1 the term 'drift' or 'glacial deposit' were used for the deposits as a whole1 the term 'boulder clay' being restricted to the plastic portions and not including pockets of sand1 gravel and other non-plastic materials4 ,boulder clays--using this term in the limited sense just mentioned--consist of variable quantities of sand and clay1 stones and gravel being generally associated with them4 ,the stones may usually be removed by careful picking1 and the gravel by means of a 'clay cleaner' which forces the plastic material through apertures too small to allow the gravel to pass4 ,the plastic material so separated is far from being a pure clay and may contain almost half its weight of sand1 the greater part of which is readily separated by washing the material4 ,boulder clays1 when freed from stones and gravel1 are sufficiently plastic to meet the needs of most users1 without being so highly plastic and contractile as to necessitate admixture with sand or similar material4 ,some boulder clays contain limestone in the form of gravel or as a coarse powder produced by the crushing of larger fragments4 ,these are less suitable for manufacturing purposes as the lime produced when the articles are burned in the kilns is liable to swell and to disintegrate them on exposure4 ,owing to their origin and the nature of the impurities they contain1 boulder clays are never pure and when burned are irregular in colour and somewhat fusible unless subjected to some process of purification4 ,,chapter ,v ,,some ,,clays ,,of ,,commercial ,,importance ,although clays occur in deposits of almost all geological periods1 many of them are of little or no commercial value4 ,this may be due to their situation or to their composition and other characteristics4 ,thus1 a ,coal ,measure clay is ordinarily quite inaccessible1 and to sink a shaft specially to obtain it may be an unprofitable undertaking2 if1 however1 a shaft is sunk for coal the clays in the neighbourhood of the coal seams are rendered accessible and1 usually1 a certain amount of such clays is brought to the surface in order to remove it out of the way of the coal miners4 ,again1 a clay deposit may be so far removed from human habitations as to make it practically valueless1 but if1 for any reason1 the population of the district in which the clay is situated grows sufficiently1 the clay may become of considerable value4 ,it not infrequently happens1 therefore1 that the commercial importance of a clay deposit is one which fluctuates considerably1 yet1 in spite of this fact1 there are certain kinds of clay which are nearly always of some commercial value4 ,the most important of these are the kaolins 7china clays71 the pottery and stoneware clays1 the refractory clays 7fireclays71 the brick and terra-cotta clays and shales1 and the clays used in the manufacture of ,portland cement4 ,the origin and manner in which these clays have been accumulated have been described in the previous chapters2 it now remains to indicate their characteristics from the point of view of their commercial value4 ,commercial china clays and kaolins in the ,united ,kingdom are not simple natural products but1 in the state in which they are sold commercially1 have all been subjected to a careful treatment with water1 followed by a process of sedimentation whereby the bulk of the impurities have been removed4 ,according to the extent to which this treatment has been carried out1 they will contain #aj per cent4 or more mica and quartz1 with little or no tourmaline1 felspar and undecomposed granite4 ,in some parts of ,europe and ,america1 kaolins are found in a state of sufficient purity to need no treatment of this kind unless they are to be used for the very highest class of wares4 ,illustration3 ,magnified #bbj ,diameters ,magnified #ibj ,diameters ,magnified #bbj ,diameters ,magnified #bbj ,diameters ,crystals of ,kaolinite ,magnified #ibj ,diameters ,crystals of ,secondary ,muscovite4 ,fig4 #ag4 ,kaolinite and ,mica4 7,after ,g4 ,hickling 7#cf747 ,mica is usually the chief impurity as its particles are so small and their density resembles that of the purified china clay more closely than do the other minerals4 ,in commerce the term china clay is almost invariably used to denote the washed material obtained from the 'china clay rock1' but at the pits the word 'clay' is used indiscriminately for the carclazite 7p4 #gh7 and for the material obtained from it4 ,as the term 'kaolin' is used indifferently abroad for the crude 'deposit' and for the purified commercial article1 it should be understood that the following information relates solely to the substance as usually sold and not to the crude material4 ,commercial china clay or kaolin is a soft white or faintly yellowish substance1 easily reduced to an extremely fine powder1 which when mixed with twice its weight of water will pass completely through a ,no4 #bjj sieve4 ,its specific gravity is #bfe1 but the minuteness and nature of its smallest particles and their character are such that it will remain in suspension in water for several days2 it thus appears to possess colloidal properties1 at any rate so far as the smaller particles are concerned4 ,it is almost infusible1 but shows signs of softening at #ahhj ,c4 7,seger ,cone #ci7 or at a somewhat lower temperature1 according to the proportion of impurities present4 ,when heated with silica or with various metallic oxides it fuses more readily owing to the formation of silicates4 ,china clays and kaolins are not appreciably affected by dilute acids1 but some specimens are partially decomposed by boiling concentrated hydrochloric acid 7#bf7 and all are decomposed by boiling sulphuric acid1 the alumina being dissolved and the silica liberated in a form easily soluble in solutions of caustic soda or potash4 ,this has led to the conclusion that some kaolins may have been produced by weathering1 as the bulk of true kaolinitic clays 7such as ,cornish china clay7 is not affected by boiling hydrochloric acid 7p4 #ha74 ,owing to the exceptional minuteness of its particles1 it is extremely difficult to ascertain whether pure china clay or kaolin is crystalline or amorphous4 ,johnson and ,blake 7#ba7 found that all the specimens they examined 'consisted largely of hexagonal plates' and that in most kaolins 'these plates are abundant--evidently constituting the bulk of the substance4' ,this observation is contrary to the experience of most investigators1 the majority of whom have found the bulk of the material to be amorphous and sponge-like1 but a small portion of it to consist of hexagonal or rhombic crystals4 ,mellor 7#bb7 has proposed the name clayite for this amorphous material1 the crystalline portion being termed kaolinite as suggested by ,johnson and ,blake4 ,both kaolinite 7crystalline7 and clayite 7amorphous7 yield the same results on analysis and correspond very closely to the formula ,h#d;,al#b,si#b,o#i or ,al#b,o#cb,si,o#bb;,h#b,o1 so that it is most probable that they are the same substance in different physical states4 ,according to ,hickling 7#cf7 the general impression that the particles of china clay are amorphous is due to the use of microscopes of insufficient power4 ,with an improved instrument1 ,hickling claims to have identified the 'amorphous' portion of china clay with crystalline kaolinite1 the clay particles 7fig4 #ag7 being in the form of irregular1 curved1 hexagonal prisms or in isolated plates4 ,the former show strong transverse cleavages4 ,the index of refraction and that of double refraction agree with those of ,anglesea kaolinite crystals1 as does the specific gravity4 ,in spite of their great purity1 commercial china clays and kaolins are almost devoid of plasticity1 nor can this property be greatly increased by any artificial treatment4 ,this has led to the conclusion that plasticity is not an essential characteristic of the clayite or kaolinite molecules1 but is due to physical causes not shown by any investigation of the chemical composition of the material4 ,in addition to the specially purified kaolins just described1 alkaline kaolins1 siliceous kaolins and ferruginous kaolins are obtained from less pure rocks and do not undergo so thorough a treatment with water4 ,some of these varieties are not improbably derived from transported kaolins1 as they occur in ,tertiary strata1 and so bear some resemblance to the white fireclays on the ,carboniferous limestone of ,staffordshire1 ,derbyshire and ,north ,wales1 though the latter are far more plastic4 ,to be of value1 a china clay or kaolin must be as white as possible and must be free from more than an insignificant percentage of metallic oxides which will produce a colour when the clay is heated to bright redness4 ,if the material is to be used in the manufacture of paper1 paint or ultra-marine1 these colour-producing oxides are of less importance providing that the clay is sufficiently white in its commercial state4 ,the manufacturer of china-ware and porcelain requires china clay or kaolin which1 in addition to the foregoing characteristics1 shall be highly refractory4 ,it must1 therefore1 be free from more than about #b per cent4 of lime1 magnesia1 soda1 potash1 titanic acid and other fluxes4 ,it is a mistake to suppose that all white clays of slight plasticity are china clays or kaolins4 ,some pipe clays have these characteristics1 but they contain so large a proportion of impurities as to be useless for the purposes for which china clay is employed and are consequently of small value4 ,users of china clays and kaolins generally find it necessary to carry out a lengthy series of tests before accepting material from a new source1 as such a material may possess characteristics not readily shown by ordinary methods of analysis1 but which are sufficiently active to make it useless for certain purposes 7see p4 #adc74 ,pottery clays are1 as their name implies1 those used in the manufacture of pottery1 and comprise the china clays already mentioned 7p4 #ajd71 the ball clays and the less pure clays used in the manufacture of coarse red ware1 flower pots1 etc4 ,the china clays 7p4 #ajd7 are not used alone in pottery manufacture as they lack plasticity and cohesion4 ,in the production of china-ware or porcelain they are mixed with a fluxing material such as ,cornish stone1 pegmatite1 or felspar1 together with quartz or bone ash4 ,thus1 ,english china ware is produced from a mixture of approximately equal parts of bone ash1 china clay and ,cornish stone1 whilst felspathic or hard porcelain is made from a mixture of kaolin1 felspar and quartz1 a little chalk being sometimes added4 ,the ball clays 7p4 #fd7 form the basis of most ordinary pottery1 though some china clay is usually added in order to produce a whiter ware4 ,flint is added to reduce the shrinkage--which would otherwise be inconveniently great--and the strength of the finished ware is increased1 its texture is rendered closer and its capability of emitting a ringing sound when struck are produced by the inclusion of ,cornish stone or felspar in the mixture4 ,small quantities of cobalt oxide are also added to improve the whiteness in the better classes of ware4 ,illustration3 ,fig4 #ah4 ,mining best ,potter's clay in ,devonshire4 7,photo by ,mr ,g4 ,bishop47 ,the ball clays are characterised by their remarkably high plasticity1 their fine texture and their freedom from grit4 ,they are by no means so pure as the china clays1 and unless carefully selected can only be used for common ware4 ,the better qualities burn to a vitrified mass of a light brownish tint1 but when mixed with the other materials used in earthenware manufacture they should produce a perfectly white ware4 ,the inferior qualities are used for stoneware1 drain pipes1 etc4 ,it should be noted that the term 'ball clay' is used for clays of widely differing characteristics though all obtained from one geological formation2 when ordering it is necessary to state the purpose for which the clay is required or an entirely unsuitable material may be supplied4 ,for the same reason1 great care is needed in any endeavour to sell a ball clay from an hitherto unworked deposit4 ,coarse pottery clays#aa are usually found near the surface and whilst they may be derived from any geological formation1 those most used in ,england are of ,triassic or ,permian origin1 though some small potteries use material of other periods1 including alluvial or surface clays4 ,these clays are closely allied to those used for brickmaking1 but are somewhat finer in texture and more plastic4 ,in some cases they are prepared from brick clays by treating the latter in a wash-mill1 the coarser particles being then removed1 whilst the finer ones1 in the state of a slip or slurry1 are run into a settling tank and are there deposited4 ,footnote #aa3 ,coarse pottery has been defined as that made from natural clay without the addition of any material other than sand and water4 ,the presence of a considerable proportion of iron oxide results in the formation of red ware1 which is necessarily of a porous nature1 as the fluxes in the clay are such that they will not permit of its being heated to complete vitrification without loss of shape4 ,to render it impervious the ware is covered with a glaze1 usually producing red1 brown or black ware 7,rockingham ware74 ,the stoneware or drain-pipe clays1 are the most important of the vitrifiable clays and owe their value to the fact that they can be readily used for the manufacture of impervious ware without the necessity of employing a glaze4 ,they are1 therefore1 used in the manufacture of vessels for holding corrosive liquids such as acids and other chemicals1 for sanitary appliances1 sewerage pipes and in many other instances where an impervious material is required4 ,owing to the lime1 magnesia1 potash and soda they contain1 the stoneware clays undergo partial fusion at a much lower temperature than is required by some of the purer clays4 ,the fused portion fills the pores or interstices of the material1 making--when cold--a ware of great strength and impermeability4 ,the chief difficulty experienced in the manufacture of stoneware is the liability of the articles to twist and warp when heated4 ,for this reason it is necessary to burn them very carefully and to select the clays with circumspection4 ,some clays are quite unsuitable for this branch of pottery manufacture because of the practical impossibility of producing ware which is correct in shape and is free from warping4 ,what is required are clays in which the partial fusion will commence at a moderate temperature and will continue until all the pores are filled with the fused material without the remaining ingredients being attacked or corroded sufficiently to cause the ware to lose its shape4 ,as the temperature inside a potter's kiln is continually rising1 the great tendency is for the production of fused material to take place at an ever-increasing rate1 so that the danger of warping becomes greater as the firing nears completion4 ,some clays commence to vitrify at a moderate temperature and can be heated through a long range of temperature before an appreciable amount of warping occurs2 such clays are said to possess a 'long range of vitrification' 7p4 #ch74 ,in other clays the difference between the temperature at which vitrification commences and that at which loss of shape occurs is only a few degrees2 such clays are useless for the manufacture of stoneware1 as their vitrification range is too short4 ,it is therefore essential that1 for the manufacture of stoneware1 a clay should contain a large proportion of refractory material which will form a 'skeleton1' the interstices of which will be filled by the more fusible silicates produced by the firing4 ,it is generally found that of all the fluxes present in vitrifiable clays1 soda and potash compounds--the so-called 'alkalies'--and all lime compounds are the most detrimental1 as in association with clay they form a material with a very short range of vitrification4 ,magnesia1 on the contrary1 accompanies a long vitrification range4 ,the clays used in ,great ,britain for the manufacture of the best stoneware are the ,devonshire and ,dorset ball clays1 the upper portions of these deposits being used for this purpose as they are somewhat less pure than the lower portions used in the manufacture of white ware4 ,for coarser grades of stoneware1 clays of other geological formations are employed1 especially where the finished ware may be coloured1 as the purity of the clay is of less importance4 ,providing a clay has a sufficiently long vitrification range1 a suitable colour when burned1 and that it is capable of being readily formed into the desired shapes1 its composition and origin are of small importance to the stoneware manufacturer4 ,in actual practice1 however1 the number of sources of good stoneware clay is distinctly limited1 and many manufacturers are thus compelled to add suitable fluxes to refractory clays in order to meet some of their customers' requirements4 ,for this purpose a mixture of fireclay with finely powdered felspar or ,cornish stone is used4 ,chalk--which is a cheaper and more powerful flux--or powdered glass cannot be used as the range of vitrification of the mixture would be too short4 ,some manufacturers take the opposite course and add fireclay1 flint1 or other refractory material to a readily fusible clay4 ,this is satisfactory if the latter clay is relatively low in lime and owes its fusibility to potash1 soda or magnesia in the form of mica or felspar4 ,the mica and felspar grains enter so slowly into combination with the clay that a long range of vitrification occurs1 whereas with lime1 or with some other soda and potash compounds1 the combination occurs with great rapidity and the shape of the ware is spoiled4 ,the refractory clays are commonly known as fireclays on account of their resistance to heat4 ,the china clays and kaolins are also refractory1 but are too expensive and are not sufficiently plastic to be used commercially in the same manner as fireclays1 except to a very limited extent1 though bricks have been made for many years from the inferior portions of china clay rock at ,tregoning ,hill in ,cornwall4 ,the geological occurrence of the fireclays of the ,coal ,measures has already been described on p4 #ec4 ,in addition1 there are the refractory clays occurring in pockets or depressions in the ,mountain ,limestone of ,north ,wales1 ,staffordshire1 ,derbyshire and ,ireland1 which consist of siliceous clays and sands1 the insoluble residue of the local dissolution of the limestone1 intermixed with the dbris of the overlying ,millstone ,grit 7see p4 #ed74 ,these clays and sands can be mixed to produce bricks of remarkably low shrinkage1 but the pockets are only large enough to enable comparatively small works to be erected and the clays are so irregular both in composition and distribution as to render their use somewhat speculative4 ,a third type of refractory clay--termed flint clay--is used in large quantities in the ,united ,states1 but is seldom found in ,great ,britain4 ,when moistened1 flint clays do not soften1 but remain hard and flint-like with a smooth shell-like fracture4 ,for use they are ground extremely fine1 but even then they develop little plasticity4 ,they are considered by ,ries 7#f7 to have been formed by solution and re-precipitation of the clay subsequent to its primary formation1 in a manner similar to flint4 ,they are somewhat rich in alumina and many contain crystals of pholerite 7,al#b,o#cb,si,o#bc;,h#b,o74 ,the ,coal ,measure fireclays 7p4 #ec7--which are by far the most important--are divided into two sections by the coal seams1 those above the coal being shaly and fissile in structure whilst those below 7underclays7 are without any distinct lamination4 ,both these clays may be equally refractory1 but the underclays are those to which the term fireclay is usually applied4 ,the lowest portions are usually more silicious and in some areas are so rich in silica as to be more appropriately termed silica rock or ganister4 ,fireclays may1 in fact1 be looked upon as a special term for the grey clays of the ,coal ,measures1 interstratified with and generally in close proximity to the seams of coal4 ,they are known locally as clunches and underclays and were at one time supposed to represent the soil that produced the vegetation from which the coal was formed1 but are now considered by many authorities to be of estuarine origin4 ,it is important to notice that whilst the coals almost invariably occur in association with underclays1 some fireclays are found at a considerable distance from coal4 ,the fireclays of the ,coal ,measures have a composition varying within comparatively wide limits even in contiguous strata2 those chiefly used having an average of #bj to #cj per cent4 of alumina and #ej to #gj per cent4 of silica4 ,they appear to consist of a mixture of clay and quartz with a small proportion of other minerals1 but in some of them a portion of the clay is replaced by halloysite--another hydro-alumino-silicate with the formula ,h#f;,al#b,si#b,o#aj or ,al#b,o#c4b,si,o#b4c;,h#b,o4 ,their grey colour is largely due to vegetable 7carbonaceous7 matter and to iron compounds4 ,the latter--usually in the form of pyrites--is detrimental to the quality of the goods as it forms a readily fusible slag4 ,unlike the iron in red-burning clays it can seldom be completely oxidized and so rendered harmless4 ,the fireclays must therefore be carefully selected by the miners4 ,on the ,continent1 and to a much smaller extent in ,great ,britain1 refractory articles are made from mixtures of grog or burned fireclay with just sufficient raw clay to form a mass of the required strength4 ,for this purpose a highly plastic1 refractory clay is required and the ,tertiary ball clays of ,devon and ,dorset 7p4 #fd7 are particularly suitable4 ,the most important characteristics of a fireclay are that it shall be able to resist any temperature to which it may be exposed and that the articles into which it is made shall not be affected by rapid changes in temperature4 ,other characteristics of importance in some industries are the resistance to corrosive action of slags and vapours1 to cutting and abrasion by dust in flue-gases or by the implements used in cleaning the fires4 ,for those purposes it is necessary that a fireclay should possess high infusibility 7p4 #cb71 a low burning shrinkage 7p4 #bi7 and a high degree of refractoriness 7p4 #cd71 and before it is used these characteristics should be ascertained by means of definite tests1 as they cannot be determined by inspection of a sample or from a study of its chemical analysis4 ,several grades of fireclay have long been recognized on the ,continent and in the ,united ,states of ,america1 but the recent ,specification of the ,institution of ,gas ,engineers is the only official recognition in ,great ,britain of definite grades4 ,this specification defines as ,no4 #a grade a fireclay which shows no signs of fusion when heated to #afgj ,c4 or ,cone #cj at the rate of #aj ,c4 per minute1 and as ,no4 #b grade fireclay those which show no signs of fusion when similarly heated to #aehj ,c4 or ,cone #bf4 ,it is regarded as a sign of fusion if a test piece with sharp angles loses its angularity after heating to a predetermined temperature 7see p4 #ce74 ,it is customary to regard as 'fireclay' all clays which1 when formed into the shape of a ,seger ,cone 7fig4 #f7 do not bend on heating slowly until a temperature of #aehj ,c4 7,cone #bf7 is reached4 ,any clays comprised within this definition and yet not sufficiently refractory to be of the ,no4 #b grade just mentioned may be regarded as ,no4 #c grade fireclays4 ,many of the last named are well suited for the manufacture of blocks for domestic fireplaces1 for glazed bricks and for firebricks not intended to resist furnace temperatures4 ,to resist sudden changes in temperature the material must be very porous--the article being capable of absorbing at least one-sixth of its volume of water4 ,for this reason it is customary to mix fireclays with a large proportion of non-plastic material of a somewhat coarse texture1 the substance most generally employed being fireclay which has been previously burned and then crushed4 ,this material is known as grog or chamotte and has the advantage over other substances of not affecting the composition of the fireclay to which it is added1 whilst greatly increasing its technical usefulness4 ,the addition of grog also reduces the shrinkage of the clay during drying and ensures a sounder article being produced4 ,the most serious impurities in refractory clays are lime1 magnesia1 soda1 potash and titanium and their compounds as they lower the refractoriness of the material4 ,iron1 in the state of ferric oxide is of less importance1 but pyrites and all ferrous compounds are particularly objectionable4 ,pyritic and calcareous nodules may1 to a large extent1 be removed by picking1 and by throwing away lumps in which they are seen to occur4 ,there is1 at present1 no other means of removing them4 ,fireclays may be ground directly they come from the mine1 but it is usually better to expose them to the action of the weather as this effects various chemical and physical changes within the material1 which improves its quality as well as reduces the power required to crush it4 ,to take full advantage of the refractory qualities of a clay it is necessary to select it with skill1 prepare and mould it with care1 to burn it slowly and steadily1 to finish the heating at a sufficiently high temperature and to cool the ware slowly4 ,rapidly heated fireclay is seldom so resistant to heat under commercial conditions as that which has been more steadily fired4 ,rapid or irregular heating causes an irregular formation and distribution of the fused material during the process of vitrification 7p4 #cg7 and so produces goods which are too tender to be durable4 ,it is1 therefore1 necessary to exercise great care in the firing4 ,shales are rocks which have been subjected to considerable pressure subsequent to their deposition and are1 consequently1 laminated and more readily split in one direction than in others4 ,some shales are almost entirely composed of silica or calcareous matter1 but many others are rich in clay1 the term referring to physical structure and not to chemical composition4 ,the clay-shales occur chiefly in the ,silurian and ,carboniferous formations1 the latter being more generally used by clayworkers4 ,clay-shales are valued according to 7a7 the proportion of oil which can be distilled from them1 those rich in this respect being termed oil shales2 7b7 the colour when burned1 as in brickmaking and terra-cotta shales2 7c7 the refractoriness1 as in fireclay shales and 7d7 the facility with which they are decomposed on exposure or on heating and form sulphuric acid as in alum shales4 ,oil shales contain so much carbonaceous matter that on distillation at a low red heat they yield commercially remunerative quantities of a crude oil termed shale tar4 ,in composition they are intermediate between cannel coal and a purely mineral shale4 ,to be of value they should not yield less than #cj gallons of crude oil per ton of shale1 with ammonia and illuminating gas as by-products4 ,they are of ,silurian1 ,carboniferous or ,oolitic origin1 the ,kimeridge shale associated with the last-named being very valuable in this respect4 ,the most important oil shales occur in ,scotland4 ,the fireclay shales have already been described on pages #ec and #aaf4 ,the brickmaking shales are those which are sufficiently rich in clay to form a plastic paste when ground and mixed with water4 ,they can be made into bricks of excellent colour and great strength1 but for this purpose require the use of powerful crushing and mixing machinery4 ,they are usually converted into a stiff paste of only moderate plasticity and are then moulded by machinery in specially designed presses1 though some firebricks are made from crushed shale mixed into a soft paste with water and afterwards moulded by hand4 ,some shales1 such as the knotts at ,fletton near ,peterborough are not made into a paste1 the moist powdered shale being pressed into bricks by very powerful machinery4 ,brickmaking shales may be found in any of the older geological formations1 though they occur chiefly in the ,silurian1 ,permian1 ,carboniferous and ,jurassic systems4 ,the purer shales of the ,coal ,measures burn to an agreeable cream or buff colour1 the less pure ones and those of the other formations mentioned produce articles of a brick-red or blue-grey colour4 ,where the shales are of exceptionally fine grain and their colour when burned is very uniform and of a pleasing tint they are known as terra-cotta shales1 the red terra-cottas being chiefly made from those occurring in ,wales and the buff ones from the lower grade fireclays of the ,coal ,measures4 ,alum shales are characterised by a high proportion of pyrites1 which1 on roasting1 form ferrous sulphate and sulphuric acid4 ,the latter combines with the alumina in the shale and when the roasted ore is extracted with water a solution of iron sulphate and aluminium sulphate is obtained4 ,from this solution 7after partial evaporation7 alum crystals are obtained by the addition of potassium or ammonium sulphate4 ,the chief alum shales are those of the ,silurian formation in ,scotland and ,scandinavia4 ,the ,liassic shales of ,whitby were at one time an equally important source of alum4 ,during recent years a large amount of alum has been obtained from other sources or has been made from the lower grade ,dorset and ,devonshire ball clays by calcining them and then treating them with sulphuric acid4 ,these clays being almost free from iron compounds yield a much purer alum at a lower cost4 ,brick clays are those which are not suitable--either from nature or situation--for the manufacture of pottery or porcelain and yet possess sufficient plasticity to enable them to be made into bricks4 ,the term is used somewhat loosely1 and geologists not infrequently apply it to clays which are quite unsuitable for brickmaking on account of excessive shrinkage and the absence of any suitable non-plastic medium4 ,large portions of the ',london clay' are of this nature and can only be regarded as of use to brick- and roofing-tile-manufacturers when the associated ,bagshot sands are readily accessible4 ,similarly1 some of the very tough surface clays of the ,northern and ,midland counties are equally valueless1 though designated 'brick clays' in numerous geological and other reports4 ,it is1 therefore1 necessary to remember that1 as ordinarily used1 the term 'brick clay' merely indicates a material which appears at first sight to be suitable for brickmaking1 but that more detailed investigations are necessary before it can be ascertained whether a material so designated is actually suitable for the purpose4 ,it is also important to observe that local industrial conditions may be such that a valuable clay may be used for brickmaking because there is a demand for bricks1 but not for the other articles for which the clay is equally suitable4 ,for instance1 a considerable number of houses in ,northumberland and ,durham were built of firebricks at a time when it was more profitable to sell these articles for domestic buildings than for furnaces4 ,in many ways the bricks used for internal structural work form the simplest and most easily manufactured of all articles made from clay4 ,the colour of the finished product is of minor importance and so long as a brick of reasonably accurate shape and of sufficient strength is produced at a cheap rate1 little else is expected4 ,impurities--unless in excessively large proportions--are of small importance and1 indeed1 sand may almost be considered an essential constituent of a material to be used for making ordinary bricks4 ,it is1 therefore1 possible to utilize for this purpose some materials containing so little 'clay' as to make them scarcely fit to be included in this term4 ,so long as the adventitious materials consist chiefly of silica and chalk and the mixture is sufficiently plastic to make strong bricks1 it may be used satisfactorily in spite of its low content of clay1 but if the so-called 'brick clay' contains limestone1 either in large grains or nodules1 it will be liable to burst the bricks or to produce unsightly 'blow-holes' on their surfaces4 ,if too much sand or other non-plastic material is present1 the resulting bricks will be too weak to be satisfactory4 ,no brick clay can be regarded as 'safe' if it contains nodules of limestone--unless these can be removed during the preparation of the material--or if the resulting bricks will not show a crushing strength of at least #he tons per square foot4 ,the introduction of machinery in place of hand-moulding and of kilns instead of clamps has greatly raised the standard of strength1 accuracy in shape and uniformity in colour in many districts1 and many builders in the ,midlands now expect to sort out from the 'common bricks' purchased1 a sufficient number of superior quality to furnish all the 'facing bricks' they require4 ,apart from this1 and in districts where buildings are faced with stone or with bricks of a superior quality1 the 'stock' or 'common brick' may be made from almost any clay which will bear drying and heating to redness without shrinking excessively or cracking4 ,a linear shrinkage of #a in4 per foot 7 #h-a_/b per cent47 may be regarded as the maximum with most materials used for brickmaking4 ,clays which shrink more than this must have a suitable quantity of grog1 sand1 chalk1 ashes or other suitable non-plastic material added4 ,if the clay contains much ferric oxide it will produce red or brown bricks according to the temperature reached in the kiln1 but if much chalk is also present 7or is added purposely7 a combined lime-iron-silicate is produced and the bricks will be white in colour4 ,if only a small percentage of ferric oxide is present a clay will produce buff bricks1 which will be spotted with minute black specks or larger masses of a greyish black slag if pyrites are also present or if ferrous silicate has been produced by the reduction of the iron compounds and their subsequent combination with silica4 ,further information on brick earths will be found on page #fg4 ,a description of the processes used in the manufacture of bricks being outside the scope of the present work1 the reader requiring information on this subject should consult ,modern ,brickmaking 7#be7 or some similar treatise4 ,roofing tiles require clays of finer texture than those which may be made into bricks4 ,stones1 if present1 must be removed by washing or other treatment1 as it is seldom that they can be crushed to a sufficiently fine powder1 unless only rough work is required4 ,if sufficiently fine1 the clay used for roofing tiles may be precisely the same as that used for bricks and is treated in a similar manner4 ,it must1 however1 be of such a nature that it will not warp or twist during the burning2 it must1 therefore1 have a long range of vitrification 7p4 #ch74 ,terra-cotta is an ,italian term signifying baked earth1 but its meaning is now limited to those articles made of clay which are not classed as pottery1 such as statues1 large vases1 pillars1 etc41 modelled work used in architecture1 or for external decoration4 ,although the distinction cannot be rigidly maintained1 articles made of clay may be roughly divided into 7a7 ,pottery 7faience7 and porcelain 7glazed71 7b7 ,terra-cotta 7unglazed71 7c7 ,bricks and unglazed tiles devoid of decoration4 ,in this sense1 terra-cotta occupies an intermediate position between pottery and bricks1 but no satisfactory definition has yet been found for it4 ,thus1 bricks with a modelled or moulded ornament are1 strictly1 terra-cotta1 yet are not so named1 and some pottery is unglazed and yet is never classed as terra-cotta1 whilst glazed bricks are never regarded as pottery4 ,again during the past few years1 what is termed 'glazed terra-cotta' has been largely used for architectural purposes1 yet this is really 'faience4' ,although this overlapping of terms may appear confusing to the reader1 it does not cause any appreciable amount of inconvenience to the manufacturers or users1 as it is not difficult for a practical clayworker to decide in which of the three classes mentioned a given article should be placed4 ,partly on account of the lesser weight1 but chiefly in order to reduce the tendency to crack and to facilitate drying and burning1 terra-cotta articles are usually made hollow4 ,it is necessary that clays used in the manufacture of terra-cotta should be of so fine a texture that the finest modelling can be executed4 ,such clays occur naturally in several geological formations1 and some may be prepared from coarser materials by careful washing1 whereby the larger grains of sand1 stones1 etc41 are removed4 ,some shales1 when finely ground1 make excellent clays for architectural terra-cotta1 portions of all the better known fireclay deposits being used for this purpose4 ,it is1 however1 necessary to use only those shales which are naturally of fine texture1 as mechanical grinding cannot effect a sufficient sub-division of the particles of some of the coarser shales4 ,the finer ,triassic 'marls' are also admirable for terra-cotta work1 the most famous deposit being the ,etruria ,marl ,series in the ,upper ,coal ,measures near ,ruabon4 ,the most important characteristics required in terra-cotta clays are 7a7 fine texture1 or at any rate the ability to yield a fine1 dense surface1 7b7 small shrinkage with little tendency to twist1 warp or crack in firing1 7c7 pleasing and uniform colour when fired1 and 7d7 a sufficient proportion of fluxes to make it resistant to weather without giving a glossy appearance to the finished product4 ,in large pieces of terra-cotta some irregularity of shape is almost unavoidable1 but1 if care is taken in the selection and manipulation of the material1 this need not be unsightly4 ,the durability of terra-cotta is largely dependent on the nature of the surface4 ,the most suitable clays1 when fired1 have a thin 'skin' of vitrified material which is very resistant to climatic influences1 and so long as this remains intact the ware will continue in perfect condition4 ,if this 'skin' is removed1 rain will penetrate the material and under the influence of frost may cause rapid disintegration4 ,in the manufacture of very large pieces of terra-cotta a coarse1 porous clay is used for the foundation and interior1 and this is covered with the finer clay4 ,by this means a greater resistance to changes in temperature is secured1 the drying and the burning of the material in the kiln are facilitated and the risks of damage in manufacture are materially reduced4 ,cement clays are those used in the manufacture of ,portland cement and of so-called natural cements4 ,they are largely of an alluvial character and are of two chief classes3 7a7 those which contain chalk or limestone dust and clay in proportions suitable for the manufacture of cement and 7b7 those to which chalk or ground limestone must be added4 ,they vary in composition from argillaceous limestones containing only a small proportion of clay to almost pure clays4 ,the manufacture of ,portland cement has assumed a great importance and owing to the large amount of investigations made in connection with it1 it may be said to represent the chief cement made from argillaceous materials1 the others being convenient though crude modifications of it4 ,the essential constituents are calcium carbonate 7introduced in the form of chalk or powdered limestone7 and clay1 the composition of the naturally occurring materials being modified by the addition of a suitable proportion of one or other of these ingredients4 ,the material is then heated until it undergoes partial fusion and a 'clinker' is formed4 ,this clinker1 when ground1 forms the cement4 ,in ,kent1 the ,medway mud is mixed with chalk2 in ,sussex1 a mixture of gault clay and chalk is employed2 in the ,midlands and ,south ,wales1 ,liassic shales and limestone are used2 in ,northumberland a mixture of ,kentish chalk and a local clay is preferred1 and in ,cambridgeshire a special marl lying between the ,chalk and the ,greensand is found to be admirable for the purpose because it contains the ingredients in almost exactly the required proportions4 ,for cement manufacture1 clays should be as free as possible from material which1 in slip form1 will not pass through a ,no4 #ajj sieve1 as coarse sand and other rock dbris are practically inert4 ,the proportion of alumina and iron should be about one-third1 but not more than one-half1 that of the silica1 and in countries where the proportion of magnesia in a cement is limited by standard specifications1 it will be found undesirable to use clays containing more than #c per cent4 of magnesia and alkalies4 ,whilst calcareous clays usually prove the most convenient in the manufacture of cement1 it is by no means essential to use them1 and where a clay almost free from lime occurs in convenient proximity to a suitable chalk or limestone deposit an excellent cement may usually be manufactured4 ,the 'clays' from which the so-called 'natural' or ',roman cements' are made by simple calcination and crushing1 usually fuse at a lower temperature than do the mixtures used for ,portland cement1 and unless their composition is accurately adjusted they yield a product of such variable quality as to be unsuitable for high class work4 ,fuller's earth is a term used to indicate any earthy material which can be employed for fulling or degreasing wool and bleaching oil4 ,true fuller's earth is obtained chiefly from the neighbourhood of ,reigate1 ,surrey1 ,woburn ,sands1 ,bedfordshire and from below the ,oolite formation near ,bath1 but owing to the scarcity of the material and the irregularity of its behaviour1 china clay is now largely used for the same purpose4 ,true fuller's earth is much more fusible than the white clays usually substituted for it1 and when mixed with water it does not form a plastic paste but falls to powder4 ,as the chief requirement of the fuller is the grease-absorbing power of the material there is no objection to the substitution of other earths of equal efficiency4 ,fuller's earth does not appear to be a true clay1 though its constitution and mineralogical composition are by no means clearly known4 ,t4 ,j4 ,porter considers that it is chiefly composed of montmorillonite 7,al#b,o#cd,si,o#b;,h#b,o71 anauxite1 7#b;,al#b,o#ci,si,o#bf;,h#b,o71 and chalk with some colloidal silica and a little quartz4 ,it therefore appears to resemble the less pure kaolins1 but to contain little or no true clay1 though in many respects it behaves in a manner similar to a kaolin of unusually low plasticity4 ,other clays of commercial importance1 with further details of the ones just mentioned1 are described in the author's ,british ,clays1 ,shales and ,sands 7#b74 ,,chapter ,,vi ,,clay ,,substance3 ,,theoretical ,,and ,,actual ,having indicated the origin1 modes of accumulation and general characteristics of the numerous materials known as 'clay1' it now remains to ascertain what substance1 if any1 is contained in all of them and may be regarded as their essential constituent1 to which their properties are largely due4 ,just as the value of an ore is dependent to a very large extent on the proportion of the desired metal which it contains1 and just as coal is largely1 though not entirely1 esteemed in proportion to the percentage of carbon and hydrogen in it1 so there may be an essential substance in clays to which they owe the most important of their characteristics4 ,the proportion of metal in an ore or of hydrocarbon in a coal can be ascertained without serious difficulty by some means of analysis1 but with clay the difficulties are so great that1 to some extent at least1 they must be regarded as being1 for the present1 insurmountable4 ,this is in no small measure due to the general recognition of all minerals or rocks which become plastic when kneaded with water as 'clays' without much regard being paid to their composition4 ,consequently materials of the most diverse nature in other respects are termed clays if they are known to become plastic under certain conditions4 ,there is1 in fact1 at the present time1 no generally accepted definition of clay which distinguishes it from mixtures of clay and sand or other fine mineral particles4 ,the usual geological definitions are so broad as to include many mixtures containing considerably less than half their weight of true clay or they avoid the composition of the material altogether and describe it as a finely divided product of the decomposition of rocks4 ,many attempts have been made to avoid this unfortunate position1 which is alike unsatisfactory to the geologist1 the mineralogist and the chemist as well as to the large number of people engaged in the purchase and use of various clays2 and1 whilst the end sought has not been reached as completely as is desirable1 great progress has been made and much has been accomplished during the last twenty years4 ,one of the earliest attempts to ascertain whether there is an essential constituent of all clays was made by ,seger 7#g7 who used two methods of separating some of the ingredients of natural clays from the remaining constituents4 ,the first of these methods consists in an application of the investigations of ,schulze1 ,schloesing and ,schoene on soils1 viz4 the removal of the finest particles by elutriation2 the second is an extension of the method of ,forschammer and ,fresenius1 viz4 the treatment of the material with sulphuric acid4 ,to the product containing the clay when either of these methods is used ,seger gave the name clay substance1 but the material so separated is by no means pure clay4 ,the term clay substance must1 therefore1 be confined to the crude product containing the clay together with such other impurities as are in the form of extremely small particles or are soluble in sulphuric acid4 ,it has not yet been found possible to isolate pure clay from ordinary clays1 so that in investigating the nature of what ,seger was endeavouring to produce when he obtained the crude clay substance1 indirect methods are necessary4 ,it has long been known that if a sample of 'clay'--using this word in the broadest sense--is rubbed in a considerable quantity of water so as to form a thin slip or slurry1 it may readily be divided into a number of fractions each of which will consist of grains of different sizes4 ,this separation may be effected by means of a series of sieves through which the slurry is poured1 or the slurry may be caused to flow at a series of different speeds1 the material left behind at each rate of speed being kept separate2 or1 finally1 the slurry may be allowed to stand for a few seconds and may then be carefully decanted into another vessel in which it may remain at rest for a somewhat longer period1 these times of resting and decantation1 if repeated1 providing a series of fractions the materials in which are more or less different in their nature4 ',clays' containing a considerable proportion of coarse material are most conveniently separated into a series of fractions by means of sieves1 whereby they are divided into 7i7 stones1 7ii7 gravel1 7iii7 coarse sand1 7iv7 medium sand1 7v7 fine sand and 7vi7 a slurry consisting of such small particles that they can no longer be separated by sifting4 ,if the residues on the sieves are carefully washed free from any adhering fine material and are then dried1 they will be found on examination to be quite distinct from anything definable as clay4 ,they may consist of a considerable variety of minerals or may be almost entirely composed of quartz1 but with the possible exception of some shales of great hardness1 they are undoubtedly not clay4 ,this simple process therefore serves to remove a proportion of material which in the case of some 'clays' is very large but in others is insignificant2 thus #dj per cent4 of sand-like material may be removed from some brick-clays whilst a ball clay used for the manufacture of stoneware or pottery may pass completely through a sieve having #bjj meshes per linear inch4 ,the material which passes through the finest sieve employed will contain all the true clay in the material2 that is to say1 the coarser portion will1 as already mentioned1 be devoid of the ordinary characteristics of clay4 ,at the same time1 this very fine material will seldom consist exclusively of clay1 but will usually contain a considerable proportion of silt1 extremely fine mineral particles and1 in the case of calcareous clays1 a notable proportion of calcium carbonate in the form of chalk or limestone particles4 ,only in the case of the purest clays will the material now under consideration consist entirely of clay1 so that it must be again separated into its constituents4 ,this is best accomplished1 as first suggested by ,schoene1 by exposing the material to the action of a stream of water of definite speed4 ,h4 ,seger 7#g7 investigated this method very thoroughly and his recommendations as to the manner in which this separation by elutriation should be carried out remain in use at the present time4 ,briefly1 all material sufficiently fine to be carried away by a stream of water flowing at the rate of #jdc in4 per minute was found by ,seger to include the whole of the clay in the samples he examined1 but1 as was later pointed out by ,bischof1 it is not correct to term the whole of this material 'clay substance1' as when examined under the microscope1 it contains material which is clearly not clay4 ,processes of decantation of the finest material obtained after elutriation still fail to separate all the non-clay material1 and ,vogt has found that when the material has been allowed to stand in suspension for nine days some particles of mica are still associated with the clay4 ,it would thus appear that no process of mechanical separation will serve for a complete purification of a clay2 indeed1 there are good reasons for supposing that extremely fine particles of quartz and mica render physical characteristics an uncertain means of accurately distinguishing clays from other rock dust4 ,when chemical methods of investigation are employed the problem is not materially altered1 nor is its solution fully attained4 ,it is1 of course1 obvious that any chemical method should be applied to the product obtained by treating the raw material mechanically as above described1 for to do otherwise is to create needless confusion4 ,yet by far the greater number of published analyses of 'clays' report the ultimate composition of the whole material1 no attempt being made to show how much of the various constituents is in the form of sand1 stones or other coarse particles of an entirely non-argillaceous character4 ,if the particles of a 'clay' which are sufficiently small to be carried away by a stream of water with a velocity of only #jdc in4 per minute are analysed1 it will be found that their composition will vary according to the origin of the clay and the subsequent treatment to which it has been subjected during its transport and deposition4 ,if the clay is fairly free from calcareous material and is of a white-burning nature it may be found to have a composition like china clays4 ,red-burning clays1 on the contrary1 will vary greatly in composition1 so that it becomes difficult to find any close analogy between these kinds of clay4 ,this difference is partly due to the extremely fine state of division in which ferric oxide occurs in clays1 the particles of this material corresponding in minuteness to those of the purest clays and so being inseparable by any mechanical process4 ,in #ahgf ,h4 ,seger 7#g7 published what he termed a method of 'rational analysis1' which consisted in treating the clay with boiling sulphuric acid followed by a treatment with caustic soda4 ,he found that the purer china clays 7kaolins7 and ball clays were made soluble by this means and that felspar1 mica and quartz were to a large extent unaffected4 ,later investigators have found that this method is only applicable to a limited extent and that its indications are only reliable when applied to the clays just named1 but the principle introduced by ,seger has proved invaluable in increasing our knowledge of the composition of clays4 ,by means of this so-called rational analysis ,seger found that the purer clays yielded results of remarkable similarity and uniformity1 the material entering into solution having a composition agreeing very closely with the formula ,al#b,o#cb,si,o#bb;,h#b,o which is generally recognized as that of the chief constituent or constituents of china clay 7kaolin7 and the purer ball clays4 ,this crude substance1 obtainable from a large number of clays by the treatment just described1 was named clay substance by ,seger1 who regarded it as the essential constituent of all clays4 ,red-burning clays when similarly treated do not yield so uniform a product1 and the ferric oxide entering into solution makes the results very discordant4 ,moreover1 even with the china clays or kaolins a small proportion of alkalies1 lime and other oxides enter into solution and a number of minerals analogous to clay1 but quite distinct from it1 are also decomposed and dissolved4 ,for these reasons the 'rational analysis' has been found insufficient2 it is now considered necessary to make an analysis of the portion rendered soluble by treatment with sulphuric acid in order to ascertain what other ingredients it may contain in addition to the true clay present4 ,as the china clays 7kaolins7 and ball clays on very careful elutriation all yield a product of the same ultimate composition1 viz4 #ci per cent4 of alumina1 #df per cent4 of silica1 #ac per cent4 of water1 and #b per cent4 of other oxides1 they are generally regarded as consisting of practically pure clay with a variable amount of impurities4 ,many years ago ,fresenius suggested that these non-clayey constituents of clays should be calculated into the minerals to which they appeared likely to correspond so as to obtain a result similar to that obtained by ,seger without the disadvantages of the treatment with sulphuric acid and as supplementary to such treatment in the case of red-burning and some other clays4 ,more recent investigators have found that if a careful microscopic examination of the clay is made the results of estimating the composition from the proportion of the different minerals recognizable under the microscope and by calculation from the analysis of the material agree very closely and are1 as ,bischof 7#bh7 and1 more recently1 ,mellor have pointed out1 more reliable than the 'rational analysis' in the case of impure clays4 ,if care is taken to make a microscopical examination identifying the chief impurities present the calculation from the analysis may usually be accepted as sufficiently accurate1 but it is very unsatisfactory to assume1 as some chemists do1 that the alkalies and lime in the clay are all in the form of felspar and that the silica remaining in excess of that required to combine with the alkalies1 lime and alumina is free quartz4 ,some clays are almost destitute of felspar but comparatively rich in mica1 whilst others are the reverse1 so that some means of identifying the extraneous minerals is essential4 ,when this is not used1 the curious result is obtained that ,german chemists calculate the alkalies1 etc4 to felspar whilst the ,french chemists1 following ,vogt1 calculate them to mica2 ,english ceramic chemists appear undecided as to which course to follow1 and some of them occasionally report notable amounts of felspar in clays quite destitute of this mineral6 ,a statement of the composition of a 'clay' based on a mechanical separation of the coarser ingredients followed by an analysis of the finer ones and a calculation of the probable constituents of the latter1 as already described1 is known as a proximate analysis in order to distinguish it from an ultimate analysis which states the composition of the whole material in terms of its ultimate oxides4 ,a proximate analysis therefore shows the various materials entering into the composition of the clay in the following or similar terms3 ,stones per cent4 ,gravel ,coarse sand ,medium sand ,fine sand ,silt ,felspar or mica dust ,silica dust ',true clay#ab' ,moisture ,carbon ,other volatile matter ,footnote #ab3 ,in analytical reports a note should be appended stating that the figure under this term shows the proportion of the nearest approximation to true clay at present attainable4 ,for some purposes it is necessary to show the proportion of calcium1 iron and other compounds as in an ordinary ultimate analysis4 ,a comparison of the foregoing with an ultimate or 'ordinary' analysis of a clay 7p4 #af7 will show at once the advantage of the former in increasing our knowledge of the essential constituent of all clays1 if such a substance really exists4 ,its absolute existence is by no means proved1 for1 as will have been noticed1 its composition is largely based on assumption even in the most thorough investigations1 particularly of the admittedly less pure clays4 ,in the purer clays the problem is much simpler and in their case an answer of at least approximate accuracy can be given to the question ',what is clay8' ,even with these purer clays it is not sufficient to study an analysis showing the total amount of the silica1 alumina and other oxides present2 it is still necessary to effect some kind of separation into the various minerals of which they are composed4 ,when1 however1 the accessory minerals do not exceed #e per cent4 of the total ingredients their influence is less important and the nature and characteristics of the 'clay substance' itself can be more accurately studied4 ,by careful treatment of well selected china clays1 for example1 it is possible to obtain a material corresponding to the formula ,al#b,o#cb,si,o#bb;,h#b,o within a total error of #a per cent41 the small amount of impurity being1 as far as can be ascertained1 composed of mica4 ,so pure a specimen of clay is found on microscopical examination to consist of minute irregular grains of no definite form1 together with a few crystals of the same composition and identifiable as the mineral 'kaolinite' 7p4 #ajg74 ,this 'amorphous' material1 which appears to be the chief constituent of all china clays and kaolins1 has been termed clayite by ,mellor 7#bb74 ,johnson and ,blake1 ,aron and other observers have stated that the majority of the particles in china clays and kaolins are crystalline in form4 ,owing to their extreme smallness it is exceedingly difficult to prove that they are not so1 though for all ordinary purposes they may be regarded as amorphous1 the proportion of obviously crystalline matter present in ,british china clay of the highest qualities being so small as to be negligible4 ,hickling 7#cf71 using an exceptionally powerful microscope1 claims to have identified this 'amorphous' substance in china clay as 'worn and fragmental crystals of kaolinite1' and recently ,mellor and ,holdcroft and ,rieke have shown that the apparently amorphous material shows the same endo- and exothermal reactions as crystalline kaolinite4 ,so far as china clays or kaolins are concerned1 kaolinite or an amorphous substance of the same composition appears to be identical with the 'ideal clay' or 'true clay' whose characters have so long been sought4 ,this term--clayite--is very convenient when confined to china clays and kaolins1 but it is scarcely legitimate to apply it1 as has been suggested1 to material in other clays until it has been isolated in a sufficiently pure form to enable its properties to be accurately studied4 ,this restriction is the more necessary as in one very important respect clayite obtained from china clay and some kaolins differs noticeably from the nearest approach to it obtainable from the more plastic clays3 namely1 in its very low plasticity4 ,this may be explained by the fact that it is only obtainable in a reasonably pure form in clays of a primary character1 whilst the plastic clays have usually been transported over considerable areas and have been subjected to a variety of treatments which have had a marked effect on their physical character4 ,moreover1 the fact that the purest 'clay' which can be isolated from plastic clays appears to be amorphous and to some extent colloidal greatly increases the difficulty of obtaining it in a pure state1 especially as no liquid is known which will dissolve it without decomposing it4 ,the fact that it is not an elementary substance1 but a complex compound of silica1 alumina and the elements of water1 also increases the intricacy of the problem1 for these substances occur in other combinations in a variety of other minerals which are clearly distinct from clay4 ,ever since the publication of ,seger's memorable papers 7#g71 and to a small extent before that time1 it has been generally understood that china clay or kaolin represented the true essential constituent of clays1 but several investigators have been so imbued with the idea that all true clay substance must have a crystalline form that they have frequently used the term 'kaolinite' to include the 'amorphous' substance in plastic clays4 ,this is unfortunate as it is by no means proved that the latter is identical with kaolinite1 and a distinctive term would be of value in preventing confusion4 ,other investigators have used the word 'kaolin' with equal freeness1 so that whilst it originally referred to material from a particular hill or ridge in ,china#ac it has now entered into general use for all clays whose composition approximates to that of china clay 7p4 #af7 in which the plasticity is not well developed4 ,thus1 in spite of the difference in origin between many ,german and ,french kaolins and the china clays of ,cornwall1 it is the custom in ,europe generally to term all these materials 'kaolin4' ,yet they are very different in many respects from the material originally imported from ,china4 ,footnote #ac3 ,kao-ling is ,chinese for a high ridge or hill4 ,as the essential clay substance has not yet been isolated in a pure form from the most widely spread plastic clays1 but is largely hypothetical as far as they are concerned1 the author prefers the term pelinite#ad when referring to that portion of any plastic clays or mixtures of clays with other minerals which may be regarded as being the constituent to which the argillaceous portion of the material owes its chief properties4 ,in china clay and kaolin the 'true clay' is identical with clayite--or even with kaolinite 7p4 #ajh7--;and there is great probability that this identity also holds in the case of the more plastic clays of other geological formations1 but until it is established it appears wisest to distinguish the hypothetical or ideal clay common to all clays 7if there is such a substance7 by different terms according to the extent to which its composition and characters of the materials most closely resembling it are experimentally known4 ,footnote #ad3 ,from the ,greek3 plinos made of clay4 ,the substances most resembling this 'ideal clay' which have1 up to the present been isolated1 are3 7a7 ,kaolinite4 ,found in a crystalline form in china clays and kaolins 7p4 #ajg74 7b7 ,clayite4 ,a material of the same chemical composition as kaolinite1 but whose crystalline nature 7if it be crystalline7 has not been identified--chiefly obtained from china clays and kaolins4 7c7 ,pelinite4 ,a material similar to clayite1 but differing from it in being highly plastic and1 to some extent1 of a colloidal nature--obtained from plastic clays4 7d7 ,laterite4 ,a material resembling clayite in physical appearance1 but containing free alumina and free silica 7p4 #hj74 7e7 ,clay ,substance4 ,a general term indicating any of the foregoing or a mixture of them2 it is also applied 7unwisely7 to the material obtained when a natural clay is freed from its coarser impurities by elutriation 7p4 #g74 ,the ,chief ,characteristics of ',true ,clay' from ,different ,sources4 ,in so far as it can be isolated true clay appears to be an amorphous1 or practically amorphous1 material which may under suitable conditions crystallize into rhombic plates of kaolinite4 ,the particles of which it is composed are extremely small1 being always less than #jjjjd in4 in diameter4 ,they adsorb dyes from solutions and show other properties characteristic of colloid substances though in a very variable degree1 some clays appearing to contain a much larger proportion of colloidal matter than do others4 ,to some extent the power of adsorption of salts and colouring matters appears to be connected with the plasticity 7p4 #da7 of the material1 but this latter property varies so greatly in clayite or pelinite from different sources as to make any generalization impossible4 ,true clay substance appears to be quite white1 any colour present being almost invariably traceable to ferric compounds or to carbonaceous matter4 ,the latter is of small importance to potters as it burns away in the kiln4 ,the specific gravity of clay substance is #bfe according to ,hecht1 the lower figures sometimes reported being too low4 ,its hardness is usually less than that of talc--the softest substance on ,mohs' scale--but some shales are so indurated as to scratch quartz4 ,it is quite insoluble in water and in dilute solutions of acids or alkalies1 but is decomposed by hydrofluoric acid and by concentrated sulphuric acid when heated1 alumina entering into solution and silica being precipitated in a colloidal condition4 ,it absorbs water easily until a definite state of saturation has been reached1 after which it becomes impervious unless the proportion of water is so large and the time of exposure so great that the material falls to an irregular mass which may be converted into a slurry of uniform consistency by gently stirring it4 ,with a moderate amount of water1 pelinite develops sufficient plasticity to enable it to be modelled with facility1 but clayite and some specimens of pelinite are somewhat deficient in this respect4 ,the pelinitic particles usually possess the capacity to retain their plasticity after being mixed with considerable proportions of sand or other non-plastic material and are then said to possess a high binding power 7p4 #bh74 ,if a large proportion of water is added to a sample of clayite or pelinite and the mixture is stirred into a slurry it will be found to remain turbid for a considerable time and will not become perfectly clear even after the lapse of several days4 ,its power of remaining in suspension is much influenced by the presence of even small amounts of soluble salts in either the water or the clay substance1 its precipitation being hastened by the addition of such salts as cause a partial coagulation of the colloidal matter present4 ,some specimens of clayite and pelinite retain their suspensibility even in the presence of salts1 but this is only true of a very limited proportion of the substance4 ,in most cases the presence of soluble salts causes the larger particles to sink somewhat rapidly and to carry the finer particles with them4 ,the rate at which a slip or 'cream' made of elutriated clay and water will flow through a small orifice is dependent on the viscosity of the liquid and this in turn depends on the amount of colloidal material present1 i4e4 on how much of the clay 7pelinite7 is in a colloidal form4 ,its viscosity is greatly affected by the addition or presence of small quantities of acid or alkali or of acidic or basic salts4 ,acids increase the viscosity2 alkalies and basic salts1 on the contrary1 make the slip more fluid4 ,neutral salts behave in different ways according to the concentration of the solution and to the amount of clay 7pelinite7 present in the slip4 ,if the slip contains so little water as to be in the form of a thin paste1 neutral salts usually have but a small action1 but when the slip contains only a small proportion of clay 7pelinite7 the presence of neutral salts will tend to cause the precipitation of the clay4 ,in this way salts act in two quite different directions according to the concentration of the slip4 ,on drying a paste made of clay and water the volume gradually diminishes until the greater part of the water has been removed2 after this the remainder of the water may be driven off without any further reduction in volume of the material4 ,this is another characteristic common to colloidal substances such as gelatin4 ,the material when drying attains a leathery consistency which is at a maximum at the moment when the shrinkage is about to cease2 on further drying the material becomes harder and more closely resembles stone4 ,providing that wet clay is not heated to a temperature higher than that of boiling water it appears to undergo no chemical change and on cooling it will again take up water#ae and be restored to its original condition except in so far as its colloidal nature may have been affected by the heating4 ,if1 however1 the temperature is raised to about #ejj ,c4 a decomposition of the material commences and water is evolved4 ,this water--which is commonly termed 'combined water'--is apparently an essential part of the clay-molecule and when once it has been removed the most important characteristics of the clay are destroyed and cannot be restored4 ,the reactions which occur when clay is heated are complex and are rendered still more difficult to study by the apparent polymerization of the alumina formed4 ,mellor and ,holdcroft 7#bi7 have recently investigated the heat reactions of the purest china clay obtainable and confirm ,le ,chatelier's view 7#aj7 that on heating to temperatures above #ejj ,c4 clay substance decomposes into free silica1 free alumina and water1 the two former undergoing a partial re-combination with formation of sillimanite 7,al#b,o#c,si,o#b7 if a temperature of #abjj ,c4 is reached4 ,mellor and ,holdcroft point out that there is no critical point of decomposition for clay substance obtained from china clay1 as it appears to lose water at all temperatures1 though its decomposition proceeds at so slow a rate below #djj ,c4 as to be scarcely appreciable4 ,footnote #ae3 ,some clays are highly hygroscopic and absorb moisture readily from the atmosphere4 ,according to ,seger 7#g7 this hygroscopicity distinguishes true clay from silt and dust4 ,after the whole of the 'combined water' has been driven off1 if the temperature continues to rise1 it is found that at a temperature of #ijj ,c4 an evolution of heat occurs4 ,this exothermal point1 together with the endothermal one occurring at the temperature at which the decomposition of the clay seems to be most rapid1 has been found by ,le ,chatelier1 confirmed by ,mellor and ,holdcroft1 to be characteristic of clay substance derived from kaolin and china clay1 and the two last-named investigators state that it serves as a means of distinguishing kaolinite or clayite from other alumino-silicates of similar composition4 ,these thermal reactions have not1 as yet1 been fully studied in connection with plastic clays2 with china clay1 as already noted1 they probably indicate a polymerization of the alumina set free by the decomposition of the clay substance1 as pure alumina from a variety of sources has been found by ,mellor and ,holdcroft to behave similarly4 ,on still further raising the temperature of pure clay 7pelinite or clayite7 no further reactions of importance occur1 the material being practically infusible4 ,if1 however1 any silica1 lime1 magnesia1 alkalies1 iron oxide or other material capable of combining with the alumina and silica is present as impurities in the clay substance1 combination begins at temperatures above #ijj ,c4 ,this causes a reduction of the heat-resisting power of the material2 the silicates and alumino-silicates produced fuse and begin to react on the remaining silica and alumina1 first forming an impermeable mass in place of the porous one produced with pure clay substance1 and gradually1 as the material loses its shape1 producing a molten slag if the 'clay' is sufficiently impure4 ,as ordinary clays are never quite free from metallic compounds other than alumina1 this formation of a fused portion--technically known as vitrification 7p4 #cg7--occurs at temperatures depending on the nature of the materials present1 so that a wide range of products is obtained1 the series commencing with the entirely unfused pure clay 7china clay71 passing through the slightly vitrified fireclays1 the more completely vitrified ball clays to the vitrifiable stoneware clays and ending with materials so rich in easily fusible matter as scarcely to be worthy of the name of clays4 ,the constitution of the clay molecule is a subject which has attracted the attention of many investigators and is being closely studied at the present time4 ,it is a subject of peculiar difficulty owing to the inertness of clay substance at all but high temperatures1 and to the complexity of reactions which take place as soon as any reagent is brought into active connection with it4 ,without entering into details regarding the various graphic formulae which have been suggested1 it is sufficient to state that the one which is most probably correct1 as far as present knowledge goes1 is ,mellor's and ,holdcroft's re-arrangement of ,groth's formula 7#cj7 ,,ho _/,o,si,o ,al#b_/ ,o _/ _/ ,,ho_/ _/ ,o,si,o_/ _/ ,,ho ,,oh which on removal of the hydroxyl groups might be expected to give the anhydride ,o _/,o,si,o ,al#b_/ _/_/ _/ ,o_/_/ ,o,si,o_/ though in practice this substance--if formed at all--appears to be instantly split up into ,al#b,o#c and ,si,o#b4 ,by regarding the aluminium as a nucleus1 as above1 and some aluminium silicates as hypothetical alumino-silicic acids1 as suggested by ,ulffers1 ,scharizer1 ,morozewicz 7#bi7 and others1 clay substance may be conveniently considered1 along with analogous substances1 as forming a special group quite distinct from the ordinary silicates4 ,in this way ,mellor and ,holdcroft 7#bi7 consider that clay substance is not a hydrated aluminium silicate--as is usually stated in the text-books--but an alumino-silicic acid1 the salts of which are the zeolites and related compounds4 ,from this hypothesis it naturally follows that clay substance is analogous to colloidal silica which has been formed by the decomposition of a silicate by means of water and an acid4 ,if this view be correct1 pure clay substance or true clay is a tetra-basic alumino-silicic acid ,h#d;,al#b,si,o#i or ,al#b,si#b,o#e7j;,h#d74 ,that its acid properties are not readily recognizable at ordinary temperatures is due to its inertness2 at higher temperatures its power of combination with lime1 soda potash and other bases is well recognized1 though the reactions which occur are often complicated by decompositions and molecular re-arrangements which occur in consequence of the elevated temperature4 ,there are a number of minerals which closely resemble clayite or pure clay substance in composition1 the chief difference being in the proportion of water they evolve on being heated4 ,thus ,rectorite ,h#b;,al#b,si#b,o#h1 ,kaolinite ,h#d;,al#b,si#b,o#i1 ,halloysite ,h#f;,al#b,si#b,o#aj and ,newtonite ,h#aj;,al#b,si#b,o#ab4 ,in the crystalline form these minerals may be distinguished from each other by means of the microscope1 but as the chief materials of which clays are composed appears to be amorphous it is impossible to ascertain with certainty whether a given specimen of clay substance is composed of a mixture of these analogous minerals in an amorphous form or whether it consists entirely of clayite1 i4e4 the clay substance obtained from china clay4 ,as already stated1 the thermal reactions which occur on heating clayite appear to be characteristic of kaolinite whilst halloysite is completely decomposed at a temperature somewhat below #bjj ,c42 but the not improbable presence of two or more of these alumino-silicic acids in clays of secondary or multary origin makes it almost impossible to determine whether clayite is an essential constituent of all clays or whether the purest clay substance 7pelinite7 obtained from some of the more plastic clays does not possess a different chemical composition as well as different physical properties4 ,the view that clays may be regarded as impure varieties of clayite is considered erroneous by several investigators for various reasons4 ,for instance1 felspar is rarely found in china clays1 but is a common constituent of secondary 7plastic7 clays4 ,j4 ,m4 van ,bemmelen 7#bf71 who has found that the alumina-silica ratio of clays produced by weathering is always higher than that in clays of the china clay type produced by hypogenic action4 ,in a number of clays examined he found that a portion was soluble in boiling hydrochloric acid whereas clayite is scarcely affected by this treatment4 ,he also found a varying proportion of alumino-silicate insoluble in hydrochloric acid but dissolved on treatment with boiling sulphuric acid and subsequently with caustic soda solution2 this latter he considers to be true clayite4 ,unfortunately1 his results were obtained by treating the crude clay with acid1 instead of first removing such non-plastic materials as can be separated by washing1 so that all that they show is that some clays contain alumino-silicates of a nature distinct from clayite in addition to any clayite which may be found in them4 ,the fact that all clays when heated to #gjj or #hjj ,c4 readily react with lime-water to form the same calcium silicates and aluminates indicates so close a resemblance between the clay substance obtainable from different sources as to constitute strong evidence of the identity of this substance with clayite or with materials so analogous to it as to be indistinguishable from it under present conditions4 ,in all probability1 the plastic clays have been derived from a somewhat greater variety of minerals than the primary clays 7p4 #ga7 and under conditions of decomposition which differ in details1 though broadly of the same nature as those producing china clays4 ,the presence of colloidal matter suggests a more vigorous action--or even a precipitation from solution--instead of the slower reactions which result in the formation of the kaolinite crystals4 ,the much smaller particles present in plastic clays also indicate a more complete grinding during the transportation of the material or some form of precipitation4 ,if1 as ,hickling suggests1 all clays are direct products of the decomposition of mica1 the fact that several varieties of mica are known and that the conditions under which these decompose must vary considerably1 afford a good1 if incomplete1 explanation of some of the widely diverse characteristics observed in different clays4 ,notwithstanding the great complexities of the whole subject and the apparently contradictory evidence concerning some clays1 there is a wide-spread feeling that whatever may be the mineral from which a given clay has been derived1 the true clay substance1 which is its essential constituent1 would 7if it could be isolated in a pure state7 prove to be of the same composition as kaolinite obtainable from china clay of exceptional purity4 ,the purest clay substances 7pelinite7 yet obtained from some of the most plastic clays are1 however1 so impure as to make any detailed investigation of their composition by present methods abortive4 ,the methods of synthesis which have proved so successful in organic chemistry have hitherto yielded few intelligible results with clays1 on account of the complexity of the accessory reactions which occur4 ,the ,difference between ,pure ,clay ,substance and ,ordinary ,clays4 ,the properties and characteristics of true clay are very seriously modified by other materials which may be associated with it4 ,this may be perceived by comparing the properties of clays mentioned in ,chapter ,i with those of various forms of true clay just given4 ,moreover1 as true clay never occurs in a perfectly pure state in nature1 the properties of clays must be largely dependent on the accessory ingredients4 ,silica1 for example1 when alone is a highly refractory material1 but in the presence of true clay it reduces the refractoriness of the latter4 ,lime has a similar effect though its chemical action on the clay is entirely different4 ,a very small proportion of some substances--notably the oxides of sodium and potassium--will greatly alter the behaviour of true clay when heated and will produce an impervious mass in place of a porous one4 ,for these reasons1 it is necessary in studying clays to pay attention to both their physical and chemical properties and to separate the material into fractions so that each of these may be studied separately and their individual as well as their collective characteristics ascertained4 ,failure to do this has been the cause of much obscurity and confusion in investigations on certain clays composed of a considerable proportion of non-argillaceous material which ought to have been separated before any attempt was made to study the true clay present4 ,there is1 therefore1 a considerable difference between a natural clay and the pure clay substance theoretically obtainable from it2 this difference being most marked in the case of low-grade brick clays of glacial origin1 which may contain #ej per cent4 or more of adventitious materials4 ,if used in a natural state they would be found to be valueless on account of their impurities giving them characteristics of a highly undesirable character1 whereas the true clay in them is found--in so far as it can be separated--to bear a close resemblance to that obtained from a high grade1 plastic1 pottery clay4 ,unfortunately1 it is1 at present1 impossible to isolate this clay substance in anything approaching a pure form1 and many clays are without commercial value because of comparatively small proportions of impurities which cannot be separated from the clay substance without destroying the latter4 ,classification of ,clays4 ,owing to the widely differing substances from which clays can1 apparently1 be formed and the peculiar difficulties which are experienced in investigating the nature of clay substance from different sources1 it is by no means easy to devise a scheme of classification of clays1 though many of these have been attempted by different scientists4 ,the classification adopted by geologists is based on the fossil remains and on the stratigraphical position of clays relative to other rocks1 as described in ,chapter ,,ii4 ,this is of great value for some purposes1 but the composition of the substances termed 'clay' by geologists differs so greatly1 even when only one formation is considered1 as to make their classification of little or no use where the value or worthlessness of the material depends upon its composition4 ,thus the so-called ,oxford clay ranges from a hard silicious shale to a comparatively pure clay2 some portions of it are so contaminated with calcareous and ferruginous matter as to make the material quite useless for the potter or clayworker4 ,a geological classification of clays is chiefly of value as indicating probable origins1 impurities and certain physical properties2 but the limits of composition and general characteristics are so wide as to make it of very limited usefulness4 ,the classification of clays on a basis of chemical composition is rendered of comparatively little value by the large number of clays which occupy ill-defined borders between the more clearly marked classes4 ,moreover1 attempts to predict the value and uses of clays from their chemical composition are generally so misleading as to be worse than useless1 unless a knowledge of some of the physical characters of the clays is available4 ,it is1 of course1 possible to differentiate some clays from others by their composition1 but not with sufficient accuracy to permit of definite and accurate classification4 ,a classification based exclusively on the composition of clays is equally unsatisfactory for other reasons1 the chief of which is the placing together of clays of widely differing physical character1 and the separation of clays capable of being used for a particular purpose4 ,to some extent the latter objection may be disregarded1 though it is of great importance in considering the commercial value of a clay4 ,classification based on the uses of clays of different kinds has been suggested by several eminent ceramists1 but is obviously unsatisfactory1 particularly as it is by no means uncommon to use mixtures of clays and other minerals for some purposes4 ,thus stoneware clays must be vitrifiable under conditions which may be defined with sufficient accuracy1 but many manufacturers of stoneware do not use clays which are naturally vitrifiable2 they employ a mixture of refractory clay and other minerals to obtain the material they require4 ,a classification based on the origin of clays regarded from the petrological point of view offers some advantages1 but is too cumbersome for ordinary purposes and suffers from the disadvantage that the origin of some important clays is by no means clearly known4 ,the author prefers a modification of ,grimsley's and ,grout's classification 7#ca7 as follows3 ,i4 ,primary clays4 7a7 ,clays produced by 'weathering' silicates--as some kaolins4 7b7 ,clays produced by lateritic action--very rich in alumina1 some of which is apparently in a free state4 7c7 ,clays produced by telluric water containing active gases 7hypogenically formed clays7--as ,cornish china clay4 ,,ii4 ,secondary clays4 7d7 ,refractory#af secondary clays--as fireclays and some pipe clays4 7e7 ,pale-burning non-refractory clays--as pottery clays1 ball clays and some shales4 7f7 ,vitrifiable clays--as stoneware clays1 paving brick clays4 7g7 ,red-burning and non-refractory clays--as brick and terra-cotta clays and shales4 7h7 ,calcareous clays or marls1 including all clays containing more than #e per cent4 of calcium carbonate4 ,,iii4 ,residual clays4 7i7 ,clays which have been formed by one of the foregoing actions and have been deposited along with calcareous or other matter but1 on the latter being removed by subsequent solution1 the clay has remained behind--as the white clays of the ,derbyshire hills4 ,footnote #af3 ,a refractory clay is one which does not soften sufficiently to commence losing its shape at any temperature below that needed to bend ,seger ,cone #bf 7approximately #afjj;,c47 7see p4 #aaf74 ,some further sub-division is necessary for special purposes1 particularly in sections e1 f and h1 but to include further details would only obscure the general scheme4 ,some clays will1 apparently1 be capable of classification in more than one section1 thus a vitrifiable clay may owe its characteristic to a high proportion of calcium carbonate and so be capable of inclusion as a calcareous clay4 ,broadly speaking1 however1 if the clay is tested as to its inclusion in each section of the scheme in turn it will be found that its highest value will be in the section which is nearest to the first in which the clay can legitimately be placed4 ,from a consideration of a classification such as the foregoing1 together with a detailed study of the physical and chemical properties of the material as a whole1 and also of the various portions into which it may be divided--particularly that which has been isolated by mechanical methods of purification and separation--it is not difficult to gain a fairly accurate idea of the nature of any clay4 ,although the present state of knowledge does not permit them to be classified with such detail as is the case with plants1 animals1 or simple chemical compounds1 the study of clays and the allied materials has a fascination peculiarly its own1 not the least interesting features of which are those properties of the clay after it has been made into articles of use or ornament4 ,these are1 however1 beyond the scope of what is commonly understood by the term 'the natural history of clay4' ,,bibliography ,a complete bibliography of clay would occupy several volumes4 ,the following list only includes the more accessible of the works quoted in the text4 #a4 ,second ,report of the ,committee on ,technical ,investigation--,rle of ,iron in ,burning ,clays4 ,orton and ,griffith4 ,indianapolis4 #aije4 #b4 ,british ,clays1 ,shales and ,sands4 ,alfred ,b4 ,searle4 ,charles ,griffin and ,co4 ,ltd4 ,london4 #aiaa4 #c4 ,transactions of the ,english ,ceramic ,society4 v4 p4 #gb4 ,hughes and ,harber4 ,longton1 ,staffs4 #aije4 #d4 ,royal ,agricultural ,society's ,journal4 ,,xi4 #e4 ,die ,tone4 ,p4 ,rohland4 ,hartleben's ,verlag4 ,vienna4 #aiji4 #f4 ,clays3 their ,occurrence1 ,properties and ,uses4 ,h4 ,ries4 ,chapman and ,hall4 ,london4 #aijh4 #g4 ,gesammelte ,schriften4 ,h4 ,seger4 ,tonindustrie ,zeitung ,verlag4 ,berlin4 #aijh4 #h4 ,tonindustrie ,zeitung4 #aijb4 p4 #ajfd4 #i4 ,tonindustrie ,zeitung4 #aijd4 p4 #ggc4 #aj4 ,treatise on ,ceramic ,industries4 ,e4 ,bourry 7,revised translation by ,a4 ,b4 ,searle74 ,scott1 ,greenwood and ,son4 ,london4 #aiaa4 #aa4 ,the ,colloid ,matter of ,clay4 ,h4 ,e4 ,ashley4 ,u4,s4,a4 ,geological ,survey ,bulletin #chh4 ,washington4 #aiji4 #ab4 ,sprechsaal4 #aije4 p4 #abc4 #ac4 ,action of ,heat on ,refractory ,materials4 ,j4 ,w4 ,mellor and ,f4 ,j4 ,austen4 ,trans4 ,eng4 ,cer4 ,soc4 ,,vi4 ,hughes and ,harber4 ,longton1 ,staffs4 #aijf4 #ad4 ,wiedermann's ,annalen4 ,,vii4 p4 #ccg4 #ae4 ,geological ,contemporaneity4 #ahfb4 #af4 ,geological ,magazine4 ,,iv4 pp4 #bda1 #bii4 #ag4 ,la ,cramique industrielle4 ,a4 ,granger4 ,gauthier ,frres4 ,paris4 #aije4 #ah4 ,american ,journal of ,science4 #ahga4 p4 #ahj4 #ai4 ,the ,hensbarrow ,district4 ,j4 ,h4 ,collins4 ,geological ,survey4 #ahgh4 #bj4 ,monographs of the ,u4,s4,a4 ,geological ,survey4 ,,xxviii4 ,c4 ,r4 van ,hise4 #ahig4 #ba4 ,on ,kaolinite and ,pholerite4 ,american ,journal of ,science4 ,,xliii4 #ahfg4 #bb4 ,the ,nomenclature of ,clays4 ,j4 ,w4 ,mellor4 ,eng4 ,cer4 ,soc4 ,,viii4 ,hughes and ,harber4 ,longton1 ,staffs4 #aijh4 #bc4 ,on the present distribution of ,coal ,balls4 ,m4 ,c4 ,stopes and ,d4 ,m4 ,s4 ,watson4 ,phil4 ,trans4 ,royal ,society4 ,b4 ,vol4 ,,cc4 #aijh4 #bd4 ,natural ,history of ,coal4 ,e4 ,a4 ,n4 ,arber4 ,cambridge ,university ,press4 #aiaa4 #be4 ,modern ,brickmaking4 ,a4 ,b4 ,searle4 ,scott1 ,greenwood and ,son4 ,london4 #aiaa4 #bf4 ,die verschiedene ,arten der ,verwitterung4 ,j4 ,m4 van ,bemmelen4 ,zeits4 angewandte ,chemie4 ,,lxvi4 ,leopold ,voss ,verlag4 ,hamburg4 #aiaj4 #bg4 ,pyrometrische ,beleuchtung4 ,carl ,bischof4 ,tonindustrie ,zeitung4 #ahgg4 #bh4 ,die feuerfeste ,tone4 ,carl ,bischof4 ,quandt and ,haendler4 ,leipzig4 #aijd4 #bi4 ,the ,chemical ,constitution of the ,kaolinite ,molecule4 ,trans4 ,eng4 ,cer4 ,soc4 ,x4 ,hughes and ,harber4 ,longton1 ,staffs4 #aiaa4 #cj4 ,tabellarische ,uebersicht der ,mineralien4 ,p4 ,groth4 ,brunswick4 #ahih4 #ca4 ,west ,virginia ,geological ,survey4 ,,iii4 #aijf4 #cb4 ,memoirs of the ,geological ,survey4 ,london4 #cc4 ,the ,publications of ,stanford's ,geographical ,institute4 ,london4 #cd4 ,handbuch der gesam4 ,tonwarenindustrie4 ,b4 ,kerl4 ,verlag der ,tonindustrie ,zeitung4 #aiaj4 #ce4 ,causal ,geology4 ,e4 ,h4 ,l4 ,schwarz4 ,blackie and ,sons1 ,ltd4 #aiaj4 #cf4 ,china ,clay3 its nature and origin4 ,g4 ,hickling4 ,trans4 ,inst4 ,mining ,engineers4 #aijh4 ,,index ,absorption1 #dj1 #aea ,absorptive power of clays1 #dj ,accumulation of clays1 #hd ,acid-proof ware1 #aac ,acids1 effect of1 #ajf1 #aea1 #aeb ,adsorption1 #dj1 #aej ,agriculture1 clays in1 #e1 #ef1 #eg1 #ei1 #fa1 #fb1 #fc1 #fg ,air1 #dc1 #he ,alkalies in clay1 #ch1 #aae1 #acc1 #adb1 #adc1 #aee ,alluvial deposits1 #fh1 #hg1 #aab1 #acb ,alum clays and shales1 #eg1 #abc1 #abd ,alum manufacture1 clays for1 #abd ,alumina1 #f ,alumina1 free1 #hj1 #hb1 #aed ,alumina-silica ratio1 #acc1 #aei ,alumino-silicic acid1 #f1 #gf1 #ha1 #aah1 #aee1 #aeg ,aluminous clays1 #hb1 #aag ',amorphous' clay1 #ajg1 #adf ,analyses of clays1 #af1 #ada1 #add ,anauxite1 #acd ,architectural ware1 #abi1 #acj ,argillaceous earths1 #a ,argillaceous limestone1 #hh1 #acb ,associated rocks1 #dh ,bagshot clays and sands1 #fd1 #abe ,ball clays1 #f1 #ai1 #bh1 #fb1 #fd1 #hb1 #aaj1 #aae1 #aai1 #abe1 #ach1 #ada1 #aef1 #aff ,bending of clay1 #cc ,bibliography1 #afh ,binding power1 #bh1 #aea ,binds1 #ec ,bituminous shales1 #eg1 #ei ,black spots1 #ad1 #abh ,black ware1 #aac ,bleaching oil1 #acd ,blue bricks1 #ac1 #ef ,bone-ash1 #aaj ,boulder clays1 #c1 #g1 #aj1 #fe1 #aja ,bovey ,tracey clay1 #fb ,brick clays1 earths and shales1 #a1 #b1 #e1 #aj1 #aa1 #ab1 #ac1 #ca1 #cg1 #dj1 #df1 #ef1 #eg1 #ei1 #fa1 #fc1 #fe1 #fg1 #fh1 #ia1 #ajj1 #aja1 #ajd1 #aab1 #aag1 #abc1 #abe1 #abi1 #ach1 #afb1 #aff ,brittleness1 #df ,brown ware1 #aac ,buff bricks1 #abh ,burned clay1 #bh1 #ca1 #da1 #aai1 #aba ,calcareous clays1 #ch1 #fa1 #fh1 #hh1 #acc1 #aci1 #aff ,calcareous sands1 #hh ,calcium1 see ,lime compounds ,cambrian clays1 #ea ,carbon in clay1 #ae1 #aai1 #add ,carbonates in clay1 #aj1 #hb ,carboniferous clays and shales1 #eb1 #abd ,carboniferous limestone1 #eb1 #ajh ,carclazite1 #gh1 #ajf ,cellulose in clays1 #bg ,cement clays1 #eg1 #ajd1 #aca ,chalcopyrite1 #ad ,chalk1 #aj1 #aa1 #fa1 #fg1 #fh1 #hh1 #aaf1 #abg1 #abh1 #acb1 #acd1 #aci ,chamotte1 #aba ,chemical properties of clay1 #f ,china clay rock1 #gh1 #ajf1 #aaf ,china clays1 #b1 #e1 #f1 #g1 #i1 #bb1 #bg1 #dj1 #di1 #fd1 #ga1 #ge1 #gh1 #hb1 #hd1 #ajd1 #aaj1 #aaf1 #ada1 #adf1 #adg1 #adh1 #aef1 #afe ,china-ware1 #aji1 #aaj ,chinese clay1 #gc ,classification of clays1 #afc ,clay molecule1 #aef ,clay-shales1 #abb ,clay substance1 #ace et seq4 ,clay substance1 defined1 #aej ,clayite1 #hc1 #ajg1 #adg1 #adi ,clinker1 #acb ,clunches1 #aah ,coagulated clays1 #ig ,coagulation1 #dc1 #aeb ,coal ,measure clays and shales1 #ec1 #if1 #ajc1 #aag1 #abd1 #acj ,coarse pottery1 #aab ,cobalt1 #aaj ,colloid theory1 #ig ,colloidal properties of clay1 #be1 #ha1 #hb1 #ig1 #ajf1 #adg1 #aej1 #aeb ,colloidal silica1 #ha1 #acd1 #aeg ,colloids1 #bd1 #da1 #dc1 #gf1 #afj ,colluvial clays1 #ii ,colours of burned ware1 #ai1 #abc1 #aca ,colours of clays and shales1 #ai1 #ei1 #aae1 #aai1 #abd1 #abf ,combined water1 #de1 #aed ,common clays1 #c ,composition of clays1 #d1 #f1 #af1 #bc1 #ce1 #dd1 #ajg1 #aag1 #aah1 #acc1 #acd1 #add1 #aef1 #afd ,composition of clays 7burned71 #df ,cornish stone1 #aaj1 #aaf ,cracked ware1 #df1 #abg1 #acj1 #aca ',cream1' #ci1 #dc1 #aeb ,cretaceous clays1 #fa ',crumb' of clay1 #bd ,crushing clay1 #de ',crystalline' clay1 #ajg1 #adf1 #adh ,crystals in clay-ware1 #df ,decantation1 #aci ,decomposition of clay1 #aed ,definitions of clay1 #b-e1 #abj1 #ace1 #adi1 #aej ,de-greasing wool1 #acd ,deposition of clays1 #di1 #ea1 #ij1 #ii ,devonian clays1 #ea ,diluvial clays1 #ii ,dinas rock1 #ed ,disintegration1 #ajb ,distribution of clays1 #a ,drain-pipe clays1 #aab1 #aac ,drift1 #fe1 #aja ,drift clays1 #aja ,drying clays1 #bg1 #abg1 #aec ,durability1 #aca ,dyes1 #da1 #aej ,earth movements1 #he1 #if ,earthenware1 #cg1 #aab ,earths for bricks1 see ,brick clays ,electrolytes1 #dc ,elutriation1 #h1 #acg1 #adj ,eocene clays1 #fc ,epigenic clays1 #hb ,erosion1 #hi1 #ii1 #ajj ,estuarine clays1 #ij1 #ic1 #aah ,etruria marls1 #ee1 #acj ,expansion1 #cb ,exposure1 #dc ,faience1 #abi ,farewell ,rock1 #ed ,fat clays1 #bi ,felspar1 #g1 #h1 #da1 #gd1 #ajd1 #aaj1 #aaf1 #ada1 #add1 #aei ,ferric and ,ferrous compounds1 #ab1 #aba1 see ,iron ,fine clays1 #aab ,fineness1 see ,texture ,firebricks1 #ad1 #ed1 #fa1 #aaf ,fireclay1 #cc1 #ce1 #eb1 #ed1 #hb1 #ajd1 #ajh1 #aaf1 #abc1 #aef1 #aff ,fissile clays1 #aag ,flint1 #aaj1 #aaf ,flint clays1 #aag ,floods1 #he1 #hg1 #ii ,flower-pot clays1 #eg1 #aaj ,fluoric vapours1 #ge1 #gg1 #afe ,fluviatile clays1 #hh1 #ib ,fluxes1 #h1 #aa1 #ch1 #ci1 #aae1 #aaf1 #aca ,food-clays1 #a ,formation of clays1 #dh1 #gj ,formula of clay1 #aef ,free alumina1 #hj1 #aed ,free silica1 #g1 #hj1 #aed1 #afa ,frost1 #dc1 #hf ,fuller's earth1 #ei1 #acc ,fulling cloth1 #a1 #acc ,fusibility1 #cb1 #eh1 #aac1 #aaf ,fusible clays1 #aaf ,fusing point1 #cb ,fusion1 #dg1 #aac1 #abj1 #acb1 #aee ,ganister1 #eb1 #ed1 #aah ,gault1 #fa1 #acb ,geological classification1 #afc ,geological nature of clay1 #d1 #ej ,glacial clays1 #fe1 #ajj1 #afb ,glaciers1 #he1 #hi1 #ajj ,glass1 #aaf ,glassy structure1 #dg ,glazed bricks1 #aai ,glazed pottery1 #abi ,glazed terra-cotta1 #ef1 #abi ,grades of fireclay1 #abj ,gravel1 #g1 #fb1 #fe1 #hi1 #ajj1 #aja1 #ajb1 #ach1 #add ,green colour1 #ad ,greensand1 #acc ,grinding1 #hj1 #aba ,grit1 #aab1 see also ,millstone ,grit ,grog1 #bh1 #ca1 #da1 #aai1 #aba ,growan1 #gh ,gypsum1 #aj1 #ab1 #fb ,halloysite1 #aah1 #aeh ,hardness1 #de ,heat1 effects of1 #bh1 #cg1 #ci1 #de1 #hj1 #abb1 #adf1 #aec1 #aed1 #aeh1 #aei ,hydrargillite1 #hj ,hydro-alumino-silicates1 #f ,hydrocarbons in clay1 #ae ,hydrolysis1 #gh1 #ig ,hygroscopic clays1 #aec ,hypogenic clays1 #afe ,ice-action1 #he1 #ajj ,ideal clay1 #adf ,impermeability1 #dj1 #aac ,impervious articles1 #aac1 #aee ,impurities in clays1 #g1 #di1 #hb1 #ajb1 #ajd1 #aji1 #aba1 #abf1 #adb1 #adc1 #aee1 #afb1 #afc ,ions1 #dc ,indurated clays1 #ah ,infusibility1 #ajf1 #aai1 see ,refractoriness ,iron compounds1 #g1 #aj1 #ab1 #ac1 #bj1 #fb1 #aab1 #aai1 #aba1 #abh1 #acc1 #ada1 #ade1 #afd ,ironstone1 #fb ,irregularity in shape1 #aca ,jurassic clays and shales1 #eg1 #abd ,kao-ling1 #adh ,kaolinite1 #ai1 #hj1 #aje1 #ajg1 #adf1 #adi1 #aeh ,kaolinization1 #gf1 #gg1 #gi ,kaolins1 #i1 #ba1 #di1 #fd1 #ga1 #gc1 #gf1 #gi1 #hb1 #hd1 #ajd1 #aaf1 #ada1 #adf1 #adg1 #adh1 #afe ,keele series1 #ee ,kellaways clay1 #ei1 #fa ,keuper marls1 #eg ,kiln shrinkage1 #cj ,kimeridge clays1 #ei ,knotts1 #abd ,lacustrine clays1 #ij1 #ia ,lake-deposited clays1 #he1 #hh1 #ia ,lakes1 #he1 #hh ,laminated clays1 #ec1 #aag1 #abb ,laterite1 #hj1 #adi ,lateritic action1 #hj1 #afe ,lateritic clays1 #hb ,lean clays1 #bi ,liassic clays and shales1 #eg1 #abe1 #acb ,lime1 #g1 #aj1 #ajb1 #aei ,lime compounds1 #aj1 #aa1 #ch1 #da1 #dg1 #aac1 #aae1 #aaf1 #aba1 #abg1 #aci1 #adb1 #adc1 #ade1 #aee1 #aeg1 #afb1 #afd ,limestone1 #aj1 #aa1 #eb1 #ei1 #fa1 #fb1 #hh1 #ajb1 #aag1 #abg1 #acb1 #aci ,lime troubles1 #aa ,loam1 #eg1 #fg1 #hh ,london clay1 #fb1 #fc1 #abe ,ludwig's chart1 #ce ,magnesium compounds1 #g1 #aj1 #aa1 #da1 #dg1 #aac1 #aae1 #aaf1 #aba1 #acc1 #aee ,malm-bricks1 #aa ,malms1 #aj1 #fh ,marcasite1 #aj1 #ac ,marine clays1 #fa1 #ic ,marls1 #aj1 #ea1 #ed1 #ee1 #eg1 #fg1 #fh1 #hh1 #acj1 #acb1 #aff ,mechanical analysis1 #acg ,medway mud1 #acb ,melting point1 #ca1 #cb ,mica1 #g1 #h1 #gf1 #ajd1 #aje1 #aaf1 #adj1 #ada1 #add1 #afj ,microscopical examination1 #ah1 #aje1 #adc1 #aeh ,millstone grit1 #ed1 #ee1 #aag ,mineral nature of clay1 #c ,minerals resembling clay1 #aeh ,mining ball clay1 #aaa ,modelling clays1 #acj ,moisture1 #ae1 #add ,molecular attraction1 #bb ,molecular constitution of clay1 #ba1 #aef ,montmorillonite1 #acd ,mundic1 #ac ,muscovite1 #aje ,newtonite1 #aeh ,nodules1 #aba1 #abg ,non-plastic material1 #dc1 #aba1 #aea ,non-refractory clays1 #aff ,occurrence of clays1 #dh1 #aaf ,ocean currents1 action of1 #hi ,odour of clay1 #ai ,oil1 bleaching1 #acd ,oil shales1 #fa1 #abb1 #abc ,oolite clays1 #ei1 #acd ,ooze1 #ie1 #ii ,organic matter1 #ai1 #aai ,origins of clays1 #dh1 #ga1 #afj1 #afe ,oxford clay1 #ei1 #ie1 #afd ,oxides in clay1 #aj1 #hb ,paint1 clays for1 #aji ,paper1 clays for1 #aji ,particles1 nature of1 #ah1 #ca1 #ajf1 #ajg1 #aej ,paving brick clays1 #aff ,pelagic ooze1 #ie1 #ii ,pelinite1 #hc1 #adh1 #adi ,permian clays and shales1 #eg1 #aab1 #abd ,pholerite1 #aag ,physical characters of clays1 #ag ,picking clay1 #aba ,pipe clays1 #fd1 #fe1 #hb1 #aji1 #aff ,plant-extracts in clays1 #bf ,plastic clays1 #b1 #dc1 #fe1 #fg1 #hb1 #hh1 #ajb1 #aab1 #abc1 #adg1 #adh1 #afj ,plasticity1 #bj-bg1 #da1 #df1 #ig1 #ih1 #ii1 #ajh1 #aji1 #aab1 #aag1 #abc1 #abe1 #abg1 #aea1 #afj ,pleistocene clays1 #fg ,pockets1 #fe1 #he1 #aja1 #aaf1 #aff ,porcelain1 #cg1 #df1 #gc1 #aji1 #aaj1 #abe1 #abi ,pores in clay1 #cj1 #aad ,porosity1 #cj1 #ci1 #aba1 #aca1 #aee ,portland cement1 #aca ,potash compounds1 #g1 #aj1 #aac1 #aae1 #aaf1 #aba1 #aeg1 see ,alkalies ,pottery clays1 #a1 #e1 #ca1 #df1 #ff1 #ajj1 #aja1 #ajd1 #aaj1 #aab1 #aad1 #abe1 #abi1 #afb1 #aff ,precambrian clays1 #ea ,precipitated clays1 #ig1 #aeb ,primary clays1 #gj1 #ga1 #hd1 #afe ,proximate analysis1 #af1 #add ,purbeck clays1 #ei ,pure clays1 #e1 #f1 #g1 #adb1 #aee1 #aef ,purification of clay1 #g1 #ff1 #gh1 #ajd1 #aac1 #abh1 #adj ,pyrites1 #aj1 #ac1 #dd1 #ef1 #eg1 #aai1 #abd1 #abh ,quartz1 #h1 #ajd1 #aaj1 #aah1 #adj1 #ada1 #adc ,rain1 #dd1 #he1 #hf ,rational analysis1 #ada ,reading clays1 #fc ,recent clays1 #fg ,rectorite1 #aeh ,re-deposited clays1 #ih ,red bricks1 #ab ,red burning clays1 #ada1 #adb1 #aff ,red iron oxide1 #ab ,red ware1 #aac ,reduction in volume1 #cj ,refractoriness1 #cd1 #aai1 #abj1 #abc1 #aee ,refractory articles1 #e1 #aai ,refractory clays1 #i1 #cb1 #cc1 #ce1 #ch1 #eb1 #fe1 #hb1 #ajd1 #aaf1 #afe1 #aff ,residual clays1 #gj1 #hd1 #aff ,resistance to abrasion1 #aai ,resistance to corrosion1 #aai ,resistance to crushing1 #df ,resistance to cutting1 #aai ,resistance to temperature1 see ,refractoriness ,resistance to weathering1 #gf ,ringing sound1 #aaj ,river-deposited clays1 #hh ,rivers1 #he1 #hg ,rock binds1 #ec ,rockingham1 #aac ,rock-like clays1 #b ,rocks associated with clay1 #dh ,roman cements1 #acc ,roofing tiles1 #eg1 #fc1 #abf1 #abh ,sagger marls1 #ed ,sand1 #g1 #ca1 #da1 #fb1 #hb1 #hi1 #ajj1 #aja1 #aag1 #acc1 #ach1 #add ,sandstones1 #ec ,sandy clays1 #fh ,sandy loams1 #hh ,sandy marls1 #hh ,sanitary articles1 #e1 #aac ,sawdust1 #dj ,scum1 #aj ,sea1 action of1 #he1 #hi1 #ii ,sea-deposited clays1 #ic ,secondary clays1 #gj1 #hb1 #hc1 #aff ,sedimentary rocks1 #dh ,sedimentation of clay1 #dc1 #hh1 #ij1 #ajd ,seger cones1 #cc1 #cd ,selection of clay1 #abb ,selenite1 #aj ,separation of clays1 #ij1 #ade ,settling1 #dc ,sewerage pipes1 #aac ,shale oil1 #ae1 #fa ,shale tar1 #abc ,shales1 #b1 #e1 #ea1 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#ajb1 #abh1 #ach1 #add ,streams1 #he1 #hf ,strength1 #bc1 #de1 #aac ,sub-surface clays1 #e ,sulphates in clay1 #aj1 #ab1 #hb ,sulphides in clay1 #aj1 #hb ,sulphuric acid1 #abd ,sunlight1 #de ,surface clays1 #b1 #e1 #eb1 #aab ,suspension of clay1 #dc1 #ij1 #adj1 #aeb ,swelling1 #ae1 #ajb ,tannin in clay1 #be1 #bf1 #da ,telluric water1 #afe ,temperature1 resistance to1 #aai1 #abj ,tensile strength1 #bc1 #de ,terra-cotta clays and shales1 #e1 #aj1 #ab1 #ca1 #df1 #ef1 #fc1 #ia1 #ajd1 #abc1 #abd1 #abi1 #aff ,tertiary clays1 #fb ,texture1 #aab1 #acj ,thermal reactions1 #adf1 #aed1 #aeh ,tiles1 #a1 #e1 #eg1 #ia1 #aja1 #abi ,titanium compounds1 #aba ,tourmaline1 #gf1 #ajd ,transportation of clays1 #di1 #hf1 #ih1 #ii1 #ajj ,transported clays1 #gj ,triassic clays1 #eg1 #aab1 #acj ,true clay1 #add1 #adf1 #adi1 #aej1 #afj ,twisted ware1 #aad1 #abi1 #aca ,types of clay1 #hb ,ultimate analysis1 #af1 #add ,ultra-marine1 clays for1 #aji ,underclays1 #ec1 #aag1 #aah ,uses of clay1 #a1 #afe ,valuation 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