Chapter VI: Clay Substance: Theoretical and Actual

Having indicated the origin, modes of accumulation and general characteristics of the numerous materials known as 'clay,' it now remains to ascertain what substance, if any, is contained in all of them and may be regarded as their essential constituent, to which their properties are largely due. Just as the value of an ore is dependent to a very large extent on the proportion of the desired metal which it contains, and just as coal is largely, though not entirely, esteemed in proportion to the percentage of carbon and hydrogen in it, so there may be an essential substance in clays to which they owe the most important of their characteristics.

The proportion of metal in an ore or of hydrocarbon in a coal can be ascertained without serious difficulty by some means of analysis, but with clay the difficulties are so great that, to some extent at least, they must be regarded as being, for the present, insurmountable. 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 composition. Consequently materials of the most diverse nature in other respects are termed clays if they are known to become plastic under certain conditions.

There is, in fact, at the present time, no generally accepted definition of clay which distinguishes it from mixtures of clay and sand or other fine mineral particles. 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 rocks.

Many attempts have been made to avoid this unfortunate position, which is alike unsatisfactory to the geologist, the mineralogist and the chemist as well as to the large number of people engaged in the purchase and use of various clays; and, whilst the end sought has not been reached as completely as is desirable, great progress has been made and much has been accomplished during the last twenty years.

One of the earliest attempts to ascertain whether there is an essential constituent of all clays was made by Seger (7) who used two methods of separating some of the ingredients of natural clays from the remaining constituents. The first of these methods consists in an application of the investigations of Schulze, Schloesing and Schoene on soils, viz. the removal of the finest particles by elutriation; the second is an extension of the method of Forschammer and Fresenius, viz. the treatment of the material with sulphuric acid.

To the product containing the clay when either of these methods is used Seger gave the name _clay substance_, but the material so separated is by no means pure clay. The term clay substance must, therefore, 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 acid.

It has not yet been found possible to isolate pure clay from ordinary clays, so that in investigating the nature of what Seger was endeavouring to produce when he obtained the crude clay substance, indirect methods are necessary.

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 slurry, it may readily be divided into a number of fractions each of which will consist of grains of different sizes. This separation may be effected by means of a series of sieves through which the slurry is poured, or the slurry may be caused to flow at a series of different speeds, the material left behind at each rate of speed being kept separate; or, finally, 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 period, these times of resting and decantation, if repeated, providing a series of fractions the materials in which are more or less different in their nature.

'Clays' containing a considerable proportion of coarse material are most conveniently separated into a series of fractions by means of sieves, whereby they are divided into (i) stones, (ii) gravel, (iii) coarse sand, (iv) medium sand, (v) fine sand and (vi) a slurry consisting of such small particles that they can no longer be separated by sifting. If the residues on the sieves are carefully washed free from any adhering fine material and are then dried, they will be found on examination to be quite distinct from anything definable as clay. They may consist of a considerable variety of minerals or may be almost entirely composed of quartz, but with the possible exception of some shales of great hardness, they are undoubtedly not clay. 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 insignificant; thus 40 per cent. 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 200 meshes per linear inch.

The material which passes through the finest sieve employed will contain all the true clay in the material; that is to say, the coarser portion will, as already mentioned, be devoid of the ordinary characteristics of clay. At the same time, this very fine material will seldom consist exclusively of clay, but will usually contain a considerable proportion of silt, extremely fine mineral particles and, in the case of calcareous clays, a notable proportion of calcium carbonate in the form of chalk or limestone particles. Only in the case of the purest clays will the material now under consideration consist entirely of clay, so that it must be again separated into its constituents. This is best accomplished, as first suggested by Schoene, by exposing the material to the action of a stream of water of definite speed. H. Seger (7) 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 time. Briefly, all material sufficiently fine to be carried away by a stream of water flowing at the rate of 0·43 in. per minute was found by Seger to include the whole of the clay in the samples he examined, but, as was later pointed out by Bischof, it is not correct to term the whole of this material 'clay substance,' as when examined under the microscope, it contains material which is clearly not clay.

Processes of decantation of the finest material obtained after elutriation still fail to separate all the non-clay material, 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 clay.

It would thus appear that no process of mechanical separation will serve for a complete purification of a clay; indeed, 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 dust.

When chemical methods of investigation are employed the problem is not materially altered, nor is its solution fully attained. It is, of course, obvious that any chemical method should be applied to the product obtained by treating the raw material mechanically as above described, for to do otherwise is to create needless confusion. Yet by far the greater number of published analyses of 'clays' report the ultimate composition of the whole material, no attempt being made to show how much of the various constituents is in the form of sand, stones or other coarse particles of an entirely non-argillaceous character.

If the particles of a 'clay' which are sufficiently small to be carried away by a stream of water with a velocity of only 0·43 in. per minute are analysed, 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 deposition. 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 clays. Red-burning clays, on the contrary, will vary greatly in composition, so that it becomes difficult to find any close analogy between these kinds of clay. This difference is partly due to the extremely fine state of division in which ferric oxide occurs in clays, the particles of this material corresponding in minuteness to those of the purest clays and so being inseparable by any mechanical process.

In 1876 H. Seger (7) published what he termed a method of 'rational analysis,' which consisted in treating the clay with boiling sulphuric acid followed by a treatment with caustic soda. He found that the purer china clays (kaolins) and ball clays were made soluble by this means and that felspar, mica and quartz were to a large extent unaffected. 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 named, but the principle introduced by Seger has proved invaluable in increasing our knowledge of the composition of clays. By means of this so-called rational analysis Seger found that the purer clays yielded results of remarkable similarity and uniformity, the material entering into solution having a composition agreeing very closely with the formula Al2O3·2SiO2·2H2O which is generally recognized as that of the chief constituent or constituents of china clay (kaolin) and the purer ball clays. This crude substance, obtainable from a large number of clays by the treatment just described, was named _clay substance_ by Seger, who regarded it as the essential constituent of all clays.

Red-burning clays when similarly treated do not yield so uniform a product, and the ferric oxide entering into solution makes the results very discordant. Moreover, even with the china clays or kaolins a small proportion of alkalies, lime and other oxides enter into solution and a number of minerals analogous to clay, but quite distinct from it, are also decomposed and dissolved. For these reasons the 'rational analysis' has been found insufficient; 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 present.

As the china clays (kaolins) and ball clays on very careful elutriation all yield a product of the same ultimate composition, viz. 39 per cent. of alumina, 46 per cent. of silica, 13 per cent. of water, and 2 per cent. of other oxides, they are generally regarded as consisting of practically pure clay with a variable amount of impurities. 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 clays. 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 are, as Bischof (28) and, more recently, Mellor have pointed out, more reliable than the 'rational analysis' in the case of impure clays. 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 accurate, but it is very unsatisfactory to assume, as some chemists do, 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 alkalies, lime and alumina is free quartz. Some clays are almost destitute of felspar but comparatively rich in mica, whilst others are the reverse, so that some means of identifying the extraneous minerals is essential. When this is not used, the curious result is obtained that German chemists calculate the alkalies, etc. to felspar whilst the French chemists, following Vogt, calculate them to mica; English ceramic chemists appear undecided as to which course to follow, and some of them occasionally report notable amounts of felspar in clays quite destitute of this mineral!

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 latter, as already described, 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 oxides. A proximate analysis therefore shows the various materials entering into the composition of the clay in the following or similar terms:

Stones per cent.
Gravel "
Coarse sand "
Medium sand "
Fine sand "
Silt "
Felspar or mica dust "
Silica dust "
'True clay[12]' "
Moisture "
Carbon "
Other volatile matter "

[Footnote 12: 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 attainable.]

For some purposes it is necessary to show the proportion of calcium, iron and other compounds as in an ordinary ultimate analysis.

A comparison of the foregoing with an ultimate or 'ordinary' analysis of a clay (p. 16) will show at once the advantage of the former in increasing our knowledge of the essential constituent of all clays, if such a substance really exists. Its absolute existence is by no means proved, for, as will have been noticed, its composition is largely based on assumption even in the most thorough investigations, particularly of the admittedly less pure clays.

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 clay?'

Even with these purer clays it is not sufficient to study an analysis showing the total amount of the silica, alumina and other oxides present; it is still necessary to effect some kind of separation into the various minerals of which they are composed. When, however, the accessory minerals do not exceed 5 per cent. of the total ingredients their influence is less important and the nature and characteristics of the 'clay substance' itself can be more accurately studied. By careful treatment of well selected china clays, for example, it is possible to obtain a material corresponding to the formula Al2O3·2SiO2·2H2O within a total error of 1 per cent., the small amount of impurity being, as far as can be ascertained, composed of mica. So pure a specimen of clay is found on microscopical examination to consist of minute irregular grains of no definite form, together with a few crystals of the same composition and identifiable as the mineral 'kaolinite' (p. 107). This 'amorphous' material, which appears to be the chief constituent of all china clays and kaolins, has been termed _clayite_ by Mellor (22).

Johnson and Blake, Aron and other observers have stated that the majority of the particles in china clays and kaolins are crystalline in form. Owing to their extreme smallness it is exceedingly difficult to prove that they are not so, though for all ordinary purposes they may be regarded as amorphous, the proportion of obviously crystalline matter present in British china clay of the highest qualities being so small as to be negligible.

Hickling (36), using an exceptionally powerful microscope, claims to have identified this 'amorphous' substance in china clay as 'worn and fragmental crystals of kaolinite,' and recently Mellor and Holdcroft and Rieke have shown that the apparently amorphous material shows the same endo- and exothermal reactions as crystalline kaolinite.

So far as china clays or kaolins are concerned, 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 sought.

This term--clayite--is very convenient when confined to china clays and kaolins, but it is scarcely legitimate to apply it, as has been suggested, to material in other clays until it has been isolated in a sufficiently pure form to enable its properties to be accurately studied. 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 clays: namely, in its very low plasticity. This may be explained by the fact that it is only obtainable in a reasonably pure form in clays of a primary character, 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 character. Moreover, 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 state, especially as no liquid is known which will dissolve it without decomposing it. The fact that it is not an elementary substance, but a complex compound of silica, alumina and the elements of water, also increases the intricacy of the problem, for these substances occur in other combinations in a variety of other minerals which are clearly distinct from clay.

Ever since the publication of Seger's memorable papers (7), and to a small extent before that time, it has been generally understood that china clay or kaolin represented the true essential constituent of clays, 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 clays. This is unfortunate as it is by no means proved that the latter is identical with kaolinite, and a distinctive term would be of value in preventing confusion. Other investigators have used the word 'kaolin' with equal freeness, so that whilst it originally referred to material from a particular hill or ridge in China[13] it has now entered into general use for all clays whose composition approximates to that of china clay (p. 16) in which the plasticity is not well developed. Thus, in spite of the difference in origin between many German and French kaolins and the china clays of Cornwall, it is the custom in Europe generally to term all these materials 'kaolin.' Yet they are very different in many respects from the material originally imported from China.

[Footnote 13: _Kao-ling_ is Chinese for a high ridge or hill.]

As the essential clay substance has not yet been isolated in a pure form from the most widely spread plastic clays, but is largely hypothetical as far as they are concerned, the author prefers the term _pelinite_[14] 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 properties. In china clay and kaolin the 'true clay' is identical with clayite--or even with kaolinite (p. 108)--and there is great probability that this identity also holds in the case of the more plastic clays of other geological formations, but until it is established it appears wisest to distinguish the hypothetical or ideal clay common to all clays (if there is such a substance) by different terms according to the extent to which its composition and characters of the materials most closely resembling it are experimentally known.

[Footnote 14: From the [Greek: pêlinos] = made of clay.]

The substances most resembling this 'ideal clay' which have, up to the present been isolated, are:

(_a_) _Kaolinite._ Found in a crystalline form in china clays and kaolins (p. 107).

(_b_) _Clayite._ A material of the same chemical composition as kaolinite, but whose crystalline nature (if it be crystalline) has not been identified--chiefly obtained from china clays and kaolins.

(_c_) _Pelinite._ A material similar to clayite, but differing from it in being highly plastic and, to some extent, of a colloidal nature--obtained from plastic clays.

(_d_) _Laterite._ A material resembling clayite in physical appearance, but containing free alumina and free silica (p. 80).

(_e_) _Clay Substance._ A general term indicating any of the foregoing or a mixture of them; it is also applied (unwisely) to the material obtained when a natural clay is freed from its coarser impurities by elutriation (p. 7).

The Chief Characteristics of 'True Clay' from Different Sources.

In so far as it can be isolated _true clay_ appears to be an amorphous, or practically amorphous, material which may under suitable conditions crystallize into rhombic plates of kaolinite. The particles of which it is composed are extremely small, being always less than 0·0004 in. in diameter. They adsorb dyes from solutions and show other properties characteristic of colloid substances though in a very variable degree, some clays appearing to contain a much larger proportion of colloidal matter than do others. To some extent the power of adsorption of salts and colouring matters appears to be connected with the plasticity (p. 41) of the material, but this latter property varies so greatly in clayite or pelinite from different sources as to make any generalization impossible.

True clay substance appears to be quite white, any colour present being almost invariably traceable to ferric compounds or to carbonaceous matter. The latter is of small importance to potters as it burns away in the kiln. The specific gravity of clay substance is 2·65 according to Hecht, the lower figures sometimes reported being too low. Its hardness is usually less than that of talc--the softest substance on Mohs' scale--but some shales are so indurated as to scratch quartz. It is quite insoluble in water and in dilute solutions of acids or alkalies, but is decomposed by hydrofluoric acid and by concentrated sulphuric acid when heated, alumina entering into solution and silica being precipitated in a colloidal condition.

It absorbs water easily until a definite state of saturation has been reached, 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 it. With a moderate amount of water, pelinite develops sufficient plasticity to enable it to be modelled with facility, but clayite and some specimens of pelinite are somewhat deficient in this respect. 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 (p. 28).

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 days. 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 substance, its precipitation being hastened by the addition of such salts as cause a partial coagulation of the colloidal matter present. Some specimens of clayite and pelinite retain their suspensibility even in the presence of salts, but this is only true of a very limited proportion of the substance. In most cases the presence of soluble salts causes the larger particles to sink somewhat rapidly and to carry the finer particles with them.

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 present, _i.e._ on how much of the clay (pelinite) is in a colloidal form. Its viscosity is greatly affected by the addition or presence of small quantities of acid or alkali or of acidic or basic salts. Acids increase the viscosity; alkalies and basic salts, on the contrary, make the slip more fluid. Neutral salts behave in different ways according to the concentration of the solution and to the amount of clay (pelinite) present in the slip. If the slip contains so little water as to be in the form of a thin paste, neutral salts usually have but a small action, but when the slip contains only a small proportion of clay (pelinite) the presence of neutral salts will tend to cause the precipitation of the clay. In this way salts act in two quite different directions according to the concentration of the slip.

On drying a paste made of clay and water the volume gradually diminishes until the greater part of the water has been removed; after this the remainder of the water may be driven off without any further reduction in volume of the material. This is another characteristic common to colloidal substances such as gelatin. The material when drying attains a leathery consistency which is at a maximum at the moment when the shrinkage is about to cease; on further drying the material becomes harder and more closely resembles stone.

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[15] and be restored to its original condition except in so far as its colloidal nature may have been affected by the heating. If, however, the temperature is raised to about 500° C. a decomposition of the material commences and water is evolved. 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 restored. 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 formed. Mellor and Holdcroft (29) have recently investigated the heat reactions of the purest china clay obtainable and confirm Le Chatelier's view (10) that on heating to temperatures above 500° C. clay substance decomposes into free silica, free alumina and water, the two former undergoing a partial re-combination with formation of sillimanite (Al2O3SiO2) if a temperature of 1200° C. is reached. Mellor and Holdcroft point out that there is no critical point of decomposition for clay substance obtained from china clay, as it appears to lose water at all temperatures, though its decomposition proceeds at so slow a rate below 400° C. as to be scarcely appreciable.

[Footnote 15: Some clays are highly hygroscopic and absorb moisture readily from the atmosphere. According to Seger (7) this hygroscopicity distinguishes true clay from silt and dust.]

After the whole of the 'combined water' has been driven off, if the temperature continues to rise, it is found that at a temperature of 900° C. an evolution of heat occurs. This exothermal point, together with the endothermal one occurring at the temperature at which the decomposition of the clay seems to be most rapid, has been found by Le Chatelier, confirmed by Mellor and Holdcroft, to be characteristic of clay substance derived from kaolin and china clay, and the two last-named investigators state that it serves as a means of distinguishing kaolinite or clayite from other alumino-silicates of similar composition. These thermal reactions have not, as yet, been fully studied in connection with plastic clays; with china clay, as already noted, they probably indicate a polymerization of the alumina set free by the decomposition of the clay substance, as pure alumina from a variety of sources has been found by Mellor and Holdcroft to behave similarly.

On still further raising the temperature of pure clay (pelinite or clayite) no further reactions of importance occur, the material being practically infusible. If, however, any silica, lime, magnesia, alkalies, iron oxide or other material capable of combining with the alumina and silica is present as impurities in the clay substance, combination begins at temperatures above 900° C. This causes a reduction of the heat-resisting power of the material; the silicates and alumino-silicates produced fuse and begin to react on the remaining silica and alumina, first forming an impermeable mass in place of the porous one produced with pure clay substance, and gradually, as the material loses its shape, producing a molten slag if the 'clay' is sufficiently impure. As ordinary clays are never quite free from metallic compounds other than alumina, this formation of a fused portion--technically known as _vitrification_ (p. 37)--occurs at temperatures depending on the nature of the materials present, so that a wide range of products is obtained, the series commencing with the entirely unfused pure clay (china clay), passing through the slightly vitrified fireclays, 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 clays.

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 time. It is a subject of peculiar difficulty owing to the inertness of clay substance at all but high temperatures, and to the complexity of reactions which take place as soon as any reagent is brought into active connection with it.

Without entering into details regarding the various graphic formulae which have been suggested, it is sufficient to state that the one which is most probably correct, as far as present knowledge goes, is Mellor's and Holdcroft's re-arrangement of Groth's formula (30)

HO\ /OSiO\
\Al2/ \O
/ \ /
HO/ /\ \OSiO/
/ \
HO OH

which on removal of the hydroxyl groups might be expected to give the anhydride

O\\ /OSiO\
\\Al2/ \
// \ /
O// \OSiO/

though in practice this substance--if formed at all--appears to be instantly split up into Al2O3 and SiO2.

By regarding the aluminium as a nucleus, as above, and some aluminium silicates as hypothetical alumino-silicic acids, as suggested by Ulffers, Scharizer, Morozewicz (29) and others, clay substance may be conveniently considered, along with analogous substances, as forming a special group quite distinct from the ordinary silicates. In this way Mellor and Holdcroft (29) consider that clay substance is not a hydrated aluminium silicate--as is usually stated in the text-books--but an alumino-silicic acid, the salts of which are the zeolites and related compounds. 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 acid.

If this view be correct, pure clay substance or true clay is a tetra-basic alumino-silicic acid H4Al2SiO9 or Al2Si2O5(0H4). That its acid properties are not readily recognizable at ordinary temperatures is due to its inertness; at higher temperatures its power of combination with lime, soda potash and other bases is well recognized, though the reactions which occur are often complicated by decompositions and molecular re-arrangements which occur in consequence of the elevated temperature.

There are a number of minerals which closely resemble clayite or pure clay substance in composition, the chief difference being in the proportion of water they evolve on being heated. Thus _Rectorite_ H2Al2Si2O8, _Kaolinite_ H4Al2Si2O9, _Halloysite_ H6Al2Si2O10 and _Newtonite_ H10Al2Si2O12. In the crystalline form these minerals may be distinguished from each other by means of the microscope, 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 clayite, _i.e._ the clay substance obtained from china clay. As already stated, the thermal reactions which occur on heating clayite appear to be characteristic of kaolinite whilst halloysite is completely decomposed at a temperature somewhat below 200° C.; 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 (pelinite) obtained from some of the more plastic clays does not possess a different chemical composition as well as different physical properties.

The view that clays may be regarded as impure varieties of clayite is considered erroneous by several investigators for various reasons. For instance, felspar is rarely found in china clays, but is a common constituent of secondary (plastic) clays. J. M. van Bemmelen (26), 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 action. In a number of clays examined he found that a portion was soluble in boiling hydrochloric acid whereas clayite is scarcely affected by this treatment. 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 solution; this latter he considers to be true clayite. Unfortunately, his results were obtained by treating the crude clay with acid, instead of first removing such non-plastic materials as can be separated by washing, 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 them.

The fact that all clays when heated to 700 or 800° C. 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 conditions.

In all probability, the plastic clays have been derived from a somewhat greater variety of minerals than the primary clays (p. 71) and under conditions of decomposition which differ in details, though broadly of the same nature as those producing china clays. 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 crystals.

The much smaller particles present in plastic clays also indicate a more complete grinding during the transportation of the material or some form of precipitation. If, as Hickling suggests, all clays are direct products of the decomposition of _mica_, the fact that several varieties of mica are known and that the conditions under which these decompose must vary considerably, afford a good, if incomplete, explanation of some of the widely diverse characteristics observed in different clays.

Notwithstanding the great complexities of the whole subject and the apparently contradictory evidence concerning some clays, there is a wide-spread feeling that whatever may be the mineral from which a given clay has been derived, the _true clay substance_, which is its essential constituent, would (if it could be isolated in a pure state) prove to be of the same composition as kaolinite obtainable from china clay of exceptional purity. The purest clay substances (pelinite) yet obtained from some of the most plastic clays are, however, so impure as to make any detailed investigation of their composition by present methods abortive. The methods of synthesis which have proved so successful in organic chemistry have hitherto yielded few intelligible results with clays, on account of the complexity of the accessory reactions which occur.

The Difference between Pure Clay Substance and Ordinary Clays.

The properties and characteristics of _true clay_ are very seriously modified by other materials which may be associated with it. This may be perceived by comparing the properties of clays mentioned in Chapter I with those of various forms of true clay just given. Moreover, as true clay never occurs in a perfectly pure state in nature, the properties of clays must be largely dependent on the accessory ingredients.

Silica, for example, when alone is a highly refractory material, but in the presence of true clay it reduces the refractoriness of the latter. Lime has a similar effect though its chemical action on the clay is entirely different. 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 one.

For these reasons, 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 ascertained. 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 present.

There is, therefore, a considerable difference between a natural clay and the pure clay substance theoretically obtainable from it; this difference being most marked in the case of low-grade brick clays of glacial origin, which may contain 50 per cent. or more of adventitious materials. 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 character, 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 grade, plastic, pottery clay. Unfortunately, it is, at present, impossible to isolate this clay substance in anything approaching a pure form, 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 latter.

Classification of Clays.

Owing to the widely differing substances from which clays can, apparently, be formed and the peculiar difficulties which are experienced in investigating the nature of clay substance from different sources, it is by no means easy to devise a scheme of classification of clays, though many of these have been attempted by different scientists.

The classification adopted by geologists is based on the fossil remains and on the stratigraphical position of clays relative to other rocks, as described in Chapter II. This is of great value for some purposes, but the composition of the substances termed 'clay' by geologists differs so greatly, even when only one formation is considered, as to make their classification of little or no use where the value or worthlessness of the material depends upon its composition. Thus the so-called Oxford clay ranges from a hard silicious shale to a comparatively pure clay; some portions of it are so contaminated with calcareous and ferruginous matter as to make the material quite useless for the potter or clayworker. A geological classification of clays is chiefly of value as indicating probable origins, impurities and certain physical properties; but the limits of composition and general characteristics are so wide as to make it of very limited usefulness.

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 classes. Moreover, attempts to predict the value and uses of clays from their chemical composition are generally so misleading as to be worse than useless, unless a knowledge of some of the physical characters of the clays is available. It is, of course, possible to differentiate some clays from others by their composition, but not with sufficient accuracy to permit of definite and accurate classification.

A classification based exclusively on the composition of clays is equally unsatisfactory for other reasons, the chief of which is the placing together of clays of widely differing physical character, and the separation of clays capable of being used for a particular purpose. To some extent the latter objection may be disregarded, though it is of great importance in considering the commercial value of a clay.

Classification based on the uses of clays of different kinds has been suggested by several eminent ceramists, but is obviously unsatisfactory, particularly as it is by no means uncommon to use mixtures of clays and other minerals for some purposes. Thus stoneware clays must be vitrifiable under conditions which may be defined with sufficient accuracy, but many manufacturers of stoneware do not use clays which are naturally vitrifiable; they employ a mixture of refractory clay and other minerals to obtain the material they require.

A classification based on the origin of clays regarded from the petrological point of view offers some advantages, but is too cumbersome for ordinary purposes and suffers from the disadvantage that the origin of some important clays is by no means clearly known.

The author prefers a modification of Grimsley's and Grout's classification (31) as follows:

I. Primary clays.

(_a_) Clays produced by 'weathering' silicates--as some
kaolins.

(_b_) Clays produced by lateritic action--very rich in
alumina, some of which is apparently in a free state.

(_c_) Clays produced by telluric water containing active gases
(hypogenically formed clays)--as Cornish china clay.

II. Secondary clays.

(_d_) Refractory[16] secondary clays--as fireclays and some
pipe clays.

(_e_) Pale-burning non-refractory clays--as pottery clays,
ball clays and some shales.

(_f_) Vitrifiable clays--as stoneware clays, paving brick
clays.

(_g_) Red-burning and non-refractory clays--as brick and
terra-cotta clays and shales.

(_h_) Calcareous clays or marls, including all clays
containing more than 5 per cent. of calcium carbonate.

III. Residual clays.

(_i_) Clays which have been formed by one of the foregoing
actions and have been deposited along with calcareous or
other matter but, on the latter being removed by subsequent
solution, the clay has remained behind--as the white clays
of the Derbyshire hills.

[Footnote 16: 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 26 (approximately 1600°C.) (see p. 116).]

Some further sub-division is necessary for special purposes, particularly in sections _e_, _f_ and _h_, but to include further details would only obscure the general scheme. Some clays will, apparently, be capable of classification in more than one section, thus a vitrifiable clay may owe its characteristic to a high proportion of calcium carbonate and so be capable of inclusion as a calcareous clay. Broadly speaking, however, 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 placed.

From a consideration of a classification such as the foregoing, together with a detailed study of the physical and chemical properties of the material as a whole, 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 clay. Although the present state of knowledge does not permit them to be classified with such detail as is the case with plants, animals, or simple chemical compounds, the study of clays and the allied materials has a fascination peculiarly its own, not the least interesting features of which are those properties of the clay after it has been made into articles of use or ornament. These are, however, beyond the scope of what is commonly understood by the term 'the natural history of clay.'

BIBLIOGRAPHY

A complete bibliography of clay would occupy several volumes. The following list only includes the more accessible of the works quoted in the text.

1. "Second Report of the Committee on Technical Investigation--Rôle
of Iron in Burning Clays." Orton and Griffith. Indianapolis.
1905.

2. "British Clays, Shales and Sands." Alfred B. Searle. Charles
Griffin and Co. Ltd. London. 1911.

3. "Transactions of the English Ceramic Society." v. p. 72. Hughes
and Harber. Longton, Staffs. 1905.

4. "Royal Agricultural Society's Journal." XI.

5. "Die Tone." P. Rohland. Hartleben's Verlag. Vienna. 1909.

6. "Clays: their Occurrence, Properties and Uses." H. Ries. Chapman
and Hall. London. 1908.

7. "Gesammelte Schriften." H. Seger. Tonindustrie Zeitung Verlag.
Berlin. 1908.

8. "Tonindustrie Zeitung." 1902. p. 1064.

9. "Tonindustrie Zeitung." 1904. p. 773.

10. "Treatise on Ceramic Industries." E. Bourry (Revised translation
by A. B. Searle). Scott, Greenwood and Son. London. 1911.

11. "The Colloid Matter of Clay." H. E. Ashley. U.S.A. Geological
Survey Bulletin 388. Washington. 1909.

12. "Sprechsaal." 1905. p. 123.

13. "Action of Heat on Refractory Materials." J. W. Mellor and F. J.
Austen. Trans. Eng. Cer. Soc. VI. Hughes and Harber. Longton,
Staffs. 1906.

14. "Wiedermann's Annalen." VII. p. 337.

15. "Geological Contemporaneity." 1862.

16. "Geological Magazine." IV. pp. 241, 299.

17. "La Céramique industrielle." A. Granger. Gauthier Frères. Paris.
1905.

18. "American Journal of Science." 1871. p. 180.

19. "The Hensbarrow District." J. H. Collins. Geological Survey.
1878.

20. "Monographs of the U.S.A. Geological Survey." XXVIII. C. R. van
Hise. 1897.

21. "On Kaolinite and Pholerite." American Journal of Science.
XLIII. 1867.

22. "The Nomenclature of Clays." J. W. Mellor. Eng. Cer. Soc. VIII.
Hughes and Harber. Longton, Staffs. 1908.

23. "On the present distribution of Coal Balls." M. C. Stopes and D.
M. S. Watson. Phil. Trans. Royal Society. B. Vol. CC. 1908.

24. "Natural History of Coal." E. A. N. Arber. Cambridge University
Press. 1911.

25. "Modern Brickmaking." A. B. Searle. Scott, Greenwood and Son.
London. 1911.

26. "Die verschiedene Arten der Verwitterung." J. M. van Bemmelen.
Zeits. angewandte Chemie. LXVI. Leopold Voss Verlag. Hamburg.
1910.

27. "Pyrometrische Beleuchtung." Carl Bischof. Tonindustrie Zeitung.
1877.

28. "Die feuerfeste Tone." Carl Bischof. Quandt and Haendler.
Leipzig. 1904.

29. "The Chemical Constitution of the Kaolinite Molecule." Trans.
Eng. Cer. Soc. X. Hughes and Harber. Longton, Staffs. 1911.

30. "Tabellarische Uebersicht der Mineralien." P. Groth. Brunswick.
1898.

31. "West Virginia Geological Survey." III. 1906.

32. "Memoirs of the Geological Survey." London.

33. "The Publications of Stanford's Geographical Institute." London.

34. "Handbuch der gesam. Tonwarenindustrie." B. Kerl. Verlag der
Tonindustrie Zeitung. 1910.

35. "Causal Geology." E. H. L. Schwarz. Blackie and Sons, Ltd. 1910.

36. "China Clay: its nature and origin." G. Hickling. Trans. Inst.
Mining Engineers. 1908.

INDEX

Absorption, 40, 151
Absorptive power of clays, 40
Accumulation of clays, 84
Acid-proof ware, 113
Acids, effect of, 106, 151, 152
Adsorption, 40, 150
Agriculture, clays in, 5, 56, 57, 59, 61, 62, 63, 67
Air, 43, 85
Alkalies in clay, 38, 115, 133, 142, 143, 155
Alluvial deposits, 68, 87, 112, 132
Alum clays and shales, 57, 123, 124
Alum manufacture, clays for, 124
Alumina, 6
Alumina, free, 80, 82, 154
Alumina-silica ratio, 133, 159
Alumino-silicic acid, 6, 76, 81, 118, 155, 157
Aluminous clays, 82, 117
'Amorphous' clay, 107, 146
Analyses of clays, 16, 141, 144
Anauxite, 134
Architectural ware, 129, 130
Argillaceous earths, 1
Argillaceous limestone, 88, 132
Associated rocks, 48

Bagshot clays and sands, 64, 125
Ball clays, 6, 19, 28, 62, 64, 82, 110, 115, 119, 125, 138, 141, 156, 166
Bending of clay, 33
Bibliography, 168
Binding power, 28, 151
Binds, 53
Bituminous shales, 57, 59
Black spots, 14, 128
Black ware, 113
Bleaching oil, 134
Blue bricks, 13, 56
Bone-ash, 110
Boulder clays, 3, 7, 10, 65, 101
Bovey Tracey clay, 62
Brick clays, earths and shales, 1, 2, 5, 10, 11, 12, 13, 31, 37, 40, 46,
56, 57, 59, 61, 63, 65, 67, 68, 91, 100, 101, 104, 112, 117, 123, 125,
129, 138, 162, 166
Brittleness, 46
Brown ware, 113
Buff bricks, 128
Burned clay, 28, 31, 41, 119, 121

Calcareous clays, 38, 61, 68, 88, 133, 139, 166
Calcareous sands, 88
Calcium, see _Lime compounds_
Cambrian clays, 51
Carbon in clay, 15, 119, 144
Carbonates in clay, 10, 82
Carboniferous clays and shales, 52, 124
Carboniferous limestone, 52, 108
Carclazite, 78, 106
Cellulose in clays, 27
Cement clays, 57, 104, 131
Chalcopyrite, 14
Chalk, 10, 11, 61, 67, 68, 88, 116, 127, 128, 132, 134, 139
Chamotte, 121
Chemical properties of clay, 6
China clay rock, 78, 106, 116
China clays, 2, 5, 6, 7, 9, 22, 27, 40, 49, 64, 71, 75, 78, 82, 84, 104,
110, 116, 141, 146, 147, 148, 156, 165
China-ware, 109, 110
Chinese clay, 73
Classification of clays, 163
Clay molecule, 156
Clay-shales, 122
Clay substance, 135 _et seq._
Clay substance, defined, 150
Clayite, 83, 107, 147, 149
Clinker, 132
Clunches, 118
Coagulated clays, 97
Coagulation, 43, 152
Coal Measure clays and shales, 53, 96, 103, 117, 124, 130
Coarse pottery, 112
Cobalt, 110
Colloid theory, 97
Colloidal properties of clay, 25, 81, 82, 97, 106, 147, 150, 152
Colloidal silica, 81, 134, 157
Colloids, 24, 41, 43, 76, 160
Colluvial clays, 99
Colours of burned ware, 19, 123, 131
Colours of clays and shales, 19, 59, 115, 119, 124, 126
Combined water, 45, 154
Common clays, 3
Composition of clays, 4, 6, 16, 23, 35, 44, 107, 117, 118, 133, 134,
144, 156, 164
Composition of clays (burned), 46
Cornish stone, 110, 116
Cracked ware, 46, 127, 130, 131
'Cream,' 39, 43, 152
Cretaceous clays, 61
'Crumb' of clay, 24
Crushing clay, 45
'Crystalline' clay, 107, 146, 148
Crystals in clay-ware, 46

Decantation, 139
Decomposition of clay, 154
Definitions of clay, 2-5, 120, 135, 149, 150
De-greasing wool, 134
Deposition of clays, 49, 51, 90, 99
Devonian clays, 51
Diluvial clays, 99
Dinas rock, 54
Disintegration, 102
Distribution of clays, 1
Drain-pipe clays, 112, 113
Drift, 65, 101
Drift clays, 101
Drying clays, 27, 127, 153
Durability, 131
Dyes, 41, 150

Earth movements, 85, 96
Earthenware, 37, 112
Earths for bricks, see _Brick clays_
Electrolytes, 43
Elutriation, 8, 137, 140
Eocene clays, 63
Epigenic clays, 82
Erosion, 89, 99, 100
Estuarine clays, 90, 93, 118
Etruria marls, 55, 130
Expansion, 32
Exposure, 43

Faience, 129
Farewell Rock, 54
Fat clays, 29
Felspar, 7, 8, 41, 74, 104, 110, 116, 141, 144, 159
Ferric and Ferrous compounds, 12, 121, see _Iron_
Fine clays, 112
Fineness, see _Texture_
Firebricks, 14, 54, 61, 116
Fireclay, 33, 35, 52, 54, 82, 104, 108, 116, 123, 156, 166
Fissile clays, 117
Flint, 110, 116
Flint clays, 117
Floods, 85, 87, 99
Flower-pot clays, 57, 110
Fluoric vapours, 75, 77, 165
Fluviatile clays, 88, 92
Fluxes, 8, 11, 38, 39, 115, 116, 131
Food-clays, 1
Formation of clays, 48, 70
Formula of clay, 156
Free alumina, 80, 154
Free silica, 7, 80, 154, 161
Frost, 43, 86
Fuller's earth, 59, 133
Fulling cloth, 1, 133
Fusibility, 32, 58, 113, 116
Fusible clays, 116
Fusing point, 32
Fusion, 47, 113, 120, 132, 155

Ganister, 52, 54, 118
Gault, 61, 132
Geological classification, 163
Geological nature of clay, 4, 50
Glacial clays, 65, 100, 162
Glaciers, 85, 89, 100
Glass, 116
Glassy structure, 47
Glazed bricks, 119
Glazed pottery, 129
Glazed terra-cotta, 56, 129
Grades of fireclay, 120
Gravel, 7, 62, 65, 89, 100, 101, 102, 138, 144
Green colour, 14
Greensand, 133
Grinding, 80, 121
Grit, 112, see also _Millstone Grit_
Grog, 28, 31, 41, 119, 121
Growan, 78
Gypsum, 10, 12, 62

Halloysite, 118, 158
Hardness, 45
Heat, effects of, 28, 37, 39, 45, 80, 122, 146, 153, 154, 158, 159
Hydrargillite, 80
Hydro-alumino-silicates, 6
Hydrocarbons in clay, 15
Hydrolysis, 78, 97
Hygroscopic clays, 153
Hypogenic clays, 165

Ice-action, 85, 100
Ideal clay, 146
Impermeability, 40, 113
Impervious articles, 113, 155
Impurities in clays, 7, 49, 82, 102, 104, 109, 121, 126, 142, 143, 155,
162, 163
Ions, 43
Indurated clays, 18
Infusibility, 106, 119, see _Refractoriness_
Iron compounds, 7, 10, 12, 13, 20, 62, 112, 119, 121, 128, 133, 141,
145, 164
Ironstone, 62
Irregularity in shape, 131

Jurassic clays and shales, 57, 124

Kao-ling, 148
Kaolinite, 19, 80, 105, 107, 146, 149, 158
Kaolinization, 76, 77, 79
Kaolins, 9, 21, 49, 64, 71, 73, 76, 79, 82, 84, 104, 116, 141, 146,
147, 148, 165
Keele series, 55
Kellaways clay, 59, 61
Keuper marls, 57
Kiln shrinkage, 30
Kimeridge clays, 59
Knotts, 124

Lacustrine clays, 90, 91
Lake-deposited clays, 85, 88, 91
Lakes, 85, 88
Laminated clays, 53, 117, 122
Laterite, 80, 149
Lateritic action, 80, 165
Lateritic clays, 82
Lean clays, 29
Liassic clays and shales, 57, 125, 132
Lime, 7, 10, 102, 159
Lime compounds, 10, 11, 38, 41, 47, 113, 115, 116, 121, 127, 139, 142,
143, 145, 155, 157, 162, 164
Limestone, 10, 11, 52, 59, 61, 62, 88, 102, 117, 127, 132, 139
Lime troubles, 11
Loam, 57, 67, 88
London clay, 62, 63, 125
Ludwig's chart, 35

Magnesium compounds, 7, 10, 11, 41, 47, 113, 115, 116, 121, 133, 155
Malm-bricks, 11
Malms, 10, 68
Marcasite, 10, 13
Marine clays, 61, 93
Marls, 10, 51, 54, 55, 57, 67, 68, 88, 130, 132, 166
Mechanical analysis, 137
Medway mud, 132
Melting point, 31, 32
Mica, 7, 8, 76, 104, 105, 116, 140, 141, 144, 160
Microscopical examination, 18, 105, 143, 158
Millstone grit, 54, 55, 117
Mineral nature of clay, 3
Minerals resembling clay, 158
Mining ball clay, 111
Modelling clays, 130
Moisture, 15, 144
Molecular attraction, 22
Molecular constitution of clay, 21, 156
Montmorillonite, 134
Mundic, 13
Muscovite, 105

Newtonite, 158
Nodules, 121, 127
Non-plastic material, 43, 121, 151
Non-refractory clays, 166

Occurrence of clays, 48, 116
Ocean currents, action of, 89
Odour of clay, 19
Oil, bleaching, 134
Oil shales, 61, 122, 123
Oolite clays, 59, 134
Ooze, 95, 99
Organic matter, 19, 119
Origins of clays, 48, 71, 160, 165
Oxford clay, 59, 95, 164
Oxides in clay, 10, 82

Paint, clays for, 109
Paper, clays for, 109
Particles, nature of, 18, 31, 106, 107, 150
Paving brick clays, 166
Pelagic ooze, 95, 99
Pelinite, 83, 148, 149
Permian clays and shales, 57, 112, 124
Pholerite, 117
Physical characters of clays, 17
Picking clay, 121
Pipe clays, 64, 65, 82, 109, 166
Plant-extracts in clays, 26
Plastic clays, 2, 43, 65, 67, 82, 88, 102, 112, 123, 147, 148, 160
Plasticity, 20-27, 41, 46, 97, 98, 99, 108, 109, 112, 117, 123, 125,
127, 151, 160
Pleistocene clays, 67
Pockets, 65, 85, 101, 116, 166
Porcelain, 37, 46, 73, 109, 110, 125, 129
Pores in clay, 30, 114
Porosity, 30, 39, 121, 131, 155
Portland cement, 131
Potash compounds, 7, 10, 113, 115, 116, 121, 157, see _Alkalies_
Pottery clays, 1, 5, 31, 46, 66, 100, 101, 104, 110, 112, 114, 125,
129, 162, 166
Precambrian clays, 51
Precipitated clays, 97, 152
Primary clays, 70, 71, 84, 165
Proximate analysis, 16, 144
Purbeck clays, 59
Pure clays, 5, 6, 7, 142, 155, 156
Purification of clay, 7, 66, 78, 104, 113, 128, 140
Pyrites, 10, 13, 44, 56, 57, 119, 124, 128

Quartz, 8, 104, 110, 118, 140, 141, 143

Rain, 44, 85, 86
Rational analysis, 141
Reading clays, 63
Recent clays, 67
Rectorite, 158
Re-deposited clays, 98
Red bricks, 12
Red burning clays, 141, 142, 166
Red iron oxide, 12
Red ware, 113
Reduction in volume, 30
Refractoriness, 34, 119, 120, 123, 155
Refractory articles, 5, 119
Refractory clays, 9, 32, 33, 35, 38, 52, 65, 82, 104, 116, 165, 166
Residual clays, 70, 84, 166
Resistance to abrasion, 119
Resistance to corrosion, 119
Resistance to crushing, 46
Resistance to cutting, 119
Resistance to temperature, see _Refractoriness_
Resistance to weathering, 76
Ringing sound, 110
River-deposited clays, 88
Rivers, 85, 87
Rock binds, 53
Rockingham, 113
Rock-like clays, 2
Rocks associated with clay, 48
Roman cements, 133
Roofing tiles, 57, 63, 126, 128

Sagger marls, 54
Sand, 7, 31, 41, 62, 82, 89, 100, 101, 117, 133, 138, 144
Sandstones, 53
Sandy clays, 68
Sandy loams, 88
Sandy marls, 88
Sanitary articles, 5, 113
Sawdust, 40
Scum, 10
Sea, action of, 85, 89, 99
Sea-deposited clays, 93
Secondary clays, 70, 82, 83, 166
Sedimentary rocks, 48
Sedimentation of clay, 43, 88, 90, 104
Seger cones, 33, 34
Selection of clay, 122
Selenite, 10
Separation of clays, 90, 145
Settling, 43
Sewerage pipes, 113
Shale oil, 15, 61
Shale tar, 123
Shales, 2, 5, 51, 52, 53, 57, 61, 96, 104, 122, 130, 132, 138, 151,
162, 166
Shrinkage, 11, 29, 58, 68, 102, 110, 117, 119, 121, 127, 131, 153
Sifting, 7, 138
Silica, 6, 7, 80, 154, 155, 161
Silica rock, 118
Silicates, 8, 82
Siliceous clays, 117
Sillimanite, 46, 154
Silt, 90, 91, 99, 139, 144
Silurian clays and shales, 51, 124
Sintering, 38
Size of particles, 18, 21, 31, 106, 107, 150
'Skeleton,' 115
'Skin' on ware, 131
Slag in bricks, etc., 11, 13, 119, 155
Slates, 51
Slurry, 39, 43, 152
Snow, 85
Soda compounds, 7, 10, 113, 115, 116, 121, 157, see _Alkalies_
Softening point, 33
Soil, see _Agriculture_
Solubility of clay, 151, 159
Soluble salts, 10
Sorting, 90
Sources of clays, 85
Specification of fire clays, 120
Specific gravity, 18, 106, 151
Staffordshire bricks, 13, 56
Standard clay, 4
Stone, Cornish, 110, 116
Stoneware clays, 104, 112, 113, 156, 165, 166
Stones, 7, 65, 100, 102, 128, 138, 144
Streams, 85, 86
Strength, 23, 45, 113
Sub-surface clays, 5
Sulphates in clay, 10, 12, 82
Sulphides in clay, 10, 82
Sulphuric acid, 124
Sunlight, 45
Surface clays, 2, 5, 52, 112
Suspension of clay, 43, 90, 140, 152
Swelling, 15, 102

Tannin in clay, 25, 26, 41
Telluric water, 165
Temperature, resistance to, 119, 120
Tensile strength, 23, 45
Terra-cotta clays and shales, 5, 10, 12, 31, 46, 56, 63, 91, 104, 123,
124, 129, 166
Tertiary clays, 62
Texture, 112, 130
Thermal reactions, 146, 154, 158
Tiles, 1, 5, 57, 91, 101, 129
Titanium compounds, 121
Tourmaline, 76, 104
Transportation of clays, 49, 86, 98, 99, 100
Transported clays, 70
Triassic clays, 57, 112, 130
True clay, 144, 146, 149, 150, 160
Twisted ware, 114, 129, 131
Types of clay, 82

Ultimate analysis, 16, 144
Ultra-marine, clays for, 109
Underclays, 53, 117, 118
Uses of clay, 1, 165

Valuation of clay, 103, 109, 123, 126, 162, 165
Vegetable matter, 15, 119
Veins, 85
Viscosity, 152
Verifiable clays, 113, 156, 166
Vitrification, 15, 20, 37, 112, 113, 114, 156
Vitrification range, 38, 114, 115, 116, 129
Volcanoes, 85

Warp, 99
Warped ware, 114, 129, 131
Washing, 7, 79
Water, effect of, 74, 76, 81, 85, 86, 151
Water in clays, 15, 17, 29, 39, 45, 154
Wealden clay, 62
Weathering, 44, 74, 76, 79, 80, 97, 107, 165
White bricks, 68, 128
White clays, 70, 166
Wind, 86

Zeolites, 157

CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS

End of Project Gutenberg's The Natural History of Clay, by Alfred B. Searle