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CHAPTER I

INTRODUCTION. THE CHEMICAL AND PHYSICAL PROPERTIES OF CLAY

The chief uses of clay have been recognized since the earliest periods of civilization; the ancient Assyrian and Egyptian records contain numerous references to the employment of clay for the manufacture of bricks and for fulling or whitening cloth.

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 regions. The use of certain white clays as a food, or at any rate as a means of staving off hunger, is common among some tribes of very primitive peoples. The more important uses of clays for building and other purposes are naturally confined to the more civilized nations.

The term clay (A.S. cloeg; Welsh clai; Dutch kley) although used in a scientific sense to include a variety of argillaceous earths (Fr. argile = clay) used in the manufacture of bricks, tiles, pottery and ceramic products (Gr. keramos = potter's earth) generally, is really a word of popular origin and use. Consequently, it is necessary to bear in mind, when considering geological or other problems of a scientific nature, that this term has been incorporated into scientific terminology and that its use in this connection not infrequently leads to confusion. In short, whilst almost every dictionary includes one or more definitions of clay, and most text-books on geology, mineralogy, and allied sciences either attempt a definition or assume the reader's knowledge of one, there is no entirely satisfactory limitation in regard to the substances which may or may not be included under the term.

Clay is a popular term for a variety of substances of very varied origins, of great dissimilarity in their composition and in many of their chemical and physical properties, and differing greatly in almost every conceivable respect. It is commonly supposed that all clays are plastic, 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 degree. Shales, on the one hand—whilst clearly a variety of clay—are hard and rock-like, requiring to be reduced to powder and very thoroughly mixed with water before they become plastic; many impure surface deposits, on the other hand, are so highly plastic as to necessitate the addition of other (sandy) materials before they can be used for the manufacture of bricks and tiles.

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 water,' but this definition is so wide as to be almost impracticable, and leads to the inclusion of many substances which have no real connection with clays. The limitation of the use of the word 'clay' to the plastic or potentially plastic materials of any single geological epoch is also impracticable, for clays appear to have been deposited in almost every geological period, though there is some difference of opinion as to the time of the formation of certain clays known as kaolins.

Clay is not infrequently termed a mineral, but this does not apply at all accurately to the many varieties of earths known as 'common clays,' which, together with the 'boulder clays,' contain many minerals and so cannot, as a whole, be included under this term.

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 (including brick clay) are exempt—there can be no doubt that, scientifically, clay is not a mineral but a rock. Whatever mineral (if any) may give the chief characteristic property to the clays as a class must be designated by a special title, for the general term 'clay' will not serve for this purpose. Geologically, the clays are sedimentary rocks, some being unaltered, whilst others—the slates—are notably metamorphosed and can seldom be used for the purposes for which clays are employed.

Most clays may be regarded as a mixture of quartz grains, undecomposed rock débris and various decomposition products of rocks; if the last-named consists chiefly of certain hydrous alumino-silicates, they may be termed 'clay substance' (see Chapter VI). 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 30 per cent. of the latter substance.

It is, at the present time, quite impossible to construct an accurate definition of the term 'clay.' The most satisfactory hitherto published defines 'clay' as 'a solid rock composed mainly of hydro-alumino-silicates or alumino-silicic acids, but often containing large proportions of other materials; the whole possessing the property of becoming plastic when treated with water, and of hardening to a stone-like mass when heated to redness.'

From what has already been written, it will be understood that there is no such entity as a standard clay, for the varieties are almost endless, and the differences between them are sometimes so slight as to be scarcely distinguishable.

A further consideration of this branch of the subject may, however, conveniently be deferred to a subsequent chapter.

The best-known clays are the surface clays, loams and marls, the shales and other sub-surface clays, and the pottery and china clays. The values of these different materials vary enormously, some being almost worthless whilst others are highly valued.

The surface clays are chiefly used for the manufacture of bricks and tiles (though some are quite unsuitable for this purpose) and form the soil employed in agriculture in many districts.

The sub-surface clays and shales are harder, and usually require mechanical treatment before they can be used for brick and terra-cotta manufacture, or for the production of refractory and sanitary articles.

The pottery and china clays are usually more free from accessory constituents, and are regarded as the 'purest' clays on the market, though a considerable amount of latitude must be allowed in interpreting the term 'pure.' China clays are by no means pure in the state in which they occur, and require careful treatment before they can be sold.

Further information with regard to the characteristics of certain clays will be found in Chapter V.

The Chemical Properties of Clay.

The chief constituents of all clays are alumina and silica, the latter being always in excess of the former. These two oxides are, apparently, combined to form a hydro-alumino-silicate or alumino-silicic acid corresponding to the formula H₄Al₂Si₂O₉[1], but many clays contain a much larger proportion of silica than is required to form this compound, and other alumino-silicates also occur in them in varying proportions (see Chapters V and VI).

All clays may, apparently, be regarded as consisting of a mixture of one or more hydrous alumino-silicates with free silica and other non-plastic minerals or rock granules, and their chemical properties are largely dependent on the nature and proportion of these accessory ingredients.

The purest forms of clay (china clays and ball clays) approximate to the formula above-mentioned, but others differ widely from it, as will be seen from the analyses on p. 16. The chemical properties of pure clay are described more fully in Chapter VI.

[1] This formula is commonly written Al₂O₃2SiO₂2H₂O, but although this is a convenient arrangement, 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—Na₂CO₃24H₂O, or zinc sulphate crystals—ZnSO₄7H₂O (see Chapter VI).

Taking china clay, which has been carefully purified by levigation, as representative of the composition of a 'pure' clay, it will be found that the chief impurities in clays are (a) stones, gravel and sand—removable by washing or sifting; (b) felspar, mica and other silicates and free silica—which cannot be completely removed without affecting the clay and (c) lime, magnesia, iron, potash and soda compounds, together with minute quantities of other oxides, 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 purification.

To give a detailed description of the effect of each of the impurities just referred to would necessitate a much larger volume than the present, but a few brief notes on the more important ones are essential to a further consideration of the natural history of clay.

Stones, gravel and sand are most noticeable in the boulder clays, but they occur in clays of most geological ages, though in very varying proportions. Sometimes the stones are so large that they may be readily picked out by hand; in any case the stones, 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 sieve, or by allowing it to remain stationary for a few moments and then allowing the supernatant liquid to run off into a settling tank. 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 2 ft. per hour. Even then, the clay so removed may be found to contain minute grains of silt, much of which may be removed by a series of sedimentations for various periods, though a material perfectly free from non-plastic granules may be unattainable.

Most of the sand found associated with clays is in the form of fragments of quartz crystals (fig. 1), though it may be composed of irregular particles of other minerals or of amorphous silica.

Felspar, mica and other adventitious silicates occur in many natural clays in so fine a state of division that their removal would be unremunerative. In addition to this they act as fluxes when the clays are heated in kilns, binding the less fusible particles together and forming a far stronger mass than would otherwise be produced. Consequently, they are valuable constituents in clays used for the manufacture of articles in which strength or imperviousness is important. 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 page, the purification of the clay is not difficult. Usually, however, the most careful treatment fails to remove all these minerals; their presence may then be detected by microscopical examination and by chemical analysis. For most of the purposes for which clays are used, small proportions of these silicates are unimportant, but where clays of a highly refractory nature are required; and for most of the purposes for which china clays (kaolins) are employed, they must not be present to the extent of more than 5 per cent., smaller proportions being preferable.

Fig. 1. Quartz crystals, natural size. (From Miers' Mineralogy by permission of Macmillan & Co.)

Oxides, sulphides, sulphates and carbonates of various metals form the third class of impurities in clays. Of these, the most important are calcium oxide (lime), calcium carbonate (chalk and limestone), calcium sulphate (gypsum and selenite), the corresponding magnesia, magnesium carbonate, and sulphate, the various iron oxides, ferrous carbonate and iron sulphides (pyrite and marcasite) (p. 13).

Potash and soda compounds are commonly present as constituents of the felspar, mica, or other silicates present, and need no further description, though small proportions of soluble salts—chiefly sodium, potassium, calcium and magnesium sulphates—occur in most clays and may cause a white scum on bricks and terra-cotta made from them.

Lime and magnesia compounds may occur as silicates (varieties of felspar, mica, etc.), but their most important occurrence is as chalk or limestone. Chalk is a constant constituent of malms[2] and of many marls, but the latter may contain limestone particles. Limestone occurs in many marls and to a smaller extent in other clays. In the boulder clays it frequently forms a large portion of the stony material. If the grains are very small (as in chalk), the lime compounds act as a flux, reducing the heat-resisting power of the clay and increasing the amount of vitrification; they produce in extreme cases a slag-like mass when the clay is intensely heated. If, on the contrary, the grains are larger (as frequently occurs with limestone), they are converted into lime or magnesia when the clay is 'burned' in a kiln, and the lime, on exposure to weather, absorbs moisture (i.e. slakes), swells, and may disintegrate the articles made from the clay. Limestone (except when in a very finely divided state) is almost invariably objectionable in clays, but chalk is frequently a valuable constituent.

[2] A malm is a natural mixture of clay and chalk (p. 68).

Chalk is added to clay in the manufacture of malm-bricks to produce a more pleasing colour than would be obtained from the clay alone, to reduce the shrinkage of the clay to convenient limits and, less frequently, to form a more vitrifiable material. Chalk, on heating, combines with iron oxide and clay, forming a white silicate, so that some clays which would, alone, form a red brick, will, if mixed with chalk, form a white one.

Lime compounds have the serious objection of acting as very rapid and powerful fluxes, so that when clays containing them are heated sufficiently to start partial fusion, a very slight additional rise in temperature may easily reduce the whole to a shapeless, slag-like mass. Magnesia compounds act much more slowly in this respect and so are less harmful.

Gypsum—a calcium sulphate—occurs naturally in many sub-surface clays, often in well-defined crystalline masses. It reduces the heat-resisting power of the clays containing it and may, under some conditions, rise to the surface of the articles made from the clay, in the form of a white efflorescence or scum, such as is seen on some brick walls.

Iron compounds are highly important because they exercise a powerful influence on the colour of the burned clays. The red oxide (ferric oxide) is the most useful form in burned clay, but in the raw material ferrous oxide and ferrous carbonate may also occur, though they are converted into the red oxide on heating. The red iron oxide, which is closely related to 'iron rust,' occurs in so finely divided a state that its particles appear to be almost as small as those of the finest clays. Hence attempts to improve the colour of terra-cotta and bricks by the addition of commercial 'iron oxide' are seldom satisfactory, the finest material obtainable being far coarser than that occurring in clays.

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 firing, and although a 'base' and capable of reducing the heat-resisting power of clays, it does not appear to do so as long as the conditions in the kiln are sufficiently oxidizing. It is this which enables red bricks and other articles to be obtained with remarkable uniformity of colour combined with great physical strength. In a reducing atmosphere, on the contrary, ferrous oxide readily forms and attacks the clay, forming a dark grey vitreous mass. If the iron particles are separated from each other they will, on reduction, form small slag-like spots, but if they are in an extremely fine state of division and well distributed, the brick or other article will become slightly glossy and of an uniform black-grey tint. The famous Staffordshire 'blue' bricks owe their colour to this characteristic; they are not really 'blue' in colour. The effect of chalk on the colour of red-burning clays has already been mentioned.

Iron pyrite (fig. 2) and marcasite (fig. 3)—both of which are forms of iron sulphide—occur in many clays, particularly those of the Coal Measures. Mundic is another form of pyrites which resembles roots or twigs, but when broken show a brassy fracture. 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 cubic, root-like and spherical forms; the latter only show a brass-like sheen when broken. Even when only a small proportion of mundic, pyrite or marcasite is present, it is highly objectionable for several reasons. In the first place, half the sulphur present is given off at a dark red heat and is liable to cause troublesome defects on the goods. Secondly, because the remaining sulphur and iron are not readily oxidized, so that there is a great tendency to form slag-spots of ferrous silicate, owing to the iron attacking the clay at the same moment as it parts with its remaining sulphur. For this reason, clays containing any iron sulphide seldom burn red, 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 firebricks. If chalcopyrite (copper-iron sulphide) is present the spots may be bright green in colour.

Fig. 2. Pyrite. Fig. 3. Marcasite.

Slightly magnified.

(From Miers' Mineralogy by permission of Macmillan & Co.)

Carbon, either free or as hydrocarbons (chiefly vegetable matter) or in other forms, is a constituent of most clays, though seldom reported in analyses. Its presence exercises an important influence in several respects. On heating the clay, with an ample supply of air, the carbonaceous matter may distil off (as shale oil), but more usually it decomposes and burns out leaving pores in the material. If the air-supply is insufficient and the heating is so rapid and intense that vitrification commences before the carbon is all burned away, the pores become filled with the fused ingredients of the clay, air can no longer reach the carbon particles and a black 'core' or heart is produced. Under peculiarly disadvantageous conditions the material may also swell greatly. This is a serious defect in many classes of clay used for brickmaking, and its causes and prevention have been exhaustively studied by Orton and Griffiths (1)[3] but, beyond the brief summary given above, these are beyond the scope of the present work.

Water is an essential constituent of all unburned clays, though the proportion in which it occurs varies within such wide limits that no definite standard can be stated. This water is found in two conditions: (a) as moisture or mechanically mixed with the clay particles and (b) in a state of chemical combination.

[3] References to original papers, etc. will be found in the appendix.

ANALYSES OF TYPICAL CLAYS

The samples were all dried at 105° C.

Clay	China Clay	Ball Clay	Fireclay	Brick Clay	Boulder Clay	Marl
Locality	Cornwall	Dorset	Yorkshire	Midlands	Lancs.	Suffolk
Ultimate Analysis:
Silica	47·1	49·1	68·9	57·7	63·7	43·7
Alumina	39·1	33·7	19·3	24·3	20·4	15·5
Ferric oxide	·6	1·2	1·0	5·0	3·0	5·2
Titanium oxide	—	·2	1·8	·1	·2	—
Lime	·4	·8	·9	3·7	4·3	16·3
Magnesia	·2	·3	·3	2·5	2·7	2·1
Potash and Soda	·3	2·5	·9	2·8	2·9	·7
Carbon	2·6	4·3	1·8	1·6	·4	1·6
Water	9·3	7·7	4·8	2·0	2·2	2·4
Other Matter	·4	·2	·3	·3	·2	12·5
Total	100·0	100·0	100·0	100·0	100·0	100·0
Proximate Analysis:
Gravel and Sand	—	8·4	4·6	22·1	23·1	9·2
Silt	—	4·8	9·0	3·1	8·4	16·0
Felspar- and mica-dust	5·2	15·4	10·3	24·3	18·5	8·9
Silica-dust	3·1	4·0	38·0	3·1	12·6	2·0
Free calcium carbonate	—	—	—	2·1	·2	28·4
Free iron oxide and pyrites	·4	·9	·7	4·2	1·6	3·9
'True clay'	91·3	66·5	37·4	41·1	35·6	31·6
Total	100·0	100·0	100·0	100·0	100·0	100·0

For other analyses the books in the Bibliography at the end of the present volume should be consulted, particularly No. 2, i.e. British Clays, Shales and Sands.

The amount of mechanically mixed water will naturally vary with the conditions to which the clay has been subjected; it will be greatest in wet situations and will diminish as the clay is allowed to dry.

The 'combined water,' on the contrary, appears to be a function of the true clay present in the material, and reaches its highest proportions in the china clays and kaolins, which contain approximately 13 per cent. On heating a clay to 105° C. the moisture or mechanically mixed water is evaporated, but the combined water remains unaffected[4] until the temperature is raised to more than 600° C., 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 (see Chapter VI).