Read Ebook: The Natural History of Clay by Searle Alfred B Alfred Broadhead

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Quartz and other forms of free silica expand on heating, so that clays containing them in large quantities shrink very slightly or may even expand.

As clays shrink equally in all directions it is usual to state the contraction in linear instead of volume form. Thus instead of stating that a certain clay when moulded into bricks, dried and burned, shrinks 18 per cent. by volume, it is customary to state that it shrinks 3/4 in. per foot. For many purposes, it is sufficient to regard the linear shrinkage as one-third the volume-shrinkage, but this is not strictly accurate.

In practice, a suggestion made many years ago by Seger is used; the clay to be tested is made into a small tetrahedron , heated slowly until it bends over and the point of the test-piece is almost on a level with the base. 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 described. In spite of the apparent crudeness of the test this 'softening point' appears to be fairly constant for most refractory clays.

The bending of a test-piece in this manner is the result of the action of all fluxes in the clay, 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 temperature, nor is the softening effect under one rate of rise in temperature the same as that at another rate. Nevertheless a study of the behaviour of various clays heated simultaneously is valuable and the method forms a convenient means of comparing different materials.

The temperature may be measured by means of a pyrometer, but for the reason just stated it is more convenient and in some respects more accurate to use standard mixtures known as Seger Cones , and to state the softening point in terms of the 'cone' which behaves like the clay being tested. A medium fireclay will not soften below Seger Cone 26 and a really good one will have a softening point of cone 34 or 35 .

Various attempts have been made to ascertain the relationship between the refractoriness of clays and their chemical composition. If attention is confined strictly to the more refractory clays, some kind of relationship does appear to exist. Thus Richter found that the refractoriness of clay is influenced by certain oxides in the following order: magnesia, lime, ferrous oxide, soda and potash, but this only applies to clays containing less than 3 per cent. of all these oxides. Cramer, in 1895, found that free silica also interfered with the action of these oxides and more recently Ludwig has devised a chart , on the upright sides of which are plotted the equivalents of the lime, magnesia and alkalies, whilst the silica equivalents are plotted on the horizontal base. In each case the 'molecular formula' of the clay is calculated from its percentage composition, and this 'formula' is reduced so as to have one 'molecule' of alumina, thereby fixing the alumina as a constant and reducing the number of variables to two--the metallic oxides and the silica. Unfortunately Ludwig's chart is only applicable to the more refractory clays and cannot be relied upon even for these, though it is extremely useful for comparing clays from identical or similar geological formations.

Attempts to express the refractoriness of clays by means of formulae proving abortive, there only remains the direct test of heating a clay under definite conditions in the manner previously described.

The extent to which a given clay will vitrify depends on the amount of fluxing material it contains, on the smallness of its particles and on the duration and intensity of the heating. Clays containing alkalies and lime compounds vitrify with great rapidity when once the necessary temperature has been reached, 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 slag. Refractory clays, on the contrary, vitrify more slowly and at much higher temperatures so that accidental overheatings of them are far less common.

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 clay. In some cases the clay melts as soon as vitrification becomes noticeable, in others the vitrification occurs at a dull red heat, but the material does not lose its shape until after a prolonged heating at the highest temperature of a firebrick kiln or testing furnace.

Calcareous clays have the melting and sintering points close together, so that it is almost impossible to produce vitrified and impervious ware from them, as they lose their shape too readily. If, however, the difference between the sintering and fusing temperatures can be enlarged--that is, if the vitrification range can be extended--more impervious ware can be made. The easiest means of extending the vitrification range consists in regulating the proportion of large and small particles. The former increase and the latter diminish the range.

Basic compounds and fluxes cause a lowering of the melting-point and a shortening of the vitrification range.

When heated moderately, clay forms a porous material and, unless the heating is excessive, it will absorb about one-eighth of its weight of water. 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 porous.

Porosity may thus be regarded as the opposite of vitrification; porous goods being relatively light and soft whilst vitrified ones are dense and hard. For some purposes, porosity is an important characteristic: for example, building bricks which are moderately porous are preferable to those which are vitrified. The manufacture of porous blocks for the construction of light, sound-proof partitions, etc. has increased rapidly of late. They are made by adding sawdust or other combustible material to the clay. The added substances burn out on firing the goods in a kiln.

Clays which are porous can be dried more readily and with less risk of cracking than those which are more dense. For this reason, some clayworkers mix non-plastic material such as sand or burned clay with their raw material.

Ashley has endeavoured to measure the plasticity of clays by determining their adsorption capacity for various aniline dyes, but his untimely decease prevented the investigation being completed. There is reason to suppose that the relation between adsorption and plasticity is extremely close in many clays and that the former may, to an important extent, be used as a measure of the latter. In some clays, however, this relationship does not exist.

Sand and burned clay only show faint adsorption phenomena; felspar shows them to a slight and almost negligible extent and most of the other non-plastic ingredients of clays are non-adsorptive.

Selective adsorption being an important characteristic of colloidal substances, the possession of this power by plastic clays supports the claim that plasticity is due, at least in part, to the presence of colloids.

The addition of small quantities of a solution of certain substances to a stiff clay paste usually reduces its stiffness, and in some cases turns it into a liquid. 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 paste. In a few moments the mixture will be sufficiently liquid to pour readily, but it may be rendered quite stiff again by adding sufficient acid to neutralize the alkali previously used. Weber 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 patented.

The effect of adding water to a dry clay is curious. At first the particles in contact with the water become sticky and plastic, and if the proportion of water added is suitable and the mixing is sufficiently thorough a plastic mass will be produced, the characteristics of which will depend on the nature of the clay used. This process of mixing clay with a limited amount of water is known as 'tempering.' The proportion of water required to make a paste of suitable consistency for modelling appears to be constant for each clay. If, however, a larger proportion of water is added the particles of clay will be separated so widely from each other that they lose their cohesion, and instead of a plastic mass, the material will form a liquid of cream-like consistency. If a piece of stiff clay paste is suspended in a large volume of water without stirring, disintegration will still occur and the clay will be deposited as a sediment at the bottom of the vessel. The leaner the clay or the larger the proportion of non-plastic material it contains, the more rapidly will this disintegration take place. A highly plastic clay will become almost impervious and will retain its shape indefinitely.

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 use. All clays are rendered more workable by exposure, but some of them are damaged by the oxidation of some impurities in them, though in other clays this very oxidation, if followed by the leaching action of rain, effects an important purification of the material.

Weathering appears to have no effect on the chemical composition of the particles of true clay in the material, though it may decompose the impurities present. On the clay itself its action is largely physical and consists chiefly in separating the particles slightly from each other, thereby enabling water to penetrate the material more readily and facilitating the production of a plastic paste. 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 water.

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 occur, but compared with the purely physical action of weathering the amount of such chemical decomposition must be relatively unimportant in most cases. It may, however, account for the presence of free silica and free alumina in some clays.

The extent to which clays are ordinarily heated and the conditions under which they are cooled do not usually induce the formation of crystals; the object of the clayworker being to produce a homogeneous mass, the particles of which are securely held together. 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 substance. The silicates formed are, therefore, in a condition of solid, super-cooled solution in which the tendency to crystallize is restrained by viscosity.

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 occur. The facility with which crystallization occurs varies greatly with the composition of the fused material, those silicates which are rich in lime and magnesia crystallizing more readily than those containing potash or soda. Vogt has stated that small quantities of alumina promote the formation of a glassy structure, and Morozewicz has shown that a large excess of this substance must be present if crystallization is to occur.

The study of the reactions which occur when clays are heated is, however, extremely complex, not only on account of the variety of substances present, but also on account of the high temperatures at which it is necessary to work, so that for a further consideration of it the reader should consult special treatises on the fusion of silicates. This subject has now become an important branch of physical chemistry.

CLAY AND ASSOCIATED ROCKS

Clay, as already mentioned, is geologically a rock and not a mineral, and belongs to the important group of sedimentary rocks which have been derived from the igneous or primary ones by processes of weathering, suspension in water and subsequent deposition or sedimentation.

Whatever may be the primary origin of clay, its chief occurrence is in geological formations which have undoubtedly been formed by aqueous action. 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 reduced.

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 temperatures.

Some clays have only been carried by small streams and for short distances; these are seldom highly plastic and resemble the lean china clays and kaolins. Others have been carried by rapidly moving rivers and have been discharged into lakes or into the sea; they have thus undergone a process of gradual purification by elutriation, 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 ocean. The nature of such deposits will, naturally, differ greatly from each other, the materials at first associated with the clay, or becoming mixed with it at a later stage, exercising an important influence on its texture, composition and properties. If the transporting stream flows through valleys whose sides are formed of limestone, chalk, sandstone or other materials, these will become mixed with the clay, and to so great an extent has the mixing occurred that very few clays occur in a state even approximating to purity. The majority of clays are contaminated with iron oxide, 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 clay. 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 earth, and that these subterranean changes bring about tilting, folding, overturning and other secondary changes, which, later, cause a fresh set of materials to be mixed with the clays. Further than this, the action of the weather, of rivers and of the sea never ceases, so that a process of re-mixing and re-sorting of materials is continuously taking place, and has been doing so for countless ages. It is, therefore, a legitimate cause for wonder that such enormous deposits of clays of so uniform a character should occur throughout the length and breadth of Europe, and practically throughout the world. For although the composition of many of these beds is of a most highly complex nature, the general properties such as plasticity, behaviour on heating, etc., remain remarkably constant over large areas of country, 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 country. Considerable differences undoubtedly exist, but these are insignificant in comparison with the vastly different circumstances under which the deposits were accumulated.

Leaving the consideration of the modes of formation of the various clay deposits to later chapters , it is convenient here to enumerate some of the chief characteristics of the different clay deposits and their associated rocks. In this connection it is not proposed to enter into minute details, but rather to indicate in broad outline the chief characteristics of the clays from the different deposits. 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 world.

Valuable Coal Measure clays occur in enormous quantities in Northumberland, Durham, Yorkshire, Nottinghamshire, Derbyshire, Staffordshire, near Stourbridge, in Warwickshire, Shropshire, North and South Wales and South West Scotland. In Ireland, on the contrary, the Coal Measure clays are of little value except in the neighbourhood of Coal Island, co. Tyrone. The position of the 'Sagger Marls' of North Staffordshire , relative to the 'Farewell Rock' or Millstone Grit, is shown in fig. 8 in which the horizontal lines represent coal-seams and ironstone veins.

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 formation, but the number of clays and shales which have been examined is too small for any accurate conclusion to be drawn.

For many industrial purposes, particularly for the manufacture of refractory goods, the clays and shales of the Carboniferous System are highly important. The less valuable burn to a reddish colour, often spoiled with many grey spots of ferrous silicate derived from the pyrites in the clay, but the purer varieties burn to a delicate primrose or pale buff tint and are amongst the most heat-resisting materials known. The Coal Measure clays of Yorkshire are particularly esteemed for their refractory properties; for the manufacture of glazed bricks and for blocks for architectural purposes somewhat ambiguously termed 'glazed terra-cotta.' The inferior qualities are largely used for the manufacture of red engineering bricks, some of them competing successfully with the more widely known 'blue bricks' of Staffordshire.

The Coal Measure clays of Shropshire are noted for the manufacture of red roofing tiles, especially in the neighbourhood of Broseley.

Agriculturally, the Coal Measure clays are usually poor, but are occasionally of good quality. The shales produce heavy, cold clays and the yellow subsoil produces soils of a light, hungry character so that the two should, if possible, be mixed together.

Agriculturally, the Permian clays are a free working loam yielding large crops of most of the ordinary farm products.

They are specially known amongst clayworkers as the material from which the Midland red bricks of Nottinghamshire and Leicestershire and the Somersetshire tiles are prepared.

Agriculturally, the Lias clays are laid down for grass, but the lighter soils are useful for arable purposes.

Agriculturally, the Kimeridge clays resemble gault and are difficult to work as arable land, though they form first-rate pasturage.

Agriculturally, Oxford clay is difficult to work and, while much of it is valuable, large portions are poor and cold. When well exposed to frost it is made much lighter, but even then is not very suitable for wheat and autumn sown crops.

Agriculturally, the Cretaceous clays form good arable soil where they are not too exposed, but they suffer from drought.

Agriculturally, the Wealden clay produces stiff, yellowish soils of a wet and poor character, but sometimes loams of a highly productive nature occur.

The Eocene clays are composed of a variety of clays, many of which are only distinguishable by the different fossils they contain. The most important are the Reading clays, the London clay and the Bagshot clays.

The 'blue' and 'black' ball clays are the most valued by potters, but the quality is usually ascertained by a burning test.

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 plasticity. They are, therefore, largely used in the manufacture of all kinds of earthenware of which they form the foundation material.

In composition, ball clays appear to consist chiefly of a hydro-alumino-silicate corresponding to the formula H4Al2Si2O9, and in this they very closely resemble the china clays . The latter are, however, but slightly plastic whilst the ball clays are amongst the most plastic clays known. The china clays are also much more refractory than the ball clays owing to the somewhat larger proportion of alkalies in the latter.

Agriculturally, drift or boulder clays are poor soils, but by judicious management and careful mixing they may be made more fertile. Where it contains chalk--as in Norfolk and Suffolk--boulder drift forms an excellent arable soil.

Agriculturally, they are of considerable importance.

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 derived, and distinguish them from the 'secondary' or 'transported' clays which have been carried some distance away from their place of origin.

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