The Natural History of Clay

by Alfred B. Searle

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 (though a much longer time will be required) 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.

If a mixture of clay and water in the form of a cream or slurry be allowed to rest, the larger and less plastic particles will settle, but many of the particles of true clay will remain suspended for several hours and some of them for several days. Some particles of clay are so small that it is doubtful if they would ever settle completely unless some coagulant were added, and as they readily pass through all ordinary filtering media it is extremely difficult to collect them in a pure state. These turbid suspensions of clay may be rapidly cleared by the addition of sodium chloride which increases the surface tension of the solution. The fine particles behave in the same way as colloidal substances, i.e. as if they possessed an electrostatic charge. Hence the addition of a salt (electrolyte), whose ions annul the opposite charges of the electric double layer assumed by Helmholtz to be present, enables the particles to coagulate in accordance with the ordinary laws of surface tension ([14]).

Exposure to the action of air and frost has a marked effect on many clays. When freshly dug these may be hard and difficult to crush, but after exposure they break up readily into small fragments. Clays differ greatly in the extent to which they are affected by exposure; some are completely disintegrated by standing 48 hours in the open air, whilst others are scarcely affected by exposure in bleak places through several years of storm, sunshine and frost. Usually, however, the effect of a couple of nights exposure to hard frost will produce a marked disintegration of the material.

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 (e.g. pyrites) 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 action of the weather on rocks, resulting in the formation of clays, is described in [Chapter III.]

Heat effects remarkable changes in the physical character of clays; the most important of these have already been noted. At a gentle heat, the clay is dried and retains most of its power of becoming plastic when moistened; very little, if any, decomposition occurs. At a higher temperature it loses its 'combined water,' the clay molecule apparently dissociating, and a hard stony mass—consisting of particles of free silica and free alumina cemented together by the more easily fusible impurities present—is formed. If the heating is continued the hardness of the material is increased owing to more molten silicate having been produced from the impurities present, and on cooling, its tensile strength and resistance to crushing will be found to be enormously greater than those of the original clay. All potential plasticity is destroyed by heating to 700° C. and no method of restoring it has yet been devised. As clays are abundant, this is not a serious disadvantage for the specially desired characteristics of bricks, terra-cotta, pottery and porcelain are all such as to be incompatible with plasticity. The latter is extremely valuable in the shaping of the wares mentioned, but after the manufacture is completed, the destruction of the plasticity is an essential feature of their usefulness.

If the heating is very prolonged or is repeated several times, clays change other of their physical characters and become brittle and liable to crack under sudden changes of temperature. This is partly due to the further fusion (vitrification) which occurs and partly to the formation of crystalline silicates, notably Sillimanite ([13]).

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.