Clays are mineral substances, very extensively found in nature, soft to the touch, yielding under slight pressure, and of very varied colours: white, yellow, red, green, blue, grey, or black, according to the nature of the impurities contained in them. They possess as common and distinctive characteristics: first, their plasticity, which causes them to preserve received impressions - this quality varies with the nature of the clay; second, their property of forming with water a tenacious paste, with a peculiar smell, which paste can be modelled, and hardened by drying. The clay then clings to the tongue, and if it is again suitably moistened, becomes plastic once more; but if it is exposed to a high temperature, its nature completely changes; it can no longer be diluted with water, it undergoes a considerable contraction, and, at the same time, acquires occasionally a very great degree of hardness; for certain clays, when highly baked, will give a spark on being struck by steel.
Pure clay, which is white and refractory, is composed of silica, aluminium, and water, this latter being in a state of combination and not as a hydrate. The proportions of these three substances are very variable, and lie within the following limits: -
45 to 75 Per cent.
38 to 16 „
Water in combination....
6 to 19 „
These pure clays are very rare. Ordinary clays, such as are found in great profusion, contain other substances in more or less quantity; especially iron oxides and sulphuret (pyrites), salts of lime, particularly the carbonate (limestone) and the sulphate (gypsum), magnesium, alkalies (potash and soda), organic substances, etc. We shall refer later on to the properties given to clays by the presence of these ingredients.