Clay is the name of various earths, which consist of hydrated silicate of aluminium, with small proportions of the silicates of iron, calcium, magnesium, potassium, and sodium. All the varieties are characterized by being firmly coherent, weighty, compact, and hard when dry, but plastic when moist and comprising very fine-grained materials (less than 0.004 mm in diameter), smooth to touch, not readily diffusible in water, but when mixed not readily subsiding in it. Their tenacity and ductility when moist and their hardness when dry has made them from the earliest times the materials of bricks, tiles, pottery, etc.
Of the chief varieties porcelain-clay, kaolin, or china-clay, a white clay with occasional gray and yellow tones, is the purest. Potter's-clay and pipe-clay, which are similar but less pure, are generally of a yellowish or grayish colour, from the presence of iron. Fire-clay is a very refractory variety, always found lying immediately below the coal; it is used for making fire-bricks, crucibles, etc. Loam is the same substance mixed with sand, oxide of iron, and various other foreign ingredients. The boles, which are of a red or yellow colour from the presence of oxide of iron, are distinguished by their conchoidal fracture. The ochres are similar to the boles, containing only more oxide of iron. Other varieties are f'uller's-earth, Tripoli, and boulder-clay, the last a hard clay of a dark-brown colour, with rounded masses of rock of all sizes embedded in it, the result of glacial action.
The distinctive property of clays as ingredients of the soil is their power of absorbing ammonia and other gases and vapours generated on fertile and manured lands; indeed no soil will long remain fertile unless it has a fair proportion of clay in its composition. The best wheats both in Britain and the European continent are grown on calcareous clays. Research Clay
Abbreviations
Rocks & Minerals
Research Clay