CLAY, the name of various earths, which consist of hydrated silicate of alu minum, with small proportions of the silicates of iron, calcium, magnesium, potassium, and sodium. All the varieties are characterized by being firmly co herent, weighty, compact, and hard when dry, but plastic when moist, smooth to touch, not readily diffusable 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 ma terials of bricks, tiles, pottery, etc. Of the chief varieties porcelain Clay, kaolin, or China Clay, a white Clay with occa sional 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 color, from the pres ence of iron. Fire Clay is a very refrac tory variety, always found lying imme diately 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 for eign ingredients. The boles, which are of a red or yellow color from the pres ence 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 fuller's-earth, Tripoli, and boulder Clay, the last a hard Clay of a dark-brown color, with rounded masses of rock of all sizes embedded in it, the result of glacial ac tion. The distinctive property of Clays as ingredients of the soil is their power of absorbing ammonia and other gases and vapors generated on fertile and manured lands; indeed no soil will long remain fertile unless it has a fair pro portion of Clay in its composition. The best wheats both in America and Europe are grown on calcareous Clays, as also the finest fruits and flowers of the rosa ceous kind.
The following shows the value of the Clay industry in the United States for the calendar year 1919 (est.) : The total imports of Clay products in 1919 were valued at $7,366,535, of which $7,230,061 were pottery products, and $136,474 brick, tile, etc. In the same year the total exports were valued at $6,582,284.