CLAY (AS. claw, Ger. Mei: ultimately con nected with Lat. plus, gluten, glue, Gk. leXoth, gloios, gum, OCh. Slay. gl(nu, slime). A term applied to earthy material or soil which shows plasticity when wet, thus permitting it to be molded into any desired form, which it retains when dry. Its distinguishing character is a physical one for clay varies widely in other respects, being made up of fine mineral frag ments, the most prominent of which may he the mineral kaolinite, a hydrated silicate of alu mina. Clay is formed primarily by the decompo sition of feldspathic rock in situ. :Ind such a clay is said to he residual in its nature. The material. however, is often washed down into the lakes or (mean by the surface-waters, and there spread out over the bottom as an alumi nuns sediment ; such a clay deposit being known as a sedimentary one, which is not only strati fied, but may also be more extensive than a residual. Sedimentary clay sometimes becomes consolidated by the pressure of other sediments which have heen deposited on top of it, arid it is then termed shale. These shales, on grinding and mixing with water, develop the same plas ticity as does soft clay.
The chief chemical constituents of clay are siliea and alumina; but, in addition to these, variable quantities of iron oxide, lime, mag nesia. alkalies, water, and even rarer substances are often present. These ingredients affect the physical properties of the clay; such as its color when !mimed. air and fire shrinkage. refractori ness, plasticity, and thus indirectly its uses. The lime, magnesia, iron oxide, and alkalies exert a fluxing action in burning, and the great er their quantity the lower the fusing-point of the clay. Silica decreases the air and fire shrinkage of a clay, while alumina and water have the re verse effect. The property of plasticity, to gether with that of hardening under fire, makes clay an article of great value in the plastic arts. Those clays which are low in plasticity are said to be 'lean.' while the highly plastic varieties are 'fat.' Clay does not fuse suddenly. but soft
ens gradually under the influence of heat. In very fusible clays this sintering may begin at 1500° or 1700° F.. while in very refractory kinds it does not take place until a temperature of 5000° F. or more is reached. The red color of a burned clay is due to considerable iron oxide, while buff is produced by a small quantity of iron, or by an excess of lime. The following table gives the composition of several grades of clay: Clay is used in the manufacture of common, pressed. and paving brick: terra-cotta, fireproof big, terra-cotta lumber; roofing, floor, and glazed tile; firebrick, retorts, crucibles, muffles, and other refractory goods: all grades of pottery. stoves. semer-pipe, door-knobs. electrical insu lators, turbine-wheels, closets, and bathtubs and washtubs: filters, mineral paint, food-adulter ants. Portland cement. paper - fillers, emery wheels (as a cement therein), ultramarine, mod eling, soap. etc.
Clay is widely distributed geographically, and also geologically—i.e. in the rock-formations of different ages. In the United States deposits are found at a great number of localities. The Cre taceous clays of New Jersey are much used in the manufacture of refractory goods and white mare, while the Carboniferous clays of Pennsyl vania and Ohio are also extensively employed for making firebrick. Kaolin is quarried at several points in North Carolina, Georgia, Mary land, and Wisconsin; much white ball-clay is found in Florida, and stoneware-clay in Illinois and Mssouri. Clays suitable for brick, terra cotta, and sewer-pipe occur at many points all over the United States. In the Central States, clays suitable for the manufacture of vitrified paving-briek are actively worked. With all this supply. however, much ball-clay and kaolin is imported, the material coning chiefly from Eng land. About $70.000,000 worth of clay products are produced annually in the United States alone.
The different varieties of clay arc as follows: