Clay

Clay (A.S. clæg; of the same root as 'clag,' 'claggy'), a term applied, in a vague way, to those kinds of earth or soil which, when moist, have a notable degree of tenacity and plasticity. The clays are not easily distinguished as rock species, but they all appear to owe their origin to the decomposition of various rocks, and to consist chiefly of aluminic silicate, along with other ingredients, which vary in character with the nature of the parent rock from the degradation of which they are derived. Thus common clay is a mixture of kaolin or China clay (which is a hydrated clay), and the fine powder of some felspathic mineral, which is anhydrous and not decomposed. Clays vary much in plasticity—all being more or less plastic when moistened with water, and capable of being moulded into any form. But while many retain their shape when dried by heat, others, which in the damp state would ordinarily be called clays, tend to fall to powder when all the water is driven off. Clay is eaten by the Botocudos and other savage tribes, as also in Georgia and the Carolinas by negroes and 'poor whites.' The plastic clays are used for many purposes, as for making pottery of all kinds, bricks and tiles, tobacco-pipes, firebricks, &c. The following are the commoner varieties of clay and clay-rocks: China clay or Kaolin (q.v.); Pipeclay (q.v.), very like kaolin, but containing a larger percentage of silica; potter's clay, not so pure as the preceding; sculptor's clay or modelling clay, a fine potter's clay, sometimes mixed with fine sand; plastilina, a potter's clay from Italy, supposed to be composed of oil, glycerine, rosin, and powdered clay, which, as it does not shrink, need not be kept wet; brick-clay (see BRICKS), an admixture of clay and sand with some ferruginous matter; Fireclay (q.v.), containing little or no lime, alkaline earth, or iron (which act as fluxes), and hence infusible or highly refractory; Shale (q.v.), a laminated clay-rock; clay-slate, an indurated cleaved clay-rock; Loam (q.v.), a non-plastic mixture of clay and sand; Marl (q.v.), a clay containing much calcareous matter. Clay at the deepest sea-bottom is largely of animal origin. See PETROGRAPHY.