The Columbia Encyclopedia, Sixth Edition. Columbia Encyclopedia World Factbook American Heritage Coll. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, e.g., potassium, sodium, calcium, magnesium, or iron, in small amounts. 1 Properties and Classification Properties of the clays include plasticity, shrinkage under firing and under air drying, fineness of grain, color after firing, hardness, cohesion, and capacity of the surface to take decoration. The purest clays are the china clays and kaolins.
Bentonites are clays composed of very fine particles derived usually from volcanic ash.
See also fuller's earth . 2 Individual clay particles are always smaller than 0.004 mm. Clays are easily molded into a form that they retain when dry, and they become hard and lose their plasticity when subjected to heat. One of the commonest processes of clay formation is the chemical decomposition of feldspar . 4 Clay consists of a sheet of interconnected silicates combined with a second sheetlike grouping of metallic atoms, oxygen, and hydroxyl, forming a two-layer mineral such as kaolinite .