Any naturally occurring mineral substance consisting preponderantly of hydrous aluminum silicates, which divided and mixed with water, yields a more or less plastic mass which can be formed and molded, and which will retain its shape on drying. Clays vary greatly in composition but in their purest forms they approach the composition of Kaolinite, A12O3.2Si2.2H20. They are used as compounding ingredients in rubber, some of them having mild reinforcing properties.