CLAY, ARGIL (Spanish Arcilla , French Argile , Italian & Portuguese Argilla , Catalan Argila , Esperanto Argilo (from Latin Argilla ), German Ton ). Naturally occurring hydrated aluminium silicate, in pure form or with iron oxides impurities; when saturated with water, it becomes plastic, but contracts and hardens on burning (ENCYCL.).

Clay's mineral aggregates give rise to rocks containing aluminosilicates in powder form. Its most significant property is that, when mixed with water, it produces a more or less plastic paste, which can be worked using a variety of means. On drying it acquires a certain cohesion and decreases in volume; on heating, it loses constitutional water, contracts, hardens and becomes practically inalterable. These characteristics led to it being one of the first raw materials to be worked by Man, and, today, it forms the basis for all the ceramics industries.