Sometimes the latter sheetlike structure is found sandwiched between two silica sheets, forming a three-layer mineral such as vermiculite. In the lithification process, compacted clay layers can be transformed into shale. From prehistoric times, clay has been indispensable in architecture, in industry, and in agriculture. Clays are also of great industrial importance, e.g., in the manufacture of TILE for wall and floor coverings, of porcelain, china, and earthenware, and of pipe for drainage and sewage. Properties of the clays used in such products that must be taken into consideration include plasticity, shrinkage under firing and under air drying, fineness of grain, color after firing, hardness, cohesion, capacity of the surface to take decoration.
Bentonites are clays composed of very fine particles derived usually from volcanic ash.
Highly absorbent, bentonite is much used in foundry work for facing the molds and preparing the molding sands for casting metals. The less absorbent bentonites are used chiefly in the oil industry, e.g., as filtering and deodorizing agents in the refining of petroleum and, mixed with other materials, as drilling muds to protect the cutting bit while drilling. Other uses are in the making of fillers, sizings, and dressings in construction, in clarifying water and wine, in purifying sewage, and in the paper, ceramics, plastics, and rubber industries. Clay is one of the three principal types of soil, the other two being sand and loam. A certain amount of clay is a desirable constituent of soil, since it binds other kinds of particles together and makes the whole retentive of water.