Clay, 1 of 3 main types of earth, found in layers under the earth's crust and often at river mouths. Most clays consist of very small particles of hydrated aluminum silicates (kaolinites) that are usually produced by the weathering of rocks. Clay is easily malleable when wet and retains its shape when dried. If it is fired (baked) in a high-temperature oven or kiln it becomes extremely hard and, if first coated with a glaze, nonporous. Electrical insulators, sewage pipes, cement, kitchen tiles, chinaware, bricks, and paper manufacture all require clay. Clay is essential to the soil, holding moisture and preventing organic material from being washed away.