The American Ceramic Society has defined clay as "a fine-grained rock which, when suitably crushed and pulverized, becomes plastic when wet, leather-hard when dried and on firing is converted to a permanent rock-like mass." In general, clay is considered to be hydrated aluminum silicate and can be represented by the general formula Al2O32SiO22H2O. leather hard. clay that has dried to the point where it is stiff enough to retain its shape, but wet enough for further work to be done on it. bone dry. Leather-hard. A stage of drying where all shrinkage in the clay has been completed. The clay is slightly damp and feels soft to the touch. ... Ceramics Vocabulary. 42 ... Man-made materials are often used as building materials. Some examples of man-made materials used in buildings are fired bricks, clay blocks, concrete, fabric, foam, glass, metal and ceramics. Scientists have managed to make some creative man-made materials. For example, they have created titanium foam. Terra Sigillata (or terra sig. to cool potters) is a slip made of extremely fine flat clay particles. The terra sig is applied to the "leather-hard", or "bone-dry" greenware, and can be burnished (or polished) with a stick, smooth pebble, back of a spoon, soft towel, chamois, or whatever works.