Clay

Clay is a generic term for an aggregate of hydrous silicate particles less than 4 µm (micrometres) in diameter. Clay consists of a variety of phyllosilicate minerals rich in silicon and aluminium oxides and hydroxides which include variable amounts of structural water. Clays are generally formed by the chemical weathering of silicate-bearing rocks by carbonic acid, but some are formed by hydrothermal activity.

Clays are heavy in texture yet soft to the touch. Clay is a malleable substance when wet, which means it can be shaped easily with the hands. When dry, it becomes firm and when "fired," or hardened by intense heat, clay becomes permanently solid. A fireplace specifically designed for hardening clay is called a kiln. These properties make clay an ideal substance for making durable pottery items, both practical and decorative, with different types of clay and firing conditions giving earthenware, stoneware and porcelain.

One of the earliest artifacts ever uncovered is a drinking vessel made of sun-dried clay. Depending on the content of the soil, clay can appear in various colors, from a dull gray to a deep orange-red.