Clay is, of course, the essential material from which pottery is formed. It is one of the most common materials on earth. Most clays began as feldspatic and granitic rock which were decomposed millions of years ago by hydrothermal action and weathering. Clay is primarily composed of the minerals alumina and silica with small amounts of other minerals and metal oxides. Clays found in various geological formations vary widely in their purity and in such important properties as plasticity. Most potters today use clays that are blends of several clays to get the particular properties they want. Mined clays are normally classified as either primary (found in the location where they were formed) or secondary (eroded or carried away by the movements of water and earth and redeposited in another location). Primary clays are often called china clay or kaolin and secondary clays are often called ball clays. In blended form, as used by potters, clays are grouped by the temperature at which they are fired. Earthenware and terracotta are fired to low temperatures and still absorbs water after firing; stoneware and porcelain are fired to much higher temperatures and absorb very little water after firing. Raku clays can mature over a range of temperature and are compounded primarily to withstand a high degree of thermal shock inherent in the raku firing process; they are normally quite porous after firing. For most of my work, I have chosen a commercially compounded stoneware clay which matures at cone 6 (2280 degrees Fahrenheit).