The Process, Continued - The Clay
Most potters don’t mine their own clay, though many of us have an abashed admiration for those who do.
Commercial clays often contain a mixture of various substances, and are formulated to be used at different temperatures and for different purposes. There are a remarkable number of different clays that are available commercially--usually sold as 25-pound rectangular blocks.
Clays range from a pure creamy white to black. Various shades of white, cream, brown, red and yellow clays are available. Some clays are made to survive the extreme, fast temperature changes of raku firing. Others are translucent when thrown very thinly. Clays that are designed to be used for sculpture often include grog, which consists of finely-ground particles of already-fired clay. This gives the clay more strength while wet, giving the sculptor and potter more leeway in creating shapes that defy gravity.
Choosing a Clay
The potter selects a clay, taking into account the purpose of the pot, the temperature and technique planned for the firing and for the decoration of the clay's surface.
Each kind of clay is designed to be fired at a specific temperature. When it reaches that temperature in the kiln it will be "fully vitrified." If you fire a clay past its designed temperature, it weakens, and can even melt, which is why it's always vital to know what a clay's designed firing temperature is, before firing it.