There are over thirty types of clay, but in pottery these types are divided into the three main classes of earthenware clay bodies, mid-fire stoneware clay bodies and high-fire stoneware clay bodies. These three classes are sometimes described as earthenware, stoneware and porcelain, respectively, after the type of pottery these clays make.Clays fired at higher temperatures tend to be less permeable.
These categories of clay are based on how long and at what temperature the clay must be fired in order to reach maturation, or its optimal hardness. Each category includes clays with different colors and degrees of workability. Clay colors are caused by impurities. For instance, red clay generally contains a high level of iron relative to other clays, and black clay contains manganese. Clay workability refers to how plastic the clay is. Higher levels of kaolin make clays less plastic, but also less porous.
Porcelain clays have the highest firing temperature. They are made of fine-grained white clays with high levels of kaolin. Stoneware clays fire at a medium temperature and are exceptionally durable and non-porous. Stoneware clays are often buff, brown or gray due to low levels of impurities. Earthenware clays have the lowest firing temperatures, but they are often more porous and less durable. Because of the low firing temperature achievable in a simple pit fire, earthenware was the first form of fired ceramic invented by humans. Earthenware is often called terra cotta or simply pottery.