Stoneware clays are plastic, secondary or sedimentary clays whose verifications is obtained between 2200 oF and 2400 oF. Fired, their color goes from chamois or clear gray to darker gray or to brown. They vary much in plasticity and firing temperatures and there is not a clear distinction between fire clays and stoneware. In fact, the classification of the types of clay depends more on its ceramic use than on its true chemical or physical nature or on its geological origin. The same clay can very well be used as fire clay, to make some refractory brick, and to manufacture stoneware fired at high temperature.
Many clays are usable to manufacture stoneware without any modification. They can have the exact necessary plasticity to be turned and present the specific characters of stoneware in drying and firing. The small workshops of the last century, which produced utility stoneware, usually used a clay body extracted in the area and prepared without adding any other material. This kind of natural stoneware bodies can give very pleasant colors and textures and can well accept slips and glazes for stoneware fired at high temperatures.