Stoneware is a rather broad term for pottery or other ceramics fired at a relatively high temperature. A modern technical definition is a vitreous or semi-vitreous ceramic made primarily from stoneware clay or non-refractory fire clay. Whether vitrified or not, it is nonporous (does not soak up liquids); it may or may not be glazed. Historically, across the world, it has usually been developed after earthenware and before porcelain, as kiln and has often been used for high-quality as well as utilitarian wares. With the right materials all that is necessary to move from earthenware to stoneware is a higher firing temperature in the kiln, so the development may well have occurred independently in various regions.
As a rough guide, modern earthenwares are normally fired at temperatures in the range of about 1,000°C (1,830 °F) to 1,200 °C (2,190 °F); stonewares at between about 1,100 °C (2,010 °F) to 1,300 °C (2,370 °F); and porcelains at between about 1,200 °C (2,190 °F) to 1,400 °C (2,550 °F). Historically, reaching high temperatures was a long-lasting challenge, and temperatures somewhat below these were used for a long time. Earthenware can be fired effectively as low as 600°C, achievable in primitive pit firing, but 800 °C (1,470 °F) to 1,100 °C (2,010 °F) was more typical.
Stoneware is not recognised as a category in traditional East Asian terminology, and much Asian stoneware, such as Chinese Ding ware for example, is counted as porcelain by local definitions. Terms such as „porcellaneous“ or „near-porcelain“ may be used in such cases. One widely recognised definition of stoneware is from the Combined Nomenclature of the European Communities, a European industry standard. It states:
- „Stoneware, which, though dense, impermeable and hard enough to resist scratching by a steel point, differs from porcelain because it is more opaque, and normally only partially vitrified. It may be vitreous or semi-vitreous. It is usually coloured grey or brownish because of impurities in the clay used for its manufacture, and is normally glazed.“
Five basic categories of stoneware have been suggested:
- Traditional stoneware – a dense and inexpensive body. It is opaque, can be of any colour and breaks with a conchoidal or stony fracture. Traditionally made of fine-grained secondary, plastic clays which can used to shape very large pieces.
- Fine stoneware – made from more carefully selected, prepared, and blended raw materials. It is used to produce tableware and art ware.
- Chemical stoneware – used in the chemical industry, and when resistance to chemical attack is needed. Purer raw materials are used than for other stoneware bodies. Ali Baba is a popular name for a large chemical stoneware jars of up to 5,000 litres capacity used to store acids.
- Thermal shock resistant stoneware – has additions of certain materials to enhance the thermal shock resistance of the fired body.
- Electrical stoneware – historically used for electrical insulators, although it has been replaced by electrical porcelain.