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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, and has often been used for high-quality as well as utilitarian wares.
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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:
Materials and firing
The key raw material in stoneware is either naturally occurring stoneware clay or non-refractory fire clay. The mineral kaolinite is present but disordered, and although mica and quartz are present their particle size is very small. Stoneware clay is often accompanied by impurities such as iron or carbon, giving it a "dirty" look, and its plasticity can vary widely. Non-refractory fire clay may be another key raw material. Fire clays are generally considered refractory, because they withstand very high temperatures before melting or crumbling. Refractory fire clays have a high concentration of kaolinite, with lesser amounts of mica and quartz. Non-refractory fire clays, however, have larger amounts of mica and feldspar.
Formulations for stoneware vary considerably, although the vast majority will conform to: plastic fire clays, 0 to 100 percent; ball clays, 0 to 15 percent; quartz, 0 to 30 percent; feldspar and chamotte, 0 to 15 percent.
Stoneware can be once-fired or twice-fired. Maximum firing temperatures can vary significantly, from 1100 °C to 1300 °C depending on the flux content. Typically, temperatures will be between 1180 °C to 1280 °C, the higher end of which equate to Bullers Rings 38 to 40 or Seger cones 4 to 8. To produce a better quality fired glaze finish, twice-firing can be used. This can be especially important for formulations composed of highly carbonaceous clays. For these, biscuit firing is around 900 °C, and glost firing (the firing used to form the glaze over the ware) 1180–1280 °C. Water absorption of stoneware products is less than 1 percent.
Another type, Flintless Stoneware, has also been identified. It is defined in the UK Pottery (Health and Welfare) Special Regulations of 1950 as: "Stoneware, the body of which consists of natural clay to which no flint or quartz or other form of free silica has been added."
Traditional East Asian thinking only classifies pottery into "low-fired" and "high-fired" wares, equating to earthenware and porcelain, without the intermediate European class of stoneware, and the many local types of stoneware were mostly classed as porcelain, though often not white and translucent.
History and notable examples
The earliest examples of stoneware have been dated to 1900 BC in the Indus Valley Civilization. An industry of a nearly industrial-scale mass-production of stoneware bangles flourished in the Indus Valley throughout the civilization's Mature Period (2600–1900 BC).
Another very early example of stoneware has been found in China, naturally as an extension of higher temperatures achieved from early development of reduction firing. From the various definitions of high-fired ceramics, it is agreed that the earliest stoneware is encountered in the late Shang dynasty in China, with large quantities in production by the Han dynasty.
In both medieval China and Japan, stoneware was very common, and several types became admired for their simple forms and subtle glaze effects. Japan did not make porcelain until about 1600, and north China (in contrast to the south) lacks the appropriate kaolin-rich clays for porcelain on a strict Western definition. Jian ware in the Song dynasty was mostly used for tea wares, and appealed to Buddhist monks. Most Longquan celadon, a very important ware in medieval China, was stoneware. Ding ware comes very close to porcelain, and even modern Western sources are notably divided as to how to describe it, although it is not translucent and the body often grey rather than white.
Many modern commercial tablewares and kitchenwares use stoneware rather than porcelain or bone china, and it is common in craft and studio pottery. American stoneware was the predominant houseware of 19th century North America, where the alternatives were less developed.
Other notable historical European types include: