Stoneware

Jian ware tea bowl with "hare's fur" glaze, southern Song dynasty, 12th century, Metropolitan Museum of Art (see below)[1]

Stoneware is a rather broad term for pottery or other ceramics fired at a relatively high temperature.[2] A modern technical definition is a vitreous or semi-vitreous ceramic made primarily from stoneware clay or non-refractory fire clay.[3] Whether vitrified or not, it is nonporous (does not soak up liquids);[4] it may or may not be glazed.[5] Historically, across the world, it has been developed after earthenware and before porcelain, and has often been used for high-quality as well as utilitarian wares.

As a rough guide, modern earthenwares are normally fired in a kiln 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.[6] Stoneware also needs certain types of clays, more specific than those able to make earthenware, but can be made from a much wider range than porcelain.

Glazed Chinese stoneware storage jar from the Han Dynasty

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.[7] Terms such as "porcellaneous" or "near-porcelain" may be used in such cases. One 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.[4][8]

In industrial ceramics, five basic categories of stoneware have been suggested:[9]

  • 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.[10]
  • 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.

Materials and firing

American stoneware jug with Albany slip glaze on the top, c. 1900, Red Wing, Minnesota[11]

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.[12] 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.[13]

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.[14]

A Staffordshire pottery stoneware plate from the 1850s with white glaze and transfer printed design. Visually this hardly differs from earthenware or porcelain equivalents.

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.[15] Typically, temperatures will be between 1180 °C and 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.[16]

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."[17]

Traditional East Asian thinking classifies pottery only 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.[18]

Methods of forming stoneware bodies include moulding, slipcasting and wheel throwing.[19] Underglaze and overglaze decoration of many types can be used. Much tableware in stoneware is white-glazed and decorated, and it is then visually highly similar to porcelain or faience earthenware.

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