Stoneware - Definition and Description

Definition and Description

One widely-recognized definition 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."

Formulations for stoneware vary considerably, although the vast majority will conform to: plastic fire clays 0–100%, ball clays 0–15%, quartz 0–30% feldspar and chamotte 0 –15%

Stoneware is generally once-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 & Seger cones 4 to 8. To produce a better quality fired glaze finish twice firing can be used, and this can be especially important for formulations composed of highly carbonaceous clays. For these, biscuit firing is around 900 °C and glost firing 1180–1280 °C. Water absorption of stoneware products is less than 1%.

Five categories 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 elsewhere 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• Electrical stoneware - historically has been used for electrical insulators, although has been replaced by electrical porcelain.

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."