Stoneware

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A Staffordshire stoneware plate from the 1850s with transferred copper print - (From the home of JL Runeberg)

Stoneware is a category of clay and a type of ceramic distinguished primarily by its firing and maturation temperature (from about 1200°C to 1315 °C). In essence, it is man-made stone. One widely recognized definition is from the Combined Nomenclature of the European Communities which 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".

In contrast, earthenware is fired at lower temperatures and is not impervious to liquids. Porcelain, which some consider to be a type of stoneware, is distinguished as being whiter than stoneware and always vitreous. Kaolin, or China Clay, has a lower content of impurities than many other clays. It is also fired to a vitreous state, transforming the constituent silica into glass. Some porcelain bodies are translucent after firing. Firing a piece of pottery to too high a temperature will result in warping or melting. Vitreous clay bodies can be made at different temperatures ranges, but they are typically fired in the stoneware/porcelain range. Fired stoneware absorbs up to 5% water, porcelain 0%, and earthenware up to 10%. Earthenware, when moist, is typically not freeze resistant.

A Chinese stoneware vase from the Song Dynasty, 11th century.

Clay refers to group of minerals that generally exhibit plasticity when mixed with water, and which chemically primarily consist of alumina and silica. Potters refer to combinations of clays mixed with other materials as clay bodies. Different kinds of clay bodies are created by mixing additives, such as feldspar, grog, quartz, flint, many other minerals are used and these can include spodumene, wollastonite to modify clays. Clay bodies can thereby be formulated to fire at a range of temperatures. Darker clays often contain iron and other metal oxide impurities. The clay used for porcelain and white stoneware clay bodies contain very little of these impurities.

Glaze may be applied to stoneware pottery before a second firing at a different temperature, or a glaze may be applied before a single, raw firing. Salt-glazed stoneware became the dominant houseware of nineteenth century America.