Fired clay

Fire clay is a range of refractory clays used in the manufacture of ceramics, especially fire brick. The United States Environmental Protection Agency defines fire clay very generally as a "mineral aggregate composed of hydrous silicates of aluminium (Al₂O₃·2SiO₂·2H₂O) with or without free silica."

High-grade fire clays can withstand temperatures of 1775 °C (3227 °F), but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1,515 °C (2,759 °F). Fire clays range from flint clays to plastic fire clays, but there are semi-flint and semi-plastic fire clays as well. Fire clays consist of natural argillaceous materials, mostly Kaolinite group clays, along with fine-grained micas and quartz, and may also contain organic matter and sulphur compounds.

Fire clay is resistant to high temperatures, having fusion points higher than 1,600 °C (2,910 °F), therefore it is suitable for lining furnaces, as fire brick, and for manufacture of utensils used in the metalworking industries, such as crucibles, saggars, retorts and glassware. Because of its stability during firing in the kiln, it can be used to make complex items of pottery such as pipes and sanitary ware.

The chemical composition typical for fire clays are 23-34% Al₂O₃, 50-60% SiO₂ and 6-27% loss on ignition together with various amounts of Fe₂O₃, CaO, MgO, K₂O, Na₂O and TiO₂. Chemical analyses from two 19th-century sources, shown in table below, are somewhat lower in alumina although a more contemporary source quotes analyses that are closer.

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