Fire clay

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.

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.

Its stability during firing in the kiln means that 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.