Fire 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."¹

High-grade fire clays can withstand temperatures of 1,775 °C (3,227 °F), but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1,515 °C (2,759 °F).² 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. 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, 6-27% ignition loss and various amounts of Fe₂O₃, CaO, MgO, K₂O, Na₂O, and TiO₂.² Chemical analyses from two 19th-century sources, shown in table below, are somewhat lower in alumina³⁴ although a more contemporary source quotes analyses that are closer.⁵

Unlike conventional brick-making clay, some fire clays (especially flint clays) are mined at depth, found as a seatearth, the underclay associated with coal measures. Though many are extracted through open pit methods⁶. Many fireclay deposits are associated with coal even if the seam is small and of low quality but not all fire clays form around coal⁷. While many commercial mines focus on huge deposits sometimes spanning 100s of miles wide and meters thick found in sedimentary rocks associated with coals⁸, formed by primary clay that has been eroded and transported downstream in a somewhat pure form or has been chemically weathered after deposition(from ground water itself or from the organic acids from said coal) ⁹. These deposits are similar to ball clay deposits in every way except iron content, particle size, and fired color ¹⁰ . Although smaller deposits occur in innumerable spots across almost every U.S. state, as well as every country, these can occur as primary clay deposits from chemical weathering, much like kaolin deposits but from parent rocks with more iron¹¹. More commonly they occur in a wide variety of sedimentary rocks and can vary quite differently in terms of plasticity, color, sand content, and mineral content¹² and can be found next to or within meters of clays of other or similar types. Many of these deposits were abandoned during the turn of the 20th century and lay abandoned still or covered by development ¹³.