Fire clay is a range of refractory clays used in the manufacture of ceramics, especially

fire brick A fire brick, firebrick, or refractory is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high temperature, but will also usually have a low thermal con ...

. 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."

Properties

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 .Minerals Zone, World Mineral Exchange.
Retrieved 2011-6-23. Fire clays range from ''

flint clay Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and start fires ...

s'' 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 Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...

group clays, along with fine-grained micas and quartz, and may also contain

organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...

matter and

sulphur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

compounds. Fire clay is resistant to high temperatures, having

fusion point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase_(matter), phase exist in Thermodynamic equilibr ...

s higher than ; therefore it is suitable for lining

furnace A furnace is a structure in which heat is produced with the help of combustion. Furnace may also refer to: Appliances Buildings * Furnace (central heating): a furnace , or a heater or boiler , used to generate heat for buildings * Boiler, used t ...

s, as fire brick, and for manufacture of utensils used in the metalworking industries, such as crucibles, saggars,

retort In a chemistry laboratory, a retort is a device used for distillation or dry distillation of substances. It consists of a spherical vessel with a long downward-pointing neck. The liquid to be distilled is placed in the vessel and heated. The n ...

s 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.

Chemical composition

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 Cao or CAO may refer to: Mythology *Cao (bull), a legendary bull in Meitei mythology Companies or organizations * Air China Cargo, ICAO airline designator CAO *CA Oradea, Romanian football club *CA Osasuna, Spanish football club *Canadian Assoc ...

MgO Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ions ...

, 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.

Extraction

Unlike conventional

brick A brick is a type of block used to build walls, pavements and other elements in masonry construction. Properly, the term ''brick'' denotes a block composed of dried clay, but is now also used informally to denote other chemically cured cons ...

-making clay, some fire clays (especially flint clays) are mined at depth, found as a

seatearth Seatearth is a British coal mining term, which is used in the geological literature. As noted by Jackson,Jackson, J.A., 1997, ''Glossary of geology'', 4th ed. American Geological Institute, Alexandria. a seatearth is the layer of sedimentary rock u ...

, the underclay associated with coal measures.

References

{{DEFAULTSORT:Fire Clay Ceramic materials Silicates Refractory materials Bricks Clay