Fire clay is a range of

refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

s used in the manufacture of

ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...

s, especially

fire brick A fire brick, firebrick, fireclay brick, or refractory brick is a block of ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. Made of primarily oxide materials like silica and alumina in varying ratios, these insulati ...

. The

United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent agency of the United States government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it began operation on De ...

defines fire clay very generally as a "mineral aggregate composed of hydrous

silicates A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used for an ...

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 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 Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina () ...

group clays, along with fine-grained

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

s and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

, and may also contain organic matter and

sulphur Sulfur (American spelling and the preferred IUPAC name) or sulphur (English in the Commonwealth of Nations, Commonwealth spelling) is a chemical element; it has Symbol (chemistry), symbol S and atomic number 16. It is abundance of the chemical ...

compounds. Fire clay is resistant to high temperatures, having fusion points higher than ; therefore it is suitable for lining furnaces, as

, and for manufacture of utensils used in the

metalworking Metalworking is the process of shaping and reshaping metals in order to create useful objects, parts, assemblies, and large scale structures. As a term, it covers a wide and diverse range of processes, skills, and tools for producing objects on e ...

industries, such as

crucible A crucible is a container in which metals or other substances may be melted or subjected to very high temperatures. Although crucibles have historically tended to be made out of clay, they can be made from any material that withstands temperat ...

s, saggars,

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

s, and

glassware upTypical drinkware. This list of glassware includes drinking vessels (drinkware), tableware used to set a table for eating a meal and generally glass items such as vases, and glasses used in the catering industry. It does not include laboratory ...

. Its stability during firing in the

kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or Chemical Changes, chemical changes. Kilns have been used for millennia to turn objects m ...

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 ...

, MgO, K₂O, Na₂O, and TiO₂. Chemical analyses from two 19th-century sources, shown in table below, are somewhat lower in

alumina Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...

although a more contemporary source quotes analyses that are closer.

Extraction

Unlike conventional

brick A brick is a type of construction material used to build walls, pavements and other elements in masonry construction. Properly, the term ''brick'' denotes a unit primarily composed of clay. But is now also used informally to denote building un ...

-making clay, some fire clays (especially flint clays) are mined at depth, found as a seatearth, the underclay associated with

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

measures.

References

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