Fire clay is a term applied to a range of refractory

Refractory

A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...

 (heat resistant) clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

s used in the manufacture of ceramic

Ceramic

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

s, especially fire brick

Fire brick

A fire brick, firebrick, or refractory brick is a block of refractory 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 conductivity for greater energy efficiency...

.

High grade fire clays can withstand temperatures of 1775°C (3227°F, PCE 35), but to be referred to as a "fire clay" the material must withstand a minimum temperature of 1515°C (2759°F, PCE 19). Fire clays range from flint clays (non-plastic) 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

Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...

 group clays, along with fine-grained micas and quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, and may also contain organic

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 matter and sulphur compounds.

Properties

It is resistant to high temperatures, having fusion points higher than 1,600°C, therefore it is suitable for lining furnace

Furnace

A furnace is a device used for heating. The name derives from Latin fornax, oven.In American English and Canadian English, the term furnace on its own is generally used to describe household heating systems based on a central furnace , and sometimes as a synonym for kiln, a device used in the...

s, as fire brick, and manufacture of utensils used in the metalworking

Metalworking

Metalworking is the process of working with metals to create individual parts, assemblies, or large scale structures. The term covers a wide range of work from large ships and bridges to precise engine parts and delicate jewelry. It therefore includes a correspondingly wide range of skills,...

 industries, such as crucible

Crucible

A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...

s, saggars, retort

Retort

In a chemistry laboratory, a retort is a glassware 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...

s, and glassware

Glassware

This list of glassware includes drinking vessels , tableware, such as dishes, and flatware used to set a table for eating a meal, general glass items such as vases, and glasses used in the catering industry whether made of glass or plastics such as polystyrene and...

. Because of its stability during firing in the kiln

Kiln

A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...

, it can be used to make complex items of pottery such as pipes and sanitary ware.

Chemical composition

Fire clay chemical compositions have a range of 23-34wt% aluminium oxide

Aluminium oxide

Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...

 (Al₂O₃), 50-60wt% silicon oxide

Silicon oxide

Silicon oxide may refer to either of the following:*Silicon dioxide, SiO2, very well characterized*Silicon monoxide, SiO, not very well characterized...

 (SiO₂), and 9-12wt% L.O.I. (Loss On Ignition) as volatiles

Volatiles

In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...

, mostly H₂O (water vapor

Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

) released from hydrated minerals

Mineral hydration

Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....

 upon "ignition" (heating). Fire clays usually contain Fe₂O₃ (iron(III) oxide

Iron(III) oxide

Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...

) (maximum 25wt%), along with Calcium Oxide

Calcium oxide

Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

 (CaO), Magnesium Oxide

Magnesium oxide

Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...

 (MgO), Potassium Oxide

Potassium oxide

Potassium oxide is an ionic compound of potassium and oxygen. This pale yellow solid, the simplest oxide of potassium, is a rarely encountered, highly reactive compound...

 (K₂O), Sodium Oxide

Sodium oxide

Sodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....

 (Na₂O), and Titanium Dioxide

Titanium dioxide

Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...

 (TiO₂).

Extraction

Unlike conventional brick

Brick

A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

-making clay, some fire clays (especially flint clays) are mined

Mining

Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

 at depth, found as a seatearth

Seatearth

Seatearth is a British coal mining term, which is used in the geological literature. As noted by Jackson, a seatearth is the layer of sedimentary rock underlying a coal seam. Seatearths have also been called seat earth, "seat rock", or "seat stone" in the geologic literature. Depending on its...

, the underclay associated with coal

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

measures.