What is Fire Clay?

Mary McMahon
Mary McMahon

Fire clay is a type of clay which is used in the production of heat resistant clay items, such as the crucibles used in metals manufacturing. This type of clay is commonly mined from areas around coal mines, although other natural deposits are also available as potential sources, with many nations having deposits of clays suitable for use in high temperature applications. This clay can also be refined and treated to make it suitable for specialty applications.

Fire clay is commonly mined around coal mines.
Fire clay is commonly mined around coal mines.

This type of clay has a very high fusion and melting point. Once it is worked and fired, it will hold up to extremely high temperatures such as those found in kilns, furnaces, and retorts, in addition to the high temperatures of some production lines. Fire clay can also be used to create fire resistant chimney and flue liners, and fire resistant pads for safety, as seen when a hearth in front of a fireplace is made with fire clay to reduce the risk of fire.

This clay contains high percentages of alumina and silica, with minimal amounts of trace impurities. It tends to be pale to creamy yellow in color, due to the balance of minerals in the clay, although it can also be colored for various applications. Although fire clay will grow sooty with use, underneath the layer of soot, the clay will remain intact. Damage to the clay can occur when it sustains trauma or when temperatures climb outside its safety range.

Numerous companies make bricks and other products out of fire clay. They specify the tolerances of their products so that people know which settings they are appropriate for, and to help potential consumers make the best purchasing choices. These companies can also make custom products by request for people who need objects of a particular shape and size. For example, a metalworker might need a custom crucible for a project.

Like other clays, this clay is highly malleable in raw form. It can be molded, extruded, shaped by hand, and stamped. Various additives can be mixed in to make it more coarse, and it can be ground to be smoother. Slip and scraps for fire clay manufacturing can also be recycled, as long as they are not fired, and worked into new batches of clay for use. Fire clay also shrinks after it has been molded and during the firing process, which is something to be aware of when working with raw clay.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments

anon88294

Fireclay is a seatearth for coal, not necessarily "near" coal mines.

A seatearth is the layer of sedimentary rock underlying a coal seam.

anon55229

Fire clay is often found near natural coal deposits and one wonders why. Coal is, I think, fossilised trees and branches, etc., but why this should be associated with fire clay is an interesting question.

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