Fire brick

A Fire brick, firebrick, or refractory brick is a block of refractory ceramic material used in lining furnaces and kilns.

A refractory brick is built primarily to withstand temperature. This does not usually accompany resistance to heat flow; in fact, most refractory bricks usually have the highest thermal conductivities. This is logical, as thermally-related fractures are caused by tensions within the mass of the material. In turn, these fractures are caused by different degrees of thermal expansion within the mass; these varying degrees are caused by different degrees of heat being absorbed by thermally non-conductive materials. Using a thermally conductive material negates the whole causality chain presented here by allowing the material to absorb heat uniformly, thus dilating uniformly and avoiding the internal tensions which lead to fracture. It is important for refractory brick to have a high resistance to erosion by ash-laden gases and to the fluxing action of molten slag; it should not spall badly under rapid temperature change, and its structural strength should hold up well under rapid temperature changes.

Fireclay is baked in the kiln until it is partly vitrified, and for special purposes may also be glazed. Fire-bricks usually contain 30-40% aluminium oxide or alumina and 50% silicon dioxide or silica. They can also be made of chamotte and other materials. For bricks of extreme refractory character, the aluminium oxide content can be as high as 50-80% (with correspondingly less silica), and silicon carbide may also be present. The standard size of fire-brick is 9 x 4.5 x 2.5 in. (228 mm x 115 mm x 64 mm)

The silica firebricks that line steel-making furnaces are used at temperatures up to 1650 °C (3000 °F), which would melt many other types of ceramic, and in fact part of the silica firebrick liquifies. HRSI, a material with the same composition, is used to make the insulating tiles of the space shuttle.

A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces.

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