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, but should also usually have a low thermal conductivity to save energy.

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)

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