Firebrick is a block of refractory ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high heat, but should also usually have a low thermal conductivity to save energy. Usually dense firebricks are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a wood-fired kiln or a furnace, which is subject to abrasion from wood, fluxing from ash or slag, and high temperatures.
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.
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 liquefies. A range of other materials find use as firebricks for lower temperature applications. Magnesium oxide is often used as a lining for furnaces. Common red clay brick are used for chimneys and wood-fired ovens.