Refractory bricks 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. Fire bricks usually contain 30-40% aluminium oxide or alumina and 50% silicon dioxide or silica.
Fireclay is baked in the kiln until it is partly vitrified, and for special purposes may also be glazed.