firebrick, brick that can withstand high temperatures, used to line flues, stacks, furnaces, and fireplaces. In general, such bricks have high melting points that range from about 2,800°F (1.540°C) for fireclay to 4,000°F (2,200°C) for silicon carbide. They also should resist the chemicals in slags and not spall, i.e., flake under wide temperature changes. Clay bricks of fireclay or kaolin are common. Silica has good strength at high temperatures, but since it spalls, it is used in furnaces that remain hot continuously. High alumina bricks can stand high temperatures and high loads. To withstand alkalis there are magnesite bricks. Firebricks are not good insulators. A special insulating firebrick of highly porous fireclay or kaolin has low conductivity and low heat content but a melting point of only about 1,600°F to 2,800°F (870°C–1,540°C). It makes a thin, light wall that saves fuel because it heats quickly. A separate layer of insulating material backs up other kinds of firebrick. Refractory mortar binds firebricks together. Additional support comes from metal anchors projecting from the metal casing. Special coatings give bricks extra protection from slags and dust-laden gases. Plastic or castable mixes are used for repairs or special shapes.