Bricks are used for structural purposes in buildings, for paving, and for lining furnaces. The latter type of brick is known as refractory or fire-brick. Hard-burned brick should be used for face work exposed to the weather, and soft brick for filling, foundations, and the like. The standard brick measures approximately 2.25" x 4" x 8", and has a crushing strength of between 1000 and 3000 pounds per square inch depending on quality. A highly impervious and ornamental surface may be laid on brick either by salt glazing, in which salt is added during the burning process, or by the use of a "slip," which is a glaze material into which the bricks are dipped. Subsequent reheating in the kiln fuzes the slip into a glazed surface integral with the brick base.
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. 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. Fire-brick is baked in the kiln until it is partly vitrified, and for special purposes may also be glazed. Fire-bricks usually contain 30-40% alumina and 50% silica. For bricks of extreme refractory character, alumina 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".