(1) An earthy, ceramic material of low thermal conductivity that is capable of withstanding extremely high temperature (1650–2200C) without essential change. There are three broad groups of these: (a) acidic (silica, fireclay), (b) basic (magnesite, dolomite), and (c) amphoteric (alumina, carbon, and silicon carbide). Their primary use is for lining steel furnaces, coke ovens, glass lehrs, and other continuous high-temperature applications. They are normally cast in the form of brick and are sometimes bonded to assure stability. The outstanding property of these materials is their ability to act as insulators. The most important are fireclay (aluminum silicates), silica, high alumina (70–80% Al2O3), mullite (clay-sand), magnesite (chiefly MgO), dolomite(CaO-MgO), forsterite (MgO-sand), carbon, chrome ore–magnesite, zirconia, and silicon carbide. (2) Characterizing the ability to withstand extremely high temperature, e.g., tungsten and tantalum are refractory metals, clay is a refractory earth, ceramics are refractory mixtures.
