CERAMICS

the art or process of making useful and ornamental articles from clay by shaping and then hardening them by firing at high temperatures. Ceramics under this definition are generally known as pottery (q.v.). More broadly, ceramics denotes the manufacture of any product that is made from a nonmetallic mineral by firing at high temperatures. Industrial ceramics can thus be said to comprise all industrially used solid materials that are neither metallic nor organic. Chief among these are whitewares such as earthenware, porcelain, and fine china; structural clay products such as brick, tile, and terra-cotta; refractories; cement; and certain abrasives. The properties that make ceramics so useful are their mechanical strength in spite of brittleness; their chemical durability (at both normal and high temperatures) against the effects of oxygen, water, and various chemicals; their hardness; their nonconductivity of heat and electricity; and their ability to be decorated. Among the many structural products made from naturally occurring clay materials are common and face bricks, terra-cotta, sewer pipe, drain and hollow tile, glazed architectural tile, and roofing tile. The general term whitewares includes a wide range of products, such as art pottery and stoneware; dinnerware, including fine china and earthenware; and such industrially useful substances as sanitary plumbing ware, wall and floor tile, chemical porcelain, and electrical porcelain. Ceramics are highly useful as refractories, i.e., materials that can retain their chemical and physical identity and mechanical functions even at extremely high temperatures. Refractory ceramicsmade from natural materials such as clay, silica, chromite, or magnesite or from synthetic materials such as zirconia, alumina, or silicon carbideare widely used in metallurgy to line blast furnaces, converters, smelters, and ore roasters, as well as in boilers and in kilns that are used to make ceramics. Alumina and silicon carbide are also classified as abrasives, as are boron carbide, boron nitride, and such naturally occurring materials as diamond and corundum. Among the cements, the most important by far is portland cement, a roasted mixture of lime, silica, and alumina. Portland cement is the basic ingredient of concrete.