FIRECLAY
Clays that will withstand high temperatures without melting or cracking have been used for lining furnaces, flues, and for making firebricks and lining tiles. Common fireclays are usually silicate of alumina. Theoretically these clays contain 45.87% alumina and 54.13 silica, but in general they contain considerable iron oxide, lime, and other impurities. Most U.S. clays are from New Jersey, Kentucky, Pennsylvania, Ohio, and Missouri. They are largely Al2O3 ■ SiO2, with CaO, Fe2O3, and TiO2. Those low in iron oxide, lime, and magnesia, and alkalies are chosen. The clays are grouped as low-duty, intermediate-duty, high-duty, and super-duty The low-duty has low alumina and silica with high impurities and is limited to a temperature of 1600°F (871°C). Standard types are good for temperatures of 2400 to 2700°F (1316 to 1482°C), and the super-duty to temperatures of 2700 to 3000°F (1482 to 1649°C). Kiln-burned clay should have a balanced proportion of coarse, intermediate, and fine grain sizes. Clays with an excess of silica are also used. The German Klingenberg clay used for crucibles has about 60% silica. The term refractory clay embraces nearly all clays having a melting point above 320°F (160°C). But the clays alone are likely to shrink and crack, and they may be mixed with other clays, sand, or graphite. Firebrick is made in various shapes and sizes and is usually white or buff color. Common firebrick from natural clays will melt from 2800 to 3100°F (1538 to 1704°C).
Insulating firebrick is made with fireclay and a combustible material, such as sawdust, which burns out to leave a porous structure. The weight is 1.25 to 4 lb (0.57 to 1.81 kg) per brick compared with 8 lb (3.6 kg) for regular firebrick. Firebrick containing more than 47.5% alumina is not classified as fireclay brick but as alumina brick. SL firebrick of Manville Corp. is a kaolin firebrick with 62% silica, and it can be used up to 2300°F (1260°C). Spalling is a common failure of fireclay brick, but it can be reduced under long soaking at 1650°F (899°C) or alternating periods of heating and cooling at higher temperatures. Alamo brick and Varnon brick, of Harbison-Walker Refractories Co., are high-duty firebrick. Kaosil firebrick, of the same company, is designated as a semisilica firebrick. It is made from low-alkali siliceous kaolin of Pennsylvania, rotary-fired at high temperature. The nominal composition is 75.6% silica, 21.8 alumina, 1.7 titania, 0.5 iron oxide, 0.27 magnesia, 0.15 lime, and 0.10 alkalies. The brick can be used in a soaking heat of 2700°F (1482°C), is resistant to spalling and to fluxing by alkali slags, and has high load-carrying ability. Korundal, of the same company, is a corundum-mullite brick for temperatures to 3425°F (1880°C). It contains 91% corundum alumina, 8 silica, and less than 1 iron oxide, lime, magnesia, and alkalies. It melts at 3668°F (2020°C), converting all the mullite to corundum, but slow cooling returns the brick to the original mixture.
Some other materials used in making firebrick are chromite, bauxite, diatomaceous earth, and magnesite, or the artificial materials silicon carbide and aluminum oxide, but brick made of these is designated by the name of the material or by trade names. Chromite brick will withstand temperatures up to 3700°F (2038°C), and magnesia brick up to 3900°F (2149°C), while silicon-carbide brick without a clay binder will withstand heats to 4000°F (2204°C). Firecrete, of Manville Corp., is a lightweight refractory consisting of calcined high-alumina clay used for furnace doors and floors. It will withstand temperatures of 2400°F (1316°C) in continuous operation. Insuline, of Quigley Co., is a calcined fireclay in small cellular particles. In insulating brick it is called Insulbrix, and as a lightweight concrete it is known as Insulcrete. Allmul firebrick, of Babcock & Wilcox Co., for glass furnaces, is mullite with no free silica.

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