fire clay, clay that has a high degree of resistance to heat. By the best standards it should have a fusion point higher than 1,600°C. The term "fire clay" is commonly held to exclude kaolin and other refractory potter's clays. Fire clay should contain high percentages of silica and alumina, with as little as possible of such impurities as lime, magnesia, soda, and potash, which lower the fusion point of the clay. Fire clay often forms the bed layer of earth under seams of coal. Two types are recognized—flint clay, exceedingly hard, nonplastic, and resembling flint in appearance, occurring in the United States; and plastic fire clay. The principal uses of fire clay are in the manufacture of firebrick and of various accessory utensils, such as crucibles, saggers, retorts, and glass pots, used in the metalworking industries.