This composite is an especially usual refractory material. Due to easy availability and low cost aluminia-silica composite is extensively utilized in industry today. From pure alumina to pure silica, the percentage composition might vary. Fireclay is created along with alumina varying from 20 to 40 percent. Among these limits of alumina and depending on degree of firing four classes of fireclay are acquired. They are medium duty, super duty, high duty and low duty. Fireclay is utilized in form of bricks. Pure silica brick is usually acid. Within composite of silica-alumina the acidic nature varies to neutral along with increasing proportion of silica. Acidic fireclay along ith high alumina content in between 50-99% has high fusion temperature than fireclay bricks. They are employed for more severe furnace condition. The acidic fireclay bricks are more expensive.
Basic refractories consist mostly of magnesia or MgO and lime or CaO, chrome ore or mixture of two or more of these materials. Basic bricks have superior density, high melting point temperature and fine resistance to chemical attach via basic slag and oxides. Such refractories, containing 92 to 95 percent magnesia are utilized as linings in the basic oxygen steel making furnace. Though, refractories are costlier.