This U.S. industry comprises establishments primarily engaged in manufacturing clay refractory, mortar, brick, block, tile, and fabricated clay refractories, such as melting pots. A refractory is a material that will retain its shape and chemical identity when subjected to high temperatures and is used in applications that require extreme resistance to heat, such as furnace linings. Cross-References. Establishments primarily engaged in manufacturing nonclay refractories are classified in U.S. Industry 327125, Nonclay Refractory Manufacturing.