The development of the basic oxygen steel-making furnace (BOF) in the late 1950s and early 1960s ushered in pitch-bonded doloma, doloma-magnesia, magnesia-doloma, and magnesia brick (the latter with purity levels from 70 to 95 wt% MgO) for construction of the vessel linings; magnesia and doloma gun mixes continued to be used as maintenance materials