Slag

Vitreous substance composed of earthy or refuse matter that is separated from metals in the process of smelting. Slag is formed from the combination of silica with flux materials such as soda and potash, along with metal oxides and other impurities that float to the surface. In ceramic kilns, slag is formed by the reaction of the fluxing materials with the refractory lining. Objects made from ancient iron and copper alloys often contain small pieces of slag that were incompletely separated from the metal. Slag is a useful residue incorporated in the manufacture of concrete, mineral wool, slag brick, slag glass, portland cement, and cinder block.