A product of smelting, containing, mostly as silicates, the substances not sought to be produced as matte or metal, and having a lower specific gravity than the latter; -- called also, esp. in iron smelting, cinder. The slag of iron blast furnaces is essentially silicate of calcium, magnesium, and aluminium; that of lead and copper smelting furnaces contains iron.
By-product of heating / melting iron and steel. Consists of oxides and other impurities that are not generally wanted. The process of forming a slag is essential to remove impurities at almost every stage of iron and steel production. A flux such as lime, limestone or fluorspar is added to the furnace and this attracts many of the impurities from the metal. Types of slag are currently used in road surfacing.