SLAG (2)

Slag (?), n. (Metal.) 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.