(1):

(v. t.) The scoria of a volcano.

(2):

(v. t.) The dross, or recrement, of a metal; also, vitrified cinders.

(3):

(v. i. & t.) To form, or form into, a slag; to agglomerate when heated below the fusion point.

(4):

(n.) 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.