vt the scoria of a volcano.
2. slag ·vt the dross, or recrement, of a metal; also, vitrified cinders.
3. slag ·add. ·vi & ·vt to form, or form into, a slag; to agglomerate when heated below the fusion point.
4. slag ·add. ·noun 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.