Slag (?), n. [Sw. slagg, or LG. slacke, whence G. schlacke; originally, perhaps, the splinters struck off from the metal by hammering. See Slay, v. t.] 1. The dross, or recrement, of a metal; also, vitrified cinders.
2. The scoria of a volcano.
Slag furnace, orSlag hearth(Metal.), a furnace, or hearth, for extracting lead from slags or poor ore. — Slag wool, mineral wool. See under Mineral.