What is "slag hearth"

Slag \Slag\ (sl[a^]g), 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.

3. (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.

   Slag furnace, or Slag hearth (Metal.), a furnace, or hearth, for extracting lead from slags or poor ore.

   Slag wool, mineral wool. See under Mineral.