From The Collaborative International Dictionary of English v.0.48:
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.
[1913 Webster]
2. The scoria of a volcano.
[1913 Webster]
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.
[Webster 1913 Suppl.]
Slag furnace, or Slag hearth (Metal.), a furnace, or
hearth, for extracting lead from slags or poor ore.
From The Collaborative International Dictionary of English v.0.48:
Slag \Slag\, v. i. & t. [imp. & p. p. Slagged; p. pr. & vb. n.
Slagging.] (Metal.)
To form, or form into, a slag; to agglomerate when heated
below the fusion point.
[Webster 1913 Suppl.]