Webster’s Revised Unabridged Dictionary · 1913
Slag
Slag, n. (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, 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.