Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

slag
     n : the scum formed by oxidation at the surface of molten metals
         [syn: {scoria}, {dross}]
     [also: {slagging}, {slagged}]

slagging
     See {slag}