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.]