SCUM

skum (chel'ah; Septuagint ios, "poison" or "verdigris"; compare Plato Rep. 609a): The word is only found in Ezekiel 24:6,11-12, where the Revised Version (British and American) translates it "rust." The fact, however, that the caldron is of brass and therefore not liable to rust, and the astonishment expressed that the fire did not remove it (Ezekiel 24:12), would seem to point to the preferability of the translation "scum," the residue of dirt adhering to the caldron from previous use.

From the International Standard Bible Encyclopedia
Edited by James Orr, published in 1939 by Wm. B. Eerdmans Publishing Co.