Scum

skum (chel'ah; Septuagint ios, "poison" or "verdigris"; compare Plato Rep. 609a): The word is only found in Eze 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 (Eze 24:12), would seem to point to the preferability of the translation "scum," the residue of dirt adhering to the caldron from previous use.

⇒See the definition of scum in the KJV Dictionary

⇒See also the McClintock and Strong Biblical Cyclopedia.