The impurities of silver separated from the one in the process of melting (Proverbs 25:4; 26:23; Psalm 119:119). It is also used to denote the base metal itself, probably before it is smelted, in Isaiah 1:22, 25.

Noah Webster's New International Dictionary of the English Language

1. (n.) The scum or refuse matter which is thrown off, or falls from, metals in smelting the ore, or in the process of melting; recrement.

2. (n.) Rust of metals.

3. (n.) Waste matter; any worthless matter separated from the better part; leavings; dregs; refuse.