Waste formed by melting metal.

Son of man, the house of Israel is to me become DROSS: all they are brass, and tin, and iron, and lead, in the midst of the furnace; they are even the DROSS of silver. Therefore thus saith the Lord GOD; Because ye are all become DROSS, behold, therefore I will gather you into the midst of Jerusalem. ( Ezekiel 22:18-19 )