'Rust' in the Bible

Therefore, this is what the Lord God says:Woe to the city of bloodshed,the pot that has rust inside it,and whose rust will not come off!Empty it piece by piece;lots should not be cast for its contents.

Set the empty pot on its coalsso that it becomes hot and its copper glows.Then its impurity will melt inside it;its rust will be consumed.

It has frustrated every effort;its thick rust will not come off.Into the fire with its rust!