'Rust' in the Bible

"This is what the Lord GOD says: "How terrible it is for that blood-filled city, to the pot whose rust remains in it, whose rust won't come off. Empty it one piece at a time. Don't let a lot fall on it.

Make the pot stand empty on the coals until its bronze glows red, its rust can be scoured off, and its dross completely removed.

The pot wearies me, but its thick rust won't come off, even with fire.