'Iron smelting' in the Bible

You, however, the Lord has selected and brought from Egypt, that iron-smelting furnace, to be his special people as you are today.

After all, they are your people and your special possession whom you brought out of Egypt, from the middle of the iron-smelting furnace.

Those are the terms that I charged your ancestors to keep when I brought them out of Egypt, that place which was like an iron-smelting furnace. I said at that time, "Obey me and carry out the terms of the agreement exactly as I commanded you. If you do, you will be my people and I will be your God.