'Iron smelting' in the Bible

For the LORD took you and brought you out of the iron-smelting furnace out of Egypt to be the people of his inheritance, 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.