'Bronze' in the Bible

"I myself will go before you, and he will make the mountains level; I'll shatter bronze doors and cut through iron bars.

Because I knew that you are obstinate, and because your neck is an iron sinew, and your forehead is bronze,

"Instead of bronze, I'll bring gold, and instead of iron, I'll bring silver; instead of wood, bronze, and instead of stones, iron. I'll appoint peace as your supervisor and righteousness as your taskmaster.