'Bronze' in the Bible

“I will go before you and make the rough places smooth;I will shatter the doors of bronze and cut through their iron bars.

“Because I know that you are obstinate,And your neck is an iron sinewAnd your forehead bronze,

“Instead of bronze I will bring gold,And instead of iron I will bring silver,And instead of wood, bronze,And instead of stones, iron.And I will make peace your administratorsAnd righteousness your overseers.