Numbers 31:21-23

21Then Eleazar the priest said to the men who were in the battle, “The Lord has given Moses this legal requirement:

22Anything made of gold, silver, bronze, iron, tin, or lead—

23that is, all metals that do not burn—must be passed through fire in order to be made ceremonially pure. These metal objects must then be further purified with the water of purification. But everything that burns must be purified by the water alone.