Numbers 31:22-24

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.

24On the seventh day you must wash your clothes and be purified. Then you may return to the camp.”