Numbers 31:19-24

19And all of you who have killed anyone or touched a dead body must stay outside the camp for seven days. You must purify yourselves and your captives on the third and seventh days.

20Purify all your clothing, too, and everything made of leather, goat hair, or wood.”

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.

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