You shall purify every garment, every article of skin, all work of goats’ hair, and every article of wood.”
Only the gold, and the silver, the brass, the iron, the tin, and the lead. These are excepted, and, though unclean, are not to be purified by washing, but in another way, by fire, as follows; all these metals were well known in those early times, and were made use of for instruments and vessels of various sorts.
Num 31:19-20 Purification of the Warriors, the Prisoners, and the Booty. - Moses commanded the men of war to remain for seven days outside the camp of the congregation, to carry out upon the third and seventh day the legal purification of such persons and things as had been rendered unclean through contact with dead bodies.
Commenting on Numbers 31:19-20