John Gill Commentary Numbers 31:22

"howbeit the gold, and the silver, the brass, the iron, the tin, and the lead," — Numbers 31:22 (ASV)

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.