Albert Barnes Commentary Numbers 31:22

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

Brass - Render copper. See Genesis 4:22 note. The verse is noteworthy, as it illustrates the variety of metals in use at this early date for domestic purposes. All these metals were common in Egypt centuries before the date of the Exodus.