22 Gold, silver, bronze, iron, tin, and lead 23 - and anything else that can survive fire - must be passed through the fire; then it will be ritually purified. It must also be ritually washed in the Water-of-Cleansing. Further, whatever cannot survive fire must be put through that water.
Matthew Henry's Commentary on Numbers 31:22-23
Commentary on Numbers 31:19-24
(Read Numbers 31:19-24)
The Israelites had to purify themselves according to the law, and to abide without the camp seven days, though they had not contracted any moral guilt, the war being just and lawful, and commanded by God. Thus God would preserve in their minds a dread and detestation of shedding blood. The spoil had been used by Midianites, and being now come into the possession of Israelites, it was fit that it should be purified.