Every thing that may abide the fire, ye shall make it go through the fire, and it shall be clean: nevertheless it shall be purified with the water of separation: and all that abideth not the fire ye shall make go through the water. (Numbers 31:23)
Water is not an adequate cleanser of metals, but fire is. The various utensils taken from the Midianites were unclean by reason of contact with dead bodies, and also because they had been used by the heathen.