Read Numbers 31:23 Using Other Translations
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.
everything that can stand the fire, you shall pass through the fire, and it shall be clean. Nevertheless, it shall also be purified with the water for impurity. And whatever cannot stand the fire, you shall pass through the water.
that is, all metals that do not burn—must be passed through fire in order to be made ceremonially pure. These metal objects must then be further purified with the water of purification. But everything that burns must be purified by the water alone.