Adam Clarke on Proverbs 17:3

The fining pot is for silver - When silver is mixed, or suspected to be mixed, with base metal, it must be subjected to such a test as the cupel to purify it. And gold also must be purified by the action of the fire. So God tries hearts. He sends afflictions which penetrate the soul, and give a man to see his state, so that he may apply to the spirit of judgment and the spirit of burning, to destroy what cannot stand the fire, to separate and burn up all the dross.