Adam Clarke on Proverbs 27:22

Though thou shouldest bray a fool - Leaving all other conjectures, of which commentators are full, I would propose, that this is a metaphor taken from pounding metallic ores in very large mortars, such as are still common in the East, in order that, when subjected to the action of the fire, the metal may be the more easily separated from the ore. However you may try, by precept or example, or both, to instruct a stupid man, your labor is lost; his foolishness cannot be separated from him. You may purge metals of all their dross; but you cannot purge the fool of his folly.