The Lord identifies himself with a smelter of metals. Metals, such as silver, gold, iron, copper, are contained in ore. To extract the metal, heat is applied. By means of heat, the metal is separated from the ore, which leaves pure metal on the one hand and slag or dross on the other. The result is pure metal. The purpose of the refiner’s fire, then, is to purify metals.
When the Lord identifies himself as a refiner’s fire, he refers to his purpose of purifying his people, as will become evident in the next verse.
2 But who can endure the day of his coming, and who can stand when he appears? For he is like a refiner’s fire and like fullers’ soap.