Gold frequently has silver with it in varying amounts. How these were separated in Bible times is not known, but a distinction in the methods of treating the two seems to be noted in Proverbs 17:3 and 27:21: “The refining pot is for silver and the furnace for gold.” Nitric acid was evidently not discovered until the ninth century C.E.; so, previously gold was purified by other means. For example, if lead was present with the gold, the impurities could be fluxed off as a slag while the gold would be held by the lead. Then by slowly boiling off the lead (an operation known as cupelling) pure gold would remain behind. This process requires considerable skill, for if the temperature is too high or the boiling off too rapid, the gold is carried away with the lead. The operator learns to judge and control the refining by the color of the molten metal. (Compare Ps 12:6; Jer 6:28-30; Eze 22:18-22.) The use of lye in the refining of silver is alluded to in the Scriptures.​—Mal 3:2, 3.

Jehovah himself is referred to as a refiner. His Word is highly refined. (2Sa 22:31; Ps 18:30; 119:140; Pr 30:5) This tried and tested Word is one means by which God purifies his people in removing all sinful dross of uncleanness. (Ps 17:3; 26:2; 105:19; Da 12:9, 10; Mal 3:3)