To separate and purify metals, and the craftsman who does it. By repeated melting in clay refining pots called crucibles, the slag and impurities were removed from the desired metal. (Ps 12:6; Pr 17:3; 27:21) Remnants of slag dumps have been found in the region around ancient Succoth, where some of Solomon’s mining and smelting operations were located. Sometimes impurities were burned off; at other times refiner’s lye (see LAUNDRYMAN) was used to consolidate the scummy dross so it could be skimmed off the surface. (Isa 1:25; Mal 3:2) The refiner sat in front of his furnace and supplied the charcoal fire with a forced draft by means of bellows.​—Jer 6:29; Mal 3:3.

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.

If the copper-bearing ore was an oxide or a carbonate, mixing the crushed ore with charcoal and burning it freed the copper in the metallic state. However, if the copper ore was a sulfide, preliminary roasting was necessary first to burn off the sulfur as sulfur dioxide and at the same time convert the copper sulfide to copper oxide. Then it could be reduced with charcoal to obtain the free metal.

Extraction of iron was more difficult, because of the tremendous heat required. Iron melts at 1,535° C. (2,795° F.) The ancients, however, built smelting furnaces equipped with bellows to give a forced draft similar to present-day blast furnaces. (De 4:20; Jer 6:29; Eze 22:20-22) There are no details as to Hebrew iron furnaces, but they may have been similar to those known to have existed in ancient India. Made of clay, pear-shaped, about 1.2 m (4 ft) in diameter at the bottom, narrowing to 0.3 m (1 ft) at the top, they had goatskin bellows equipped with nozzles attached to clay tubes that supplied air to the bottom of the furnace. Charged with charcoal, the fire was started and the ore was added. Another layer of charcoal was then added on top, and the forced heat was continued for three or four hours. With the run completed, the front of the furnace was broken down to remove the bloom of metal.

It is a simple matter to win lead from its common ore, galena, that is, lead sulfide. First the ore is roasted with an injection of air to convert the lead sulfide to lead oxide; the sulfur combines with oxygen and forms the gas sulfur dioxide. The lead oxide is then mixed with charcoal and charged into a blast furnace; then the carbon dioxide is driven off, leaving the liquid lead behind in the crucible.

Figurative Use. 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) Fiery trials also refine the faithful. (Isa 48:10; Da 11:35; Zec 13:9; compare 1Pe 1:6, 7.) The wicked, on the other hand, are judged as nothing more than scummy dross, fit only for the worthless slag heap.​—Ps 119:119; Pr 25:4, 5; Eze 22:18-20.