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.

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.