The first step in making copper metal is mining the ore. In ancient times the ore was then crushed in grinding stones and heated at the bottom of a furnace in heat-resistant clay crucibles covered with charcoal. Using blow tubes, or tuyeres, air was forced into the furnace chamber so that temperatures reached 1083° Celsius (1981° F) and the ore was smelted. Smelting is the process by which ore turns into metal. A percentage of the ore is converted to metal; the remainder is converted to almost worthless slag (a waste product composed of impurities that are separate from the metal).
In 2005, I went to southern India to study a traditional metal workshop with similarities to the ancient process, which helped me to understand ancient metal production at Khirbat en-Nahas. Even the slag was crushed after smelting so that every minute piece of metal that had been trapped in the slag was collected, put in crucibles and re-melted in a furnace. The hot metal was then poured into simple open “molds”, about 80 centimeters by 15 centimeters (31.5 by 6 inches), etched by hand into the ground. The result was the production of metal ingots, strikingly similar in shape to Iron Age ingots, that could be stored for later re-melting and use in casting copper objects. It is likely that the process I observed in India is virtually identical to the activities carried out in the four-room workhouse at Khirbat en-Nahas thousands of years ago.