Toward end of the Bronze Age, about 1500 BC, roasting was added to convert the more abundant sulfide ores of copper to the oxides, considerably increasing the copper supply. In the roasting process the iron sulfide is converted to oxide and then by reaction with silicates present to a slag which separates from the copper oxide in the reduction step. After the reduction or smelting step, copper can be refined by blowing air through the molten metal. If the refining is carried out at too high a temperature or the airblast is too strong, some of the copper is oxidized as well and the metal is made brittle by the oxide present. The production of copper from its ores today is still virtually the same as in the ancient world.
Modern refineries use an electrolytic step to effect further purification of the copper.
This step is the most important example of industrial electrolytic purification of metals. The electrolytic purification step is the anodic dissolution of the blister copper and simultaneous deposition of pure copper at the cathode. The electrolyte is a concentrated aqueous copper sulfate solution, the anode is a slab of blister copper, and the cathode is a thin sheet of pure copper. The reactive metal impurities dissolve with the copper but are not redeposited and remain in the solution. The purity of copper produced by this process is extremely high.