Lead is produced mainly from its sulphide, known as galena. The smelting of this ore is complicated by the presence of arsenic, copper, iron, zinc, or silver, and like the smelting of copper sulphides, the process is one of desulphurization. The blast furnace, usually of rectangular cross-section, is now generally used for smelting lead, though if the ore is rich in sulphur, it is first roasted in a reverberatory furnace to drive off sulphur and oxidize some of the ore. A flux of iron oxide and limestone assist in reducing the ore, and the impure metal is drawn off through an opening in the base of the furnace leading outward and upward from the bottom of the hearth.