Vertical shaft furnace that produces liquid metals by the reaction of air introduced under pressure into the bottom of the furnace with a mixture of metallic ore, fuel, and flux fed into the top. Blast furnaces are used to produce pig iron from iron ore for subsequent processing into steel; they are also employed in processing lead, copper, and other metals. The current of pressurized air maintains rapid combustion. Blast furnaces were used in China as early as 200 BC, and appeared in Europe in the 13th century, replacing the bloomery process. Modern blast furnaces are 70–120 ft (20–35 m) high, have 20–45-ft (6–14-m) hearth diameters, use coke fuel, and can produce 1,000–10,000 tons (900–9,000 metric tons) of pig iron daily. See alsometallurgy, smelting.