A furnace for smelting iron ores, such as haematite (Fe₂O₃) or magnetite (Fe₃O₄), to make pig iron.

The furnace is a tall refractory-lined cylindrical structure that is charged at the top with the dressed ore, coke, and a flux, usually limestone.

The conversion of the iron oxides to metallic iron is a reduction process in which carbon monoxide and hydrogen are the reducing agents. The overall reaction can be summarized thus
Fe₂O₃ + 2CO₂ + 2H₂ ^__> 3F_e +2CO₂ + 2H₂O.

The CO is obtained within the furnace by blasting the coke with hot air from a ring of tuyeres about two-thirds of the way down the furnace. The reaction producing the CO is 2C + O₂ ^__> 2CO.

In most blast furnaces hydrocarbons (oil, gas, tar, etc.) are added to the blast to provide a source of hydrogen. In the modern direct-reduction process the CO and H₂ may be produced separately so that the reduction process can proceed at a lower temperature. The pig iron is produced by a blast furnace contains about 4% carbon and further refining is usually required to produce steel or cast iron.