In the blast furnace limestone is the ratio of ore coke 8 :4: 1.At 673 -973 K, called the zone of reduction, spongy iron is formed after the reduction of Fe₂O₃ to Fe. At the zone of heat absorption, 1073 1273 K, CO₂ is reduced to CO, limestone decomposes and so formed lime reacts with impurity form the slag, CaSiO₃. At 1473 1573 K, coke burns to give CO₂ porous iron melts and if iron oxide, any is present gets reduced to Fe. Above 1573 K  molten  iron is at the bottom of the hearth. Slag floats over it. Waste gases called blast furnace gases get out from the outlet which are burnt and the heat obtained is used to heat the blast of air going into tuyers. Thus pig iron is oblained.