Iron ores are always reduced in the blast furnace, the parts and operation of which have been described (Pars. 53 and 57. The process of smelting iron is very simple, as the ores used are oxides, uncombined with other metals, and merely mixed with gangue, which is removed by combining with the flux in the charge. The simplicity of the process consists in the fact that all of the chemical changes take place in one operation.

The important changes are as follows: The highly heated air forced in at the tuyeres at once spreads through the charge which fills the furnace. Coming in contact with the incandescent fuel, the oxygen of the blast and the carbon of the fuel unite, forming Co2. The Co2 meets more carbon as it rises and gives up part of its oxygen, forming CO. This is a powerful reducing agent and, with some complexity of reaction, it reduces the ore, and at the same time the flux combines with the gangue. Metallic iron and slag are thus formed, and as they sink into the fusion zone, both melt and run down.

In the intense heat of the fusion zone, some of the compounds of silicon, sulphur and manganese, and usually all the phosphorus compounds are decomposed, and these elements enter the molten iron. At the high heat of the modern blast furnace molten iron also dissolves some carbon, hence when the iron settles on the furnace hearth it has carried down with it small amounts of silicon, phosphorus, sulphur, manganese and carbon, which are always present in ore, flux, or fuel, but which have a great affinity for iron and remain with it when tapped from the furnace.