Much earthy material--gangue--containing silica and silicates is always found with iron ores. These are infusible, and something must be added to render them fusible. CaO forms with SiO2 just the flux needed. See page 132. Ca0 + Si02 = ? Which of these is the basic, and which the acidic compound? CaO results from heating CaCO3; hence the latter is employed instead of the former. In what case would Si02 be used as the flux? Into the blast furnace are put, in alternate layers, the fuel, the flux, and the ore. The fire, once kindled, is kept burning for months or years. Hot air is driven in through the tuyeres (tweers). O unites with C of the fuel, forming CO2 and CO. The C also reduces the ore. Fe2O3 + 3 C = ? CO accomplishes the same thing. 3 CO + Fe2O3 = ? The intense heat fuses CaO and SiO2 to a silicate which, with other impurities, forms a slag; this, rising to the surface of the molten mass, is drawn off. The iron is melted, falls in drops to the bottom, and is drawn off into sand molds. See Figure 47. This is pig-iron. It contains as impurities, C, Si, S, P, Mn, etc. If too much S or P is present in an ore, it is worthless. This is why the abundant mineral FeS2 cannot be used as a source of iron. From the top of the furnace N, CO, CO2, H2O, etc., escape. These gases are used to heat the air which is forced through the tuyeres, and to make steam in boilers. 267. Steel.--The manufacture of steel and wrought-iron consists in removing most of the impurities from pig-iron. It will be seen that the most common compounds of C, S, Si, and P, are their oxides, and these are for the most part gases. Hence these elements are removed by oxidation. Bessemer steel is prepared by melting pig-iron and blowing hot air through it. A converter (Fig. 48) lined with siliceous sand, and holding several tons, is partially filled with the molten metal; blasts of hot air are driven into it, and the C and other impurities, together with a little of the Fe, are oxidized. The exact moment when the process has gone far enough, and most of the impurities have been removed, is indicated by the appearance of the escaping flame. It usually takes from five to ten minutes. The blast is then stopped, and the metal has about the composition of wrought-iron; it contains some uncombined O. A white pig-iron (spiegeleisen), which contains a known quantity of C and of Mn, is at once added. Mn removes part of the extra O, and, though it remains, does not injure the metal. The C is "dissolved" by the Fe, which is then run into molds (ingots). This process, the Bessemer, invented in 1856, has revolutionized steel manufacture. No less than ten tons of iron have been converted into steel, in five minutes, in a single converter.