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26                         METALLURGY   OF   CAST   IRON.
facture of cast iron and worked to an economical advantage generally contains from 50 to 65 per cent of iron, and it is rare that ore of sufficient quantity to keep a furnace going steadily on a fair uniform product can be obtained containing more than 70 per cent of iron.
The pi^iron which the founder uses (barring ferro-silicon, etc.) generally contains from 92 to 96 per cent of metallic iron, with 4 to 8 per cent of impurities, chiefly carbon, silicon, manganese, sulphur, and phosphorus. These impurities, while called such, are really the elements which make iron of any practical value in the various industries. According to changes in the proportions of these so-called impurities, we are given the different grades of pig iron so essential to meet varying conditions called for in our widely diversified use of iron.
Silica ranges in ores from a trace to 20 per cent, and often higher. The ores generally used for ordinary pig metals contain from 3 to 8 per cent of silica. Next to the iron in the ore, silica is the largest constituent in nearly all ores used. The combined silica in the ores, fuel, and flux gives the silicon to the iron. Where high or ferro-silicon iron is desired, high silicious ores are used in connection with a greater amount of fuel and higher temperature in the furnace. With like fuels, ores, and fluxes the higher the temperature in a furnace, the higher silicon will be found in the iron. The higher the temperature desired, the more fuel it is necessary to use. Furnaces may work so cold by reduction of fuel, or bad working, as to cause the greater part of the silica to be carried off with the slag, instead of its making silicon in the iron. a soft coke that would'                ,jj