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Charcoal fuel contains no sulphur, and if the ore and flux are likewise free from it an iron will be obtained free of sulphur — something which cannot be said of coke or anthracite iron. Let charcoal iron be melted in an " air furnace " instead of a cupola, where the iron must be mixed with coke .or coal, and it can then clearly demonstrate its superiority over coke or anthracite iron. To melt charcoal in a cupola greatly impairs its superior qualities and brings it largely on a level with coke or anthracite iron. Coke or anthracite will often answer well for an approximation, but to obtain the very best mixture for chilled work, guns, etc., charcoal iron will ever remain the king metal of cast irons, when melted in an air furnace, unless modern advance arranges to eliminate sulphur, etc., from metal and " refine " the iron before it is cast into pigs in such a manner as to be relied upon, or while .being re-melted in the cupola. For analyses of charcoal iron, see pages 268, 269 and 299.
Refining iron means the lowering or removal of some impurities—carbon, silicon, and manganese being classed with them in this instance. The process, of course, increases the percentage of iron in the product but, for casting purposes, should not be carried too far. Unfortunately, sulphur and phosphorus will not go as readily as manganese and silicon, in fact, in the ordinary refining of a bed they will not go at all; hence the value of refining is to be looked for in the removal of the mechanically mixed slag, the lowering of the silicon and manganese, and, in some cases, the carbon contents, with the consequent increase in the combined carbon of the product and the closing up of the grain.