We have seen a parcel of pig iron that was marked with a species of white efflorescence, ascertained on examination to be silica; this was rejected for its hardness by the founder, but on being manufactured by the process of puddling, gave bar iron of good quality.
From what has just been stated, it appears that the other metals more generally exist in cast iron, in a state of alloy with pure iron, which is intimately mixed with the carburet. Thus as a general rule, the pig which contains them, will be more likely to be grey in colour than that which does not, but it may, notwithstanding, be injured in quality. The exact effect of such alloys upon cast iron, does not appear to have been fully examined.
The ores whence iron is obtained, are all oxides, with the exception of a carbonate whence steel is in a few places obtained directly. They contain, in combination with the iron, or forming parts of a heterogeneous aggregate, a variety of earthy substances. In the reduction of these ores, two objects are to be accomplished, the separation of the oxygen, and the fusion of the earthy mass. Carbon, in some one of its native or artificial forms, is used to effect the former purpose, upon the same principle that it is applied to the other metallic oxides. Thus a furnace in which a fire of carbonaceous matter is kept up and urged to the highest possible degree of intensity by blowing machines, is necessary. When the earths are pure, even the highest heat of furnaces is incapable of fusing them, and although the oxides of the ancient metals, and among the rest, the oxide of iron, increase the fusibility of one of the earths; still, if but one earth be present, it is only in a few cases that the simple ore will furnish the means of its own fusion. We are therefore compelled to make use of the property possessed by the earths, of rendering each other more fusible.
Silica is the earth to which we have referred, as being susceptible of fusion when mixed with the oxide of iron. Silica, also, when mixed with the other earths, renders them more fusible than is its own mixture with oxide of iron. Hence it may be stated as a general rule, that ores which do not contain silica, cannot be decomposed without the addition of that earth. The most of our American ores contain silex in sufficient abundance; hence it is usual to add to them, in the process of reduction, carbonate of lime, which is called flux. Did not the ore contain silica, this would not produce its effect, and a due admixture of the three earths, silica, alumina, and lime, appears to be necessary to cause the most advantageous results.
The remarks of Karsten on this head are new and worthy of attention.
"It is upon the choice and the just proportion of the flux, that the profit of the manufacturer in a great degree depends. Employed in too great quantities they fail in the important purpose of giving to the scoriæ a proper consistence. It is very difficult to fix their proportions exactly, and, in truth, these ought to vary with the manner in which the furnace works; but a proportion determined for a state of the furnace when the temperature is neither too high nor too low, is usually adopted.
"Chemists and metallurgists, have endeavoured to determine the degree of fusibility of the earths when mixed with each other; but their researches have shed but little light upon the management of blast furnaces. We are, in spite of them, still compelled to have recourse to experience. Far, however, be it from me to depreciate the attempts of Achurd, Bergman, Chaptal, Cramer, &c.; they are valuable at least, in pointing out the road that is to be pursued in the experiments.
"It follows, in general terms, from these experiments, that lime, silica, alumina, and magnesia, are infusible when not mixed with each other; that no mixture of earths is fusible without the presence of silica; that the fusion of the oxides of iron cannot take place by the addition of any simple earth other than silica; that ternary mixtures are more fusible than binary; that quaternary mixtures vitrify even more readily, and that the oxide of manganese promptly determines the liquefaction of all the earths.
"The theory of the vitrification of oxides, aided by trials on a small scale, points out the kind of earthy mixture which ought to be employed, but it cannot fix the exact proportion of the different earths that ought to be adopted; nor does it teach the means of replacing an earth by its chemical equivalent, as, for instance lime, by magnesia. The solution of the question will depend rather upon the properties of the silicates of lime and magnesia at high temperatures, than upon the action of these silicates upon iron. It is hardly probable that the iron obtained from all ores, could be equally good, even if the most proper fluxes could be added to these ores. Those who have maintained this opinion, have erroneously imagined that the reduction of the ore could always be effected under the same circumstances, which would not be the case, even if these fluxes were ascertained and made use of."