Iron contains in its ores many impurities of different natures, according to circumstances, and is in its preparation exposed to several others; by these its quality is frequently much affected. Its valuable ores all contain the iron in the state of oxide. The oxygen, it is generally believed, is not wholly separated even in the best malleable iron, but enough still remains to impair in some degree its good qualities. In its manufacture it is exposed to the action of carbon, with which it is capable of combining. Much iron appears to contain some of the combinations of this sort, existing in the form of hard particles, technically known by the name of pins.
Of inflammable bodies, sulphur and phosphorus are frequently contained in the ores of iron; and when pit coal is used in the manufacture, the former substance is present, and may influence the product. The union of sulphur, in very small quantities, with the iron, creates the defect called red short, although it is probably not the only substance that produces the same fault; but when it is caused by sulphur, all the good properties of the iron are impaired, which is not always the case when it arises from other impurities. The defect of breaking when cold, has been attributed to the presence of phosphorus by high authority. There are, however, ores in this country, containing a phosphate of lime, which yield iron of excellent quality.
A mixture of sulphur and carbon deprives iron of its property of welding, and in the highest proportion gives the opposite defects of being both red and cold short.
Ores of iron contain the earths, silex, alumina, lime, and magnesia. With the bases of these earths the metal is capable of forming alloys; those of the three first are often thus combined. Silicium has been discovered combined with iron to the extent of 3-1/2 per cent. It has been found to render this metal harder, more brittle, and more similar in structure to steel; so small a quantity as 1/2 per cent. has been sufficient to render it liable to break when cold; and it appears probable, that by far the greater part of the cold short irons owe this fault to the presence of silex, rather than to that of phosphorus. Iron obtained from the ores by means of coal, is, under circumstances of equality in other respects, more likely to be combined with silicium than when made with charcoal. Karsten infers that a combination with aluminum produces similar defects, and denies the assertion of Faraday, that the good qualities of a steel brought from India are due to an alloy with this earthy base. A combination with the metallic base of lime, lessens the property that iron possesses of being welded, but does not render it more liable to fracture, either under the hammer or when cold.
Of the metals proper:—
Copper renders iron red short.
Lead combines with iron with great difficulty, so that its presence in the ores can hardly be considered dangerous, but when the combination is formed, the iron is both liable to break when red-hot and when cold.
A very small quantity of tin destroys the strength of iron in a great degree when cold, but still leaves it fit to be forged.
Wrought iron does not appear to unite with zinc, but its presence in the ores is injurious to the manufacture, for a reason that will be hereafter stated.
Antimony renders iron cold short, the alloy is harder and more fusible, and approaches in character to cast iron.