Most of the ores of iron require, before they are subjected to the process of reduction, a preparatory operation called roasting. This consists in exposing them to a comparatively low heat. The more important use of this process is to render the mass more susceptible of mechanical division, but it also serves in many cases to separate the sulphur and arsenic that may exist in the ore. There are some ores, as, for instance, those of a number of mines in Morris and Sussex counties, New-Jersey, which are so free from impurities, and which yield so readily to the mechanical means employed for separating them, that this process is wholly unnecessary; but such ores are rare, and the process of roasting must, generally speaking, be performed.
The mechanical division, which exposes a larger surface to the action of heat and of the chemical agents, is called stumping; this is usually performed by appropriate machinery, but was in the infancy of the art effected by hand.
The reduction of rich ores of iron, such as are almost wholly made up of its oxides, and contain but little earthy matter, may be performed in a common smith's forge. The reduction in this case takes place immediately in the blast of the bellows, where the intensely heated ore is in contact with the burning charcoal; and if a carburet be formed, it is immediately decomposed, and pure iron is the result. Such is probably the more ancient of all the processes for obtaining malleable iron, and it is still used to a certain extent even at the present day. The hearth in which the operation is at present performed, differs from the forge of a common smith only in its greater size, and in the increased power of its bellows. A cavity is prepared, in which a charcoal lire is lighted, and to which the nozzle or tuyere of the bellows is directed; ore in minute fragments is thrown upon the ignited fuel, fresh coal and ore are added from time to time, and the latter being reduced to the malleable state descends, as the charcoal burns away, to the bottom of the cavity. Here the successive portions, still kept hot by the fuel above them, agglutinate, and form a porous mass, containing in its cavities a black vitreous substance, which is composed of the earthy matter rendered fusible by the metallic oxide. This porous mass is called the Loup.
It would be unsafe to subject the loup immediately to the action of heavy hammers of iron. It is, therefore, after being withdrawn from the fire, beaten with wooden mallets, to bring its parts into closer contact, and press out the vitreous matter. While this is performed, it cools so much as to require to be again heated, which is done in the same fire. Indeed, the same forge is used in all the successive heats that the iron in this process requires.
After the loup has been again heated, it may be subjected to the hammer. This unquestionably was anciently one moved by hand; but now, in all manufactories of this character, a heavy mass of case hardened iron is employed for the purpose; this is lifted by machinery impelled by a water wheel, and permitted to fall upon the loup. The loup is again heated, and again beaten into an irregular octangular prism, called the cingle; this, after a third heat, is formed into a rectangular block, called a bloom; and the whole, or a proper proportion of this is drawn into a bar, at three successive heats; the middle being beaten out first, and the two ends in succession. Thus, in addition to the heat employed in the original reduction, the iron must be at least six times reheated before it becomes a finished marketable bar.
In this manner the ore of Elba is still manufactured in Catalonia and Tuscany, and there can be little doubt that it is identical with the original rude process, by which the iron of that most ancient of known mines was prepared to be an object of commerce. The processes in these two districts differ from each other in some minute particulars, and are known on the continent of Europe as the processes à la Catalane and à l'Italienne. This method is known in the United States by the name of blooming.
Bloomeries are frequent in the United States, being found in many parts of the primitive country, where the magnetic ore of iron is abundant. The iron manufactured by blooming is, generally speaking, remarkable for its nerve, being strong and tenacious in the highest degree, unless the ore be in fault. It is not, however, homogeneous, being liable to contain what are called pins, or grains that have the hardness and consistence of steel.
Blooming is comparatively an expensive process. It requires, indeed, little original capital, but the product in proportion to the capital employed is but small. It is wholly impracticable with poor ores, and demands a great length of time and expenditure of fuel, unless the ore be very fusible. Another objection to it is common to a process we shall hereafter describe, that of refining, and lies in the numerous successive heats, which the small extent of fire, and the slow process of hammering render necessary, before the bar is finished. It has been attempted in New-Jersey to lessen the expense attending these heats, by performing them in reverberatory furnaces. A saving of fuel to a small amount would probably thus be effected, but the number of heats would still remain the same. A more important and useful improvement has superseded the last; the process of rolling, which will be hereafter described, has been introduced, and by means of it a bar may be drawn out at a single heat, and at far less expense of manual labour. Such establishments exist at Dover and Rockaway, New-Jersey, which receive the iron completely reduced from the neighbouring forges, and fashion it into bars.
A forge fire, and, consequently, the process of blooming, is insufficient to convert poor ores, or those that contain much earthy matter, into iron. Treated in this way, those ores, if fusible at all, would become a mass of slag, as the earth would require, at the temperature of a forge fire, the whole, or the greater part of the metallic oxide for its fusion.
Iron being introduced, and its valuable applications known, it became necessary, in those countries that do not afford rich ores, to discover a method by which the poorer might be reduced. This could only be effected by giving such a degree of heat, as would render the earthy matter capable of melting, at a less expense of metal. To increase the mass of fuel, by increasing the depth of the cavity, and actually forming it of walls, thus enabling it to contain a greater quantity, would be obvious means of attaining this end. The ore must be added in smaller proportions, and, being longer in contact with the heated charcoal, would become carbureted; the carbon must therefore be finally burned away, before malleable iron could be attained. A rude but efficient process of this sort, is described by Gmelin as in use among the Tartars; an analogous method, whose use has been superseded by iron imported from Europe, was found among the nations of Guinea; and Mungo Park saw a more perfect application of the same principle at Camalia, on the Gambia. Furnaces of similar character, but more skilfully constructed, are still used in some parts of Germany, and are called stuckoffen.