Part 11

The ores of iron, which are all oxides, are reduced by exposing them to the action of carbonaceous matter, at a high temperature. The carbon first separates the oxygen from the ore, which becomes metallic, but as it has for the carbon a high affinity, that substance tends to combine with it. The iron combined with carbon is rendered far more fusible than it is when pure, and thus readily melts; when the heat of the furnace is little more than is sufficient for effecting this fusion, the two substances are uniformly mixed, and probably form a compound analogous to a metallic alloy; this is the white cast iron. When the compound is exposed to a heat higher than is sufficient to melt it, a separation appears again to take place, the carbon tending to assume in part the form of plumbago, the iron to retain no more of carbon than is sufficient to keep it liquid at the new temperature, and thus passes from the state of cast iron to that of steel, and finally approaches to that of malleable iron. If the cooling take place slowly, the carbon, obeying its own law of crystallization, arranges itself in thin plates, and the iron, consolidating afterwards, fills up all the interstices with grains or imperfect crystals; and thus the mass assumes a dark grey colour, partly owing to the natural colour of the iron, but in a greater degree to the plumbago. When the cooling is rapid, the carbon still disseminated throughout the mass, does not crystallize separately, but the two substances again form an uniform compound.

Thus, according to the theory, there is no essential difference in the proportion of carbon between grey and white cast iron, but the former is a mechanical mixture of crystals of carbon, nearly pure, with iron containing a less proportion of carbon than the white, while the white iron is a homogeneous alloy of carbon and iron.

Upon this theory may be explained all the facts which have been found wholly irreconcilable with the other.

1. The more intense the heat of the furnace, the deeper the colour, and consequently the higher quality of the cast iron.

2. The changes that take place from grey to white cast iron, merely by difference in the rate of cooling.

3. The reconversion of the white variety into grey, by simply heating it above its melting temperature, and allowing it to cool gradually.

4. The formation of imperfect crystals of plumbago (_kish_) on the surface of grey iron.

5. The approach to malleability of the grey iron, which is utterly irreconcilable with its being a homogeneous compound, more charged with carbon than the white.

The basis of white cast iron, appears to be a definite chemical compound, of two atoms of iron to one of carbon, and is therefore analogous in its chemical constitution to carburet of hydrogen and carburet of sulphur, but like all metallic alloys it is capable of containing an excess of one of the substances in a state of mixture during fusion, and which does not separate on rapid cooling. The iron alone is found in excess in this substance.

Steel appears to contain but half the quantity of carbon in its chemical proportions that white cast iron does, but, like it, is susceptible of a variety of mixtures; if the proportion of carbon amount to three per cent., it loses the property of malleability, if the proportion fall as low as one per cent. it can no longer be tempered, and is identical with the harder varieties of bar-iron. As the carburets of iron, whether in the form of pig or of steel, may be considered as alloys, if they be presented to other metals, the results must necessarily be different from what occurs when pure iron is exposed to the same substance. The union that may take place in the one instance may not occur in the other. It may often happen, that when the iron is pure, a true chemical combination will occur, while in the other case, no more than a mechanical mixture can be effected. For the same reason, the consequence may be totally different when the third substance is presented to the iron when first deoxidated, in the presence merely of an excess of carbon, and when the combination with that substance has actually occurred.

If reduced at the same time with the iron, the other metals will unite with it more readily than with the carburet, and they may afterwards prevent its union with carbon, for there are few, if any metals, besides iron, which have any affinity for carbon.

Cast iron may contain the bases of the earths that form a part of its ores. Of these, silicium is the most usual, and there is probably no cast iron that does not contain a portion of it. It appears to render this form of the metal harder and less suitable for the purposes of the moulder, but is separated almost wholly when it is converted into wrought iron.

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.

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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 scoriae 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."

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 _a la Catalane_ and _a 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_.

As a carburet, or actual cast-iron, must be formed in these processes, and, as the separation of carbon at the bottom of a deep cylinder, and where the metal would probably be covered by a vitreous liquid, is difficult, the iron might sometimes resist the efforts made to render it malleable, and run from the furnace in a liquid form. It might therefore have readily occurred, that it would be less costly to finish the process in a forge. The _stuckoffen_ were therefore converted into _flossoffen_, or melting furnaces, whence the liquid carburet was withdrawn, and afterwards converted into bar iron. Such was probably the cause that led to the original discovery of cast iron, a discovery that cannot be traced further back than the end of the fifteenth century.

The uses of cast iron for purposes to which wrought iron is inapplicable, and the readiness with which it is fashioned, by pouring it into moulds, led to the increase of the size of the _flossoffen_, and in the power of the blowing apparatus, which has caused the introduction of the blast furnace. This forms the basis of the methods by which iron in all its forms is chiefly prepared at the present day, and is hence worthy of particular consideration.

The difference between the blast furnace proper, and the ancient fires from which it gradually took its rise, consists wholly in its superior height, and in the greater power of the blowing machines, by which its combustion is supplied with air.

This increase of height adds to the mass of the contained combustible,--additional air is therefore required for effecting its complete inflammation, and the joint effect is, that a much higher temperature is generated. By this, the earthy matters either contained in the ores, forming portions of the combustible, or added as _fluxes_, are rendered fusible at a less expense of oxide of iron; the carburet formed, becomes more fluid, and the product is more likely to assume the character of grey pig-iron.

Charcoal, as in the other processes, was the fuel originally employed, and is still principally used in most countries. But coal deprived of its volatile parts, and charred or converted into coke, has been substituted in some regions, as will hereafter be stated. Each of these combustibles requires a furnace of appropriate character, and demands a difference in the mode of management.

A blast-furnace is a hollow chamber enveloped, generally speaking, in a mass of masonry, of the form of a truncated pyramid. The chamber is composed essentially of three parts; the upper has the figure of a truncated cone, whose greatest base is lowest: this may be called the body of the furnace; the middle portion has also the figure of a truncated cone, whose greater base is uppermost, and is common to it and the upper portion: this contraction is called the _boshes_ of the furnace; the lower position is called the hearth, and is usually enclosed on three sides by walls of refractory substances, on the fourth it is bounded by two stones, one serving as a lintel, which is called the tymp, the other resting on the foundation, and known by the name of the _dam_. Such at least is the shape of the blast furnaces in common use, and which will suffice for our present purpose.

The blast is introduced into the hearth, at a small distance above the level of the upper edge of the dam, and is now generally performed by means of two _tuyeres_; in the more ancient furnaces, there was but one. The furnace being completely dried, a fire is lighted in the hearth, and fuel gradually added, until the whole is filled to the _trundle head_, which is the open and lesser base of the truncated cone that forms the body of the furnace. The blast may then be applied, slowly and gently at first, and increasing gradually, until it reach its maximum of intensity. As the blast proceeds, the charcoal gradually burns, and descends; its place is supplied at top by fresh fuel, by ore, and by the earthy matter used as a flux. This is styled _charging_ the furnaces. The earlier charges often contain no ore, but are wholly composed of charcoal and flux, and, in all cases, the proportion of ore and flux is at first small, and is gradually augmented. The charges are made as often as the mixed mass in the furnace descends sufficiently low to admit the quantity that is chosen as the proper amount. The charcoal is thrown in first, and the ore and flux are spread and mixed upon its surface. The principles which govern the amount of the charge, are as follows:--

"The volume of the charges depends upon the capacity of the furnace. If they be too large, they cool the upper part of the furnace, which will cause great inconveniences, particularly if zinc exist in the ore. On the other hand, small charges of charcoal will be cut or displaced by the ore, which will occasion a descent by sudden falls, in an oblique direction, or in a confused manner. It follows that the volume of the charge, although proportioned to the volume of the furnace, must be augmented: when the charcoal is light and susceptible of being displaced; and with the friability, the weight, and the shape of the fragments of the ore."

"The heat, considered in any given horizontal section of the furnace, will be intense in proportion to the thickness of the layer of charcoal that reaches it. It follows, that the fusible ore requires smaller charges of charcoal than one that is more refractory. If the beds of charcoal and mineral are too thick, the upper part of the furnace will not be sufficiently heated. Hence it is obvious, that there must be a maximum and minimum charge for every different dimension of furnace, and for every different species of ore and fuel." _Karsten_.