Chapter V: Front Matter (5)

2.--_Report of the Select Committee (of the Senate of the
United States,) to whom was referred "the petition of
upwards of three hundred mechanics, Citizens of the City and
County of Philadelphia, employed in the various branches of
the manufacture of iron," and also, the petition of the
"Journeymen blacksmiths of the City and County of
Philadelphia, employed in manufacturing anchors and chain
cables."_

3.--_Report of the minority of the Select Committee on
certain memorials to reduce the duty on imported iron._

4.--_Remarks of the majority of the Select Committee on the
blacksmiths' petition in reply to the arguments of the
minority._

5.--_Manuel de la Metallurgie de fer par_ C. I. B. KARSTEN,
_traduit de l'Allemand, par_ F. I. CULMAN, _seconde edition,
entierement refondue, &c._ 3 vols. 8vo. pp. 504, 496, & 488.
Mme. Thirl: 1830: Metz.

6.--_Voyage Metallurgique en Angleterre, par_ MM. DUFRENOY
_et_ ELIE DE BEAUMONT. 1 vol. 8vo. pp. 572. Bachelier:
Paris: 1827.

The discussion contained in the petitions and legislative reports which we have prefixed to this article, is one of the most powerful interest, not merely to those concerned in the manufacture of iron, and the articles of commerce of which it is the material, but to the whole community. Iron, if the cheapest and most abundant, is intrinsically the most valuable of the metals. It may supersede, and gradually has, in its applications, superseded the greater part of the rest, and has taken the place of wood and stone in a great variety of mechanical structures; it is indispensable in the modern arts of the attack and defence of nations; and its possession is the distinctive difference between civilized man and the savage. Well was it said to Croesus exhibiting his golden treasures, that he who possessed more iron, would speedily make himself master of them, and the truth of the maxim was even more powerfully verified, when the accumulated riches of the Aztecs and Incas were acquired at the cost of a few pounds of Toledo steel.

When we compare the state of manners and arts of the Mexicans and Peruvians with that of their Spanish conquerors, we are almost compelled to admit, that the possession of iron was perhaps the only real superiority in civilization which the latter possessed. Gunpowder played but a small part in the contests where handfuls of men routed myriads; the courage of the Indian warrior is not less firm than that of the descendant of the Goths.

The sciences and arts which are now the boast of European civilization, were then but awakening from a slumber of ages; in the latter, the workmanship of Europe was in many instances inferior to that of the new world, and in the former, to take as an instance that which occupies the highest place, astronomy, the civil year of the Mexicans was intercalated and restored to the solar, by a process more perfect than that we even now employ; and the latter was not introduced into Europe until half a century after the throne of Montezuma fell. The bloody human sacrifices which excited to such a degree the abhorrence of the conquerors, were not greater marks of savage cruelty, than were their own _auto da fes_, and the tortures inflicted on Guatemozin. Yet if not superior in bravery, in the arts, the sciences, and the more distinctive attribute of civilization, humanity, the possession of iron was sufficient to ensure the triumph of the Spaniards.

Of all the metallurgic arts, that by which iron is prepared from its ores, demands the greatest degree of practical skill, and is the most difficult to bring to perfection. Although ages have elapsed since it first became an object of human industry, its manipulation and preparation are yet receiving improvements, while those of the other ancient metals appear hardly susceptible of modification or advancement. Copper and its alloys, tin, lead, and mercury, were as well and as cheaply prepared by the ancients as by the moderns; and the reduction of the precious metals has received no important change, since the process of amalgamation was first applied to them,--while the preparation of iron is daily improving under our eyes, and its cost diminishing. It may even be doubted whether the iron we first find mentioned in history, was an artificial product, and not obtained from the rare masses in which it is found existing in the native state, and which are supposed to be of meteoric origin.

The original use of iron is ascribed in the sacred writings to Tubal Cain, who lived before the flood;--but we have no proof that he did not employ a native iron of this description. Be this as it may, the united testimony of antiquity exhibits to us an alloy of copper used for the purposes to which we apply iron, and the latter metal as comparatively scarce, and of high value. The qualities of iron were known and appreciated, but the art of preparing it was not understood. The reason is obvious; those ores of iron which have an external metallic aspect, are difficult of fusion and reduction, those which are more readily converted, are dull, earthy in their appearance, and unlikely to attract attention,--while gold and silver manifest in their native state their brilliant characters, and the ores of copper and lead exhibit a higher degree of lustre than the metals themselves.

If, then, history does not show us the ancient nations employing iron for their arms and instruments, it is because they were unable to prepare it. Even in the middle ages, we find copper in use for arms, because the nations that employed it, could not conquer the difficulties that attend the preparation of iron.

The books of Moses, however, show that iron was known at that era to the Egyptians, and the distinction he draws between it and brass, seems in favour of our view of the origin of that which was then employed. The stones of the promised land were to be iron, but brass was to be dug from the hills. Twelve hundred years before Christ, if we receive the testimony of Homer, who, if he be rejected as an historian, must still be admitted as a faithful painter of manners. The Greeks used an alloy of copper for their arms, but were unacquainted with iron, which they estimated of much higher value.

Autar Peleides thechen solon autochoonon,
hon prin men riptaske mega sthenos Eetionos.
Alla etoi ton epephne podarchos dios Achilleus,
Ton d aget enneessi sun alloisin chteatessin.
Ste d orthos chai muthon en Argeioisin eeipen.
Ornusth, hoi chai toutou aethlou peiresesthe!
&c. Iliad, Book XXIII, 1. 826.

From this passage and the following lines, we learn the two-fold fact: 1. That a mass of iron of no greater weight than could be used as a quoit, by a man of great strength, was esteemed of sufficient value to be cited as an important article in the spoil of a prince: 2. That its use was confined to agricultural purposes, and not applied in war. Hence the more valuable form steel, and its tempering, were unknown.

Five hundred years later, Lycurgus attempted to introduce the use of iron, as money, into Sparta. The reasons usually cited for this act, do not seem to apply; and we ought not to accuse that lawgiver of the want of knowledge in political economy that is usually ascribed to him, in endeavouring to give a base material a conventional value to which it was not entitled. The iron was still, probably, more costly than brass, and the error of Lycurgus did not lie in ascribing to it a value beyond its actual cost, but in depriving it of the property of convertibility to useful purposes, which was necessary to maintain its price.

In the construction of the temple by Solomon, 130 years before the aera of Lycurgus, iron was employed in great abundance; and, from the cost lavished upon that building, we are almost warranted in considering it as still bearing a high value, even in that country, so far in the advance of Greece in the arts of civilized life.

Herodotus ascribes the discovery of the art of welding iron to Glaucus of Chio, 430 years before the Christian aera. But, before this period, the Greeks had carried the art of working it into Italy, Spain, and Africa; and the famous mines of Elba, that are still worked, were probably opened 700 years before Christ.

It is from the working of these mines that we are to date the introduction of iron in such abundance as to reduce its price, bring it into general use, and finally cause it to supersede wholly the alloys of copper. This ore is of extremely easy reduction, by processes of great simplicity, which furnish iron of excellent quality, and are, as we shall hereafter see, still in use. We cannot, indeed, infer with certainty, that these were the processes used by the ancients; but their simplicity is a strong argument in favour of their remote invention.

Steel seems to have been known as different in qualities from iron, at a very remote period; that is to say, it was understood that there were varieties of iron, which when tempered, became hard, whilst others remained soft. The intentional preparation of it, as a different species, seems to have taken its rise among the Chalybes, a people of Asia Minor, and it was afterwards obtained from Noricum. We still find in the latter country, (Styria,) an ore that furnishes steel, by processes as simple as those by which the iron is obtained from the ore of Elba, and hence can form some tolerable guess at the mode in which the steel of the ancients was obtained.

The third form in which we find iron as an article of commerce, namely, cast iron, is of far more recent origin. It has been traced to the banks of the Rhine, and it is certain that stove-plates were cast in Alsace in A. D. 1494. From this epoch, then, dates the great improvement in the preparation of iron, by which its price has been so far lessened, as to render it available for innumerable purposes, from which a small addition to its present cost would exclude it.

* * * * *

Iron, as may be inferred from what has been stated, is known in commerce in three distinct forms--wrought or bar iron, cast or pig iron, and steel. The received chemical theory on this subject is, that the former is metallic iron nearly in a pure state, and that the two latter are chemical compounds of iron and carbon. How far this is true will be examined in the sequel.

When wrought iron is nearly pure, it has, when in bars of not less than an inch square, or plates not less than half an inch in thickness, a granular structure. From the appearance of these grains, an estimate may be had of its quality; grains without any determinate form, neither presenting, when broken, crystalline faces, nor arranging themselves in plates; and which, in the fracture of the bar, exhibit points, and even filaments, manifesting the resistance they have opposed, are marks of the best quality. If, when broken, a crystalline character is exhibited, the quality is bad, and will, according to a disposition difficult to describe in words, either break under the hammer when heated, or be subject to rupture when cold. These two opposite defects are, in the language of our manufacturers, called red and cold short, or shear. The former fault unfits it for being easily worked; the latter destroys its most important usefulness. When the manufacture has been badly conducted, crystals will appear mingled with tenacious grains, and a want of uniform consistence will render it unfit for being cut and worked by the file. Iron of the latter character may, notwithstanding, possess great tenacity.

In still smaller bars, good iron, in breaking, exhibits filaments like those shown by a piece of green wood when broken across; this is technically called nerve; and as it does not show itself in larger bars, it has been supposed that it is the result of the process of drawing out the bars. This is partially true, although the iron that presents a crystalline structure will not acquire nerve, however frequently hammered. To obtain nerve in larger masses, it is necessary to form them of bundles of smaller bars, a process known under the name of faggoting.

* * * * *

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.

Arsenic produces a great waste in the manufacture of iron, and when alloyed with it, injures or destroys its capability of being welded.

Ores which contain titanium, according to universal experience in this country, give an iron inclining to the defect of red short, but possessing the highest degree of tenacity. Such are several of the ores of the northern part of New-Jersey, and of Orange County, New-York.

Manganese in small quantities renders iron harder, but injures none of its good qualities. Many of our ores contain manganese, but when carefully manufactured the iron appears to contain but an insensible trace of this _metal_.

Nickel unites with iron in all proportions, and gives a soft and tenacious alloy; no good property of the iron appears to be injured by it. United with steel it gives an alloy of excellent quality. Nickel is rare among the ores of iron that are not of meteoric origin. But native malleable iron is occasionally found in large masses alloyed with this metal, and its extrinsic source has been fully ascertained. The masses are sometimes of very great size; we have already expressed our opinion that the iron that first came into use was derived from this source, and had been employed for ages before the processes for preparing it from its more abundant ores were discovered.

Cast iron is distinguished into two varieties, which are obviously distinct in character, the grey and the white; a mixture of the two forms that which is called mottled. It is generally believed, and usually stated in the books, that both of these are combinations of iron with carbon, and that their difference in appearance and quality grows out of the difference in the proportions in which the two substances exist; that the grey iron contains the greatest dose of carbon, and the white the least. There is, as will be seen, good reason to question the latter part of this statement.

The grey iron requires the greatest degree of heat for its fusion, is more fluid when melted, is softest, best fitted for castings which require to be turned or filed, and for those that must be thin; the white iron is very hard and brittle; the greatest degree of strength and tenacity is due to the mixture, or mottled iron, and to that variety of mottled in which the grey rather predominates.

The different varieties are readily convertible, for the grey iron when melted and suddenly cooled becomes white, when cooled more slowly is mottled, and when carefully preserved from rapid loss of heat, retains its colour. On the other hand, experiments on a small scale have shown, that white cast iron, subjected to a heat equal to that at which the grey melts, and allowed to cool slowly, becomes grey. Hence their difference can hardly be ascribed to chemical constitution. Neither can the presence of a greater or less quantity of oxygen, as is sometimes supposed, produce the difference, for under circumstances in all other respects similar, except the rate at which they are cooled, iron of the three different varieties may be produced, We therefore feel warranted in rejecting the usual theory, particularly as the reception of it has rather impeded than advanced the manufacture of iron.

The theory of Karsten is far more consistent with the facts, and is directly applicable to the practical purposes of the iron master. We shall endeavour to give a succinct exposition of this theory, introducing all that is necessary for its full explanation.

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.

* * * * *

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_.

The charge of charcoal being determined upon such principles, it is added by measure, and always in equal quantities, while the proportion of ore and flux is made to vary, not only by a gradual increase at the beginning of the operation, but according to the working of the furnace. The manner in which the furnace is working can be inferred, even before its products are ascertained, by the appearance of the flame at the trundle-head, and at the tymp, by the manner in which the charge descends, and more surely still, by the appearance of the scoriae. By a strict attention to these circumstances the proportion of the charge of ore may be regulated. A fortnight usually elapses from the time of the first charge until it reaches a regular state of working, and variations will occur even after that period, in consequence of the greater or less moisture of the combustible and minerals, the continual wearing away of the sides of the furnace, the variations in the state of the atmosphere, and in the play of the blowing machines, the greater or less attention of the workmen, and numerous other accidental circumstances.

The mode of proceeding when coke is the fuel employed, rests upon the same principles, but the dimensions of furnace that are best suited to the different combustibles are different. As a general principle, the height of furnaces must depend upon the force of the blast and the density of the fuel. If the fuel be dense, and the blowing machine weak, the furnace must not have a great height; and even if the blast can be made strong, too high a furnace is disadvantageous for light charcoal. Coke, on the other hand, may be used in furnaces of greater height than any species of charcoal, provided the blast be of sufficient power. So long as the imperfect bellows were used in blowing, the height of the furnace was limited wholly by their action. More powerful apparatus in the form of cylinders, analogous in form and arrangement to those of steam-engines, and like them, either single or double acting, have now been introduced; the intensity of the blast is in them only limited by the moving power, which is applied to them, and when this is the steam engine, it may be said, that no limit can arise from the want of blast. We may, therefore, at the present day, regulate the height of furnaces by the nature of the fuel that is consumed in them.

The greater part of the furnaces in our country still retain the ancient and imperfect form of bellows, hence their height is restricted to the limits of from eighteen to twenty-four feet, and rarely or never reaches thirty. But when the apparatus is such as to supply a proper quantity of air, it has been found that even with light and porous charcoal, such as is given by white pine, the height ought not to be less than thirty feet, and when hard woods are used should be as great as thirty-six feet. Furnaces of even forty feet have been found to answer an excellent purpose, where the charcoal was prepared from oak. When coke is used, furnaces have been made as high as fifty, or even as seventy feet; but experience in England has shown, that from forty-five to forty-eight feet is the proper limit. This height is not at present exceeded in that country, even when the furnace has the greatest dimensions in other respects, and has been found efficacious, even when the vast quantity of eighteen tons has been furnished daily by a single furnace.

The force of the blast will depend upon the nature of the fuel, the volume of air, the quantity of mixed material the furnace holds; and thus furnaces in which coke is used, will require the most powerful blast, whether we have regard to the volume or the intensity. The latter may be measured by a column of mercury adapted in a syphon tube to the air pipes, exactly as the gauge is adapted to the pipes of the steam engine.

The reduction and liquefaction of the metal take place progressively, as the charges descend in the furnace. The separation of the oxygen is due to the presence of carbonaceous matter at high temperatures, begins at the surface of the pieces of ore, and proceeds gradually inwards; the earthy parts of the ore, of the fuel employed, and the flux, unite and melt; they are thus separated, and being sooner fused than the metal, make their way through the charcoal, and descend first to the hearth. The reduced metal, continuing in contact with the burning carbon, acquires a greater or less portion of that substance, becomes fusible, melts, and follows the liquified earths. Dropping into the hearth that already contains the liquid vitrified earths, it passes by its superior gravity to the bottom, and is protected by them from the blast. Even at the bottom of the hearth, the heat is sufficient to retain the carbureted metal in a liquid state, and this is permitted gradually to accumulate, until it rises nearly to the level of the dam.

It now becomes necessary to withdraw or _cast_ the metal. This is done by forcing a way through a channel left beneath the dam in the masonry of the hearth, and closed with clay; the inner portion of this is baked hard, and requires to be broken through with a steel point. As soon as the passage is opened, the metal runs out, and is received in a long trench formed in the sand floor of the moulding house, to which are adapted a number of less trenches, at right angles, each containing about one hundred weight of metal. The metal in the longer trench is also broken into pieces of the same size, and the ingots thus formed are called _pigs_, whence the term for this variety, _pig iron_.

From one to three days will elapse from the time of the first charge until the furnace can be tapped, and pigs cast. From that time the casting succeeds with tolerable regularity, according to the working of the furnace, and at intervals depending upon the volume of the charge, and the capacity of the hearth.

It appears probable that the fusion of the iron is effected always by a direct chemical union of that metal with carbon, in the proportion of two atoms of the former to one of the latter. This constitutes, as we have seen, the white variety of pig iron. But as it continues, generally speaking, in the furnace, long after its fusion takes place, it acquires a temperature higher than its proper melting point, and a tendency to separation takes place, the iron retaining in combination no more of the carbon than is necessary to maintain it in a fluid state at the increased temperature. Thus the grey variety of pig iron is formed; and on casting it, the carbon, in a form similar to that of plumbago, is disseminated throughout the mass, or forms on its surface the efflorescence that is called kish, and which is always a sign of a high quality in the iron it accompanies.

In conformity with this theory, we find that a high temperature in the furnace always produces grey cast iron; and that a low temperature, from whatever cause it may arise, renders the iron more or less inclining to white. So also if the metal be not exposed to the heat for a sufficient length of time, it becomes white.

Karsten classes these several causes of whiteness in the product, in the following order:--

"In conformity with the observations that have hitherto been
made, white cast iron is obtained:

"1. By the use of ores that are too easily fusible, or which
is the same thing, by an excess of flux, by a want of
density in the charcoal, and by too strong a blast, even
when the working of the furnace is regular.

"2. By a surcharge of ore, which deranges the action of the
furnace, and produces impure cinder, containing uncombined
iron.

"3. By boshes of too rapid a slope, and a blast of too great
a velocity; and this may occur even where the cinder is
pure.

"4. By too low a temperature, even when the cinder is pure,
and the furnace works regularly.

"5. By a derangement in the action of the furnace, arising
not from a surcharge of ore, but from an irregularity in the
descent of the charge.

"6. By the substances contained in the body of the furnace
exercising too great a pressure upon those beneath; the heat
in this case, concentrated in the hearth, cannot reach the
boshes, and the upper part of the furnace; the working may
be regular, the cinder and flame may in this case give no
sign of derangement.

"7. By too great a breadth in the furnace.

"8. When coke is used, it may arise from too great a
quantity of ashes, or of fossil charcoal, (anthracite,)
being contained in it. The presence of these will keep down
the heat of the furnace. An excess of ashes may be remedied,
by using the ore and flux in proper proportions to fuse
them, but a diminution in the charge must be made; the
cinder becomes viscid, and likely to obstruct the descent of
the charges.

"9. By an accidental cooling, arising from humidity, and
other similar causes."