Part 12

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."

Among the last may be reckoned the presence of zinc in the ore. This metal, although volatile, is not separated at the temperature given in the process of roasting, nor does it sublime in the upper and cooler parts of the furnace. But, as the ore descends, it passes into the state of vapour, and requires for its conversion, great quantities of heat that becomes latent. It hence cools the lower part of the furnace far more rapidly than even wet coal, or moist ores. The cooling thus caused, may not be effected until the melted metal reach the hearth, and may there cause it to become solid. Thus the solid mass called a salamander, may, in some cases, be formed; and thus may be explained the fact, that ores of iron that contain the more easily fusible metal zinc, are more liable to interrupt the action of the furnace in this manner, than others. The volatilized zinc rises to the upper part of the furnace, where the heat is often insufficient to retain it in the state of vapour, and is then deposited on the sides. In this position, it will also disturb the action of the furnace.

Coke being more dense than charcoal, will, in its combustion, furnish a more intense heat;--hence it is hardly possible to obtain by a charcoal fire, iron of as deep a colour as may be procured by the use of the former fuel. It will also resist the pressure of far greater weights than charcoal, and hence the proportion of ore may be much greater when it is used; containing more and less fusible earthy matters than charcoal, it requires a greater quantity of flux.

In the manufacture of cast iron then, coke gives iron better suited for small castings, for those which require turning or filing, and yields a far greater quantity from a furnace. Hence arises the very great superiority which Great Britain has, until recently, possessed over most other countries, in those fabrics in which these qualities are valuable; and hence it has been found until lately, in this country, hardly possible to manufacture fine machinery that requires workmanship after it is cast, without the aid of the higher qualities of Scotch iron, which, in these qualities, exceeds even the English. Recently, however, iron fully equal to the best Scotch, but like it wanting in tenacity, has been manufactured at the Bennington furnace in Vermont:--so also at the Greenwood furnace in Orange county, N. Y., and at West Point, iron approaching to the Scotch in softness, but very superior in strength, has been produced. In these cases, the height of the furnace has been carried up to the limits we have before laid down, and powerful blowing cylinders substituted for the ancient bellows.

When the pig iron is to be used for re-casting, every effort ought to be used to obtain it of the deepest possible colour. This, as may be seen from what has been already stated, will be effected by keeping the furnace at the highest possible temperature, and exposing the metal to it a sufficient length of time. In effecting this, however, certain defects may arise:--thus a longer exposure to a high heat, will cause the reduction of other oxides that may be present, as of manganese and the metallic bases of the earths; and the iron in becoming more soft, and approaching in fact more nearly to the form of the pure metal, will combine and form alloys with these bases. In this way, it will, as has been stated, become cold short; and to this may be attributed the want of strength in the greater part, if not all, of the British iron. The use of coke as a fuel, tends to increase this defect, in consequence of the great quantity of earthy matter it contains.

When the ores are pure, cast iron manufactured by charcoal, is not liable to such a fault. Hence the cast iron of Sweden and the United States, manufactured from the magnetic iron, or, in some cases in this country, from rich haematites, has very superior tenacity, insomuch that these two nations have alone been able to use this material in the construction of field pieces. When white iron is obtained from a furnace, it may have two different qualities. The first arises from a mere defect of heat, where all other circumstances are favourable, and the ore is completely reduced. The second arises when the reduction is not complete, and the separation of the earths and other oxides has not been fully effected. Of all the varieties of cast iron, this latter is by far the worst. It is indeed more easily converted into wrought iron than the other species, but the product is always of very inferior quality; it is rarely or never produced by furnaces fed with charcoal, but may be obtained by accident or design in those where coke is used, by a surcharge of ore, or by too great a proportion of flux, and sometimes cannot be avoided in warm and moist weather, where the air is rarefied and charged with vapour.

The grey iron obtained by the use of each of the different kinds of fuel, has its own peculiar advantages; that made with coke possessing, as a general rule, when melted, a higher degree of fluidity which adapts it for more delicate castings; being softer and better suited for fitting; while that manufactured with charcoal, possesses a greater degree of strength. One solitary instance has been quoted, in which a manufacturer of great intelligence has obtained by the use of charcoal, from a very pure ore, a union of both these valuable properties, and another, in which iron as soft as that made with coke, has been produced by means of charcoal.

In spite of this apparent balance in the properties of the two fuels, the introduction of coke into the art of reducing iron has been attended with the most important advantages. These lie in the superior economy of the process, and in the enormous quantity of the product. The manufacture of iron by charcoal is limited, by the growth of the forests, which replace themselves only at distant periods, by the large space they occupy, and the consequent labour of transportation; by the cost of cutting the wood and preparing the coal; and finally, even when the fuel can be obtained in abundance, and at small cost, the burden of the furnace, and the heat obtained in a given space are less than when coke is used, and the quantity of metal yielded is in consequence comparatively small. The coke furnaces of Great Britain, have therefore supplied cast iron in such abundance and at such diminished prices as to have brought it into use for a great variety of purposes, to which, until recently, it was hardly considered applicable.

In England, as in other countries, charcoal was the only fuel at first used; and after bloomeries had been in vogue for centuries, the blast furnace was introduced from the shores of the Rhine. For many years the growth of the forests proved sufficient to supply the demand, but at length the increase of population caused them to be encroached upon by cultivation; the growth of the manufacture was first prevented, and finally, almost extinguished.

The method by charcoal appears to have reached its acme of prosperity, at the close of the reign of the First James, when the furnaces of the kingdom yielded 180,000 tons of pig iron. About this period, Dudley first proposed the use of pit coal; but the time had not yet arrived in which it was absolutely necessary to seek for a new process, in consequence of the failure of the old one.

In 1745, or in the course of one hundred and thirty years, the forests had been so far encroached upon, that the product of the furnaces had fallen to 17,000 tons per annum, and in 1788, the quantity made with charcoal had dwindled as low as 13,000 tons. At this epoch, coke was introduced into blast furnaces, and in eight years the whole quantity produced by both methods had mounted up to 150,000 tons, or increased more than tenfold.

At nearly the lowest ebb of the British manufacture, the art of preparing iron was introduced into her then provinces, the present United States; and in 1737 it was attempted to obtain permission to introduce the product into England. The attempt failed, and in 1750 an act was passed to protect the exportation of English iron to America, and to prevent the establishment of forges. Had the other policy prevailed, England would probably have seen her manufacture of iron transferred to the United States, and with great immediate advantage both to herself and her then most valuable colony; but she would probably have seen herself at the present day degraded from her high stand in the scale of nations, to the secondary place in which the extent of her territory would keep her, were it not for the superiority of her manufacturing industry, of which iron is the basis. The quantity of iron now produced in England, exceeds that furnished by the rest of the world united, and does not fall short of 800,000 tons. It has a value even in its raw state of near four millions sterling, and is of far greater intrinsic worth, in consequence of the spur which its abundance gives to every other branch of industry.

Bar iron is at the present day principally manufactured from the pig. The process originally used for this purpose is called refining. The fire in which it is performed is a forge, similar in form and character to that employed in blooming. In blooming, the iron must be reduced, combines with carbon, and is subsequently decarbureted; while in the refining, the latter part of the operation alone remains. In this last process, while the carbon is burning away, the metallic bases of the earths are then oxidated, combine with oxide of iron, and form a vitreous substance. Hence, when it is carefully conducted, by far the greater part of the impurities contained in the cast iron may be removed. Refined iron, if made from ore of equal purity, is not inferior in tenacity to bloomed, and is superior in other respects, being more homogeneous, free from pins, and more easily treated by the smith. As a general rule, it is also less costly, that is to say, the same quantity of charcoal and workmanship will furnish a greater quantity of refined iron. It requires, however, a much greater capital, and the labour of transporting the coal from the greater distances which the increased consumption of a single blast furnace and several refineries will demand, may swell the cost of that article. A bloomery fire does not require more than 2000 acres of woodland, while a blast furnace will use the charcoal of 5000. Thus it happens, that it may be more advantageous to spread a number of bloomeries over a given district of country, than to unite a blast furnace and an equal number of refineries in a single place. The celebrated iron of Sweden and Russia is refined, and our country furnishes iron prepared in the same manner not inferior in quality. The principle objection to the process is the great expense of the fuel employed, in the successive heats to which the iron must be exposed in drawing it into bars, after the processes of conversion and the separation of impurities have been effected.

As charcoal became scarce in England, it was attempted to employ coke in lieu of it, in the refineries. This, however, constantly failed, in consequence of the great intensity of the heat, by which the pig was melted suddenly instead of being exposed to the blast, long enough to burn away the carbon. Reverberatory furnaces were next tried, and with partial success, but a combined process has finally been introduced which has been successful and which is called, from a part of the operation, the method of _puddling_.

The manufacture of wrought iron, by means of bituminous coal, is executed at three successive processes, and is facilitated by very great improvements in the machinery. Where hammers are still used, they are much increased in weight, and driven with greater velocity; but by far the greater part of the operation of drawing the bars is effected by means of rollers. The plan of these is in some measure borrowed from the slitting mill, in which bar iron is reduced into rods and thin rolls for various uses. These rollers are in sets, composed each of two of equal diameter, lying in a horizontal position, and placed one vertically above the other. Grooves corresponding to each other are cut in the two rollers, between which the heated iron is drawn by their revolution, and forced to assume a section that just fills up the two grooves. By passing in succession through grooves gradually decreasing in size, any form or magnitude may be given to the bars; and the operation is so rapid, that the bar may be drawn from the loup at a single heat.

The first operation to which the pig iron is subjected, consists in melting it in a fire called a finery, similar in form and character to the bloomeries and refineries of which we have spoken, but in which the fuel is coke. The melted metal is drawn off by tapping the furnace from beneath, and is cast into thin plates. In this way it assumes the characters of the white cast iron, which has been described as formed, when the reduction of the metal is complete, a form that cannot be given when the blast furnace in which it is made is supplied with coke. The rapidity of the cooling is increased, by throwing water on the surface of the plates. It thus appears, that this operation is adopted in order to bring the cast iron into a slate that it may often assume when manufactured by charcoal, and which cannot be given to it by coke. In conformity with this view of the subject, it has been found, that when wrought iron is manufactured by puddling, from American pig prepared by charcoal, this preliminary operation is unnecessary.