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.
The fine metal, obtained in the manner we have described, is next broken into pieces, and subjected to heat in a reverberatory furnace. A rapid heat is given at first to liquefy the iron, and is then diminished by means of dampers; the melted mass is violently stirred to expose it to the action of air and heat, by which the carbon is burnt away, and a part of the oxides of iron and the earthy bases combined and vitrified; as the carbon is separated, the metal gradually loses its liquidity, and finally dries, or assumes the consistence of sand: this shows that the carbon is separated, and the iron has assumed its malleable nature. The addition of water aids the oxidation of the several substances, and facilitates the process. The heat is again increased, and the metal collected under it, and rolled together into parcels suited to the action of the drawing machinery, and to the size of the bar that is to be made; these are pressed together, and a partial union takes place among their particles. When they have attained a white heat, they are withdrawn in succession. In some cases, where the number of puddling furnaces is great, they are immediately carried to the rollers and drawn down. But where quality is more regarded than quantity, they are first subjected to the action of the hammer, and finally rolled. The latter process has the advantage of separating more completely the vitrefied oxides, than can be done by rolling alone, but it will often require a second heat, which is given in a forge fire called the chaffery. When rollers are used alone, a minute and half is sufficient to form the bar; and a power of thirty houses will roll two hundred tons per week.
The iron in this state is still of very inferior quality, although its external appearance may be good. It is, notwithstanding, sometimes thrown into the market, and this has given rise to the impression that prevails in this country of the bad quality of English rolled iron. It may, however, be used in some cases, where it need not be fashioned by forging; thus, where it requires no more than to be cut into lengths, or where the original bars will answer the purpose, its cheapness may recommend it. Iron for rail-roads is of this quality; and the punching of holes, by which it may be fastened down, is effected by a simple addition of steel teeth, at proper distances, to the last groove through which it is passed. In this form, ready to lay down, rail-road iron may be shipped from England at the low price of 7l. 10s. sterling per ton; and a similar quality in the simple bar may probably be afforded at about 7l. We have never heard of its being sold so low as is stated in the evidence before the Committee of Congress, say 5l. 5s. There was, however, a period, when an excess of production, caused by a competition between the manufacturers of Wales and Staffordshire, entailed ruin on many of them, and their articles were sold far below the price of production. The price which we have stated is lower than that which has recently been paid in England for rail-road iron, and is that of some shipped from Liverpool, 1st March, 1831, when a considerable fall had taken place.
In order to render the iron which has undergone this process merchantable, it is subjected to the third of the operations which we have enumerated. For this purpose, the bars are made from three to four inches in breadth, and half an inch in thickness. These are cut into lengths, proportioned to the weight of the bar of finished iron that is to be made, and piled together by fours, in a reverberatory furnace, similar in character to the puddling furnace. Here they are exposed to a white heat, by which the four pieces of each pile are made to adhere; they are then withdrawn, and subjected to rollers similar to those used after the puddling process, but of more careful workmanship. The cost of finishing bar iron in this way, when the pig is made by the manufacturer himself, as ascertained upon the spot by Dufrênoy and de Beaumont, is, in Wales, 8l. 15s., in Staffordshire, 9l. 12s. The cost of making pig iron in Wales is 4l. 7s., or about half that of the finished bar iron, and in Staffordshire 5l 2s.
The iron prepared by the three processes of which we have spoken, although merchantable, and suited for various common purposes, is still far from good. We give the characters by which it is distinguished, from the work of Karsten:—
"The iron prepared in the English manner, appears dense and exempt from cracks and flaws. But this goodness is only apparent; the uniform pressure to which the bars are subjected at every point, masks their defects. If a piece of this kind be taken, that in its fracture appears dense and homogeneous, and it be heated in order to be drawn out under a common forge hammer, it dilates and exhibits numerous flaws, that sometimes increase to such a degree, that the bar will fall to pieces under the hammer. It is probable that the cause of this phenomenon is due to the scoriæ, which, in this mode of working, remain mixed in the mass."
The translator adds:—