The first step in the conversion of the pig iron usually taken has been, and to a certain extent even is still, to remelt the metal in what is termed a finery furnace, a kind of forge in which a charcoal fire is urged by a cold blast, and so regulated that an excess of oxygen is supplied, or rather more than would suffice to convert all the carbon of the fuel into carbonic acid; although this is perhaps not absolutely necessary, as carbonic acid would itself supply oxygen by suffering reduction to carbonic oxide. At any rate the melted metal is exposed to an oxidizing atmosphere and constantly stirred. Many different arrangements of the furnace and details of the process have been used. For instance, where the finest quality of malleable iron was not aimed at, coke has been the fuel employed, and many shapes of furnaces, etc., have been contrived, and various additions of ores, oxides, etc., made to the charge, according to local practice and the nature of the crude iron. One marked effect of the operation is the final removal of nearly all the silicon, which is burnt or oxidized into silica, and this at once unites with oxide of iron, which is also formed, to produce a readily fusible slag of silicate of iron, and in the production of this silicate any sand attached to the pig will also take part. Much of the carbon, amounting sometimes to more than half, is also eliminated as carbonic oxide, and of what is left but little remains in the graphitic state. The action on the phosphorus is usually less marked, but there is always a notable reduction of the quantity. The sulphur is also lessened in some degree, although when coke is used, the fuel has the disadvantage of itself containing sulphur, phosphates, and other deleterious matters. Sometimes a little lime is added to the charge to take up the sulphur from the coke. The operation lasts some hours, the fused metal being frequently stirred with an iron rod, until it assumes a pasty granular condition, when the workman gradually collects it upon the end of the rod into a ball of about three-quarters of a cwt. in weight. These balls, or blooms as they are called, are removed from the furnace while still intensely hot, and at once submitted to powerful pressure by means of some suitable mechanical arrangement, the effect being to squeeze out the liquid slag and force the particles of metal together by which the whole becomes partially welded into a more compact mass. Then this mass is, while still hot, either hammered with gradually increased force of the strokes, or in the more modern practice, passed between iron rollers (these we shall presently describe), by which it is shaped into a bar. The bars are afterwards cut into lengths, reheated without contact of fuel, again hammered or re-rolled; and this process is several times repeated when the best product is required. During the first treatment of the blooms, and also in the subsequent hammering or rolling, the oxygen of the atmosphere acts on the surface of the glowing metal, so as to cover it with thin scales of oxide, and these, carried into the interior of the mass, will give up their oxygen to any residual silicon, carbon, etc., producing a little more slag, carbonic oxide, phosphate of iron, etc., which by the pressure of the hammers or rolls are ultimately forced out of the metal. It will be observed that in producing the pig iron the chemical action is the separation of oxygen from the metal, while conversely an oxidizing action is set up in the finery and subsequent treatment, in order to burn off the foreign ingredients. But this cannot be done without at the same time re-oxidizing some of the iron itself, of which therefore there is always a considerable loss, by its formation into slag (silicate), cinder, foundry scale (oxide), etc. The quantity of iron lost depends of course on many conditions, such as the care exercised in the operations, but it occurs in all the processes that have been devised for the conversion in question, even in the most modern: its amount may be taken to range between 10 and 20 per cent. The reader is requested to bear in mind the nature of the chemical actions that have just been described, for in even the most recently invented processes the principle is the same in nature and effect. So completely can the foreign elements be eliminated by this, or some analogous process, such as we shall presently mention, that the finest Swedish bar iron contains more than 99½ per cent. of the metal, and in some cases only a very little carbon and a mere trace of phosphorus remain, amounting together to less than 1 part in 2000. Such metal is made from very pure ore, containing no sulphur and scarcely any phosphorus, while charcoal is the fuel used in all the operations. As already mentioned, the objection to the use of coke is the sulphur, phosphates, and siliceous matters it contains. Toward the close of the eighteenth century an invention came into use which obviated the disadvantages of the cheaper fuel for converting crude iron. This was the puddling furnace, brought into use after much experimenting by Henry Cort in 1784. In it the pig iron is fused in a reverberatory furnace, the form of which will be understood from Fig. 19, which is a diagram showing such a furnace in section, where f is the fire, a an aperture at which the fuel is introduced, p the ash pit, b is a low wall of refractory material called the “bridge,” over which the flame passes, and is by the low arched roof reflected or reverberated downwards upon the charge, c, which is laid on a hearth, or iron floor, having spaces below it where air circulates in order to prevent it becoming too hot. In Cort’s original arrangement the bed of the hearth was formed of sand, which gave rise to much inconvenience by producing a quantity of the very fusible silicate of iron, that speedily attacked the masonry of the furnace, and therefore a very important improvement was devised some years later by S. B. Rogers, who made the bed of his furnace of a layer of oxide of iron, spread on a cast iron plate 1½ inches thick. In later times it has become usual to cover the iron hearth with certain other refractory mixtures varied according to circumstances, of oxide, ore, cinder, lime, etc. There is one of these mixtures significantly designated “bull-dog” by the workmen. We may mention here that it has, in more recent times, when very high temperatures are obtainable, been found unnecessary to cause even the flame to come into contact with the substances on the hearth, inasmuch as the heat radiated from the flame and the intensely heated roof of the furnace suffices, so that in consequence of this the roofs are now constructed nearly flat. In the puddling furnace the melted metal is constantly stirred, and no little skill is required to regulate the fire by the damper on the chimney, and to admit the proper amount of air to mix with the flame. The pig iron softens and melts gradually, until at length it becomes perfectly liquid, at which stage it swells up and appears to boil owing to the escape of carbonic oxide in numerous jets, which burn with the characteristic pale blue flame. The puddler then briskly stirs the mass to cause more complete oxidation of the carbon, silicon, etc., by bringing the superficially formed oxide of iron into the interior. As the iron loses its carbon, it assumes much the texture of porridge, consisting of pasty lumps of malleable iron implexed with the liquid slag (silicate of iron, etc.) which drips from the spongy balls as the puddler collects them at the end of his stirring rod, as in the finery operation. The next thing is to run the mass immediately between powerful rolls (puddling rolls) by which the slag is squeezed out, as before, and finally through the finishing rolls that shape it into bars or plates.

When a comparatively impure pig iron is used or when a better quality of malleable metal is desired, the crude iron is submitted to a preliminary treatment before puddling. This treatment, by a technical distinction, called refinery, is practically identical with the finery process already described, except that instead of being collected into blooms, the fluid metal is run out to form a layer 2 or 3 inches thick, and this, before becoming quite solid, is suddenly cooled by having water thrown over it, the result being a white, hard, brittle mass, which broken into pieces is ready for the puddling furnace.

The operation that has been described is known as hand puddling, in contradistinction to later methods in which it has been sought to substitute some form of machine that will produce the same result automatically, such as revolving furnaces, etc. It has been found difficult to maintain these in good working order, and in England at least mechanical puddling has never found much favour, but in the great iron works of Creusot, in France, large revolving furnaces were in use about 1880, which could turn out 20 tons of converted iron in 24 hours, whereas the old hand puddling furnaces could in the same period produce only 2½ or 3 tons, with two sets of men, the puddler and one assistant. Of these mechanical furnaces it is unnecessary to give any account, especially as the puddling process itself has nearly gone out of use, having been superseded by more economical methods.

The use of rolls for treating the product of the puddling furnace, and for making it into bars, was also an invention of Henry Cort’s, for which he obtained a patent in 1783. This was in many respects an immense improvement on the older system of hammering; it is still practised, and by it shapes can be given to the metal scarcely possible on the older system, while the tenacity of the metal is increased by the uniformity given to the grain. The difference of chemical composition between cast and wrought iron the reader has already been made acquainted with, and there is quite as great a difference in their textures. The former, when broken across, shows a distinctly crystalline structure, which we may compare to that of loaf-sugar, while the latter exhibits grain, not unlike that of a piece of wood. This fibrous structure depends upon the mechanical treatment of the iron, and in rolled bars the fibres always arrange themselves parallel to the length of the bar. Fig. 20 shows this fibrous structure in a piece of iron where a portion has been wrenched off. Like wood, wrought iron has much greater tenacity along the fibres than across them; that is, a much less force is required to tear the fibres asunder than to break them transversely. Consequently, to obtain the greatest advantage from the strength of wrought iron, the metal must be so applied that the chief force may act upon it in the direction of the fibres. Near the beginning of our article on Iron Bridges (q.v.) the reader will find some illustrations of the very different resisting powers of cast and wrought iron.

Nothing in the way of inventions can be compared to those of Cort’s as to the effect they have had in promoting the iron industry, until we reach a period some years after the middle of our century; but we must not neglect to recognize the scarcely inferior importance of Rogers’ improvement. Singularly enough, neither of these men reaped any benefit from his inventions. Cort died in the last year of the eighteenth century, quite a poor man, having been supported only by a niggardly pension of some £160 from the Government, and leaving his family in indigent circumstances. Yet a most eminent authority on iron questions (Sir W. Fairbairn) estimated—some time about the middle of our era—that the two inventions of Cort’s alone, the rolling-mill and the reverberatory puddling furnace, had by that time added to the wealth of Great Britain by an amount equivalent to six hundred million pounds sterling. For many iron-masters had profited by these inventions, amassing very great fortunes, in some instances also acquiring titles of honour. Clearly to Cort and Rogers may be applied the sic vos non vobis saying.

We shall now turn to the improvements that have been effected in the blast furnace, and of these none perhaps has been more marked than that made by Neilson, when in 1828 he substituted heated air for the ordinary cold air that had before always supplied the blast. It will be remembered that the heat is due to the combination of only the oxygen of the air with the carbon of the coke, but the greater part of the air—the four-fifths of nitrogen—take no part in the action, beyond abstracting a large proportion of the heat; but when the air is heated to a high temperature before entering the furnace, the cooling effect of the nitrogen is greatly obviated, and consequently a much higher temperature is obtained at the place of combustion, and the requisite intensity of heat is at once produced, which is most effective in completing the fusion and separation from each other of the slags and iron, and also in accomplishing the reduction of the oxide. But Neilson found that the net result of burning some fuel to heat the air before entering the furnace was a great economy of the total fuel required for smelting the ore. He had to encounter many difficulties in carrying his invention into practice; the iron ovens first used for heating the air were rapidly oxidized; and when thick cast iron pipes were substituted, these were liable to leak at the joints on account of the expansions and contractions caused by changes of temperature. Then the new invention had as usual to contend with established prejudices and misconceptions; but it soon came into use in Scotland, where it effected a great saving; inasmuch as it was found possible to use with the hot blast raw coal of a certain kind, plentiful in Scotland, because the heat retained by the ascending gases sufficed to convert the coal at the top of the charge into coke.

It will be remembered that the active agent in the reduction of the ore is the carbonic oxide gas formed by the incomplete combustion of the carbon of the fuel; or what comes to the same thing, the absorption by carbonic acid first produced of another proportion of carbon. The carbon oxide robs the iron oxide of its oxygen to become itself changed into carbonic acid. In reality however the action is more complex than this in its chemical relations; for instance, metallic iron will under certain circumstances act conversely on carbonic acid, and rob it of half its oxygen. The net result of the reactions between carbon, iron, iron oxide, and these gases depends mainly upon the temperature and pressure and upon the relative quantities of each substance present. In the gases escaping from the blast furnace there is always a large quantity (nearly one-third) of carbonic oxide. At the blast furnaces in work during the first half of our century the combustible gases were allowed to burn to waste as they issued from the top of the furnace, in the manner shown in Fig. 17, and at night the flames used to form a weird and striking feature in the prospect of an iron-smelting region.

Instead of allowing the escaping gases to burn to waste, it became the practice about 1860, and so continues, to draw them off and burn them under steam boilers or use their flames for heating the blast. An effective method of withdrawing the gases is shown in Fig. 21, which is a section through the upper part of a smelting furnace, with the “cup and cone” arrangement. The mouth of the furnace is covered by a shallow iron cone a, open at the bottom, into which fits another cone b, attached to a chain c, sustained by an arm of the lever d, which is firmly held in position by the chain e, and is also provided with a counterpoise f. When the mouth of the furnace is thus closed, the gases find an exit by the opening g, seen behind the cones, and leading into a downward passage, through which they are drawn by the draught of a tall chimney to the place where they are burnt. The charge for the furnace is filled into the hopper a, and at the proper time the chain, e, is slackened when the weight of the material resting on the suspended cone overcomes that of the counterpoise, and the charge slides down over the surface of the cone b, which is immediately drawn up again by the counterpoise, so that the opening into the air is at once closed.

The march of improvement in the blast furnace has been characterized particularly in Britain and the United States by a great increase of dimensions, which is found to promote economy in fuel, etc. In the former country the furnace of the latter part of our century is commonly from 70 to 80 feet high, and some have even been built with a height of more than 100 feet, while in the States the tendency to build very high furnaces is still more marked. A single large furnace may turn out as much as 1,500 tons of pig iron in a week, and some in America, it is said, actually produce as much as 2,500 tons. The more usual output of a blast furnace is however much less than these amounts; but if we say only one-half, or even one-third of these quantities, a state of things is indicated very different from what obtained about 1837, when the best Welsh furnaces produced only 200 tons a week. If we go back to the beginning of the century, the difference is much more marked, for the blast furnaces of that period could turn out only about 30 tons in a week.

The proportions of fuel, ore, and limestone charged into the furnace vary greatly according to the composition of the ore, the quality of iron aimed at, and the practice of each manufacturer. It is usual previously to calcine the carbonate ores and others also, in order to expel the carbonic acid and the moisture, of which last all contain a considerable amount: and sometimes the limestone is mixed with the ore to undergo this preliminary process. The charge being conveyed from the roasting kilns to the blast furnace while still hot effects an obvious economy of fuel in the latter. In the case of hÆmatite ore the quantities of materials in one charge may be something like 54 cwt. of ore, 9 cwt. of limestone, and 33 cwt. of coke. It is quite common to use mixtures of different kinds of ore, so as to modify the quality of the product according to particular requirements. The use of the limestone is to take up silica, and the slag is found to consist mainly of silicates of lime and alumina. The amount flowing from a blast furnace of course varies much according to the conditions, and is larger than would commonly be supposed; for the production of one ton of pig iron involves the production of from ½ to 1½ tons of slag.

Fig. 22 represents in section the later type of blast furnace, which of course is circular in plan. Its height may be taken as 80 feet, and the diameter at the widest part of the interior as 22½ feet, narrowed to 20 feet near the top. The lowest portion, C, is called the crucible, the bottom of which is the hearth, both formed of the most refractory materials obtainable. The conical widening, B, above the crucible is the boshes, and at the top is seen the “cup and cone” apparatus already described, A, surmounted by the short cylindrical iron mouth, through apertures in which the charges are tipped from the gallery, D, these having been raised there in small trucks by hydraulic or other elevators. The escaping gases leave the furnace by the exit, E, which leads into the “down-come,” G, and they are conducted from it to the “regenerative stoves” and dealt with as presently to be described. Our section represents the masonry of the furnace as sustained by pillars, P, at the outside of the lower part; these pillars support a strong ring of iron plates upon which the wall rests. This arrangement has the advantage of allowing the workmen the greatest freedom of access to parts about the crucible, which require much attention. Here, at the lowest part, is an aperture from which the liquid iron is allowed to run out every five or six hours, it being plugged in the meantime by clay and sand. The slag being much lighter than the iron, floats above it, and runs off at a higher level over the tympstone. Opening into the hearth are several orifices to admit the hot blast from the nozzles of the tuyÈres, which of course do not project into the furnace itself; but they are so near to the region of intensest heat that they would be rapidly destroyed unless they were surrounded by a casing through which a current of water is constantly running. The tuyÈres, of which there may be 3 or 5, are supplied from the pipe seen at K. The earlier plans of heating the air did not permit of a very high temperature being given to the hot blast, about 600° F. being the limit; but the “regenerative” stoves can supply a blast of more than 1,600° F., or not far below the melting point of silver. Another great increase has been in the pressure of the blast; 2 or 3 lbs. per square inch sufficed in the earlier practice; but the lofty modern furnaces have to be supplied with the blast at a pressure of 10 lbs. per square inch, and over. Even when comparatively low pressures were the rule, a large ironworks required much blowing power. The works formerly at Dowlais, in South Wales, for instance, had an engine of 650 horse-power for the blowing engine, in which a piston of 12 feet diameter moved in a cylinder 12 feet in length. The quantity of air that passes into a blast furnace amounts to thousands of tons per week, its weight being much greater than that of all the ore, coke, and limestone put together.

It need scarcely be said that great care and expense are bestowed on the construction of these furnaces. Only the best and most refractory materials, such as firebricks, are used for the lining, and the exterior is a casing of solid masonry, strengthened with iron bands. When a new furnace is finished it takes a month or six weeks to put it into operation; but when this is done it will remain in action night and day continuously for a long period—perhaps for eight or ten years—before the necessity for repairs requires a “blow out.” And the blow out and restarting, without the cost of repairs, entail an outlay of several hundred pounds.

The gases leaving the throat of the furnace consist mainly of nitrogen and a little carbonic acid, together with about one-third of their volume of the combustible gases, carbonic oxide, and some hydrogen; but these last do not leave the furnace in an ignited state, because the oxygen there has already been consumed. They are conducted by the “down-come” pipe, G, Fig. 22, to a point at which, by means of a valve, they can be directed to one or other of two circular towers entirely filled with firebricks, arranged chequerwise, so as to form innumerable passages between them. The furnace gases are admitted at the bottom of the Cowper tower, or “regenerative stove,” into a flue to which a regulated quantity of air has access, and there they are fired: the flame ascending the flue to the upper part of the tower, thence descends, communicating its heat to the firebricks, which soon acquire a very high temperature, especially where the flame first enters, and the burnt gases leave the tower for a tall chimney, leaving most of their heat in the firebricks. When this action has continued for a sufficient time, the connection of the regenerator with the throat of the furnace is cut off, and the escaping gases are directed into the other regenerator, and at the same time the blast from the blowing engine is made to ascend among the firebricks of the first, where gaining increasing temperature as it ascends—the stove being hottest at the top—the air leaves the tower to be conducted to the tuyÈres at such high temperature as already mentioned. While the one regenerator is thus heating the blast, the other is in its turn accumulating heat from the flames of the escaping gases; and thus they are worked alternately, the action being constantly reversed after suitable intervals.

When iron is combined with a much smaller proportion of carbon than in cast iron, and contains little or no graphitic or uncombined carbon, we have the very useful compound known as steel. In the earlier half of the century it was customary to distinguish steel from malleable iron on the one hand, and cast iron on the other. If the compound contained from 0·5 to 1·5 per cent. of carbon, it was called steel by some authorities, while others extended these limits a little on either side. Later it was found that the presence of elements other than carbon can confer steely properties on iron, and indeed it is possible to have a metal containing no carbon, but possessing the characteristic properties of steel. Sir Joseph Whitworth proposed to classify a piece of metal according to its tensile strength, without any regard to either its chemical composition or its mode of manufacture: if it could not bear more than 30 tons per square inch it should be considered iron, but if it had a higher tensile strength, it should then be regarded as steel. To estimate the engineering value a figure depending upon the elongation or stretching of the specimen before breaking was to be added to the number of tons of the breaking load. This stretching power of steel is in some cases of as much importance as the tensile strength: the ordnance maker, for instance, considers a steel with a breaking strength of 53 tons under an elongation of 5 per cent. as for his purposes to be rejected: while a specimen showing a breaking strain of only 30 tons along with an elongation of 35 per cent., on 2 inches of length, he will regard as good. The tensile strength of steel depends in part on its composition, in part on the mode of manufacture, and in part on the subsequent treatment. The average tensile strength of a wrought iron bar per square inch of section is about 25 tons (30 is the maximum); while the like average for steel is 43 tons, and some kinds of cast steel will bear nearly 60 tons. Steel bars of a certain temper subjected by Sir Joseph Whitworth to a process of hardening in oil showed a tensile strength of even 90 tons per square inch. These figures will suffice to show the great utility of steel in structures and machines. But steel has besides a characteristic property which makes it extremely valuable in a great variety of applications, namely, its capability of being tempered. If a piece of steel is heated to dull redness and suddenly cooled by plunging it into cold water, it becomes so extremely hard that it cannot be acted on by a file; nay, its hardness may be made to rival that of the diamond, which is the hardest substance known. Now by a second operation this hardness can be reduced to any required degree: this is done by re-heating the metal to a certain moderate degree between 430° F. and 630° F. and again cooling it by immersion in some cooling medium. In this “letting down” process, it is the highest temperature that produces the greatest softening, and the properties of the tempered steel will depend upon the precise degree to which the metal has been reheated. For example, if the product be required for making into sword blades, or watch-springs, and to possess much elasticity, the proper temperature is between 550° F. and 570° F.; but if the steel is to be suitable for saws the temperature must range within a few degrees of 600° F., according to the fineness of the tool intended; a lower temperature would give a metal too hard for them to be sharpened with a file. On the other hand, sharp cutting instruments and tools for working metals are obtained hard by tempering at lower degrees than springs. In practice the index of the temperature is taken from the colour of the film of oxide that gradually forms on a polished surface of the metal as the heat is raised, and begins by a very pale yellow (at 430° F.), passing through deeper shades into brown, then through purple into deep blue (at 570° F.), etc. The reader will now see why watch and clock springs have their deep blue colour, and he can observe for himself the whole series of colours by very gradually heating a piece of polished steel over a small flame.

If we compare the chemical composition of wrought iron and of cast iron with that of steel as regards the content of carbon, we see at once that steel holds an intermediate position, so that if in the puddling furnace we could arrest the decarbonization at a certain point we should obtain steel; or if, on the other hand, we could put back into chemical combination with the decarbonized wrought iron a due percentage of carbon we should in that way also obtain steel. And it will be observed that the oldest primitive furnaces could not have failed sometimes to have produced steel as the net or final result of such actions. In fact, steel always has been and still is produced on one or other of these two principles, applied in divers ways, but severally and distinctly directed to that end. Of the many more or less modified processes of steel-making that have been in use, we need here but briefly mention a few which were the processes of the first sixty years of our century, and are to a considerable extent still in operation, although eclipsed in importance by two other processes that, since the date referred to, have been supplying the metal in enormously increased quantities, and which will have to be particularly described.

The most usual of the older processes of steel-making, still carried on at Sheffield and elsewhere, is known as the cementation process: it consists in heating bars of the best wrought iron in contact with charcoal, at a high temperature, for three or four weeks. At Sheffield the iron bars and charcoal are packed in alternate layers into troughs 14 ft. long by 3½ ft. deep and wide, constructed of slabs of siliceous sandstone 6 in. thick. The last layer of charcoal at the top is covered to a certain depth with a layer of refractory matter, and the flames from a furnace beneath are made to envelop the stone troughs or pots, as they are technically called, for a period of a week or more according to the thickness of the bars operated upon. These are generally 3 in. broad and from five- to six-eighths of an inch thick. When it is found by withdrawing a test bar for examination that the operation is complete, the fire is gradually diminished and the whole allowed to cool slowly, which requires about a fortnight. Instead of only charcoal, a mixture of powdered charcoal or soot with a little salt has been used by some makers—which mixture, technically called cement powder, has given its name to the process. In some works 16 tons or more of iron are treated in one operation. The bars are found unchanged in form, but increased in weight by perhaps 27 lbs. per ton, for carbon has combined with the iron, being apparently transferred in the iron from one particle to another. The surface of the bars becomes rough and uneven from a multitude of blebs or blisters, and hence they are called blister bars, and the steel of which they now consist is named blister steel. In this conversion we may suppose that the iron at its outer surface first enters into combination with carbon taken from the carbonic oxide gas, which would be produced by combustion of the charcoal with the limited quantity of air in its interstices, and the oxygen thus set free would immediately seize again on the surrounding charcoal, and by repeated changes of this kind in which the oxygen acts as a carrier of carbon to the iron, in which it is transferred inwards from particle to particle. The cause of the blisters has been much discussed: probably the cause is the formation and escape of a volatile compound of carbon and sulphur at the surface of the soft metal; for it is known that nearly the whole of the little sulphur in the wrought iron disappears in the cementation process. Blister steel is never homogeneous, for near the surface it always contains more carbon than within; the bars are therefore broken up into short lengths which are carefully assorted, bound together with wire, heated, welded together under a hammer or by rolling, and finally formed into a bar, which is stamped with the outline of a pair of shears, and is then known as shear steel, because this product was generally found the most suitable for making the shears used in dressing cloth.

Another method of dealing with the blister steel is to charge crucibles or pots having covers with 50 or 100 lbs. weight of the broken-up bars, and subject the crucibles to a strong heat in a reverberatory furnace, when the metal melts, and at the proper moment the contents of a great number of pots are almost simultaneously poured into a mould to form an ingot. The result is a very uniform steel of the finest texture, known and highly esteemed as cast steel or crucible steel. This steel is much more fusible than iron, but less so than cast iron.

The production of steel by arresting at a certain stage the decarbonizing of cast iron in the puddling furnace requires much experience on the part of the workman, who has to learn when the desired point has been reached by certain indications, such as the appearance of the flame, or by the examination of a small sample of the fluid metal withdrawn and rapidly cooled. Various additions to the charge in definite proportions are generally made, such as scales of iron oxide, or a quantity of an oxide ore (hÆmatite, etc.) or other materials, the most essential for a good product consisting of a little manganese in some form. The result is puddled steel; and this, like blister steel, can be converted into cast steel by fusion in crucibles, running into ingot moulds, and subsequent treatment by hammering, pressing, rolling, etc. In 1864 puddled steel was described as an article of great commercial importance, but this it soon lost by the introduction of simpler, cheaper, and more reliable processes. The methods and improvements proposed for the production of steel have been exceedingly numerous, as is shown by the records of the English Patent Office alone, which contain up to the end of 1856 specifications of ninety-two patents for different steel-manufacturing processes, while from 1857 to 1865, the epoch-marking period of steel making, seventy-four more patents were obtained for this purpose. It would be quite beyond our limits to make special reference to these, and to the numerous patents which have since been granted, but there is one of great importance in steel-making which must be mentioned, and that is the patent for the employment in the cementation process of carbide of manganese, taken out by J. M. Heath in 1839. This made England almost independent of the former large importations of Swedish and Russian iron, and it caused an immediate reduction of £40 in the price per ton of good steel, effecting a saving which up to 1855 is calculated at not less than £2,000,000. Heath was one of those who fail to benefit by their inventions, for his was boldly appropriated by another person who took advantage of a verbal flaw in the specification, and Heath did not obtain any redress from the law courts until, after ten years’ litigation, a majority of Exchequer judges reversed all the previous decisions against him (1853). In the meantime the man had died, but as the patent was about to expire his widow was on petition granted an extension of it for seven years. The nature of the influence of manganese on steel-making has not been fully explained, and there is some diversity of opinion on the subject, as it is said—on the one hand, merely to remove or counteract the injurious effects of sulphur or phosphorus; on the other, to impart to the steel greater ductility, strength, and power of welding, tempering, etc.

The manufacture of crucible or cast steel has been carried on at Essen in Prussia by the firm of A. Krupp & Co., on a scale surpassing anything attempted elsewhere,—theirs being the largest steel-works in the world, and remarkable for the variety and excellence of its products. It began in so small a way that it is said only a single workman was employed. To the Great Exhibition of 1851, at London, Krupp’s firm sent a block of crucible cast steel weighing 2¼ tons, a larger mass of the metal than had ever been shown before, and looked upon with no little astonishment, for at that time steel was a precious commodity, the price of refined steel ranging from £45 to £60 per ton. At the next London Exhibition, in 1862, the Essen Works showed a block of cast steel 20 tons in weight, and at the Vienna Exhibition of 1873, one of 52 tons. This casting, which was first made of a cylindrical shape, was forged into an octagonal form under an immense steam-hammer, larger than the Woolwich hammer described on a previous page, for the weight of the moving part is no less than 50 tons. This huge mass of cast steel was of the finest quality; the forging into the prismatic form was to show its malleability, for it was intended for the body of a gun to have a bore of 14 inches. Since the period referred to, ingots of more than 100 tons have been cast. That shown at Vienna was the product of some 1,800 crucibles, each containing 65 lbs. of melted steel, which had to be poured into the mould in a regular and continuous stream, so that the metal might solidify into a perfectly uniform mass. Such work can be done only by trained men, who act in regular ranks with military precision, and in pairs emptying their crucibles into channels previously assigned, then filing off to the other end of the rank to receive another crucible, while the pair of men who were behind are pouring out theirs, and so on in succession. The crucibles are emptied into a number of channels formed of iron lined with fire-clay, and leading down into the mould. Many precautions have to be taken to ensure the regular progress of the operations, and all the time required to fill the huge moulds may be counted by minutes.

The headpiece to our chapter on Fire-Arms gives but a very inadequate idea of the magnitude of the Essen Works about 1870. A better notion will be obtained from a few figures which we select from a list giving some of the contents of the Essen Works in 1876. There were 1,109 furnaces of various kinds, of which 250 were for smelting; 77 steam hammers, 294 steam engines, 18 rolling mills, 365 turning lathes, and 700 other machine tools; 24 miles of ordinary gauge railway for traffic within the works; together with 10 miles of narrow gauge railway; 38 miles of telegraph lines, with 45 Morse apparatus, etc. (J. S. Jeans’ Steel: its History, etc., 1880). These figures belong, be it observed, to the state of things in 1876; but we learn from a later authority that in 1894 these works employed 15,000 men, and we must suppose that the plant has been proportionately increased since the earlier period, when 10,000 men were employed.

In the year 1854 a regular system of records began to be kept of the amounts of coal and ores raised in Great Britain, and also of the quantities of the various metals produced. These show that in 1894 very nearly three times as much coal was raised as in 1854, and that in the same period the quantity of British pig iron smelted annually had increased four-fold; these increases look small when compared with the expansion of the steel production in Britain within the same period of forty years, for this had enlarged thirty-fold. This extraordinary development is attributable to the introduction of two processes by either of which various steels of excellent quality, and adapted to a great range of applications, can be produced cheaply and with certainty. These processes are respectively known as the Bessemer and the Open Hearth, and the reader should observe that with the main principles involved in these he has already been made acquainted.

Henry Bessemer, who first saw the light in England in 1813, may be said to have been born an inventor, for his father was one before him—a Frenchman employed in the royal mint at Paris, afterwards appointed by the Revolutionary authorities to superintend a public bakery; on an accusation of giving short weight, thrown into prison, from which, and probably from the guillotine, he escaped, and found employment in the English mint. Subsequently he devised some notable improvements in the art of producing letterpress type, and for many years carried on a prosperous business as a typefounder. The son developed inventive faculties at a very early age: in lathe engraving, dies, dating stamps, etc. His name became familiar to everyone by his production of the metallic powder long known as “Bessemer’s Gold Paint.” It became known to Bessemer that the raw material of this substance, which was then sold at £5, 10s. per lb., really cost only about one shilling per lb., and he set himself to discover its composition and mode of manufacture. He succeeded in this so well that he could produce the article at the insignificant cost of four shillings a pound, and his first order for a supply of it was at the rate of £4 per lb., and the business was continued, realising profits of something like 1,000 per cent. at first. For this article no patent was taken out, but Bessemer himself, assisted by two trustworthy workmen, carried on the manufacture in secret, and he some time afterwards rewarded the fidelity of his men by handing over the business to them as a free gift. Then he took out patents for improvements in the manufacture of oils, varnishes, sugar, plate glass, etc. Several of his machines for these purposes were shown at the London Exhibition of 1851. Bessemer is said to have obtained altogether some 150 patents, including those granted for inventions connected with our subject. He may be regarded as the type of the very fortunate inventor, since on the patents of the one process we are going to describe he ultimately obtained royalties to the value of more than £1,057,000, and this irrespective of profits derived from commercially working it himself.

At the time of the Crimean War, Bessemer had some experiments made at Vincennes with cylindrical projectiles he had devised for firing from smooth-bore guns, yet so as to impart to the projectile at the same time rotation about its axis. The experiments were successful, but it was pointed out that the guns of cast iron then in use would not bear heavy projectiles, and he was induced, at the suggestion of the Emperor Napoleon III., to undertake some researches with the view of finding metal more suitable for artillery. Bessemer, having then little knowledge of the metallurgy of iron, applied himself on his return to England to the study of the best books on the subject, visited the principal iron-working districts, and began a series of experiments at a small experimental installation he set up in London. There, after repeated failures, he did at length succeed in producing a metal much tougher than the cast iron then used, and a small model gun was submitted to the Emperor, who encouraged Bessemer to persevere with his experiments; which he did, though the expense was a great tax on his capital, continued as the experiments were for two years and a half. But by this time he had acquired a knowledge of many important facts, and these gradually led him to the experimental realization of the idea he had conceived, but only after many trials in which several thousand pounds were expended. At length the agenda of the British Association for the Cheltenham meeting of 1856 announced that a paper would be read by H. Bessemer, entitled “The Manufacture of Iron and Steel without Fuel.” It will be easily understood that a title in such terms would give rise to much derisive incredulity; and we may imagine the iron-masters on that occasion crowding into Section G, while asking each other in the spirit of certain philosophers of old, “What will this babbler say?” Some of what he did say may here be quoted, as at once explanatory and historically memorable.

“I set out with the assumption that crude iron contains about 5 per cent. of carbon; that carbon cannot exist at a white heat in the presence of oxygen without uniting therewith and producing combustion; that such combustion would proceed with a rapidity dependent on the amount of surface of carbon exposed; and lastly, that the temperature which the metal would acquire would be also dependent on the rapidity with which the oxygen and carbon were made to combine; and consequently, that it was only necessary to bring the oxygen and carbon together in such a manner that a vast surface should be exposed to their mutual action, in order to produce a temperature hitherto unattainable in our largest furnaces.

“With a view of testing practically this theory, I constructed a cylindrical vessel of 3 ft. in diameter and 5 ft. in height, somewhat like an ordinary cupola furnace (see Fig. 23). The interior is lined with firebricks, and at about 2 in. from the bottom of it I inserted five tuyÈre pipes, the nozzles of which are formed of well-burned fire-clay, the orifice of each tuyÈre being about three-eighths of an inch in diameter; they are so put into the brick lining (from the outer side) as to admit of their removal and renewal in a few minutes, when they are worn out. At one side of the vessel, about half-way up from the bottom, there is a hole made for running-in the crude metal, and on the opposite side there is a tap-hole, stopped with loam, by means of which the iron is run out at the end of the process. In practice this converting vessel may be made of any convenient size, but I prefer that it should not hold less than one nor more than five tons of fluid iron at each charge; the vessel should be placed so near to the discharge hole of the blast furnace as to allow the iron to flow along a gutter into it. A small blast cylinder is required capable of compressing air to about 8 lbs. or 10 lbs. to the square inch. A communication having been made between it and the tuyÈres before named, the converting vessel will be in a condition to commence work; it will however on the occasion of its first being used after re-lining with firebricks be necessary to make a fire in the interior with a few baskets of coke, so as to dry the brickwork and heat up the vessel for the first operation, after which the fire is to be all carefully raked out at the tapping-hole, which is again to be made good with loam: the vessel will then be in readiness to commence work, and may be so continued without any use of fuel until the brick lining, in the course of time, becomes worn away, and a new lining is required. I have before mentioned that the tuyÈres are situated nearly close to the bottom of the vessel, the fluid metal will therefore rise some 18 in. or 2 ft. above them; it is therefore necessary, in order to prevent the metal from entering the tuyÈre holes, to turn on the blast before allowing the fluid crude iron to run into the vessel from the blast furnace. This having been done, and the metal run in, a rapid boiling up of the metal will be heard going on within the vessel, the metal being tossed violently about and dashed from side to side, shaking the vessel by the force with which it moves; from the throat of the converting vessel flame will immediately issue, accompanied by a few bright sparks such as are always seen rising from the metal when running into the pig-beds. This state of things will continue for about fifteen minutes, during which time the oxygen in the atmospheric air combines with the carbon contained in the iron, producing carbonic oxide, or carbonic acid gas, and at the same time evolving a powerful heat. Now, as this heat is generated in the interior of, and is diffused in innumerable fiery bubbles through, the whole fluid mass, the metal absorbs the greater part of it, and its temperature becomes immensely increased, and by the expiration of the fifteen minutes before named that part of the carbon which appears mechanically mixed and diffused throughout the crude iron has been entirely consumed: the temperature however is so high that the chemically combined carbon now begins to separate from the metal, as is at once indicated by an immense increase in the volume of flame rushing out of the throat of the vessel. The metal in the vessel now rises several inches above its natural level, and a light frothy slag makes its appearance and is thrown out in large foam-like masses. This violent eruption of cinder generally lasts about five or six minutes, when all further appearance of it ceases, a steady and powerful flame replacing the shower of sparks and cinder which always accompanies the boil. The rapid union of carbon and oxygen which thus takes place adds still further to the temperature of the metal, while the diminished quantity of carbon present allows a part of the oxygen to combine with the iron, which undergoes combustion and is converted into an oxide. At the excessive temperature that the metal has now acquired, the oxide as soon as formed undergoes fusion, and forms a powerful solvent of those earthy bases that are associated with the iron; the violent ebullition which is going on mixes most intimately the scoria and metal, every part of which is thus brought in contact with the fluid oxide, which will thus wash and cleanse the metal most thoroughly from the silicon and other earthy bases which are combined with the crude iron, while the sulphur and other volatile matters which cling so tenaciously to iron at ordinary temperatures are driven off, the sulphur combining with the oxygen and forming sulphurous acid gas.

“The loss in weight of crude iron during its conversion into an ingot of malleable iron was found, on a mean of four experiments, to be 12½ per cent., to which will have to be added the loss of metal in the finishing rolls. This will make the entire loss probably not less than 18 per cent. instead of about 28 per cent., which is the loss on the present system. A large portion of this metal is however recoverable by heating with carbonaceous gases the rich oxides thrown out of the furnace during the boil. These slags are found to contain innumerable small grains of metallic iron, which are mechanically held in suspension in the slags and may be easily recovered.

“I have before mentioned that after the boil has taken place a steady and powerful flame succeeds, which continues without any change for about ten or twelve minutes, when it rapidly falls off. As soon as this diminution of flame is apparent the workman will know that the process is completed, and that the crude iron has been converted into pure malleable iron, which he will form into ingots of any suitable size and shape by simply opening the tap-hole of the converting vessel and allowing the fluid malleable iron to flow into the iron ingot moulds placed there to receive it. The masses of iron thus formed will be free from any admixture of cinder, oxide, or other extraneous matters, and will be far more pure and in a forwarder state of manufacture than a pile formed of ordinary puddle bars. And thus it will be seen that by a single process, requiring no manipulation or particular skill, and with only one workman, from three to five tons of crude iron pass into the condition of several piles of malleable iron in from thirty to thirty-five minutes, with the expenditure of about a third part the blast now used in a finery furnace, with an equal charge of iron, and with the consumption of no other fuel than is contained in the crude iron.

“To those who are best acquainted with the nature of fluid iron, it may be a matter of surprise that a blast of cold air forced into melted crude iron is capable of raising its temperature to such a degree as to retain it in a perfect state of fluidity after it has lost all its carbon and is in the condition of malleable iron, which, in the highest heat of our forges, only becomes softened into a pasty mass. But such is the excessive temperature that I am enabled to arrive at with a properly shaped converting vessel and a judicious distribution of the blast, that I am enabled not only to retain the fluidity of the metal, but to create so much surplus heat as to remelt all the crop-ends, ingot-runners, and other scrap that is made throughout the process, and thus bring them, without labour or fuel, into ingots of a quality equal to the rest of the charge of new metal....

“To persons conversant with the manufacture of iron, it will be at once apparent that the ingots of the malleable metal which I have described will have no hard or steely parts, such as are found in puddled iron, requiring a great amount of rolling to blend them with the general mass, nor will such ingots require an excess of rolling to expel cinder from the interior of the mass, since none can exist in the ingot, which is pure and perfectly homogeneous throughout, and hence requires only as much rolling as is necessary for the development of fibre; it therefore follows that, instead of forming a merchant bar, or rail, by the union of a number of separate pieces welded together, it will be far more simple and less expensive to make several bars or rails from a single ingot. Doubtless this would have been done long ago had not the whole process been limited by the size of the ball which the puddler could make.

“The facility which the new process affords of making large masses will enable the manufacturer to produce bars that, in the old mode of working, it was impossible to obtain; while at the same time it admits of the use of more powerful machinery, whereby a great deal of labour will be saved and the process be greatly expedited.... I wish to call the attention of the meeting to some of the peculiarities which distinguish cast steel from all other forms of iron, viz., the perfectly homogeneous character of the metal, the entire absence of sand-cracks or flaws, and its greater cohesive force and elasticity, as compared with the blister steel from which it is made,—qualities which it derives solely from its fusion and formation into ingots, all of which properties malleable iron acquires in like manner by its fusion and formation into ingots in the new process; nor must it be forgotten that no amount of rolling will give the blister steel, although formed of rolled bars, the same homogeneous character that cast steel acquires by a mere extension of the ingot to some ten or twelve times its original length....

“I beg to call your attention to an important fact connected with the new process which affords peculiar facilities for the manufacture of cast steel. At that stage of the process immediately following the boil the whole of the crude iron has passed into the condition of cast steel of ordinary quality. By the continuation of the process the steel so produced gradually loses its small remaining portion of carbon, and passes successively from hard to soft steel, and from soft steel to steely iron, and eventually to very soft iron; hence, at a certain period of the process, any quality of metal may be obtained. There is one in particular which by way of distinction I call semi-steel, being in hardness about midway between ordinary cast steel and soft malleable iron. This metal possesses the advantage of much greater tensile strength than soft iron; it is also more elastic, and does not readily take a permanent set, while it is much harder and is not worn or indented so easily as soft iron; at the same time it is not so brittle or hard to work as ordinary cast steel. These qualities render it eminently well adapted to purposes where lightness and strength are specially required, or where there is much wear, as in the case of railway bars, which from their softness and lamellar texture soon become destroyed. The cost of semi-steel will be a fraction less than iron, because the loss of metal that takes place by oxidation in the converting vessel is about 2½ per cent. less than it is with iron; but as it is a little more difficult to roll, its cost per ton may fairly be considered to be the same as iron; but as its tensile strength is some 30 or 40 per cent. greater than bar iron, it follows that for most purposes a much less weight of metal may be used than that so taken. The semi-steel will form a much cheaper metal than any we are at present acquainted with. These facts have not been elicited from mere laboratory experiments, but have been the result of working on a scale nearly twice as great as is pursued in our largest iron works, the experimental apparatus doing 7 cwt. in thirty minutes, while the ordinary puddling furnace makes only 4½ cwt. in two hours, which is made into six separate balls, while the ingots or blooms are smooth, even prisms, 10 in. square by 30 in. in length, weighing about equal to ten ordinary puddle balls.”

The startling novelty of the methods and results described in this paper had the effect of paralyzing discussion at the time. But soon the voice of detraction was heard; many iron-masters ridiculed the idea of producing iron and steel without fuel, and indeed it may have been observed, the title of the paper notwithstanding, that first the silicon and carbon, and then the iron itself, really supplied the fuel. And we must remember that malleable iron in a molten state was then deemed an impossibility, for the hottest furnaces then known could not effect the fusion, however prolonged their action might be, yet Bessemer was to obtain five tons in this condition in the short space of half an hour with no other aid than cold air. Then it was said that Bessemer’s process of forcing air into melted cast iron had no claim of novelty, for it had been tried before and found valueless. Some iron-masters on trying experiments on a small scale and with imperfect appliances met with failures, and discredited the process at once; but five large establishments paid for licences sums amounting to £26,500 within three weeks of the reading of the paper. At the works of the Dowlais Iron Co., in South Wales, who were the first licensees, the first converter was set up under Bessemer’s personal superintendence, and at the first operation five tons of iron were produced direct from the blast furnace pig. This apparently satisfactory result proved quite otherwise when this iron came to be practically tested; for it was found quite useless! It was both “cold-short” and “red-short,” to use the technical terms,—the former of which means that although the sample may be welded, it is when cold brittle and rotten; the latter means that at a low red heat it breaks and crumbles under the hammer. Further trials were made, new experiments instituted, but the success that attended Bessemer’s early experiments could not be repeated, and as yet no one knew the reason why. Now it so happened that in the preliminary experiments an exceptionally pure pig iron had been made use of containing little or no phosphorus or sulphur, substances very deleterious in iron, and still more so in steel. With the capital obtained by the sale of his licences Bessemer quietly set to work to investigate the cause of his non-success, making daily experiments with a ton or two of metal at a time. These experiments extended over a period of two and a half years, and upon them Bessemer and his partner spent about £16,000, besides the £4,000 the preliminary researches had cost. But all difficulties were at length overcome, and the process was now found capable of turning out pure iron and steel when the pure pig iron of Sweden was used in the converter. In the meantime the licensees had made no attempts practically to carry out the process, which began to be denounced as visionary: it was “a mare’s nest”; it was “a meteor that had passed through the metallurgical world, but had gone out with all its sparks.” When Bessemer again brought the subject before the public, he found that no one believed in it; everyone said, “Oh, this is the thing that made such a blaze two or three years ago, and which was a failure.” Neither iron-makers nor steel-makers would now take it up. Bessemer and his partner thereupon joined with three other gentlemen to establish at Sheffield a steel-works of their own, where the invention should be carried into full practice. In due time works were erected, and they commenced to sell steel, receiving at first very paltry orders, for such quantities as 28 lbs. or 56 lbs.; but the orders soon became larger, and afterwards very much larger, for they were underselling the Sheffield manufacturers by £20 a ton, and their steel was undistinguishable from the higher priced article. Bessemer had now bought his licences back again, and in the course of his second set of experiments had patented each improvement as it occurred to him, finally bringing the mechanical apparatus to the degree of efficiency requisite for practical working, without which his primary idea would have been valueless. Before directing the reader’s attention to the form the apparatus had assumed, we may transcribe what Mr. Jeans, in the work above referred to, has told about the commercial success of the Bessemer steel-making firm:—

“On the expiration of the fourteen years’ term of partnership of this firm the works, which had been greatly increased from time to time out of revenues, were sold by private contract for exactly twenty-four times the amount of the whole subscribed capital, notwithstanding that the firm had divided in profits during the partnership a sum equal to fifty-seven times the gross capital, so that by the mere commercial working of the process, apart from the patent, each of the five partners retired after fourteen years from the Sheffield works with eighty-one times the amount of his subscribed capital, or an average of nearly cent. per cent., every two months,—a result probably unprecedented in the annals of commerce.”

Fig. 24.—Bessemer Converter.
A, Front view, showing the mouth, c; B, Section.