White.White.Mottled.Grey.
Iron88·8189·30493·2990·376
Combined carbon4·942·4572·781·021
Graphite, or uncombined carbon...0·8711·992·641
Silicon0·751·1240·713·061
Sulphurtrace2·516trace1·139
Phosphorus0·120·9131·230·928
Manganese5·382·815trace0·834

The reader will observe that the last item in the table above is a substance that he has not yet made the acquaintance of, namely, manganese. This is a metal which in many of its chemical relations much resembles iron, and ferruginous ores usually contain a greater or less proportion of it. Manganese is of great importance in the manufacture of steel, as we shall presently see; but as a separate metal it has no application, and is obtainable in the metallic state with much difficulty. One of its oxides has however very extensive applications in the chemical arts, and others form acid radicles, which in combination with potash or soda give rise to useful products. The well-known “Condy’s fluid” is a solution of one of these.

We have seen how malleable iron or steely iron may be directly obtained from the ores, but it has been found that on the large scale it is necessary and more economical to operate on the pig iron produced by the blast furnaces in such a manner as to remove the greater part of the foreign substances.

Fig. 19.—Section of a Reverberatory Furnace.

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.