“Iron and coal,” it has been well said, “are kings of the earth”; and this is true to such an extent that there is scarcely an invention claiming the reader’s attention in this book but what involves the indispensable use of these materials. Again, in their production on the large scale it will be seen that there is a mutual dependence, and that this is made possible only by means of the invention we have begun with; for without the steam engine the deep coal mines could not have the water pumped out of them,—it was indeed for this very purpose that the steam engine was originally contrived,—nor could the coal be efficiently raised without steam power. Before the steam engine came into use iron could not be produced or worked to anything like the extent attained even in the middle of the nineteenth century, for only by steam power could the blast be made effective and the rolling mill do its work. On the other hand, the steam engine required iron for its own construction, and this at once caused a notable increase in the demand for the metal. Once more, the engine itself supplies no force; for without the fuel which raises steam from the water in the boiler it is motionless and powerless, and that fuel is practically coal. In consequence of thus providing power, and also of supplying a requisite for the production of iron, coal has acquired supreme industrial importance, so that all our great trades and places of densest population are situated in or near coal-fields. But what we have further to say about coal may be conveniently deferred to a subsequent article, while we proceed to treat of iron, and of the contrivances in which it plays an essential part.

Iron has also been called “the mainspring of civilization,” and the significance of the phrase is obvious enough when we consider the enormous number and infinite variety of the things that are made of it: the sword and the ploughshare; all our weapons of war and all our implements of peace; the slender needle and the girders that span wide rivers; the delicate hair-spring of the tiny watch and the most tenacious of cables; the common utensils of domestic life and the huge battle-ships of our fleets; the smoothest roads, the loftiest towers, the most spacious pleasure palaces. Such extensive applications of iron for purposes so diverse have been rendered possible only by the greater facility and cheapness of production, together with the better knowledge of the properties of the substance and increased skill in its treatment, that have particularly distinguished our century. Apart again from the constructive uses of iron, it enters essentially into another class of inventions of which the age is justly proud, namely, those which utilize electricity in the production of light, mechanical power, and chemical action; for it is on a quality possessed by iron, and by iron alone, that the generation of current by the electric dynamo ultimately depends. This peculiar property of iron, which was first announced by Arago in 1820, and has since proved so fertile in practical applications, is that a bar of the metal can, under suitable conditions, be instantly converted into the most powerful of magnets, and as quickly demagnetized. What these conditions are will be explained when we come to treat of electricity.

Besides the unique property of iron just referred to, and its superlative utility in arts and industries, there are other circumstances that give a peculiar interest to this metal. It is the chief constituent of many minerals, and traces or small quantities are found in most of the materials that make up the crust of the earth; it is present also in the organic kingdoms, being especially notable in the blood of vertebrate (back-boned) animals, of which it is an essential component. Notwithstanding its wide diffusion, iron is not found native, that is, as metal, but has to be extracted from its ores, which are usually dull stony-looking substances, as unlike the metal as can be conceived. In this respect it differs from gold, which is not met in any other than the metallic state, in the form of nuggets, minute crystals or branching filaments, and from metals such as silver, copper, and a few others which also are occasionally found native. It is true that rarely small quantities of metallic iron have been met with in the form of minute grains disseminated in volcanic rocks; but in contrast with the practical absence of metallic iron from terrestrial accessible materials is the fact that masses of iron, sometimes of nearly pure metal, occasionally descend upon the earth from interplanetary space. These are aerolites, of which there are several varieties, some consisting only of crystalline minerals without any metallic iron, others of a mixture of minerals and metals, but the most common are of iron, always alloyed with a small quantity of nickel, and usually containing also traces more or less of a few other metals and known chemical elements. The iron in some specimens has been found to amount to 93 per cent. of the whole. These aerolites, or meteorites, as they are also called, are of irregular shape and vary greatly in size, which however is sometimes very considerable: one found in South America was calculated to weigh 14 tons, another discovered in Mexico, 20 tons. There is in the British Museum a good specimen of an iron meteorite, which is represented in Fig. 16, where it will be observed that a portion has been cut off to form a plane surface, which when polished and etched by an acid, reveals a crystalline structure quite peculiar and distinctive, so that such meteorites can be recognized with certainty, even if they did not possess surface characters which are easily observed and identified when once a specimen has been examined. The fall of meteorites to the surface of the earth is comparatively rare, but it has been witnessed by even scientific observers; as when Gassendi, the French astronomer, saw in Provence the fall of a meteorite weighing 59 lbs. In the Transactions of the Royal Society for 1802 may be found a detailed account of an instance in England of the fall which took place in Yorkshire, on the 13th December 1795, of a stone 56 lbs. in weight. Aerolites become ignited or incandescent by reason of the great velocity with which they pass through the atmosphere, whereby the air in front of them is condensed and heated, the heat often being sufficient to liquefy or even vaporize the solid matter. The so-called shooting stars are with good reason believed to be nothing but such incandescent aerolites, and the aerolites themselves are regarded as small asteroids, or scattered planetary dust, portions of which occasionally coming within the sphere of the earth’s attraction are drawn to its surface. Meteoric iron is too rare to be of any value as a source of iron, but certain specimens have been found in which the metal was malleable and of excellent quality. From such meteorites the natives of India and other places have, it is said, sometimes forged weapons of wonderful temper and keenness, and we may well imagine that when such weapons have been made from iron that had actually been observed to fall from the sky, they would be regarded as endowed with magical powers, so that we may perhaps ascribe to such circumstances the origin of some of the legends about enchanted swords, etc. It is significant also that in some Egyptian inscriptions of the very highest antiquity, the word indicating iron has for its literal meaning stone of the sky.

But as nature has hardly provided man with the metal iron, he has been obliged to find the art of extracting it from substances which are utterly unlike the metal itself. In this case, as in many others, the art has been discovered and practised ages before any scientific knowledge of the nature of the processes employed had been acquired. The idea prevails that there are such difficulties in extracting this metal; that elaborate and complex appliances, not unlike those in use in modern times, were requisite for the purpose; and therefore that the use of iron is compatible only with a somewhat late period in man’s history, and implies a comparatively advanced stage of civilization. Now there undoubtedly are facts which tend to confirm this view; for instance, the Spaniards who first colonized North America found the natives perfectly familiar with the use of copper, but without any acquaintance with iron, although the region abounded with the finest ferruginous minerals; and, again, the archæologists who have examined the relics of ancient civilizations and of pre-historic peoples about the shores of the Mediterranean, find in the earliest of these relics weapons and implements of rudely chipped stones, followed later by the use of better-shaped and polished stones; hence the periods represented by these, they have respectively designated by the terms palæolithic and neolithic—the old and the new stone ages. At some later time the stone of these implements was gradually replaced by bronze, which is a mixture of copper and tin, while as yet iron does not occur in any form among the remains. In the latest layers, however, articles of iron are found, and it is inferred that this metal came into use only after bronze had been known for an indefinite period; hence these later pre-historic periods have come to be respectively called the bronze age and the iron age. No doubt this succession really occurred in the localities where the observations were made, but it would not be justifiable to assume that the same was the case in every part of the world, for much would depend on such circumstances as the presence or absence of the essential minerals. We may also set against the supposed difficulty of obtaining iron from the ores, the still greater complexity of the methods required for the production of copper and of tin. Besides this is the fact that the ores of tin are found but in very few places in the world, and of these only the Cornwall mines, so well known in ancient times, would be likely to furnish a supply to the places where pre-historic bronzes are found; this implies that navigation and commerce must have already made considerable progress. On the other hand, iron has been produced and worked for untold ages by the negro races all over Central Africa, and the method of treating the ore has no doubt been that which is there still practised by certain scarcely civilized tribes, and it is as simple as any metallurgical operation can possibly be, requiring merely a hole dug in a clay bank, wherein the fuel and minerals are piled up, and the mere wind supplies sufficient blast to urge the fire to the needful temperature, or air is blown in from rude bellows made of a pair of skins alternately raised and compressed. These very primitive furnaces have in some places developed into permanent clay structures, seven or eight feet in height. The natives of Central Africa have therefore long known the method of extracting iron, as well as of forging and casting it.

The nature and value of what has been done during the century in the treatment of iron would not be intelligible without some description of the ordinary processes of extracting the metal from the ores; and a scientific understanding of these implies some acquaintance with chemistry. Not because metallurgy has been developed from chemistry, for the fact is rather the reverse; indeed, as we have seen, the art of extracting iron from its ores was practised ages before chemistry as a science was dreamt of. Although we may assume that many of our readers have sufficient knowledge of chemistry to attach distinct ideas to such few chemical terms as we shall have occasion to use, yet it may be of advantage to others to have some preliminary notes of the character of the chemical actions, and of some properties of the substances that will have to be referred to. It is certainly the case that people in general, and even people very well informed in other subjects, have but the vaguest notions of the nature of chemical actions, and of the meaning of the terms belonging to that science. For example, one of our most popular and justly esteemed writers, treating of the very subject of iron extraction, calls the ore a matrix, thereby implying that the iron as metal is disseminated in detached fragments throughout the mass, which is a conception inconsistent with the facts. The reader will be in a more advantageous position for understanding the relation of the ores of iron to the metal, if he will follow in imagination, or still better in reality, a few observations and experiments like the following—of which, however, he is recommended not to attempt the chemical part unless he is himself practically familiar with the performance of chemical operations, or can obtain the personal assistance of someone who is. Taking, say, a few common iron nails, let him note some obvious properties they possess: they have weight—are hard and tough so that they cannot be crushed in a mortar—are opaque to light—if a smooth surface be produced on any part, it will show that peculiar shiny appearance which is called metallic lustre, in this case without any decided colour—they are not dissolved by water as sugar or salt is—and are attracted by a magnet. If several of the nails be heated to bright redness they may be hammered on an anvil into one mass, and this may be flattened out into a thin plate, or it may be shaped into a slender rod and then drawn out into wire; or otherwise the nails may be converted into the small fragments called iron filings. In these several forms the nails, as nails, will have ceased to exist; but the material of which they were formed will remain unchanged, and each and every part of it however large or small will continue to exhibit all the properties noted above as belonging to the substance of the nails, which in the cases supposed has undergone merely physical change of shape. Treating our nails in yet another way, we may proceed to subject them to a chemical change, by an experiment very simple in itself, but involving certain precautions, by neglect of which the tyro in chemical operations would incur some personal risks; these might however be obviated by using only very small quantities of the materials (a mere pinch of iron filings and a few drops of sulphuric acid), when the results would still be sufficiently observable. A few of the iron nails having been placed in a flask of thin glass, we pour upon them a mixture of oil of vitriol (sulphuric acid) and water, which has previously been prepared by gradually adding 1 measure of the acid to 5 measures of water. The action that takes place is greatly accelerated by heat, and indeed the contents should be heated to boiling by standing the flask on a layer of fine sand spread on an iron plate and gently heated from below. The nails will soon disappear, being completely dissolved by the acid liquid, and the turbid solution should be filtered through filtering paper as rapidly as possible and while still hot. This turbid and dirty looking condition is due to foreign matters in the nails, for these never consist of pure iron. The filtered liquid is set aside to cool in a closed vessel, in which after a time will be found a deposit of crystals of a pale bluish-green colour. The liquor above these having been poured off, the crystals are to be rinsed with a very small quantity of cold water, and then dried between folds of blotting-paper, after which they are ready for examination. The quantity of the diluted acid put into the flask should have a certain proportion to the weight of the nails; about 5 fluid ounces to 1 ounce of iron will be found convenient, for if less is used the nails will not be entirely dissolved, and an excess will tend to keep the crystals in solution instead of depositing them when cold. The nails—as such—will now have passed out of existence: can we say that the iron that formed them exists in the crystals? Certainly not as the metal iron, for every property of the metal will have disappeared. The crystals are brittle, can be crushed in a mortar—they are translucent—they show no metallic lustre, but only glassy surfaces—they are readily dissolved by water—they are not attracted by a magnet. The most powerful lens will fail to show the least particle of iron in them; they have in their properties no assignable relation to the metal of the nails, but are matter of quite another sort; and be it noted that this entire otherness is the special and characteristic sign of chemical change. So complete is the transformation in the case we have been considering that it would never have been said that iron was contained in these crystals, but rather that the metal had for ever passed out of existence, but for one circumstance; and that is, that by subjecting the crystals to certain processes of chemical analysis we can again obtain from them the iron in metallic state. Nay more, we should find the weight of metal so obtained to be exactly equal to that of the pure iron dissolved from the original nails, supposing of course that we operated upon the whole of the crystalline matter so produced. The inference therefore is that although every property of the iron appeared to be absent from the crystals, the iron entering in them retained there its original weight, and the correct statement of the change would be, that in the crystals the iron had lost all its original properties SAVE ONE, namely, its weight, or gravitating force, if we choose to call it so, a property belonging to it in common with every material substance. Chemical analysis can also separate from the crystals their other constituents and weigh them apart—so much water and so much sulphuric acid—and when to these weights that of the iron is added, the sum exactly makes up the weight of the crystals.

A still simpler experiment, which may be performed by anyone with the greatest ease, may serve as a further illustration of the profound nature of the change in the properties of bodies brought about by chemical combination, and it will also serve as the occasion of directing attention to a remarkable circumstance that invariably characterizes such changes, and one that should always be present in our minds when we are considering them. A yard of flat magnesium wire can be bought for a few pence, and after its metallic character has been observed in the silvery lustre disclosed by scraping the dull white surface, a few inches is to be held vertically by a pair of tongs, or by inserting one extremity in a cleft at the end of a stick, then the lower part is brought into contact with a candle or gas flame. The metal will instantly burn with a dazzlingly brilliant light, and some white smoke (really fine white solid particles) will float into the air; but if a plate be held under the burning metal, some of the smoke will settle upon it, together with white fragments that have preserved some shape of the metallic ribbon, but which a touch will reduce into a fine white powder, identical with the well-known domestic medicine called “calcined magnesia”—a substance totally different from the metal magnesium. The reader will scarcely require to be told that in this burning the metal is entering into combination with the oxygen of the air—by which that invisible gas somehow becomes fixed in these solid white particles, so entirely unlike itself. But this experiment might be so arranged that the quantities of magnesium and oxygen entering into the magnesia could be weighed. For this purpose special appliances would be required in order to ensure complete combustion of the metal, for in the experiment as just described some small particles are liable to be shielded from the oxygen by a covering of magnesia, and the arrangement would have to be such that the whole of the white powder could be gathered up and weighed. In the absence of such appliances, and of a delicate balance, together with the skill requisite for their use, the reader must for the time be contented to take our word for what would be the result. In every experiment the magnesia would be found heavier than the metal burned in the proportion of 5 to 3; in other words, magnesia always contains (so the phrase runs) 3 parts of magnesium combined with 2 of oxygen: never more nor less. A definite proportion between the weights of the constituent substances characterizes every chemical combination, and when this is once determined in a single sample of the compound, it is determined for every portion of the same, wherever found or however produced. But each compound has its own particular proportion, that is, the quantitative relations are different for each. For example, the two constituents of water, hydrogen and oxygen, are combined in the ratio of 1 to 8, etc.; and oxygen combines with metals in a ratio different in each case. Then occasionally the same ratio of constituents occurs in compounds of different composition. The elementary student is apt to suppose that this is because of the law which he finds stated, probably in almost the first page of his text-book: “Every compound contains its elements in definite and invariable proportions”; and even well-educated people entertain the idea of the fact being “governed by” or “obeying” the law just quoted,—a misconception arising from the other use of the word “law,” as signifying an enactment. The real case however is the converse; namely, that a multitude of facts like that above stated have governed the law, and caused it to be what it is—the general statement of many observed facts.

We have assumed that the reader’s chemical knowledge had already made him aware that in every case of ordinary combustion the oxygen of the atmosphere is in the act of entering into combination with the burning body: as with the magnesium, so with a coal fire, a gas flame, or a burning candle; only in these last cases the products of the combustion pass away invisibly. The candle by burning disappears from sight, but its matter is not lost, and as in the case of magnesium, the compounds it forms weigh more than the unburnt candle. The experiment is commonly shown in courses of elementary lectures on chemistry, of so burning a candle that the invisible products are retained in the apparatus, instead of being dissipated in the atmosphere, and the increase of weight of the burnt candle over the original one is demonstrated by the balance. Important as is the part played by oxygen in all chemical actions on the earth, the composition of the atmosphere was not understood until the end of the eighteenth century, and it was well on into the nineteenth before the quantities of its constituents were accurately determined. Now everyone knows that air is mainly made up of a mixture of the two gases oxygen and nitrogen. A mixture of two or more things is very different from a chemical combination of them; for in the former each ingredient retains its own properties. (See Air in Index.) Nitrogen being an inert gas that takes no part in combustion, or in the ordinary chemical actions of the air, acts therein simply as a diluent of the oxygen. It is necessary in relation to our present subject to bear this in mind, as well as the relative quantities of the two gases in air. For our immediate purpose we may neglect the minor constituents of air—such as watery vapour, carbonic acid, etc., of which the total weight does not exceed one hundredth part of the whole—and consider air as a mixture of 23 parts by weight of oxygen with 77 of nitrogen, or calculated in volumes, 21 measures of oxygen with 79 of nitrogen. Compounds of oxygen with nearly every one of the other seventy or more chemical elements are known, and these compounds, which are called oxides, are arranged by chemists under five or six classes, forming as they do basic radicles, acid radicles, saline oxides, etc. With some of these compounds belonging to different classes, we must make acquaintance after noticing the elementary substance with which the oxygen is united.

We begin with carbon, which forms the chief constituent of all our combustibles. Some specimens of graphite, plumbago, or “blacklead” consist of almost pure carbon (98 per cent.), and some varieties of wood charcoal exceptionally contain 96 per cent.; but in ordinary charcoal the percentage is much less. Coal, the most familiar of our solid fuels, varies greatly in composition, carbon being the predominating constituent, in amount from 57 to 93 per cent. Coke, another fuel much used in metallurgical operations, is made by heating coal without access of air, when a large quantity of gaseous substances is expelled. Coke burns with an intense and steady heat without emitting any visible smoke, but it does not ignite as readily as coal. Carbon forms two different compounds with oxygen: both are invisible gases, but they differ in the proportions of the constituents, and present different properties. When carbon (coal, coke, or charcoal) is completely burnt, that is, with an abundant supply of air, the product is carbonic acid gas, in which 3 parts of carbon are combined with 8 of oxygen: when, on the other hand, the carbon is burnt with a sufficiently restricted access of air, the result is carbonic oxide gas, in which 3 parts of carbon are united with only 4 of oxygen. The reader will here observe that the former contains just twice as much oxygen as the latter for the same quantity of carbon. This fact and numberless others like it are expressed or summed up by another law of chemical combination which states that when two elements combine in several different proportions these are invariably such that the ratios in the several compounds will be found to have exact and simple numerical relations; that is, such as may, when reduced to their lowest terms, be expressed by the simple integers 1, 2, 3, etc., as 1 : 2, 3 : 2; ... 8 : 9, etc. It comes to the same thing if we compare together the weights A and A´ which are united in each compound with any one identical weight of B, giving of course the ratio A : B ÷ A´ : B. For instance, in the case just given, of carbon and oxygen, 3 : 4 ÷ 3 : 8 = 2 : 1. This, which is simply stating the facts, is called the law of multiple proportions. On a later page will be found another illustration (see Index, Nitrogen and Oxygen Compounds), and its expression in terms of the atomic theory, which goes behind the facts (so to speak), but is extremely useful by comprehending many other groups of facts in chemistry and in other sciences. Carbonic acid gas is of course incombustible, but carbonic oxide gas burns by uniting with the additional proportion of oxygen and becoming carbonic acid. On the other hand, carbonic acid gas passing over red-hot coals takes up from them the additional proportion of carbon, and is, we may say, unburnt into carbonic oxide. When we see a pale blue flame flickering over the bright embers in a fire grate, it is carbonic oxide burning back again by taking more oxygen from the air above the coals. Carbonic oxide combines directly with two or three of the metals, as, for instance, it forms a volatile compound with nickel, at a certain temperature, and this is decomposed again at a higher temperature. The like takes place with iron, although in very small quantities, but the observation throws some light on the processes of reduction. Carbonic oxide is neither acid nor basic, but carbonic acid is an acid oxide, and as such unites with oxides of the basic class to form another range of compounds. Thus, for example, the oxide of the metal calcium is quicklime, which is strongly basic, and this directly combines with carbonic acid, forming a neutral substance called in systematic chemistry calcium carbonate, or more commonly but less correctly, carbonate of lime, familiar to everyone in the compact state as limestone, and marble, and in a more or less pulverent condition as chalk. When any of these is heated to redness, carbonic acid is expelled and quicklime remains. Like most oxides, quicklime forms a compound with water, the combination being attended with the extrication of much heat, the compact quicklime swelling and crumbling into slaked lime. The chemist’s term for a compound of a basic oxide with water is hydrate, while that of an acid oxide with water is for him properly an acid, or in order to particularly distinguish this class, an oxy-acid. It was however the older practice to give the name of acid to the oxide alone, and this naming having found its way into popular language is much more familiar to the non-scientific reader. The systematic names of the two compounds of carbon and oxygen are carbon monoxide and carbon dioxide, but we shall use here the more familiar terms carbonic oxide and carbonic acid.

We have now to call attention to a substance which contributes by far the largest part to the solid crust of our globe. It is called silica, from silic-, the Latin word for flint (without case suffix): it is seen in flint, and very pure in rock crystal, quartz, agate, and calcedony. It forms the essential part of every kind of sand and sandstone, and is the principal ingredient of clay, granite, slate, basalt, and many other minerals. Silica is the oxide of a quasi-metal called silicon, which can be obtained from silica with difficulty, and only by roundabout processes, presenting itself in different conditions according to the process used. Silica is an acid oxide, and it readily unites with most of the basic oxides when heated with them, forming a class of compounds of different properties which are much modified in admixtures containing two or more. Very few of these silicates are soluble in water, most of them are not: they are all fusible at various temperatures, except silicate of alumina, of which fire-clay is chiefly constituted. Alumina, it should be stated, is the oxide of the metal aluminium. The silicates of lime and of magnesia fuse only with great difficulty; but the silicates of iron and of manganese are easily fused, and silicate of lead still more so. Glass is a mixture of silicates, often of lime, soda, and alumina; sometimes of lead and potash mainly; porcelain and pottery consist chiefly of silicate of alumina with varying proportions of silicates of iron, of lime, etc.

It now remains only to mention two non-metallic elements that are nearly always present in crude iron, but which the metallurgist strives to eliminate, as they are in general very injurious to the quality of the material even when their amount is very small. The first is sulphur, well known as brimstone, also as flowers of sulphur, a yellow coloured solid, which burns in the air. The product of the combustion is an invisible gas of a readily recognized pungent odour: this is an acid-forming oxide containing equal weights of sulphur and oxygen. There is another oxide in which the weight of oxygen is one and a half times that of the sulphur, and this is the radicle of the very active sulphuric acid or oil of vitriol. Sulphur, like oxygen, unites with most of the other elements, forming compounds called sulphides. Of these the iron compound called pyrites is the best known, and its occurrence in coal prevents the use of that material as fuel in contact with iron or other metals. Phosphorus is an element that occurs naturally only in combination; in its separated state it is a very inflammable solid. It combines directly with other substances and is taken up by some fused metals in large quantities. In many cases a very small proportion of it existing in a metal greatly modifies the properties of that metal. Phosphorus forms several oxides, and these are radicles of powerful acids, among which is phosphoric acid that combines with basic oxides to form phosphates.

We have now, in the few last paragraphs, set before the reader the minimum of chemical knowledge that will enable him to follow the rationale of such processes of the modern treatment of iron and its ores as we can here give an outline of. Although there are numberless minerals from which some iron can be extracted, the name of iron ore is confined to such as contain a sufficient amount to make the extraction commercially profitable, and this requires that the mineral should be capable of yielding at least one-fifth of its weight. The ores are very abundant in many parts of the world, and they consist mainly of oxides and their hydrates, or of carbonate, or of carbonate mixed with clay and silicates, sometimes also with coaly matters in addition. The carbonate iron ores are often mixed with oxides. Each class of ore is liable to be contaminated with phosphates and with sulphur. The richest ore is the magnetic iron ore, which is found in enormous masses in Sweden, Russia, and North America. It is an oxide containing 72·41 per cent. of iron. Red hæmatite and specular ore are varieties of another oxide with 70 per cent. of iron: the former is a very pure ore when compact. It is found in Lancashire, Cumberland, and South Wales, and much has been imported from Spain, while America has abundant supplies near Lake Superior. Specular iron ore forms brilliant steel-like crystals which show the red colour of hæmatite only when scratched or powdered. Elba was famous for this ore, which occurs also in Russia and Sweden, and large deposits are met with in both North and South America. Brown hæmatite is a hydrate of the former, containing 60 per cent. of iron; it abounds in France and Spain, where some kinds are associated with a noteworthy quantity of phosphate of iron. Spathic or sparry iron ore is, when pure, a collection of nearly colourless transparent crystals, consisting of carbonate of iron; it contains about 48 per cent. of iron, and also some of the metal manganese, which last circumstance makes it, as we shall see, particularly suitable for producing certain kinds of steel—indeed it is sometimes called steel ore. Large beds of it occur in Styria and Carinthia. Clay iron stone, or clay-band, has been extensively mined in Britain. It is found abundantly in Staffordshire, Yorkshire, Derbyshire, and South Wales. It consists of carbonate of iron intimately mixed with clay. The quantity of iron in some samples falls as low as 17 per cent., but it rises with variations to as much as 50 per cent. Much of its importance arises from the fact of its occurring in beds alternating with layers of coal, limestone, and clay, so that the same pit is sometimes able to supply firebricks for building the furnace, fuel for the smelting, and limestone for the flux,—a combination of advantages that for long enabled iron to be produced in England cheaper than elsewhere. The like is true of the blackband ore, which, in addition to the same ferruginous composition as the last, contains also so much combustible or bituminous material that it can be calcined (roasted) without additional fuel. The deposits of blackband in Lanarkshire and Ayrshire, which were discovered only in 1801, have given great industrial importance to the district. Yet another British ore must be noticed, namely, the Cleveland ironstone of the North Riding of Yorkshire. This is a carbonate of a grey or bluish colour caused by the presence of a little iron silicate. It contains also a considerable amount of phosphorus.

How simple is the operation of obtaining iron from the ore has already been stated—that it is necessary only to surround lumps of ore by fuel in a fire urged by a natural or artificial blast, and then to hammer the mass extracted from the furnace so as to weld together the scattered particles of the metal, and at the same time squeeze out the associated slag and cinders, in order to obtain a coherent malleable piece, which can be reheated in a smith’s fire, and forged into any required form. It is no wonder therefore that iron was so produced by the ancient Britons; at any rate Cæsar found them well provided with iron implements and weapons. No doubt the Romans brought their more advanced skill to the working of the metal; but in the matter of treating the original ore, the methods they pursued on an extensive scale in Britain were of the rude kind already described. Indeed in localities where the Romans were known to have carried on their operations, the remains of their workings are almost always found on high ground, so that it may be inferred that they relied upon the winds to fan their fires, and their operations were incomplete and wasteful. The most extensive of them appear to have been in Sussex and Monmouthshire, in which last county there are places where the ground is in large areas covered by their cinders and refuse, and in this about 30 or 40 per cent. of iron occurs, so that for some centuries this material was found capable of being profitably reworked as a source of the metal. Iron continued to be produced in England during the middle ages with charcoal for fuel, but its export was forbidden, and whatever steel was required had to be imported from abroad. Afterwards German artisans were brought over for making steel, and soon afterwards the importation of shears, knives, locks, and other articles was prohibited. The native production of iron continued, and this consumed the forests so rapidly for the supply of charcoal, that various Acts were passed to restrain the iron-makers, in order to preserve the timber. In spite of these, the arts of smelting and working iron advanced apace: bellows were used for the blast, and then the works were brought down into the valleys, where water power could be employed to work them. The scarcity of charcoal fuel caused many attempts to supply its place with pit coal, but these met with small success, partly on account of the coal containing so much sulphur, and partly from the difficulty of obtaining with it a sufficiently high temperature, especially as the blowing apparatus was as yet very imperfect. At length, in the first half of the seventeenth century, the problem was solved by Dud Dudley, whose process was kept secret, but is believed to have consisted in supplying coal at the top of a higher furnace, in such a manner that the coal was converted into coke by the heat of the escaping gases before it reached the reducing zone of the furnace. This innovation was violently opposed by the charcoal smelters, who persecuted the inventor in every way, until their resistance was successful. But before the middle of the next century coke was regularly used in iron smelting, the process having been made successful by Darby at Coalbrookdale, and then many new applications of cast iron came into vogue. Coke being a substance burning less freely than charcoal, bellows were found inadequate to give the necessary blast, and were displaced by blowing cylinders, actuated at first by water wheels, but this uncertain and comparatively feeble source of power was soon superseded by the steam engine, the “fire engine,” for which, as we have seen, Watt obtained his patent in 1769. The furnaces were not then all engaged in producing the fusible metal now called cast or pig iron as are the huge blast furnaces we see at the present time. Indeed it was much to the disgust of the old iron smelter that occasionally his product turned out to be of the fusible kind, unworkable by the hammer, which therefore he regarded as worthless. At what date cast iron was first used is uncertain; but probably it was not long before the fourteenth century. The furnaces in use up to that time were small square walled-in structures only 3 or 4 feet high, and their effect would not greatly exceed that of a smith’s forge: but as improved blowing apparatus gave more power, they soon became enlarged into oval or round brick towers from 10 to 15 feet high, and they, like the small furnaces, could be made to yield either smith iron or steel by modifying the charge and the manner of applying the blast; while furnaces of dimensions exceeding a certain limit could no longer be trusted to turn out malleable metal, but they produced instead the cruder substance we call white pig iron, and this requires much subsequent treatment before it is converted into malleable or “merchant iron.” Nevertheless the demand for cast iron as such, and more particularly the adoption of improved methods of deriving malleable iron from it, caused further increase in the size and numbers of blast furnaces, until in the early part of our century 30 feet was not an unusual height, the highest one in England in 1830 attaining 40 feet. The total make of pig iron in England was in that year nearly 700,000 tons, perhaps about fifty times as much as it was a century before, and thirty years later (1860) it had risen to nearly 4,000,000 tons. These figures show the extraordinary expansion of the British iron manufacture in the earlier part of the century; and the still more extensive applications of iron during the next twenty years had the effect of almost doubling the produce in 1880, and of increasing also three-fold the amount of foreign metal imported, raising it to 2,500,000 tons. The reader will now, it is hoped, be prepared to follow with some interest a brief account of the principal inventions which have brought about results of such importance.

Deferring for the moment any description of the latest blast furnaces, we invite his attention to Fig. 17, which represents the furnace used in the first half of our century, but which now is of an obsolete type, Fig. 18 being the section and plan of the same. The lower part of Fig. 17 shows where the molten metal has been allowed to run out of the furnace into channels made in dry sand; first a main stream, then branches to right and left, each of these with smaller offsets on each side of it. These smaller channels are the moulds for the pigs, so called because of the fancied resemblance of their position with regard to the branch that supplied them, to the litter of a sow. They are easily broken off from the larger mass, and then form pieces about 3 ft. long with a ᗜ-shaped section, 4 in. wide, the weight being from 60 to 80 lbs. This is iron of the crudest kind, and though it is often referred to as “cast iron,” it is, as a matter of fact, not used in this state for any castings, except those of the very roughest and largest kind: a certain amount of purification is requisite in most cases. This is given by fusing the metal—along with some form of oxide and often other matters—in a cupola furnace, which is like a small blast furnace, being from 8 ft. to 20 ft. high and uses coke for fuel with a cold blast.

So far from being simply iron, pig contains a large and variable proportion of other matters amounting often to 10 or 12 per cent.; and these confer upon it its fusibility. The principal one is carbon, which is found in the metal partly in the state of chemical combination with it, and partly in the form of small crystals similar to those of graphite or plumbago, disseminated through the mass. When there is a comparatively small proportion of the carbon combined with the iron, the substance is grey, and it can be filed or drilled or turned in a lathe. In white cast iron the combined carbon predominates, or is sometimes accompanied by scarcely any graphitic carbon; it is brittle and so very hard that a file makes no impression. It fuses at a lower temperature than the other varieties. A third kind is the mottled cast iron, which shows a large coarse grain when broken, and distinct points of separate graphite particles; it is tougher than the others, and therefore when cannon were made of cast iron this variety was preferred. The following table giving the percentage composition of four samples of crude cast iron will show their diversities.

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.

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.