one of the imperfect metals, but the hardest and most useful as well as the most plentiful of them all, is of a livid whitish colour inclining to grey, and internally composed to appearance of small facets; susceptible of a fine polish, and capable of having its hardness more increased or diminished by certain chemical processes than any other metal.
It is very generally diffused throughout the globe, being frequently found mixed with sand, clay, chalk, and over the being likewise the colouring matter of a great number of stones and earth. It is found also in the ashes of vegetables, and in the blood of animals, in such abundance, that some authors have attributed both the colours of vegetables and of the vital fluid itself to the iron contained in them. In consequence of this abundance the iron ores are extremely numerous.
1. Native iron, formerly thought not to have an existence anywhere, is now certainly known to have been met with in several places. It is, however, by negal, &c., no means common, but occurs sometimes in iron mines. Margraaff found a fibrous kind of it at Eibenstock in Saxony, and Dr Pallas found a mass in Siberia weighing 1600 pounds. Mr Adanson likewise informs us, that native iron is common about Senegal; but some naturalists are of opinion that these pieces which have been taken for native iron, are in reality artificial, and have been accidentally buried in the earth. The large piece mentioned by Dr Pallas is of that species called red flint, which is malleable when cold, but brittle when red hot.—A mass of a similar nature is said to have been lately found in South America.
This American mass of iron was discovered by some Indians in the district of Santiago del Estero in the midst of a wide extended plain. It projected about a foot above the ground, and almost the whole of its upper surface was visible; and the news of its being found in a country where there are no mountains, nor even even the smallest stone within a circumference of 100 leagues, could not but be very surprising. Though the journey was attended with great danger on account of the want of water, and abundance of wild beasts in these deserts, some private persons, in hopes of gain, undertook to visit this mass; and having accomplished their journey, sent a specimen of the metal to Lima and Madrid, where it was found to be very pure soft iron.
As it was reported that this mass was only the extremity of an immense vein of the metal, a commission was given to Don Michael Ribbin de Celis to examine the spot; and the following is an abstract of his account.
"The place is called Otumpa, in lat. 27° 28' S. and the mass was found almost buried in pure clay and ashes. Externally it had the appearance of very compact iron; but internally was full of cavities, as if the whole had been formerly in a liquid state. I was confirmed in this idea (says our author), by observing, on the surface of it, the impressions of human feet and hands of a large size, as well as of the feet of large birds, which are common in this country. Though these impressions seem very perfect, yet I am persuaded that they are either a lucifer nature, or that impressions of this kind were previously upon the ground, and that the liquid mass of iron falling upon it received them. It resembled nothing so much as a mass of dough; which having been stamped with impressions of hands and feet, and marked with a finger, had afterwards been converted into iron.
"On digging round the mass, the under surface was found covered with a coat of scoriae from four to six inches thick, undoubtedly occasioned by the moisture of the earth, because the upper surface was clean. No appearance of generation was observed in the earth below or round it to a great distance. About two leagues to the eastward is a brackish mineral spring, the only one to be met with in all the country. Here there was a very gentle ascent of between four and five feet in height, running from north to south; all the rest being as perfect a level as can be imagined. The earth in every part about this spring, as well as near the mass, is very light, loose, and greatly resembling ashes even in colour. The grass of the adjacent parts is very short, small, and extremely unpalatable to cattle; but that at a distance is long and extremely grateful to them: from all which circumstances it is probable that this mass was produced by a volcanic explosion. Its weight might be estimated at about 300 quintals.—It is likewise an undoubted fact, that in these forests there exists a mass of pure iron in the shape of a tree with its branches. At a little depth in the earth are found stones of quartz of a beautiful red colour, which the honey-gatherers, the only persons who frequent this country, make use of as flints to light their fires. They had formerly carried some of them away on account of their peculiar beauty, being spotted and studded as it were with gold. One of these, weighing about an ounce, was ground by the governor of the district, who extracted from it a drachm of gold."
The native iron said to have been found about Senegal has a cubical form; and out of this the black inhabitants make different kinds of vessels for their own use. Some masses have been found in a polyhedral granulated form, and of a bright yellow colour; but which, on being polished, show the proper colour of the metal. Mr Bergman informs us, that the great mass of native metal found in Siberia resembles forged iron in its composition, a centenary, or 63 grains, yielding 49 cubic inches of inflammable air; and from many experiments it appears, that ductile iron yields from 48 to 51 cubic inches of the same kind of air. Dr Matthew Guthrie informs us, that "the pores of this iron were filled with a yellow vitreous matter, of such hardness as to cut glass." The cells are lined with a kind of varnish contiguous to the glairy substance within.
2. The calciform ores are either composed of the Calciform, blackish, blackish-brown, or red calx of the metal; the former being in some measure magnetic, in consequence of the phlogiston it contains; the latter showing nothing of this property until it be roasted.
The name of calciform may be applied to all the ores of this metal, excepting the native iron already mentioned, and the native Prussian blues, of which we shall afterwards treat. All of them are mixed with different minerals, and generally take their colour from that of the calx of iron which is prevalent in them. Mr Kirwan enumerates a great many different species.
3. Steel ore, Stahlerz, the ferrum chalybeatum Steel ore Linnei, and minera ferri nigra of Cronstedt. This is of a dark colour, solid, and compact, but with difficulty striking fire with steel; reducible to a black powder, obedient to the magnet, and somewhat malleable when red hot; affording from 60 to 80 per cent. of good iron. It is met with in Sweden, the Isle of Elbe, and North America. The ferrum tellurium and minera ferri crysallizata of Wallerius, belongs to this species, but is somewhat less magnetic. Our author denominates it crystallized iron ore in an octohedral or cubic form.
4. The magnet, according to Fourcroy, is a muddy Magnet's iron ore, which, however, some authors suppose to be very near the metallic state. Mr Kirwan says it differs but little from the foregoing, only that it has less lustre. There are two kinds, the fine and the coarse grained, of which the latter lose their power the soonest. When heated red hot, it smells of sulphur. Our author thinks it may contain nickel, as this seems to contain metal is found to possess a magnetic property when purified to a certain degree.
5. The brown calx of iron combined with plumbago Brown ore, go, black eisen glimmer, schwartz, eisen baren or eisenmar, consists of black shining scales more or less magnetic, affording, according to Mr Rinnan, 26 per cent. of iron, the rest being plumbago.
6. The brown calx of iron united with the white White calx of manganese, and mild calcareous earth in various ores proportions. These constitute the white ores of iron, on which Mr Bergman has given a dissertation.—"They have received (says he) divers denominations from the singular heat with which they are accompanied. Their texture is almost the same with that of the calcareous stone, yet it is rarely found compact, and composed of impalpable particles. It is sometimes squamous, sometimes granulated with small distinct particles, some of them shining, but in general spathous. This description, however, is not meant for their complete and perfect state; for the figure of their parts is more or less destroyed by spontaneous calcination." Iron; nay, the whole mass is at length resolved into a powder: sometimes it is found stalactitic, sifulous and ramous, cellular, or even germinating like moss. Sometimes, though very seldom, they have sufficient hardness to strike fire with steel; but though, when found mixed with flint and newly dug up, they are of this kind, yet they soon lose the property we speak of. When perfect, they generally resemble the calcareous stone, unless when exposed for some time to the air, by which the union of their parts are gradually diminished. Their colour is white, but the surface which comes into contact with the air grows gradually brown, or even blackish; yet as long as the iron which is converted into an ochre remains in them, they have a ferruginous hue; but though the surface is thus changed, the internal parts remain the same, and, on being filed or broken, exhibit the natural colour.
This change is effected by the air, not upon the iron, as is commonly believed, but on the white calx of manganese which is depilogiticated by the atmosphere.
"The specific gravity of the ore, when perfect, varies between 3,640 and 3,810, and is diminished according to the degree of calcination. The ore whose particles are quite separated is from 2.5 to 2.9; but that which is not perfectly corroded, from 3.3 to 3.6. It is rarely attracted by the magnet, whether perfect or calcined, though the metallic part sometimes amounts to nearly one half the weight.
The white ores of iron are found, though in very small quantity, in Sweden. The Suart-begger, or Black Mountain, in Dalecarlia, has its name from its surface, which is grown black by calcination. It is high, and naked on the summit, which is crossed by a broad calcareous vein with shining particles of spar, and a white ore of iron, together with a galena, pseudogalena, black ore of iron, pyrites, scheel, and garnet intermixed. In the old mines at Halleforo, or the eastern mines, the rock itself appears to consist of a white ore of iron; but in other places it is either found in small quantity, or very poor in metal. Many mountains about Smialkald in Germany contain these ores. In one called Stabbagge, a broad vein occurs almost horizontal, and from 25 to 30 fathoms thick. It consists of an irregular spar, in which are dispersed quartz and pieces of the ore, which are found of a better quality in proportion as they are more deeply seated. The uppermost side, which is pendant, consists of a sandy stone from 9 to 20 fathom high; but the lower is margaceous, and is found more indurated towards the lower parts; and at the very lowest is extended by a blue mica: the sides scarcely cohere to the vein. The whole mountain in Naufavia consists of a yellowish ore of iron, certain veins of which are accompanied with copper, and others with hematites. The hill of Arzberg, situated at Eifenartz in Upper Silesia, is 6000 fathoms in circuit, 900 in diameter, and 450 in height. According to some accounts the ore is irregularly accumulated and concreted, consisting of masses of quartz charged with argillaceous earth and white ore of iron; but, according to others, the ore is found there not only in heaps, but in various veins."
This ore, when analysed, gave 38 parts of the brown calx of iron, 24 of the white calx of manganese, and 50 of mild calcareous earth. Another from West Silvathreg, yielded 22 of the brown calx of iron, 28 of the white calx of manganese, and 50 of mild calcareous earth. The aerial acid is used, and is united not only to the earth, but also to the metallic calx. The above proportions of the crude materials in the ore of Eifenartz, would yield, according to Mr Kirwan, 38 parts of calcareous earth, 38 of iron in its metallic state, and 24 of manganese. Many others are poorer, and come to such a degree as scarcely to deserve the name of an ore. They abound also in France and Spain, and are found sometimes in heaps, sometimes also forming veins, strata, or even whole mountains. Mr Bergman never found them contain any organized bodies; a mark (says he) by which the most ancient productions of the earth have been distinguished. When this iron ore bears a stalactitical appearance, and is very white, it is called floes ferri, and eisenbluth. An hundred parts of it yield 65 of calcareous earth, and 35 of calx of iron; which, according to Rinman, produce 27 of iron in its metallic state.
7. Magnetic sand. Of this kind is the black sand of Virginia, whose specific gravity is about 4,600, and contains half its weight of metal.
From an account inserted in the Philosophical Transactions for 1763, we are informed, that there are very large quantities of this sand-iron ore in Virginia; perhaps as large as of any other kinds of iron-ore. It is so pure, that it requires a mixture of bog-ore, or of flags from other smeltings, to reduce it to a metallic form. The iron and steel produced from it were above 60 per cent. or from 50 to 85; the quality of both extremely good; and two small bars were sent as a sample to the museum of the Royal Society of London. Large strata of black sand-iron-ore are found in Portugal, even at a considerable distance from the seashore, or from any running waters. A very great part of this black sand is attracted by the magnet. There is also found, particularly in France, a black, heavy, unmagnetic sand, of the siliceous kind, which is said to contain iron and zinc in great quantity. Mr Kirwan, p. 143, of his Mineralogy, speaks of a siliceous sand consolidated by semiflologiticated calx of iron, which does not crumble into sand when powdered. It is generally of a black or brown colour; but grows reddish or yellowish, and moulders by exposure to the air. It does not effervesce with acids, unless it contains tefaceous particles, which is frequently the case; it is even frequently covered with shells. He adds, that the agglutinating power of solutions of iron has been shown by a stony concretion of this sort that had been long buried in the sea, and is mentioned in a paper of Mr Edward King in the Philosophical Transactions for 1779. Mr Rinman, however, has found that depilogiticated calces of iron, and particularly its solutions in mineral acids, have no binding power.
8. Red calx of iron indurated and combined with a small quantity of clay, frequently with manganese.—Indurated red ore. Fourcroy calls this a muddy iron-ore, which seems to be formed in the manner of stalactites, and deriving its name from its colour, which is commonly red, or the colour of blood, though not without variations. Mr Kirwan says, that "it is generally of a red, yellow, purple, or brown colour, of a metallic lustre, and very hard, though seldom capable of giving fire with steel." Fourcroy tells us, that it is usually composed of layers which cover each other, and are themselves formed of convergent needles, the external part being covered with tubercles; and that it is not only distinguished by the colour, but by the form, as the haematites botrytes, in the form of bunches of grapes. Mr Kirwan tells us, that its structure is either solid, granular, scaly, or fibrous; that it occurs in shapeless masses, in a stalactitical form; or, according to Gmelin, crystalized in regular forms, though M. de Lisle denies this. In some places it forms whole mountains, and affords from 40 to 80 per cent. of iron. Mr Gerhard extracted alum from it, which affords a proof of its containing clay; and Mr Hilan found it also to contain manganese. In its natural state it is not affected by the magnet; but by torrefaction it becomes black and magnetic.
9. Haematitical, red, yellow, and brown ochres
These are, by Mr Kirwan, intitled "haematites in a loose form, mixed with a notable proportion of argill" (clay.) They are distinguished, he says, from clays, by containing a larger proportion of martial particles. To this species belong the ores which become brown by calcination, and likewise magnetic. They are sometimes mixed with clay or calcareous earths; in which case these ores effervescence with acids. The haematites, or blood stones, have their names, not on account of their external colours, but because, when reduced to powder, they produce a red or blood-colour. The yellow haematites, however, only produce the same colour by pulverisation. They are productive of very good iron, and are found in great abundance in the province of Galiza in Spain. The inhabitants of Compostella, the capital, make a good commerce of these haematites of the hardest kind for the furnishing gold leaves, and various other metals. A dark blue kind, somewhat similar to black-lead, is principally employed for these purposes. They are found in many parts of Europe, sometimes forming whole mountains. The most extraordinary ores of this kind, both on account of their forms and of their various and brilliant colours, are found in the island of Elba near the coast of Tuscany. The crystallized ores are here the most beautiful and the most common, though not to be met with anywhere else. They exhibit various gradations of the finest colours, as red, violet, blue, green, yellow, brown, and black; inasmuch that, according to Coudrai's expression, they look like so many clusters of emeralds, sapphires, diamonds, rubies, and topazes. E. Peni and Mongez affirm, that these ores are mineralized only by the aerial acid; tho' Coudrai is of opinion, that they contain sulphur also. Besides these beautiful crystallized ores, this island contains also many others; being indeed little other than a group of iron-mountains. The ores in general produce the very best kind of iron.
10. Emery, smyrnis, is a grey or reddish iron-ore found in great quantity on the islands of Jersey and Guernsey. It is extremely hard, yielding in this respect no substance except the diamond itself. It is also very refractory, and for these reasons is not used for the sake of the metal it contains, nor indeed is it well known what proportion is contained in it. "The best sort (says Mr Kirwan) is of a dark grey colour, but becomes brown, and in great measure magnetic, by calcination; other sorts are of a rusty reddish white or yellowish colour. Its specific gravity is from 3,000 to 4,000. It is used in polishing glass and metals; for which purpose it must first be ground down and levigated in mills.
11. The argillaceous ores. These comprehend the Bog ores, ochres, and more particularly, those mentioned by Fourcroy under the name of bog-ores of iron, which are commonly met with disposed in beds, and seemingly deposited by waters. Mr Fourcroy informs us, that this kind of ore is very often in the form of spherical bodies either regular or irregular. Organic matters, such as wood, leaves, bark, flax, &c. are not unfrequently found in the state of bog-ores. This kind of transition seems to indicate an analogy between iron and organic substances. In the wood of Boulogne near Auteuil there is a mine of bog-ore of iron, in which vegetable substances become mineralized almost immediately under our eyes.
Mr Kirwan distinguishes two principal varieties of these; one found on mountains, and such as are met with in swampy grounds or low lands overflowed with water; both of them very heavy, and some absorbing water like clays.
The Highland argillaceous ochres are either yellow, Highland red, brown, or greyish, indurated and friable, or loose argillaceous and powdery, or in grains; they are composed chiefly of the red or yellow calx of iron, or of a grey iron ore called Torfien, in a loose form mixed with clay. Hence they often contain manganese or siderite, and in France are said to be mixed with a calx of zinc. They do not obey the magnet before calcination, and rarely after it. They effervescence with acids only in consequence of being mixed with calcareous earths; they are soluble with difficulty in the acids, but the most soluble are the best. The iron produced from them is of very different quality, according to the nature of the ore from whence it is produced. To this species belong the hornstone overloaded with iron, and a white iron ore mentioned by Rinman found in Kent. It is mixed with clay or marl, and is scarcely soluble in acids. It affords 47 per cent. of brittle iron.
The swampy argillaceous ore, according to Mr Kirwan, are found in irregular lumps of a brown or brownish-black, and sometimes in round balls, porous or solid, or in flat round pieces or in grains, and sometimes in slender triangular prisms parallel to each other, and very brittle. It is mixed with clay and extractive, and becomes magnetic by calcination; during which operation it gives out a quantity of aerated volatile alkali, and loses one-fourth of its weight. The crude ore affords about 36 per cent. of metal, and 50 per cent. after calcination. The iron produced from it, at least in Sweden, is that called cold-iron. According to Mr Hjalm some sorts of this ore contain 28 per cent. of manganese.
12. Red calcareous iron ore is found loose in many parts of England, effervesces strongly with acids, and is used as a paint under the name of red ochre.
13. Martial calamite. Though calamite is properly an ore of zinc, it sometimes contains such a large proportion of iron as to make it worth while to extract the iron. The ore consists of a mixture of quartz and clay, with the calces of iron and zinc. It is of moderate hardness, and a yellow, red, or brown colour.
14. Martial pyrites. This has its name from its property of giving fire with steel. It is commonly in pyrites. small red masses, sometimes regularly formed, and usually cubical, spherical, or dodecahedral, though their form varies considerably. Some are brown on the outside, others of the colour of iron, some yellowish, and resembling the ores of copper, even on their surface; but all of them are yellow, and as it were coppery within, and for the most part composed of needles, or pyramids of several sides, whose summits converge to a common centre. The pyrites are commonly dispersed, and particularly in copper mines in the neighbourhood of iron-mines, and in clays and coal mines, the upper stratum of the latter being almost always pyritous. They are all easily decomposed, and yield green vitriol, as is explained under the article Chemistry.
15. Iron mineralized by arsenic. This combination takes place either by the combination of arsenic alone with the metal, or in conjunction with sulphur. The former is called in Germany mispickel, and speise by the Bohemians; it is of a bright white colour, sometimes, though rarely, variegated like a pigeon's neck, and is not easily altered by exposure to the air. It is not magnetic either before or after calcination; it is soluble in acids, and affords arsenic by distillation in the proportion of 30 or 40 per cent. and sometimes contains a small proportion of copper and silver. It is frequently found in indurated clay, quartz, spar, schorl, &c., and mixed with other metallic ores. When this metal contains less than 1/20th of arsenic, it is magnetic, according to Scheffer; whence, if the calcination be pushed to a sufficient length, the ore must remain magnetic.
That species of ore which consists of iron mineralized by sulphur and arsenic together, contains the white, grey or bluish grey pyrites or marcasite. It is found either in solid compact masses of a moderate size, or in grains, and gives fire with steel. When burnt it affords a blue flame and the smell of arsenic, with orpiment or realgar, instead of pure arsenic by distillation in close vessels. It is not magnetic either before or after calcination, and contains much more arsenic than sulphur.
16. Native Prussian blue consists of clay mixed with iron, and coloured with some unknown tingeing substance, generally found in swampy grounds or bogs. It is at first white, but when exposed to the air becomes either of a light or deep blue. By heat it turns greenish, and emits a slight flame, becoming afterward red and magnetic. It is soluble both in alkalies and acids; but the alkaline solution is precipitated by acids, and the acid solution by alkalies. The precipitate at first is greenish, and gradually affumes a white hue, but regains its blue colour on being mixed with vegetable astringents. Mr Woulfe found this kind of ore in Scotland on the surface of the earth. The greatest part of marshy grounds containing turf, likewise have some of this.
17. The terre verte, or green earth of Verona and Normandy, is used as a pigment, and contains iron in some unknown state, mixed with clay, and sometimes with chalk and pyrites; alum and selenite being likewise accidentally mixed with it. It is soluble with difficulty in acids, is not magnetic before calcination, and becomes of a coffee-colour by heat.
18. Mr Fourcroy informs us, that "it has been discovered some years ago, that iron is often united naturally with the phosphoric acid. The muddy or bog ores are sometimes of this nature; a portion of this compound remaining in the iron gives it the property of being brittle when cold. Iron in this state was called siderite by Bergman, and it has since been called water-iron."
There are several other kinds of iron ore enumerated by mineralogists; but those already mentioned are the most remarkable.
The following observations on iron in its different states, with an account of the methods of manufacturing it, &c., are extracted from Magellan's Notes on Cronstedt's Mineralogy.
1. Iron is employed in three different states, each having its peculiar properties, by which they are each more particularly applicable to various purposes. The first is cast iron, the second is wrought or malleable iron, and the third is called steel.
According to Bergman, cast iron, which may be called unripe or raw-iron, contains the smallest share of phlogiston. The malleable iron contains the greatest quantity; and the steel a middling share between both, neither so much as the malleable, nor so little as the cast-iron. This last is called also pig-iron, and yettin in England.
2. The richest ores of iron are the compact and ponderous, of a brownish, reddish-brown, or red colour. Some of these ores, in colour and appearance, do not ill resemble iron itself; as the grey ores of Derbyshire, and the bluish of the Forest of Dean in Gloucestershire. Most of the Swedish ores are likewise of this kind. Others are blackish, brown, red, yellowish, or rusty-coloured; these are the most common in England and Germany. There is one very singular species of a friated texture, and of a pale yellowish or greyish colour, oftentimes white, and in some degree pellucid; which, although in its crude state, promises nothing metallic, nevertheless, on being moderately calcined, discovers, by the deep colour it affumes, that it abounds in iron. Cramer informs us, that it gives out by fusion from 30 to 60 per cent. But some richer ores yield no less than 70 and 80 on the hundred.
3. Different kinds of iron ore are found adhering in some mines to the tops of caverns in form of icicles or strix, sometimes irregularly cluttered together, sometimes hanging down like the bristles of a brush; from whence the name of brush-iron-ore. Other particular forms of the iron stone have occasioned a variety of fanciful names, that are met with in some of the metallurgic writers.
4. The iron of Great Britain is made from three different kinds of ores: 1. From the iron-ore called the Lancashire ore, from the country where it is found in greatest abundance. This ore is very heavy, of a fibrous or lamellated texture; it is of a dark purple, approaching to a shining black; and when reduced to powder, it becomes of a deep red; it lies in veins like the ores of other metals. 2. The bog-ore, which resembles a deep yellow ochre clay, and seems to be the deposition of some ferruginaceous rivulets, whose currents had formerly been over the surface of those flat marshy plains. It lies in beds of irregular thickness, commonly from 12 to 20 inches, and very various in their breadths from side to side, never being of great dimensions. 3. The iron-stones, however, have no regular... gular appearance, and do not in the least resemble a metal in their external surface. They lie often in beds of great extent, like other stony matters, and are sometimes stratified with seams of pit-coal, forming alternate layers.
5. The ores of iron are commonly calcined previous to the fusion, even the harder ones, though they should contain nothing sulphureous or arsenical, in order to calcine the hard adhering matrices, and render the masses soft enough to be easily broken into fragments of a convenient size for melting. After the mineral is duly prepared, it must be melted in furnaces of large capacities, from 16 to 25 feet high, and from 10 to 14 wide; the most approved shape nearly resembles that of a hen's egg, with the largest end undermost, below which is a square cavity to contain the melted metal, and at the top a very short vent about 20 inches in diameter. The inner wall is built of firestone, which endures very strong heat with little risk of melting, and all the joints are cemented with mortar composed of sand and clay. This is surrounded with more building, which deviates more and more from a circular form, and becomes a square building of about 20 feet at the base, and gradually converges to the top.
6. Near the bottom is an aperture, for the insertion of the pipe of a large bellows, worked by water or by other machines that may produce a strong current of air. Some very powerful ones, as those in the iron works at Colebrook-dale and at Carron, consist of two or more iron cylinders, about upwards of two feet wide, whose pistons are alternately moved by a small fire engine or by a water wheel: but Mr Wilkinson very ingeniously adapted to his own a large vaulted receiver surrounded by water, which produces a very regular and uniform blast. Two or more holes are also left ready to be occasionally opened at the bottom of the furnace, to permit at a proper time the scoria and the metal to flow out, as the process may require. Charcoal, or coke with lighted brushwood, is first thrown in: and when the inside of the furnace has acquired a strong ignition, the ore is thrown in by small quantities at a time; with more of the fuel; and commonly a portion of lime-stone is thrown also as a flux. The ore gradually subdues into the hottest part of the furnace, where it becomes fused; and the metallic parts being revived by the coal, pass through the scoria, and fall to the lower part or bottom of the furnace, where a passage is open for taking off the scum or dross. The metal now in strong fusion is let out by a tap-hole into furrows made in a bed of sand; the large mass, which sets in the main furrow, is called by the workmen a sow, and the lesser ones pigs of iron. Chimney-backs, stoves, garden-rollers, &c. are formed of this rough metal, taken out of the receiver with ladles, and cast into moulds made of fine sand."
It is proper to observe, that the excessive and long-continued ignition kept up in these furnaces gradually wastes the materials of which they are composed, rendering their sides thinner until at last they become unable to sustain the weight of the melted metal; so that it has sometimes been known to burst out suddenly in a violent and most destructive stream. At certain intervals, therefore, the fire ought to be allowed to go out, whatever may be the expense of rekindling it, and the furnace examined and repaired.
7. The quantity of fuel, the additions, and the heat, must be regulated, in order to obtain iron of good quality; and this quality must likewise be in the first product be necessarily different, according to the nature of the parts that compose the ore.
8. Two or three tons, viz. 4000 or 6000 pounds weight of iron, are now run off in 24 hours, at some large furnaces, after the application of the large bellows; whilst scarcely an hundred weight could be obtained in a day before that application, because a large quantity of the metal was lost in the dross; hence in some places the flags of different ores, left by old operators in former times, are now remelted to advantage along with fresh ore; and on account of the richness of these old flags of different ores, some people have been misled into the opinion, that the metal was regenerated in them.
9. Peat and turf has been found to answer tolerably well, mixed with charcoal, for the smelting of iron ores; but an attempt to use it on a large scale has at last been found not to answer the expectations that had been conceived from the first trials. Pit-coal, if applied to the same purpose, renders the iron hard and brittle; but this inconvenience is prevented, by previously soaking the coal, and employing it in the state of true coak. Cramer, in his Art of Assaying, p. 347, says, that pit-coals, kent-coals, and Scotch-coals, which burn to a white ash like wood, and abound more in bitumen, may be used in the first fluxion of the iron from its ore; and if the iron proves not so malleable as required, this property may be given to it by melting the metal a second time with wood.
10. The best cast-iron or raw-iron, as much freed from heterogeneous matters as the usual process of melting can effect it, is not at all malleable, and to hard as perfectly to withstand the file.
11. In general the impure cast-iron, as run from the ore, is melted down a second time in another furnace, intermixed with charcoal. A strong blast of air being impelled on the surface of the metal, its fusion is remarkably promoted; the iron thickens into a mass called a loop, which is conveyed under a large hammer raised by the motion of a water-wheel. The iron is there beaten into a thick square form, is then heated again until almost ready to melt, and is forged; by a few repetitions of this process, it becomes completely malleable, and is at length formed into bars for sale.
12. Iron in this state of malleability is much softer than before, and of a fibrous texture. But if it is still crude and brittle after the above process, it shows that there have remained heterogeneous matters, being hidden in its interstices, which must be expelled; for this purpose the iron must be stratified with charcoal-dust within a proper furnace, heaped up in good quantity in strata; then the fire must be blown pretty strongly, so as to bring it to a fusion, which is to be helped by the addition of fusible scorias or of sand. The fire must not be much greater than necessary to make all these melt as equally as possible; to obtain this end, the melted mats must be agitated here and there with poking rods of wrought iron, in order to make every part feel alike the action of the fire and air; and the increasing scoria taken out once or twice.
13. In the mean time, a great many sparkles will be thrown out from the iron, which diminish the more as the iron comes nearer to the desired degree of purity, but they never cease entirely. The burning coals being then removed, and the scoria conveyed out of the fire through a channel made for that purpose, the iron, by lessening the violence of the fire, grows solid, and must be taken out red-hot, and tried by striking it with a hammer. If it proves crude still, let the melting be repeated; and when it is at last sufficiently purified, it is to be hammered, and extended various ways, by making it red-hot many times over; this done, it will no longer be brittle, even when cold, as Cramer affirms.
14. Cast-iron has of late been brought into the malleable state by passing it through rollers instead of forging it. Indeed this seems to be a real improvement in the process, as well in point of dispatch, as in its not requiring that skill and dexterity which forgemen only acquire by long practice. If the purposes of commerce should require more iron to be made, it will be easy to fabricate and erect rolling machines, though it might be impracticable to procure expert forgemen in a short time.
15. This method was discovered by Henry Cort of Gosport, who obtained an exclusive privilege granted by the king's patent. By this process the raw or cast-iron is freed from the impurities, which are not discharged in the common methods of rendering this metal malleable; for iron is in itself a simple homogeneous metal; and all iron must become equally good, if it be purified from the heterogeneous and unmetallic particles that are any ways mixed with it.
16. The ordinary method of converting cast-iron into malleable, is, as we have seen, by employing great quantities of charcoal, which furnishes phlogiston, and remetallizes the particles, which are unmetallized and mixed with the heterogeneous matters contained in the fused mass: but in Cort's method there is no need of charcoal, instead of which only sea-coal is employed; because the object is not to remetallize, but only to expel what is unmetallic, instead of endeavouring to restore the calcined parts with charcoal at a great expense, and still leaving the bofines undone. In this method the iron is only heated and wrought simply by the heat of the flame, instead of being mixed with the burning fuel and ashes, which are not easily disengaged afterwards from the metal. The squeezing it between the rollers, forces out the melted flags from the metallic pores, and brings its metallic fibres into a perfect solidity and close contact, so that they are obliged to cohere much more perfectly to each other, than by the interrupted and partial action of the hammer. By the operation of being long stirred, the sulphurous particles are more disposed to be disengaged, and are burned away in the form of blue sparks; the metal then begins to curdle, and to lose its fusibility, like folder when it just begins to settle; the metallic particles meeting and coalescing together, much like the churning of milk, where the cream is separated by the union formed between the fibrous particles of the cheese. The curdles formed into a connected mass become what is called loops. The process is as follows:
17. Five or six hundred weight of raw cast-iron (and even of cold short iron) is brought into a low fusion, on a kind of hearth or low furnace, in which it lies to the depth of about 6 inches. One or two workmen continually stir this fused mass with long iron pokers for about 4 or 5 hours. The heat is then lowered: the men fashion the iron into narrow pieces of about 3½ feet long, and 3 inches square, with long knives or chisels made for that purpose. They are then heated to the welding degree, and hammered to expel and scatter the unmetallic dross. These slabs are then formed to a wedge-point at one end, in order to adapt them to be received between the rollers: they are malleable already, but they contain still some dross.
18. They are then heated again to the hottest welding heat in the air furnace: and immediately passed through large iron-rollers, turned by a water-wheel or by horses. If the end presented to the rollers should slip instead of entering, a boy, who stands ready, throws some sand upon the iron, and it goes in easily. Much foreign and heterogeneous matter is squeezed out by the rollers; and the iron comes out in a purer malleable state. The same heat will serve to pass the iron through two sets of rollers, which are grooved so as to fashion it into nail-rods or other forms according to the required purposes.
19. Various and repeated severe trials have been made in the royal dock-yards of England, in the presence of persons of knowledge and rank, to prove the strength, malleability, and softness or toughness of this new iron; and it has proved to be equal, and even sometimes superior, to the best Swedish iron. But it is not easy to conceive by what singular fatality so great an improvement in manufacturing this most useful metal has not yet been generally adopted by the iron-masters.
20. Steel is iron in an intermediate state between cast-iron and malleable iron, which is soft and tough. The iron run from some German ores is found to be a good steel when forged only to a certain point.
But the best steel is usually made by cementation from the best forged iron, with matters chiefly of the inflammable kind. Two parts of pounded charcoal and one of wood ash is esteemed a good cement. The charcoal dust may be made of bones, horns, leather, and hairs of animals, or of any of these ingredients after they are burned in a close vessel till they are black: these being pulverized, and mixed with wood-ashes, must be well mixed together. The iron should be of pure metal, not over thick, and quite free from heterogeneous matters: their flexibility, both when hot and when cold, is a very good sign thereof. A deep crucible, two or three inches higher than the bars, is to receive part of the cement, well pressed at the bottom, the height of 1½ inch; and the bars are to be placed perpendicularly, about one inch distant from the sides of the vessel and from each other. All the interfaces are to be filled with the same cement, and the whole covered to the top with it; then a tile is applied to cover the vessel, stopping the joints with thin lute. 21. The crucible is then to be put in the furnace, and a strong fire is to be made, that it be kept moderately red hot for six or ten hours together; at the end of which time they will be found converted into steel. If the cementation be continued too long, the steel will become excessively brittle, incapable of being welded, and apt to crack and fly in forging. On the contrary, steel cemented with absorbent earths is reduced to the state of forged iron.
22. Steel is further purified for making the nicest kinds of instruments, such as lancets, pen-knives, razors, and various pieces, for the best kind of watches, time-keepers, or chronometers, and astronomical regulators. This purification of steel consists in melting it again with a strong but regular fire in a crucible, the better to free it from the heterogeneous parts, and little flaws that may be contained in it. It is then called cast-steel when fused into bars; which name, however, does not imply that the pieces, for instance the cast-steel razors, have been really cast in their present shape; for they must be forged from the bar after it is cast. The fusion must have been perfect, so that the metallic parts be rendered uniform. The metal diminishes a little by this process, for a bar of common steel 36 inches long, will afterwards produce another only of 35, if properly fused and purified.
23. The cast-steel will not bear more than a red heat; otherwise it runs away, like sand under the hammer, if the heat is pushed to the welding degree. Dr Watson says, that this manufacture of cast-steel was introduced at Sheffield only about 40 years ago by one Waller. This man was still living about the year 1765; he dwelt at St Bartholomew's close, and was a galloon-wire drawer by trade. The difficulty of procuring small cylinders of good steel to flatten the wire for lace-work in his business, whose defect proceeded from the bad texture of the steel, set his imagination on the enquiry after a method of purifying the metal to a greater perfection; and he thought that a new fusion of it was the most likely to accomplish his views. After some trials, he at last succeeded; but it was soon known to others, who got the advantages for themselves; of which ill fate the real inventor very bitterly complained till the end of his life. His own name was even forgotten, as one Huntman practised this art to such an extent, that cast steel was known under his sole name afterwards.
24. But before this discovery made by Waller in England, this kind of steel was made already in Germany, as Watson affirms; and from thence some small quantities were brought to England at a considerable price. Since that time this branch of business is carried on advantageously at Sheffield; for the manufacturers there furnish a great abundance of broken tools and old bits of steel, at a penny a pound, which, after fusion and purification, sell for 10 or 12 times as much.
25. It is a valuable property of iron, after it is reduced into the flate of steel, that though it is sufficiently soft when hot, or when gradually cooled, to be formed without difficulty into various tools and utensils; yet it may be afterwards rendered more or less hard, even to an extreme degree, by simply plunging it, when red-hot, into cold water. This is called tempering. The hardness produced is greater in proportion as the steel is hotter and the water colder. Hence arises the superiority of this metal for making mechanic instruments or tools, by which all other metals, and even itself, are filed, drilled, and cut. The various degrees of hardness given to iron, depend on the quantity of ignition it possesses at the moment of being tempered, which is manifested by the succession of colours, exhibited on the surface of the metal, in the progress of its receiving the increasing heat. They are the yellowish-white, yellow, gold-colour, purple, violet, and deep-blue; after which, the complete ignition takes place. They proceed from a kind of scoriification on the surface of the heated metal.
26. A bar of clean white steel may be made to assume all the above colours at once, by placing one end in the fire, and keeping the other end out, which is supposed of a proper length to remain cold.
27. These colours serve as signs to direct the artist in tempering this metal. For though ignited steel, suddenly quenched in very cold water, proves extremely hard and brittle; yet it may be reduced to the required degree of temper by heating it till it exhibits a known colour. This is the method employed in this process by the artists. As soon as the piece of steel is completely ignited, they plunge it in a very cold water; and as soon as it looses its fiery appearance, they take it out, rub it quickly with a file, or on a plate covered with sand, that it may have a white surface. The heat, which is still within the metal, soon begins to produce the succession of colours. If a hard temper is desired, as soon as the yellow tinge appears, the piece is dipped again, and stirred about in the cold water. If the purple appears before the dipping it, the temper will be fit for tools employed in working upon metals; if dipped while blue, it will be proper for springs; and for other instruments fit to cut all sorts of soft substances; but if the last pale colour be waited for, the steel will not be hard at all.
28. It deserves notice, that a piece of iron is rendered considerably warm by hammering, so as even to become red hot. But after the iron has been completely hammered once, it is asserted that it cannot be rendered again red hot by the same operation, because no further compression can then be made. Hard steel is the only metal that, being struck flatwise with the sharp edge of a flint, or of another hard stone, produces sparks of fire.
29. Iron is often manufactured so as to be 150 times, and even above 630 times, more valuable than gold. On weighing some common watch pendulum-springs at Mr Tho. Wright's, watch-maker to the king, such as are sold at half a crown by the London artists for common work, ten of them weighed but one single grain. Hence one pound avoirdupois (= 7000 gr.) contains ten times as many of these springs; which, at half a crown a-piece, amount to 8750l. Sterling. The troy ounce of gold sells at 4l. Sterling, and the pound (= 5760 gr.) at 48l. Sterling, which gives 58,33 (or 58l. 6s. 7d.) for each pound avoirdupois of gold; and of course \( \frac{8750}{58} = 150 \). But the pendulum-springs of the best kind of watches sell at half a guinea each; and at this rate the above-mentioned value must be increased in the ratio of 1 to 4.2; viz. of half a crown to half a guinea; which will amount to 36,750l. Sterling; and this sum divided by by the value of this pound of gold, gives above 630 to the quotient.
Under the article Electricity, we have taken notice of a curious experiment of burning iron in dephlogisticated air; of which an account is also given under Aerology, where the experiments of Dr Priestley are related. In the last number of the Chemical Annals we find the subject particularly treated of by M. Lavoisier. "The beautiful experiment of Mr Ingenhouz (says he) is now well known. A piece of very fine iron wire is turned into a spiral form; one end of it is fixed in a bottle-cork; to the other a piece of agaric is fastened: when this has been done, a bottle is filled with vital air; the agaric is lighted, and it is then, along with the iron wire, quickly introduced into the bottle, which is stoppered with the cork. As soon as the agaric is plunged into the vital air, it begins to burn with a dazzling light; the inflammation is communicated to the iron, which also burns, throwing off bright sparks that fall to the bottom of the bottle in round globules. These globules become black as they cool, and preserve some remains of their metallic lustre. The iron thus burnt is more brittle than glass itself; it powders easily; is attractable by the magnet, but less so than before the operation."
M. Lavoisier, in order to observe more fully the changes which happened to the metal on this occasion, repeated the experiment upon a scale considerably larger. He immersed chips of iron turned into a spiral form into a vessel filled with pure air which contained about 12 quarts; fixing to the end of each chip a small bit of agaric, and a particle of phosphorus weighing scarce 1/9th of a grain. Having set fire to the phosphorus and agaric, the iron is wholly consumed to the very last particle with a bright white light resembling flares in rockets. The heat in this combustion melts the iron, which falls down in globules of different sizes. In the first instant of the combustion there is a slight dilatation of the air; but this is succeeded by a very rapid diminution; and when the quantity of iron is sufficient, and the air very pure, almost the whole gas is absorbed. Our author recommends only small quantities of iron to be burnt at a time; because the heat produced by its combustion is so great, that the glass is apt to fly. A dram, or a dram and an half, is sufficient for a jar holding four gallons, which ought to be very strong in order to reflect the weight of the mercury with which it is to be filled. The increase of weight in the iron, by being burnt in this manner, is, according to our author, about 35 per cent. It is then in a state of ethiops, and may be powdered in a mortar. When the air in which the combustion has been performed is very pure, there is no great difference between that in which the iron has been burnt and the original quantity, excepting only a small mixture of fixed air from the little portion of charcoal contained in the iron.
In this work also we find some observations on the solubility of iron in pure water from Crell's Annals for the year 1788. It has generally been supposed that pure water is incapable of dissolving or holding iron in solution; but the fact seems now to be established by the following experiment. A pound of fresh distilled water was poured upon two ounces of iron-filings into narrow-necked glass retort; the vessel was then put in a sand heat, and the liquid evaporated to one half; after which the mouth was slightly stoppered with a cork, and the matter left to digest in a gentle heat. On opening the vessel it was found that the water had become fayptic, and had a ferruginous taste; whence it appeared that part of the metal was dissolved. Phlogisticated alkali had no effect upon this solution until a few drops of pure distilled acetic acid were added, when a little Prussian blue fell to the bottom. Soon after making this experiment, our author met with a natural mineral water which contained iron in solution, though it would not precipitate any thing until a few drops of acid were added. This solubility of iron in pure water has been also taken notice of by M. Landriani and M. Monnet.
Iron is easily calcinable by fire, and is soluble in all the acids, even that of fixed air. By exposure to the atmosphere it is attacked by the pure part of the surrounding fluid, which thus becomes converted into fixed air, the metal in the mean time being changed into a yellowish brown powder called rust. Common iron is much more subject to rust than steel; and this facility of calcination renders it a matter of great importance to discover some effectual method of preventing it from taking place. Various compositions have been recommended, but none have been found more effectual than common oil. As the use of this, however, must be on many occasions troublesome and disagreeable, a still more commodious method has been fallen upon. It is known that the metal, after having undergone that kind of calcination in which it combines with the base of dephlogisticated air, or begins to combine with it, is not subject to rust. By giving it a coating of this kind, therefore, it is effectually preserved from any action of the air; and this is done by heating it till it assumes a blue colour, which indicates a partial calcination on the outside; and thus utensils are made capable of being preserved from rust for a long time; though even these, when exposed wet, or even a long time to the atmosphere, will be covered with rust and decay like others. For the chemical properties of iron, see Chemistry; for its electrical and magnetic ones, see Electricity and Magnetism.
Iron-Moulds, and spots of ink in linen, may be taken out by dipping the stained part in water, sprinkling it with a little of the powdered essential salt of wood-ash, then rubbing on a pewter plate, and washing the spot out with warm water.
Iron-Sick, in the sea-language, is said of a ship or boat, when her bolts or nails are so eaten with rust, and so worn away, that they occasion hollows in the planks, whereby the vessel is rendered leaky.
Iron-Wood, in botany. See the article Sideroxylum.
Iron-Work, in botany. See the article Sideritis.