Chapter XXII: Iron, Cobalt, and Nickel (1)

Judging from the atomic weights, and the forms of the higher oxides of the elements already considered, it is easy to form an idea of the seven groups of the periodic system. Such are, for instance, the typical series Li, Be, B, C, N, O, F, or the third series, Na, Mg, Al, Si, P, S, Cl. The seven usual types of oxides from R_{2}O to R_{2}O_{7} correspond with them (Chapter XV.) The position of the eighth group is quite separate, and is determined by the fact that, as we have already seen, in each group of metals having a greater atomic weight than potassium a distinction ought to be made between the elements of the even and uneven series. The series of even elements, commencing with a strikingly alkaline element (potassium, rubidium, cæsium), together with the uneven series following it, and concluding with a haloid (chlorine, bromine, iodine), forms a large period, the properties of whose members repeat themselves in other similar periods. The elements of the eighth group are situated between the elements of the even series and the elements of the uneven series following them. And for this reason elements of the eighth group are found in the middle of each large period. The properties of the elements belonging to it, in many respects independent and striking, are shown with typical clearness in the case of iron, the well-known representative of this group.

_Iron_ is one of those elements which are not only widely diffused in the crust of the earth, but also throughout the entire universe. Its oxides and their various compounds are found in the most diverse portions of the earth's crust; but here iron is always found combined with some other element. Iron is not found on the earth's surface in a free state, because it easily oxidises under the action of air. It is occasionally found in the native state in meteorites, or aerolites, which fall upon the earth.

_Meteoric iron_ is formed outside the earth.[1] Meteorites are fragments which are carried round the sun in orbits, and fall upon the earth when coming into proximity with it during their motion in space. The meteoric dust, on passing through the upper parts of the atmosphere, and becoming incandescent from friction with the gases, produces that phenomenon which is familiar under the name of falling stars.[2] Such is the doctrine concerning meteorites, and therefore the fact of their containing rocky (siliceous) matter and metallic iron shows that outside the earth the elements and their aggregation are in some degree the same as upon the earth itself.

[1] The composition of meteoric iron is variable. It generally contains
nickel, phosphorus, carbon, &c. The schreibersite of meteoric
stones contains Fe_{4}Ni_{2}P.

[2] Comets and the rings of Saturn ought now to be considered as
consisting of an accumulation of such meteoric cosmic particles.
Perhaps the part played by these minute bodies scattered throughout
space is much more important in the formation of the largest
celestial bodies than has hitherto been imagined. The investigation
of this branch of astronomy, due to Schiaparelli, has a bearing on
the whole of natural science.

The question arises as to why the iron in meteorites is in a free
state, whilst on earth it is in a state of combination. Does not
this tend to show that the condition of our globe is very different
from that of the rest? My answer to this question has been already
given in Volume I. p. 377, Note 57. It is my opinion that inside
the earth there is a mass similar in composition to
meteorites--that is, containing rocky matter and metallic iron,
partly carburetted. In conclusion, I consider it will not be out of
place to add the following explanations. According to the theory of
the distribution of pressures (see my treatise, _On Barometrical
Levelling_, 1876, pages 48 _et seq._) in an atmosphere of mixed
gases, it follows that two gases, whose densities are _d_ and
_d__{1}, and whose relative quantities or partial pressures at a
certain distance from the centre of gravity are _h_ and _h__{1},
will, when at a greater distance from the centre of attraction,
present a different ratio of their masses _x_ : _x__{1}--that is,
of their partial pressures--which may be found by the equation
_d__{1}(log(_h_) - log(_x_)) = _d_(log(_h__{1}) - log(_x__{1})).
If, for instance, _d_ : _d__{1} = 2 : 1, and _h_ = _h__{1} (that is
to say, the masses are equal at the lower height) = 1000, then when
_x_ = 10 the magnitude of _x__{1} will not be 10 (_i.e._ the mass
of a gas at a higher level whose density = 1 will not be equal to
the mass of a gas whose density = 2, as was the case at a lower
level), but much greater--namely, _x__{1} = 100--that is, the
lighter gas will predominate over a heavier one at a higher level.
Therefore, when the whole mass of the earth was in a state of
vapour, the substances having a greater vapour density accumulated
about the centre and those with a lesser vapour density at the
surface. And as the vapour densities depend on the atomic and
molecular weights, those substances which have small atomic and
molecular weights ought to have accumulated at the surface, and
those with high atomic and molecular weights, which are the least
volatile and the easiest to condense, at the centre. Thus it
becomes apparent why such light elements as hydrogen, carbon,
nitrogen, oxygen, sodium, magnesium, aluminium, silicon,
phosphorus, sulphur, chlorine, potassium, calcium, and their
compounds predominate at the surface and largely form the earth's
crust. There is also now much iron in the sun, as spectrum analysis
shows, and therefore it must have entered into the composition of
the earth and other planets, but would have accumulated at the
centre, because the density of its vapour is certainly large and it
easily condenses. There was also oxygen near the centre of the
earth, but not sufficient to combine with the iron. The former, as
a much lighter element, principally accumulated at the surface,
where we at the present time find all oxidised compounds and even a
remnant of free oxygen. This gives the possibility not only of
explaining in accordance with cosmogonic theories the predominance
of oxygen compounds on the surface of the earth, with the
occurrence of unoxidised iron in the interior of the earth and in
meteorites, but also of understanding why the density of the whole
earth (over 5) is far greater than that of the rocks (1 to 3)
composing its crust. And if all the preceding arguments and
theories (for instance the supposition that the sun, earth, and all
the planets were formed of an elementary homogeneous mass, formerly
composed of vapours and gases) be true, it must be admitted that
the interior of the earth and other planets contains metallic
(unoxidised) iron, which, however, is only found on the surface as
aerolites. And then assuming that aerolites are the fragments of
planets which have crumbled to pieces so to say during cooling
(this has been held to be the case by astronomers, judging from the
paths of aerolites), it is readily understood why they should be
composed of metallic iron, and this would explain its occurrence in
the depths of the earth, which we assumed as the basis of our
theory of the formation of naphtha (Chapter VIII., Notes 57-60).

The most widely diffused terrestrial compound of iron is iron bisulphide, FeS_{2}, or _iron pyrites_. It occurs in formations of both aqueous and igneous origin, and sometimes in enormous masses. It is a substance having a greyish-yellow colour, with a metallic lustre, and a specific gravity of 5·0; it crystallises in the regular system.[2 bis]

[2 bis] Immense deposits of iron pyrites are known in various parts of
Russia. On the river Msta, near Borovitsi, thousands of tons are
yearly collected from the detritus of the neighbouring rocks. In
the Governments of Toula, Riazan, and in the Donets district
continuous layers of pyrites occur among the coal seams. Very thick
beds of pyrites are also known in many parts of the Caucasus. But
the deposits of the Urals are particularly vast, and have been
worked for a long time. Amongst these I will only indicate the
deposits on the Soymensky estate near the Kishteimsky works; the
Kaletinsky deposits near the Virhny-Isetsky works (containing 1-2
p.c. Cu); on the banks of the river Koushaivi near Koushvi (3-5
p.c. Cu), and the deposits near the Bogoslovsky works (3-5 p.c.
Cu). Iron pyrites (especially that containing copper which is
extracted after roasting) is now chiefly employed for roasting, as
a source of SO_{2}, for the manufacture of chamber sulphuric acid
(Vol. I. p. 291), but the remaining oxide of iron is perfectly
suitable for smelting into pig iron, although it gives a sulphurous
pig iron (the sulphur may be easily removed by subsequent
treatment, especially with the aid of ferro-manganese in Bessemer's
process). The great technical importance of iron pyrites leads to
its sometimes being imported from great distances; for instance,
into England from Spain. Besides which, when heated in closed
retorts FeS_{2} gives sulphur, and if allowed to oxidise in damp
air, green vitriol, FeSO_{4}.

The oxides are the principal ores used for producing metallic iron. The majority of the ores contain ferric oxide, Fe_{2}O_{3}, either in a free state or combined with water, or else in combination with ferrous oxide, FeO. The species and varieties of iron ores are numerous and diverse. Ferric oxide in a separate form appears sometimes as crystals of the rhombohedric system, having a metallic lustre and greyish steel colour; they are brittle, and form a red powder, specific gravity about 5·25. Ferric oxide in type of oxidation and properties resembles alumina; it is, however, although with difficulty, soluble in acids even when anhydrous. The crystalline oxide bears the name of _specular iron ore_, but ferric oxide most often occurs in a non-crystalline form, in masses having a red fracture, and is then known as _red hæmatite_. In this form, however, it is rather a rare ore, and is principally found in veins. The hydrates of ferric oxide, ferric hydroxides,[3] are most often found in aqueous or stratified formations, and are known as _brown hæmatites_; they generally have a brown colour, form a yellowish-brown powder, and have no metallic lustre but an earthy appearance. They easily dissolve in acids and diffuse through other formations, especially clays (for instance, ochre); they sometimes occur in reniform and similar masses, evidently of aqueous origin. Such are, for instance, the so-called bog or lake and peat ores found at the bottom of marshes and lakes, and also under and in peat beds. This ore is formed from water containing ferrous carbonate in solution, which, after absorbing oxygen, deposits ferric hydroxide. In rivers and springs, iron is found in solution as ferrous carbonate through the agency of carbonic acid: hence the existence of chalybeate springs containing FeCO_{3}. This ferrous carbonate, or _siderite_, is either found as a non-crystalline product of evidently aqueous origin, or as a crystalline spar called _spathic iron ore_. The reniform deposits of the former are most remarkable; they are called spherosiderites, and sometimes form whole strata in the jurassic and carboniferous formations. _Magnetic iron ore_, Fe_{3}O_{4} = FeO,Fe_{2}O_{3}, in virtue of its purity and practical uses, is a very important ore; it is a compound of the ferrous and ferric oxides, is naturally magnetic, has a specific gravity of 5·1, crystallises in well-formed crystals of the regular system, is with difficulty soluble in acids, and sometimes forms enormous masses, as, for instance, Mount Blagodat in the Ural. However, in most cases--for instance, at Korsak-Mogila (to the north of Berdiansk and Nogaiska, near the Sea of Azov), or at Krivoi Rog (to the west of Ekaterinoslav)--the magnetic iron ore is mixed with other iron ores. In the Urals, the Caucasus (without mentioning Siberia), and in the districts adjoining the basin of the Don, Russia possesses the richest iron ores in the world. To the south of Moscow, in the Governments of Toula and Nijninovgorod, in the Olonetz district, and in the Government of Orloffsky (near Zinovieff in the district of Kromsky), and in many other places, there are likewise abundant supplies of iron ores amongst the deposited aqueous formations; the siderite of Orloffsky, for instance, is distinguished by its great purity.[4]

[3] The hydrated ferric oxide is found in nature in a dual form. It is
somewhat rarely met with in the form of a crystalline mineral
called _göthite_, whose specific gravity is 4·4 and composition
Fe_{2}H_{3}O_{4}, or FeHO_{2}--that is, one of oxide of iron to one
of water, Fe_{2}O_{3},H_{2}O; frequently found as brown ironstone,
forming a dense mass of fibrous, reniform deposits containing
2Fe_{2}O_{3},3H_{2}O--that is, having a composition
Fe_{4}H_{6}O_{9}. In bog ore and other similar ores we most often
find a mixture of this hydrated ferric oxide with clay and other
impurities. The specific gravity of such formations is rarely as
high as 4·0.

[4] The ores of iron, similarly to all substances extracted from veins
and deposits, are worked according to mining practice by means of
vertical, horizontal, or inclined shafts which reach and penetrate
the veins and strata containing the ore deposits. The mass of ore
excavated is raised to the surface, then sorted either by hand or
else in special sorting apparatus (generally acting with water to
wash the ore), and is subjected to roasting and other treatment. In
every case the ore contains foreign matter. In the extraction of
iron, which is one of the cheapest metals, the dressing of an ore
is in most cases unprofitable, and only ores rich in metal are
worked--namely, those containing at least 20 p.c. It is often
profitable to transport very rich and pure ores (with as much as 70
p.c. of iron) from long distances. The details concerning the
working and extraction of metals will be found in special treatises
on metallurgy and mining.

Iron is also found in the form of various other compounds--for instance, in certain silicates, and also in some phosphates; but these forms are comparatively rare in nature in a pure state, and have not the industrial importance of those natural compounds of iron previously mentioned. In small quantities iron enters into the composition of every kind of _soil_ and all rocky formations. As ferrous oxide, FeO, is isomorphous with magnesia, and ferric oxide, Fe_{2}O_{3}, with alumina, isomorphous substitution is possible here, and hence minerals are not unfrequently found in which the quantity of iron varies considerably; such, for instance, are pyroxene, amphibole, certain varieties of mica, &c. Although much iron oxide is deleterious to the growth of vegetation, still plants do not flourish without iron; it enters as an indispensable component into the composition of all higher _organisms_; in the ash of plants we always find more or less of its compounds. It also occurs in blood, and forms one of the colouring matters in it; 100 parts of the blood of the highest organisms contain about 0·05 of iron.

The _reduction_ of the ores of iron into metallic iron is in principle very simple, because when the oxides of iron are strongly heated with charcoal, hydrogen, carbonic oxide, and other reducing agents,[5] they easily give metallic iron. But the matter is rendered more difficult by the fact that the iron does not melt at the heat developed by the combustion of the charcoal, and therefore it does not separate from those mechanically mixed impurities which are found in the iron ore. This is obviated by the following very remarkable property of iron: at a high temperature it is capable of combining with a small quantity (from 2 to 5 p.c.) of carbon, and then forms _cast iron_, which easily _melts_ in the heat developed by the combustion of charcoal in air. For this reason metallic iron is not obtained directly from the ore, but is only formed after the further treatment of the cast iron; the first product extracted from the ore being cast iron. The fused mass disposes itself in the furnace below the slag--that is, the impurities of the ore fused by the heat of the furnace. If these impurities did not fuse they would block up the furnace in which the ore was being smelted, and the continuous smelting of the cast iron would not be possible;[6] it would be necessary periodically to cool the furnace and heat it up again, which means a wasteful expenditure of fuel, and hence in the production of cast iron, the object in view is to obtain all the earthy impurities of the ore in the shape of a fused mass or slag. Only in rare cases does the ore itself form a mass which fuses at the temperature employed, and these cases are objectionable if much iron oxide is carried away in the slag. The impurities of the ores most often consist of certain mixtures--for instance, a mixture of clay and sand, or a mixture of limestone and clay, or quartz, &c. These impurities do not separate of themselves, or do not fuse. The difficulty of the industry lies in forming an easily-fusible slag, into which the whole of the foreign matter of the ore would pass and flow down to the bottom of the furnace above the heavier cast iron. This is effected by mixing certain _fluxes_ with the ore and charcoal. A flux is a substance which, when mixed with the foreign matter of the ore, forms a fusible vitreous mass or slag. The flux used for silica is limestone with clay; for limestone a definite quantity of silica is used, the best procedure having been arrived at by experiment and by long practice in iron smelting and other metallurgical processes.[7]

[5] The reduction of iron oxides by hydrogen belongs to the order of
reversible reactions (Chapter II.), and is therefore determined by
a limit which is here expressed by the attainment of the same
pressure as in the case where hydrogen acts on iron oxides, and as
in the case where (at the same temperature) water is decomposed by
metallic iron. The calculations referring to this matter were made
by Henri Sainte-Claire Deville (1870). Spongy iron was placed in a
tube having a temperature _t_, one end of which was connected with
a vessel containing water at 0° (vapour tension = 4·6 mm.) and the
other end with a mercury pump and pressure gauge which determined
the limiting tension attained by the dry hydrogen _p_ (subtracting
the tension of the water vapour from the tension observed). A tube
was then taken containing an excess of iron oxide. It was filled
with hydrogen, and the tension _p__{1} observed of the residual
hydrogen when the water was condensed at 0°.

_t_ = 200° 440° 860° 1040°
_p_ = 95·9 25·8 12·8 9·2 mm.
_p_{1} = -- -- 12·8 9·4 mm.

The equality of the pressure (tension) of the hydrogen in the two
cases is evident. The hydrogen here behaves like the vapour of iron
or of its oxide.

By taking ferric oxide, Fe_{2}O_{3}, Moissan observed that at 350°
it passed into magnetic oxide, Fe_{3}O_{4}, at 500° into ferrous
oxide, FeO, and at 600° into metallic iron. Wright and Luff (1878),
whilst investigating the reduction of oxides, found that (_a_) the
temperature of reaction depends on the condition of the oxide
taken--for instance, precipitated ferric oxide is reduced by
hydrogen at 85°, that obtained by oxidising the metal or from its
nitrate at 175°; (_b_) when other conditions are the same, the
reduction by carbonic oxide commences earlier than that by
hydrogen, and the reduction by hydrogen still earlier than that by
charcoal; (_c_) the reduction is effected with greater facility
when a greater quantity of heat is evolved during the reaction.
Ferric oxide obtained by heating ferrous sulphate to a red heat
begins to be reduced by carbonic oxide at 202°, by hydrogen at
260°, by charcoal at 430°, whilst for magnetic oxide, Fe_{3}O_{4},
the temperatures are 200°, 290°, and 450° respectively.

[6] The primitive methods of iron manufacture were conducted by
intermittent processes in hearths resembling smiths' fires. As
evidenced by the uninterrupted action of the steam boiler, or the
process of lime burning, and the continuous preparation and
condensation of sulphuric acid or the uninterrupted smelting of
iron, every industrial process becomes increasingly profitable and
complete under the condition of the continuous action, as far as
possible, of all agencies concerned in the production. This
continuous method of production is the first condition for the
profitable production on the large scale of nearly all industrial
products. This method lessens the cost of labour, simplifies the
supervision of the work, renders the product uniform, and
frequently introduces a very great economy in the expenditure of
fuel and at the same time presents the simplicity and perfection of
an equilibrated system. Hence every manufacturing operation should
be a continuous one, and the manufacture of pig iron and sulphuric
acid, which have long since become so, may be taken as examples in
many respects. A study of these two manufactures should form the
commencement of an acquaintance with all the contemporary methods
of manufacturing both from a technical and economical point of
view.

[7] The composition of slag suitable for iron smelting most often
approaches the following: 50 to 60 p.c. SiO_{2}, 5 to 20
Al_{2}O_{3}, the rest of the mass consisting of MgO, CaO, MnO, FeO.
Thus the most fusible slag (according to the observations of
Bodeman) contains the alloy Al_{2}O_{3},4CaO,7SiO_{2}. On altering
the quantity of magnesia and lime, and especially of the alkalis
(which increases the fusibility) and of silica (which decreases
it), the temperature of fusion changes with the relation between
the total quantity of oxygen and that in the silica. Slags of the
composition RO,SiO_{2} are easily fusible, have a vitreous
appearance, and are very common. Basic slags approach the
composition 2RO,SiO_{2}. Hence, knowing the composition and
quantity of the foreign matter in the ore, it is at once easy to
find the quantity and quality of the flux which must be added to
form a suitable slag. The smelting of iron is rendered more complex
by the fact that the silica, SiO_{2}, which enters into the slag
and fluxes is capable of forming a slag with the iron oxides. In
order that the least quantity of iron may pass into the slag, it is
necessary for it to be reduced before the temperature is attained
at which the slags are formed (about 1000°), which is effected by
reducing the iron, not with charcoal itself, but with carbonic
oxide. From this it will be understood how the progress of the
whole treatment may be judged by the properties of the slags.
Details of this complicated and well-studied subject will be found
in works on metallurgy.

Thus the following materials have to be introduced into the furnace where the smelting of the iron ore is carried on: (1) the iron ore, composed of oxide of iron and foreign matter; (2) the flux required to form a fusible slag with the foreign matter; (3) the carbon which is necessary (_a_) for reducing, (_b_) for combining with the reduced iron to form cast iron, (_c_) principally for the purpose of combustion and the heat generated thereby, necessary not only for reducing the iron and transforming it into cast iron, but also for melting the slag, as well as the cast iron--and (4) the air necessary for the combustion of the charcoal. The air is introduced after a preparatory heating in order to economise fuel and to obtain the highest temperature. The air is forced in under pressure by means of a special blast arrangement. This permits of an exact regulation of the heat and rate of smelting. All these component parts necessary for the smelting of iron must be contained in a vertical, that is, _shaft furnace_, which at the base must have a receptacle for the accumulation of the slag and cast iron formed, in order that the operation may proceed without interruption. The walls of such a furnace ought to be built of fireproof materials if it be designed to serve for the continuous production of cast iron by charging the ore, fuel, and flux into the mouth of the furnace, forcing a blast of air into the lower part, and running out the molten iron and slag from below. The whole operation is conducted in furnaces known as _blast furnaces_. The annexed illustration, fig. 93 (which is taken by kind permission from Thorpe's Dictionary of Applied Chemistry), represents the vertical section of such a furnace. These furnaces are generally of large dimensions--varying from 50 to 90 feet in height. They are sometimes built against rising ground in order to afford easy access to the top where the ore, flux, and charcoal or coke are charged.[8]

[8] The section of a blast furnace is represented by two truncated
cones joined at their bases, the upper cone being longer than the
lower one; the lower cone is terminated by the hearth, or almost
cylindrical cavity in which the cast iron and slag collect, one
side being provided with apertures for drawing off the iron and
slag. The air is blown into the blast furnace through special
pipes, situated over the hearth, as shown in the section. The air
previously passes through a series of cast-iron pipes, heated by
the combustion of the carbonic oxide obtained from the upper parts
of the furnace, where it is formed as in a 'gas-producer.' The
blast furnace acts continuously until it is worn out; the iron is
tapped off twice a day, and the furnace is allowed to cool a little
from time to time so as not to be spoilt by the increasing heat,
and to enable it to withstand long usage.

Blast furnaces worked with charcoal fuel are not so high, and in
general give a smaller yield than those using coke, because the
latter are worked with heavier charges than those in which charcoal
is employed. Coke furnaces yield 20,000 tons and over of pig iron a
year. In the United States there are blast furnaces 30 metres high,
and upwards of 600 cubic metres capacity, yielding as much as
130,000 tons of pig iron, requiring a blast of about 750 cubic
metres of air per minute, heated to 600°, and consuming about 0·85
part of coke per 1 part of pig iron produced. At the present time
the world produces as much as 30 million tons of pig iron a year,
about 9/10 of which is converted into wrought iron and steel. The
chief producers are the United States (about 10 million tons a
year) and England (about 9 million tons a year); Russia yields
about 1-1/5 million tons a year. The world's production has doubled
during the last 20 years, and in this respect the United States
have outrun all other countries. The reason of this increase of
production must be looked for in the increased demand for iron and
steel for railway purposes, for structures (especially
ship-building), and in the fact that: (_a_) the cost of pig iron
has fallen, thanks to the erection of large furnaces and a fuller
study of the processes taking place in them, and (_b_) that every
kind of iron ore (even sulphurous and phosphoritic) can now be
converted into a homogeneous steel.

In order to more thoroughly grasp the chemical process which takes
place in blast furnaces, it is necessary to follow the course of
the material charged in at the top and of the air passing through
the furnace. From 50 to 200 parts of carbon are expended on 100
parts of iron. The ore, flux, and coke are charged into the top of
the furnace, in layers, as the cast iron is formed in the lower
parts and flowing down to the bottom causes the whole contents of
the furnace to subside, thus forming an empty space at the top,
which is again filled up with the afore-mentioned mixture. During
its downward course this mixture is subjected to increasing heat.
This rise of temperature first drives off the moisture of the ore
mixture, and then leads to the formation of the products of the dry
distillation of coal or charcoal. Little by little the subsiding
mass attains a temperature at which the heated carbon reacts with
the carbonic anhydride passing upwards through the furnace and
transforms it into carbonic oxide. This is the reason why carbonic
anhydride is not evolved from the furnace, but only carbonic oxide.
As regards the ore itself, on being heated to about 600° to 800° it
is reduced at the expense of the carbonic oxide ascending the
furnace, and formed by the contact of the carbonic anhydride with
the incandescent charcoal, so that the reduction in the blast
furnace is without doubt brought about _by_ the formation and
decomposition of _carbonic oxide_ and not by carbon itself--thus,
Fe_{2}O_{3} + 3CO = Fe_{2} + 3CO_{2}. The reduced iron, on further
subsidence and contact with carbon, forms cast iron, which flows to
the bottom of the furnace. In these lower layers, where the
temperature is highest (about 1,300°), the foreign matter of the
ore finally forms slag, which also is fusible, with the aid of
fluxes. The air blown in from below, through the so-called
_tuyeres_, encounters carbon in the lower layers of the furnace,
and burns it, converting it into carbonic anhydride. It is evident
that this develops the highest temperature in these lower layers of
the furnace, because here the combustion of the carbon is effected
by heated and compressed air. This is very essential, for it is by
virtue of this high temperature that the process of forming the
slag and of forming and fusing the cast iron are effected
simultaneously in these lower portions of the furnace. The carbonic
acid formed in these parts rises higher, encounters incandescent
carbon, and forms with it carbonic oxide. This heated carbonic
oxide acts as a reducing agent on the iron ore, and is reconverted
by it into carbonic anhydride; this gas meets with more carbon, and
again forms carbonic oxide, which again acts as a reducing agent.
The final transformation of the carbonic anhydride into carbonic
oxide is effected in those parts of the furnace where the reduction
of the oxides of iron does not take place, but where the
temperature is still high enough to reduce the carbonic anhydride.
The ascending mixture of carbonic oxide and nitrogen, CO_{2}, &c.,
is then withdrawn through special lateral apertures formed in the
upper cold parts of the furnace walls, and is conducted through
pipes to those stoves which are used for heating the air, and also
sometimes into other furnaces used for the further processes of
iron manufacture. The fuel of blast furnaces consists of wood
charcoal (this is the most expensive material, but the pig iron
produced is the purest, because charcoal does not contain any
sulphur, while coke does), anthracite (for instance, in
Pennsylvania, and in Russia at Pastouhoff's works in the Don
district), coke, coal, and even wood and peat. It must be borne in
mind that the utilisation of naphtha and naphtha refuse would
probably give very profitable results in metallurgical processes.

The process just described is accompanied by a series of other
processes. Thus, for instance, in the blast furnace a considerable
quantity of cyanogen compounds are formed. This takes place because
the nitrogen of the air blast comes into contact with incandescent
carbon and various alkaline matters contained in the foreign matter
of the ores. A considerable quantity of potassium cyanide is formed
when wood charcoal is employed for iron smelting, as its ash is
rich in potash.

The _cast iron_ formed in blast furnaces is not always of the same quality. When slowly cooled it is soft, has a grey colour, and is not completely soluble in acids. When treated with acids a residue of graphite remains; it is known as _grey_ or soft cast iron. This is the general form of the ordinary cast iron used for casting various objects, because in this state it is not so brittle as in the shape of _white cast iron_, which does not leave particles of graphite when dissolved, but yields its carbon in the form of hydrocarbons. This white cast iron is characterised by its whitish-grey colour, dull lustre, the crystalline structure of its fracture (more homogeneous than that of grey iron), and such hardness that a file will hardly cut it. When white cast iron is produced (from manganese ore) at high temperatures (and with an excess of lime), and containing little sulphur and silica but a considerable amount of carbon (as much as 5 p.c.), it acquires a coarse crystalline structure which increases in proportion to the amount of manganese, and it is then known under the name of 'spiegeleisen' (and 'ferro-manganese').[9]

[9] The specific gravity of white cast iron is about 7·5. Grey cast
iron has a much lower specific gravity, namely, 7·0. Grey cast iron
generally contains less manganese and more silica than white; but
both contain from 2 to 3 p.c. of carbon. The difference between the
varieties of cast iron depends on the condition of the carbon which
enters into the composition of the iron. In white cast iron the
carbon is in combination with the iron--in all probability, as the
compound CFe_{4} (Abel and Osmond and others extracted this
compound, which is sometimes called 'carbide,' from tempered steel,
which stands to unannealed steel as white cast iron does to grey),
but perhaps in the state of an indefinite chemical compound
resembling a solution. In any case the compound of the iron and
carbon in white cast iron is chemically very unstable, because when
slowly cooled it decomposes, with separation of graphite, just as a
solution when slowly cooled yields a portion of the substance
dissolved. The separation of carbon in the form of graphite on the
conversion of white cast iron into grey is never complete, however
slowly the separation be carried on; part of the carbon remains in
combination with the iron in the same state in which it exists in
white cast iron. Hence when grey cast iron is treated with acids,
the whole of the carbon does not remain in the form of graphite,
but a part of it is separated as hydrocarbons, which proves the
existence of chemically-combined carbon in grey cast iron. It is
sufficient to re-melt grey cast iron and to cool it quickly to
transform it into white cast iron. It is not carbon alone that
influences the properties of cast iron; when it contains a
considerable amount of sulphur, cast iron remains white even after
having been slowly cooled. The same is observed in cast iron very
rich in manganese (5 to 7 p.c.), and in this latter case the
fracture is very distinctly crystalline and brilliant. When cast
iron contains a large amount of manganese, the quantity of carbon
may also be increased. Crystalline varieties of cast iron rich in
manganese are in practice called ferro-manganese (p. 310), and are
prepared for the Bessemer process. Grey cast iron not having an
uniform structure is much more liable to various changes than dense
and thoroughly uniform white cast iron, and the latter oxidises
much more slowly in air than the former. White cast iron is not
only used for conversion into wrought iron and steel, but also in
those cases where great hardness is required, although it be
accompanied by a certain brittleness; for instance, for making
rollers, plough-shares, &c.

Cast iron is a material which is either suitable for direct application for casting in moulds or else for working up into _wrought iron_ and _steel_. The latter principally differ from cast iron in their containing less carbon--thus, steel contains from 1 p.c. to 0·5 p.c. of carbon and far less silicon and manganese than cast iron; wrought iron does not generally contain more than 0·25 p.c. of carbon and not more than 0·25 p.c. of the other impurities. Thus the essence of the working up of cast iron into steel and wrought iron consists in the removal of the greater part of the carbon and other elements, S, P, Mn, Si, &c. This is effected by means of oxidation, because the oxygen of the atmosphere, oxidising the iron at a high temperature, forms solid oxides with it; and the latter, coming into contact with the carbon contained in the cast iron, are deoxidised, forming wrought iron and carbonic oxide, which is evolved from the mass in a gaseous form. It is evident that the oxidation must be carried on with a molten mass in a state of agitation, so that the oxygen of the air may be brought into contact with the whole mass of carbon contained in the cast iron, or else the operation is effected by means of the addition of oxygen compounds of iron (oxides, ores, as in Martin's process). Cast iron melts much more easily than wrought iron and steel, and, therefore, as the carbon separates, the mass in the furnace (in puddling) or hearth (in the bloomery process) becomes more and more solid; moreover the degree of hardness forms, to a certain extent, a measure of the amount of carbon separated, and the operation may terminate either in the formation of steel or wrought iron.[10] In any case, the iron used for industrial purposes contains impurities. _Chemically pure iron_ may be obtained by precipitating iron from a solution (a mixture of ferrous sulphate with magnesium sulphate or ammonium chloride) by the prolonged action of a feeble galvanic current; the iron may be then obtained as a dense mass. This method, proposed by Böttcher and applied by Klein, gives, as R. Lenz showed, iron containing occluded hydrogen, which is disengaged on heating. This galvanic deposition of iron is used for making galvanoplastic _clichés_, which are distinguished for their great hardness. Electro-deposited iron is brittle, but if heated (after the separation of the hydrogen) it becomes soft. If pure ferric hydroxide, which is easily prepared by the precipitation of solutions of ferric salts by means of ammonia, be heated in a stream of hydrogen, it forms, first of all, a dull black powder which ignites spontaneously in air (pyrophoric iron), and then a grey powder of pure iron. The powdery substance first obtained is an iron suboxide; when thrown into the air it ignites, forming the oxide Fe_{3}O_{4}. If the heating in hydrogen be continued, more water and pure iron, which does not ignite spontaneously, will be obtained. If a small quantity of iron be fused in the oxyhydrogen flame (with an excess of oxygen) in a piece of lime and mixed with powdered glass, pure molten iron will be formed, because in the oxyhydrogen flame iron melts and burns, but the substances mixed with the iron oxidise first. The oxidised impurities here either disappear (carbonic anhydride) in a gaseous form, or turn into slag (silica, manganese, oxide, and others)--that is, fuse with the glass. Pure iron has a silvery white colour and a specific gravity of 7·84; it melts at a temperature higher than the melting-points of silver, gold, nickel, and steel, _i.e._ about 1400°-1500° and below the melting point of platinum (1750°).[11] But pure iron becomes soft at a temperature considerably below that at which it melts, and may then be easily forged, welded, and rolled or drawn into sheets and wire.[11 bis] Pure iron may be rolled into an exceedingly thin sheet, weighing less than a sheet of ordinary paper of the same size. This ductility is the most important property of iron in all its forms, and is most marked with sheet iron, and least so with cast iron, whose ductility, compared with wrought iron, is small, but it is still very considerable when compared with other substances--such, for instance, as rocks.[12]

[10] This direct process of separating the carbon from cast iron is
termed _puddling_. It is conducted in reverberatory furnaces. The
cast iron is placed on the bed of the furnace and melted; through
a special aperture, the puddler stirs up the oxidising mass of
cast iron, pressing the oxides into the molten iron. This
resembles kneading dough, and the process introduced in England
became known as puddling. It is evident that the puddled mass, or
bloom, is a heterogeneous substance obtained by mixing, and hence
one part of the mass will still be rich in carbon, another will be
poor, some parts will contain oxide not reduced, &c. The further
treatment of the puddled mass consists in hammering and drawing it
out into flat pieces, which on being hammered become more
homogeneous, and when several pieces are welded together and again
hammered out a still more homogeneous mass is obtained. The
quality of the steel and iron thus formed depends principally on
their uniformity. The want of uniformity depends on the oxides
remaining inside the mass, and on the variable distribution of the
carbon throughout the mass. In order to obtain a more homogeneous
metal for manufacturing articles out of steel, it is drawn into
thin rods, which are tied together in bundles and then again
hammered out. As an example of what may be attained in this
direction, imitation Damascus steel may be cited; it consists of
twisted and plaited wire, which is then hammered into a dense
mass. (Real damascened wootz steel may be made by melting a
mixture of the best iron with graphite (1/12) and iron rust; the
article is then corroded with acid, and the carbon remains in the
form of a pattern.)

Steel and wrought iron are manufactured from cast iron by
puddling. They are, however, obtained not only by this method but
also by the _bloomery process_, which is carried out in a fire
similar to a blacksmith's forge, fed with charcoal and provided
with a blast; a pig of cast iron is gradually pushed into the
fire, and portions of it melt and fall to the bottom of the
hearth, coming into contact with an air blast, and are thus
oxidised. The bloom thus formed is then squeezed and hammered. It
is evident that this process is only available when the charcoal
used in the fire does not contain any foreign matter which might
injure the quality of the iron or steel--for instance, sulphur or
phosphorus--and therefore only wood charcoal may be used with
impunity, from which it follows that this process can only be
carried on where the manufacture of iron can be conducted with
this fuel. Coal and coke contain the above-mentioned impurities,
and would therefore produce iron of a brittle nature, and thus it
would be necessary to have recourse to puddling, where the fuel is
burnt on a special hearth, separate from the cast iron, whereby
the impurities of the fuel do not come into contact with it. The
manufacture of steel from cast iron may also be conducted in
fires; but, in addition to this, it is also now prepared by many
other methods. One of the long-known processes is called
_cementation_, by which steel is prepared from wrought iron but
not from cast iron. For this process strips of iron are heated
red-hot for a considerable time whilst immersed in powdered
charcoal; during this operation the iron at the surface combines
with the charcoal, which however does not penetrate; after this
the iron strips are re-forged, drawn out again, and cemented anew,
repeating this process until a steel of the desired quality is
formed--that is, containing the requisite proportion of carbon.
The _Bessemer_ process occupies the front rank among the newer
methods (since 1856); it is so called from the name of its
inventor. This process consists in running melted cast iron into
converters (holding about 6 tons of cast iron)--that is,
egg-shaped receivers, fig. 94, capable of revolving on trunnions
(in order to charge in the cast iron and discharge the steel), and
forcing a stream of air through small apertures at a considerable
pressure. Combustion of the iron and carbon at an elevated
temperature then takes place, resulting from the bubbles of oxygen
thus penetrating the mass of the cast iron. The carbon, however,
burns to a greater extent than the iron, and therefore a mass is
obtained which is much poorer in carbon than cast iron. As the
combustion proceeds very rapidly in the mass of metal, the
temperature rises to such an extent that even the wrought iron
which may be formed remains in a molten condition, whilst the
steel, being more fusible than the wrought iron, remains very
liquid. In half an hour the mass is ready. The purest possible
cast iron is used in the Bessemer process, because sulphur and
phosphorus do not burn out like carbon, silicon, and manganese.

The presence of manganese enables the sulphur to be removed with
the slag, and the presence of lime or magnesia, which are
introduced into the lining of the converter, facilitates the
removal of the phosphorus. This basic Bessemer process, or _Thomas
Gilchrist process_, introduced about 1880, enables ores containing
a considerable amount of phosphorus, which had hitherto only been
used for cast iron, to be used for making wrought iron and steel.
Naturally the greatest uniformity will be obtained by re-melting
the metal. Steel is re-melted in small wind furnaces, in masses
not exceeding 30 kilos; a liquid metal is formed, which may be
cast in moulds. A mixture of wrought and cast iron is often used
for making cast steel (the addition of a small amount of metallic
Al improves the homogeneity of the castings, by facilitating the
passage of the impurities into slag). Large steel castings are
made by simultaneous fusion in several furnaces and crucibles; in
this way, castings up to 80 tons or more, such as large ordnance,
may be made. This molten, and therefore homogeneous, steel is
called _cast steel_. Of late years the _Martin's process_ for the
manufacture of steel has come largely into use; it was invented in
France about 1860, and with the use of regenerative furnaces it
enables large quantities of cast steel to be made at a time. It is
based on the melting of cast iron with iron oxides and iron
itself--for instance, pure ores, scrap, &c. There the carbon of
the cast iron and the oxygen of the oxide form carbonic oxide, and
the carbon therefore burns out, and thus cast steel is obtained
from cast iron, providing, naturally, that there is a requisite
proportion and corresponding degree of heat. The advantage of this
process is that not only do the carbon, silicon, and manganese,
but also a great part of the sulphur and phosphorus of the cast
iron burn out at the expense of the oxygen of the iron oxides.
During the last decade the manufacture of steel and its
application for rails, armour plate, guns, boilers, &c., has
developed to an enormous extent, thanks to the invention of cheap
processes for the manufacture of large masses of homogeneous cast
steel. Wrought iron may also be melted, but the heat of a blast
furnace is insufficient for this. It easily melts in the
oxyhydrogen flame. It may be obtained in a molten state directly
from cast iron, if the latter be melted with nitre and
sufficiently stirred up. Considerable oxidation then takes place
inside the mass of cast iron, and the temperature rises to such an
extent that the wrought iron formed remains liquid. A method is
also known for obtaining wrought iron directly from rich iron ores
by the action of carbonic oxide: the wrought iron is then formed
as a spongy mass (which forms an excellent filter for purifying
water), and may be worked up into wrought iron or steel either by
forging or by dissolving in molten cast iron.

Everybody is more or less familiar with the _difference in the
properties of steel and wrought iron_. Iron is remarkable for its
softness, pliability, and small elasticity, whilst steel may be
characterised by its capability of attaining elasticity and
hardness if it be cooled suddenly after having been heated to a
definite temperature, or, as it is termed, _tempered_. But if
tempered steel be re-heated and slowly cooled, it becomes as soft
as wrought iron, and can then be cut with the file and forged, and
in general can be made to assume any shape, like wrought iron. In
this soft condition it is called _annealed steel_. The transition
from tempered to annealed steel thus takes place in a similar way
to the transition from white to grey cast iron. Steel, when
homogeneous, has considerable lustre, and such a fine granular
structure that it takes a very high polish. Its fracture clearly
shows the granular nature of its structure. The possibility of
tempering steel enables it to be used for making all kinds of
cutting instruments, because annealed steel can be forged, turned,
drawn (under rollers, for instance, for making rails, bars, &c.),
filed, &c., and it may then be tempered, ground and polished. The
method and temperature of tempering and annealing steel determine
its hardness and other qualities. Steel is generally tempered to
the required degree of hardness in the following manner: It is
first strongly heated (for instance, up to 600°), and then plunged
into water--that is, hardened by rapid cooling (it then becomes as
brittle as glass). It is then heated until the surface assumes a
definite colour, and finally cooled either quickly or slowly. When
steel is heated up to 220°, its surface acquires a yellow colour
(surgical instruments); it first of all becomes straw-coloured
(razors, &c.), and then gold-coloured; then at a temperature of
250° it becomes brown (scissors), then red, then light blue at
285° (springs), then indigo at 300° (files), and finally sea-green
at about 340°. These colours are only the tints of thin films,
like the hues of soap bubbles, and appear on the steel because a
thin layer of oxides is formed over its surface. Steel rusts more
slowly than wrought iron, and is more soluble in acids than cast
iron, but less so than wrought iron. Its specific gravity is about
7·6 to 7·9.