Copper renders iron red short.
Lead combines with iron with great difficulty, so that its presence in
the ores can hardly be considered dangerous, but when the combination is
formed, the iron is both liable to break when red-hot and when cold.
A very small quantity of tin destroys the strength of iron in a great
degree when cold, but still leaves it fit to be forged.
Wrought iron does not appear to unite with zinc, but its presence in the
ores is injurious to the manufacture, for a reason that will be
hereafter stated.
Antimony renders iron cold short, the alloy is harder and more fusible,
and approaches in character to cast iron.
Arsenic produces a great waste in the manufacture of iron, and when
alloyed with it, injures or destroys its capability of being welded.
Ores which contain titanium, according to universal experience in this
country, give an iron inclining to the defect of red short, but
possessing the highest degree of tenacity. Such are several of the ores
of the northern part of New-Jersey, and of Orange County, New-York.
Manganese in small quantities renders iron harder, but injures none of
its good qualities. Many of our ores contain manganese, but when
carefully manufactured the iron appears to contain but an insensible
trace of this _metal_.
Nickel unites with iron in all proportions, and gives a soft and
tenacious alloy; no good property of the iron appears to be injured by
it. United with steel it gives an alloy of excellent quality. Nickel is
rare among the ores of iron that are not of meteoric origin. But native
malleable iron is occasionally found in large masses alloyed with this
metal, and its extrinsic source has been fully ascertained. The masses
are sometimes of very great size; we have already expressed our opinion
that the iron that first came into use was derived from this source, and
had been employed for ages before the processes for preparing it from
its more abundant ores were discovered.
Cast iron is distinguished into two varieties, which are obviously
distinct in character, the grey and the white; a mixture of the two
forms that which is called mottled. It is generally believed, and
usually stated in the books, that both of these are combinations of iron
with carbon, and that their difference in appearance and quality grows
out of the difference in the proportions in which the two substances
exist; that the grey iron contains the greatest dose of carbon, and the
white the least. There is, as will be seen, good reason to question the
latter part of this statement.
The grey iron requires the greatest degree of heat for its fusion, is
more fluid when melted, is softest, best fitted for castings which
require to be turned or filed, and for those that must be thin; the
white iron is very hard and brittle; the greatest degree of strength and
tenacity is due to the mixture, or mottled iron, and to that variety of
mottled in which the grey rather predominates.
The different varieties are readily convertible, for the grey iron when
melted and suddenly cooled becomes white, when cooled more slowly is
mottled, and when carefully preserved from rapid loss of heat, retains
its colour. On the other hand, experiments on a small scale have shown,
that white cast iron, subjected to a heat equal to that at which the
grey melts, and allowed to cool slowly, becomes grey. Hence their
difference can hardly be ascribed to chemical constitution. Neither can
the presence of a greater or less quantity of oxygen, as is sometimes
supposed, produce the difference, for under circumstances in all other
respects similar, except the rate at which they are cooled, iron of the
three different varieties may be produced, We therefore feel warranted
in rejecting the usual theory, particularly as the reception of it has
rather impeded than advanced the manufacture of iron.
The theory of Karsten is far more consistent with the facts, and is
directly applicable to the practical purposes of the iron master. We
shall endeavour to give a succinct exposition of this theory,
introducing all that is necessary for its full explanation.
The ores of iron, which are all oxides, are reduced by exposing them to
the action of carbonaceous matter, at a high temperature. The carbon
first separates the oxygen from the ore, which becomes metallic, but as
it has for the carbon a high affinity, that substance tends to combine
with it. The iron combined with carbon is rendered far more fusible than
it is when pure, and thus readily melts; when the heat of the furnace is
little more than is sufficient for effecting this fusion, the two
substances are uniformly mixed, and probably form a compound analogous
to a metallic alloy; this is the white cast iron. When the compound is
exposed to a heat higher than is sufficient to melt it, a separation
appears again to take place, the carbon tending to assume in part the
form of plumbago, the iron to retain no more of carbon than is
sufficient to keep it liquid at the new temperature, and thus passes
from the state of cast iron to that of steel, and finally approaches to
that of malleable iron. If the cooling take place slowly, the carbon,
obeying its own law of crystallization, arranges itself in thin plates,
and the iron, consolidating afterwards, fills up all the interstices
with grains or imperfect crystals; and thus the mass assumes a dark grey
colour, partly owing to the natural colour of the iron, but in a greater
degree to the plumbago. When the cooling is rapid, the carbon still
disseminated throughout the mass, does not crystallize separately, but
the two substances again form an uniform compound.
Thus, according to the theory, there is no essential difference in the
proportion of carbon between grey and white cast iron, but the former is
a mechanical mixture of crystals of carbon, nearly pure, with iron
containing a less proportion of carbon than the white, while the white
iron is a homogeneous alloy of carbon and iron.
Upon this theory may be explained all the facts which have been found
wholly irreconcilable with the other.
1. The more intense the heat of the furnace, the deeper the colour, and
consequently the higher quality of the cast iron.
2. The changes that take place from grey to white cast iron, merely by
difference in the rate of cooling.
3. The reconversion of the white variety into grey, by simply heating it
above its melting temperature, and allowing it to cool gradually.
4. The formation of imperfect crystals of plumbago (_kish_) on the
surface of grey iron.
5. The approach to malleability of the grey iron, which is utterly
irreconcilable with its being a homogeneous compound, more charged with
carbon than the white.
The basis of white cast iron, appears to be a definite chemical
compound, of two atoms of iron to one of carbon, and is therefore
analogous in its chemical constitution to carburet of hydrogen and
carburet of sulphur, but like all metallic alloys it is capable of
containing an excess of one of the substances in a state of mixture
during fusion, and which does not separate on rapid cooling. The iron
alone is found in excess in this substance.
Steel appears to contain but half the quantity of carbon in its chemical
proportions that white cast iron does, but, like it, is susceptible of a
variety of mixtures; if the proportion of carbon amount to three per
cent., it loses the property of malleability, if the proportion fall as
low as one per cent. it can no longer be tempered, and is identical with
the harder varieties of bar-iron. As the carburets of iron, whether in
the form of pig or of steel, may be considered as alloys, if they be
presented to other metals, the results must necessarily be different
from what occurs when pure iron is exposed to the same substance. The
union that may take place in the one instance may not occur in the
other. It may often happen, that when the iron is pure, a true chemical
combination will occur, while in the other case, no more than a
mechanical mixture can be effected. For the same reason, the consequence
may be totally different when the third substance is presented to the
iron when first deoxidated, in the presence merely of an excess of
carbon, and when the combination with that substance has actually
occurred.
If reduced at the same time with the iron, the other metals will unite
with it more readily than with the carburet, and they may afterwards
prevent its union with carbon, for there are few, if any metals, besides
iron, which have any affinity for carbon.
Cast iron may contain the bases of the earths that form a part of its
ores. Of these, silicium is the most usual, and there is probably no
cast iron that does not contain a portion of it. It appears to render
this form of the metal harder and less suitable for the purposes of the
moulder, but is separated almost wholly when it is converted into
wrought iron.
We have seen a parcel of pig iron that was marked with a species of
white efflorescence, ascertained on examination to be silica; this was
rejected for its hardness by the founder, but on being manufactured by
the process of puddling, gave bar iron of good quality.
From what has just been stated, it appears that the other metals more
generally exist in cast iron, in a state of alloy with pure iron, which
is intimately mixed with the carburet. Thus as a general rule, the pig
which contains them, will be more likely to be grey in colour than that
which does not, but it may, notwithstanding, be injured in quality. The
exact effect of such alloys upon cast iron, does not appear to have been
fully examined.
* * * * *
The ores whence iron is obtained, are all oxides, with the exception of
a carbonate whence steel is in a few places obtained directly. They
contain, in combination with the iron, or forming parts of a
heterogeneous aggregate, a variety of earthy substances. In the
reduction of these ores, two objects are to be accomplished, the
separation of the oxygen, and the fusion of the earthy mass. Carbon, in
some one of its native or artificial forms, is used to effect the former
purpose, upon the same principle that it is applied to the other
metallic oxides. Thus a furnace in which a fire of carbonaceous matter
is kept up and urged to the highest possible degree of intensity by
blowing machines, is necessary. When the earths are pure, even the
highest heat of furnaces is incapable of fusing them, and although the
oxides of the ancient metals, and among the rest, the oxide of iron,
increase the fusibility of one of the earths; still, if but one earth be
present, it is only in a few cases that the simple ore will furnish the
means of its own fusion. We are therefore compelled to make use of the
property possessed by the earths, of rendering each other more fusible.
Silica is the earth to which we have referred, as being susceptible of
fusion when mixed with the oxide of iron. Silica, also, when mixed with
the other earths, renders them more fusible than is its own mixture with
oxide of iron. Hence it may be stated as a general rule, that ores which
do not contain silica, cannot be decomposed without the addition of that
earth. The most of our American ores contain silex in sufficient
abundance; hence it is usual to add to them, in the process of
reduction, carbonate of lime, which is called _flux_. Did not the ore
contain silica, this would not produce its effect, and a due admixture
of the three earths, silica, alumina, and lime, appears to be necessary
to cause the most advantageous results.
The remarks of Karsten on this head are new and worthy of attention.
"It is upon the choice and the just proportion of the flux,
that the profit of the manufacturer in a great degree
depends. Employed in too great quantities they fail in the
important purpose of giving to the scoriae a proper
consistence. It is very difficult to fix their proportions
exactly, and, in truth, these ought to vary with the manner
in which the furnace works; but a proportion determined for
a state of the furnace when the temperature is neither too
high nor too low, is usually adopted.
"Chemists and metallurgists, have endeavoured to determine
the degree of fusibility of the earths when mixed with each
other; but their researches have shed but little light upon
the management of blast furnaces. We are, in spite of them,
still compelled to have recourse to experience. Far,
however, be it from me to depreciate the attempts of Achurd,
Bergman, Chaptal, Cramer, &c.; they are valuable at least,
in pointing out the road that is to be pursued in the
experiments.
"It follows, in general terms, from these experiments, that
lime, silica, alumina, and magnesia, are infusible when not
mixed with each other; that no mixture of earths is fusible
without the presence of silica; that the fusion of the
oxides of iron cannot take place by the addition of any
simple earth other than silica; that ternary mixtures are
more fusible than binary; that quaternary mixtures vitrify
even more readily, and that the oxide of manganese promptly
determines the liquefaction of all the earths.
"The theory of the vitrification of oxides, aided by trials
on a small scale, points out the kind of earthy mixture
which ought to be employed, but it cannot fix the exact
proportion of the different earths that ought to be adopted;
nor does it teach the means of replacing an earth by its
chemical equivalent, as, for instance lime, by magnesia. The
solution of the question will depend rather upon the
properties of the silicates of lime and magnesia at high
temperatures, than upon the action of these silicates upon
iron. It is hardly probable that the iron obtained from all
ores, could be equally good, even if the most proper fluxes
could be added to these ores. Those who have maintained this
opinion, have erroneously imagined that the reduction of the
ore could always be effected under the same circumstances,
which would not be the case, even if these fluxes were
ascertained and made use of."
Most of the ores of iron require, before they are subjected to the
process of reduction, a preparatory operation called roasting. This
consists in exposing them to a comparatively low heat. The more
important use of this process is to render the mass more susceptible of
mechanical division, but it also serves in many cases to separate the
sulphur and arsenic that may exist in the ore. There are some ores, as,
for instance, those of a number of mines in Morris and Sussex counties,
New-Jersey, which are so free from impurities, and which yield so
readily to the mechanical means employed for separating them, that this
process is wholly unnecessary; but such ores are rare, and the process
of roasting must, generally speaking, be performed.
The mechanical division, which exposes a larger surface to the action of
heat and of the chemical agents, is called stumping; this is usually
performed by appropriate machinery, but was in the infancy of the art
effected by hand.
The reduction of rich ores of iron, such as are almost wholly made up of
its oxides, and contain but little earthy matter, may be performed in a
common smith's forge. The reduction in this case takes place immediately
in the blast of the bellows, where the intensely heated ore is in
contact with the burning charcoal; and if a carburet be formed, it is
immediately decomposed, and pure iron is the result. Such is probably
the more ancient of all the processes for obtaining malleable iron, and
it is still used to a certain extent even at the present day. The hearth
in which the operation is at present performed, differs from the forge
of a common smith only in its greater size, and in the increased power
of its bellows. A cavity is prepared, in which a charcoal lire is
lighted, and to which the nozzle or _tuyere_ of the bellows is directed;
ore in minute fragments is thrown upon the ignited fuel, fresh coal and
ore are added from time to time, and the latter being reduced to the
malleable state descends, as the charcoal burns away, to the bottom of
the cavity. Here the successive portions, still kept hot by the fuel
above them, agglutinate, and form a porous mass, containing in its
cavities a black vitreous substance, which is composed of the earthy
matter rendered fusible by the metallic oxide. This porous mass is
called the _Loup_.
It would be unsafe to subject the loup immediately to the action of
heavy hammers of iron. It is, therefore, after being withdrawn from the
fire, beaten with wooden mallets, to bring its parts into closer
contact, and press out the vitreous matter. While this is performed, it
cools so much as to require to be again heated, which is done in the
same fire. Indeed, the same forge is used in all the successive heats
that the iron in this process requires.
After the loup has been again heated, it may be subjected to the hammer.
This unquestionably was anciently one moved by hand; but now, in all
manufactories of this character, a heavy mass of case hardened iron is
employed for the purpose; this is lifted by machinery impelled by a
water wheel, and permitted to fall upon the loup. The loup is again
heated, and again beaten into an irregular octangular prism, called the
cingle; this, after a third heat, is formed into a rectangular block,
called a bloom; and the whole, or a proper proportion of this is drawn
into a bar, at three successive heats; the middle being beaten out
first, and the two ends in succession. Thus, in addition to the heat
employed in the original reduction, the iron must be at least six times
reheated before it becomes a finished marketable bar.
In this manner the ore of Elba is still manufactured in Catalonia and
Tuscany, and there can be little doubt that it is identical with the
original rude process, by which the iron of that most ancient of known
mines was prepared to be an object of commerce. The processes in these
two districts differ from each other in some minute particulars, and are
known on the continent of Europe as the processes _a la Catalane_ and _a
l'Italienne_. This method is known in the United States by the name of
_blooming_.
Bloomeries are frequent in the United States, being found in many parts
of the primitive country, where the magnetic ore of iron is abundant.
The iron manufactured by blooming is, generally speaking, remarkable for
its nerve, being strong and tenacious in the highest degree, unless the
ore be in fault. It is not, however, homogeneous, being liable to
contain what are called pins, or grains that have the hardness and
consistence of steel.
Blooming is comparatively an expensive process. It requires, indeed,
little original capital, but the product in proportion to the capital
employed is but small. It is wholly impracticable with poor ores, and
demands a great length of time and expenditure of fuel, unless the ore
be very fusible. Another objection to it is common to a process we shall
hereafter describe, that of refining, and lies in the numerous
successive heats, which the small extent of fire, and the slow process
of hammering render necessary, before the bar is finished. It has been
attempted in New-Jersey to lessen the expense attending these heats, by
performing them in reverberatory furnaces. A saving of fuel to a small
amount would probably thus be effected, but the number of heats would
still remain the same. A more important and useful improvement has
superseded the last; the process of rolling, which will be hereafter
described, has been introduced, and by means of it a bar may be drawn
out at a single heat, and at far less expense of manual labour. Such
establishments exist at Dover and Rockaway, New-Jersey, which receive
the iron completely reduced from the neighbouring forges, and fashion it
into bars.
A forge fire, and, consequently, the process of blooming, is
insufficient to convert poor ores, or those that contain much earthy
matter, into iron. Treated in this way, those ores, if fusible at all,
would become a mass of slag, as the earth would require, at the
temperature of a forge fire, the whole, or the greater part of the
metallic oxide for its fusion.
Iron being introduced, and its valuable applications known, it became
necessary, in those countries that do not afford rich ores, to discover
a method by which the poorer might be reduced. This could only be
effected by giving such a degree of heat, as would render the earthy
matter capable of melting, at a less expense of metal. To increase the
mass of fuel, by increasing the depth of the cavity, and actually
forming it of walls, thus enabling it to contain a greater quantity,
would be obvious means of attaining this end. The ore must be added in
smaller proportions, and, being longer in contact with the heated
charcoal, would become carbureted; the carbon must therefore be finally
burned away, before malleable iron could be attained. A rude but
efficient process of this sort, is described by Gmelin as in use among
the Tartars; an analogous method, whose use has been superseded by iron
imported from Europe, was found among the nations of Guinea; and Mungo
Park saw a more perfect application of the same principle at Camalia, on
the Gambia. Furnaces of similar character, but more skilfully
constructed, are still used in some parts of Germany, and are called
_stuckoffen_.
As a carburet, or actual cast-iron, must be formed in these processes,
and, as the separation of carbon at the bottom of a deep cylinder, and
where the metal would probably be covered by a vitreous liquid, is
difficult, the iron might sometimes resist the efforts made to render it
malleable, and run from the furnace in a liquid form. It might therefore
have readily occurred, that it would be less costly to finish the
process in a forge. The _stuckoffen_ were therefore converted into
_flossoffen_, or melting furnaces, whence the liquid carburet was
withdrawn, and afterwards converted into bar iron. Such was probably the
cause that led to the original discovery of cast iron, a discovery that
cannot be traced further back than the end of the fifteenth century.
The uses of cast iron for purposes to which wrought iron is
inapplicable, and the readiness with which it is fashioned, by pouring
it into moulds, led to the increase of the size of the _flossoffen_, and
in the power of the blowing apparatus, which has caused the introduction
of the blast furnace. This forms the basis of the methods by which iron
in all its forms is chiefly prepared at the present day, and is hence
worthy of particular consideration.
The difference between the blast furnace proper, and the ancient fires
from which it gradually took its rise, consists wholly in its superior
height, and in the greater power of the blowing machines, by which its
combustion is supplied with air.
This increase of height adds to the mass of the contained
combustible,--additional air is therefore required for effecting its
complete inflammation, and the joint effect is, that a much higher
temperature is generated. By this, the earthy matters either contained
in the ores, forming portions of the combustible, or added as _fluxes_,
are rendered fusible at a less expense of oxide of iron; the carburet
formed, becomes more fluid, and the product is more likely to assume the
character of grey pig-iron.
Charcoal, as in the other processes, was the fuel originally employed,
and is still principally used in most countries. But coal deprived of
its volatile parts, and charred or converted into coke, has been
substituted in some regions, as will hereafter be stated. Each of these
combustibles requires a furnace of appropriate character, and demands a
difference in the mode of management.
A blast-furnace is a hollow chamber enveloped, generally speaking, in a
mass of masonry, of the form of a truncated pyramid. The chamber is
composed essentially of three parts; the upper has the figure of a
truncated cone, whose greatest base is lowest: this may be called the
body of the furnace; the middle portion has also the figure of a
truncated cone, whose greater base is uppermost, and is common to it and
the upper portion: this contraction is called the _boshes_ of the
furnace; the lower position is called the hearth, and is usually
enclosed on three sides by walls of refractory substances, on the fourth
it is bounded by two stones, one serving as a lintel, which is called
the tymp, the other resting on the foundation, and known by the name of
the _dam_. Such at least is the shape of the blast furnaces in common
use, and which will suffice for our present purpose.
The blast is introduced into the hearth, at a small distance above the
level of the upper edge of the dam, and is now generally performed by
means of two _tuyeres_; in the more ancient furnaces, there was but one.
The furnace being completely dried, a fire is lighted in the hearth, and
fuel gradually added, until the whole is filled to the _trundle head_,
which is the open and lesser base of the truncated cone that forms the
body of the furnace. The blast may then be applied, slowly and gently at
first, and increasing gradually, until it reach its maximum of
intensity. As the blast proceeds, the charcoal gradually burns, and
descends; its place is supplied at top by fresh fuel, by ore, and by the
earthy matter used as a flux. This is styled _charging_ the furnaces.
The earlier charges often contain no ore, but are wholly composed of
charcoal and flux, and, in all cases, the proportion of ore and flux is
at first small, and is gradually augmented. The charges are made as
often as the mixed mass in the furnace descends sufficiently low to
admit the quantity that is chosen as the proper amount. The charcoal is
thrown in first, and the ore and flux are spread and mixed upon its
surface. The principles which govern the amount of the charge, are as
follows:--
"The volume of the charges depends upon the capacity of the
furnace. If they be too large, they cool the upper part of
the furnace, which will cause great inconveniences,
particularly if zinc exist in the ore. On the other hand,
small charges of charcoal will be cut or displaced by the
ore, which will occasion a descent by sudden falls, in an
oblique direction, or in a confused manner. It follows that
the volume of the charge, although proportioned to the
volume of the furnace, must be augmented: when the charcoal
is light and susceptible of being displaced; and with the
friability, the weight, and the shape of the fragments of
the ore."
"The heat, considered in any given horizontal section of the
furnace, will be intense in proportion to the thickness of
the layer of charcoal that reaches it. It follows, that the
fusible ore requires smaller charges of charcoal than one
that is more refractory. If the beds of charcoal and mineral
are too thick, the upper part of the furnace will not be
sufficiently heated. Hence it is obvious, that there must be
a maximum and minimum charge for every different dimension
of furnace, and for every different species of ore and
fuel." _Karsten_.
The charge of charcoal being determined upon such principles, it is
added by measure, and always in equal quantities, while the proportion
of ore and flux is made to vary, not only by a gradual increase at the
beginning of the operation, but according to the working of the furnace.
The manner in which the furnace is working can be inferred, even before
its products are ascertained, by the appearance of the flame at the
trundle-head, and at the tymp, by the manner in which the charge
descends, and more surely still, by the appearance of the scoriae. By a
strict attention to these circumstances the proportion of the charge of
ore may be regulated. A fortnight usually elapses from the time of the
first charge until it reaches a regular state of working, and variations
will occur even after that period, in consequence of the greater or less
moisture of the combustible and minerals, the continual wearing away of
the sides of the furnace, the variations in the state of the atmosphere,
and in the play of the blowing machines, the greater or less attention
of the workmen, and numerous other accidental circumstances.
The mode of proceeding when coke is the fuel employed, rests upon the
same principles, but the dimensions of furnace that are best suited to
the different combustibles are different. As a general principle, the
height of furnaces must depend upon the force of the blast and the
density of the fuel. If the fuel be dense, and the blowing machine weak,
the furnace must not have a great height; and even if the blast can be
made strong, too high a furnace is disadvantageous for light charcoal.
Coke, on the other hand, may be used in furnaces of greater height than
any species of charcoal, provided the blast be of sufficient power. So
long as the imperfect bellows were used in blowing, the height of the
furnace was limited wholly by their action. More powerful apparatus in
the form of cylinders, analogous in form and arrangement to those of
steam-engines, and like them, either single or double acting, have now
been introduced; the intensity of the blast is in them only limited by
the moving power, which is applied to them, and when this is the steam
engine, it may be said, that no limit can arise from the want of blast.
We may, therefore, at the present day, regulate the height of furnaces
by the nature of the fuel that is consumed in them.
The greater part of the furnaces in our country still retain the ancient
and imperfect form of bellows, hence their height is restricted to the
limits of from eighteen to twenty-four feet, and rarely or never reaches
thirty. But when the apparatus is such as to supply a proper quantity of
air, it has been found that even with light and porous charcoal, such as
is given by white pine, the height ought not to be less than thirty
feet, and when hard woods are used should be as great as thirty-six
feet. Furnaces of even forty feet have been found to answer an excellent
purpose, where the charcoal was prepared from oak. When coke is used,
furnaces have been made as high as fifty, or even as seventy feet; but
experience in England has shown, that from forty-five to forty-eight
feet is the proper limit. This height is not at present exceeded in that
country, even when the furnace has the greatest dimensions in other
respects, and has been found efficacious, even when the vast quantity of
eighteen tons has been furnished daily by a single furnace.
The force of the blast will depend upon the nature of the fuel, the
volume of air, the quantity of mixed material the furnace holds; and
thus furnaces in which coke is used, will require the most powerful
blast, whether we have regard to the volume or the intensity. The latter
may be measured by a column of mercury adapted in a syphon tube to the
air pipes, exactly as the gauge is adapted to the pipes of the steam
engine.
The reduction and liquefaction of the metal take place progressively, as
the charges descend in the furnace. The separation of the oxygen is due
to the presence of carbonaceous matter at high temperatures, begins at
the surface of the pieces of ore, and proceeds gradually inwards; the
earthy parts of the ore, of the fuel employed, and the flux, unite and
melt; they are thus separated, and being sooner fused than the metal,
make their way through the charcoal, and descend first to the hearth.
The reduced metal, continuing in contact with the burning carbon,
acquires a greater or less portion of that substance, becomes fusible,
melts, and follows the liquified earths. Dropping into the hearth that
already contains the liquid vitrified earths, it passes by its superior
gravity to the bottom, and is protected by them from the blast. Even at
the bottom of the hearth, the heat is sufficient to retain the
carbureted metal in a liquid state, and this is permitted gradually to
accumulate, until it rises nearly to the level of the dam.
It now becomes necessary to withdraw or _cast_ the metal. This is done
by forcing a way through a channel left beneath the dam in the masonry
of the hearth, and closed with clay; the inner portion of this is baked
hard, and requires to be broken through with a steel point. As soon as
the passage is opened, the metal runs out, and is received in a long
trench formed in the sand floor of the moulding house, to which are
adapted a number of less trenches, at right angles, each containing
about one hundred weight of metal. The metal in the longer trench is
also broken into pieces of the same size, and the ingots thus formed are
called _pigs_, whence the term for this variety, _pig iron_.
From one to three days will elapse from the time of the first charge
until the furnace can be tapped, and pigs cast. From that time the
casting succeeds with tolerable regularity, according to the working of
the furnace, and at intervals depending upon the volume of the charge,
and the capacity of the hearth.
It appears probable that the fusion of the iron is effected always by a
direct chemical union of that metal with carbon, in the proportion of
two atoms of the former to one of the latter. This constitutes, as we
have seen, the white variety of pig iron. But as it continues, generally
speaking, in the furnace, long after its fusion takes place, it acquires
a temperature higher than its proper melting point, and a tendency to
separation takes place, the iron retaining in combination no more of the
carbon than is necessary to maintain it in a fluid state at the
increased temperature. Thus the grey variety of pig iron is formed; and
on casting it, the carbon, in a form similar to that of plumbago, is
disseminated throughout the mass, or forms on its surface the
efflorescence that is called kish, and which is always a sign of a high
quality in the iron it accompanies.
In conformity with this theory, we find that a high temperature in the
furnace always produces grey cast iron; and that a low temperature, from
whatever cause it may arise, renders the iron more or less inclining to
white. So also if the metal be not exposed to the heat for a sufficient
length of time, it becomes white.
Karsten classes these several causes of whiteness in the product, in the
following order:--
"In conformity with the observations that have hitherto been
made, white cast iron is obtained:
"1. By the use of ores that are too easily fusible, or which
is the same thing, by an excess of flux, by a want of
density in the charcoal, and by too strong a blast, even
when the working of the furnace is regular.
"2. By a surcharge of ore, which deranges the action of the
furnace, and produces impure cinder, containing uncombined
iron.
"3. By boshes of too rapid a slope, and a blast of too great
a velocity; and this may occur even where the cinder is
pure.
"4. By too low a temperature, even when the cinder is pure,
and the furnace works regularly.
"5. By a derangement in the action of the furnace, arising
not from a surcharge of ore, but from an irregularity in the
descent of the charge.
"6. By the substances contained in the body of the furnace
exercising too great a pressure upon those beneath; the heat
in this case, concentrated in the hearth, cannot reach the
boshes, and the upper part of the furnace; the working may
be regular, the cinder and flame may in this case give no
sign of derangement.
"7. By too great a breadth in the furnace.
"8. When coke is used, it may arise from too great a
quantity of ashes, or of fossil charcoal, (anthracite,)
being contained in it. The presence of these will keep down
the heat of the furnace. An excess of ashes may be remedied,
by using the ore and flux in proper proportions to fuse
them, but a diminution in the charge must be made; the
cinder becomes viscid, and likely to obstruct the descent of
the charges.
"9. By an accidental cooling, arising from humidity, and
other similar causes."
Among the last may be reckoned the presence of zinc in the ore. This
metal, although volatile, is not separated at the temperature given in
the process of roasting, nor does it sublime in the upper and cooler
parts of the furnace. But, as the ore descends, it passes into the state
of vapour, and requires for its conversion, great quantities of heat
that becomes latent. It hence cools the lower part of the furnace far
more rapidly than even wet coal, or moist ores. The cooling thus caused,
may not be effected until the melted metal reach the hearth, and may
there cause it to become solid. Thus the solid mass called a salamander,
may, in some cases, be formed; and thus may be explained the fact, that
ores of iron that contain the more easily fusible metal zinc, are more
liable to interrupt the action of the furnace in this manner, than
others. The volatilized zinc rises to the upper part of the furnace,
where the heat is often insufficient to retain it in the state of
vapour, and is then deposited on the sides. In this position, it will
also disturb the action of the furnace.
Coke being more dense than charcoal, will, in its combustion, furnish a
more intense heat;--hence it is hardly possible to obtain by a charcoal
fire, iron of as deep a colour as may be procured by the use of the
former fuel. It will also resist the pressure of far greater weights
than charcoal, and hence the proportion of ore may be much greater when
it is used; containing more and less fusible earthy matters than
charcoal, it requires a greater quantity of flux.
In the manufacture of cast iron then, coke gives iron better suited for
small castings, for those which require turning or filing, and yields a
far greater quantity from a furnace. Hence arises the very great
superiority which Great Britain has, until recently, possessed over most
other countries, in those fabrics in which these qualities are valuable;
and hence it has been found until lately, in this country, hardly
possible to manufacture fine machinery that requires workmanship after
it is cast, without the aid of the higher qualities of Scotch iron,
which, in these qualities, exceeds even the English. Recently, however,
iron fully equal to the best Scotch, but like it wanting in tenacity,
has been manufactured at the Bennington furnace in Vermont:--so also at
the Greenwood furnace in Orange county, N. Y., and at West Point, iron
approaching to the Scotch in softness, but very superior in strength,
has been produced. In these cases, the height of the furnace has been
carried up to the limits we have before laid down, and powerful blowing
cylinders substituted for the ancient bellows.
When the pig iron is to be used for re-casting, every effort ought to be
used to obtain it of the deepest possible colour. This, as may be seen
from what has been already stated, will be effected by keeping the
furnace at the highest possible temperature, and exposing the metal to
it a sufficient length of time. In effecting this, however, certain
defects may arise:--thus a longer exposure to a high heat, will cause
the reduction of other oxides that may be present, as of manganese and
the metallic bases of the earths; and the iron in becoming more soft,
and approaching in fact more nearly to the form of the pure metal, will
combine and form alloys with these bases. In this way, it will, as has
been stated, become cold short; and to this may be attributed the want
of strength in the greater part, if not all, of the British iron. The
use of coke as a fuel, tends to increase this defect, in consequence of
the great quantity of earthy matter it contains.
When the ores are pure, cast iron manufactured by charcoal, is not
liable to such a fault. Hence the cast iron of Sweden and the United
States, manufactured from the magnetic iron, or, in some cases in this
country, from rich haematites, has very superior tenacity, insomuch that
these two nations have alone been able to use this material in the
construction of field pieces. When white iron is obtained from a
furnace, it may have two different qualities. The first arises from a
mere defect of heat, where all other circumstances are favourable, and
the ore is completely reduced. The second arises when the reduction is
not complete, and the separation of the earths and other oxides has not
been fully effected. Of all the varieties of cast iron, this latter is
by far the worst. It is indeed more easily converted into wrought iron
than the other species, but the product is always of very inferior
quality; it is rarely or never produced by furnaces fed with charcoal,
but may be obtained by accident or design in those where coke is used,
by a surcharge of ore, or by too great a proportion of flux, and
sometimes cannot be avoided in warm and moist weather, where the air is
rarefied and charged with vapour.
The grey iron obtained by the use of each of the different kinds of
fuel, has its own peculiar advantages; that made with coke possessing,
as a general rule, when melted, a higher degree of fluidity which adapts
it for more delicate castings; being softer and better suited for
fitting; while that manufactured with charcoal, possesses a greater
degree of strength. One solitary instance has been quoted, in which a
manufacturer of great intelligence has obtained by the use of charcoal,
from a very pure ore, a union of both these valuable properties, and
another, in which iron as soft as that made with coke, has been produced
by means of charcoal.
In spite of this apparent balance in the properties of the two fuels,
the introduction of coke into the art of reducing iron has been attended
with the most important advantages. These lie in the superior economy of
the process, and in the enormous quantity of the product. The
manufacture of iron by charcoal is limited, by the growth of the
forests, which replace themselves only at distant periods, by the large
space they occupy, and the consequent labour of transportation; by the
cost of cutting the wood and preparing the coal; and finally, even when
the fuel can be obtained in abundance, and at small cost, the burden of
the furnace, and the heat obtained in a given space are less than when
coke is used, and the quantity of metal yielded is in consequence
comparatively small. The coke furnaces of Great Britain, have therefore
supplied cast iron in such abundance and at such diminished prices as to
have brought it into use for a great variety of purposes, to which,
until recently, it was hardly considered applicable.
In England, as in other countries, charcoal was the only fuel at first
used; and after bloomeries had been in vogue for centuries, the blast
furnace was introduced from the shores of the Rhine. For many years the
growth of the forests proved sufficient to supply the demand, but at
length the increase of population caused them to be encroached upon by
cultivation; the growth of the manufacture was first prevented, and
finally, almost extinguished.
The method by charcoal appears to have reached its acme of prosperity,
at the close of the reign of the First James, when the furnaces of the
kingdom yielded 180,000 tons of pig iron. About this period, Dudley
first proposed the use of pit coal; but the time had not yet arrived in
which it was absolutely necessary to seek for a new process, in
consequence of the failure of the old one.
In 1745, or in the course of one hundred and thirty years, the forests
had been so far encroached upon, that the product of the furnaces had
fallen to 17,000 tons per annum, and in 1788, the quantity made with
charcoal had dwindled as low as 13,000 tons. At this epoch, coke was
introduced into blast furnaces, and in eight years the whole quantity
produced by both methods had mounted up to 150,000 tons, or increased
more than tenfold.
At nearly the lowest ebb of the British manufacture, the art of
preparing iron was introduced into her then provinces, the present
United States; and in 1737 it was attempted to obtain permission to
introduce the product into England. The attempt failed, and in 1750 an
act was passed to protect the exportation of English iron to America,
and to prevent the establishment of forges. Had the other policy
prevailed, England would probably have seen her manufacture of iron
transferred to the United States, and with great immediate advantage
both to herself and her then most valuable colony; but she would
probably have seen herself at the present day degraded from her high
stand in the scale of nations, to the secondary place in which the
extent of her territory would keep her, were it not for the superiority
of her manufacturing industry, of which iron is the basis. The quantity
of iron now produced in England, exceeds that furnished by the rest of
the world united, and does not fall short of 800,000 tons. It has a
value even in its raw state of near four millions sterling, and is of
far greater intrinsic worth, in consequence of the spur which its
abundance gives to every other branch of industry.
Bar iron is at the present day principally manufactured from the pig.
The process originally used for this purpose is called refining. The
fire in which it is performed is a forge, similar in form and character
to that employed in blooming. In blooming, the iron must be reduced,
combines with carbon, and is subsequently decarbureted; while in the
refining, the latter part of the operation alone remains. In this last
process, while the carbon is burning away, the metallic bases of the
earths are then oxidated, combine with oxide of iron, and form a
vitreous substance. Hence, when it is carefully conducted, by far the
greater part of the impurities contained in the cast iron may be
removed. Refined iron, if made from ore of equal purity, is not inferior
in tenacity to bloomed, and is superior in other respects, being more
homogeneous, free from pins, and more easily treated by the smith. As a
general rule, it is also less costly, that is to say, the same quantity
of charcoal and workmanship will furnish a greater quantity of refined
iron. It requires, however, a much greater capital, and the labour of
transporting the coal from the greater distances which the increased
consumption of a single blast furnace and several refineries will
demand, may swell the cost of that article. A bloomery fire does not
require more than 2000 acres of woodland, while a blast furnace will use
the charcoal of 5000. Thus it happens, that it may be more advantageous
to spread a number of bloomeries over a given district of country, than
to unite a blast furnace and an equal number of refineries in a single
place. The celebrated iron of Sweden and Russia is refined, and our
country furnishes iron prepared in the same manner not inferior in
quality. The principle objection to the process is the great expense of
the fuel employed, in the successive heats to which the iron must be
exposed in drawing it into bars, after the processes of conversion and
the separation of impurities have been effected.
As charcoal became scarce in England, it was attempted to employ coke in
lieu of it, in the refineries. This, however, constantly failed, in
consequence of the great intensity of the heat, by which the pig was
melted suddenly instead of being exposed to the blast, long enough to
burn away the carbon. Reverberatory furnaces were next tried, and with
partial success, but a combined process has finally been introduced
which has been successful and which is called, from a part of the
operation, the method of _puddling_.
The manufacture of wrought iron, by means of bituminous coal, is
executed at three successive processes, and is facilitated by very great
improvements in the machinery. Where hammers are still used, they are
much increased in weight, and driven with greater velocity; but by far
the greater part of the operation of drawing the bars is effected by
means of rollers. The plan of these is in some measure borrowed from the
slitting mill, in which bar iron is reduced into rods and thin rolls for
various uses. These rollers are in sets, composed each of two of equal
diameter, lying in a horizontal position, and placed one vertically
above the other. Grooves corresponding to each other are cut in the two
rollers, between which the heated iron is drawn by their revolution, and
forced to assume a section that just fills up the two grooves. By
passing in succession through grooves gradually decreasing in size, any
form or magnitude may be given to the bars; and the operation is so
rapid, that the bar may be drawn from the loup at a single heat.
The first operation to which the pig iron is subjected, consists in
melting it in a fire called a finery, similar in form and character to
the bloomeries and refineries of which we have spoken, but in which the
fuel is coke. The melted metal is drawn off by tapping the furnace from
beneath, and is cast into thin plates. In this way it assumes the
characters of the white cast iron, which has been described as formed,
when the reduction of the metal is complete, a form that cannot be given
when the blast furnace in which it is made is supplied with coke. The
rapidity of the cooling is increased, by throwing water on the surface
of the plates. It thus appears, that this operation is adopted in order
to bring the cast iron into a slate that it may often assume when
manufactured by charcoal, and which cannot be given to it by coke. In
conformity with this view of the subject, it has been found, that when
wrought iron is manufactured by puddling, from American pig prepared by
charcoal, this preliminary operation is unnecessary.
The fine metal, obtained in the manner we have described, is next broken
into pieces, and subjected to heat in a reverberatory furnace. A rapid
heat is given at first to liquefy the iron, and is then diminished by
means of dampers; the melted mass is violently stirred to expose it to
the action of air and heat, by which the carbon is burnt away, and a
part of the oxides of iron and the earthy bases combined and vitrified;
as the carbon is separated, the metal gradually loses its liquidity, and
finally dries, or assumes the consistence of sand: this shows that the
carbon is separated, and the iron has assumed its malleable nature. The
addition of water aids the oxidation of the several substances, and
facilitates the process. The heat is again increased, and the metal
collected under it, and rolled together into parcels suited to the
action of the drawing machinery, and to the size of the bar that is to
be made; these are pressed together, and a partial union takes place
among their particles. When they have attained a white heat, they are
withdrawn in succession. In some cases, where the number of puddling
furnaces is great, they are immediately carried to the rollers and drawn
down. But where quality is more regarded than quantity, they are first
subjected to the action of the hammer, and finally rolled. The latter
process has the advantage of separating more completely the vitrefied
oxides, than can be done by rolling alone, but it will often require a
second heat, which is given in a forge fire called the _chaffery_. When
rollers are used alone, a minute and half is sufficient to form the bar;
and a power of thirty houses will roll two hundred tons per week.
The iron in this state is still of very inferior quality, although its
external appearance may be good. It is, notwithstanding, sometimes
thrown into the market, and this has given rise to the impression that
prevails in this country of the bad quality of English rolled iron. It
may, however, be used in some cases, where it need not be fashioned by
forging; thus, where it requires no more than to be cut into lengths, or
where the original bars will answer the purpose, its cheapness may
recommend it. Iron for rail-roads is of this quality; and the punching
of holes, by which it may be fastened down, is effected by a simple
addition of steel teeth, at proper distances, to the last groove through
which it is passed. In this form, ready to lay down, rail-road iron may
be shipped from England at the low price of 7_l._ 10_s._ sterling per
ton; and a similar quality in the simple bar may probably be afforded at
about 7_l._ We have never heard of its being sold so low as is stated in
the evidence before the Committee of Congress, say 5_l._ 5_s._ There
was, however, a period, when an excess of production, caused by a
competition between the manufacturers of Wales and Staffordshire,
entailed ruin on many of them, and their articles were sold far below
the price of production. The price which we have stated is lower than
that which has recently been paid in England for rail-road iron, and is
that of some shipped from Liverpool, 1st March, 1831, when a
considerable fall had taken place.
In order to render the iron which has undergone this process
merchantable, it is subjected to the third of the operations which we
have enumerated. For this purpose, the bars are made from three to four
inches in breadth, and half an inch in thickness. These are cut into
lengths, proportioned to the weight of the bar of finished iron that is
to be made, and piled together by fours, in a reverberatory furnace,
similar in character to the puddling furnace. Here they are exposed to a
white heat, by which the four pieces of each pile are made to adhere;
they are then withdrawn, and subjected to rollers similar to those used
after the puddling process, but of more careful workmanship. The cost of
finishing bar iron in this way, when the pig is made by the manufacturer
himself, as ascertained upon the spot by Dufrenoy and de Beaumont, is,
in Wales, 8_l._ 15_s._, in Staffordshire, 9_l._ 12_s._ The cost of
making pig iron in Wales is 4_l._ 7_s._, or about half that of the
finished bar iron, and in Staffordshire 5_l_ 2_s._
The iron prepared by the three processes of which we have spoken,
although merchantable, and suited for various common purposes, is still
far from good. We give the characters by which it is distinguished, from
the work of Karsten:--
"The iron prepared in the English manner, appears dense and
exempt from cracks and flaws. But this goodness is only
apparent; the uniform pressure to which the bars are
subjected at every point, masks their defects. If a piece of
this kind be taken, that in its fracture appears dense and
homogeneous, and it be heated in order to be drawn out under
a common forge hammer, it dilates and exhibits numerous
flaws, that sometimes increase to such a degree, that the
bar will fall to pieces under the hammer. It is probable
that the cause of this phenomenon is due to the scoriae,
which, in this mode of working, remain mixed in the mass."