combustibles. Some specimens of graphite, plumbago, or “blacklead”
consist of almost pure carbon (98 per cent.), and some varieties of wood
charcoal exceptionally contain 96 per cent.; but in ordinary charcoal
the percentage is much less. Coal, the most familiar of our solid fuels,
varies greatly in composition, carbon being the predominating
constituent, in amount from 57 to 93 per cent. Coke, another fuel much
used in metallurgical operations, is made by heating coal without access
of air, when a large quantity of gaseous substances is expelled. Coke
burns with an intense and steady heat without emitting any visible
smoke, but it does not ignite as readily as coal. Carbon forms two
different compounds with oxygen: both are invisible gases, but they
differ in the proportions of the constituents, and present different
properties. When carbon (coal, coke, or charcoal) is completely burnt,
that is, with an abundant supply of air, the product is _carbonic acid_
gas, in which 3 parts of carbon are combined with 8 of oxygen: when, on
the other hand, the carbon is burnt with a sufficiently restricted
access of air, the result is _carbonic oxide_ gas, in which 3 parts of
carbon are united with only 4 of oxygen. The reader will here observe
that the former contains just twice as much oxygen as the latter for the
same quantity of carbon. This fact and numberless others like it are
expressed or summed up by another _law_ of chemical combination which
states that when two elements combine in several different proportions
these are invariably such that the ratios in the several compounds will
be found to have exact and simple numerical relations; that is, such as
may, when reduced to their lowest terms, be expressed by the simple
integers 1, 2, 3, etc., as 1 : 2, 3 : 2; ... 8 : 9, etc. It comes to the
same thing if we compare together the weights A and A´ which are united
in each compound with any one identical weight of B, giving of course
the ratio A : B ÷ A´ : B. For instance, in the case just given, of
carbon and oxygen, 3 : 4 ÷ 3 : 8 = 2 : 1. This, which is simply stating
the facts, is called the _law of multiple proportions_. On a later page
will be found another illustration (see Index, _Nitrogen and Oxygen
Compounds_), and its expression in terms of the _atomic theory_, which
goes behind the facts (so to speak), but is extremely useful by
comprehending many other groups of facts in chemistry and in other
sciences. Carbonic acid gas is of course incombustible, but carbonic
oxide gas burns by uniting with the additional proportion of oxygen and
becoming carbonic acid. On the other hand, carbonic acid gas passing
over red-hot coals takes up from them the additional proportion of
carbon, and is, we may say, _unburnt_ into carbonic oxide. When we see a
pale blue flame flickering over the bright embers in a fire grate, it is
carbonic oxide burning back again by taking more oxygen from the air
above the coals. Carbonic oxide combines directly with two or three of
the metals, as, for instance, it forms a volatile compound with nickel,
at a certain temperature, and this is decomposed again at a higher
temperature. The like takes place with iron, although in very small
quantities, but the observation throws some light on the processes of
reduction. Carbonic oxide is neither acid nor basic, but carbonic acid
is an acid oxide, and as such unites with oxides of the basic class to
form another range of compounds. Thus, for example, the oxide of the
metal _calcium_ is quicklime, which is strongly basic, and this directly
combines with carbonic acid, forming a _neutral_ substance called in
systematic chemistry _calcium carbonate_, or more commonly but less
correctly, carbonate of lime, familiar to everyone in the compact state
as limestone, and marble, and in a more or less pulverent condition as
chalk. When any of these is heated to redness, carbonic acid is expelled
and quicklime remains. Like most oxides, quicklime forms a compound with
water, the combination being attended with the extrication of much heat,
the compact quicklime swelling and crumbling into _slaked lime_. The
chemist’s term for a compound of a _basic_ oxide with water is
_hydrate_, while that of an _acid_ oxide with water is for him properly
an _acid_, or in order to particularly distinguish this class, an
_oxy-acid_. It was however the older practice to give the name of acid
to the oxide alone, and this naming having found its way into popular
language is much more familiar to the non-scientific reader. The
systematic names of the two compounds of carbon and oxygen are _carbon
monoxide_ and _carbon dioxide_, but we shall use here the more familiar
terms carbonic oxide and carbonic acid.
We have now to call attention to a substance which contributes by far
the largest part to the solid crust of our globe. It is called _silica_,
from _silic-_, the Latin word for flint (without case suffix): it is
seen in flint, and very pure in rock crystal, quartz, agate, and
calcedony. It forms the essential part of every kind of sand and
sandstone, and is the principal ingredient of clay, granite, slate,
basalt, and many other minerals. Silica is the oxide of a quasi-metal
called _silicon_, which can be obtained from silica with difficulty, and
only by roundabout processes, presenting itself in different conditions
according to the process used. Silica is an acid oxide, and it readily
unites with most of the basic oxides when heated with them, forming a
class of compounds of different properties which are much modified in
admixtures containing two or more. Very few of these _silicates_ are
soluble in water, most of them are not: they are all fusible at various
temperatures, except silicate of alumina, of which fire-clay is chiefly
constituted. Alumina, it should be stated, is the oxide of the metal
aluminium. The silicates of lime and of magnesia fuse only with great
difficulty; but the silicates of iron and of manganese are easily fused,
and silicate of lead still more so. Glass is a mixture of silicates,
often of lime, soda, and alumina; sometimes of lead and potash mainly;
porcelain and pottery consist chiefly of silicate of alumina with
varying proportions of silicates of iron, of lime, etc.
It now remains only to mention two non-metallic elements that are nearly
always present in crude iron, but which the metallurgist strives to
eliminate, as they are in general very injurious to the quality of the
material even when their amount is very small. The first is _sulphur_,
well known as _brimstone_, also as _flowers of sulphur_, a yellow
coloured solid, which burns in the air. The product of the combustion is
an invisible gas of a readily recognized pungent odour: this is an
acid-forming oxide containing equal weights of sulphur and oxygen. There
is another oxide in which the weight of oxygen is one and a half times
that of the sulphur, and this is the radicle of the very active
_sulphuric acid_ or _oil of vitriol_. Sulphur, like oxygen, unites with
most of the other elements, forming compounds called _sulphides_. Of
these the iron compound called _pyrites_ is the best known, and its
occurrence in coal prevents the use of that material as fuel in contact
with iron or other metals. _Phosphorus_ is an element that occurs
naturally only in combination; in its separated state it is a very
inflammable solid. It combines directly with other substances and is
taken up by some fused metals in large quantities. In many cases a very
small proportion of it existing in a metal greatly modifies the
properties of that metal. Phosphorus forms several oxides, and these are
radicles of powerful acids, among which is _phosphoric acid_ that
combines with basic oxides to form _phosphates_.
We have now, in the few last paragraphs, set before the reader the
minimum of chemical knowledge that will enable him to follow the
_rationale_ of such processes of the modern treatment of iron and its
ores as we can here give an outline of. Although there are numberless
minerals from which some iron can be extracted, the name of _iron ore_
is confined to such as contain a sufficient amount to make the
extraction commercially profitable, and this requires that the mineral
should be capable of yielding at least one-fifth of its weight. The ores
are very abundant in many parts of the world, and they consist mainly of
_oxides_ and their _hydrates_, or of _carbonate_, or of carbonate mixed
with clay and silicates, sometimes also with coaly matters in addition.
The carbonate iron ores are often mixed with oxides. Each class of ore
is liable to be contaminated with phosphates and with sulphur. The
richest ore is the _magnetic iron ore_, which is found in enormous
masses in Sweden, Russia, and North America. It is an oxide containing
72·41 per cent. of iron. _Red hæmatite_ and _specular ore_ are varieties
of another oxide with 70 per cent. of iron: the former is a very pure
ore when compact. It is found in Lancashire, Cumberland, and South
Wales, and much has been imported from Spain, while America has abundant
supplies near Lake Superior. Specular iron ore forms brilliant
steel-like crystals which show the red colour of hæmatite only when
scratched or powdered. Elba was famous for this ore, which occurs also
in Russia and Sweden, and large deposits are met with in both North and
South America. Brown hæmatite is a hydrate of the former, containing 60
per cent. of iron; it abounds in France and Spain, where some kinds are
associated with a noteworthy quantity of _phosphate_ of iron. _Spathic_
or _sparry iron_ ore is, when pure, a collection of nearly colourless
transparent crystals, consisting of carbonate of iron; it contains about
48 per cent. of iron, and also some of the metal _manganese_, which last
circumstance makes it, as we shall see, particularly suitable for
producing certain kinds of steel—indeed it is sometimes called _steel
ore_. Large beds of it occur in Styria and Carinthia. _Clay iron stone_,
or _clay-band_, has been extensively mined in Britain. It is found
abundantly in Staffordshire, Yorkshire, Derbyshire, and South Wales. It
consists of carbonate of iron intimately mixed with clay. The quantity
of iron in some samples falls as low as 17 per cent., but it rises with
variations to as much as 50 per cent. Much of its importance arises from
the fact of its occurring in beds alternating with layers of coal,
limestone, and clay, so that the same pit is sometimes able to supply
firebricks for building the furnace, fuel for the smelting, and
limestone for the _flux_,—a combination of advantages that for long
enabled iron to be produced in England cheaper than elsewhere. The like
is true of the _blackband ore_, which, in addition to the same
ferruginous composition as the last, contains also so much combustible
or bituminous material that it can be _calcined_ (roasted) without
additional fuel. The deposits of _blackband_ in Lanarkshire and
Ayrshire, which were discovered only in 1801, have given great
industrial importance to the district. Yet another British ore must be
noticed, namely, the _Cleveland ironstone_ of the North Riding of
Yorkshire. This is a carbonate of a grey or bluish colour caused by the
presence of a little iron silicate. It contains also a considerable
amount of phosphorus.
How simple is the operation of obtaining iron from the ore has already
been stated—that it is necessary only to surround lumps of ore by fuel
in a fire urged by a natural or artificial blast, and then to hammer the
mass extracted from the furnace so as to weld together the scattered
particles of the metal, and at the same time squeeze out the associated
slag and cinders, in order to obtain a coherent malleable piece, which
can be reheated in a smith’s fire, and forged into any required form. It
is no wonder therefore that iron was so produced by the ancient Britons;
at any rate Cæsar found them well provided with iron implements and
weapons. No doubt the Romans brought their more advanced skill to the
working of the metal; but in the matter of treating the original ore,
the methods they pursued on an extensive scale in Britain were of the
rude kind already described. Indeed in localities where the Romans were
known to have carried on their operations, the remains of their workings
are almost always found on high ground, so that it may be inferred that
they relied upon the winds to fan their fires, and their operations were
incomplete and wasteful. The most extensive of them appear to have been
in Sussex and Monmouthshire, in which last county there are places where
the ground is in large areas covered by their cinders and refuse, and in
this about 30 or 40 per cent. of iron occurs, so that for some centuries
this material was found capable of being profitably reworked as a source
of the metal. Iron continued to be produced in England during the middle
ages with charcoal for fuel, but its export was forbidden, and whatever
steel was required had to be imported from abroad. Afterwards German
artisans were brought over for making steel, and soon afterwards the
importation of shears, knives, locks, and other articles was prohibited.
The native production of iron continued, and this consumed the forests
so rapidly for the supply of charcoal, that various Acts were passed to
restrain the iron-makers, in order to preserve the timber. In spite of
these, the arts of smelting and working iron advanced apace: bellows
were used for the blast, and then the works were brought down into the
valleys, where water power could be employed to work them. The scarcity
of charcoal fuel caused many attempts to supply its place with pit coal,
but these met with small success, partly on account of the coal
containing so much sulphur, and partly from the difficulty of obtaining
with it a sufficiently high temperature, especially as the blowing
apparatus was as yet very imperfect. At length, in the first half of the
seventeenth century, the problem was solved by Dud Dudley, whose process
was kept secret, but is believed to have consisted in supplying coal at
the top of a higher furnace, in such a manner that the coal was
converted into _coke_ by the heat of the escaping gases before it
reached the reducing zone of the furnace. This innovation was violently
opposed by the charcoal smelters, who persecuted the inventor in every
way, until their resistance was successful. But before the middle of the
next century coke was regularly used in iron smelting, the process
having been made successful by Darby at Coalbrookdale, and then many new
applications of cast iron came into vogue. Coke being a substance
burning less freely than charcoal, bellows were found inadequate to give
the necessary blast, and were displaced by _blowing cylinders_, actuated
at first by water wheels, but this uncertain and comparatively feeble
source of power was soon superseded by the steam engine, the “fire
engine,” for which, as we have seen, Watt obtained his patent in 1769.
The furnaces were not then all engaged in producing the fusible metal
now called _cast_ or _pig iron_ as are the huge blast furnaces we see at
the present time. Indeed it was much to the disgust of the old iron
smelter that occasionally his product turned out to be of the fusible
kind, unworkable by the hammer, which therefore he regarded as
worthless. At what date _cast iron_ was first used is uncertain; but
probably it was not long before the fourteenth century. The furnaces in
use up to that time were small square walled-in structures only 3 or 4
feet high, and their effect would not greatly exceed that of a smith’s
forge: but as improved blowing apparatus gave more power, they soon
became enlarged into oval or round brick towers from 10 to 15 feet high,
and they, like the small furnaces, could be made to yield either smith
iron or steel by modifying the charge and the manner of applying the
blast; while furnaces of dimensions exceeding a certain limit could no
longer be trusted to turn out malleable metal, but they produced instead
the cruder substance we call white pig iron, and this requires much
subsequent treatment before it is converted into malleable or “merchant
iron.” Nevertheless the demand for cast iron as such, and more
particularly the adoption of improved methods of deriving malleable iron
from it, caused further increase in the size and numbers of blast
furnaces, until in the early part of our century 30 feet was not an
unusual height, the highest one in England in 1830 attaining 40 feet.
The total make of pig iron in England was in that year nearly 700,000
tons, perhaps about fifty times as much as it was a century before, and
thirty years later (1860) it had risen to nearly 4,000,000 tons. These
figures show the extraordinary expansion of the British iron manufacture
in the earlier part of the century; and the still more extensive
applications of iron during the next twenty years had the effect of
almost doubling the produce in 1880, and of increasing also three-fold
the amount of foreign metal imported, raising it to 2,500,000 tons. The
reader will now, it is hoped, be prepared to follow with some interest a
brief account of the principal inventions which have brought about
results of such importance.
[Illustration:
FIG. 17.—_Blast Furnace (Obsolete Type)._
]
[Illustration:
FIG. 18.—_Section and Plan of Blast Furnace (Obsolete Type)._
]
Deferring for the moment any description of the latest blast furnaces,
we invite his attention to Fig. 17, which represents the furnace used in
the first half of our century, but which now is of an obsolete type,
Fig. 18 being the section and plan of the same. The lower part of Fig.
17 shows where the molten metal has been allowed to run out of the
furnace into channels made in dry sand; first a main stream, then
branches to right and left, each of these with smaller offsets on each
side of it. These smaller channels are the moulds for the _pigs_, so
called because of the fancied resemblance of their position with regard
to the branch that supplied them, to the litter of a sow. They are
easily broken off from the larger mass, and then form pieces about 3 ft.
long with a ᗜ-shaped section, 4 in. wide, the weight being from 60 to 80
lbs. This is iron of the crudest kind, and though it is often referred
to as “cast iron,” it is, as a matter of fact, not used in this state
for any castings, except those of the very roughest and largest kind: a
certain amount of purification is requisite in most cases. This is given
by fusing the metal—along with some form of oxide and often other
matters—in a _cupola furnace_, which is like a small blast furnace,
being from 8 ft. to 20 ft. high and uses coke for fuel with a cold
blast.
So far from being simply iron, pig contains a large and variable
proportion of other matters amounting often to 10 or 12 per cent.; and
these confer upon it its fusibility. The principal one is carbon, which
is found in the metal partly in the state of chemical combination with
it, and partly in the form of small crystals similar to those of
graphite or plumbago, disseminated through the mass. When there is a
comparatively small proportion of the carbon combined with the iron, the
substance is grey, and it can be filed or drilled or turned in a lathe.
In white cast iron the combined carbon predominates, or is sometimes
accompanied by scarcely any graphitic carbon; it is brittle and so very
hard that a file makes no impression. It fuses at a lower temperature
than the other varieties. A third kind is the _mottled_ cast iron, which
shows a large coarse grain when broken, and distinct points of separate
graphite particles; it is tougher than the others, and therefore when
cannon were made of cast iron this variety was preferred. The following
table giving the percentage composition of four samples of crude cast
iron will show their diversities.
┌─────────────────────────────────┬────────┬────────┬────────┬────────┐
│ │ White.│ White.│Mottled.│ Grey.│
├─────────────────────────────────┼────────┼────────┼────────┼────────┤
│Iron │ 88·81│ 89·304│ 93·29│ 90·376│
│Combined carbon │ 4·94│ 2·457│ 2·78│ 1·021│
│Graphite, or uncombined carbon │ ...│ 0·871│ 1·99│ 2·641│
│Silicon │ 0·75│ 1·124│ 0·71│ 3·061│
│Sulphur │ trace│ 2·516│ trace│ 1·139│
│Phosphorus │ 0·12│ 0·913│ 1·23│ 0·928│
│Manganese │ 5·38│ 2·815│ trace│ 0·834│
└─────────────────────────────────┴────────┴────────┴────────┴────────┘
The reader will observe that the last item in the table above is a
substance that he has not yet made the acquaintance of, namely,
_manganese_. This is a metal which in many of its chemical relations
much resembles iron, and ferruginous ores usually contain a greater or
less proportion of it. Manganese is of great importance in the
manufacture of steel, as we shall presently see; but as a separate metal
it has no application, and is obtainable in the metallic state with much
difficulty. One of its oxides has however very extensive applications in
the chemical arts, and others form acid radicles, which in combination
with potash or soda give rise to useful products. The well-known
“Condy’s fluid” is a solution of one of these.
We have seen how malleable iron or steely iron may be directly obtained
from the ores, but it has been found that on the large scale it is
necessary and more economical to operate on the pig iron produced by the
blast furnaces in such a manner as to remove the greater part of the
foreign substances.
[Illustration:
FIG. 19.—_Section of a Reverberatory Furnace._
]