WHAT SHELLS ARE MADE OF
The body of a shell is made of steel of a fairly strong variety. That is
to say, it is stronger than that used for shipbuilding and for bridges
and such work: but it is less so than some of the higher grades of
steel, such as that used for making wire ropes. Owing to so much of this
steel being rolled during the war, "shell quality" has come to be as
well known to the general engineer as any of the many varieties which he
has been accustomed to since his apprentice days. Many people wondered,
at one time, why the cheaper and more easily worked cast iron could not
be used for shells. There was a period when the steel works were quite
unable to cope with the demands for steel, yet the iron foundries were
crying out for work. This question then arose in many minds, Why not
make cast iron shells? The answer is that cast iron is too weak: it
would blow into fragments too soon.
Just think what a shell is and what it has to do. It is a metal case
filled with explosive. It is thrown from a gun and is intended to blow
itself to pieces on arrival at its destination. It is that
self-destruction which carries destruction to all around as well. It is
necessary, in order to obtain the best result, that an appreciable time
should elapse between the ignition of the explosive and the bursting of
the case. The force of the most sudden explosion is not really developed
at once, but takes an appreciable time. After ignition, therefore, as
the explosion gradually becomes complete, the pressure inside the shell
is growing, and too weak a shell would go to pieces before the maximum
pressure had been attained. Thus much of the energy of the explosion
would simply be liberated into the air instead of being employed in
hurling the fragments of shell with enormous force.
That is, of course, not a complete explanation of the whole action of a
high-explosive shell, but it indicates generally the reason why a
special quality of steel is required in order to get the best results.
Steel having been dealt with in another chapter, we will pass to the
other metals which play important if not essential parts in the
production of modern projectiles. So important are several of these that
the lack of one or two of them would, under modern conditions, mean
certain defeat for a nation.
Let us first of all take copper, of which is made the driving bands of
the shells and which in combination with zinc forms brass of which noses
and other important parts are made.
Its ore is found in many parts of the world, notably in the United
States, Chile and Spain. The ores are of several kinds, the simpler ones
to deal with being oxides and carbonates of copper, meaning compounds
of copper with oxygen and with oxygen and carbon respectively.
It will be remembered that ores of iron are usually of the same nature,
namely, oxides and carbonates, and consequently we find that the method
of obtaining copper from these ores resembles the methods employed to
obtain iron from its ores.
The ore is thrown into a large furnace, like the blast furnaces of the
ironworks, and in the heat of the fire the bonds between copper and
oxygen are loosened and the superior attractions of the carbon in the
fuel entice the oxygen away, leaving the metal comparatively pure.
Unfortunately, however, copper is found most plentifully in combination
with sulphur with which it forms what is termed sulphide. This copper
sulphide is called by miners "copper pyrites." Another trouble is that
mixed with the copper pyrites there is usually more or less of iron
pyrites, or sulphate of iron, so that to obtain the copper not only has
the sulphur to be got rid of but also the iron. This complicates the
operations very much, the ore having to be subjected to repeated
roastings and meltings during which the sulphur passes off in the form
of sulphur dioxide (a material from which sulphuric acid can be
obtained), leaving oxygen in its place. Thus the copper sulphide becomes
copper oxide, after which the oxygen is carried away by carbon, leaving
the relatively pure metal. Moreover, at each operation various
substances are thrown into the furnace called fluxes, which do not
mingle with the metal but float on the top in the form of slag, and
into the slag the iron passes, so that finally the copper is obtained
alone.
Zinc is another important material for shell-making. Its ores used to be
found in great plenty in Silesia, but the chief source of supply is now
Australia. It is what is called "zinc blende," and consists of zinc
sulphide, or zinc and sulphur in combination. In all these names, it may
be interesting to mention, at this point, the termination "ide"
indicates a compound of two substances, so that we can safely conclude
that the "ides" consist of the two elements named in their titles and no
others. Thus zinc sulphide is zinc and sulphur and nothing else, iron
sulphide is iron and sulphur, copper oxide is copper and oxygen, and so
on.
The blende is first roasted in huge furnaces specially built for the
purpose. To ensure its being thoroughly treated it has to be "rabbled"
or turned over and over, since otherwise all of it might not be brought
into contact with the necessary oxygen. At one time done by men with
rakes, it is now generally accomplished by mechanical means.
A description of one such furnace will be of interest. It consists of a
long rectangular building of brickwork bound together with steel
framework. Inside it is divided up into low chambers, the roof of each
forming the floor of the one above.
At intervals along its length mighty shafts of iron pass up from
underneath right through all the floors, emerging finally above the
topmost, while along underneath the furnace there runs a shaft the
action of which turns the vertical shafts slowly round and round.
Attached to the vertical shafts are long strong arms of iron, one arm to
each floor, and upon the arms are placed rabbles, as they are termed,
pieces of iron shod sometimes with fireclay, resembling most of any
familiar objects a small ploughshare.
As the arms slowly revolve, at the rate of once or twice per minute, the
arms are carried round and round and the rabbles plough up and turn over
and over the layer of ore lying upon the floor.
There are arms on the top of the furnace, too, sometimes, where the ore
is first laid so that it may be dried by the heat escaping from the
furnace beneath, an interesting example of economy effected by utilizing
heat which would otherwise be wasted.
The whole of the furnace, from end to end and on every floor, is thus
swept continually by the rotating arms with their dependent rabbles, and
the latter are cunningly shaped so that they not only turn the ore over
and over, but gradually pass it along the different floors or hearths.
It is fed automatically by a mechanical feeder which pushes it on, a
small quantity at a time, to the drying hearth on the top. Then the
rabbles take charge of it and gradually pass it from the area swept by
one shaft to that of the next until it has passed right along the top
and has become thoroughly dried. Arrived there it falls through a hole
on to the topmost hearth or floor, along which it travels by the same
means but in the contrary direction until it again falls through a hole
on to the top floor but one. And so it goes on until at last, fully
roasted, it falls from the bottom floor of the furnace into trucks or
other provision for carrying it away.
Some kinds of ore require to be heated by means of gas which is
generated in a "gas-producer" near by. In others, however, the sulphur
in the ore acts as the fuel, and so the furnace, having been once
started, can be kept up for long periods without the expenditure of any
coal at all. Very little attention is needed by furnaces such as these,
so that with no fuel to pay for and very little labour, they are
extremely economical.
Owing to the great heat, too, the arms would stand a very good chance of
getting melted were they not kept cool by a continual stream of water
flowing through the shafts and arms. This furnishes a continual supply
of hot water which is sometimes used for other purposes in the works.
The process of roasting, whether carried on in furnaces such as these or
not, results in the formation of oxide instead of sulphide; in other
words, the sulphur is turned out and oxygen takes its place. The
dislodged sulphur then joins up with some more oxygen and forms sulphur
dioxide, which can be led away to the sulphuric acid plant and there, by
union with water, turned into that extremely valuable substance,
sulphuric acid.
We cannot, however, treat zinc oxide as we would iron oxide or copper
oxide, for zinc is volatile, and so, instead of accumulating in the
bottom of a blast furnace as the iron and copper do, would pass off up
the chimney.
The oxide is therefore mixed with coal or some other form of carbon and
placed in retorts made of fireclay. These retorts are fixed in rows one
above the other like the retorts at a gasworks, and hot gases from a
gas-producer down below pass around and among them. To the mouth of each
retort is fitted a condenser, also made of fireclay.
Now what happens in the retorts is this: the heat loosens the bonds
between the zinc and the oxide, the latter passing into union with some
carbon from the coal. The zinc at the same time becomes vapour and
passes into the condenser, the lower temperature of which turns it into
a liquid which the workmen remove at intervals in ladles. On being
poured into moulds and allowed to solidify this metal is called by the
name of "spelter," which bears to zinc the same relation that pig-iron
does to the more highly developed forms of iron. Spelter is simply zinc
in its crudest form.
Tin, although less important in war than copper and zinc, plays a not
unimportant part. It has been found for centuries in Cornwall. The
Romans used to trade with the natives of Britain for tin. Although
considerable quantities of it is still obtained from there, the greatest
tin-producing country of all at present is the Federated Malay States.
Australia also furnishes ore, as does Bolivia and Nigeria.
In Cornwall the ore occurs as rock in veins or lodes filling up what
must once have been fissures in granite rocks. That near the surface has
long been taken, so that to-day the mines are very deep and costly to
work. Some can only afford to operate when the market price of tin is
above a certain limit. Much of the ore from the newer districts--the
Malay States, for example--is in small fragments mixed with gravel in
beds near the surface. Such is called alluvial or stream tin, since the
deposits were undoubtedly put in their present position by streams or
rivers. So long as they last these easily accessible alluvial deposits
will always be cheaper to work than the deep mines. On the other hand,
they may give out, and recent explorations underground seem to indicate
that there is still much valuable ore not only of tin but of other
metals too, to be obtained from the old mines of Cornwall.
The ore of tin, like so many other ores, is generally oxide. It is first
roasted to expel sulphur and arsenic which are often present as
impurities, and then it is melted in a reverberatory furnace such as
that described for the manufacture of wrought iron. As usual, the oxygen
combines with carbon, the impurities form slag which floats on the top,
and the pure metal falls to the bottom of the furnace from whence it can
be drawn off.
Mixed with or in the neighbourhood of tin ore there is sometimes found
another mineral called wolfram, which plays an extremely important part
in modern warfare, so much so that the British and other Governments
engaged in the war were at times hard put to it to find enough. Its
value resides in the fact that it contains tungsten, an element which
has wonderful powers in hardening steel.
It consists of tungsten and oxygen, but is not an oxide since there is
also iron in the partnership. This fact is very useful, however, since
it enables the particles of wolfram to be picked out from the mass of
other stuff among which they are found by a magnet.
There are some very wonderful machines called magnetic separators, made
for this express purpose. In one, with which I am familiar, there is an
endless band stretched horizontally upon two rollers. One of the rollers
being driven round the belt travels along so that the mineral being fed
on to it in a stream is carried along under several magnets. These
magnets are very different from the ordinary magnet, inasmuch as they
are revolving. We might almost describe them as small magnetized
flywheels. As they spin round they pick up slightly the particles of ore
which contain iron, but have no effect at all upon those which do not
contain iron. They do not actually lift the particles up on to
themselves: they just exercise a slight pull upon them, and by virtue of
the fact that they are revolving, pull them off the band and throw them
to one side. The wheels can be set closer or farther from the belt at
will so as to make them act more or less strongly, and thus the most
magnetic particles can be separated from those less magnetic, these
latter being still kept separate from the wholly non-magnetic particles.
Thus by simple and purely mechanical means are the precious bits of
wolfram obtained from the other less valuable or worthless minerals with
which they are mixed.
The same method is used with other minerals besides wolfram: it can be
applied to all those which exhibit in some small degree the magnetic
properties which we usually associate with iron.
This sorting out of one mineral from others continually crops up in
connection with nearly all the metals except iron. Iron is practically
the only one whose ore occurs in vast masses which need simply to be dug
up and thrown into the furnace. The others, where they occur as rock in
veins, have to be crushed to detach what is wanted from what is not
wanted, and then the two have to be sorted in some way. Magnetic
separation is but one of these ways. Another takes advantage of the fact
that we seldom find two things together which have precisely the same
specific gravity. Consequently, if we throw the mixture on to a shaking
table the heavier particles will behave differently from the lighter
ones and the two will separate. The same result can be obtained by
throwing the mixture into a stream of water, the water acting
differently upon the lighter and upon the heavier particles. Another way
which may be mentioned is founded upon the fact that some things can be
readily wetted with oil while others throw the oil off and refuse to be
wetted by it. If a mixture of these two sorts of thing be stirred
violently in a suitable oily liquid the former will be found eventually
in the froth, while the latter will sink to the bottom. All these
different methods are employed, as they are found necessary in preparing
the ores of the various metals to which we have been referring.
Except in the case of alluvial ores which have been broken already by
the action of ancient streams of water, nearly all ores (except iron)
have to be crushed before the ores can be separated out. Some of this
work is done by the very simplest contrivances, showing how in some
cases invention has almost come to a stop through the machines having
been reduced to their simplest form. A notable instance of this is the
stamp mill, in which heavy timbers are lifted up by machinery and then
allowed to slide down upon the ore, just like gigantic pestles. More
elaborate grinding machines are sometimes used, however, but it is
impossible to mention them all here.
The action of sorting out the fragments of ore from the miscellaneous
assortment of crushed rocks is termed "concentrating," and the sorted
ores are called "concentrates."
Another metal which has proved itself of immense importance in war is
aluminium, and it fittingly comes at the close of the list since it is
dealt with in a manner peculiar to itself. Practically all the others
are obtained from their ores by means of heat and heat alone. Aluminium
is obtained by electricity acting in the process called electrolysis.
It is surprising to learn that aluminium is one of the very commonest
things on the face of the earth. Clay and many common rocks are very
largely made of it. Clay, to be precise, is a silicate of alumina, a
term which is interesting when it is explained. Silica is the name given
to oxide of silicon. Sand is mostly silica. Alumina, too, is oxide of
aluminium. Silicate of alumina is a combination of the two.
Any clay, therefore, could be used as an ore from which to obtain
aluminium, but of course there are certain minerals specially suitable
for the purpose, since in them the metal is plentiful and easily
extracted.
In another chapter reference is made to the production of caustic soda
from a solution of common salt by electrolysis. The same principle,
precisely, is used to obtain the metal aluminium from its ore, which is
generally an oxide.
Common salt, let me remind you, is sodium and chlorine combined. When
you dissolve it in water it becomes ionized, which means that each
molecule of salt splits up into two ions one of which is electrically
positive and the other electrically negative. Then, when we introduce
two electrodes into the solution and connect them to a battery or
dynamo, all the positive ions go to one electrode and all the negative
ions to the other.
We cannot dissolve aluminium ore in water, but we can in a bath of
molten cryolite, and for some reason or other, whether because of the
heat or not we cannot say, the ore becomes ionized, the aluminium atoms
being one sort and the oxygen atoms the other sort. These ions then sort
themselves out, the oxygen ions being taken into combination with the
carbon rod which forms the positive electrode, while the metal ions
collect upon the negative electrode. Since this latter is a slab of
carbon which forms the bottom of the vessel in which the process is
carried on, the result is that pure aluminium gradually accumulates in
the bottom of the vessel and can be drawn off from time to time.
Aluminium is always produced in places where electric power can be
obtained cheaply, such as near waterfalls.