Part 10 (1/2)

CHAPTER XI

WHAT Sh.e.l.lS ARE MADE OF

The body of a sh.e.l.l is made of steel of a fairly strong variety. That is to say, it is stronger than that used for s.h.i.+pbuilding 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, ”sh.e.l.l 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 sh.e.l.ls. 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 sh.e.l.ls? The answer is that cast iron is too weak: it would blow into fragments too soon.

Just think what a sh.e.l.l 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 sh.e.l.l is growing, and too weak a sh.e.l.l 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 sh.e.l.l with enormous force.

That is, of course, not a complete explanation of the whole action of a high-explosive sh.e.l.l, 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 pa.s.s 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 sh.e.l.ls and which in combination with zinc forms bra.s.s 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 pa.s.ses 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 pa.s.ses, so that finally the copper is obtained alone.

Zinc is another important material for sh.e.l.l-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 t.i.tles 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 pa.s.s 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 pa.s.s it along the different floors or hearths.

It is fed automatically by a mechanical feeder which pushes it on, a small quant.i.ty at a time, to the drying hearth on the top. Then the rabbles take charge of it and gradually pa.s.s it from the area swept by one shaft to that of the next until it has pa.s.sed 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.