[Illustration: FIG. 17.—OLD METHOD. 100 PIECES.]
[Illustration: FIG. 18.—NEW METHOD. ONE SOLID BLOCK.]
At the New Explosives Company's Stowmarket Works large solid blocks of gun-cotton are pressed up under a new process, whereby blocks of gun- cotton, for use in submarine mines or in torpedo warheads, are produced. Large charges of compressed gun-cotton have hitherto been built up from a number of suitably shaped charges of small dimensions (Fig. 17), as it has been impossible to compress large charges in a proper manner. The formation of large-sized blocks of gun-cotton was the invention of Mr A. Hollings. Prior to the introduction of this method, 8 or 9 lbs. had been the limit of weight for a block. This process has been perfected at the Stowmarket factory, where blocks varying from the armour-piercing shell charge of a few ounces up to blocks of compressed gun-cotton mechanically true, weighing 4 to 5 cwts. for torpedoes or submarine mines, are now produced. At the same time the new process ensures a uniform density throughout the block, and permits of any required density, from 1.4 downwards, being attained; it is also possible exactly to regulate the percentage of moisture, and to ensure its uniform distribution. The maximum percentage of moisture depends, of course, upon the density. By the methods of compression gun-cotton blocks hitherto employed, blocks of a greater thickness than 2 inches, or of a greater weight than 9 lbs., could not be made, but with the new process blocks of any shape, size, thickness, or weight that is likely to be required can be made readily and safely. The advantages which are claimed for the process may be enumerated as follows:—(1.) There is no space wasted, as in the case with built-up charges, through slightly imperfect contact between the individual blocks, and thus either a heavier charge—i.e., about 15 per cent. more gun- cotton—can be got into the same space, or less space will be occupied by a charge of a given weight. (2.) The metallic cases for solid charges may be much lighter than for those built-up, since with the former their function is merely to prevent the loss of moisture from wet gun-cotton, or to prevent the absorption of moisture by dry gun-cotton. They can thus be made lighter, as the solid charge inside will prevent deformation during transport. With built-up charges the case must be strong enough to prevent damage, either to itself or to the charge it contains. For many uses a metal case, however light, may be discarded, and one of a thin waterproof material substituted. (3.) The uniform density of charges made by this process is very favourable to the complete and effective detonation of the entire mass, and to the presence of the uniform amount of moisture in every part of the charge. (4.) Any required density, from the maximum downwards, may be obtained with ease, and any required amount of moisture left in the charge. These points are of great importance in cases where, like torpedo charges, it is essential to have the centre of gravity of the charge in a predetermined position both vertically and longitudinally, and the charge so fixed in its containing case that the centre of gravity cannot shift. The difficulty of ensuring this with a large torpedo charge built up from a number of discs and segments is well known. Even with plain cylindrical or prismatic charges a marked saving in the process of production is effected by this new system. The charges being in one block they are more easily handled for the usual periodical examination, and they do not break or chafe at the edges, as in the case of discs and cubes in built-up charges. A general view of the press is given in Fig. 19. The gun-cotton in a container is placed on a cradle fixed at an angle to the press. The mould is swivelled round, and the charge pushed into it with a rammer, and it is then swivelled back into position. The mould is made up of a number of wedge pieces which close circumferentially on the enclosed mass, which is also subjected to end pressure. Holes are provided for the escape of water.
[Illustration: FIG. 19.—A 4-CWT. BLOCK OF GUN-COTTON BEING TAKEN FROM
HYDRAULIC PRESS.]
~The Waltham Abbey Process.~—At the Royal Gunpowder Factory, Waltham Abbey, the manufacture of gun-cotton has been carried out for many years. The process used differs but little from that used at Stowmarket. The cotton used is of a good quality, it is sorted and picked over to remove foreign matters, &c., and is then cut up by a kind of guillotine into 2-inch lengths. It is then dried in the following manner. The cotton is placed upon an endless band, which conducts it to the stove, or drying closet, a chamber heated by means of hot air and steam traps to about 180° F.; it falls upon a second endless band, placed below the first; it travels back again the whole length of the stove, and so on until delivered into a receptacle at the bottom of the farther end, where it is kept dry until required for use. The speed at which the cotton travels is 6 feet per minute, and as the length of the band travelled amounts to 126 feet, the operation of drying takes twenty-one minutes. One and a quarter lb. are weighed out and placed in a tin box; a truck, fitted to receive a number of these boxes, carries it along a tramway to a cool room, where it is allowed to cool.
~Dipping.~—Mixed acids are used in the proportion of 1 to 3, specific gravity nitric acid 1.52, and sulphuric acid 1.84. The dipping tank is made of cast iron, and holds 220 lbs. of mixed acids, and is surrounded on three sides by a water space in order to keep it cool. The mixed acids are stored in iron tanks behind the dipping tanks, and are allowed to cool before use. During the nitration, the temperature of the mixed acids is kept at 70° F., and the cotton is dipped in quantities of 1-1/2 lb. at a time. It is put into a tin shoot at the back of the dipping tank, and raked into the acids by means of a rabble. It remains in the acids for five or six minutes, and is then removed to a grating at the back, pressed and removed. After each charge of cotton is removed from the tank, about 14 lbs. of fresh mixed acids are added, to replace amount removed by charge. The charge now weighs, with the acids retained by it, 15 lbs.; it is now placed in the pots, and left to steep for at least twenty-four hours, the temperature being kept as low as possible, to prevent the formation of soluble cotton, and also prevent firing. The proportion of soluble formed is likely to be higher in hot weather than cold. The pots must be covered to prevent the absorption of moisture from the air, or the accidental entrance of water, which would cause decomposition, and consequent fuming off, through the heat generated by the action of the water upon the strong acids.
The excess of acids is now extracted by means of hydro-extractors, as at Stowmarket. They are worked at 1,200 revolutions per minute, and whirled for five minutes (10-1/2 lbs. of waste acids are removed from each charge dipped). The charge is then washed in a very similar manner to that previously described, and again wrung out in a centrifugal extractor (1,200 revolutions per minute). The gun-cotton is now boiled by means of steam in wooden tanks for eight hours; it is then again wrung out in the extractors for three minutes, boiled for eight hours more, and again wrung out; it is then sent to the beater and afterwards to the poacher. The poachers hold 1,500 gals. each, or 18 cwt. of cotton. The cotton remains six hours in the poachers. Before moulding, 500 gals. of water are run into the poacher, and 500 gals. of lime water containing 9 lbs. of whiting and 9 gals. of a caustic soda solution. This mixture is of such a strength that it is calculated to leave in the finished gun-cotton from 1 to 2 per cent. of alkaline matter.
By means of vacuum pressure, the pulp is now drawn off and up into the stuff chest—a large cylindrical iron tank, sufficiently elevated on iron standards to allow room for the small gauge tanks and moulding apparatus below. It holds the contents of one poacher (18 cwt.), and is provided with revolving arms to keep the pulp stirred up, so that it may be uniformly suspended in water.
Recently a new process, invented by J.M. and W.T. Thomson (Eng. Pat. No. 8,278, 1903), has been introduced at the Waltham Abbey Factory. The object of this invention is the removal of the acids of nitration from the nitrated material after the action has been completed, and without the aid of moving machinery, such as presses, rollers, centrifugals, and the like. The invention consists in the manufacture of nitrated celluloses by removing the acids from the nitrated cellulose directly by displacement without the employment of either pressure or vacuum or mechanical appliances of any kind, and at the same time securing the minimum dilution of the acids. It was found that if water was carefully run on to the surface of the acids in which the nitro-cellulose is immersed, and the acids be slowly drawn off at the bottom of the vessel, the water displaces the acid from the interstices of the nitro-cellulose without any undesirable rise in temperature, and with very little dilution of the acids. By this process almost the whole of the acid is recovered in a condition suitable for concentration, and the amount of water required for preliminary washing is very greatly reduced. The apparatus which is used for the purpose consists of a cylindrical or rectangular vessel constructed with a perforated false bottom and a cock at its lowest point for running off the liquid. Means are also provided to enable the displacing water to be run quietly on to the surface of the nitrating acids.[A]
[Footnote A: In a further patent (Eng. Pat. 7,269, 1903, F.L. Natham), J.M. Thomson and W.T. Thomson propose by use of alcohol to replace the water, used in washing nitro-cellulose, and afterward to remove the alcohol by pressing and centrifuging.]