[Footnote C: Costs from £10 to £25 a ton. In his description of the "Preparation of Cotton-waste for the Manufacture of Smokeless Powder," A. Hertzog states that the German military authorities require a cotton which when thrown into water sinks in two minutes; when nitrated, does not disintegrate; when treated with ether, yields only 0.9 per cent. of fat; and containing only traces of chlorine, lime, magnesia, iron, sulphuric acid, and phosphoric acid. If the cotton is very greasy, it must be first boiled with soda-lye under pressure, washed, bleached with chlorine, washed, treated with sulphuric acid or HCl, again washed, centrifugated, and dried; if very greasy indeed a preliminary treatment with lime-water is desirable. See also "Inspection of Cotton-Waste for Use in the Manufacture of Gun-cotton," by C.E. Munro, Jour. Am. Chem. Soc., 1895, 17, 783.]
~Drying the Cotton.~—This operation is performed in either of two ways. The cotton may either be placed upon shelves in a drying house, through which a current of hot air circulates, or dried in steam-jacketed cylinders. It is very essential that the cotton should be as dry as possible before dipping in the acids, especially if a wholly "insoluble" nitro-cellulose is to be obtained. After drying it should not contain more than 0.5 per cent. of moisture, and less than this if possible. The more general method of drying the cotton is in steam-jacketed tubes, i.e., double cylinders of iron, some 5 feet long and 1-1/2 foot wide. The cotton is placed in the central chamber (Fig. 10), while steam is made to circulate in the surrounding jacket, and keeps the whole cylinder at a high temperature (steam pipes may be coiled round the outside of an iron tube, and will answer equally well). By means of a pipe which communicates with a compressed air reservoir, a current of air enters at the bottom, and finds its way up through the cotton, and helps to remove the moisture that it contains. The raw cotton generally contains about 10 per cent. of moisture and should be dried until it contains only 1/2 per cent. or less. For this it will generally have to remain in the drying cylinder for about five hours. At the end of that time a sample should be taken from the top of the cylinder, and dried in the water oven (100° C.[A]) for an hour to an hour and a half, and re-weighed, and the moisture then remaining in it calculated.
[Footnote A: It is dried at 180° C. at Waltham Abbey, in a specially constructed drying chamber.]
[Illustration: FIG. 10.—COTTON DRIER.]
It is very convenient to have a large copper water oven, containing a lot of small separate compartments, large enough to hold about a handful of the cotton, and each compartment numbered, and corresponding to one of the drying cylinders. The whole apparatus should be fixed against the wall of the laboratory, and may be heated by bringing a small steam pipe from the boiler-house. It is useful to have a series of copper trays, about 3 inches by 6 inches, numbered to correspond to the divisions in the steam oven, and exactly fitting them. These trays can then be taken by a boy to the drying cylinders, and a handful of the cotton from each placed in them, and afterwards brought to the laboratory and weighed (a boy can do this very well), placed in their respective divisions of the oven, and left for one to one and a half hours, and re-weighed.
When the cotton is found to be dry the bottom of the drying cylinder is removed, and the cotton pushed out from the top by means of a piece of flat wood fixed on a broom-handle. It is then packed away in galvanised- iron air-tight cases, and is ready for the next operation. At some works the cotton is dried upon shelves in a drying house through which hot air circulates, the shelves being of canvas or of brass wire netting. The hot air must pass under the shelves and through the cotton, or the process will be a very slow one.
~Dipping and Steeping.~—The dry cotton has now to be nitrated. This is done by dipping it into a mixture of nitric and sulphuric acids. The acids used must be strong, that is, the nitric acid must be at least of a gravity of 1.53 to 1.52, and should contain as little nitric oxide as possible. The sulphuric acid must have a specific gravity of 1.84 at 15° C., and contain about 97 per cent. of the mono-hydrate (H_{2}SO_{4}). In fact, the strongest acids obtainable should be used when the product required is gun-cotton, i.e., the highest nitrate.
The sulphuric acid takes no part in the chemical reaction involved, but is necessary in order to combine with the water that is liberated in the reaction, and thus to maintain the strength of the nitric acid. The reaction which takes place is the following:—
2(C_{6}H_{10}O_{5}) + 6HNO_{3} = C_{12}H_{14}(NO_{3}){6} + 6 H{2}O.
324 378 = 594 108.
Cellulose. Gun-Cotton.
Theoretically,[A] therefore, 1 part of cellulose should form 1.8 part of gun-cotton. Practically, however, this is never obtained, and 1.6 lb. from 1 lb. of cellulose is very good working. The mixture of acids used is generally 1 to 3, or 25 per cent. nitric acid to 75 per cent. sulphuric acid.