1902 Encyclopedia > Gun Cotton

Gun Cotton

GUN-COTTON (Pyroxilin, Coton poudre, Fulmi colon, Schiessbaumwolle). In 1838 Pelouze observed that when cotton fabrics or paper were immersed in cold concentrated nitric acid for a short time, the free acid being subsequently removed by washing, these materials became, without im-portant alteration of structure, converted into substances possessed of explosive properties. These were at the time accepted as closely allied to the substance named xyloidin, described some years previously by Braconnet, which is obtained by adding water to a solution of starch in cold nitric acid. But subsequent observation established the identity of these explosive products with the explosive cotton, or gun-cotton, of which in 1845 Schonbein announced the discovery, and which he at once proposed as a substitute for gunpowder. Soon after this announcement Bottger and Otto published the method of producing gun-cotton by immersing carded cotton in cold concentrated nitric acid, and subsequently Knop introduced the more advantageous method of treating the cotton wool with a mixture of nitric and sulphuric acids, the latter being used as a dehydrater of the nitric acid, and as an absorbent of the water eliminated by the nitrification of the cellulose or cotton fibre. The composition of gun-cotton was subse-quently made the subject of study by Bottger, Pelouze, Peligot, Von Kirchhoff, Sobrero, B^champ, Porret, Crum, Gladstone, Hadow, and others, and various formulae were proposed as representing its composition. The divergence of opinion on this point arose partly from difficulties attend-ing the preparation of uniform products, and the obtaining of trustworthy analytical results with these, and partly from differences of opinion regarding the nature of the chemical reaction, whereby the cellulose becomes converted into an explosive body. The products obtained in the earlier investigations differed very much as regards their solubility in mixtures of alcohol and ether, and also with respect to the proportion which their weight bore to that of the cotton wool employed in the experimental operations. Crum was the first to entertain the view that gun-cotton might be re-garded as cellulose, in which the two or three atoms of hydrogen are replaced by their equivalent of nitric peroxide. This view was afterwards also advanced by Gerhardt, and it received strong support from the researches of Hadow, whose results established the fact that several distinct varieties of pyroxilin could be produced by varying the pro-portions of nitric and sulphuric acids used, and who definitely established the composition of three of these, the most explosive of which constituted the chief proportion of | the product ordinarily obtained as gun-cotton, and had the composition expressed by the name trinitrocellulose. This highest nitro-product in its pure state is insoluble in mix-tures of ether and alcohol, whereas the lower products (one of which is the so-called collodion gun-cotton, used for photographic purposes, see COLLODION, vol.vi. p. 149) differed in regard to their ready solubility in different mixtures of those solvents. Crum's formula for pyroxilin, thus con-firmed by Hadow, was afterwards strongly supported by Schrotter, Bedtenbacher, and Schneider, in their joint investigation of gun-cotton manufactured in Austria by the improved process of Von Lenk, and though again disputed by Pelouze and Maury, and by Champion and Pellet, the correctness of the formula C6H5.N3Ou (or C6HV053N2), originally proposed by Crum, was conclusively established in 1866 by the exhaustive analytical and synthetical experi-ments of Abel. In the manufacture of gun-cotton, even when most carefully conducted, the most explosive product, trinitrocellulose, is never obtained in a condition even approaching purity ; it always contains an admixture (rang-ing from 4 to 10 per cent, in the products of highest quality) of the lower nitrocellulose products, i.e., the soluble varieties of gun-cotton. In addition to these im-purities it contains, even when the cotton employed has been submitted to purification by treatment with alkali, small proportions of nitrogenized matters, soluble in alcohol, formed from resinous or fatty substances retained within the cotton fibre. These substances are very much more prone to undergo decomposition (with development of nitrogen acids) by exposure to heat or light than the cellulose derivatives themselves; and Abel's experiments demonstrated that the uncertain stability of gun-cotton, which brought this material into bad repute not long after its discovery, from the occurrence of disastrous explosions arising apparently from its spontaneous decomposition, was ascribable primarily to the development of free acid in the gun-cotton by the action of comparatively moderate heat or of light upon these impurities.

The occurrence of a violent explosion at the works of Messrs Hall of Faversham, not long after they had com-menced the manufacture of Schonbein's gun-cotton wool, followed by a similar casualty in France, led to the abandon-ment of endeavours to apply this substance, within a brief period of its discovery, except in Austria, where Von Lenk persevered in attempts to devise means for obtaining ic in a purer and therefore more stable condition, as well as for bringing its explosive action sufficiently under control to permit of its advantageous employment as a substitute for gunpowder, not only for destructive but also for projectile purposes. The system of manufacture elaborated by Von Lenk consisted in loosely spinning long staple cotton into yarn of various sizes and different compactness; this yarn was converted into gun-cotton by very careful treatment with a large excess of the strongest nitric and sulphuric acids, the product being immersed for many weeks in run-ning water, and then treated with weak alkali; the gun-cotton yarn and thread were either wound more or less compactly on reels or cores, for employment in firearms, or made up into very compact ropes with hollow cores, or into plaits, of lamp-wick form, for employment in shells or mines. The rapidity of explosion of the gun-cotton, in open air, or under slight confinement, was thus brought to a great extent under control, but if the resistance opposed to the expansion of the highly heated gases upon the first ignition of the confined gun-cotton developed sufficient pressure to cause them at once to penetrate the inner struc-ture of gun-cotton fibre which composed a charge, a sudden and violent explosion was thus brought about. Hence no practical advance was made in the reduction of the violence of action of gun-cotton by Von Lenk's researches. By the system which Abel has more recently elaborated the fibre after its conversion into gun-cotton is reduced to a very fine state of division; when in this condition the explosive substance is readily converted into sheets or granules, or by compression into homogeneous masses of various degrees of compactness, and of any desired form. In this manner the rapidity of action or explosion of gun-cotton may be reduced to a minimum, though uniformity of action in fire-arms is still very difficult to attain with it. As the reduc-tion of the gun-cotton fibre to a very fine state of division greatly facilitates the removal by washing, and by alkaline treatment, of the small quantities of unstable impurities already spoken of, the stability of gun-cotton as now manu-factured is much greater than that of former products. Compressed gun-cotton needs, like the other forms in which this explosive has been used, very strong confinement for the development of violent explosion, but this can be readily accomplished without any confinement of the substance, through the agency of an initiative detonation; the explosion of 2 grains of strongly confined mercuric fulminate in close contact with the compressed material suffices to ensure this result (see Detonation, article EXPLOSIVES, vol. viii. p. 809). Gun-cotton contains in the normal (air dry) condition 2 per cent, of water. The compressed material of the ordinary density ( = about 1) contains from 25 to 30 per cent, of water when saturated; even with 15 per cent, it is uninflammable, and when containing 17-20 per cent, it may be cut and drilled with perfect safety. If gun-cotton contains more than the normal 2 per cent, of water it can no longer be detonated by the minimum quantity of fulminate, and the strength of the initiative detonation has to be increased in proportion to the amount of water it contains; when it contains 17 per cent, of water, its detonation cannot be accomplished with less than 200 grains of confined ful-minate. An initiative charge of 1 ounce of air dry gun-cotton (detonated by means of an ordinary fulminate fuze) suffices, however, to ensure the detonation of wet gun-cotton '{as used in mines, torpedoes, rockets, &c). The susceptibility of gun-cotton to detonation when wet (and there-fore perfectly uninflammable) gives this substance a great advantage over other explosive agents, as it may be stored in a perfectly harmless condition (wet gun-cotton being quite unalterable) and at once used in that state as a powerful destructive agent through the agency of a deton-ating charge. The explosive action of wet gun-cotton is somewhat sharper than that of the dry material, the deton-ation being transmitted through its mass with greater rapidity in consequence of the displacement of air in its pores by the incompressible liquid. Gun-cotton, if it con-sists entirely of trinitrocellulose, does not contain sufficient oxygen for the complete oxidation of its carbon; hence more work can be accomplished with a given weight of gun-cotton if a solid oxidizing agent (a nitrate or a chlorate) be incorporated with it in proportion sufficient for complete oxidation. The compressed preparations (chlorated or nitrated gun-cotton) are as sensitive to detonation as gun-cotton itself, but are less sudden or sharp in their action. These preparations, first manufactured by Abel, were extensively experimented with some years ago, and one of them, prepared with barium nitrate, is now manufactured under the name of tonite for blasting purposes. Prepara-tions allied to gun-cotton, in the production of which wood-fibre is used as the starting point, are manufactured for sporting and blasting purposes under the name of Schulze's powder, sawdust powder, and patent gunpowder, (F. A. A.)

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