Gun Cotton , an explosive substance obtained by subjecting common cotton to the action of strong nitric acid, first brought to public notice in 1846 by Prof. Schonbein of Basel, Switzerland. Several products are known under this name, possessing distinct properties, though differing slightly in constitution. Cotton fibre is nearly pure cellulose, C18H30O15, and by the action of the acid a number of equivalents of nitric oxide, N204, or N02, are substituted for an equal number of hydrogen. The number of equivalents substituted varies from 0 to 9, according to the strength of the acid. Thus, the substitution of 9 equivalents would give C18H21(N02)9O15, or more simply, C6H7(N02)3 05. Water is also generated in the reaction, by the union of hydrogen with the oxygen set free in reducing nitric acid to oxide. In preparing the gun cotton, two or three parts of strong sulphuric acid are mixed with one part of nitric acid, to absorb this water and thus prevent further hydration of the nitric acid. The following formulas show the constitution of the products obtained by using two or three parts of sulphuric and one of nitric acid, while the quantity of water is varied : 1. With monohydrated acids : C18H30O15+9(HN-03)=C18H21(N02)9015 + 9H20. 2. With 3 to 4 parts water: C18H30O15 + 8(HN03) = C18H22 (N02)8015 + 8H2O. 3. With 4 to 5 parts water: C18H30O15 + r(HN03)=C18H23(N02)70157H20. 4. With 5 to 6 parts water: C18H30O15+6(H N03)=C18H24(N02)6015 + 6H20. The first variety is the well known detonating gun cotton.
It is insoluble in common ether and alcohol, and is not attacked by acetic acid, but is soluble in acetic ether. The second is a less explosive variety, soluble in common ether with one eighth of alcohol added, but insoluble in acetic acid. The third is inflammable, but not ordinarily explosive, and is soluble both in ether and in glacial acetic acid. This variety is used for collodion. The fourth is always dissolved in the acid, from which hitherto it has not been isolated. In the manufacture of gun cotton the substitutions are never exactly in accordance with the foregoing formulas, but most probably different degrees of substitution take place in the different molecules of the same mass of cotton; and a careful analysis shows invariably an intermediate constitution, though in many cases one of the typical constitutions is very closely approached. Gun cotton is not readily distinguishable in appearance from the unaltered cotton; but it is slightly harsher to the touch and the compression of the fingers. When moistened with a solution of iodine in potassium iodide, and touched with a drop of dilute sulphuric acid, it turns yellow, while unchanged cotton under the same test turns blue.
The variety here to be considered is the detonating product, C6H7(N02)305. To prepare it the strongest commercial acids are required, the nitric acid having a density of near 1.50, and the sulphuric a density of 1.847. An immersion of the cotton for a few seconds yields an explosive material; but to insure the highest degree of explosiveness the immersion should be prolonged. Baron Lenk protracted it to 48 hours. On withdrawing it from the bath, as much acid as possible is squeezed out between porcelain plates, and the cotton is repeatedly and thoroughly washed in water. The fibres are capillary, and during the immersion the tubes absorb acid, which it is difficult to remove entirely. As the so-called spontaneous ignition of gun cotton has been frequently attributed to the retention of acid, the original method of washing the unbroken fibre in water has been abandoned, and a new mode of treatment, devised by Mr. F. A. Abel, has been extensively applied in England. The fibre is first subjected to two or three rinsings in a large volume of water, and is freed from water as far as possible after each rinsing, by a centrifugal drying machine. It is then reduced to pulp by an engine similar to that used in reducing paper pulp.
The access of water to the capillary tubes is thus greatly facilitated, and to insure the most searching purification, the pulp is transferred to a "poaching machine," where it is beaten about, and kept suspended in a large volume of warm water, continuously renewed, and rendered slightly alkaline at the close of the operation. This operation lasts about 48 hours, and the quantity treated is half a ton. In this way a thorough intermixture of the products of many dippings is obtained, and the average constitution is rendered uniform. The" pulp is then compacted into the desired forms by a preliminary moulding and a subsequent pressing by hydraulic power, ranging from four to six tons per square inch. On leaving the press the slab contains about 20 per cent. of water, in which condition it may be safely cut with circular or band saws, and even hot iron may be used without danger to burn holes in it. When wet it may be kept for an indefinite period without change, and, whether wet or dry, it is affected by very few reagents. Its stability is superior to that of gunpowder in every respect save the all-important one of immunity from inexplicable explosion. Repeated accidents have destroyed the confidence of all but the most sanguine in its safety.
It was for a time supposed that the ingenious process of Mr. Abel had removed the causes of distrust; but a terrible and unexplained explosion at Stow-market in 1871 revived the feeling, which still prevails. How far the repeated disasters attending the use and storage of gun cotton are due to the carelessness of those who have it in charge, is unknown, for it leaves no witnesses to testify to the origin of the explosions, and the very mystery which surrounds them, and our frequent inability even to conjecture a probable cause, are sufficient reasons for regarding it as a treacherous servant. If massive gun cotton be ignited by a coal or flame of low intensity, it burns in the open air inexplosively. If ignited by a powerful flame, it flashes like gunpowder; but if ignited by a fulminate, it detonates with tremendous violence. This "sympathetic" quality, by virtue of which the total combustion seems to follow the character of the ignition, is difficult to explain, and also presents some anomalies.
According to Mr. Abel, about live grains of fulminate of mercury is required to produce detonation; but ten times that quantity of chloride of nitrogen, strongly confined, is required to produce the same result, while 350 grains of nitro-glyce-rine, exploded in contact with massive gun cotton, fails to produce any other result than the mechanical disintegration of the mass. The rate at which detonation is propagated along a row of gun-cotton disks has been investigated by Mr. Abel and Capt. A. Noble, who found it to be about 18,000 feet a second. A sound wave would travel through such a mass with less than one fourth of this velocity, and such rapidity of transmission seems explicable only on the assumption that ignition is carried along the surface by the expanding gases, driven by their tension. The products of combustion of gun cotton vary with the conditions under which it is exploded. They consist of varying proportions of carbonic oxide, carbonic acid, marsh gas, nitric oxide, nitrogen, and water.
Two analyses by Lieut. Von Karolyi of the Prussian army gave, by volume:
The first analysis is that of gases obtained by exploding the cotton in vacuo, and the second by exploding it in strong iron tubes placed in a mortar. - Many attempts have been made to substitute gun cotton for gunpowder in military operations. A brief examination of the phenomena of explosions will show that it can never be advantageously used as an agent for the propulsion of projectiles. In the explosion of all detonating compounds the conversion of the solid or liquid material into elastic gases is so rapid that it may be regarded as practically, though not strictly, instantaneous. The subsequent expansion of these gases by their elastic force must be the motive power of the shot; and at the instant of concussion this is so great, in the case of gun cotton confined in a small receptacle, that no material can withstand it, unless the chamber be much larger than the bulk of the gun cotton. If the explosive energy be reduced by using a large chamber, or by mixing the compound with some inert material, then the total elastic effort is no greater than that of gunpowder.
It can be rendered serviceable in this relation only by depriving it of the very excess of energy which can alone give greater velocity to a projectile; and there is no probability that any means can be employed to compensate for this loss of energy, as may be done with gunpowder. As a bursting charge for hollow projectiles, the destructive efficiency of gun cotton is very great; but experience has shown its extreme liability to explode prematurely by the shock of the discharge. Its detonation in contact with hard materials shatters or disintegrates them. "Wood and rock are completely pulverized by it, while hollow projectiles are often blown into innumerable fragments. But these effects extend to a small distance only from the centre of explosion, and at the distance of a few feet the effect is no greater than that obtained from gunpowder. - The use of gun cotton in blasting-is favored by its indifference to water, and by its great explosive effect, estimated by Combes and Flandin to be fourfold, by Seguier sixfold, and by Tamper double that of gunpowder. These estimates are based upon practical experiment. They indicate a saving in practice by the use of shallower bore holes.
But the relative costliness of gun cotton, the danger of premature explosions in charging holes, its varying quality, its liability to spontaneous decomposition, and its too rapid combustion, which gives it a tendency to shatter or pulverize rather than lift and loosen the rock, have proved great obstacles to its use. Many of these objections have been obviated, it is asserted, by the manufacture of compound gun cotton above described, and by the admixture of less explosive or non-explosive substances, such as common cotton. Extensive and successful use was made of gun cotton in the quarries of Co-morn and in the removal of the ancient walls of Vienna. In the former case, the cotton was wound in solid cylinders; in the latter the cylinders were hollow. Punshon, an English manufacturer, claims that he makes an article of definite explosive power in grades, suited to any use, the quality of each grade being uniform. He also asserts that his gun cotton preparations will not explode or decompose spontaneously, and can be stored or transported without danger. In these preparations, the gun cotton is coated with a powder of sugar, or potash or other salts, which separates the fibres.
By changing the quality of this powder the desired grade of exploding force is obtained. - Bleekrode found that gun cotton, when moistened with an inflammable liquid, like carbon sulphide, ether, benzine, or alcohol, and ignited by the electric spark or otherwise, does not explode, but burns slowly. Hence he recommends that when stored it should he covered with such a liquid, which could be removed by evaporation. A new variety of gun cotton is made by immersing cotton for 15 minutes in a saturated solution of chlorate of potassa. It is chemically more allied apparently to the chlorate of potassa powders. Little is known of its application. A new gun cotton, known as gadoxyline, is manufactured in Wolverhampton, England. Its composition is unknown. Gun cotton has been manufactured into an explosive paper, to reduce the danger of handling and charging.