[Footnote A: The paper by Prof. Lunge, Jour. Amer. Chem. Soc., 1901, 23[8], 527-579, contains valuable information on this subject.]
Besides the nitrate, A. Luck[A] has proposed to use other esters of cellulose, such as the acetate, benzoate, or butyrate. It is found that cellulose acetate forms with nitro-glycerine a gelatinous body without requiring the addition of a solvent. A sporting powder is proposed composed of 75 parts of cellulose nitrate (13 per cent. N.) mixed with 13 parts of cellulose acetate.
[Footnote A: Eng. Pat. 24,662, 22nd November 1898.]
The discovery of gun-cotton is generally attributed to Schönbein (1846), but Braconnot (in 1832) had previously nitrated starch, and six years later Pelouse prepared nitro-cotton and various other nitro bodies, and Dumas nitrated paper, but Schönbein was apparently the first chemist to use a mixture of strong nitric and sulphuric acids. Many chemists, such as Piobert in France, Morin in Russia, and Abel in England, studied the subject; but it was in Austria, under the auspices of Baron Von Lenk, that the greatest progress was made. Lenk used cotton in the form of yarn, made up into hanks, which he first washed in a solution of potash, and then with water, and after drying dipped them in the acids. The acid mixture used consisted of 3 parts by weight of sulphuric to 1 part of nitric acid, and were prepared some time before use. The cotton was dipped one skein at a time, stirred for a few minutes, pressed out, steeped, and excess of acid removed by washing with water, then with dilute potash, and finally with water. Von Lenk's process was used in England at Faversham (Messrs Hall's Works), but was given up on account of an explosion (1847).
Sir Frederick Abel, working at Stowmarket and Waltham Abbey, introduced several very important improvements into the process, the chief among these being pulping. Having traced the cause of its instability to the presence of substances caused by the action of the nitric acid on the resinous or fatty substances contained in the cotton fibre, he succeeded in eliminating them, by boiling the nitro-cotton in water, and by a thorough washing, after pulping the cotton in poachers.
Although gun-cottons are generally spoken of as nitro-celluloses, they are more correctly described as cellulose nitrates, for unlike nitro bodies of other series, they do not yield, or have not yet done so, amido bodies, on reduction with nascent hydrogen.[A] The equation of the formation of gun-cotton is as follows:—
2(C_{6}H_{10}O_{5}) + 6HNO_{3} = C_{12}H_{14}O_{4}(NO_{3}){6} + 6OH{2}. Cellulose. Nitric Acid. Gun-Cotton. Water.
The sulphuric acid used does not take part in the reaction, but its presence is absolutely essential to combine with the water set free, and thus to prevent the weakening of the nitric acid. The acid mixture used at Waltham Abbey consists of 3 parts by weight of sulphuric acid of 1.84 specific gravity, and 1 part of nitric acid of 1.52 specific gravity. The same mixture is also used at Stowmarket (the New Explosive Company's Works). The use of weaker acids results in the formation of collodion- cotton and the lower nitrates generally.
[Footnote A: "Cellulose," by Cross and Bevan, ed. by W.R. Hodgkinson, p. 9.]
The nitrate which goes under the name of gun-cotton is generally supposed to be the hexa-nitrate, and to contain 14.14 per cent. of nitrogen; but a higher percentage than 13.7 has not been obtained from any sample. It is almost impossible (at any rate upon the manufacturing scale) to make pure hexa-nitro-cellulose or gun-cotton; it is certain to contain several per cents. of the soluble forms, i.e., lower nitrates. It often contains as much as 15 or 16 per cent., and only from 13.07[A] to 13.6 per cent. of nitrogen.