Chapter III: NITRO-CELLULOSE, &c (2)

~Packing.~--The finished slabs and discs are dipped into a solution of soda and carbolic acid, and packed in special wood metal-lined cases. When it is to be sent abroad, the metal lining, which is made of tinned copper, is soldered down, but both the outer wooden and inner metal cases are fitted with air-tight screw-plugs, so that when necessary water can be added without unfastening the cases.

~Reworked gun-cotton~ does not make such good discs as new pulped gun- cotton, probably because the fibrous tenacity of the gun-cotton has been destroyed by the amount of pressure it has previously undergone, so that when repulped it resembles fine dust, and a long time is required to press it into any prescribed form. It is generally boiled for eight hours to open up the fibre and remove alkali, then broken up by hand with wooden mallets, pulped, and then used with fresh gun-cotton in the proportion of 1 to 5 parts.

~Manufacture at Le Bouchet.~--At Le Bouchet gun-cotton was made thus:--200 grms. of cotton were steeped for an hour in 2 litres of a mixture of 1 volume concentrated nitric and 2 volumes sulphuric acid. The cotton was then removed and pressed, whereby 7/10ths of the waste acids was recovered. After this it was washed for one to one and a half hours in running water, strongly pressed again; allowed to lie for twenty-four hours in wood-ash lye; then well washed in running water; pressed, and finally dried on a wide linen sheet, through which was forced air heated to 60 deg. C. The average yield from 100 parts of cotton was 165 parts of gun- cotton. The strong pressings of the gun-cotton, while still impregnated with acids, caused subsequent washings to be difficult and laborious.

~Granulation of Gun-Cotton.~--Gun-cotton is often required in the granulated form for use either alone or with some form of smokeless powder. This is done under the patent of Sir Frederick Abel in the following manner:--The gun-cotton from the poacher is placed in a centrifugal machine, very similar to the hydro-extractors before mentioned, and used for wringing out the acids. In this machine it loses water until it only contains 33 per cent., and is at the same time reduced to a more or less fibrous state. It is then taken to the granulating room, where it is first passed through sieves or perforations, which break up the mass into little pieces like shot. The material is then transferred to a revolving drum made of wood or stout leather, which is kept constantly revolving for some time. The material is occasionally sprinkled with water. The drum in turning, of course, carries the granules partially round with it, but the action of gravity causes them to descend constantly to the lowest point, and thus to roll over one another continually. The speed of the drum must not be too rapid. None of the granules must be carried round by centrifugal force, but it must be fast enough to carry them some little distance up the side of the drum. After removal from the drum the granules are dried upon shelves in the drying house.

Gun-cotton is also dissolved in acetone or acetic ether until it has taken the form of a jelly. It is then rolled into thin sheets, and when dry cut up into little squares. In the manufacture of smokeless powders from nitro-cellulose, nitro-lignine, &c., the various substances are mixed with the gun-cotton or collodion-cotton before granulating.

~Collodion-Cotton.~--In the manufacture of collodion or soluble cotton the finer qualities of cotton-waste are used and the acids used in the dipping tanks are much weaker. The manufacture of collodion-cotton has become of more importance than gun-cotton, by reason of its use for the manufacture of the various forms of gelatine, such as gelatine dynamite, gelignite, forcite, &c., and also on account of its extensive use in the manufacture of many of the smokeless powders. It is also used for the manufacture of "collodion," which is a solution of collodion-cotton in ether-alcohol; for the preparation of celluloid, and many other purposes. It is less explosive than gun-cotton, and consists of the lower nitrates of cellulose. It is soluble in nitro-glycerine, and in a mixture of 2 parts of ether and 1 of alcohol; also in acetone, acetic ether, and other solvents. MM. Menard and Domonte were the first to prepare a soluble gun- cotton, and its investigation was carried on by Bechamp, who showed that its properties and composition were different to those of gun-cotton.

~Manufacture.~--The cotton used is cotton-waste.[A] It is thought by some that Egyptian cotton is preferable, and especially long fibre varieties. The strength of the acids used is, however, of more importance than the quality of the cotton. The percentage composition of the acid mixture which gives the best results is as follows:--Nitric acid, 23 per cent.; sulphuric acid, 66 per cent.; and water, 11 per cent; and has a specific gravity of 1.712 (about). It can be made by mixing sulphuric acid of specific gravity 1.84 with nitric acid of specific gravity 1.368 in the proportions of 66 per cent. and 34 per cent. respectively. (The production of the penta-nitro-cellulose is aimed at if the collodion-cotton is for use as an explosive.) If the acids are much weaker than this, or potassium nitrate and sulphuric acid is used, the lower nitrates will be formed. The product, while being entirely soluble in ether-alcohol or nitro-glycerine, will have a low nitrogen content, whereas a material with as high a nitrogen as 12 or 12.6 is to be aimed at.

[Footnote A: Raw cotton is often used.]

The cotton should not be allowed to remain in the dipping tanks for more than five minutes, and the acid mixture should be kept at a temperature of 28 deg. C. or thereabouts; and the cotton should be removed after a few minutes, and should not be pressed out, as in the case of gun-cotton, but at once transferred to the pots and allowed to steep for forty-eight hours. (Some prefer twenty-four hours, but there is more chance in this case of the product containing non-nitrated cellulose.) When the nitration is complete, the collodion-cotton is removed from the pots, and treated in exactly the same manner as described under gun-cotton. The produce should be entirely soluble in ether-alcohol and nitro-glycerine, and contain as near 12.7 per cent. of nitrogen as possible. The theoretical nitrogen is for the penta-nitro-cellulose 12.75 per cent. This will, however, seldom if ever be obtained. The following are some of the results I have obtained from different samples:--

Nitrogen. (1.) (2.) (3.) German make 11.64 11.48 11.49 per cent. Stowmarket 12.57 12.60 11.22 " Walsrode 11.61 12.07 11.99 " Faversham 12.14 11.70 11.60 "

and the following was the analysis of a sample (No. 1) of German-made collodion-cotton, which made very good blasting gelatine:--

_ Soluble cotton (collodion) 99.118 per cent.| Nitrogen = 11.64 per cent. Gun-cotton 0.642 " _| Non-nitrated cotton 0.240 " Total ash 0.25 "

It should contain as little non-nitrated or unconverted cotton and as little gun-cotton as possible, as they are both insoluble in nitro- glycerol. The quality and composition of any sample of collodion-cotton can be quickly inferred by determining the percentage of nitrogen by means of the nitrometer and the use of the solubility test.[A] A high nitrogen content coupled with a high solubility is the end to be aimed at; a high nitrogen with a low solubility shows the presence of gun-cotton, and a low nitrogen, together with a low solubility, the presence of unnitrated cotton. Where complete solubility is essential and the percentage of nitrogen less important, Dr Lunge recommends nitration with a mixture of equal parts of sulphuric and nitric acids containing from 19 to 20 per cent. of water.

[Footnote A: See Analysis of Explosives.]

Mr T.R. France claims to have invented some improvements in the manufacture of soluble nitro-cellulose. His object has been to produce an article as uniform as possible. His explanation of the imperfect action of the acids is that, however uniform the mixed acids may be in strength and proportions, and however carefully the operations of nitrating, &c., may be conducted, there are variable elements found in different samples of cotton. The cotton fibre has for its protection a glazed surface. It is tubular and cellular in structure, and contains a natural semi-fluid substance composed of oil or gum, which varies in nature according to the nature of the soil upon which the cotton is grown. The tubes of the fibre seem to be open at one end only when the fibre is of normal length. When, therefore, the cotton is subjected to the action of the mixed acids, the line of least resistance seems to be taken by them, viz., the insides of the tubes constituting the fibre of the cotton, into which they are taken by capillary attraction, and are subject to change as they progress, and to the increased resistance from the oil or gum, &c., in their progress, and therefore to modified action, the result of which is slower and slower action, or chemical change. He also thinks it is possible that the power of capillary attraction is balanced in the tubes by air contained therein, after a little, sufficiently so to prevent the acids from taking full effect. To get over this, Mr France uses his cotton in a fine state, almost dust, in fact, and then nitrates in the usual mixture of acids at 40 deg. to 90 deg. F., the excess of acids being removed by pressure. He says he does not find it necessary to wash this fine cotton dust in an alkaline solution previous to nitration. His mixed acids consist of 8 parts HNO_{3} = 42 deg. B., and 12 parts H_{2}SO_{4} = 66 deg. B., and he stirs in the dipping tank for fifteen minutes, the temperature being 50 deg. F. to 100 deg. F., the temperature preferred being 75 deg. F.

~"Nitrated" Gun-Cotton.~--The nitrates that are or have been mixed with gun-cotton in order to supply oxygen are potassium nitrate, ammonium nitrate, and barium nitrate (tonite). The total combustion of gun-cotton by potassium nitrate corresponds to the equation:--

10[C_{24}H_{18}(NO_{3}H)_{11}O_{9}] + 82KNO_{3} = 199CO_{2} + 41K_{2}CO_{3} + 145H_{2}O + 96N_{2},

or 828 grms. of nitrate for 1,143 grms. of gun-cotton, or 42 per cent. nitrate and 58 per cent. gun-cotton. The explosive made at Faversham by the Cotton Powder Company, and known as tonite No. 1, consists of very nearly half gun-cotton and half barium nitrate. The relations by weight of total combustion would be 51.6 of gun-cotton to 48.4 of barium nitrate. The average composition of tonite I have found by analysis to be 51 per cent. gun-cotton to 49 per cent. barium nitrate. The heat liberated is practically the same as for an equivalent weight of KNO_{3}; but the barium nitrate mixture weighs 2,223 grms. instead of 1,971 grms., or one-eighth more. The advantage in mixing a nitrate with gun-cotton is that it supplies oxygen, and by converting all the carbon into carbonic acid, prevents the formation of the poisonous gas carbonic oxide (CO). The nitrates of potassium and barium are also used admixed with nitro- cellulose in several of the sporting smokeless powders.

~The Manufacture of Tonite.~--The explosive tonite was patented by Messrs Trench, Faure, and Mackie, and is manufactured at Faversham and Melling at the works of the Cotton Powder Company, and at San Francisco by the Tonite Powder Company. It consists of finely divided and macerated gun-cotton incorporated with finely ground nitrate of barium which has been carefully recrystallised. It is made by acting upon carbonate of barium[A] with nitric acid. The wet and perfectly purified, finely pulped gun-cotton is intimately mixed up between edge runners with about the same weight of nitrate, and the mixing and grinding continued until the whole has become an intimately mixed paste. This paste is then compressed into cartridges, formed with a recess at one end for the purpose of inserting the detonator. The whole is then covered with paraffined paper.

[Footnote A: Witherite, BaCO_{3} + 2HNO_{3} = Ba(NO_{3})_{2} + CO_{2} + H_{2}O.]

The tonite No. 2 consisted of gun-cotton, nitrates of potash and soda, charcoal and sulphur. Tonite No. 3[A] is composed as follows:--Gun-cotton, 19 per cent.; di-nitro-benzol, 13 per cent.; and barium nitrate, 68 per cent. or similar proportions. It is a yellowish colour, and being slower in its explosive action, is better adapted for blasting soft rock.

[Footnote A: Tonite No. 1 was patented by Messrs Trench, Faure, and Mackie, and tonite Nos. 2 and 3 by Trench alone.]

Tonite is extensively used in torpedoes and for submarine blasting, also for quarries, &c. Large quantities were used in the construction of the Manchester Ship Canal. Among its advantages are, that the English railways will take tonite on the same footing as gunpowder; it is a very dense material; if wetted it can easily be dried in the sun; it very readily explodes by the use of a proper detonator; while it burns very slowly and without the least danger; the cartridges being waterproofed, it can be employed in wet bore holes, and it can be tamped with water; and finally, as it contains sufficient oxygen to oxidise the carbon, no carbonic oxide (CO) gas is formed, i.e., its detonation is perfect. It is a very safe explosive to use, being little susceptible to either blows or friction.

Not long ago, a committee, composed of Prof. P. Bedson, Drs Drummond and Hume, Mr T. Bell, one of H.M. Inspectors of Coal Mines, and others, in considering the problem whether the fumes produced by the combustion of tonite were injurious to health, carried out a series of experiments in coal mines for this purpose. The air at the "intake" was analysed, also the air of the "return," and the smoky air in the vicinity of the shot holes. The cartridge was surrounded by the flame-extinguishing mixture, and packed in a brown paper bag. During the first experiment nineteen shots were fired (= 6.29 lbs. tonite). The "return" air showed only a trace of carbonic oxide gas (CO). At the second experiment thirteen shots were fired (= 4.40 lbs. tonite), and analysis of the air of the "return" showed that CO was present in traces only, whilst the fumes contained only 1.9 to 4.8 parts per 10,000.

~Dangers in connection with the Manufacture of Guncotton, &c.~--Of all the nitro compounds, the least dangerous to manufacture are gun-cotton and collodion-cotton. The fact that the Stowmarket Factory is within five minutes' walk of the town shows how safe the manufacture of this explosive is regarded. With the exception of the nitration and the compression into blocks or discs, the whole process is worked with a large excess of water, and the probability of an explosion is thus reduced to a minimum. Among the precautions that should, however, be taken, are--first, the careful extraction of the resinous and soluble substances from the cotton before nitration, as it was shown many years ago by Sir F.A. Abel that the instability of the gun-cotton first manufactured in England and Austria was chiefly due to these compounds. They are generally removed by boiling the cotton in a soda solution.

The actual nitration of cotton is not a dangerous operation, but the operations of wringing in the hydro-extractors, and washing the nitro- cotton after it leaves the first centrifugal machine, are somewhat so. Great care should be taken that the wrung-out nitro-cotton at once comes in contact with a large excess of water, i.e., is at once immersed entirely in the water, since at this stage it is especially liable to decomposition, which, once started, is very difficult to stop. The warmer the mixture and the less water it contains, the more liable it is to decomposition; hence it is that on warm and damp days the centrifugal machines are most likely to fire. The commencement of decomposition may be at once detected by the evolution of red fumes. Directly the gun-cotton is immersed in the large quantity of water in the beater and poacher it is safe.

In order that the final product may be stable and have good keeping qualities, it is necessary that it should be washed completely free from acid. The treatment in the beater and poacher, by causing the material to assume the state of a fine pulp, in contact with a large quantity of water, does a good deal to get rid of the free acid, but the boiling process is absolutely necessary. It has been proposed to neutralise the free acid with a dilute solution of ammonia; and Dr C.O. Weber has published some experiments bearing upon this treatment. He found that after treatment with ammonia, pyroxyline assumed a slightly yellowish tinge, which was a sure sign of alkalinity. It was then removed from the water, and roughly dried between folds of filter paper, and afterwards dried in an oven at 70 deg. C. After three hours, however, an explosion took place, which entirely destroyed the strong copper oven in which the nitro- cotton (about one oz.) had been drying. The explosion was in some respects remarkable. The pyroxyline was the di-nitro-cellulose (or possibly the penta-nitro?), and the temperature was below the igniting point of this material (40 deg. C. would have been a better temperature). Dr Weber determined the ignition point of his di-nitro-cellulose, and found it to be 194 deg. to 198 deg. C., and he is therefore of opinion that the explosion was due to the treatment of the partially washed material with ammonia. A certain quantity of ammonium nitrate was probably formed, and subsequently dried upon the nitro-cellulose, in a state of very fine subdivision. The faintest trace of acid would then be sufficient to bring about the explosive ignition of the ammonium nitrate.

The drying of gun-cotton or collodion-cotton is also a somewhat dangerous operation. A temperature of 40 deg. C. (104 deg. F.) should not be exceeded, and thermometers should be placed in the nitro-cotton, and the temperature frequently observed. An electric alarm thermometer is also a useful adjunct to the cotton drying house. Great care must also be taken that there are no exposed hot-water pipes or stoves in the drying house, as the fine gun-cotton dust produced by the turning or moving of the material upon the shelves would settle upon such pipes or stoves, and becoming hot, would be very sensitive to the least friction. The floor also should be covered with linoleum or indiarubber. When hot currents of air are made to pass over the surface of gun-cotton, the gun-cotton becomes electrified. It is important, therefore, to provide some means to carry it away. Mr W.F. Reid, F.I.C., was the first to use metal frames, carriers, and sieves, upon which is secured the cloth holding the gun-cotton, and to earth them.

The compression of gun-cotton into blocks, discs, &c., is also attended with considerable risk. Mr O. Guttmann, in an interesting paper upon "The Dangers in the Manufacture of Explosives" (_Jour. Soc. Chem. Ind._, No. 3, vol. xi., 1892), says: "The compression of gun-cotton into cartridges requires far more care than that of gunpowder, as this is done in a warm state, and gun-cotton even when cold, is more sensitive than gunpowder. When coming out of the centrifugal machines, the gun-cotton should always pass first through a sieve, in order to detect nails or matches which may by chance have got into it. What has been said as to gunpowder presses applies still more to those for gun-cotton, although the latter are always hydraulic presses. Generally the pistons fit the mould perfectly, that is to say, they make aspiration like the piston of a pump. But there is no metal as yet known which for any length of time will stand the constant friction of compression, and after some time the mould will be wider in that part where the greatest compression takes place. The best metal for this purpose has proved to be a special steel made by Krupp, but this also is only relatively better; for pistons I prefer hard cast iron. If the position of the moulds and pistons is not exactly the same in all cases, what the Germans call 'Ecken' (English 'binding') will take place, viz., the mould will stand obliquely to the piston, and a dangerous friction will result." "Of course, it is necessary to protect the man working the hydraulic valves during compression. At Waltham Abbey they have a curtain made of ship's hawsers, which is at the same time elastic and resistant." Mr Guttmann has found that a partition wall 12 inches thick, made of 2-inch planks, and filled with ground cinders, gives very effective protection. A door in this partition enables the workman to get to the press, and a conical tube penetrates the wall, enabling the man to see the whole work from a safe standpoint. The roof, or one side of the building, should be of glass, so as to give the explosion a direction.

~Trench's Fire-extinguishing Compound~ is manufactured by the Cotton Powder Company at Faversham, and is the invention of Mr George Trench, F.C.S., the manager of the Company. The object of the invention is to surround the cartridges of tonite, when used in coal mines, with a fire- extinguishing compound. If a charge of tonite, dynamite, or gelatine dynamite is put inside a few ounces of this mixture, and then fired, not the least trace of flame can be observed, and experiments appear to show that there is no flame at all. The compound consists of sawdust impregnated with a mixture of alum and chlorides of sodium and ammonia. Fig. 22 shows the manner of placing the tonite cartridge in the paper bag, and surrounding it with the fire-extinguishing compound, _aa_. The attachment of the fuse and detonator is also shown.

The following report (taken from the _Faversham News_, 22nd Oct. 1887) of experiments conducted in the presence of several scientific and mining men will show its value:--"A large wrought-iron tank, of 45 cubic feet capacity, had been sunk level with the ground in the middle of the yard; to this tank the gas had been laid on, for a purpose that will be explained later on. The charges were fired by means of electricity, a small dynamo firing machine being placed from 30 to 40 yards away from the 'mine.'" Operations were commenced by the top of the tank being covered over and plastered down in order to make it air-tight; then a sufficient quantity of coal gas was placed in it to make it highly inflammable and explosive, the quantity being ascertained by a meter which had been fixed specially for the purpose. Whilst the gas was being injected the cartridge was prepared.

The first experiment was to try whether a small charge of tonite--fired without the patent extinguisher--would ignite the gas. The gas having been turned on, a miner's lamp was placed in the "tank," but this was extinguished before the full quantity of gas had gone through the meter. However, the gas being in, the charge of 1-1/4 oz. tonite was placed in the "mine," the detonator was connected by means of long wires to the dynamo machine, and the word was given to "fire." With a tremendous report, and a flash of fire, the covering of the mine flew in all directions, clearly showing that the gas had exploded. The next cartridge (a similar charge) was prepared with the patent compound. First of all a brown paper case of about 2 inches diameter was taken, and one of the tonite cartridges was placed in the centre of it, the intervening space between the charge and-the case being packed with the "fire-extinguishing compound." The mine having had another supply of gas injected, the protected cartridge was placed inside and fired. The result was astonishing, the explosion not being nearly so loud, whilst there was not the least flash of fire. "Protected" and "unprotected" charges were fired at intervals, gas being turned into the tank on each occasion. Charges of tonite varying from 1 to 6 oz. were also used with the compound. The report was trifling, whilst no flash could be seen.

~Uses of Collodion-Cotton.~--The collodion or soluble gun-cotton is used for a variety of purposes. The chief use is, however, for the manufacture of the various explosive gelatine compounds, of which blasting gelatine is the type. It is also very extensively used in the manufacture of smokeless powders, both military and sporting--in fact, very few of them do not contain it. In some, however, nitro-lignose or nitrated wood is used instead. This, however, is chemically the same thing, viz., nitro- cellulose, the cellulose being derived from the wood fibre. It is more used in this connection than the higher nitrate gun-cotton. Another use to which it has been applied very extensively, of recent years, is in the manufacture of "celluloid." It is used in photography for the preparation of the films on the sensitised plates, and many other purposes. Dissolved in a solution of two parts ether and one of alcohol, it forms the solution known as collodion, used for a variety of purposes, such as a varnish, as a paint for signals; in surgery, for uniting the edges of wounds.

Quite lately, Mr Alfred Nobel, the well-known inventor of dynamite, has patented the use of nitro-cellulose, hydro- or oxy-cellulose, as an artificial substitute for indiarubber. For this purpose it is dissolved in a suitable non-volatile or slightly volatile "solvent," such as nitro- naphthalene, di-nitro-benzene, nitro-toluene, or its homologues; products are obtained varying from a gelatinous consistency to the hardness of ebonite. The proportions will vary from about 20 per cent. of nitro- cellulose in the finished product, forming a soft rubber, to 50 per cent. nitrating celluloid, and the "solvent" chosen will depend on the use to which the rubber substitute is to be put, the liquids giving a more elastic substance, whilst mixtures of solids and liquids may be employed when the product is to be used at high temperatures. By means of rollers steam heated, the incorporation may be accomplished without the aid of a volatile liquid, or the nitro-cellulose may be employed wet, the water being removed after "solution."

It is advisable to use the cellulose nitrated only just enough to render it suitable, in order to reduce the inflammability of the finished product. Mr W. Allen, M.P., of Gateshead, proposed to use celluloid for cartridge cases, and thus to lighten ammunition, and prevent jambing, for the case will be resolved into gases along with the powder. Extractors will also be done away with.

~Celluloid~ is an intimate mechanical mixture of pyroxyline (gun-cotton or collodion-cotton) with camphor, first made by Hyatt, of Newark, U.S.A., and obtained by adding the pyroxyline to melted camphor, or by strongly compressing the two substances together, or by dissolving the constituents in an appropriate solvent, e.g., alcohol or ether, and evaporating to dryness. A combination of the two latter methods, i.e., partial solution, with pressure, is now usually adapted. The pyroxyline employed is generally the tetra- and penta-nitrated cellulose, the hexa-nitrate (gun-cotton) being but seldom used on account of its explosive properties.

Care is taken to prevent the formation of the hexa-nitrate by immersing the cellulose in only moderately strong nitric acid, or in a warm mixture of nitric and sulphuric acids. The paper, either in small pieces or in sheets, is immersed for about twenty-five minutes in a mixture of 2 parts of nitric acid and 5 parts of sulphuric acid, at a temperature of about 30 deg. C., after which the nitrated cellulose is thoroughly washed with water to remove the last traces of free acid, pressed, and whilst still moist, mixed with the camphor.

In the process of Trebouillet and De Besancele, the cellulose, which may be in the form of paper, cotton, or linen, is twice nitrated--first in the acid mixture employed in a previous operation; and secondly, in a fresh mixture of 3 parts sulphuric acid of 1.83 specific gravity, and 2 parts concentrated nitric acid containing nitrous acid. After each nitration the mass is subjected to pressure, and is then carefully washed with water, to which, at the last, a small quantity of ammonia or caustic soda is added to remove the final traces of acid. The impregnation of the pyroxyline with the camphor is effected in a variety of ways.

The usual proportion of the constituents is 2 parts pyroxyline and 1 part camphor. In Trebouillet and De Besancele's process, 100 parts of pyroxyline are intimately mixed with from 40 to 50 parts camphor, and moulded together by strong pressure in a hot press, and afterwards dried by exposure to air, desiccated by calcium chloride or sulphuric acid. The usual method is, however, to dissolve the camphor in the least possible quantity of alcohol, and sprinkle the solution over the dry pyroxyline, which is then covered with a second layer of pyroxyline, and the whole again treated with the camphor solution, the addition of pyroxyline and camphor solution being repeated alternately until the requisite amount of celluloid mixture is obtained.

The mass, which sinks together in transparent lumps, is worked for about an hour between cold iron rollers, and then for the same period between rollers which can be gently heated by steam. The layer of celluloid surrounding the rollers is then cut away and again pressed, the resulting cake, which is now about 1 cm. thick, being cut into plates of about 70 cm. long and 30 cm. broad. These are placed one above the other, and strongly pressed together by hydraulic pressure at a temperature of about 70 deg. for twenty-four hours. The thick cakes are once more cut into plates of the desired thickness, and placed in a chamber heated from 30 deg. to 40 deg. for eight to fourteen days, whereby they become thoroughly dry, and are readily made into various articles either by being moulded while warm under pressure, cut, or turned. Occasionally other liquids, e.g., ether and wood spirit, are used in place of alcohol as solvents for the camphor.

Celluloid readily colours, and can be marbled for manufacturing purposes, &c. It is highly inflammable and not explosive even under pressure, and may be worked under the hammer or between rollers without risk. It softens in boiling water, and may be moulded or pressed. Its specific gravity varies slightly with its composition and with the degree of pressure it has received. It is usually 1.35. It appears to be merely a mixture of its components, since by treatment with appropriate solvents the camphor may be readily extracted, and on heating the pyroxyline burns away while the camphor volatilises.

The manufacture of pyroxyline for the purpose of making celluloid has very much increased during recent years, and with this increase of production improved methods of manufacture have been invented. A series of interesting papers upon the manufacture of pyroxyline has been published by Mr Walter D. Field, of New York, in the _Journal of the American Chemical Society_[A] from which the following particulars are taken:--

[Footnote A: Vol. xv., No. 3, 1893; Vol. xvi., No. 7, 1894; Vol. xvi., No. 8, 1894. Figs. 19, 20, 21, 22, and 23 are taken from Mr Field's paper.]

~Selection of the Fibre.~--Cotton fibre, wood fibre, and flax fibre in the form of raw cotton, scoured cotton, paper, and rags are most generally used, and give the best results. As the fibres differ greatly in their structure, they require different methods of nitrating. The cotton fibre is a flattened hollow ribbon or collapsed cylindrical tube, twisted a number of times, and closed at one end to form a point. The central canal is large, and runs nearly to the apex of the fibre. Its side walls are membraneous, and are readily penetrated by the mixed acids, and consequently the highest nitration results. In the flax fibre the walls are comparatively thick, the central canal small; hence it is to be presumed that the nitration must proceed more slowly than in the case of cotton. The New Zealand flax gives the most perfectly soluble nitrates of any of the flaxes. Cotton gives a glutinous collodion, and calico a fluid collodion. One of the largest manufacturers of pyroxyline in the States uses the "Memphis Star" brand of cotton. This is an upland cotton, and its fibres are very soft, moist, and elastic. Its colour is light creamy white, and is retained after nitration. The staple is short, and the twist inferior to other grades, the straight ribbon-like filaments being quite numerous. This cotton is used carded, but not scoured. This brand of cotton contains a large quantity of half and three-quarter ripe fibre, which is extremely thin and transparent, distributed throughout the bulk of the cotton (Monie., Cotton Fibre, 67). Mr Field says, "This is a significant fact when it is known that from this cotton an extremely soluble pyroxyline can be produced."

Pyroxyline of an inferior grade as regards colour only can be produced from the cotton wastes of the trade. They must be scoured before they are fit for nitrating. Paper made from the pulps of sulphite and sulphate processes is capable of yielding a very soluble pyroxyline. It can be nitrated at high temperatures and still yield good results. Tissue paper made from flax fibre is also used after being cut into squares.

Mowbray (U.S.P., No. 443, 105, 3rd December 1890) says that a pure cotton tissue paper less than 1/500 inch in thickness, thin as it is, takes on a glutinous or colloid surface, and thus requires some thirty minutes to enable the nitration to take place. With a thicker paper only the surface would be nitrated. He therefore uses a fibre that has been saturated with a solution of nitrate of soda, and afterwards dried slowly, claiming that the salt crystallises in the fibre, or enters by the action termed osmose, and opens up the fibre to the action of the acid. This process would only be useful when the cotton is to be nitrated at a low temperature. At a high temperature it would be unnecessary.

Dietz and Wayne (U.S.P., No. 133, 969) use ramie, rheca, or China grass for producing a soluble pyroxyline. That made from ramie is always of uniform strength and solubility, and requires a smaller quantity of solvent to dissolve it than that made from cotton. Mr Field's experience, however, is entirely contrary to this statement. Such is the influence of the physical form of the fibre on the process of nitration, that when flax fibre and cotton fibre are nitrated with acid mixtures of exactly the same strength, and at the same temperature, the solution of the first is glutinous or thick, and the second fluid or thin. By simply nitrating at a higher temperature than the cotton, the flax will yield a pyroxyline giving an equally fluid collodion.

The presence of chlorine in the fibre must be carefully avoided, as such a fibre will yield an acid product which cannot be washed neutral. The fibre must be dry before nitration; and this is best done, according to Mr Field, by using the form of drier used in drying wool.

~Nitration of the Fibre.~--Mixed cotton and flax fibre in the form of paper, from 2/1000 to 3/1000 inch thick, and cut into 1-inch squares, is nitrated by the Celluloid Manufacturing Company, and the same paper, left in long strips, 1 inch wide, is used for nitration by the Xylonite Manufacturing Company, of North Adams, Mass. (U.S.A.).

The Celluloid Company introduce the cut paper into the mixed acids by means of a hollow, rapidly revolving tube, flared at the lower end, and immersed in the mixed acids. The centrifugal force of the revolving tube throws the paper towards the sides of the vessel, leaving the centre of the vessel ready for fresh paper.

The Xylonite Company simply cut the paper into long strips, and introduce it into the mixed acids by means of forks. The arrangement used by this Company for holding the mixed acids is a cylindrical vessel divided into a number of sections, the whole revolving like a turntable, thus allowing the workman to nitrate successively each lot of paper at a given point. This Company did not remove the acid from the paper after its immersion, but plunged it immediately into the water, thus losing a large proportion of the waste acid. The Celluloid Company, by using the paper in smaller pieces, and more paper to a pound of acid, and wringing the mixed acid from the paper before immersion in water, had a better process of nitration.

Other manufacturers use earthenware vessels, and glass or steel rods, hooked at one end, having small pieces of rubber hose pulled over the other end to prevent the hand from slipping. The form of vessel in general use is that given in Fig. 23. It is large enough to nitrate 1 lb. of cotton at a time. The hook at one end of the rod enables the workman to pull the pyroxyline apart, and thus ensures saturation of the fibre. In the winter the room in which the nitrating is done must be kept at a temperature of about 70 deg. F. in order to secure equality in the batches.

The nitrating apparatus of White and Schupphaus (U.S.P., No. 418, 237, 89) Mr Field considers to be both novel and excellent. The cage (Fig. 24), with its central perforated cylinder (Fig. 25), is intended to ensure the rapid and perfect saturation of the tissue paper used for nitrating. The patentees say that no stirring is required with their apparatus. This, says Mr Field, might be true when paper is used, or even cotton, when the temperature of nitration is from 30 deg. to 35 deg. C., but would not be true if the temperature were raised to 50 deg. to 55 deg. C. The process is as follows:-- The paper is nitrated in the cage (Fig. 25), the bottom of which is formed by the flanged plate C, fastened to the bottom of the internal cylinder B. After nitration the cage is carried to a wringer, which forms the basket, and the acids removed. Finally, the cage is taken to a plunge tank, where the paper is removed from the cage by simply pulling out the central perforated cylinder B. Fig. 26 shows the nitrating pot, with its automatic cover. The plunge tank is shown in plan and section in Figs. 28 and 29. This apparatus is suitable for the nitration of cotton fibre in bulk at high or low temperatures. Other methods that have been patented are Mowbray's (U.S.P., No. 434, 287), in which it is proposed to nitrate paper in continuous lengths, and Hyatt's (U.S.P., No. 210, 611).

~The Acid Mixture.~--Various formulae have been published for producing soluble nitro-cellulose. In many instances, although the observations were correct for the single experiment, a dozen experiments would have produced a dozen different products. The composition of the acids used depends upon the substance to be nitrated, and the temperature at which the nitration will be worked. Practically there are three formulae in general use--the one used by the celluloid manufacturers; another in which the cotton is nitrated at high temperatures; and a third in which the temperature of the immersion is low, and the time of nitration about six hours. Of the three, the best method is the last one, or the one in which the cotton is immersed at a low temperature, and then the reaction allowed to proceed in pots holding from 5 to 10 lbs. of cotton. The formula used by the celluloid manufacturers for the production of the low form of nitrated product which they use is:--

Sulphuric acid 66 parts by weight. Nitric acid 17 " " Water 17 " "

Temperature of immersion, 30 deg. C. Time, twenty to thirty minutes.

The cellulose is used in the form of tissue paper 2/1000 inch thick, 1 lb. to 100 of acid mixture. The nitro-cellulose produced by this formula is very insoluble in the compound ethers and other solvents of pyroxyline, and is seemingly only converted or gelatinised by the action of the solvent. The next formula produces a mixture of tetra-and penta-nitro- celluloses hardly soluble in methyl-alcohol (free from acetone), but very soluble in anhydrous compound ethers, ketones, and aldehydes:--

Nitric acid, sp. gr. 1.435 8 lbs. Sulphuric acid, sp. gr. 1.83 15-3/4 lbs. Cotton 14 oz.

Temperature of nitration, 60 deg. C. Time of immersion, forty-five minutes.

The 60 deg. of temperature is developed by mixing the acids together. The cotton is allowed to remain in the acid until it feels "short" to the rod.

The following table, due to Mr W.D. Field, shows very plainly the great variation in the time of the immersion and the temperature by seemingly very slight causes. It extends over fourteen working days, during which time it rained four days. The formula used is that given above, except that the specific gravity of the nitric acid is somewhat lower. The product obtained differs only from that produced by using nitric acid of specific gravity 1.43 in being soluble in methyl-alcohol. From 30 to 35 lbs. of pyroxyline were produced in each of the fourteen days.

A careful examination of this table will prove very instructive. The increase in yield varies from 31 per cent. to nothing, and the loss runs as high as 10 per cent., yet care was taken to make the product uniform in quality. On the days it rained there was a loss, with the exception of the fourth day, when there was neither a loss nor a gain. On the days it was partly clear, as just before or after rain, the table shows a loss in product. We can explain this fact by reason of the moisture-absorbing qualities of the cotton. On the rainy days it would absorb the moisture from the air until, when immersed in the acids, they were weakened, and the fibre dissolved more or less in weakened acid, producing what is known as "burning" in the batch. It will also be noticed that on days which show a loss, the time of the immersion was correspondingly short, as on the a loss, the time of the immersion was correspondingly short, as on the tenth, twelfth, and seventh days.

______________________________________________________________________ | | | | | | Specific Gravity. | Time. | | |_____________________|_______________________________| | | | | | | | | | |H_{2}S0_{4}.|HNO_{3}.|Hours.|Minutes.|Hours.|Minutes.| |________________|____________|________|______|________|______|________| | | | | | | | | | 1. Clear | 1.838 | 1.4249 | ... | 20 | 4 | ... | | 2. " | 1.837 | 1.4249 | ... | 20 | 2 | ... | | 3. Cloudy | 1.837 | 1.4226 | ... | 45 | 2 | ... | | 4. Rain | 1.837 | 1.420 | ... | 20 | 1 | 20 | | 5. Clear | 1.8377 | 1.42 | 1 | 15 | 2 | ... | | 6. Rainy | 1.8391 | 1.422 | ... | 35 | 1 | 40 | | 7. Cloudy | 1.835 | 1.4226 | ... | 20 | ... | 35 | | 8. Clear | 1.835 | 1.422 | ... | 35 | 1 | 10 | | 9. Partly Clear| 1.824 | 1.4271 | ... | 20 | 1 | ... | |10. " | 1.83 | 1.4271 | ... | 10 | ... | 25 | |11. Cloudy | 1.832 | 1.425 | ... | 10 | ... | 50 | |12. Rainy | 1.822 | 1.425 | ... | 10 | ... | 20 | |13. Partly CLear| 1.8378 | 1.4257 | ... | 60 | 1 | 40 | |14. Cloudy | 1.837 | 1.4257 | 1 | 56 | 4 | 40 | |________________|____________|________|______|________|______|________| | | | | | |Temp., Deg. C. | Percentage | | |_______________|___________________| | | | | | | | | From | To | Increase. | Loss. | |________________|_______|_______|___________|_______| | | | | | | | 1. Clear | 57 deg. | 62 deg. | 31 | ... | | 2. " | 60 deg. | 62 deg. | 18 | ... | | 3. Cloudy | 60 deg. | 62 deg. | 7 | ... | | 4. Rain | 60 deg. | 63 deg. | 0 | 0 | | 5. Clear | 58 deg. | 62 deg. | 15 | ... | | 6. Rainy | 58 deg. | 62 deg. | ... | 2 | | 7. Cloudy | 62 deg. | 65 deg. | ... | 10 | | 8. Clear | 60 deg. | 62 deg. | 5 | ... | | 9. Partly Clear| 50 deg. | 60 deg. | ... | 3 | |10. " | 58 deg. | 60 deg. | ... | 10 | |11. Cloudy | 58 deg. | 60 deg. | 8 | ... | |12. Rainy | 58 deg. | 60 deg. | ... | 10 | |13. Partly CLear| 50 deg. | 58 deg. | 20 | ... | |14. Cloudy | 50 deg. | 60 deg. | 16 | ... | |________________|_______|_______|___________|_______|

The lesson this table teaches is, that it is almost impossible to nitrate cellulose in small quantities, and get uniform results, when the nitration is carried on at high temperatures. As regards the solubility of pyroxyline, Parks found that nitro-benzene, aniline, glacial acetic acid, and camphor, dissolved in the more volatile solvents methyl-alcohol and alcohol-ether, were much the best solvents for producing a plastic, as they are less volatile, and develop greater solvent action under the influence of heat. Nitro-benzene gives a solution that is granular; it seems to merely convert the pyroxyline, and not to dissolve it; but on the addition of alcohol, a solution is at once obtained, and the granular appearance disappears, and the solution becomes homogeneous. The acid mixture and the method of nitrating have much to do with the action of the various solvents, so also has the presence of water.

Dr Schupphaus found that propyl and isobutyl alcohols with camphor were active solvents, and the ketones, palmitone, and stearone in alcohol solution, also alpha- and beta-naphthol, with alcohol and anthraquinone (diphenylene diketone) in alcoholic solution, and also iso-valeric aldehyde and its derivatives, amyliden-dimethyl and amyliden-diethyl ethers.

August Sayer (U.S.P., No. 470,451) finds diethyl-ketone, dibutyl-ketone, di-pentyl-ketone, and the mixed ketones,[A] methyl-ethyl, methyl-propyl, methyl-butyl, methyl-amyl, and ethyl-butyl ketones are active solvents of pyroxyline; and Paget finds that although methyl-amyl oxide is a solvent, that ethyl-amyl oxide is not.

[Footnote A: Ketones are derived from the fatty acids by the substitution of the hydroxyl of the latter by a monad positive radical. They thus resemble aldehydes in constitution. The best-known ketone is acetone CH_{3}CO.CH_{3}. Mixed ketones are obtained by distilling together salts of two different fatty acids. Thus potassic butyrate and potassic acetate form propyl-methyl-ketone--

C(C_{2}H_{5})H_{2} | CO.CH_{3}]

The solvents of pyroxyline can be divided into general classes--First, those which are solvents without the aid of heat or solution in alcohol; second, those that are solvents when dissolved in alcohol. These solvents are those which also develop a solvent action when heated to their melting point in combination with pyroxyline.

Mr W.D. Field groups the solvents of pyroxyline into classes thus: Two of the monohydric alcohols; compound ethers of the fatty acids with monohydric alcohols, aldehydes; simple and mixed ketones of the fatty acid series. These four classes include the greater number of the solvents of pyroxyline. Those not included are as follows:--Amyl-nitrate and nitrite, methylene-di-methyl ether, ethidene-diethyl ether, amyl-chloracetate, nitro-benzene and di-nitro-benzene, coumarin, camphor, glacial acetic acid, and mono-, di-, and tri-acetin.

Richard Hale uses the following solvent:--Amyl-acetate, 4 volumes; petroleum naphtha, 4 volumes; methyl-alcohol, 2 volumes; pyroxyline, 4 to 5 ounces to the gallon of solvent. Hale used petroleum naphtha to hasten the drying qualities of the varnish, so that it would set on the article to be varnished before it had a chance to run off. It is, however, the non-hygroscopic character of the solvent that makes the varnish successful. This formula is very largely used for the production of pyroxyline varnish, which is used for varnishing pens, pencils, &c., also brass-work and silver-ware.

The body known as oxy-cellulose[A] is formed by the action of nitric acid upon cellulose when boiled with it. The quantity formed is about 30 per cent. of cellulose acted upon. When washed free from acid, it gelatinises. It is then soluble in dilute alkalies, and can be reprecipitated from solution by alcohol, acids, or saline solutions. Messrs Cross and Bevan assign to it the formula C_{18}H_{26}O_{16}. It dissolves in concentrated sulphuric acid, and with nitric acid forms a nitro body of the formula C_{18}H_{23}O_{16}3(NO_{2}), which is prepared as follows:--The gelatinous oxy-cellulose is washed with strong nitric acid until free from water, and is then diffused through a mixture of equal volumes of strong sulphuric and nitric acids, in which it quickly dissolves. The solution, after standing for about an hour, is poured in a fine stream into a large volume of water, by which the "nitro" body is precipitated as a white flocculent mass. The product, after drying at 110 deg. C., was found upon analysis to contain 6.48 per cent. nitrogen.

[Footnote A: "On the Oxidation of Cellulose," by C.F. Cross and E.J. Bevan, _Jour. Chem. Soc._, 1883, p. 22.]

MISCELLANEOUS NITRO-EXPLOSIVES.

~Nitro-Starch.~--It is only recently that, by means of the process introduced by the "Actiengesellschaft Dynamit Nobel," it has been possible to make this explosive upon the manufacturing scale. Nitro-starch has been known since 1883, when Braconnot discovered it, and called it xyloidine. Its formula is C_{6}H_{8}O_{3}(NO_{3})_{2}, but Dr Otto Muehlhaeusen has lately succeeded in preparing higher nitrated compounds, viz.:--

(_a._) C_{6}H_{7-1/2}O_{2-1/2}(NO_{3})_{2-1/2}.

(_b._) C_{6}H_{7}O_{4}(NO_{3})_{3}.

Or doubling the molecule of starch:--

Nitrogen.
i. Tetra-nitro-starch C_{12}H_{16}O_{6}(ONO_{2})_{4} 11.11 per cent.
ii. Penta-nitro-starch C_{12}H_{15}O_{5}(ONO_{2})_{5} 12.75 "
iii. Hexa-nitro-starch C_{12}H_{14}O_{4}(ONO_{2})_{6} 14.14 "

He regards them as true ethers (esters) of nitric acid. Thus on treatment with sulphuric acid, these compounds yield NO_{3}H, the residue O.NO_{2} thus appearing to be replaced by the sulphuric acid residue. On treatment with a solution of ferrous chloride, nitric oxide and "soluble" starch are regenerated. On shaking with sulphuric acid over mercury, all the nitrogen is split off as NO.

Tetra-nitro-starch is prepared upon the large scale as follows:--A quantity of potato-starch is taken and exposed in some suitable desiccating apparatus at a temperature of 100 deg. C. until all the moisture which it contains is completely driven off. It is then reduced to a fine powder by grinding, and dissolved in nitric acid of specific gravity 1.501. The vessel in which this solution is accomplished is made of lead, and must be provided with two jackets, cooled by means of water. It should further be fitted with a screw-agitator, in order to keep the nitric acid circulating freely. The charge of starch is introduced through an opening in the cover of this digesting vessel, and the proportions of acid to starch are 10 kilogrammes of starch to 100 kilos. of acid. The temperature is kept within the limits 20 deg. to 25 deg. C. When the solution of the starch is complete, the liquid is conducted into a precipitating apparatus, which is also provided with a cooling jacket, for the purpose of regulating the temperature. The bottom of this vessel is double and perforated, and here is placed a layer of gun-cotton to act as a filter. This vessel is filled with spent nitro-sulphuric acid obtained as a waste product from the nitro-glycerine manufactory, and the solution of starch in nitric acid is sprayed into it through an injector worked by compressed air, whereby the nitro-starch is thrown down in the form of a fine-grained powdery precipitate.

In order to precipitate 100 kilos. of the acid solution of starch, it is necessary to employ 500 kilos. of spent nitro-sulphuric acid. As it is precipitated the nitro-starch collects on the gun-cotton filter, and the acid liquor is run off through a tap placed beneath the perforated double bottom of the vessel, and of course below the filter pad. The precipitated starch is further cleansed from acid by repeated washings and by pressure, until all trace of acidity has been eliminated, and the substance exhibits a neutral reaction. The next step is to treat the nitro-starch with a 5 per cent. solution of soda, in contact with which it is allowed to stand for at least twenty-four hours. The product is then ground up until a sort of "milk" or emulsion is obtained, and lastly treated with a solution of aniline, so that when pressed into cake, it contains about 33 per cent. of water, and 1 per cent. of aniline.

Dr Muehlhaeusen, working on these lines in the laboratory, prepared nitro- starch which contained 10.96 and 11.09 per cent. of nitrogen. When in the state of powder it is snow-white in colour; it becomes electrified when rubbed; it is very stable, and soluble even in the cold in nitro- glycerine. He has also prepared a tetra-nitro-starch containing 10.58 and 10.50 per cent. of nitrogen, by pouring water into a solution of starch in nitric acid which had stood for several days. The substance thus produced in the laboratory had all the properties of that prepared by the other process.