Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter VII.

IN 1851 Legray first suggested the application of collodion for the receptacle of the photographic picture; and in the same year Messrs. Archer and Fry published a detailed account of the practical mode of its application. Collodion is a solution of gun-cotton in ether and alcohol; and guncotton, of which there are several varieties, is cotton or linen fiber (that is, cellulose or lignine) altered by combination with peroxide of nitrogen and probably with nitric acid. Cotton consists chemically of carbon, hydrogen, and oxygen; whilst gun-cotton contains an additional element, namely, nitrogen, which communicates explosive tendencies to several of the metalloids. The altered cotton employed for photographic purposes is not the same as gun-cotton proper; in the first place it is not so explosive; It is, secondly, almost perfectly soluble in alcohol and ether, which is not the case with gun-cotton. It is denominated pyroxyline. Pyroxyline is soluble also in acetic ether. When this soluble cotton is dissolved in a mixture of ether and alcohol, and afterward poured upon a piece of glass, it leaves on evaporation, when of a normal condition, a transparent film; whereas gun-cotton so dissolved, or xyloidine, (another form of altered cotton,) leaves an opaque film after evaporation.

Cotton or ligneous fiber is transformed into pyroxyline by immersing it in' a mixture of nitric acid and sulphuric acid; the latter seems necessary only to concentrate the nitric acid; for neither sulphur nor any of its oxides are found in pyroxyline by analysis. This, although the accepted theory, is not satisfactory, because it is found necessary to add water to certain specimens of nitro-sulphuric acid. Another reason for the use of sulphuric acid arises from the fact that pyroxyline is soluble into a gelatinous form in nitric acid, but not in the mixture of nitric and sulphuric acids. Gun-cotton may be precipitated from its ethereal and alcoholic solution into a fibrinous mass like the original, almost. This curious fact exhibits quite an analogy between solutions of salts and the mineral kingdom, and the gelatinous solutions in the organic kingdom. In the former the precipitate is either amorphous or crystalline, as in chloride of silver and carbozotate of potassa; whilst in organic solutions the precipitated ultimate atoms seem to exist, even in solution, in the form of fiber. This peculiar fibrinous deposit is thrown down by adding water to the mixed ethereal and alcoholic solution of pyroxyline, because this substance is insoluble in water. For this reason the necessity of using only Concentrated ether and alcohol is apparent; another deduction is equally apparent from this circumstance, which consists in the employment of such iodizing materials in the preparation of sensitive collodion, as are soluble in ether and alcohol, and in discarding those which are soluble principally in water, or only partially in ether and alcohol. Collodion containing a small proportion of water is thick and flows unevenly, and when dry is not quite transparent; whilst the film from anhydrous collodion is very thin, transparent, and uniform, and flows on the surface of glass very easily.

Preparation of Pyroxyline.--For this purpose the finest cotton or the best Swedish filtering-paper, or old white cotton rags are procured. These materials, especially the first, are not quite pure; a sort of resinous cement adheres with great tenacity to its fibers, and must first be dissolved before the cotton is fit for transformation into pyroxyline. The cotton is therefore boiled in a solution of carbonate of potassa in the following proportion: take one hundred parts of rainwater, two parts of cotton, and one of carbonate of potassa. These materials are maintained at a boiling temperature for a few hours, after which the cotton is taken out and thoroughly washed in several waters, and then left in clean rain-water for at least twenty-four hours, stirring the same from time to time, until every trace of the alkali is removed. It is then taken out, pressed, and dried in thin layers spread upon clean sheets of paper in the sun or on a steam-bath. Care must be taken that all moisture be entirely expelled. In this condition it is ready for the action of nitric acid. Certain rules have to be minutely observed in regard to the temperature of the nitric acid, the quantity of water which it contains, the length of time of immersion, and the intimate mixture of the ingredients; for as these conditions vary so will the pyroxyline. If, for instance, the acids are too strong, or the temperature too low, the pyroxyline will be much heavier than the weight of the cotton used, without apparently having, undergone any other outward change. Such gun-cotton will produce a thick and gelatinous collodion, giving rise to streaks in the film. If; on the contrary, the resulting pyroxyline is less in weight than the cotton introduced, or about equal to it, this indicates that the acids are too weak or the temperature too high, whereby a portion of the pyroxyline is dissolved. Such a species of gun-cotton is not wholly soluble in a mixture of ether and alcohol; it yields, however, a collodion which flows easily over the plate, is very adhesive to the glass, and yields a soft negative. Any little particles of dust that may fall on the plate are liable to produce with this collodion transparent specks on the positive or negative. The rule, therefore, on the whole is to steer between these two results, in order to obtain a pyroxyline in which the cotton fiber shows an incipient gelatinization in the acids. When the operation is successful, the weight of the dry pyroxyline will be somewhere about twenty-five per cent heavier than the cotton from which it was formed.

No. 1. Formula for the Preparation of Pyroxyline.

Commercial sulphuric acid, spec. grav., 1.843 at 60 Fahr.,. 24 fluid ounces
Commercial nitric acid, spec. grav., 1.457 at 60 Fahr.,. 8 fluid ounces
Water, 7 fluid ounces
Cotton, 1 ounce.

The vessels used in the preparation of pyroxyline may be large porcelain or glass evaporating-dishes, sitting closely in the cover of a water-bath, maintained at a temperature of 150° Fahrenheit. Each dish is furnished with a pane of glass, fitting upon it as a lid or cover. Let the water-bath be first raised to the indicated temperature; then pour the sulphuric acid into one of the dishes, add to this the water, and mix intimately by stirring with a glass rod with a rounded end; finally pour in the nitric acid, and perform the same operation to insure an intimate mixture. The temperature of this mixture will rise from 15 to 20 degrees above the point required. Remove the dish, therefore, from the bath until the temperature falls to 150°. The temperature can be lowered by stirring the mixture with cold stirring-rods or spatulas of porcelain or glass. Whilst the acids are cooling the cotton can be divided into about a dozen lots, and each lot must be gently separated into a loose condition. As soon as the proper temperature has been attained, the dish is reinstated in its position in the water-bath, and the cotton is introduced one lot at a time, so that each is carefully pressed down beneath the surface by the glass rod. As soon as all the cotton has been introduced and completely covered by the acid mixture, the lid is placed on the dish for six or eight minutes.

The thermometer used on such occasions for ascertaining the temperature of the water or mixed acids, must be strongly made, so that the bulb can be moved about in the fluid with some degree of briskness without any liability to break; it is furnished with a hinged back, which allows the lower portion to be reflected on itself, and the bulb and the lower part of the stem to be exposed. Such thermometers are manufactured for the chemist, and can be purchased at the photographic establishments.

The acids are now poured into another dish close by, allowing the largest portion to drain off, and preventing the cotton from falling out at the same time by the cover which is retained in its place. The dish containing the pyroxyline is then quickly immersed in a large tub of water, and the cotton is well stirred about so as to part with the largest portion of its acidity; it is then taken out with a pair of glass rods and plunged into fresh water in another tub, and again thoroughly washed. After this operation the pyroxyline is placed in a wooden chamber through which a current of water is kept running for twenty-four hours or more, or at least until every trace of acidity has been removed. During this time the agglutinated or adherent portions are carefully separated, so that the stream of water can more easily act upon each fiber. When blue litmus paper is no longer turned red by the water as it proceeds from the cotton, the latter is taken out, again carefully separated and placed in thin patches on sheets of paper in the sun to dry; or it may be dried on zinc plates, being part of a hot-water bath, whose temperature is maintained at about 120° Fahrenheit. At this temperature pyroxyline will not explode. In the hot days of summer, however, it can be dried quite efficaciously when placed out in the sun.

Pyroxyline, when exposed to the air, absorbs moisture; it undergoes decomposition, too, in an air-tight vessel, if light reaches it; the products of decomposition being nitric acid, peroxide of nitrogen, and probably other compounds. It has not yet been thoroughly ascertained by what means it can be preserved in a normal condition permanently; absence of moisture and of light have been found to assist in this preservation.

If a specimen of pyroxyline by keeping manifests an acid reaction, it is advisable to wash the cotton in several waters, as before, and again to dry it. To neutralize the cotton by an alkali, or a carbonated alkali, is scarcely to be recommended, because they both have a tendency to decompose it; and especially if any trace of these should be left in the fiber, decomposition is likely to ensue in the drying.

No. 2. Formula for the Preparation of Pyroxyline.

  By Weight.
Commercial sulphuric acid, spec. grav., 1.843, at 60° Fahr.,. 18 ounces,
Commercial nitric acid, spec. grav., 1.43, at 60° Fahr.,. 14 ounces
Cotton, 2 ounces

Proceed with these ingredients in all other respects as with those in Formula No. 1.

No. 3. Formula for the Preparation of Pyroxyline.

Commercial sulphuric acid, 40 ounces,
Pure nitrate of potassa, 20 ounces
Cotton, 1 ounce.

As soon as the mixture of acid and nitre has been thoroughly mixed, and almost cool, the cotton is introduced in small portions and well stirred. In about a quarter of au hour the whole mixture is thrown into a large tub full of water; in this way the pyroxyline is freed as much as possible from the acid; after this it is washed in warm water, and finally in a running stream, as in Formula No. 1.

No. 4. Formula for the Preparation of Pyroxyline.
Disdéri's Pyroxyline.

Sulphuric acid, 4000 grains.
Pulverized pure nitrate of potassa, 2000 grains

Place these in a glass vessel provided with a close-fitting cover, and stir them intimately together with a glass rod. Next add 150 grains of fine cotton-wool, in small flocks at a time, and immerse them thoroughly with the glass rod. When all the cotton has been introduced, close the vessel and set it aside for ten or fifteen minutes. After this, the pyroxyline is withdrawn by means of a pair of glass rods, and well washed, as before recommended, and dried.

In all these formulas the acids, when once used, can not be employed a second time; by distillation, the nitric acid that has not been decomposed might be obtained and used over again, if other combinations and decompositions did not result from the application of so high a temperature. In general the mixture is regarded as useless, and thrown away.