COLLODION NEGATIVE PROCESS. In this process a negative photograph is taken on a glass plate coated with collodion. The various operations are as follow :— To prepare iodized Collodion. This is made by adding to plain collodion a certain quantity of an alcoholic solution of iodide of potassium.
To make the plain collodion ; dissolve to the ounce of pure ether S. G. 765, (made by adding alcohol S. G. 845 to ether S. G. 720,) about 3; grains of pyroxyline, or more, according to the nature of the pyroxyline and the kind of film desired.
To make the iodizing solution ; dissolve 14 grains of pure iodide of potassium to the ounce of alcohol, S. G. 810. The iodide of potassium is not freely soluble in the alcohol. It must therefore be pounded in a mortar, and the bottle containing the mixture well shaken occasionally, and immersed in a basin of tepid water. All this is very troublesome The neatest and simplest way of making the iodizing solution is to procure absolute alcohol S. G. 794, which will require about the one-twentieth part of its bulk of water to raise its S. G. to 810. The iodide of potassium may therefore be dis solved in the water, and then added to the alcohol.
The S. G. of the plain collodion is about 769, and of the iodizing solution about 839. These specific gravities correspond with those of the excellent compounds made by Mr. Thomas, and called by hire " Xylo-iodide of silver," (which contains apparently neither xyloi. din, nor iodine, nor silver,) and solution X ; their chemical com position and properties being also apparently identical with Mr. Thomas's preparations.
To iodize the plain collodion ; add one part of the iodizing solu tion to three parts of the plain colloclion ; shake well together, and use the next day. If used immecliately the sensitiveness is at its highest pitch, but sometimes the negative may be a little wanting in density, and perhaps disfigured by minute floating particles which adhere to the film in consequence of the mixture not having been snowed to settle, as well as with minute white spots occasioned by undissolved crystals of iodide of potassium which have been pre cipitated by mixing the iodizer with the ether, and have not had time to be redissolved.
Iodized collodion deteriorates by keeping, and at the end of a few months is generally unfit for use, in consequence of the pyro xyline undergoing a decomposition which renders the film rotten. It is at first of a lemon yellow colour, which changes in the course of time to a deep red. In this state it is perfectly useless, and should be thrown away, as the ether obtained by redistillation from it is too impure to be used again for a similar purpose. Only small quan tities of iodized collodion should therefore be made at a time, and it should be kept in a dark place as exposure to light greatly accele rates its decomposition. The bottle which contains it should be made of orange coloured glass.
The change of colour produced in iodized collodion by keeping is due to the liberation of iodine. This is occasioned, in all probability, by the oxidation of the potassium, which forms potass, and unites with an organic acid produced by the decomposition of the ether, alcohol, and pyroxyline. The photographic effects of old, as com pared with new iodized collodion, may be imitated very closely, by adding a little acetate of potass and free iodine to the iodizing solu tion. The negatives then require a much longer exposure, and exhibit greater density in the blacks, and less gradation of tone, which are the effects produced by old iodized collodion, with the exception, however, of the increased porosity or rottenness of the film in the latter case. When old iodized collodion is used, the free iodine liberates free nitric acid from the nitrate of silver in the nitrate bath, and this lessens the sensitiveness of the film, while increased density is produced by the formation of organic salts of silver, which in all the photographic processes have the property of inten sifying the image, as well as in some degree of impairing the sen sitiveness of the excited plate or paper.