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Collodio-Gelatine

collodion, pyroxyline, alcohol, ether, film, acid, mixture, plain and quantity

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COLLODIO-GELATINE PROCESS,—A process patented in 1856 by Dr. Norris. In this the collodion film, containing the sensitive silver salts, was immersed in a solution of gelatine, albu men, caseine, or any other similar substance. The object of this was to fill up the pores of the collodion film and prevent its condensation on drying, and retain it in a sensitive and pervious state. After treatment with the- gelatine, the films were dried and exposed to the light, or could be kept for some time.

A more modern collodio-gelatine process has been devised by Messrs. Poirin and Graham. In this two emulsions are made, one of collodion and the other of gelatine. The plates are first coated with a washed collodion emulsion, and then washed in hot water, and coated with a thin film of rapid gelatine emulsion.

COLLODION.—Collodion is prepared by dissolving certain nitrated products obtained from cellulose in a mixture of alcohol and ether. The cellulose employed for the manufacture of collodion is principally clean cotton-wool nitrated with a mixture of nitric and sulphuric acid of certain strength. Gun-cotton is the product of the extreme nitration of cellulose. When treated with sulphuric and nitric acids for a short time only, cellulose is converted principally into tetra nitrate C„ H „ and penta-nitrate C, 6 (NO, both of which dissolve in a mixture of alcohol and ether. The gun-cotton used for photographic purposes is usually a mixture, more or less complex of different nitro-products.

Collodion is a viscous fluid largely used in photographic operations. It is principally employed as a vehicle to hold the sensitive salts of silver. It is manufactured by dissolving soluble gun-cotton (pyroxyline) in ether and alcohol of varied proportions. A good collodion should be limpid and structurelegs, and the film should be smooth, soft, non-contractile, and possess a certain amount of tenacity. When dried upon a surface it should leave by the evaporation of the solvents a perfectly transparent film of pyroxyline. As the quality of the collodion depends almost entirely upon the kind of pyroxyline used, it would be well to study the remarks given under that heading.

For preparing a plain collodion, the following are two serviceable formula:. The first is better adapted for winter work, and the second for the hotter summer months : In making up collodion the alcohol should be added first to the pyroxyline, and lastly the ether. By this means its solution is aided. The proportions of ether and alcohol given abovegnay be varied to suit the requirements of the operator. It should be noted that the larger the quantity of alcohol in proportion to the ether the slower will be the setting of the collodion; but if too much alcohol be added the film becomes streaky, and if the ether be in excess the film will be too contractile and liable to split in drying.

After the collodion is mixed, and the pyroxyline is dissolved, it should be allowed to remain undisturbed for two or three weeks. This will allow the insoluble particles to settle down, when the clear collodion can be decanted.

In making a plain collodion for emulsion processes, the chief points to be observed are the quality of the pyroxyline used, and the relative proportions of the alcohol and ether. If we take a sample of pyroxyline made at a low temperature and with strong nitro-sulphuric acid containing a minimum of sulphuric acid, it will be necessary to use only sufficient alcohol in the plain collodion to give the requisite solublity. This should be about a tenth or twelfth part by bulk of the amount of ether. Unless this precaution be taken, the iodized collodion will be very easily torn. It will also be so glutinous as to be difficult in working. It will adhere but loosely to the glass, and show peculiar structural markings and lines.

With a pyroxyline made from gun-cotton, with a large excess of oil of vitriol in the formula (see Pyroxyline), it is possible to use a larger quantity of alcohol. This has its advantages, as its contractility is lessened, and its setting properties are not so strong, and markings are thereby obviated. The following remarks upon the preparation of collodion, taken from Hardwich's " Photographic Chemistry," will be likely to be of service to those engaged in its preparation : It will be found that with a tough kind of pyroxyline the solubility is increased by employing the maximum quantity of alcohol, so that if the plain collodion be diluted with ether a precipitate will take place. With other kinds of pyroxyline differently prepared, the addition of ether to the plain collodion produces no precipitate. In the case of pyroxyline prepared in a sulphuric acid containing equal bulks of oil of vitriol and nitric acid, with the maximum of water, it is advisable to reduce the quantity of alcohol somewhat, for if too much alcohol be employed, the setting of the pyroxyline will be so greatly retarded that the upper edge of the film will become dry before the lower part has solidified sufficiently to take the bath without precipitation of pyroxyline. This effect would not happen if the formula containing oil of vitriol in excess be used in the preparation of the pyroxyline, because it would be impossible to use such a mixture in a state sufficiently weak to destroy the property of setting in the resulting pyroxyline; before that point was reached the cotton would dissolve in the acid.

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