484. Stripping of Film Negatives. It is some times necessary to strip the gelatine film from film negatives, even although such negatives, being extremely thin, can be printed from either side satisfactorily, provided care is taken to avoid blurring, which might result from imperfect contact between the negative image and the sensitive coating.
For this purpose, it has been proposed (Lumiere, 19°3) to swell the celluloid base by immersing it in one of its solvents, such as acetone, to which enough water has been added to prevent solution.
More recently, the following process has been recommended (Kodak Co., 1922) : The surface of the negative is coated with a 6 per cent solution of gelatine, flowed on at the lowest possible temperature. After drying, the film is placed in a strong solution of forinaline (25 to 50 per cent of the commercial liquid) for about ten minutes, and then again dried. A cut is then made with a penknife, and the negative placed in a 15 per cent solution of acetic acid, when, after a few minutes, the gelatine film can be detached from its support.
485. Transfer of the Film to a New Support. When the film is to be transferred to glass or celluloid, the latter should be previously coated with an adhesive, such as a weak solution of gum arabic, or with a solution of gelatine of about Io per cent strength' poured on while hot and allowed to set to a jelly. The film can also be transferred to a piece of perfectly clean glass which has been polished with French chalk or coated with a solution of wax in ether; after drying the film on this temporary support, it is coated with a syrupy solution of rubber, allowed to dry, and finally given a coat of collodion or celluloid varnish.
Either before or after transferring to the final support, the film should be washed with water to get rid of the salts used in the stripping process ; when the washing is done before trans ferring, a piece of plain glass, larger than the film, is placed on the bottom of the dish on which the film can be lifted out without risk of tearing it.
Care should be taken to put the right side of the film in contact with the support. The side will vary according as the film is placed on a temporary or a final support, or if it is to be used for making direct or reversed prints. If,
when the film has been taken off on a piece of paper, the upper surface is not that which must come into contact with the support, it can be transferred to another piece of paper and then applied to the prepared surface of the final support, which has been evenly wetted (or coated with a solution of gum, if this adhesive is used). The excess of liquid is removed with a squeegee or a rubber roller.
486. Removal of the Gelatine Coating on the Back of Film. When the gelatine coating on the back of a film has been stained or scratched it can be removed fairly easily (j. I. Crabtree and F. E. Ross, 1926). To do this, the emulsified side of the film is applied to an adhesive surface, and perfect contact is ensured by passing a soft rubber roller or squeegee, care being taken that there is no defect in contact at the edges of the film in a width of about A- in.
The waterproof adhesive surface can be a glass covered with rubber (coated at least 3o minutes previously with a thin layer obtained by evaporating a syrupy solution containing at least 5 per cent amyl acetate), a glass covered with a coating containing vinyl resins (§ 283), or surgical rubber strapping. The back surface is then moistened with a mixture of equal volumes of water and acetone (§ 484). When the work is finished the film is separated from the adhesive and cleaned with petrol.
487. Removal of Films from Waste Glass and Celluloid Negatives. In cleaning off the films from old negatives and for recovering the silver contained in the films, the simplest method consists in softening the gelatine by long immersion in water to which a small quantity of caustic soda has been added, and then dis solving it by placing in very hot water. The partly melted gelatine is then scraped off with a piece of wood, and the cleaning of the support completed by leaving it for a minute or two in a very weak solution of eau-de-javelle (sodium hypochlorite) followed by rinsing in cold water.
Another method is to soak the gelatine in a solution of soda carbonate of about io per cent strength to which a little caustic soda has been added when dry, the film is stripped off by treating with hydrochloric acid, it being of course unnecessary to take any of the precautions given above for proper stripping.