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Biciironlated Gelatine 674

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BICIIRONLATED GELATINE 674. General. If bichromated gelatine is coated on a support impermeable to aqueous solutions, dried, exposed under a positive, and then rinsed with water to remove excess of bichromate, the aqueous solutions of certain dyes will penetrate the gelatine where it has been protected against the action of light, but will not penetrate the relatively impermeable regions. The dye absorbed by the gelatine thus forms a positive image in tones (E. Edwards, 1875 ; C. Cros, 1880).

On the other hand, a gelatine relief, obtained in the same way as a carbon print, but by means of unpigmented gelatine (A. Chardon, 1875), or of gelatine pigmented temporarily for the pur pose of regulating the action of the light or of facilitating control of the development (A. and L. Lumiere, 1900), may be transferred to a support, or actually made on an impermeable film support, and dyed uniformly by means of dyes, which are readily fixed by almost im permeable gelatine. In this case, as in making a carbon print, a positive is obtained by printing from a negative.' It was found by Cros that the dye thus absorbed by the gelatine could be transferred to moist paper placed in contact with it, the one film of gelatine being able, after fresh impreg nation, to furnish a certain number of prints. He gave this process the name of hvdrotype. Sanger-Shepherd (1902) made a similar observa tion in the case of images obtained by dyeing a gelatine relief, using a paper coated with very soft gelatine for taking impressions. L. 1)idier (1903) worked out the dyes best lending them selves to selective absorption on undeveloped gelatine, and to the subsequent ' printing " by contact with gelatine-coated paper. This method has been put on the market under the name of Pinatype.

These methods are employed chiefly for obtaining multicolour transparencies for use as advertisements and for the production of three colour pictures on paper. The Pinatype process, by using a black dye, may be employed for mak ing a reversed negative or positive in one operation.

The images thus obtained, even when the further precaution is taken to mordant the dyes absorbed by the gelatine, are never quite fast, and so should never be exposed to the direct sun., 675. Imbibition without Development—Pina type. Fogged or doubtful dry plates may be used after removal of the silver bromide. After fixing with hypo and washing, the plates are dried, then sensitized in a 2-5 per cent solution of ammonium bicln ornate (neutralized by the addition of ammonia until it turns bright yellow), and then put aside to dry ; or the glasses from waste negatives may be gelatine coated. The perfectly cleaned glasses are first coated with a very weak solution of sodium silicate (commercial solution diluted about fifty times with water) in order to increase the adher ence of the gelatine to the glass, drained, and dried. The glasses are then levelled, silicated side upwards, and coated with a solution of hard gelatine (4 per cent for transparencies or 8 per cent for a Pinatype printing plate) in the pro portion of £3 to 18 dr. per sq. ft. After drying, the gelatine-coated plates are sensitized in the bath given above) After exposure under a positive transparency (or under a negative when making a reproduced negative), the printing being controlled with an actinometer, the plates arc washed for five minutes. To avoid diluting the dye, the plates are best dried before dyeing.

When it is not desired to print the image by transference of the dye to sized paper, the dyeing may be done in approximately 2 per cent solutions of one or another of the following dyes (L. Lemaire, 1911), or in suitable mixtures of these dyes— Red : Ponccau extra, lanafuchsiu.

Yellows: Solid acid yellow, quinoline yellow t ;reens : Solid bluish green, naphthol green. Bloc: Diamine blue.

Violet : Lanacyl violet.

Bluish or purplish blacks : Naphthol black, naphthaline black, amine blacks.

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