In the event of a separation of the gelatine film from its support during development, it is well to place it immediately in an alum bath. After a few minutes the print may be quickly rinsed and put aside to dry. The development may be continued, if necessary, after thoroughly drying.
Washing and the dipping in the alum bath may be slightly shortened, the bichromate being unable to penetrate the temporary support.
Excepting when prints are very urgently required, it is desirable that they should be allowed to dry on the temporary support, after the alum bath and washing, before being transferred. The gelatine which forms the image then possesses greater resistance, and is riot so liable to be torn or spread under the pressure of the squeegee when being transferred to the final support. In addition, the relief of the image being considerably less, contact between the half-tones and the gelatine of the final support is more easily ensured ; these half-tones form depressions in relation to the shadows. Retouching and spotting can also be done before the final transfer, and they are then practically invisible.
Drying should take place at the temperature of the surrounding air, care being taken to prevent the rays of the sun from reaching the print.
66o. Preparation of the Final Support. Double transfer papers of various kinds, thicknesses, and textures are obtainable commercially. They are coated with a thin film of gelatine, not hardened, and meet practically all usual require ments.
All papers of good quality, provided that they do not change by exposure to light or air, can be utilized as final supports. They must be coated on one face with a solution of gelatine to which a very small proportion of alum, and, at discretion, a very small quantity of glycerine, have been added. The object of the glycerine is to render the gelatinized paper flexible after drying it is almost completely removed in the soaking of the support preceding the final transfer.' All other supports on which it is proposed to transfer the image finally must be coated with a solution of gelatine. The solution should be about 6 per cent strength, and it should con tain from 9 to i8 gr. of common alum to each 20 oz. of solution (r to 2 grm. per litre). This
preparation is specially necessary for opal glass, porcelain, ivory, celluloid, or wood. Wood should have the surface made perfectly matt with glass-paper. Before coating canvas pre pared for painting, it is necessary to pumice stone the prepared surface, wash it with a solution of soda to remove any grease, rinse it, and then apply two coatings of gelatine. It should not be stretched on a frame until after the final transfer of the image.
661. Final Transfer. The double-transfer paper should be cut rather larger than the actual size of the image, but smaller than the temporary support.' About half-an-hour before the final transfer —an hour for thick papers—the double-transfer paper should be softened by being immersed. for a few moments in warm water. The tem perature should be from 85° to 95° F., excepting in the case of paper that has been kept a long time, or paper of which the gelatine coating contains a large proportion of alum, when it may be from 104° to 112° F. The paper should then be placed in cold water, in which the gelatine will continue to swell. A few minutes before transferring, the temporary support con taining the image is also immersed so that the gelatine may absorb water. The gelatinized surface of the final support is then brought into contact with the image, 2 care being taken to avoid air-bells forming between them. The temporary support with the final support cling ing to it are brought out of the water together, placed in a firm position on a marble slab or a table, covered with a sheet of india-rubber, and the greater part of the water expelled by light strokes of the squeegee. The sheet of india rubber should be lifted for a moment to ascertain if the final support is in the correct position. This verification can be made by transmitted light if the two supports are translucent. The remaining water is then expelled by somewhat firmer strokes of the squeegee, and the print put to dry spontaneously. If the two supports are flexible, hanging from a line is the preferable method ; if either of the supports is rigid, a draining rack should be used.