699. Bleaching the Image. The methods proposed to ensure the tanning of the gelatine at the points where it encloses the silver of the image, in the course of which the image is bleached by conversion of the metallic silver into silver bromide, give practically the same results.
Bleaching may be done as soon as the print has been washed free from hypo ; or it may be done after drying, in which case the operation may be postponed to any convenient time. Excellent results have been obtained on bromide prints several years old.
It is usual to employ for this operation a bath containing cupric bromide (or an equivalent mixture of potassium bromide and copper sulphate)' and a bichromate or chromic acid. The use of an acid bath, sometimes recommended for bleaching prints with excessive contrasts, tends to induce, by dissolution of the cuprous bromide or chloride and distribution of this salt throughout the layer, a general tanning of the gelatine ; subsequent swelling, and, therefore, inking, is then much more difficult. The most that should be done is to add a trace of acid to a very hard water in order to neutralize it, but it would be better to use boiled water. Amongst others the following bath may be used, pre pared only at the moment of use in sufficient quantity for the treatment of one print, and made fresh for each succeeding IO% solution copper sulphate . 5 oz. (250 c.c.) io% solution potassium bromide io oz. (5oo c.c.) 1% solution potassium bichromate 2 oz. WO c.c.) Water, to make 20 OZ. (1,000 C.C.) In this bath the image gradually weakens, and in a few minutes disappears almost com pletely if the bath is not too cold nor the blacks too dense. Wait until bleaching is complete before removing the print, and in any case keep it in the bath for at least 4 minutes to prevent irregular action. 2 If bleaching takes more than live minutes the bath is probably exhausted and should be replaced by a fresh one.
The bleached print is washed in several changes of water until the water is no longer coloured. It is then placed in a very dilute bath of sulphuric acid 3 (about i per cent) in which the last traces of the image disappear, the only sign left being a slight relief of the high-lights. A more concentrated bath weakens the tanning of the gelatine, especially in the high-lights, as does also a bath of hydrochloric acid. Then follows a further washing in several changes of water.
The print contains, in the state of silver bromide (liable to darken in light), the whole of the silver which formed the original image. Darkening of this silver bromide would produce a double tone in the image and would handicap the inking of the prints to be transferred. It is, therefore, best to dissolve this silver bromide in a fresh fixing bath (solution of hyposulphite, alone or with the addition of bisulphite but with no alum). After the usual washing, the print
should be put to dry. This drying allows the gelatine to stiffen and prevents various failures in the inking process.
70o. Swelling of the Image. Inking must be preceded by a swelling of the gelatine, causing the image to appear in bas-relief. The best results are generally obtained with the maximum degree of The process can be carried out indiscriminately on a print which has just been bleached, washed, and dried, or on an old print even one several years old.
To ensure the proper degree of swelling, it is usually sufficient to leave the print in cold water for several hours, or in tepid water F. to F.) for a quarter or half an hour. It is also possible to hasten the swelling with a solution of ammonia (4 per cent to 10 per cent of am monia), the print being rinsed afterwards in cold water. Finally, if a transfer press is available (§ 689), the wet print may be pressed between two pieces of dry blotting-paper, the print thus acquiring, on re-wetting it, a more accentuated relief, according to the number of times it is put through the press (§ 687).
701. Inking, Finishing, and Transfer. The inking of Bromoils is done as for gelatine-coated papers (§ 685), but usually a slightly softer ink is Inking is rendered more difficult according as development of the bromide print has been forced (higher Watkins factor). On the other hand, if development is incomplete, the gradation will be deficient and much detail lacking.
As already noted (§§ 686-690), prints may be or also used for transfer. Bromide prints intended for transfer must have been printed reversed, preferably by projection.
Mention may be made of the possibility of dissolving the gelatine in the whites of a Bromoil by treating it with very dilute solution of sodium hypochlorite (G. Underberg, 1925). This process inevitably causes a certain eating-out of detail in the high-lights.
Finally, mention may be made of the possi bility of transferring ink impressions in succes sion on to the same sheet of paper. The same bromide print may be used, or several prints from the same negative, and the inks may differ only slightly in colour, but be of varying con sistencies, for double-tone effects, or may be of different colours for multicolour effects. Such effects are admissible only when they are perfect, and it is therefore necessary to be sure of one's ability before attempting work which is, as a rule, successfully accomplished only by those able to achieve the results without any photo graphic base at all.'