I.Cupric sulphate200g2oz.
Water1000ccm10oz.
II.Potassium bromide200g2oz.
Water1000ccm10oz.
III.Cold saturated solution of potassium bichromate.

A concentrated bleach is made by mixing:

Solution I.3 parts
Solution II.3 parts
Solution III.1 part

To every 100 ccm of this mixture should be added 10 drops of pure hydrochloric acid (10 drops to 3½ oz.). This concentrated bleach will keep indefinitely and should be diluted before use with three to four times its volume of water. The use of a more concentrated solution is not advisable, as irregularities frequently occur in consequence of too rapid bleaching, especially towards the margins of the prints.

The color of the concentrated bleach is green, or when diluted, yellowish; the solution must be absolutely clear. When the stock solutions are mixed there is usually some cloudiness, but this is cleared up by the hydrochloric acid. By standing for a long time at low temperatures a precipitate is sometimes formed, but this is of no moment. The compounding of this bleach should be made with the greatest accuracy. Inaccuracies or modifications in its composition are serious, because although the solution does not lose in bleaching power, yet the invisible tanning action is then often not completed in the desired manner. Too great an addition of hydrochloric acid for example, accelerates the process of bleaching, but the inking-up of prints thus bleached is frequently difficult. If the bleaching of the shadows of the bromide prints goes on slowly, the reason as a rule lies in the fact that the prints were overdeveloped and have an excessively dense silver deposit.

The bromide prints should be immersed in this bleaching solution, after previous soaking in cold water. If they have been correctly made, the image rapidly grows weaker and after a few minutes its greyish-black color changes into a pale citron yellow. If the bromide print was developed too far, the bleaching takes rather longer, as the shadows, developed right through to the base, require a lengthy period for solution. If several prints are to be bleached at once, the best procedure is to place one print in the solution and turn it film side down when the first traces of bleaching are noticeable. Then the next print should be immersed with the film up and by thus proceeding gradually it is possible to bleach a large number of sheets simultaneously in the one dish. Continual movement will prevent the formation of air bells. If air bells adhere to the film, they protect those places from the action of the bleach and dark points or spots of unchanged metallic silver remain, the subsequent bleaching of which naturally prolongs the process. The same applies to prints which lie on top of one another.

With too slow bleaching, the hydrochloric acid may be gradually increased, at the most to double that prescribed; one should not hasten the bleaching process by warming the solution. The bleaching is rapidly effected in warm solutions; yet generally the film of moderately hardened papers is so altered that they swell up too much even in cold water and take the ink badly or not at all. The dilute bleaching solution will keep and may be used repeatedly as long as it acts; when it becomes exhausted, the slowing up of the bleaching cannot be hastened by the addition of hydrochloric acid. The chemical reactions in the bleaching bath are, according to Dr. P. R. von Schrott, as follows:

2CuBr₂ + Ag₂ = 2AgBr + Cu₂Br₂

The cuprous bromide, Cu₂Br₂, which is formed, reduces the bichromate as follows:

3Cu₂Br₂ + 6CrO₃ = 3CuBr₂ + 3CuCrO₄ + Cr₂O₃.CrO₃