The cliches may be too intense, either from yellow fog, over-development, or local opacities. We have nothing to say on the means employed to remove yellow fog - they do not alter the character of the picture, nor on those which attenuate the contrasts or local opacities) which, if done by redevelopment, answer pretty we)l; but we think that the reduction of cliches, generally too intense, is effected by chemical actions which are often objectionable. Among the processes employed for the purpose in question, the old one first employed, both in the collotype and the collodion process, consists in treating the cliche with a solution of potassium cyanide. It is the best, on account of its simplicity, and of not requiring much after-washing. The drawback in the gelatine process is that the solution of the commercial article has, on account of its strong alkalinity, a tendency to produce blisters and frilling by softening the film. This can, however, be easily prevented by using pure potassium cyanide in solution in alcoholised water.
The next process in order of publication is that devised by Alexis Gaudin. The reduction is effected by ferric sulphate (Monsel's salt). This process requires to eliminate the iron salt by first washing the gelatine film in a diluted solution of hydrochloric, citric, or oxalic acid, etc. It is little employed. Then comes. Spiller's process, which consists in treating the cliche by a solution of cupric bromide and common salt; that of Farmer, who uses potassium ferricyanide in solution with sodium thiosulphate; and, lastly.
Monckh oven's process, by which the reduction is effected in attacking the silver with ferric oxalate, the silver salt formed being dissolved by sodium thiosulphate, as in Farmer's process. The later processes - those of Farmer and Monck-hoven - require prolonged after-washings to eliminate the thiosulphate. For this reason the process of John Spiller is preferable, as the solvent of the cuprous and silver bromides is simply the common salt, an entirely harmless compound, should any traces of it be left in the film.
As it is seen, in all these processes the intensity is reduced by dissolving the metallic silver forming the image. If employed for the reduction of cliches generally too intense from over-development, the relative value of the local intensities may be little altered, provided the operation be conducted with great care, otherwise - and, indeed, this is of frequent occurrence - it happens that the penumbras and the details in the shadows, being formed by their layers of metallic silver, are dissolved, and, as a consequence, that the cliche, if not spoiled past remedy, has lost that admirable gradation from lights to shades, that delicacy of details, which makes a photograph such a beautiful realistic image of nature. A rational process would be to reduce the intensity not by dissolving the silver, but by transforming the colour of the image into one more actinic, or rendering the cliche more transparent, so that not only the desired de-intensification would result, but if the intensity were too much reduced it would always be possible to strengthen it.
For example, the cliche' previously soaked in water is immersed in a very diluted solution of nitro-bromhydric acid or of nitro-chlorhydric acid (aqua regia). In the solution, the image, viewed by transparency, seems to intensify on account of the formation of the silver bromide (or chloride), but when exposed to sunshine for a certain period, it becomes more transparent from the reduction of the silver haloid, and turns to a blush or violet-black, that is a more actinic' colour than the original one.
The operation should be made by diffused light, and the cliche allowed to dry before exposure to the luminous action, for, if dried in the light, the silver reduction would be of unequal intensity, the parts drying first being the lightest. In fact, when exposed under water, or, better, in presence of an absorbent of chlorine or bromine, the reduction is more complete and darker, which is generally advantageous. Of course, the de-intensification is proportionate to the more or less complete transformation of the metallic silver, the image, into silver haloid. And it is in this where resides the difficulty of the process; it requires some experience to succeed. There is, however, little danger of spoiling the cliche, for should it be too much weakened from being bleached through, the remedy is at hand; it suffices, after the insolation, to redevelop with a very weak developer - pyrogallol, eikonogen, etc, or to intensity with mercuric chloride, or by any other intensifying processes. No fixing is required after these operations.
It is only necessary to well wash the cliche.
We have also experimented with iodo-hydric acid, and, in lieu, with iodine; but the results were not generally as good. The silver iodide does not blacken in the light, and remains apparently not acted on, and when treated by the developer gives rise to a great increase of intensity. This can be regulated during the redevelopment, and an after-treatment with a solvent of silver iodide, sodium thiosulphate, etc. The iodide process has given satisfactory results for the intensification of very weak negatives. As said above, the redevelopment should be done after insolation, and with diluted developers, in order to have the action well under control. - (P. C. Duchochois.)