Another process, which while actually a carbon process, employs no light for the insolubilization of the gelatin, relying upon the action of finely divided silver on the bichromates, which causes insolubilization of gelatine in which the silver is imbedded, is known as the Raydex process. This peculiar action of silver on the bichromates was discovered by Howard Farmer in 1889, and for years lay dormant, no practical application of it being made. Briefly the process is as follows; the three constituent negatives are obtained in the usual way and, from these, prints are made on a special bromide paper. These prints are immersed in water and brought into contact with the colored pigmented tissue which has been previously soaked in some special solutions, where they are left for a short time. The print is then separated from the tissue, which is squeegeed to a temporary support, developed with warm water and then superimposed in register as described above for straight carbon work. The actual formula for the solution has never been published, but in all probability it is a compound of cupric sulphate and bichromate. The instructions issued by the Raydex Company may be summarized as follows: from the three constituent negatives three prints must be made on the special bromide paper, giving the same exposure to each; to get the correct exposure, make a trial print from the negative taken through the green filter. The exposure required to obtain a rich black, with clear highlights, and which will develop out, that is, to a point where on continuing development no apparent further action takes place, is the correct exposure for the three negatives. Development should not be too rapid, as this gives weak colors. Wet the three exposed prints and develop side by side with any good metol-hydrochinon developer. After development, transfer without washing to an acid fixing bath for at least ten minutes and then wash thoroughly and dry, or use at once for making the color prints.
Three transparent supports must be waxed at least half an hour before use. Soak the bromide prints in water and when limp place them face upwards on clean glasses (old negative glasses are suitable). The prints should be trimmed smaller than the color sheets. The back of the yellow color sheet is slightly damped and then immersed for two minutes in a special solution, then rinsed and placed on top of the correct bromide print under the surface of water, lifted out and squeegeed into contact, and set aside for twenty minutes, when the action is complete. The same procedure is gone through with the blue and the red. The color sheets should be trimmed to within about one-eighth of an inch of the size of the bromide prints and then the pairs pulled apart. Squeegee the color sheets into contact with the dry waxed supports and allow to remain for ten minutes, develop in water from 430 to 490 C. (100° to no°F.), and when development is complete rinse in clean water at 380 C. (900 F.), and hang up to dry. The bromide prints after washing can be redeveloped and again used. Single transfer paper is soaked in water for half an hour, and the yellow print squeegeed thereon and dried. The blue and red prints are painted with a special liquid cement and dried. When the paper and the yellow print are perfectly dry the paper is stripped, and the surface freed from wax by treatment with benzol. The blue print on its transparent support should be soaked in water for a few minutes with the yellow print and the two brought into contact, lifted out and lightly squeegeed into contact and dried. When dry treat with benzol and transfer the red print in the same way. The prints are reversed but for many purposes this will be of no moment. An alternative process is given, in which the prints are produced on glass and stripped with hydrofluoric acid, but this seems rather tedious. It will be seen that the process is in its essentials the carbon process pure and simple, with the important modification that the light exposure is eliminated. Some excellent results are possible by this method. The fact of being able to make the carbon prints at night is a great advantage, and also that having once gotten good bromide prints almost any number of duplicate color prints can be made from them.