CARBON PROCESS The idea of the carbon process as it is known to-day is credited to A. I,. Poitevin, who, in d. patent dated December 13, i855, describes the action of light upon a chromated gelatine mixed with d pigment. J. Pouncy is supposed to have been the first actually to produce carbon prints, his patent being dated April io, 1858. In these early processes the half-tones were mostly unsatisfactory, and modifications were made by J. C. Burnett (1858) and Fargier (186o) ; but it was not until J. W. Swan's improvement in 1864, when he patented carbon tissue, that the process became of any practical use to photographers. J. R. Johnson made further improvements in 1869, and in 1874 the flexible support used in the process was patented by J. R. Sawyer.
The carbon process differs essentially from all other methods of photographic printing. It depends for its working on the fact that gelatine, to which has been added a suitable proportion of an alkaline bichromate, becomes insoluble when exposed to light, but retains its solubility if kept in the dark. A sheet of paper is coated with a mixture of gelatine, colouring matter, and potassium bichromate, and then exposed to day light under a negative. The portions of the gelatine film that were protected by the high lights or dense parts of the negative retain their solubility, while those that receive the full force of the light through the shadow portions become insoluble. Parts exposed under the intermediate tones become partially soluble. By treating the film with hot water the soluble portions are dis solved away, while the insoluble parts remain, and form the picture. Any colouring matter may be employed, and consequently a picture may be produced in any desired colour.
Carbon tissue is a dark-surfaced paper, the colour corresponding to the deepest tone that can be obtained in the picture. No visible image is produced by exposure to light, and consequently, the exposure in the printing frame must either be timed or gauged by an actino meter. Carbon tissue sensitised in H. W. Bennett's sensitising bath requires about half the exposure necessary for printing-out silver paper to the full depth necessary for toning, or rather less than the printing-out paper requires to give a visible image resembling a finished result. The prints should be developed as soon as possible after taking from the frame (see " Continuing Action "). A piece of single transfer paper, slightly larger than the exposed print, is also required. The exposed film must be developed from the back, for the reason that the whole of the face has been rendered insoluble, excepting the extreme high-lights, while all the surface in contact with the paper has remained soluble. The film is so thick that the strongest shadow does not penetrate right through. In the deep shadows the insolubility penetrates deeply ; in the medium tones there is a moder ate thickness of insoluble gelatine, while in the very light parts there is simply a slight super ficial insolubility. This layer of soluble gelatine of varying thickness underlying the insoluble image necessitates transferring the film so that the soluble portion becomes the surface. Any attempt to develop the film on its original paper would result in its floating right off as soon as the soluble gelatine commenced to dissolve.
Transferring and developing the exposed print are simple matters. The piece of transfer paper is soaked in cold water until limp. The exposed print is then immersed in cold water for a few seconds, until it begins to become flat, and its face is then brought into contact with the pre pared surface of the transfer paper, preferably under water, the two papers brought out together, squeegeed firmly into contact, and then partially dried between blotting-paper under moderate pressure for ten or twelve minutes. The print is now ready for development. It is placed in water that has been warmed to io5° to h to° F. to C.), and kept below the surface. In about twenty or thirty seconds some of the colour will be seen oozing from the edges. This is the object of the safe edge, to preserve a mar gin of soluble gelatine ; without it the next operation would be impracticable. As soon as the colour is seen to be oozing out, the corner of the paper bearing the film is lifted, and if it comes away easily it is pulled steadily away, leaving the film on the transfer paper. This film is allowed to soak for a few minutes in the hot water, and from time to time the water is gently splashed over it, and it is taken from the water and partially drained. This treatment is continued until the print is sufficiently light, when it is drained thoroughly and then rinsed in cold water to wash off any loose gelatine and colour that may adhere. It is next immersed for about five minutes in an alum bath (I oz. to zo oz. water), washed in about three changes of water, and dried. With the exception of the developing bath, all the solutions should be cold. Care must be taken to avoid touching the sur face of the film during the working.
An objection to this method of working—the single transfer method—is that the picture is reversed, the left side becoming the right. When it is desirable to avoid this a second transfer is necessary, and the method is known as " double transfer." Instead of using the single transfer paper, a temporary support is substituted (see ` Flexible Support "). The method of working is exactly the same as described for single transfer, excepting for the preliminary waxing of the sup port. After development, treatment in the alum bath, and drying the print on the temporary support, it is ready for the second transfer. The drying on the temporary support must not be rapid, and the transfer should take place as soon as possible after drying, or else the print should be kept in a cool place, moist rather than too dry. A piece of final support or double transfer paper is soaked in cold water for at least half an hour, and then the temporary support bearing the print is similarly soaked until quite limp. Both are then immersed in warm water, about 90° P. (32° C.), for about fifteen or twenty seconds, face to face. Then they are withdrawn dinging together and squeegeed into good con tact. When thoroughly dry the two papers may be pulled apart, and the image will be firmly and permanently attached to the double transfer paper. The necessity for the second transfer is frequently obviated by the employment of a reversed negative. (See also " Carbon Tissue," " Carbon Transfer Papers," " Flexible Support," " Bennett's Carbon Sensitiser," etc.)