In the spring of 1863,[1] M. Poitevin made another important advance in carbon printing, based upon his last discovery,—that soluble organic substances might be rendered insoluble by the action of metallic salts, and recover their solubility under the action of light. It will be seen that in this operation the reactions are just the reverse of those in the chromo-gelatine processes. Paper, coated with the sensitive salt and pigment combined, simply needs exposing to the direct action of light under a transparent positive, and washing in water. The dark coating forming the shadows retains its insolubility; and in the half-tones, the film being rendered soluble through a part of its thickness, is washed away in due proportion, whilst the light having penetrated quite through the film in the whites of the picture, the color is washed away entirely, leaving the bare paper.
The mode of proceeding is as follows: Five or six parts of gelatine are dissolved in one hundred parts of water with gentle heat, and to this the necessary proportion of carbon (or some inert pigment), is added, and the paper is coated with the mixture. When required for use, these are impregnated with a solution containing ten parts of perchloride of iron, and three parts of tartaric acid in one hundred parts of water. This paper is left to dry in the dark, when it becomes insoluble, even in boiling water. It is then exposed under a positive cliché, and, under the influence of light, becomes soluble, commencing at the surface of the film. A short exposure is sufficient, and the print is then immersed in warm water, which removes the soluble matter, leaving the print with its true gradations of light and shade. It is now necessary to remove the tint given by the iron salt to the paper, and this is done by washing in a dilute
- ↑ “Photographic News,” vol. vii, p. 124.
