Carbon photographs admit of colouring in oil, water, or powder colours, without risk of damage; the manipulation is easier than that upon albumenised silver prints. Powder colours adhere very readily to the surface of these prints. By breathing on the picture a still more adherent surface is obtained.
The water colours take kindly without any preparation, washing well, and permitting tint to be worked over tint without difficulty, and the surface may be made more pleasant for working on by the application of a coating of sizing preparation. The plain carbon print so treated acquires an even, clear surface, losing all gloss without any loss of depth or trans-parencjr.
The best mode of preparing a carbon print for the reception of oil colours is by sizing it with isinglass. A solution of about 2 per cent. of isinglass in equal parts of hot water and spirits of wine, carefully applied, not too hot, to the surface of the carbon print, with a flat camel-hair brush, yields a surface upon which oil colours work admirably.
In the ordinary process of re-touching carbon prints, to remove small imperfections, it is only necessary to use the proper colour in the usual way; if a little gelatine, with a trace of a chromic salt, is employed with the colour, it will, when dry, become insoluble like the rest of the picture. If the re-touching is effected with the same materials before transferring the print, it will, when the picture is finished, be under the image, and no inequality of surface, usually apparent after touching, will be seen.