Before the Shanghai Amateur Photographio Society, Mr. J. H. Longhurst Fast night read a valuable paper illustrated' by some beautifully executed demonstrations, on the Carbon Process, Mr. Longhurst said—When one cames to explain all.the essential instractions for.carbon printing, it is surprising how. little there is to say, and yet it is the most useful and probably most. artistic printing. process in the whole range of photography.
In considering the process I have not thought it necessary to institute any comparisons between thia and. other printing processes; my aim is.to try and give those nointed with the, process some idea of how it is to be worked. It is certainly unique in. some respects. For instance, the wide range of colours in which punta may be produced; the endless variety of papers, or, other material, of the worker's own choice, thet may. form the support, for the image; the absence of all chemical aolutions, stains, ete, clean water and little powdered alum being. all that is. required. It may be mentioned, also, ed! that the prints are absolutely permanent.
Described in a general way, the carbon is a printing method, which, by the usual exposure and _development, gives a picture in which the image consists of a ent pigment embedded in a thin film or tissue of insoluble gelatine.
The principle on which the process is based is simple enough, anda brief general outline of this may help you to follow the description of the various operations better, and enable you to understand more rosily the reason for some of the details given, Gelatin ig its normal condition will absorb.cold water readily, but, will not melt; in hot water, it dissolves easily. If it is treated with potwsiim, dichromate, exposure to light so hardens it that, hot water is no longer. capable of dissolving Without exposure to light it retains its soluble character.
In preparing the film or tissue, coloured pigments are incorporated with the gelatine which is then treated with potassium bichromate, and after exposure to light under a negative, those parts of the film which remain, soluble are washed away by simple treatment with hot water, The degree of ingolubility being in proportion to the amount of light that has acted on it: the hot water causes more of the film to be dissolved away in parts forming the half tones than in the shadows, while still more is dissolved in the high lignite. In dissolving away these soluble portions, the colour that they contain is washed "away leaving only the colour and gelatine where the latter has been hardened by exposure, the result being a picture in coloured gelatine.
To enable you to clearly grasp this, I will use a rough sketch, to illustrate the effect of light action on the tissue (Fig L)
Text read from the printed page and cleaned; old type leaves the occasional error.