When a bichromate salt is mixed with an organic substance soluble in water, like gelatine, the latter has a tendency to absorb oxygen from the former, and also to become insoluble; this action is enormously accelerated by exposure to light. The carbon process, and various other processes allied to it are based on this principle of the hardening of bichromated gelatine under the influence of light. Colouring matter is mixed with the gelatine, which is spread over a sheet of paper and exposed under the negative; after which the unexposed gelatine is washed away, leaving an image in pigment similar to that used by the painter.

Printing in carbon is very simple and well adapted for the amateur worker. Except for sensitising, no chemicals are necessary, the developing agents being hot and cold water. There are no deleterious solutions requiring prolonged washing. The image is in good mineral pigment, capable of surviving the paper on which it is supported, and the various stages follow readily and are easily acquired. Bichromated gelatine is sensitive to the green rays of the spectrum; carbon tissue, therefore, needs only half the exposure of most printing-out papers. There are only two difficulties - that the paper will not keep for more than a fortnight after being sensitised, and that correct exposure cannot be judged by examination of the print itself.

Carbon Tissue

The pigmented tissue is rather troublesome to prepare, the proportions differing considerably, according to the season of the year, and according to the particular colour. An average mixture may be:

Water......... 8 oz.

Gelatine......... 2 1/2 „

Soap......... 120 gr.

White Sugar ........ 140 „

Dry Colour........ 80 ,,

The soap and sugar are dissolved in the water and the gelatine soaked till soft, when it is melted over a water bath. Meanwhile the pigment is ground up in a mortar or mill with some of the jelly; lastly the whole is stirred together and filtered, when it is ready for coating on the paper. The tissue may be obtained in all desired colours, ready cut to size from the leading manufacturers. It can only be supplied in the sensitive state by special order, owing to the fact that it must always be used within a fortnight after sensitising.

Sensitising

Float the tissue face downwards in a wide dish containing sufficient solution of bichromate of potassium (1 oz. bichromate to 20 oz. of water) which should be nearly neutral, although many contend that the tissue will keep better if rendered alkaline with a. few drops of ammonia. Agitate the dish to remove bubbles, lift and examine after a minute, and if any air bells remain sweep them away with a brush. Three minutes will be sufficient, after which the sheet may be squeegeed face downwards upon a clean sheet of glass or ferrotype, and dried in some safe cupboard free from chemical fumes or light of any kind. Tissue prepared at night should be dry and ready for use the following morning. Tissue must always dry quickly or it will be found to get tough.

Printing

Before being placed in the printing frame the negative must have a "safe edge" formed on it with a border of black varnish or black paper round the margin about a quarter of an inch wide, to prevent the pigment from splitting at the edges during development. A soft and pleasing effect is got by making this safe edge on the glass side of the plate. It is not very easy to detect any change during exposure. With light colours, such as Bartollozzi red, the faint outline of the image becomes visible; some printers claim that by breathing gently on the tissue they can tell when it is sufficiently exposed. Ordinary mortals must be content to work by an actinometer. Any good negative, with fair contrast and gradation, will give passable results on carbon paper.

If a print is removed prematurely from the printing frame and put away in the dark, the action will continue and after a few hours be found sufficient for development. Curiously enough also, if a fully exposed print is compressed in the copying press, in contact with a piece of sensitised tissue, a second image may be secured, though of course a reverse in direction. Probably the reason of this phenomenon is that certain of the subsalts, particularly the quadruple chromate chromic oxide, exercise a diffusive action through the comparatively moist layers of gelatine.

Development

The tissue must be developed, either by artificial light, or in a room protected by blinds, or otherwise, from strong daylight. The face of the tissue has become practically insoluble and, in order to develop it, we must transfer it to another surface and meanwhile remove the soluble pigment at the back of the film. This surface is termed the temporary support, and may be either a polished zinc plate, glass, or prepared paper according to the surface desired. The zinc plate, if employed, must be very smooth and especially free from scratches, each one of which would betray itself on the final print. The flexible temporary support is the best for many reasons - a smooth shellac-coated paper, which some hours before use is coated with a waxing solution.

Resin . . . . . . . . . 18 gr.

Yellow Beeswax . . . . . . 6 ,,

Spirits of Turpentine...... 1 oz.

Or celluloid sheets rubbed over with a piece of flannel steeped in:

Spermaceti . . . . . . . . 20 gr.

Benzole......... 1 oz.

Or, in default of these, common rubber solution will do. Remove the print from the printing frame and place face downwards in cold water, with a piece of flexible support of rather larger size. When the tissue begins to uncurl and float flat the two surfaces are brought into contact under water, and squeegeed together on a plate glass or other level surface, after which they are laid between blotting paper for about a quarter of an hour, or longer if desired, with a weight, such as a large book, over them.