If bichromate of potash is mixed with an organic substance such as gelatine, that gelatine becomes insoluble after exposure to light, and if that gelatine carries with it a pigment, then on becoming insoluble it holds the pigment with it. If now, paper or other material be coated with bichromate, gelatine, and pigment, and exposed to light under a negative in the usual way, the thin portions of the negative will admit of the light acting on this coating and making it insoluble, whilst the parts which are protected from light, as for instance the sky or white objects, will remain unchanged and soluble, and on being washed in water will dissolve away, leaving white paper, whilst the light-affected portions which have become insoluble remain in proportion as the light has penetrated the various densities of the negative. This then is how we obtain our print.
For fuller explanation of the paper or "tissue" and its manufacture the reader is referred to the next article.
The beginner will certainly first obtain his tissue ready made, and he can purchase it ready sensitized or otherwise. The former will be best at the outset, but it must be borne in mind that it should not be kept longer than can be helped before use, and never more than ten to fourteen days at the utmost. Various shades of blacks, browns, and reds are the usual colours, also grey, green, and blue. The tissue is rather more sensitive to light than silver paper, and should therefore be opened and handled in subdued light. It must be kept as dry as possible. A rather vigorous negative is best for carbon printing, one not too strong in contrasts. Before placing the negative in the frame, we must give it what is termed a "safe edge." This is done by making a narrow border, say of about a quarter of an inch or less, round the negative, either on the glass or film side, with opaque black varnish, or it may be done by gumming on narrow strips of paper, such as lantern-slide binders. If binders and not black varnish are used, they must be applied to the glass side.
The Carbon printing paper which will hereinafter be called the "tissue" will be found to present an unpromising appearance, and as the coating is the full colour of the pigment in which the print is eventually to appear, it follows that the progress of printing will not be visible, and a mechanical means of gauging the exposure must be resorted to. An actinometer, similar to that described in the article on Platinotype, will do, and another and simpler form is described in the next article.
Printing will occupy about one-third of the time occupied by gelatino-chloride of silver paper.
Development is conducted in daylight, but not too close to a window.
The absence of chemical solutions has been suggested as an advantage, in this process the developer being merely hot water.
It is not necessary to have this laid on, a can of hot water close at hand and a kettle on the fire or gas stove not far off are all that are required.
We shall require four or five dishes, one at least of which should be a good deal larger than the size of the prints we are to develop and several inches deep—a good-sized pie-dish or a basin will do.
Development merely consists of washing away the unaffected and therefore soluble coating, but it must be remembered that the less affected portions representing the half-tones have received their modicum of light on the surface, and therefore the soluble part of the film is underneath the part that has like a surface skin become insoluble. This necessitates the printed film or tissue being transferred to another paper or "support," so that we may develop or wash away from the back.