The Negatives.—For the carbon process the negatives should be intenser than those intended for printing out on silver paper. However, good proofs may be obtained from any negatives, so to say, by varying the strength of the bichromate solution, as, also, by using the tissue freshly sensitized for weak negatives, in order to obtain vigor, and for strong negatives, the tissue two or three days after its preparation, [pg 93] when it yields better half tones. Printing dodges are also resorted to. That the most commonly employed consists to varnish the back of the negatives with a matt varnish, or to stretch on the same a sheet of mineral paper upon which the retouches are made by rubbing graphite, chrome yellow, pink or blue colors to strengthen the shadows or the whites, as the case requires. As a rule, it is advantageous to cover the printing frame with tissue paper, whatever be the quality of the negatives.
The negatives should be bordered with deep yellow or orange-red paper to form what is termed a “safe edge” upon which should rest the tissue in order to prevent the margin from being insolubilized by the reductive action of light. If this precaution were neglected it would be impossible to strip off the paper without tearing the proof when the tissue is applied on the support upon which the image is to be developed.
Before exposing it is advisable to ascertain what the printing qualities of the negative are by making on silvered paper a proof of it—not over-printed—and another of the photometer, both being exposed at the same time and for the same period. This done, compare the proof from the negative cliché with those of the photometer, and mark the negative with the number of that of the photometer to which it corresponds, stating the shade of the proof next to it; for example: No. 2; No. 3 faint, or commences to appear, etc. This No. 2 and the observation will indicate the intensity of the negative and serve as a guide for printing on the tissue, since, as before explained, the silver paper is practically of the same sensitiveness as the tissue prepared for negatives of the ordinary intensity.
Exposure.—To print, the tissue is laid over the negative, taking care that it covers the safe edge, and a strip of silvered paper placed in the photometer, then both the printing frame and the photometer are exposed to light side by side.
Unless the negative be weak, when more vigor is obtained by exposing in sunshine, the printing should be done in the shade. It is a well-known fact that the part of the bichromated [pg 94] film corresponding to the half tones in the lights are not sufficiently impressed in comparison to the blacks while impressed in direct sun's light in this as well as in the collotype, photogravure and other processes with the chromic salts, because the luminous action through the bare glass, or nearly so, which in the negative represent the shadows and half blacks, is more energetic in proportion than through the other parts, from which it results that these parts being most acted on are made deeply insoluble through the thickness of the film, and then require to be cleared by a treatment with water at a higher temperature than the parts representing the half tints in the lights of the picture, which are but superficially and slightly insoluble, can stand.
From time to time during the exposure the print in the photometer is examined, and when a certain picture is printed to a certain shade, or when the one next by commences to appear or is faintly printed, etc., the exposure of the tissue is sufficient. This, as the reader has already inferred, is a matter of experience, the guide being the knowledge of the intensity negative tested as above explained.
Development.—The carbon prints are developed either on a sheet of paper upon which it should remain (single or simple transfer), or on a provisory support to be afterwards transferred on paper or any other material (double transfer).
Simple Transfer.—This process is quite simple: The impressed tissue and a sheet of paper coated with alumed (insoluble) gelatine are immersed face to face in cold water, and when the tissue is softened both are removed, one superposed on the other, and the whole, being placed on a glass plate and covered with a thin oil cloth, is firmly pressed into contact with the squeegee. The rationale of applying under water the tissue on the gelatinized paper is to avoid the interposition of air bubbles.
To operate by simple transfer the tissue should be impressed under a reversed negative. The reason is obvious.
Double Transfer.—By this method the carbon prints are generally developed on porcelain or opal plates, which more easily than glass plates permit one to follow the progress of [pg 95] the development and to retouch the imperfections before transferring the picture on paper.