Now two methods of working are open to the photographer, either he may sensitize his paper in a ten per cent. solution of Bichromate by immersion during one or two minutes, dry, and then coat with gum and pigment, adding to the mixture a proportion of water equal to the quantity of fluid that the Bichrornate solution would have brought if it had been added to the gum and pigment, or he may coat his paper direct with gum, pigment and Bichromate in the proportions described above. In both cases the degree of thickness of the mixture must be tried before coating, and this operation must only be performed when the mixture has shown a satisfactory behavior under this preliminary trial. It is not difficult to judge. A mixture clinging to the brush and forming ridges which cannot be softened by repeated brushings must be diluted—while if it runs over the paper, refuses to set and follows the brush in waves, gum must be added.
I do not say anything about the proportion of pigment—this is a question of personal taste.
Coat your paper in full diffused light or by bright gas light. Pin the dry sheet on a drawing board, take a flat hog's-hair brush (the fan shape seems to be the most convenient) smear the surface roughly with the sen sitive mixture, taking care not to use more than is necessary for the complete covering of the sheet, or else the coating will be too thick. This rough coating is covered with ridges and irregular brush marks; do not let the marks set but take up a goat's-hair softener and give a few strong down ward vertical strokes which will change the irregular ridges into vertical parallel lines—break these lines by several horizontal strokes perpendicular to the first—the lines will merge into one another and disappear. A few rapid and light touches here and there will finish the operation, which ought to be rapid and decisive.
Pin the paper up to dry in a dark place—absolute darkness is not necessary—and when bone dry, expose.
Correct exposure, I mean the right exposure for the desired effect, is the delicate point of the process. It can only be got at by comparative trials. I have seen men disgusted by their constant failures, who, as I found out later on, had never made these comparative trials on the same negative. They had tried every possible change in their way of coating and their proportions of mixture, but had never had the idea of trying two exposures—one of ten minutes and one of two hours—to ascertain how gum-bichromate paper acted under extremes. It is, notwithstanding, the only sure way. An actinometer is necessary, of course, and the bands, numbered, may be kept for future reference.
The average exposure in summer by diffused light for a thin negative would be twenty minutes to half an hour. Length of exposure is influenced not only by the quality of the light, the color and density of the negative, but also by the thickness of the sensitive coating. It is of course in direct ratio to this thickness.
To be able to fully understand the importance of control in the devel opment of a gum-bichromate print we must realize that the film or coating on which we are going to operate is composed of a substance uniform in appearance, but entirely soluble or semi-soluble in some parts, and insoluble, or nearly, in others. If we immerse this coated paper in a dish of water and let the solvent act undisturbed on the whole surface of this coating, it will dissolve it proportionately to the extent it has previously been rendered insoluble, and it will give us a positive duplicate of the original negative. If, on the contrary, we apply the solvent irregularly to different parts of the coated paper, if we use hot water there and cold water there; if, going even further in our personal intervention, we add local friction to the dissolving action of water locally applied, we produce a positive which has not been developed in proportion to its solubility, but proportionately to the temper ature of the agent used in developing and to the force and frequency of its application—consequently it is not a duplicate of the original negative as to tone and values, but the result, good or bad, of our own judgment.
In reality the technical or photographical part of the process is at an end when development begins. Development requires no chemicals, and no formuLe to mix them. All you have to do is to wash away, rub away, or scrape away, according to your mood, the more or less soft pigment attached to the paper. You can develop in ten minutes or several hours, wash away one side of your picture before developing the other, work with a brush, a spurt of water, or a gentle flow. So there are no rules for devel oping save those by which artists of all crafts ought to be guided.
The only indications which could be useful to a beginner are the fol lowing: Always develop the print out of the bath of water; if it is immersed it stands to reason that local development is impossible. Place the sheet of paper on a glass plate propped up at one end by some sort of wooden contrivance, the other resting on the bottom of the developing tray. Always begin by cold water, and never use higher temperatures until you have ascertained that a low temperature has no effect whatever.