On a drawing-board place a thick Bristol board (a Bristol board may be used a long time for papers of the same size), to which Iix, with four drawing-pins, the paper to be coated.
The brush is well charged with the mixture, and lightly scraped on the edge of the bowl so as to contain as nearly as possible just the quantity required to cover one sheet of paper. It is rapidly drawn the long way of the paper, the trail of mixture thus deposited being immediately spread over the whole surface of the sheet of paper by cross movements, without leaving any bare spots. The excess of mixture is immediately removed, and the coating is smoothed over with a flat brush, either hog-hair or goat-hair, fairly thick and not too supple. The coating being thus uniform, it is finished by being smoothed with another brush, similar to the preceding, which is used with less and less force on the paper, while the hoard is turned in all directions. (For this purpose the board may be placed on a coin.) Coating should not last much more than one minute ; a little less if possible.' The sheet dries in about 15 minutes. Hold it near a stove or pass it above a flame, if necessary, until the paper shows a tendency to crack.
Prepare only the number of sheets necessary for one day, or, at the most, for the next day.
669. Exposure and Development. There being no transfer of the picture, printing is done under a negative in the usual way. Printing must be controlled with an actinometer ; the time will depend on both the colour of the pigment and the thickness of the coating at the points where it is thickest in the hollows of the grained paper. Thus, it also varies with the grain of the paper, a coarse-grained paper possibly needing a time of printing double that which would suit a smooth paper. And, finally, it varies with the age of the paper. Paper which has just been prepared should be exposed a little less than paper prepared the night before.
The finest blacks are obtained, as a rule, with very slight under-exposure, the picture then developing in cold water in a very short time ; over-exposure gives harsh pictures, with greatly pronounced, and often irregular, grain.
Development must be done very shortly after exposure to the light.
For automatic development—the only plan recommended to beginners—the print is floated face down in a dish of water, and examined from time to time. The picture appears first for an
instant as a negative, caused by differences of swelling, and begins to develop after a time which, according to the exposure and especially the age of the paper, varies from five minutes (fresh paper), to more than three hours (paper 24 hours old). Without troubling further about the picture, except to renew the water from time to time in the dish, after having removed the print for a moment, development is left to itself. It may take from three hours to more than 24 hours. If it is very slow, the keeping of the print in water for several hours after it is finished will do no harm.
In case of considerable over-exposure, cold water has no action at all ; development must then be done with warm water, but only the crudest results will be obtained. If need be, development may be helped by a mixture of sawdust and water, as in the case of Artigue paper (§ 664).
When finished,' the print is put to dry, avoid ing contact of anything with the picture, which is very tender. In course of drying the definition of the picture falls off a little, owing to very slight spreading of the gum. Outlines lose their biting sharpness, but otherwise retain their character.
670. The chief merit of the gum process lies in the fact that it gives an artist the power to improve a subject by lessening or suppressing excessive details, and introducing accents here and there. Such treatment can be carried out during development, during drying, or after drying.
There is always an advantage in starting development as above, postponing any altera tions until the picture is distinctly visible. Then, by placing the print on a glass or other firm support, the development of certain parts may be hastened by sprinkling with tepid water, with a jug, or with a sponge squeezed out above the parts to be lightened. In small sizes a brush, charged with cold or warm water, may be used, but, for any small degree of lightening, and for all alterations to the principal subject, it will be found better to wait until the print is half dry, the gum then being much more resistant. After each local treatment, rinse with plenty of water in order to carry away from the picture particles of coloured gum which have been detached. For deepening of tones, colour taken from the margins of the print is used.