For the inking operation, the sheet is placed on a bed of several thicknesses of damp blotting paper; if plate-glass is not available the sup porting block may usefully be protected from the moisture by a piece of oilcloth. If an artist's easel is available, advantage may be taken of it for holding the supporting board during inking.
683. Sensitizing and Exposure to Light. The paper may be sensitized by soaking in a 2 per cent solution (in water) of potassium bichromate, or by brushing over with a 6 per cent solution of ammonium bichromate, which, at the time of use, is mixed with twice its volume of methy lated spirits (industrial spirit). The sensitized paper keeps only for about 48 hours.
In spite of the facilities offered by this process for the use of either soft or vigorous negatives, first trials should be carried out only with negatives of good gradation and not too vigorous.
Inking with litho ink is only possible with a print which has had an exposure much longer than would be suitable for inking with copper plate ink. Between these two limits all times of exposure will be suitable by determining by trial the best proportion in which the two inks should be mixed.
Although the image appears in pale brown on the yellow ground of the paper (the printing should be stopped when details appear in the high-lights), more certainty will be attained by controlling the exposure with a photometer (§ 5o8). The exposure is fairly short (from to 2 degrees Artigue), and over-exposure must by all means be avoided.
684. Washing, Swelling, and Drying. As soon as possible after exposure to light, the paper is washed in several changes of water until nearly all colour has been removed, and then placed in a solution of sulphuric acid of about i per cent strength, where the paper will be completely decolorized in two to three minutes. Then rinse in several changes of water. In this condition the sheet may be inked, or the inking may be indefinitely postponed, when it will only be necessary to re-moisten the dry sheet of paper.
To make the gelatine readily amenable to ink ing, it should be swelled, the relief being plainly visible and perceptible under the finger. This swellingmay be done byimmersion in tepid water F. to 86° F.) or in cold water to which has been added 4 per cent to io per cent of ammonia ; this is brushed over the surface of the gelatine.
If several identical papers be swelled, sensi tized under the same conditions, and exposed under the same negative for times of increasing length, it will be found that the general swelling of the gelatine is in inverse ratio to the length of the exposure, but that the relief of the image, that is to say, the difference of level between the regions most swelled and those least swelled.
(whites and blacks of the image), at first in creases with the time of exposure, then passes a maximum, and begins to decrease as the ex posure is very prolonged. The correct time of exposure corresponds with the maximum of the relief of the image.
The print being suitably swelled, it is placed on the bed of moistened blotting-paper, where it will adhere of itself, and dried by wiping (not by dabbing, which might leave rough spots) with a pad of well de-greased chamois leather or an old handkerchief.
To keep the margins white, they are covered with wet strips of parchmentized paper, cut to straight edges, and placed so as to make a rectangular frame (when the image was visible, the four corners of this rectangle may have been marked on the print with a pencil). These strips are fixed to the blotting-paper with draw ing pins.
685. Inking the Print. In two opposite cor ners of the pallette are placed small dabs (about as large as a pea) of litho ink and of copper-plate ink. These dabs of ink are then smoothed out with a knife.
In three metal boxes, weighted with pebbles, are placed the brushes, hairs uppermost, which are to be used with the hard ink, with the softer inks, also those which are to be used dry.' Finally, in full view is placed a perfect print from the same negative which has been used for the oil print.
The inking brush is pressed on the mass of hard ink, then tapped with little blows on a clean part of the glass in order to distribute the ink evenly throughout the brush. Then, held by the first three fingers just above the thickest part of the handle, the brush is pressed flat on a part of the print, including both a black and a white, pressing slightly up and down and at the same time turning it lightly round in the fingers, without, however, letting the hairs lose contact with the gelatine. If, on raising the brush, it is seen that the image is correctly rendered, with full blacks and nearly clean whites, and if the proper gradation can be obtained with light tappings, it may be con cluded that the exposure and the swelling suit the litho ink, and that one may continue to use this ink.