With an under-exposed print, in which the gum adheres very slightly to the paper, it is best to let the print first get completely dry. The gum is thus hardened, and the print may be moistened by a few minutes' immersion in water before proceeding to the retouching.
67i. Multiple Printing. A process often used (and sometimes abused) consists in superposing in register, on the same sheet of paper, several impressions in the same colour, or in more or less different colours, either for obtaining multi colour pictures or effects of double tone, or simply for modelling separately the lights and shadows of a monochrome picture. Each partial image must, obviously, be kept much less dense, and, consequently, the proportion of colour to gum should be less.
To allow of the easy registration of successive impressions with the same negative, the latter is held in a cardboard holder, from which pro trude the points of three or four drawing-pins, pushed into the card on the side corresponding with the back of the negative. The cardboard should be recessed a little, so as to accommodate the heads of the drawing-pins, and thus the paper is perforated by the points of the pins and is held in the same place on the negative during the various printings. The paper must be backed with felt, thick enough to prevent all contact of the pin-points with the hinged back of the frame (F. Grandmaitre, 1923).
After the development of each impression, the print is put aside to dry in very strong light, so as to harden the gum under the influence of the traces of bichromate retained in the film. The next coating is proceeded with, and the print is replaced in the frame for the next exposure, so that the perforations of the sheet engage anew with the drawing-pins.
672. Commercial Papers. A paper coated only with pigmented gum cannot be sensitized without dissolving an appreciable fraction of the gum, with risk of running during drying. Attempts have been made to get over this difficulty by dissolving in the gum a certain quantity of alum, which, after drying, reduces the solubility considerably, but the only gum bichromate papers which have been put on the market are those in which the gum has been mixed in a jelly of gum tragacanth prepared by boiling (Hochheimer, 190o). The develop ment has then to be done with warm water.
673. Bichromated This inter esting variation of the gum-bichromate process is particularly suitable for obtaining large pictures, by printing under enlarged negatives.
A drawing-paper, which the beginner should choose of fine grain, is first sized with a very thin film of weak gelatine solution. For this, the following mixture is used— prepared by allowing the gelatine to swell in the solution of the sugar, and dissolving it on a water bath below n5' F. The sizing is prefer ably done in a room at a temperature of at least 7o° F. ; the paper should then be at the same temperature as that of the room. It is fixed with drawing-pins to a drawing-board, placed level, and the gelatine solution poured on the centre in the proportion of 15 minims for each 16 sq. in. of surface to be covered. The
solution is spread with a swallow-tail brush, pre viously impregnated with the gelatine solution, and squeezed out on its edges. The solution is driven into the pores of the paper by vigorous operation of the brush, spreading it evenly until the sizing has a matt appearance. The sheet is then passed through the steam from a pan of boiling water till the gelatine becomes uniformly glossy. In the interval between the two sizings the brush is held in the steam to prevent the gelatine in it from setting.
Now, on a glass, roughly ground, put 15 gr. of de-greased lamp black (choose a black of brownish tint), placing the powder in the form of a tiny crater. Now pour into this 4o minims of a 5o per cent solution of gum arabic ; mix thoroughly with a flexible knife, adding gradu ally 30 minims of pure water. Grind with a glass muller for about 10 minutes, until the mixture becomes perfectly even. To cover a sheet of " royal " size (20 in. x 26 in.), about 30 gr. of this paste is placed in a very clean cup, and addition made of about in. of the strip delivered by a tube of sepia water colour. This addition gives a warmer tone, and, at the same time, improves adherence of the coating. Now add from 4 to 5 dr. (the smaller quantity in cold and damp weather, the larger in warm and (Iry weather) of the following mixture, prepared with boiling water— Crystallized sugar . 18 gr. (2 grm.) Starch . 18 gr. (z grm.) Water . 2 CZ. (I00 C.C.) After thorough mixture, this paste is placed at the centre of the gelatined sheet and spread with the swallow-tail brush previously charged with water ; when the colour begins to thicken, a flat goat-hair brush is used to finish off the coating, the first stroke of this brush being at a right angle to the last stroke of the swallow tail brush, so as to break up the streaks. Con tinue in this way until the sheet is surface-dry, and then leave to dry thoroughly by hanging from stretched cords.
The sensitizing is done, as required, by immersion in a i per cent solution of ammonium bichromate, with the addition of i per cent of neutral sodium or potassium citrate.
The negatives best suited for this mode of printing are those in which the extreme opacities are in the ratio of i to 20.
The exposure should be about equal to that required to make a P.O.P. print of depth for toning and fixing.
Before development, the print is immersed face downwards for about 10 minutes in a dish of water at 115° F., taking care to avoid ad hering air bubbles. As the water slowly cools down, the print is moved about from time to time. The print is then fixed to a rigid support, sloped to about the angle of a painter's easel, and development is begun by squirting on to the surface, with a toilet spray or an air-brush, water of a temperature about i8° F. higher than that of the soaking water. The sprayer is held from 12 in. to 16 in. from the print, and the water squirted all over the surface. As required, the development may be localized by bringing the vaporizer nearer. Development takes about 20 minutes for a surface 9 x 5 in. which has been correctly exposed.