A Gum Alternative

>From ''Photography: Theory and Practice''
by L.P.Clerc, Hon.F.R.P.S. Pub. 1954 (3rd edition) Edited by A. Kraszna - Krausz

BICHROMATED WASH - DRAWING

''This interesting variation of the gum-bichromate process is particularly suitable for obtaining large pictures, by printing under large negatives.
A drawing - paper is first sized with a very thin film of weak gelatine solution. The beginner should choose a paper with fine grain. For this the following mixture is used -

Crystalized sugar 4g
Soft photographic gelatine 4g
Water 100 ml
        prepared by allowing the gelatine to swell in the sugar solution, and dispersing it over a water bath below 115 F. The sizing is preferably done in a room at a temperature of at least 70 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 1 ml for every
16 squ '' of surface to be covered. The solution is spread with a swallow - tail brush, previously impregnated with the gelatine solution, and squeezed out in its edges. The solution is driven onto the pores of the paper by vigorous operation of the brush, spreading it evenly until the sizing has a mat appearance. The sheet is then passed through the steam from a pan of boiling water until 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, put 1 g of degreased lamp black ( choose a black of brownish tint ) on a piece of glass placing the powder in the form of a crater. Then pour into this 2.5 ml of a 50% solution of gum arabic; mix thoroughly with a flexible knife, adding gradually 2 ml of water. Grind with a glass muller for about 10 minutes, until the mixture becomes perfectly even. To cover a sheet of paper 20'' X 16'' about 2 g of this paste is placed in a clean cup, and about 0.75'' of the strip delivered by a tube of sepia water colour is added. This gives a warmer tone, and at the same time, improves the adhesion of the coating. Now add from 15 to 18 ml ( the smaller quantity in cold and damp weather, the larger in warm and dry weather ) of the following mixture prepared with boiling water -

Crystallized sugar 2 g
Starch 2 g
Water 100 ml
          After mixing thoroughly, this paste is placed at the centre of the gelatine coated 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 right angles to the last stroke of the swallow - tail brush, so as to break up the streaks. Continue 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 1% solution of ammonium bichromate, with the addition of 1% of neutral sodium or potassium citrate.
             The negatives best suited for this method of printing have a density range of about 1.3.
              The exposure should be equal to that required to make a P.O.P print for toning and fixing.
             Before development, the print is immersed face downwards for about ten minutes in a dish of water at
 115 F, taking care to avoid adhering 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 the surface with water at a temperature of about 18 F above that of the soaking water. This may be done with a toilet spray or an air brush. The spray is held from 12'' to 16'' from the print, and the water is squirted all over the surface. The development may be localized as required by bringing the vaporizer nearer. Development takes about 20 minutes for a surface 9'' X 5'' which has been correctly exposed. ''
..........................................................
Received on 05/14/06-01:29:58 AM Z