THIS is essentially a carbon process, and is usually known as the "bi-gum" process. It is purposely described separately, as it is impossible to obtain the paper commercially and one has to prepare it. After one impression is obtained, it is necessary to recoat the print with the second sensitive mixture, print, develop, and re-coat for the third print. It requires some little experience to carry this to a successful conclusion, and it is more time-consuming than straight carbon printing.
One meets again in this process the trouble of expansion of the paper, and it is possibly more accentuated. The following methods in addition to those already given may be useful. As there is no sensitive surface to take care of, one has greater latitude in treatment. The most radical treatment is to soak the paper in water for twenty-four hours and then squeegee down to a sheet of glass, the edges of which have been coated with a 10 per cent solution of gelatine, to which has been added 1.5 per cent of chrome alum. The edging should be about half an inch in width. The wet paper will adhere to this edging and on drying contracts and becomes as tight as a drumhead. It can, when dry, be painted with celluloid solution, such as:
Celluloid 12 g.
Methyl alcohol 750 ccm.
Acetone or ether 250 ccm.
Castor oil 10 ccm.
Two or three coats may be applied to the dry paper, but there should be no glaze formed; if there is, more alcohol should be added. When the solvents have completely evaporated, the surface should be freely painted with a five per cent solution of gelatine containing one per cent of chrome alum, and this must be applied warm and allowed to dry. Or, instead of the chrome alum, the paper, after having been painted with the plain gelatine solution, may be painted with 25 per cent formaldehyde solution as soon as surface dry, and then dried. If a glazed surface is required, the commercial final transfer paper for carbon work should be used, and as this can be obtained in various surfaces it saves any preliminary preparation.
The sensitive surface is prepared with gum arabic or fish glue and gum, with which the colored pigments are incorporated, then sensitized with bichromate, dried and exposed. The development is effected with cold, not hot, water, and the print is merely allowed to lie face down on the surface of the water, as both the gum and fish glue are soluble in cold water. The print is then immersed in alum solution, washed, and dried.
The pigments used are preferably those prepared for water-color painting and should be obtained from an artist's supply store, as these are ground up specially fine in water and are free from lumps or gritty particles. The preparation of the pigments is a time-consuming and laborious job; but if anyone wants to do it, the following is the correct way to set about it. A glass muller and a good sized sheet of plate glass will be required. A small heap of the powdered pigment should be placed in the middle of the glass, a few drops of water added, and the whole worked with the muller, water being added gradually, until a thick cream is obtained. Grinding with circular strokes is kept up for about half an hour, a few drops of water being occasionally added to prevent the mass getting too dry. Then the pigment should be scraped up with a palette knife and put into a wide-mouthed bottle or preserving jar, and fresh pigment ground up in this way until the stock is finished. Care must, of course, be taken not to mix the pigments. Add to the pastes in the jars about ten times their volume of water, stir or shake well, allow to stand for about ten minutes to allow the large particles to settle, and then pour off the supernatant fluid and allow to stand a week or ten days for the fine pigment to settle.
The colors used should be chrome yellow, alizarine crimson and a mixture of Prussian blue and ultramarine blue in the ratio of 4:6. The gum solution is prepared from good clean lump gum arabic (powdered gum should not be used); 200 g of the gum should be placed in the center of a piece of fine muslin, the ends of the latter gathered together so as to make a bag and tied round with a string; this is then suspended in a wide-mouthed bottle or jar, by the aid of a piece of wood across the mouth, so that the muslin bag dips sufficiently far into 600 ccm of distilled water to completely cover the gum. This should be placed in a fairly warm place and left for two or three days for the gum to dissolve. One can, of course, merely add the gum to warm water and stir untill dissolved, but as the gum nearly always contains some dirt, the method described obviates having to filter or strain the gum solution.
There are two methods of preparing the paper; one in which the sensitizer is mixed with the pigmented gum, and the other in which the latter is applied to the paper and the bichromate only applied as one wants to print; the latter is preferable, as one can prepare a reasonable stock of the pigmented paper and it will keep indefinitely, whereas with the sensitizer incorporated it will only keep a few days. If the sensitizer is to be mixed with the gum, the following is a suitable mixture:
Pigment 25 g.
Gum solution 50 ccm.
Rub up well so as to obtain a smooth mixture, using either a mortar or a large sheet of glass and a palette knife; in the latter case transfer to a tea-cup or wide-mouthed jar and add with constant stirring: