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Its Variations 666

gum, solution, mixture, coating, paper, bichromate and proportion

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ITS VARIATIONS 666. Gum-bichromate. Invented in 1858 by Pouncy, at a time when the only working methods in favour were those yielding abso lutely sharp pictures of a structure imperceptible to the eye, the gum-bichromate process fell into oblivion until Rouille-Ladeveze (1894), then R. Demachy, A. 11askell, C. Puyo, and many others showed the excellent use which could be made of it. Contrary to a widespread opinion, the gum-bichromate process does not necessarily need artistic handling. It is able to give, from a good negative, excellent prints without any personal interpretation. In France and other countries, this method has, during recent years, been almost totally abandoned in favour of methods employing greasy inks, and it must often be regretted that such has been the case in view of the success with which the process is still used.

It is best to use only raw pale Senegal gum, with the addition of a suitable antiseptic. The gum solution improves with age, and there is, therefore, no objection to preparing a certain quantity in advance. It is customary to employ the moist water-colours I supplied in metal tubes, which have the advantage of being already very finely ground in an excipient which mixes readily with the gum solution.

The bichromate may be mixed with the pig mented gum solution before being spread on the paper ; certain workers prefer, however, to impregnate the paper with a solution of about 10 per cent of potassium bichromate, and then to dry it, before coating with the mixture of gum and colour.

667. Preparation of the Mixture. In a glass jar of capacity about 20 oz., suspend a small bag of muslin in which has been put io oz. of gum, roughly broken up. Fill with cold water, and cover with a paper or cloth to prevent dust falling The solution slowly becomes acid, and its fluidity increases progressively ; after two or three weeks, dissolve in it 20 to 25 gr. of salicylic acid or thymol, in order to stop the fermentation.

It is best to choose from a small number of mineral pigments of great covering power. For black, use lamp-black (ivory-black is often too transparent), the tint of which may be warmed with a little yellow ochre or burnt umber, or tinged with blue with a little indigo for reds, red chalk and Venetian red, with some burnt sienna or burnt umber.

The average proportions of gum and bi chromate are— Gum solution, 50% . . . 3 volumes Saturated solution of potassium bichromate (8 to ro%) . . . . volume but will vary with the fluidity of the gum solution. For i volume of bichromate solution, 2 to 4 volumes of gum solution will be taken, according as the latter is very thick or very fluid. The proportion of gum will therefore vary with its age ; as a rule, the coating should con tain as much gum as is compatible with the spreading of an even film (C. Puyo, 1903).

It is difficult to give figures for the proportion of pigments in the mixture, not only on account of variations in the covering power of the various pigments (or even of one pigment, bought successively under different brands), but also because of variations in the thickness given to the film when coating.

By way of a rough guide, the best that can be said is that for black prints, the average proportion is 15 gr. of moist colour for 3 drm. of bichromated gum. It would really be more precise to say that the proportion of pigment should be such that, when coating thinly, the film should appear dark-grey (somewhat tinged by the bichromate), and not black.

The mixture is made up on the basis that about i drm. of pigmented mixture is required to cover a sheet io x 8 in. intended for pictures x 71n.

The ingredients are mixed in a basin, with a hog-hair brush, size about 2 in. broad, and rather stiff. A drop of the mixture is placed on the paper and spread with the finger in order to judge of the depth of tone.

668. Coating the Pigmented Gum. The paper must be sufficiently sized to prevent the colour penetrating its substance. All drawing papers generally are suitable, if given a further sizing with a weak mixture (about 1-5 per cent) of arrowroot or starch, spread warm with a large brush. With writing papers, strongly sized with resin, the coating of the mixture is usually difficult. Chinese or Japanese papers need delicate handling. For the first trials it is well to choose a paper of very fine grain but not absolutely glossy.

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