GELATINE 68x. General. The process of making prints in greasy inks on bichromated gelatine (called also the oil process)i introduced in 1904 by G. E. H. Rawlins, as a method for use in pic torial photography, is, in fact, only a variation of methods used since A. Poitevin (1855) in various photo-mechanical processes, coliotype, and production of prints for photo-lithographic transfer. The process consists in inking with a roller a layer of bichromated gelatine which has been exposed to light under a negative and washed in water, thus swelling in the protected parts and remaining nearly dry in the parts most completely changed by the action of light. Owing to the well-known repulsion between water and greasy substances, the ink takesonly on the dry portions (or those scarcely damp), or, at any rate, adheres effectively only in these parts.
In the Rawlins method the crude and mechan ical action of the ink roller is replaced by inking with a brush, which is easy to control and to localize. In the hands of an artist this is a method of wonderful elasticity, and can easily yield prints of great beauty. Still, there is the layer of gelatine which forms the support of the image. This drawback has been removed (R. Demachy, 1911) by transferring the image to plain paper of a quality usually used in art copper-plate printing, the gelatined paper on which the image has been made thus playing the part of a printing The image, printed from a negative in the normal way, is correct, as regards right and left, on the gelatined paper, and is thus reversed by the operation of transfer. In all cases in which it is important for the picture to be " the right way round," the print on gelatined paper must be made from a reversed negative.' 682. Materials and Apparatus. The gelatine coated paper is generally chosen from among double-transfer papers for the carbon process (§ 66o), avoiding the use of papers with a glossy coating and also those of stiff substance. The beginner will limit his choice to the smooth or matt papers, never grained papers, the use of which must also always be avoided when the image is to be transferred. The paper should be cut so as to allow a margin of about in. all round the subject ; even in good conditions inking cannot be done up to the edges of the sheet.
The inks supplied on the suggestion of Rawlins for use in this process were very fluid and quick-drying inks, covering a paper almost uniformly if the gelatine was only moderately swelled. Gradation was then obtained only by
removing the ink by tapping with a dry brush, according to the hardness of the ink. Almost immediately afterwards, R. Demachy and C. Puyo recommended the use of two types of ink of different consistencies. By using mixtures in different proportions it is easy, according as the gelatine is more or less exposed (i.e. more or less swelled), to ink the image, and the image only, without having, as a rule, to remove any ink which has once been applied. These inks are, respectively, a very hard ink (due to its very large pigment content), such as is used for lithographic machine printing, and known. as " litho ink," and a relatively fluid ink (con taining less pigment), as used for copper-plate These inks are manufactured in all shades. The beginner will be wise to limit himself to blacks, which may be rendered warmer by a little burnt sienna, or more striking by a little ultramarine blue. Copperplate inks are supplied, on request, in boxes or in metal tubes of the kind used for artists' colours. Lithographic inks, as also collotype and typographic inks, which are slightly thinner, and the use of which has sometimes been suggested, are much too hard to permit of their being supplied otherwise than in Dilution of the inks by addition of lithographic varnish (siccative linseed oil with out pigment) has often been suggested ; but this simply brings back the disadvantages of the excessively soft inks above mentioned. The most that should be done, in order to soften a copper-plate ink slightly, is to add to it a very small quantity of artists' oil colour.' Inking is done with polecat-fitch brushes, of hind's-foot shape, of diameter appropriate to the size of image to be inked (No. 9 or io for small sizes, No. 14 or 15 for large sizes), with a few brushes of the same shape but smaller (Nos. 3 to 7) for local inking. Straight polecat-fitch brushes, listed as necessary implements in some inking outfits, are considered useless by the best specialists. 2 The inks are mixed on a plate or on any piece of glass, using an artist's straight palette knife, or, failing this, an old, flexible-bladed table knife.