The print is left in water, which is renewed several times, for a long enough time to remove all colour from the ground, on which appears an extremely weak yellowish image. The print is then placed in hot water (about F.) until it shows well-marked relief 2 (two to three minutes). The print is then laid face down on several thicknesses of blotting-paper, and dried with filter paper or with a pad of butter muslin.
Immediately afterwards comes powdering with pigments, prepared by grinding and pow dering a melted mixture of resin, wax,' and a suitable pigment (lamp-black, mineral colours, etc.). These are spread with a very soft round brush, carried over the surface of the gelatine in circular movements, pressing harder on those parts where it is wished to increase the density of the image. Excess of non-adhering powder is swept away with the same brush, and any working-up of the shadows then done with a somewhat stiffer brush.
In case of lack of contrast, the print is put in cold water, and powder is removed by friction with a cotton pad. The powdering is then repeated, after having passed the print again through hot water, with the addition, if need be, of a stronger dose of ammonia, to increase the swelling.
As the pigment adheres very slightly to the damp print, retouching is easy at this stage by erasure with a moistened brush, after blotting off, but it is well to dry the print first and then to remove the last traces of bichromate by immersion in a very dilute solution of sodium bisulphite, followed by a quick rinsing. After
drying, erasing may still be done with india rubber or an erasing knife.
The colour may be fixed by passing the dry print through steam from a vessel of boiling water, the colours setting without rendering the print glossy. Use may also be made of an alcoholic fixative varnish, sprayed with an atomizer.
680. Color Process. The Color paper is coated with a layer of gelatine, in which is sus pended a light blue pigment, removable with an effervescence by an acid bath (artificial ultra marine), the evolution of gas giving the gelatine a spongy structure (which allows it to retain firmly the powdered pigments applied to its surface), and a white filling powder, such as barium sulphate. Paper thus prepared is sensi tized with bichromate, dried, exposed under a negative, and developed in tepid water. The bichromate stain is removed in a very weak solution of hydrochloric acid, and the print again washed and dried. The image is then powdered with powdered pastels, applied with a brush, this method allowing of effects in several colours being obtained. The pigment may be consolidated by means of a fixative, or trans ferred to an adhesive paper.