The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Gum-Bichromate Process
ammonium. The faint print thus obtained serves as a key or guide, and if any portion of the subject is of such a nature as to need rendering in a pure pigment, it will be easy to lay such parts completely bare in developing the key. The sheet being quite dry after the first printing, thin washes of suitable pigments mixed with gum and bichromate are plotted out upon it, and when all is once more dry the second exposure is made. Next comes a second development. After this any required number of local sensitisings and printings may be performed according to judgment. The possibilities of the method of working are only limited by the skill of the worker.
Cojnbination of Gum-Bichromate and Platinotype. Herr Silberer has recently done excellent work by printing in gum-bichromate over a platinotype, and he (The Amateur Photo-grapher, January 23rd, 1902) points out that the platinotype under-print may be the vigorous element of the combination, or the soft element, according to circumstances; but occasionally a fully printed or complete print may be made in platinotype, and only special "effects'' put in with the gum. Thus it is evident that the combination element allows of wide variation. In working from rather hard negatives, such negatives, in fact, as are ill suited for the gum process, Jacoby's platinum paper (coated on the Whatman drawing paper) was employed for the under-print. The sheet having been rather under-printed, is treated in the usual way, washed, and dried. The surface is now coated with such a gum mixture as the following: - Colour " solution " (consisting of 1 part of Aureit's tube colour for gum-bichromate and 11 parts of water),; mucilage (40 per cent.), 8 c.c.; water, 4 c.c.; saturated ammonium bichromate solution, 8 c.c. A rather long exposure is now given, and the development is partly mechanical; that is to say, it is assisted by the brush. It need scarcely be said that the above prescrip-tion may be considerably varied, an increase of gum giving harder prints. The colour used must naturally have some relation to the (black or brownish-black of the platinum under-print, blue, green, etc., being ruled out at once, although a reddish tone may sometimes be allowable. For black, a mix-ture of Paris black and lamp black is specially suitable, while the admixture of sepia or umber will give any requisite infusion of brown. It is evident that no absolute rules can be given in