The Dictionary Of Photography

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

Home | About | Photography | Contact

Gum-Bichromate Process
or intensify the shadows. Either the exposures may be about equal and the sensitive layers made of variable thickness, or the layers of sensitive material may be of equal thickness and the exposures may be varied ; again, the proportion of bichromate may be varied as indicated above. Dr. Hofmann points out that in the former case the first coating should be a thin layer of a mixture containing but little pigment and gum, but a full dose of bichromate. This will give a thin flat image. The second coat-ing should have more gum and pigment with less bichromate, which, with the same exposure as in the previous case, will in no way increase the deposit on the higher lights. The mixture for the third coating will be made with still more gum and pigment, but with less bichromate. If the exposure is the same as before, only the deeper shadows will receive a new deposit. If, on the other hand, the system of varying the exposures and using similar coatings be adopted; the first exposure must be long so as to cause a deposit as far as the lights of the print, this exposure giving a flattish picture. The second exposure must be shorter, the deposit then only reaching the middle tones, while the third exposure should be so short as only to cause a deposit on the extreme shades.
Adjusting the Print to the Negative for Multiple Impression. The coating of pigmented sensitive gum on the sheet of paper should never be so thick or dense as to make it difficult to place the negative in exact position or register for the subsequent printings. An excellent method of adjusting the sheet for subse-quent impressions is to hold the negative and sheet together in one hand, until exact correspondence is effected, and then to fix the paper in position by two or more strips of adhesive plaster, this latter being very slightly warmed.
Application of Gum-Bichromate to Colour Printing. Under the heading Photography in Natural Colours we refer to the gum-bichromate process as a means of wholly or partially making a three-colour print for the Ducos du Hauron process, and in this place we may point out how well the gum method is adapted for producing a print in colours from a single negative, the translation into colour being in this case entirely in the hands of the worker. The making of a very faint print in monochrome is the first step, the conditions for securing this being a small amount of colour solution and full dose of bichromate of