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338 HISTORY OF COLOR PHOTOGRAPHY
adjacent to the celluloid, and which penetrates but a short distance into the film. This is toned blue-green by means of iron. The green-filter separation is printed in registry, in the normal manner. This gives a surface image that does not penetrate deeply, so does not reach the lower one. The upper image is converted into a red by use of the technique just disclosed. The same idea is utilized by L. F. Douglass (U.S.P. 1632278); W. V. D. Kelley (U.S.P. 1712439 and 1810180); Max B. Dupont (Eng. P. 360109); and W. H. Peck (U.S.P. 1840524).
In all of the above disclosures, it is cupric ferrocyanide that is used as a mordant. As E. J. Wall points out in his "History of Three-Color Photography" (p. 371) it is perfectly possible to accomplish the same result by first treating the silver image with ferricyanide, then with copper salts. What is not pointed out, is that a tremendous loss of image density results. But this is not an unmixed blessing, since it allows the operator to make a more normal print for toning. After dyeing, the dye image may be fixed with tannic acid or some stronger mordant than copper ferrocyanide, and this last removed by treatment with alkali. This use of two-bath toning was suggested by F. J. Ventujol (Fr. P. 558699). A two-bath solution proposed by the Naturfarben Film g.m.b.H. (Ger. P. 393790) differs from the above in that cuprous ferricyanide is the mordant. This is formed by the action of cupric chloride upon the silver image, followed by a ferricyanide bath.
Equal in potency to the ferrocyanide, but free from the color of this image, is cuprous thiocyanate, first proposed by J. H. Christensen (U.S.P. 1447759; Eng. P. 132846 and 1354??; Ger. P. 319459* 3*9477 and 334277) The silver image is treated with the following solution:
Potassium citrate 55.5 parts
Copper sulphate 4M5 parts
Potassium thiocyanate 20 parts
Acetic acid 25 parts
Water to 1000 parts
This formula was modified to a slight extent by Seyewetz, in a paper dealing with mordant processes in general. Dr. Seyewetz is quite partial to the cuprous thiocyanate mordant. His modified formula (Brit. J. Phot., Vol. 71 (1924), p. 609) is the following:
Copper sulphate 40 parts
Potassium thiocyanate 20 parts
Potassium citrate 60 parts
Acetic acid 30 parts
Water to 1000 parts
The bleached print is a dirty gray in color. This does not modify the shade of the image, although it leads to a slight degradation. An improved formula, in that keeping qualities are much better, was given by Lyman Chalkley.