150 HANDBOOK OF PHOTOGRAPHY 



Toning Processes for Color Photography. — One obvious method of converting 

 the black-and-white silver image into a color image is by toning, with either inorganic 

 or dye tones. The emulsions carrying the three-color images are then superimposed 

 bodily on a white base. Thus the three positives are separated by the thickness of the 

 emulsions and their supports. In order to reduce the total thickness to a minimum, 

 the Chromatone process makes use of stripping films, in which the special printing 

 emulsion is coated on a very thin cellulose film which is cemented to a heavier film 

 base with an adhesive soluble in warm water. Thus the heavy bases furnish protec- 

 tion during the development and toning processes and are discarded when the emul- 

 sions are transferred to the final support. This particular process is understood to 

 use special inorganic toning solutions, in which the silver images are converted into 

 colored inorganic compounds, but dye tones, with the silver image converted into a 

 compound which could mordant the dyes, might also be used. 



Pigmented Gelatin. — Rather than use the silver image as a base for the color image, 

 it can be used to form a relief image in pigmented gelatin, similar to that obtained with 

 the various "carbon" processes. In this case, pigment of the proper color is incor- 

 porated in the emulsion during manufacture. The emulsion is exposed through the 

 transparent base, and the absorption of light by the pigment holds the image near 

 the bottom of the emulsion. Thus, when the gelatin is hardened in the vicinity of the 

 silver image, either during development or by means of a special silver bleach, and the 

 unhardened gelatin removed with warm water, there is left a relief image with thick- 

 ness proportional to the density of the silver print. The three partial color positives 

 thus obtained are then transferred to the final support. In the carbro process, the 

 differential hardening is caused by the action of the bleach on a silver image in contact 

 with the layer of pigmented gelatin. 



Imbibition Processes. — If, instead of making colored relief images which are 

 cemented together, plain gelatin reliefs are made, the3^ can be used as printing matrices 

 by saturating them with dye solutions of the proper colors, and placing them, one at a 

 time, in close contact with a gelatin layer containing a mordant to which the dyes will 

 transfer. 



Wash-off relief film has an unhardened positive emulsion containing a yellow dye 

 to control the penetration of the printing light. The film is exposed through the base, 

 and the silver image is developed to completion. It is then treated in an acid bichro- 

 mate bleach which oxidizes the silver image and hardens the adjacent gelatin. Bath- 

 ing in warm water removes the unhardened gelatin and develops the relief image, or 

 matrix. After fixing, washing, and drying, the matrix is bathed in the proper dye 

 solution. The three dyed matrices can be superimposed in register for inspection of 

 color balance and contrast. Changes in contrast can be effected by controlling the 

 acidity of the dye solutions, and slight inaccuracies in color balance can be corrected 

 by washing out part of the excess colors. When the color balance is satisfactory, the 

 matrices are squeegeed successively into close contact with a gelatin-coated paper 

 containing an aluminum mordant. Thus in the final print the three dye images are 

 contained in a single gelatin layer. Additional prints can be made merelj- by redj^eing 

 the matrices and transferring as before. 



The Technicolor process uses this type of imbibition printing, transferring the 

 successive dye images to a gelatin-coated final film. 



Catalytic Bleach. — Another method of utilizing the silver image directly in the 

 formation of the dye image makes use of the fact that certain reagents, which alone 

 have no effect on the dye, destroy it wherever there is a silver image. Thus this is in 

 effect a reversal process, giving a negative dye image from a negative, and a positive 

 dye image from a positive. However, because of the absorption of light by the dye, 

 the scnsitivitv is so low that it is suitable only for printing. 



