TECHNIQUE OF DEVELOPMENT 375 



deposit occurs, and they leave an insoluble oxidation product of the pyro along with 

 the silver image. The existence and extent of this image may be shown by removal 

 of the silver after such development. When weak, this is merely the familiar "pyro 

 stain," but conditions may be chosen which make this quite a strong brownish image. 



The first of these effects has been used to produce relief images of the type some- 

 times used in imbibition printing processes, particularly those used in color processes. 

 When the original exposure is made through the transparent emulsion support, 

 images can be produced close to that support and differing in thickness, depending 

 upon the exposure received. This latter effect, difference in thickness of deposit with 

 exposure, is enhanced by using an exposing light to which the emulsion is strongly 

 absorbent, thus using the absorption characteristics of the emulsion itself to secure 

 relief characteristics for the image; or, alternatively, to use a dyed gelatin to enhance 

 the inherent optical absorption of the emulsion and thus enhance the image depth- 

 exposure relationship. In either case, after development in the pyro developer 

 without sulphite, the soft gelatin above the image which is hardened locally by 

 development is washed off by warm water. A relief image thus results, which can be 

 used for imbibition transfers and printing. 



Several articles have been written concerning this subject. These formulas are 

 taken from a summary in "The New Photo Miniature," old series, #207, new series #2, 

 July 1935, by A. F. Odell. 



A formula proposed for this hardening effect is: 



No. 25 Hardening Pyro Developer 



Pyro 8.2 g. 1 oz. 40 gr. 



Citric acid 0. 2 g. 12 gr. 



Potassium bromide 4.0 g. 235 gr. 



Sodium hydroxide 3.4 g. 200 gr. 



Ammonium chloride 1 . 7 g. 100 gr. 



Water to 1 1. 1 gal. 



A 2-min. development at 65° has been suggested, followed by a wash at 110 to 

 130°F. to remove the soft gelatin. The exposures for this work are made through the 

 emulsion support. 



Less practical use has been made of the colored image resulting from the deposit of 

 the insoluble oxidation product of pyro along with the silver image. The fact that 

 pyro-developed images tend to produce higher printing contrast than more nearly 

 neutral-toned images of similar appearance was mentioned before. Some attempts 

 have been made to use this color image as a yellow or minus-blue image in color 

 photography, but it is not a good yellow, and its use has not been satisfactory. 



Color-forming Developers. — A method for producing colored images by color- 

 forming developers has been known for many years but has not been used to any 

 extent until some of the recent work on color photography. In 1912 some procedures 

 were described which resulted in the deposit of a colored compound along with the 

 developed silver. In addition to the insoluble pyro oxidation product noted above, 

 a wide range of colors can be produced by the use of " color formers " and color-forming 

 developers. The colored deposit is the result of coupling the oxidation product of the 

 reducing agent of the developer with a compound which may be in the developer solu- 

 tion or in the photographic emulsion itself to form in situ a new insoluble colored com- 

 pound. This compound is little in evidence while the silver image is present but is 

 very evident upon removing the silver by bleaching, and a very wide range of colors 

 may be produced. 



Only a few of the wide range of photographic developing agents lend themselves 

 to this work, but a wide range of color formers is known for addition to the developer 

 to produce many different shades. The developing agents are: 



