640 HANDBOOK OF PHOTOGRAPHY 



There is no satisfactory way of reducing the strength of the blue image of a carbro 

 print. The magenta, however, can be reduced by treating the full print in a weak 

 solution of potassium permanganate rendered slightly acid with a trace of hydro- 

 chloric acid or sulphuric acid. When sufficiently reduced, the action can be stopped 

 at once by immersion in a weak solution of sodium bisulphite followed by a slight wash. 



The yellow image can be reduced by treating the complete image in a weak solution 

 of plain hypo, again followed by careful washing. 



It is not recommended, however, to resort to such chemical reduction of the 

 magenta and yellow images because the over-all contrast is invariably disturbed. 

 These reducing agents have an even effect throughout the image and therefore can be 

 classified among the so-called "cutting" reducers, the sole purpose of which is to 

 increase the contrast. 



Small traces of bichromate always remain in suspension in the colored images and 

 a partial insolubilization of the soluble temporary support might take place if the 

 transfers are carried out in strong dajdight or sunlight. Insolubilization might also 

 occur if the last transfer onto the final support is delayed too long (over 30 hr.). A 

 safe rule is therefore to avoid too strong daylight and sunlight and to carry through 

 the various transfers within a span of a few hours. 



When properly carried out the carbro process permits results of great beauty and 

 naturalness not readily achieved by other processes. 



The room temperature for the carbro process is not critical. A comfortable value 

 of 65 to 70°F. will be found to be best. • 



Eastman Wash-off Relief. — The Eastman wash-off relief method represents a 

 typical imbibition process. 



Outline.— Vvmis, in natural color can be made by transfer of water-soluble dyes by 

 means of images produced in reUef on celluloid film. This process is based on the 

 property of the silver, forming the photographic image, to render the surrounding 

 gelatin insoluble when the development is carried out in a tanning developer or by a 

 subsequent treatment of the image with suitable bleaching chemicals. The Eastman 

 wash-off relief is typical among the processes of this type. The positive prints from 

 corresponding color-separation negatives are made on a special film carrying a silver 

 emulsion made with highly soluble gelatin. The exposure of the film must be made 

 through the celluloid base. The development of the positive film is carried out in a 

 normal positive developer, after which the films can be given a brief wash and passed 

 to the bleaching bath without intermediate fixing. 



Exposure and Development of the Positives. — The wash-off relief films are printed by 

 contact or projection from the appropriate color-separation negatives. The exposures 

 should be such as to give a faint silver deposit in the highest lights of the subject. 



The wash-off relief positives may be developed in Eastman developer DK-50 for 

 about 5 min. at 70°F. Developers DK-11 and D-76 may also be used. The three 

 films can be developed simultaneously in a tank suspended in the usual film-developing 

 hangers. Development can be also carried out in trays, treating such film individ- 

 ually in a fresh quantity of developer with uniform rocking. 



The operations of washing, bleaching, and developing of the reliefs in hot water 

 could be made to follow the development without the intermediate fixing. This 

 requires, however, that these various operations be carried through in yellow light. 

 It might be preferable, however, to rinse briefly the wash-off relief films after the 

 development and then place them to fix in the nonhardening fixing-bath formula F-24. 

 Following this they can be washed for 5 min. in running water. 



Bleaching. — The bleaching of the silver image of the film is carried through in a 

 bleaching solution (R-lOa) diluted as indicated. The three films can be placed all 

 together in a tank if suspended in the usual hangers. 



