PRINTING PROCESSES 451 



face, wliich would produce serious burns. Use the developer full strength at a tem- 

 perature of 70°F. (21°C.). 



Use a crystalline boric acid as specified. Powdered boric acid dissolves with 

 difficulty. 



Bleaching Solution 



Water.. 1 gal. 4.0 1. 



Potassium bichromate 1 J-i oz. 37 . 5 g. 



Sulphuric acid (o.p.) 1>2 A. oz. 48.0 cc. 



Use full strength at 65 to 70°F. (18 to 21°C.). For more rapid bleaching, the 

 amounts of acid and bichromate may be increased. 



Cleabing Solution 



Sodium sulphite (desiccated) 12 oz. 360 g. 



Water 1 gal. 4 1. 



Use full Strength at 65 to 70°F. (18 to 21°C.). 



Printing-out Paper. — At one time the photographer's best medium was POP, 

 printing-out paper, in which the image is visible after exposure and before develop- 

 ment. Proofs of negatives were often submitted on paper of this type because the 

 prints would fade if exposed to the light and so the person photographed could not 

 retain the proof print with any degree of satisfaction. He was forced to let the 

 photographer make a print with a more permanent image. For rendition of fine 

 detail, a glossy POP paper is superior to more modern developing-out papers and for 

 reproduction by means of half-tone engravings, P O P is still recommended. 



P O P is very slow compared to present-day printing papers. It is exposed to the 

 negative by direct daylight or very strong artificial light. It may be handled in a 

 well-lighted room. The negative should be of average contrast. Flat negatives will 

 produce weak and flat prints. Prints should be exposed somewhat longer than 

 appearances would dictate as determined by inspection during the exposure. Some 

 color is lost in subsequent processing. The parts which are to be pure white in the 

 final print should show some slight color in the correctly exposed print. 



After exposure, the prints should be washed thoroughly, i.e., until the wash water 

 no longer shows any milkiness. Five or six complete changes of water are required. 



Toning Bath. — Innumerable baths may be used to tone POP. In some there is a 

 combination of the toning and fixing processes. The following formulas are a few 

 examples of those which have been recommended. 



If the toning and fixing are to be distinct baths, the following method is recom- 

 mended by Ilford. 



Two stock solutions are necessary for making up the working toning bath. 



Sulphocyanide Solution A 



Ammonium sulphocyanide 200 gr. 12 g. 



Water 20 oz. 500 cc. 



Gold Solution B 



Gold chloride 15 gr. 1 g. 



Water 20 oz. 500 cc. 



For use, take 2 oz. each of A and B and add to 20 oz. water (50 to 400 cc. water). 

 Add the gold solution slowly and shake well. 



After about 5 or 10 min. in this bath, prints begin to lose their characteristic red 

 color and take on their final purplish color. After toning, the prints should be thor- 

 oughly washed, then immersed in hypo, 3 to 20 oz. water (75 g. to 500 cc. water), 

 and moved about for 10 min. The fixer should be used but once. Finally the prints 

 must be thoroughly washed, Ilford recommending 2 hr. 



