552 HANDBOOK OF PHOTOGRAPHY 



varjang the concentration, various degrees of intensification are possible. The great- 

 est intensification is produced with relatively small amounts of acid (2 cc. per 1.) and 

 bichromate (10 g. per 1.), whereas the degree of intensification decreases as these con- 

 centrations are increased. The quantity of acid cannot be increased indefinitely, as 

 the rate of bleaching slows down. The maximum concentration recommended by 

 Carnegie and Piper was 20 g. bichromate per liter and 40 cc. acid per liter. The 

 formula given on p. 551 is that recommended bj^ Crabtree and Muehler^ and gives an 

 increase of 40 per cent in density for a single treatment when redeveloped in D-72 

 developer. The intensification can be increased and greater speed of bleaching results 

 if 5 g. potassium bromide is added to each liter of solution. If desired, the solutions 

 can be made up for storage with ten times the concentration given above and may be 

 diluted 10 to 1 for use as required. 



Bleaching will require about }4 to 3 min. Incomplete bleaching results in rela- 

 tively greater intensification of the lower densities compared with the higher. After 

 bleaching, the negative is washed in running water until all trace of bichromate has 

 been removed. The negative is then redeveloped in a nonstaining developer. Since 

 the sulphite may result in reduction of lower densities, the developer is preferably one 

 in which the sodium sulphite concentration is low. Suitable developers include those 

 composed of Amidol, metol-hydroquinone, metol, or paraminophenol. See chapter on 

 Developers and Theory of Development. A suitable developer is the Eastman D-72, 

 which is prepared as follows : 



Hot water (approximately 125°J 500 cc. 64 oz. 



Metol (Elon) 3 . 1 g. 180 gr. 



Sodium sulphite (desiccated) 45 g. 6 oz. 



Hydroquinone 12 . 5 g. 1 oz. 260 gr. 



Sodium carbonate (desiccated) 67 . 5 g. 9 oz. 



Potassium bromide 1 .9 g- Yi oz. 



Water to make 1 1- 1 gal. 



Develop from 3 to 10 min. (depending upon degree of intensification desired) 

 at 65 to 70°F. Rinse the negative and immerse in fixing bath for 5 min., wash thor- 

 oughly, and then dry. 



The intensification process may be repeated, if desired, although the degree of 

 intensification decreases with each treatment as the number of treatments is increased. 



Mercury Intensifier. — Intensification with mercury is accomplished by depositing 

 metallic mercury or one of its compounds on the silver image. This is accompHshed 

 by first bleaching the silver image in mercuric chloride or mercuric bromide to produce 

 silver mercurous chloride. The compound thus produced is reduced, in the process of 

 development, to silver and mercury compounds or mercury and compounds of silver 

 and mercury, according to the character of the redeveloper. 



A suitable bleach consists of 



Potassium bromide 22 . .5 g. 3 oz. 



Mercuric chloride^ 22. 5 g. 3 oz. 



Water to make 1 1- 1 gal. 



1 Mercuric chloride is extremely poisonous and should be handled accordingly. 



It is essential that negatives be free from hypo before bleaching, or local stains 

 maj-- result. The negative is bleached until the image is white. After bleaching, 

 wash the negative in running water for 5 to 10 min., and redevelop. 



Several suitable redeveloping solutions are possible, but the silver cyanide redevel- 

 oper (forming with the above bleach what is known as Monckhoven's intensifier) 

 is one of the most suitable and best known. Monckhoven's intensifier results in 

 a high degree of intensification, although reduction occurs for the very low densities. 



