556 HANDBOOK OF PHOTOGRAPHY 



Iodide-hypo reducer 

 Copper reducer 



2. Proportional or zero-gradient reducers 



Ferrocyanide-hypo 



Permanganate in acid state 



Quinone I vu i u • -j 



^ . 1 , ^ Mvitn sulphuric acid 



C^umone sulphonates ) 



Deck ammonium persulphate 



Ferric salts 



3. Subproportional or negative-gradient reducers 



Alkaline persulphate (ammonium persulphate) 



Persulphate reducer 



Bensoquinone 



Erdmann's salt 



Potassium permanganate with ammonium sulphocyanide 



The literature on photographic reducers is so voluminous that no attempt will be 

 made to describe more than the most common and satisfactory reducers in general 

 use. For additional information, the reader is referred to the references given in the 

 bibliography. 



Subtractive Reducers. Farmer's Reducer. — A popular reducer introduced by 

 Howard Farmer consists of a mixture of potassium ferricyanide and sodium thio- 

 sulphate. In this reducer, the silver image is attacked and oxidized by the ferro- 

 cyanide, and silver ferrocyanide is formed. Since this silver ferrocyanide is soluble in 

 hypo, the silver removed from the negative goes into solution. 



Farmer's reducer is used as a single solution reducer, although it is made up in two 

 stock solutions since the mixture does not keep well. A satisfactory formula is 



Solution 1 



Potassium ferricyanide 37.5 g. 13^ oz. 



Water to make 500 cc. 16 oz. 



Solution 2 



Sodium thiosulphate (hypo) 480 g. 16 oz. 



Water to make 2 1. 04 oz. 



For use, 30 cc. (1 oz.) of solution 1 and 120 cc. (4 oz.) of solution 2 are diluted with 

 water to make 1 1. (32 oz.) of reducing solution. The negative is immersed in this 

 working solution until the desired reduction is obtained, after which the negative is 

 washed and dried. The reducing action continues slightly after the film is removed 

 from the reducing solution, so that it is well to remove the negative to the wash bath 

 just before the desired reduction is obtained. The action may best be carried out in a 

 shallow white tray which facilitates easy inspection of the degree of reduction taking 

 place. The mixed solution does not keep well, and if sufficient reduction is not 

 obtained after about 5 min., the film should be transferred to a fresh bath until the 

 desired result is obtained. 



While classed as a subtractive or cutting reducer, the effect produced by this 

 reducer depends to a large degree upon the nature of the reducing solution and the 

 image Avhich is being reduced. Crabtree and Muehler have found that a two-solution 

 reducer is more satisfactory and permits greater control than the Single-solution 

 Farmer's reducer. A vigorously acting formula such as one containing 1 per cent of 

 potassium ferricyanide and 10 per cent sodium thiosulphate, produces almost purely 

 subtractive reduction of the silver image. The color of the reduced image may be 

 slightly brown, but this is ordinarily not objectionable. 



