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AN Observation on the Toning of Photographic 

 Silver Images. 



By Arthur R. Riddle. 



[Read September 9, 1920.] 



Toning is a process whereby the colour of a photographic 

 silver image is changed, either by an actual change in com- 

 position, or by the deposition of another metal. 



The two methods to be discussed are: - 



1. The sulphide method of producing sepias, browns, etc. 



2. The copper method of producing purple, purple- 



brown, and red tones. 

 This note concerns a trouble which frequently occurs 

 without obvious cause. The apparent cause and remedy are 

 suggest-ed. 



The Sulphide Method. 

 Here, the silver image of the photograph is changed either 

 wholly or in part to a silver haloid, e.g., silver bromide. 

 Tliis image is then changed over to silver sulphide by the 

 a,ction of sodium sulphide. The reaction? probably conform 

 to the following equations given by Roebuck (1) : — 

 Ag3 + 2K3re (CN) , = K,Fe (CN) , + K,Ag,Fe (CN), (i.) 

 K, Ag^Fe (CN) , + 2KBr = K,Fe (CN) , + 2AgBr (ii.) 

 Na2S + 2AgBr = Ag2S + 2NaBr(iii.) 

 The formation of the silver haloid takes place in two stages. 

 Firstly, the potassium ferricyanide attacks the silver with the 

 formation of potassium silver f errocyanide ; and secondly, the 

 potassium bromide changes this over to silver bromide, on 

 account of the much greater insolubility of the latter. The 

 silver bromide image is then treated with sodium sulphide, 

 with the formation of the final silver sulphide.- 



The writer has made experiments with other haloids, in 

 one case with marked success. Smith (2), in dealing with 

 chlorine, gives the following equation : — 



KMnO^ + 8 HCl = 4H2O + KCl + MnCl^ + 5 CI (iv. ) 

 The writer makes use of this reaction for the liberation of 

 nascent chlorine, in the presence of the photographic print to 

 be toned. The chlorine combines with the silver, converting 

 "the image, wholly or in part, into silver chloride. This is 

 then changed to the sulphide as follows : — 



2 AgCl + Na^S = Ag^S + 2NaCl (v.) 



