295 



This double toning is a very marked trouble. An other- 

 wise excellent print might have its denser parts of a good 

 red-brown colour, and its lighter half-tones quite bluish. 

 The print, in addition, often loses in general density, which is 

 the obvious cause of many manuals recommending that toning 

 should be commenced with very vigorous prints, which have 

 been made somewhat darker than finally required. 



It occurred to the writer that, if the behaviour of the 

 image in toning was exactly according to what chemical 

 theory would suggest, and that the process commenced with 

 a silver image free from any thiosulphates, or salts whence 

 thiosulphates might be derived, that the troubles mentioned 

 above would not occur. Further, whereas it has always been 

 recommended that only vigorous prints of good black colour be 

 used for toning, it should be possible to tone quite delicate 

 prints, provided that no silver losses occurred in the process. 



The Cause. 



It is necessary to enquire now as to what chemical changes 

 take place after the print has been developed. Development 

 has reduced the original silver haloid in part to metallic 

 silver, which constitutes the image. The unaffected silver 

 haloid has now to be removed. The nearly universal method 

 is the use of sodium thiosulphate (Na2S203 .SH^O), unfor- 

 tunately, still frequently called sodium hyposulphite, and, 

 worse still, "hypo." 



In a solution of this salt, two well-known reactions take 

 place (4, 5, 6) : — 



AgBr + Na2S203 = NaBr + AgNaS^Og (vi.) 



2AgBr+3Na2S203-2NaBr + Ag2Na, (8^03)3 (vii.) 



The first silver sodium salt formed is practically insoluble in 

 water (7, 8), and is formed when the solution of sodium 

 thiosulphate is weak. With stronger solutions, the reaction 

 takes place according to the second equation. The silver 

 sodium salt then formed is freely soluble in water. (D The 

 concentration of the sodium thiosulphate solution is of great 

 importance. Few workers appear to realize the importance 

 of using a solution of optimum concentration. In a 50 per 

 cent, solution, the silver bromide is much less soluble than in 

 a 25 per cent, solution. It is considered by some chemists 



(1) There is apparently some confusion as to the relative 

 solubilities of the two silver sodium thiosulphates; e.g., Roscoe 

 and Schorlemmer, ''A Treatise on Chemistry," vol. i., p. 456, 

 suggest that it is the AgNaS^Og that functions in fixation of a 

 photographic image. No mention is made in this connection of 

 the AgpNa^ (8203)3. In metallurgical treatises also, on silver 

 ■extraction, the same uncertainty is frequently apparent. 



