328 



HANDBOOK OF PHOTOGRAPHY 



OH 



NH, 



OH 



+ 



NHo 



Hauff 



B. P. 11,306 (1896) 



Starnes 



B. P. 466,626 



has been introduced as the fine-grain developer Meritol. 



One of the most popular developing agents of this tj^pe is the combination of 1 mole 

 of hydroquinone with 2 moles of the sulphate salt of o-aminophenol : 



Hauff 



B. P. 27,931 



(1896) 



which is marketed as Ortol. 



Table I presents a number of the organic compoimds which have been suggested as 

 photographic developers. This list, while representative, is by no means complete. 



While the tabulation indicates the large number of organic compounds which 

 have at least been suggested as photographic developers, only a small percentage of 

 these have been commercialized to the point of being given a trade name. Of the 

 two or three dozen different agents which have appeared at one time or another on 

 the photographic market, less than a dozen different developing agents are in common 

 use today. 



Of these agents which have remained in common use, it is interesting to note that 

 two of them are the oldest of organic developers. 



Pyrogallol. — The developing action of 1 : 2 : 3-trihydroxybenzene or pyrogallol 

 (pj^ro), the oldest of organic developing agents, was observed in 1851 independently 

 by V. Regnault and S. Archer. In its fresh state, it appears as a fine white crystalline 

 powder and is extremely light. Owing to its tendency to float in the air at the slightest 

 movement of air, the crystallized form is available which is more dense and causes less 

 trouble in mixing than the older crystalline variety. The crystalline variety is also 

 more stable against oxidation by the air. Pyro has a characteristic odor and is very 

 soluble in alcohol, ether, water, and slightly soluble in warm benzene. The solubility 

 in water and 10 per cent sodium sulphite solution, both at 15°C., is about 56 per cent. 



Pyro is a soft working developer and, unless caustic hydroxides are used as the 

 alkali, is slow acting. In addition to the metallic silver image produced by develop- 

 ment with pyro, a secondary yellow-stain image comprising the oxidation products of 

 pyro is produced in situ with the silver image. The combination of black silver with 

 the yellow stain produces the brownish image associated with pyro development. 

 Owing to the presence of the blue-absorbing yellow-image portion, pyro images are 

 known to print somewhat more dense than thej^ appear visually. Increasing the con- 

 centration of sulphite inhibits the formation of the stain image, and the developed 

 image is neutral to blue-black in appearance. 



To reduce the time of development and increase contrast, both metol and hydro- 

 quinone are used with pyro, particularly for deep tank work where the combination of 

 pyro with metol only is ofttimes adopted. The keeping qualities of pyro developers 

 are generallj' poor, being classed as 1 in which, to the same scale, metol is rated as 10 

 or highest. Crystallized pyro has appeared under the trade name Piral. 



Hydroquinone. — In 1880, W. Abney discovered the developing action of the second 

 oldest developer, hydroquinone, which is used to a greater extent than any of the other 



