DEVELOPERS AND THEORY OF DEVELOPMENT 319 



accomplished commercially by the use of pyro developers with little or no sulphite. 

 The resulting differentially hardened, gelatin layers have been used for various pur- 

 poses, probably the most important of which is the production of relief images used as 

 matrices for printing by dye-imbibition processes. A second supplementary effect 

 sometimes sought and of growing importance is the deposit of a colored compound 

 along with the silver image. Special developer formulas are in general required, but 

 the resulting silver and dye images can be used for many purposes, and such dye 

 images, with the silver removed, are of rapidly increasing importance in the processes 

 of color photography. 



CHEMICAL DEVELOPMENT 



Inorganic Developers. — The history of photographic development has shown the 

 use of a few inorganic chemical agents as developers. Prior to, and for a number of 

 years after, 1851, ferrous oxalate was the common developer. This developer was 

 prepared at the time of use, by adding one part of a 25 per cent solution of ferrous 

 sulphate to three parts of a 25 per cent solution of neutral potassium oxalate, with 

 constant stirring. The reddish mixture produced can be used without bromide as a 

 developer. 



Other inorganic agents suggested as developers include solutions of sulphurous 

 acid which develop weakly and produce much fog. Copper ammonia oxalate and 

 sodium, potassium, and hydrogen peroxides in alkaline solution have also been 

 suggested. 



An energetic inorganic developer which works without much fog, unless develop- 

 ment is prolonged, is sodium hydrosulphite. A developing formula using such an 

 agent has been recommended as follows: 



Sodium hydrosulphite 20 g. 



Sodium bisulphite 30 g. 



Potassium bromide 7 g. 



Water to 11. 



Development time should not exceed 3 min. 



While ferrous oxalate was still in use at the turn of the century, organic developing 

 agents were fast replacing it, and today the use of inorganic compounds as photo- 

 graphic developers has practically disappeared. 



Developers with Organic Reducers.— Experience has lead to a very definite basic 

 pattern for developing formulas in common use. Following this pattern, modern 

 developers consist of 



Organic reducing agent or agents 



Preservative 



Accelerator 



Restrainer 



Compounds are also added for special purposes which do not fit in the four classi- 

 fications given, and at times one compound functions in a dual role resulting in less 

 compounds than the four classes listed. Instances of both types of variation will 

 appear in the subsequent pages. 



Organic Reducing Agents.— Chemically speaking, all developing agents are 

 reducers, i.e., they are themselves readily oxidized and hence tend to reduce com- 

 pounds with which they come in contact. Considered without complete analysis of 

 detail, the development process takes into solution the halide from the original silver 

 halide and furnishes an electron to the positively charged silver ion, resulting in the 

 deposit of metallic silver. The reactions in the solution which permit and complete 

 the chain of changes eventuate in the oxidation of the developing agent and the 



