CHAPTER XII 



TECHNIQUE OF DEVELOPMENT 



By D. R. White and J. R. Weber 



The two most important physical factors which influence the rate and course of 

 development are the temperature and the agitation of the solution. In addition to 

 these principal factors there are others also, each of which plays its part, having a 

 secondary influence on development. They include such factors as the position 

 of the emulsion layer in the developing solution and the effect of adjacent areas. 



Most of the mechanisms introduced in photographic development have for their 

 object adequate control of temperature and agitation. Even when no special 

 mechanical aids are used, it is still necessary to adequately control them to secure 

 the results desired. 



Control of Temperature and Effect of Variations. — In common with so many other 

 chemical reactions, an increase of temperature increases the rate of development. The 

 rate of increase of activity with increase of temperature is different for different devel- 

 oping agents. A characteristic, named the "temperature coefficient," has been used 

 as the quantitative measure of the change of activity. This is defined as the ratio of 

 the development times required to produce equal density at two temperatures differing 

 by 10°C., which is, of course, a difference of 18°F. The values obtained range from 

 1.3 for metol alone, through 1.9 for pyro and metol-hydroquinone combinations, to 

 2.5 for glycine. In many charts and guides in practical use, the results are not 

 expressed in the form of the temperature coefficient. Most frequently the tabulations 

 are in the form of specific developing times for specific temperatures. In some cases, 

 the subject is covered more broadly by giving developing time ratios or percentages 

 such that, if correct time of development is known for one temperature, it may be 

 calculated for other temperatures. These values do not always agree with the ones 

 which may be calculated from the temperature coefficients given. This may indicate 

 that the temperature coefficient is a function, not of the reducer alone, but of a specific 

 formula. If this is true, the differences may be true ones, each applying to its own 

 specific case. On the other hand, the practical tables rarely cover as wide a range as 

 18°F. (10°C.), and hence the differences found may be only differences in the precision 

 of determination of the effect of temperature. 



Table I gives the ratio of the developing time for temperature t to that at tem- 

 perature 65 °F., calculated on the basis of a temperature coefficient of 2.2. 



These values, of course, cannot be in exact agreement with all the specific recom- 

 mendations for specific formulas, but they are a fair approximation of the general 

 trend of the recommendations for metol-hj'droquinone and p-phenylenediamine devel- 

 opers and may therefore be used in varying times to compensate for temperature 

 changes of such developers with reasonable certainty that the result will be satisfac- 

 tory for practical purposes. If the work is extremely exacting, temperature variations 

 should be avoided, or, if unavoidable, values fitting the specific conditions should be 

 determined. 



It is not universally agreed that change of time can compensate for all the changes 

 introduced by change of temperature. This point is particularly strong in the con- 



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