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HANDBOOK OF PHOTOGRAPHY 



when the fihns are changed from one solution to another; (2) the removal of thio- 

 sulphate ions by virtue of the formation of complex silver thiosulphate ions as nien- 

 tioned on page 380; and (3) the accumulation of sodium iodide resulting from the 

 conversion of the silver iodide of the emulsion to silver thiosulphate. It is, of course, 

 possible to revive the bath by the addition of hypo, but, as stated above, it is usually 

 good economj' to replace the fixing bath with fresh solution when negatives recjuire 

 more than 10 to 15 min. to clear. 



The rate of agitation of the photographic material in the fixing solution has an 

 important bearing on the speed with which photographic materials are fixed. Experi- 

 ments by Crabtree and Hartt^ indicate that if the time of fixing films without agitation 

 is taken as 100 per cent, the time of fixation when agitation is made every 30 sec. may 



10 

 9 

 8 



r 7 



\ 6 

 • 5 

 : 4 

 ■ 3 

 ' 2 

 1 

 



8 

 7 

 6 

 5 

 4 

 3 

 2 

 1 



Time to clear motion picture pcinchromafic 



deg.F 

 65deg.F 



ISc/eg. F 



Time to cleoir motion picture positive fTIm 



,■^55 deg. F 



1^-65 c/eg. F 



'^-75 deg.F 



Fig. 1. 



50 100 150 200 300 400 500 600 700 

 Concentration: Grams Na2S2 03'5hi20/LIter Solution 

 -Clearing time of plain fixing bath for various temperatures and concentrations. 



reduce this time to 80 per cent. The time of fixation may be reduced to from 88 to 

 60 per cent with continuous agitation, whereas, when brush treatment is used, the 

 fixing time may be from 75 to 50 per cent of the time required without agitation, 

 depending upon the emulsion and the solution employed. 



Figure 1 also shows the effect of temperature upon the fixing time. For motion- 

 picture negative film, Fig. 1 shows that with a 30 per cent solution the fixing time is 

 decreased from about 3.75 to approximatelj' 2 min. as the temperature is raised from 

 55 to 75°F. For a 40 per cent solution, the fixing time is decreased from 3 to 2 min. 

 for the same temperature change. For positive film, an increase in temperature of 

 from 55 to 75°F. reduces the fixing time from 50 to 30 sec. for a 30 per cent fixing 

 solution. From the shape of the curves it is evident that the fixing time is approxi- 

 mately inversely proportional to the temperature of the bath for the range of tem- 

 peratures normally encountered. For minimum fixing time, it is therefore advisable 

 not to use the fixing solution too cold. At elevated temperatures, however, the 



1 Crabtree, J. I., and H. A. Hartt, Some Properties of Fixing Baths, Trans. Soc. Motion Picture 

 Engrs., 13 (No. 38), 364-405 (1929). 



