FIXING, WASHING. AND DRYING 389 



Fixing solutions suitable for tropical processing, or where the temperature of the 

 solutions cannot be reduced to 70°F., are given in baths 24 to 26 inclusive. Bath 24 

 has considerable hardening action and will keep a week at 100°F., although it has not 

 the keeping properties of the ordinary acid fixing and hardening baths. Bath 25 is a 

 chrome alum fixing bath recommended for tropical processing where the temperature 

 does not exceed 95°F. Bath 26 is a formalin solution for tropical development. The 

 hypo should be dissolved in about half the required volume of water. The sodium 

 sulphite and formalin should be added in that order, and the solution made up to 1 1. 

 by the addition of the proper amount of water. The bath keeps well, although a 

 harmless milky precipitate may form. The fumes of the formalin (40 per cent 

 formaldehyde) are irritating, and for this reason formalin should be kept in closed 

 containers as much as possible. 



A fixing bath for prints or film, which, it is claimed, will completely fix in 2 min., is 

 given as bath 27. The bath should be kept at 70°F. and should be made just prior to 

 use as it does not keep well. 



Another fixing bath suitable for rapid fixing is bath 28. 



Practical Considerations in Fixing. — The rate at which photographic materials are 

 fixed depends upon (1) the nature of the emulsion, (2) the concentration and com- 

 position of the fixing bath, (3) the degree of exhaustion or the extent to which the 

 fixing solution has been used, (4) the rate of agitation of the photographic material 

 in the rinse and fixing baths, and (5) the temperature of the solution. 



In general, coarse-grained emulsions are fixed more slowly than those whose 

 emulsions contain fine grains of silver halide. Likewise a film having a thin emulsion 

 coating fixes more rapidly than one having a heavy, thick coating. Negative mate- 

 rials, particularly fast plates and films, contain some silver iodide which dissolves more 

 slowly than the bromide, and this is another contributing factor to the relatively slow 

 fixing of fast coarse-grained emulsions. In practice, the maximum time required to 

 completely clear the film of its opalescence is about 10 to 15 min.; if the film takes 

 more than this time to clear, it is usually economical to prepare a fresh fixing solution, 

 even though the fixing solution may not be completely exhausted and would still be 

 capable of fixing the material in 20 to 30 min. For negative materials, the time of 

 fixing is often taken as twice the time required for the film to become completely 

 cleared of its opalescence. 



Printing papers may be completely fixed in about 1 min., provided the paper is 

 thoroughly agitated in a fresh fixing solution and the free flow of the fixing bath around 

 the print is not inhibited by the presence of other prints. In actual practice, however, 

 the prints are not usually very well agitated in the fixing bath, and it is difficult to 

 prevent two or more prints from adhering together. For this reason a longer time of 

 fixation is required. 



For a given emulsion and temperature, the time of fixation of a fresh hypo solution 

 depends upon the concentration of the sodium thiosulphate, as shown in Fig. 1. From 

 these curves, which are also representative of those obtained for other emulsions, it is 

 seen that the optimum concentration is not extremely critical. Negative film fixes 

 most rapidly when the concentration is 400 g. of hypo per liter, so that 40 per cent is 

 the optimum solution. The motion-picture positive film is fixed most quickly in a 

 30 per cent solution. Most of the fixing baths are made with a concentration varj'-ing 

 from 200 to 400 g. of hypo per liter of solution. Unless rapid fixation is required for 

 some particular purpose, a 30 per cent solution will be found satisfactory for practically 

 all purposes. 



As the same fixing bath is used repeatedly, the rate of fixation continually decreases. 

 The falling oft' of fixation rate is due to (1) dilution of the bath as a result of the addi- 

 tion of developer or rinse water, as well as the removal of hypo to the wash water 



