384 HANDBOOK OF PHOTOGRAPHY 



1. Nonhardening fixing baths 



a. Plain hypo baths 

 h. Acid hypo baths 



2. Acid hardening fixing baths 



The purpose of the plain hypo fixing bath is simply to dissolve the silver halide 

 from the emidsion so that the sensitized grains will not be subject to exposure after 

 development and thereby ultimately stain the photographic material. Acid fixing 

 baths fulfill the same purpose, but, in addition, the acid neutralizes the alkali which is 

 carried over from the developer into the fixing bath so that the fixing bath may be 

 used over a longer period of time. The purpose of the acid hardening fixing bath, 

 which is probably the most convenient and frequentty used form of bath, is (1) to 

 dissolve the silver halide, (2) to neutralize the alkali carried over from the developer 

 into the fixing bath by the photographic material, and (3) to harden the emulsion so 

 that it is easier to manipulate without danger of damaging it. A definite quantity of 

 acid hardener is added to a specified amount of plain hj'po solution to prevent troubles 

 which might otherwise arise during the process of fixation. 



After the negative or positive has been developed the desired length of time, it is 

 necessary to stop development promptly. One method of accomplishing this is to 

 remove the photographic material from the developing solution and rinse it in clear 

 water which dilutes the developer clinging to the photographic material and con- 

 sequently removes most of the adhering developing solution. However, this rinsing 

 is never sufficiently complete to stop development entirely as some of the developer 

 remains in the pores of the gelatin coating. If the photographic material is rinsed in 

 an acid bath instead of in clear water, the acid will neutralize the alkali of the developer 

 so that development can no longer continue. The immediate stopping of development 

 eliminates stains which might otherwise appear on negatives and prints if only a plain 

 solution of sodium thiosulphate were used in the fixing bath. 



The alkali in the developer, which is added to energize the developing agent, 

 softens the gelatin of the emulsion, thereby rendering the photographic material 

 more susceptible to damage. The acid hardener is consequently added to counteract 

 this effect by hardening the emulsion. White or potassium alum is frequently used 

 as a hardening agent. It has, however, a limited hardening effect on the gelatin 

 which, under certain conditions, is not sufficient to prevent softening of the gelatin at 

 moderately high temperatures. Where greater hardening action is required, chrome 

 alum is employed. Formalin or formaldehyde has even greater hardening effect than 

 chrome alum. Consequently chrome alum or formalin hardeners are employed where 

 considerable hardening is required or where the amount of hardening provided by 

 potassium alum is insufficient. 



To keep down the amount of oxidation which occurs during the fixing process, a 

 preservative is added to the acid hardener. The purpose of this preserv^ative is to 

 inhibit the formation of sulphur, thereby eliminating (or at least reducing) the tendency 

 for the formation of silver sulphide which turns brown and stains the negatives or 

 prints. The preservative usually added is sodium sulphite. The preservative action 

 is greater when the fixing bath is maintained slightly acid. 



Plain Fixing Baths. — A plain solution of sodium thiosulphite will dissolve the silver 

 halide grains in the developed emulsion and will, consequently, accomplish fixing, 

 provided little or none of the developing solution is carried over into the fixing bath 

 by the photographic materials. If a plain fixing bath is used, it is desirable, either 

 to wash the film thoroughly between development and fixing in a bath of clear water, 

 or to neutralize the alkali of the developer through the use of an acid stop bath or 

 rinse between development and fixing. When the latter procedure is employed, some 

 sodium sulphite must be added to the fixing bath in order to prevent sulphurization 



