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Fixing Baths — Crahtree and Ilart 365
aluminum sulfite. This precipitate forms as a result of the reaction between the alum and the sulfite when the acid in the fixing bath becomes neutralized by the alkaline developer carried in by the film.
It has been shown by Sheppard and Ballard" that this precipitate consists of basic aluminium sulfite, the ratio of sulfurous acid to alumina varying according to the conditions of precipitation. Throughout this paper the precipitate will be termed "aluminium sulfite."
(d) The bath should not produce blisters in the gelatin coating of the film. If the bath is too acid, on the addition of developer the formation of carbon dioxide gas proceeds too rapidly and gas pockets are apt to form within the gelatin layer which finally break causing blisters.
(e) The degree of hardening of the fixed-out gelatin film should be satisfactory. Hardening of the film, however, is not necessary, provided the gelatin coating of the film does not swell excessively during processing so that it can be handled and dried satisfactorily.
Hardening primarily retards the swelling of the gelatin coating so that the film can be dried faster. In the case of unswollen and hardened film there is a minimum of water present in the film to be evaporated, while a higher temperature of the drying air can be employed without danger of softening or melting the gelatin coating. In case the film is handled by the rack and tank system,* the gelatin coating of unhardened film is apt to be damaged during wiping or squeegeeing previous to drying so that hardening is usually desirable.
The addition of developer to the fixing bath also has a decided effect on the hardening properties of the bath. Therefore, the bath should be so compounded as to maintain as nearly uniform hardening as possible throughout its life in spite of the gradual accumulation of developer carried over by the film.
(f ) The cost of the bath in terms of the' quantity of film fixed in it should be as low as possible.
II. Classification of Fixing Baths
Two essential types of fixing baths are in common use, namely
(1) non-hardening fixing baths: (a) plain hypo (b) acid hypo; and
(2) acid-hardening fixing baths.