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Fixing Baths — Crahtree and Hart ' 385
VII. Factors Governing the Tendency of a Fixing Bath TO Produce Blisters
A large quantity of acid is desirable in a fixing bath because it permits the addition of a large quantity of developer before a sludge of aluminium sulfite commences to form. An excess of acid, however, may cause sulfurization which in turn can be retarded by the addition of sulfite or a buffer salt such as sodium acetate^^ but the sulfite in turn lowers the developer capacity whereas the acid was added to raise this capacity in the first place. The quantity of acid which can be added is likewise limited by the tendency of the bath to produce blisters on the films treated. The formation of blisters is largely due to the liberation of carbon dioxide or possibly sulfur dioxide formed by the action of the acid on the carbonate or sulfite in the developer retained by the film. The gases emitted in this manner form little gas bells in the gelatin which break when the pressure becomes sufficiently great producing a crater-like appearance in the gelatin.
It was found that the tendency of a fixing bath to produce blisters is governed by the following factors:
1. The quantity of sulfite and carbonate contained in the film when immersed in the fixing bath, which in turn depends upon the sulfite and carbonate content of the developer, and the extent of the rinse, if any, between developing and fixing.
2. The rate of agitation of the film when first immersed in the fixing bath. Agitation removes the excess developer from the surface of the film and hastens its diffusion out of the gelatin, thus rapidly lowering the content of alkali. Agitation of the films several seconds after immersing in the fixing bath is one of the most effective means of preventing blisters.
3. The nature and thickness of the gelatin film. Films having a thick coating of gelatin under given conditions blister much more readily than thinly coated films.
4. The duration of the rinse or wash as commonly used between developing and fixing. The longer and more complete the wash with respect to the removal of the developer, the less is the tendency of the fixing bath to blister, because the substances which cause blistering have either been removed completely or their con