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Fixing Baths — Crahtree mid Hart 393
knows, or should know, roughly the footage of film treated which has carried over a definite volume of developer to the fixing bath, assuming a constant period of drainage or rinsing between developing and fixing. Usually only one developer is used in connection with any fixing bath, but in case different developers are used, containing varying quantities of alkali, a slight correction factor should be used. The quantity of film fixed is, therefore, a measure of the quantity of developer added to the fixing bath, and therefore of the degree of exhaustion.
The point at which the revival should be made should be selected as outlined above. On an average it is advisable to revive at the half-way stage towards the sludging point. By experience the quantity of film necessary to cause a precipitate of aluminium sulfite is known and the bath should be revived when half this quantity has been treated. Thus, in the case of motion picture film if it requires 15,000 feet of film under given conditions to cause a precipitate with, say, 50 gallons of fixing bath, revival should take place after treating 7,500 feet of film.
4. The Quantity of Acid to he Added at Each Revival. As seen from Figs. 3, 4, and 5, it is necessary to increase the concentration of the acid as the concentration of the sulfite is increased in order to render the bath stable with regard to the precipitation of aluminium sulfite and for this reason it is apparent that the total acidity of a revived bath should be slightly higher than the acidity of the fresh bath in the first place. This increase in acidity cannot be definitely determined, but depends upon the procedure as followed by the individual worker, because various procedures permit the carrying over of varying quantities of developer into the fixing bath, depending upon the concentration of the developer and the period of rinse between development and fixation.
IX. Summary
1. A criterion for the comparison of various fixing baths has been developed and includes a consideration of (a) the time of fixation; (b) the degree of hardening produced; (c) the sulfurization life; (d) the developer capacity or the quantity of developer which can be added before a precipitate of aluminium sulfite is produced; (e) the propensity to blister; and (f) cost.