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484 J. I. CRABTRKE AND H. D. RUSSELL [J. S. M. P. E.
gelatin film green, but at normal temperatures no staining occurs unless the bath contains a high concentration of alum.
On the other hand, chrome alum baths possess the following advantages:
(a) Under suitable conditions, chrome alum has the property of rendering gelatin insoluble in boiling water while in all of the cases investigated in which aluminum alum was the hardening agent, it was impossible to obtain a gelatin film which would not melt at 212°F. with prolonged treatment. Chrome alum baths are therefore particularly suitable for processing at high temperatures.
(fc) Chrome alum is more efficient with regard to the quantity of alum required to produce a given degree of hardening. Lumiere and Seyewetz4 found that 0.5 per cent of chrome alum in a 15 per cent solution of hypo produced the same degree of hardening as 1.5 per cent of aluminum alum in a similar solution.
In order to determine more precisely the relative merits of chrome alum as compared with aluminum alum as a hardening agent for use with motion picture film, an extended investigation of the properties of chrome alum stop bath and fixing bath formulas seemed justified.
PART I— CHROME ALUM STOP BATHS
The Function of a Stop Bath. — During the development of photographic emulsions the gelatin is rendered alkaline by the carbonate or alkali in the developer. Part of this alkali may be removed from the film by rinsing in water between development and fixation but the gelatin still contains an appreciable quantity of alkali when the film is placed in the fixing bath, depending on the duration of the rinse. As previously shown,1 an excess of alkali destroys the hardening properties of a potassium alum fixing bath and tends to cause the formation of a precipitate of aluminum sulfite, thus rendering the bath unsatisfactory for further use. It is possible to prevent entirely the carrying over of alkali into the fixing bath by immersing the film in an acid solution termed a "short stop" or "acid stop" bath after developing and before fixing. If the stop bath contains hardening ingredients it is then possible to dispense with a hardening fixing bath and use a plain acid hypo solution for fixing purposes.
Stop baths are very necessary with certain types of motion picture film processing machines where the film is exposed to the air for several seconds in passing from the developer to the fixing bath. The stop bath not only arrests development immediately and prevents the possible formation of aerial fog and developer stain but likewise prevents sludging of the fixing bath.