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June, 1930] STOP BATHS AND FlXING BATHS 693
ture film is unnecessary, although a moderate degree of hardening is desirable so that in a relatively damp atmosphere the gelatin will not become excessively tacky and tend to produce an accumulation of gelatin on the pressure springs in the projector gate.
If the processing conditions are such that the wash water is warm or if rapid drying at high temperatures is necessary, then a degree of hardening greater than that which is usually attainable in potassium alum baths is necessary. For processing under tropical conditions the use of chrome alum baths is imperative.
(1) The question of a choice between a chrome alum stop bath or fixing bath at high temperature is largely determined by the available space. The use of a chrome alum stop bath, with or without the addition of sodium sulfate, followed by a potassium alum fixing bath is very desirable because a chrome alum fixing bath may lose its hardening properties before the fixing power of the hypo is exhausted. A chrome alum stop bath being much less expensive can be renewed more frequently. The alternative is to use a chrome alum fixing bath containing a relatively large proportion of chrome alum.
(2) The question of the desirability of using a potassium alum or chrome alum fixing bath for processing motion picture film at normal temperatures may be decided by a study of the following comparison of their properties.
(a) Chrome alum either tends to harden the gelatin so that it does not melt in boiling water, or it does not harden the gelatin at all. In some cases chrome alum merely hardens the upper layers of the gelatin coating leaving the lower substrata unhardened and in this condition the film is susceptible to reticulation. Potassium alum fixing baths produce a medium degree of hardening so that the gelatin usually melts at a temperature around 120°F. to 180°F.
(6) The hardening action of potassium alum baths does not fall off with age as is the case with many chrome alum baths and the former respond more readily and consistently to revival with acid. Fig. 6 shows the effect of reviving the F-2 potassium alum fixing bath at intervals with acid, and it is seen that the hardening varies largely between the limits of 120°F. to 180°F., the hardening of the final bath being of the same order as that of a fresh bath. The hardening properties of the potassium alum fixing bath F-2 (Fig. 6) cannot be compared directly with the hardening properties of chrome alum fixing baths on exhaustion since they were not exhausted under comparable conditions. In the case of the F-2 bath the exhaustion was