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686 J. I. CRABTREE AND H. D. RUSSELL [J. S. M. p. E.
50°F. to 65 °F. a fixation time of fifteen minutes is necessary to produce maximum hardening, while at 75 °F. to 90°F. maximum hardening is produced in five minutes.
A similar study made with Eastman panchromatic negative motion picture' film developed in D-76 and treated in fixing bath No. II indicated that the minimum time in which negative film could be hardened satisfactorily was five minutes at temperatures above 65 °F.f but at lower temperatures a longer time was required.
(9) When to Discard the Bath. — A fixing bath is usually discarded for one or more of the several following reasons: (a) it sludges, (b) it does not harden satisfactorily, (c) the time of fixation is excessive, or (d) it is muddy or stains the film.
At normal temperatures with D-16 and D-76 and suitable revival, the hardening properties are maintained up to the point when the time for fixation with positive and negative film is doubled, at which point the bath is usually discarded. The rate of fixation may be maintained by discarding a portion of the bath and adding a concentrated hypo solution at intervals, but experiments have shown that this procedure is no more economical than to discard the bath more frequently.
With the chrome alum fixing bath No. II the exhaustion can be carried to the point at which the fixing power of the hypo is exhausted, if the bath is suitably revived with acid.
(10) Fixing Bath Troubles. — Troubles with chrome alum fixing baths are roughly of the same nature as those encountered with potassium alum baths as outlined previously.1 Only those peculiar to chrome alum baths will be discussed.
(a) Sludging. — A yellowish white sludge consists of sulfur while a greenish gelatinous precipitate consists of chromium hydroxide. The former is a result of the presence of too much acid in the bath or of storing at too high a temperature. The green sludge is caused by the presence of too much developer in the fixing bath and may be prevented either by rinsing in water before fixing, by the use of an acid stop bath before fixing, or by revival of the bath with acid as recommended. With the chrome alum fixing baths I and II, however, the hardening properties are exhausted long before a green sludge is obtained, so that in actual practice this sludge should not be encountered.
If a sludge or scum of chromium hydroxide deposits on the film