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696 J. I. CRABTRBE AND H. D. RUSSELI, [J. S. M. p. E.
the chrome alum fixing bath No. I are inferior to those of No. II, and (5) the maximum degree of hardening which can be obtained on the addition of developer with potassium alum baths occurs just before the baths sludge, while with chrome alum fixing baths the hardening properties are destroyed long before the baths sludge.
(c) Although the developer capacity of some chrome alum fixing baths or the quantity of developer which can be added before sludging occurs is greater than with potassium alum baths, this advantage is offset by the fact that on revival of a chrome alum bath with sulfuric acid it is more apt to sulfurize than when adding acetic acid to potassium alum baths in the absence of adequate stirring.
(d) Both types of baths tend to deposit a scum on the film consisting of a basic aluminum or chromium sulfite, in the absence of agitation when using excessively alkaline developers. The aluminum scum is readily soluble in a weak solution of sodium carbonate but once the film is dried, the chromium scum is very difficult to remove.
(e) Film which is fixed in a fresh potassium alum bath tends to appear slightly opalescent after washing, but the opalescence disappears on drying. This opalescence does not appear with chrome alum baths at normal temperatures but this feature is of little importance in motion picture work.
SUMMARY
Owing to the fact that chrome alum either when used in a stop bath or when compounded in a suitable fixing bath is capable of hardening gelatin to an extent such that it will not melt or reticulate in boiling water, it is recommended for use with motion picture film when developing at high temperatures or whenever excessive hardening of the film is desired.5
Two varieties of chrome alum exist, namely, a violet and a green modification. It is probable that the latter is incapable of hardening gelatin. A change to the green variety occurs if the violet solution is boiled or if sodium bisulfite or an acid solution of sodium sulfite is added and the solution allowed to stand. In turn, the green variety is more or less changed back to the violet in the presence of sodium nitrite or an excess of sulfuric acid.
The difficulty involved in the use of chrome alum solutions is to maintain a constant degree of hardening since the hardening produced is influenced by a variety of factors as follows:
(a) Maximum hardening results when the solution in contact with