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May, 1930]
STOP BATHS AND FIXING BATHS
509
which exists will leave a stain and after drying this is very difficult to remove.
Any hardening or non-hardening fixing bath may be used in conjunction with the stop bath. A 30 per cent solution of hypo containing 1.0 to 2.5 per cent of sodium bisulfite or a regular potassium alum-acid or a chrome alum fixing bath may be used, although a hardening fixing bath is not necessary.
Hardening Life of Baths without Revival. — The life of a 2 per cent bath without revival with the D-16 developer is about 200 feet of motion picture positive film per gallon, that is, the bath will continue to harden the film so that it does not melt at a temperature below 212°F. up to this point. The life is only 100 feet per gallon over a period of use of two weeks.
If revival with acid is not possible or desirable, a 3 per cent bath is to be preferred in view of its longer life which, with the D-16 developer, is equal to 300 feet per gallon in a fresh bath and 200 feet per gallon during a period of use of two weeks.
Since the gelatin coating of negative motion picture film absorbs about twice as much developer as that of positive film the life of the stop baths when using negative film is proportionally less. The lives of the baths can, of course, be prolonged by rinsing in water previous to immersion in the stop bath but usually this is impractical.
With the D-76 developer the life of the baths is about one-half of that with D-16 when fresh and the hardening properties fall off more rapidly on keeping.
Hardening Life of the Baths with Revival. — If 2 per cent or 3 per cent baths are revived with sulfuric acid after processing each 100 feet of positive film or each 50 feet of negative film in either D-16 or D-76, the life of the bath is considerably prolonged as is indicated by Table III.
III. Life of Stop Baths with Exhaustion
Life of Bath (70 °F.)
Per Cent of Chrome Alum
Developer
Nature of Film Used
(Feet per Gallon)
Without
With
Revival
Revival
2
D-16
M. P. Positive
200
500
2
D-76
M. P. Pan. Negative
100
300
3
D-16
M. P. Positive
300
700
3
D-76
M. P. Pan. Negative
...
400