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J. I. CRABTREE AND H. D. RUSSELL [J. S. M. P. E.
PRACTICAL RECOMMENDATIONS
A Suitable Chrome Alum Fixing Bath. — The following formula (F-23) is recommended for processing motion picture film.
Solution A
Metric Avoirdupois
Hypo* 240 grams 100 Ibs.
Sodium sulfite (desiccated) 12.5 grams 5 Ibs. 4 oz.
Water to make 750 cc. 38 gallons
Solution B
Metric Avoirdupois
Water 150 cc. 9 gallons
Sodium sulfite (desiccated) 5 grams 2 Ibs. 1 oz.
Sulfuric acid 5%** 40 cc. . 2 gallons
Potassium chrome alum 32 grams 13 Ibs. 4 oz.
Water to make 250 cc. 12 gallons
* A bath which fixes more rapidly may be obtained by increasing the hypo concentration in the above formula to 2Y2 Ibs. per gallon (300 grams per liter).
** To prepare 5 per cent sulfuric acid add one part by volume of concentrated sulfuric acid to 19 parts by volume of cold water and mix carefully with stirring. The acid must be added to the water, otherwise an explosion is liable to occur.
Dissolve the constituents of solution A and cool to 70 °F. Then mix solution B by adding the chemicals in the order given and cool to 70° F. Then add solution B to solution A while stirring the latter thoroughly.
If the solutions are mixed in a deep tank and the hypo bath is not adequately stirred, the chrome alum solution will tend to float on top of the hypo solution and sulfurization is apt to occur. A churn type of stirring paddle is recommended. Solutions A and B must be cool when mixed, otherwise the bath will sulfurize.
It is not desirable to store the solution B as a stock hardener because it would lose its hardening properties on keeping.
Life of the Bath.
a. Sulfurization Life. — The sulfurization life of the bath without use is about two weeks when stored at 70°F., and at higher temperatures is considerably less. However, as soon as developer is added, the sulfurization life is greatly increased, by virtue of the decrease in acidity and the addition of sodium sulfite.
b. Sludging Life. — The addition of 10 per cent of D-16 developer causes a sludge of chromium hydroxide to form in the bath after storing at 1 10°F. for four days, while 15 per cent of D-16 is required to sludge the bath in one week at 70°F. No sludges were obtained at 70°F. or 110°F. with the D-76 developer.
c. Hardening Life. — The limiting quantities of unrinsed film which