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April, 1930]
TESTING FOR SODIUM THIOSUL.FATE
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washing time of 20 minutes is required under ideal washing conditions to eliminate the hypo thoroughly.
THE HYPO CONTENT OF WELL-WASHED IMAGES
The hypo content of various materials washed for increasing times under the following conditions are given in Table II. Strips four inches long of the developed and thoroughly fixed films were placed in shallow trays 4 by 6 cm. and a stream of water allowed to flow into the trays at the rate of one gallon per minute, the water being completely emptied from each tray at two minute intervals.
TABLE II Time for Elimination of Hypo
Material
Time of Washing
Hypo Content
Eastman Motion Picture Panchro
2 Minutes
12 Mg. per foot
matic Negative (Type 2)
5 Minutes
8 Mg. per foot
10 Minutes
4 Mg. per foot
20 Minutes
1 Mg. per foot
30 Minutes
No test
Eastman Motion Picture Positive
2 Minutes
8 Mg. per foot
Film
5 Minutes
2 Mg. per foot
10 Minutes
*/2 Mg. per foot
20 Minutes
No test
A number of strips of motion picture negative film secured at random from a number of Hollywood laboratories showed a maximum content of 1 mg. of hypo per foot. A test of a number of strips of positive films from different laboratories indicated the almost complete absence of hypo. These tests would indicate, therefore, that present day laboratories are washing their films satisfactorily.
Practical Recommendations. — The times of washing required to insure the stability of motion picture film as indicated above are given merely as an indication of what is required under ideal conditions. In practice, the conditions of washing vary so greatly that it is absolutely impossible to recommend specific washing times, which is the reason for determining the hypo content of films or prints washed for increasing times under any given conditions, and selecting the time which will give the necessary stability.
SUMMARY
A method of testing for the presence of sodium thiosulf ate in motion picture film has been worked out which consists of placing strips of