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April, 1930] EXHAUSTION OF DEVELOPERS 393
The acetic acid instantly stops the action of the developer and the iodide converts the undeveloped silver bromide to silver iodide which is not darkened by light. The alum serves to harden the film and the sulfite prevents the solution from becoming colored because of the action of light on the potassium iodide. The bath can be used repeatedly as long as it remains acid.
Stability of the Test Strips. — If the test strips are to be kept as a record or as a standard for comparison, they must be permanent. It has been found that motion picture positive film after 2 minutes in a stop bath is sufficiently iodized so that it is not darkened by several days' exposure to sunlight. To prevent the image from fading, the film must be washed thoroughly to remove free iodide and acid. The acid can also be removed by giving a final rinse in water containing a few drops of ammonia. Potassium iodide in an acid medium is readily decomposed by light forming free iodine which converts the fine silver grains back to silver iodide, causing the strip to fade. This explains the need for thorough washing.
It should also be noted that test strips should not be used more than a month after they are printed since the latent image on motion picture positive film is liable to show some fading in that time.
TYPICAL EXHAUSTION TESTS
In the photographs are shown test strips dipped for 1, 2, and 3 minutes at 65°F., 70°F., and 75°F., in borax developer at different stages of exhaustion and with different quantities of bromide added to the unused developer. In Fig. 1 are shown tests made at certain stages of exhaustion life of a 120 gallon tank of borax developer used for rack development of panchromatic motion picture film.4 After 80 feet per gallon had been developed in the tank, the film showed an apparent exposure about 70 per cent of that obtained with the fresh developer. This developer was used for a series of tests marked "before revival." The tank was revived by adding half the original quantity of developing agents and borax dissolved in as little water as possible with sufficient sulfite to equal 10 per cent of the solution added. This revived solution was used for the test marked "after revival." The tank of developer was then further exhausted to a total footage of 160 feet per gallon when the apparent
4 The authors are indebted to Dr. H. C. Carlton of these Laboratories for the history of these developer samples.