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422 J. I. CRABTREE AND J. F. Ross [J. S. M. P. E.
The actual quantity of sodium thiosulfate contained in a photographic film or paper is of less practical interest than the relative degree of thoroughness of washing of the material. Common practice has more or less established washing times for various materials which are known to give stable images upon keeping as indicated below, so that by comparing the hypo content of the test film with that of films washed for increasing times, a measure of the thoroughness of washing is obtained.
The test is conducted as outlined above, namely, 10 cc. of the test solution are placed in a small glass cylinder and a single frame of the motion picture film is placed in the solution, taking care to use a glass rod or other non-metallic material to push the film under the solution. Standard films which have been washed under known conditions for 10, 20, 30, 45, and 60 minutes are used for comparison. Allow the solutions to stand for 15 minutes and compare the degree of opalescence by looking down the tube from the top.
The mercuric chloride test has been used to determine the relative washing times, in comparison with standards, of motion picture negative and positive films and the test was found satisfactory in all cases.
The quantity of material and the volume of solution used in making the test can be varied depending upon the quantity of material available for the test. One frame of standard motion picture negative film in 10 cc. of solution was found sufficient for a satisfactory test, while at least twice this area of film was required for the positive film test because the emulsion thickness of positive film is less than that of negative.
THE FADING OF SILVER IMAGES
In the presence of extremely small quantities of hypo or other sulfur-containing compounds, the rate at which the silver image is converted to silver sulfide is extremely slow at normal temperatures so that it was necessary to devise an accelerated test in order to hasten the rate of fading. The most satisfactory test consisted in suspending the test samples of film in a glass fruit jar, containing carbon dioxide gas, over a small quantity of water (Fig. 1) and storing at a temperature of 110°F. The strips of film to be tested were first arranged around the edge of the jar, an excess of carbon dioxide gas passed into the jar from a gas cylinder, and the jar then rapidly sealed. Careful tests indicated that the carbon dioxide did not attack the image but merely accelerated the fading reaction.