American cinematographer (Jan-Dec 1926)

July, 1926 AMERICAN CINEM ATOGR A P HER Twenty-one days a total of one hundred 8" x 10" sheets of panchromatic film per gallon (equivalent to about 500 feet of motion picture film) were desensitized and at the end of that period a test showed an average difference of about one step on the tablet used for these tests. Inasmuch as two steps on the tablet doubled the exposure and desensitizing is proportional to concentration, this indicated a loss of about 25% in the active concentration of the bath. More stock solution could be added to keep up the strength for further use, but a difference of that magnitude should be well within the limits of safety allowed. A tank of desensitizing bath could therefore be used at least as long as a tank developer, and by occasional strengthening its life could be extended very greatly. We have no indication that the fogging action or retarding effect on development is greater in old solutions than in fresh ones of equal desensitizing power. Wooden racks become stained when used repeatedly in a desensitizing solution, and so a test was made to see if the dye were absorbed by dried cypress wood sufficiently to interfere with the activity of the bath. The effect, if any, was found to be inappreciable. Small amounts of hypo up to 0.1% were added to a pinakryptol green solution and even after considerable use there was no indication that the hypo interfered in any way with the desensitizing action of the dye. VI. The Use of Pinakryptol Green in the Developer 1 Solubility in Developers. The most serious difficulty in using desensitizers in the developer is their tendency to form a precipitate with certain developing agents. The insoluble substance formed is apparently a combination of the dye and developing agent which forms in alkaline solutions. If the developer is oxidized the reaction reverses and the dye reappears in the solution. The formation of the precipitate is greatly retarded by the presence of quinone or oxidation products of the developer. For example, if a developer is partially oxidized by standing exposed to air or if 5% to 10%. of exhausted developer is added to the fresh solution much less trouble from precipitation occurs. Hydroquinone gives the most difficulty in this respect of any developer tried in this laboratory. It can be used in elon-hydroquinone developers in which the concentration of hydroquinone is not too high. For example, in the elon-hydroquinone tank developer (MQ80 tank) it can be added to a concentration of 1-25,000, in regular MQ-80 only about half that amount will remain in a fresh developer, while with No. 16 motion picture developer less than 1-100,000 is soluble. With pyro-soda 1:1 (B. J. formula) 1-25,000 precipitates if the solution is protected from the air, but in a tray oxidation takes place so rapidly that the precipitate may not form. When diluted 1 :1 :2 as usually recommended, the precipitation takes place in the same way if the developer is kept from oxidizing. Chlorhydroquinone (adurol) gives only slightly less trouble than hydroquinone. Para aminophenol with carbonate or with caustic alkali in the form of rodinal, glycin, and elon, either do not give a precipiate, or if one forms in a concentrated stock solution it readily redissolves on dilution of the developer for use. These facts are discussed fully by Luppo-Cramer8 (Bright Light Development, p. 52). When precipitation in a developer is likely to occur it is very important to add the desensitizer very slowlv Announcing a new price, now made possible by $j increasing interest in this markable Speed Lens We also make RADAR F Anastigmat (M. P.) as well as RADAR Telephoto Anastigmat F:5.6 and a complete line of high-grade cameras and lenses for the still studio. Write for our complete catalogue UNDLACH-MANHATTAN OPTICAL CO. 900 Clinton So., Rochester, N. Y. 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