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738 Transactions of S.M.P.E., Vol XI, No. 32, 1927
since a greasy emulsion surface has a high propensity to accumulate airbells on immersion in a developer,^ many oil spots have circular white centers caused by the protective action of such airbells. The airbells may be dislodged by gently brushing the surface of the film during the initial stages of development, but this is only possible when film is developed on a continuous processing machine.
The presence of oil on the emulsion side of motion picture film can usually be confirmed by placing the film for a few seconds in water at a temperature of about 100°F. Any drops or smears of oil will retard the swelling of the gelatin and will be visible in bas-relief on viewing the surface of the film by reflected light.
The most satisfactory method of eliminating oil from the surface of motion picture film and therefore of preventing the formation of oil spots is to immerse the film in a bath of benzene or carbon tetrachloride, squeegee, and dry throughly previous to development. Such a treatment can be accomplished successfully with a film cleaning machine of the Dworsky type.'^^
The access of oil to film can usually be prevented by care in the lubrication of machinery such as cameras, printers, and processing machines. Sprockets, idlers, and spindles should be oiled whenever possible by removing the sprocket or idler from the spindle and greasing the spindle with an oily cloth rather than by applying oil with an oil can. The arrangement of the lubrication of motion picture machinery to prevent the possibility of access of oil to the film should be given the most careful consideration by designers of such machinery.
A cknowledgment The authors are indebted to Edwin 0. Wiig for his assistance in part of the experimental work.
References
1 "Moisture Markings on Motion Picture Film" by J. I. Crabtree and G.E. Matthews, T. Soc. M. P. Eng. No. 17, 29, (1923).
2 "On Hydrogen Peroxide as the Active Agent in Producing Pictures on a Photographic Plate in the Dark" by W. J. Russell, Proc. Roy. Soc. (London) 64, 409, (1899).
3 "Rack Marks and Airbell Markings on Motion Picture Film" by J. I. Crabtree and C. E. Ives, T. Soc. M. P. Eng. No. 24, 95, (1925).
4 "The Reticulation of Gelatin" by S. E. Sheppard and F. A. ElHott, J. Ind. and Eng. Chem. 10, 727, (1918).
.^ "Image Contraction and Distortion on Photographic Plates" by F. E. Ross, Astrophys. J. 52, 98, (1920).
6 ((j^ Pneumatic Film Squeegee" by J. I. Crabtree and C. E. Ives, T. Soc. M. P. Eng. XI, Ao. 50, 270, (1927).