16-mm sound motion pictures : a manual for the professional and the amateur (1953)

THE DEVELOPING MACHINE 371 permitted to dry without it. Usually there is a flannel-covered roller rotating within the cabinet against which the base side of the film gently rides as it goes through; this is known as a polishing roller. (8) The second stage drybox — where the film is slowly and finally dried by a steady blast of warm air that is cooler than the air in the first stage. The second-stage drybox may be more than twice as long as the first-stage drybox. Film is dried best if the surface is not dried too rapidly with respect to the rate of diffusion of the moisture from the inside of the film to the outside. Moisture diffuses quite slowly from inside to outside compared with surface evaporation under common drying conditions. Theoretically, the drying air temperature should be a maximum where the film enters the drybox, and should drop to approximately ambient air temperature. Staging is a practical means of accomplishing the objective. Drying is a very important function, and many commercial machines dry both improperly and insufficiently. The common error is to apply air that is too hot for too short a time, leaving the surface excessively dried and the body of the film with an excessive moisture content. After such a film is removed from the developing machine, the internal moisture diffuses to the surface making it sticky. Such a film is known in the trade as ' ' green film. ' ' The physical distortions suffered by the minute picture and sound images can be imagined when it is realized that the thermal coefficient of linear expansion of the emulsion is some 8 times that of the film base. The Eesearch Council of the Academy of Motion Picture Arts and Sciences of Hollywood issued a Technical Bulletin "Report on Film Preservative Tests" (Apr. 14, 1939) that described green film and the subsequent treatment it sometimes receives to counteract it: "Treatment given to release-print film after it has been printed, developed, and dried is commonly called ' film preserving, ' and the processes by which this treatment is given are known as 'film preservative' processes. "The gelatin of freshly developed film carries a high percentage of moisture in its pores and as long as this condition prevails is known as a ' green' emulsion. A so-called ' green' emulsion is quite soft and the slightest abrasion will cause a scratch. These scratches widen out as the gelatin dries, and cause the ' rainy' effects occasionally seen on the screen in the theater when old films are run. "As film with 'green' or soft emulsion passes through a projector, it leaves small deposits of emulsion on the tension shoes at either the aperture plate or the sound-gates, unless the tension shoes are kept thoroughly lubricated. Such deposits build up resistance to free passage of the film over them, and scratch the film during projection. ' ' When the moisture in a ' green ' emulsion is withdrawn too quickly, the gelatin shrinks and the film warps or buckles. If too great an amount of moisture is withdrawn from the gelatin, the film becomes brittle, loses its pliability, and is easily torn while being projected." The subject of "green" film is usually considered "delicate"; it is too often explained away rather than investigated and corrected. Many film preservative processes are to some extent processes that attempt to correct improper drying by other means such as emulsion hardening. Such processes do not correct improper drying; the treated film would be even better if the film were dried properly. Oftentimes "film preservatives" are applied a day or two after the film has been developed,