American cinematographer (Jan-Dec 1928)

January, 1927 AMERICAN CINEMATOGRAPHER Nine The Lubrication of Motion Picture Film When freshly developed or socalled "green" motion picture film is passed through a projector, there is a tendency for an incrustation to accumulate on the aperture plate or tension springs which retards the free passage of the film through the machine. Chemical analysis has shown that this incrustation consists largely of gelatin with more or less silver, dirt, and oil, but it contains usually only a trace of the metal or alloy of which the gate is composed. The effect of the incrustation is to increase the friction between the metal parts of the gate and the gelatin coated surface of the film. This causes excessive strains on the edges of the perforations at the pull-down sprocket which ultimately results in torn perforations and therefore a diminished projection life of the film. It is possible to reduce considerably the tendency for the formation of the gate incrustation by suitable lubrication of the film surface. This is accomplished usually by the application of a thin line of paraffin wax to the edge of the film which melts under the heat of the projector and forms an effective lubricant. However, the wax tends to wander over the picture area if applied By J. I. Crabtree and C. E. Ives (Communication No. 330 from the Eastman Kodak Research Laboratories) Photomicrographs showing appearance of surface of motion picture film by reflected light. Fig. 1 — Clear area of film. Fig. 2 — Area in region of silver image. Fig. 3 — Silver image toned with iron ferrocyanide. in excess and particularly in the case of sound record films; this is very objectionable. It is the object of this paper to discuss the various methods of lubrication employed to date and to indicate a new method which is equally satisfactory for sound record and ordinary motion picture films. Factors Affecting the Ease of Passage of Motion Picture Film through a Projector The facility with which the film passes under the pressure springs in the projector gate depends on: 1. The physical condition of the gelatin coating of the film. 2. The conditions to which the film is subjected in the projector. 1. If motion picture film is examined under a microscope by reflected light, it is seen that the gelatin surface, even in the region which is relatively free from silver, is covered with innumerable extrusions (see Fig. 1, magnification 540). The roughness of the surface is much greater in the vicinity of the silver image (see Fig. 2, magnification 790) and if the latter is toned with iron or uranium the roughness is still greater (see Fig. 3, magnification 790). This is as would be expected because the toning process intensifies the image by virtue of the Photomicrographs showing effect of burnishing, and coating the film surface with wax and then burnishing. Fig. 4 — Untreated film. Fig. 5 — Burnished. Fig. 6 — Waxed and burnished. deposition of iron or uranium ferrocyanide around the silver grains composing the image and thus enlarges them. It is possible to smooth the film surface either by grinding away or burnishing down the minute projections or by filling up the crater-like depressions. The effect of burnishing and of filling up the depressions with wax and then burnishing or polishing is strikingly shown in Figs. 4, 5, and 6. Fig. 4 shows the surface of untreated film (magnification 540). Fig. 5 shows the same film after burnishing and Fig. 6 after applying wax and burnishing. Tests have been shown that the act of burnishing or polishing the film surface without the application of a lubricant such as wax or oil does not appreciably facilitate the passage of the film through the projector gate. It is well known, however, that film which has been projected once or twice has a much less tendency to pro( Continued on Page P>) Fig. 7 — Showing partial burnishing effect on film during projection.