International projectionist (Jan-Dec 1956)

projected an equal number of times under identical conditions. At higher temperatures safety base is actually more elastic, more stable, than nitrate! Triacetate base begins to soften at 100° C. ; nitrate base at a temperature somewhat below 90° C, depending upon the age and condition of the nitrate film. (2) The superiority of triacetate safety film over nitrate in regard to permanence toward heat is also seen when the two types of base are exposed to heat of moderate intensity and tested for brittleness. When hot, nitrate film loses its rigidity and becomes so soft that it can be molded. Triacetate film retains its rigidity and becomes brittle. But when the films are cooled, nitrate becomes even more brittle than triacetate. Exposure to heat nevertheless buckles and warps triacetate film more readily than nitrate film. This effect of heat manifests itself principally in the perforation margins of film. The margins contact the hot metal of the gate runners and pressure pads and absorb a great deal of heat from them. Nitrate film softens and then hardens again, the flatness of the metal serving to remold the margins to their original flatness. Not so with triacetate, which doesn't soften. The margins warp and stay warped. Safety film then becomes edge-curled and buckled, making the picture difficult to focus. But nitrate film also buckles under such treatment because the margins are shrunk by the heat! The picture area of the film, having a greater longitudinal dimension than the margins, is forced into a wavy shape. Behavior Under Heat Polyester film, when properly formulated, is more heat-resistant than either nitrate or triacetate film. Samples tested by the writer undergo no measurable warping or shrinkage when subjected to heat sufficiently strong to produce these effects in both nitrate and triacetate stock. (3) Both nitrate and triacetate film become brittle when exposed to high temperatures and then cooled. But only nitrate gets brittle with the passage of time. Triacetate and polyester films are alike in that they can be stored for many years without acquiring any brittleness. Nitrate films also shrink, while triacetate and polyester films do not. (4) The shrinkage of motion-picture film has an important bearing upon the design of projector mechanisms. An irregular degree of shrinkage in the film causes a vertical weaving of the picture which can be minimized only by positioning the intermittent sprocket and the aperture as close together as possible. Moreover, the diameter chosen for the 16-tooth intermittent sprocket must be such that the sprocket will accommodate film having an average amount of shrinkage. Too large a sprocket results in tearing, crackling, and sizzling noises due to the disengagement of the sprocket teeth from the film perforations, while too small a sprocket, even though quiet in operation, increases the rate of perforation wear. The shrinkage of nitrate film by heat and time has always been a troublesome factor in the projection of this type of material. Older samples of nitrate film shrink up to 1.6%, but the Cronar Film Improved "Cronar," DuPont's new film base, recently underwent a second series of tests in a number of theatres in the Los Angeles area. Reports from projectionists who participated in these tests indicate a definite improvement in Cronar. Readers of the November 1955 issue of IP will remember an account of an earlier field test of Cronar conducted about a year ago. At that time a number of projectionists criticized the new film base because of a tendency to flutter out of focus when projected with powerful arc lamps. Following further research at DuPont laboratories, a second set of field tests were undertaken. The test consisted of putting special prints of the film, "Desperate Hours," composed half of Cronar base film and half of standard acetate base, through all the strains of everyday use in the projection room. Commenting on the tests, Paul Mahoney, assistant business manager of IA Local 150 in Los Angeles, states that reports from projectionists in several theatres where the test print was used were highly favorable, "especially with regard to the brilliancy of light and focus in equality to the acetate sequences." Projectionist Ted Pylet, of the Hawaii Theatre in Hollywood, who criticized the focusing aspect of Cronar film following the earlier trial, reacted differently after the more recent run. "The tendency of the film to go in and out of focus which I noted in the previous test last year, using a print of 'Country Girl,' has been eliminated." average amount of shrinkage noted in theatre-release nitrate prints during their active life is only about 0.4%. The intermittent sprocket, therefore, must accommodate shrinkages near the maximum, or at least a little above the average, hence the diameter of 0.935 inch chosen for intermittent sprockets made prior to the introduction of non-shrink triacetate film. The exclusive use of triacetate film has eliminated the shrinkage problem. But because old nitrate prints are still used from time to time, the new larger sprockets are made with a diameter of 0.943 inch to permit the running of nitrate prints as well as to give good results with triacetate prints. With the complete discontinuance of nitrate film, the diameter of the intermittent sprocket will be increased to 0.945 inch. (5) We have pointed out that nitrate film becomes brittle and shrinks with the passage of time. It also decomposes chemically, becoming discolored, very weak, sticky, bad-smelling, and spontaneously explosive. Even unused nitrate film stored under ideal conditions of temperature and humidity deteriorates to such an extent that it is practically worthless after about 50 years. Triacetate film is as permanent as any other known type of film, its life in storage being estimated at 1,000 years. And triacetate film does not require so moist an atmosphere for satisfactory storage that nitrate film does. The gelatine emulsion is thus better protected against molds and fungi that destroy it. Polyester film is also permanent and equally immune to shrinkage. Shrinkage Characteristics The processing of motion-picture films involves their immersion in various aqueous solutions, a long period of washing after development and fixing, and subsequent drying. Nitrate film slowly expands when immersed in water, and then shrinks to its original length when dried. Earlier types of safety film, notably those made of cellulose diacetate, expanded and shrank too rapidly, causing slack loops in the processing machines and film breaks in the drying cabinets. Modern triacetate safety film expands and contracts to a lesser degree, and quite as slowly, as nitrate film. Polyester film neither expands nor con{Continued on page 33) INTERNATIONAL PROJECTIONIST MAY 1956 11