International projectionist (Jan 1959-Dec 1960)

FILM STRUCTURE Continued from Page 16 dangerous nitrate film in the professional field. Triacetate Safety the Best The big break-through in safety film was the discovery of German scientists that a cellulose somewhat more highly acetylated than the diacetate* furnishes a material for film base which more nearly resembles nitrate base in working properties while retaining the advantage of non-inflammability. This is the so-called triacetate* base which is now universally used for theatrerelease prints, but first manufactured as early as 1939 by the German photographic firm of Agfa. Production of triacetate film was curtailed during World War II, but was resumed in 1948 when Eastman Kodak led the way by adopting it as a replacement for nitrate, the manufacture of which was totally discontinued in 1950. All of the professional 35-mm negative and positive film stock regularly supplied to the industry in both Europe and America is now made on triacetate safety base. Kepresentative are the acetate film stocks manufactured by Agfa ( Leverkusen ) , Agfa ( Wolf en ) , Ansco, DuPont, Eastman Kodak, Ferrania, Gevaert, Hauff, Ilford, Kodak Ltd. (London), Kodak Pathe, Lainer & Hrdliczka, Mimosa, and Perutz. In general, triacetate film is almost as good for projection purposes as nitrate; and, of course, it possesses the supreme advantage of being completely safe under all conditions of use and handling. Acetate film burns much less readily than paper or wood, and it will not support combustion at all when wound up in a roll : it is, in fact, completely devoid of fire danger. Triacetate film is superior to nitrate in at least a few other ways, e.g. it does not decompose on longterm storage, and it shrinks a bit less than nitrate with age. Modern triacetate film is almost (but not quite,) as strong as nitrate film, and it is almost (but not quite) as long-wearing in use. Triacetate is undeniably a big improvement over diacetate and acetopropionate in regard to brit tleness, but even though initially very pliant, repeated projections cause it to become more brittle than nitrate film. This has been as much of a nuisance with TV stations using 16-mm film as with theatres using standard-gauge prints. The tear strength of brittle triacetate film still in usable condition is scarcely 30 to 40 grams, whereas that of fresh triacetate stock of the same thickness (approximately S1^ mils) is close to 60 g (ASTM method D689-42T at 45 per cent relative humidity). Also, triacetate film is a trifle more liable than nitrate to permanent deformation by mechanical stress when it is heated, as by the arc-lamp beam at the aperture of a projector. Nitrate film also softens when heated, but instead of softening at relatively low temperatures (viz. 200 degree F, the softbase), it retains a fair degree of rigidity until the tempeature approaches close to 300 degree F, which is high enough to be dangerous in the case of old nitrate prints. Softening of triacetate film makes it more liable to become permanently buckled by projection with powerful arc lamps, and it becomes increasingly brittle each time it is heated and cooled.** Even though nitrate film is no longer manufactured, and is prohibited by law in most European countries it is useless to deny the fact that the desirable projection qualities of fresh nitrate stock are still the ideal toward which improvements in acetate film are aimed. Film-Shrinkage Proolem When it comes to old, well-seasoned prints, acetate films give better screen results than nitrate films which have also been much used. This is because well-worn nitrate film shrinks and deteriorates *A 10-octave frequency range means that the lowest frequency is doubled, the ■esulting' product likewise doubled, etc., "or a total of 10 times. The mathematical 'ormula is a equals 512b, in which a is ihe high-frequency limit and b is the ow-frequency limit. ***The Greek letter v ("pi") represents a definite transcendental number which cannot be exactly expressed as a rational integer. Approximately equal to 3.1116, it is the ratio of the circumference of a circle to its diameter. Its reciprocal (1/jt) equals 0.3183, approximately. See Chapter 30 of Mitchell's Manual of Practical Projection for an sasy-to-understand explanation of a few common mathematical terms with which the projectionist should be familiar. physically more than acetate does. The greatest amount of linear shrinkage of nitrate film seen by the writer was 1.7 per cent in some used theatre prints made during World War I. Film as shrunken as this really "sings" on the sprockets, making such loud buzzing and crackling noises that the projectionist may fear imminent breakage. Careful control of the nitration process and the use of superior plasticizers resulted, in later years, in nitrate positive stock which shrank only about 0.30 per cent after a 2-year storage period under ideal conditions, or 0.60 per cent after 2 years of wear and tear in the theatres. The frequently repeated statement that triacetate film is practically immune from shrinkage is, of course, completely untrue. It shrinks less than nitrate ; but the usual triacetate base formulated for theatre-release positive does shrink appreciably, and it becomes more brittle than nitrate. Tests on different brands of triacetate projection film reveal that this type of film shrinks 0.25 per cent after 2 years of storage, and about 0.40 per cent after 2 years of heavy projection duty in thetres. In other words, triacetate shrinks about 80 per cent as much as nitrate after 2 years of storage, or 65 per cent as much after a like period of normal projection use. It must be kept in mind, however, that different samples of different brands of film show a wide variance in the rate of shrinkage. A special triacetate base has been formulated for motion-picture negative which shrinks up to 0.18 per cent after 2 years of storage, and never seems to exceed the low shrinkage of 0.20 per cent no matter how long it is stored! This type of base is unfit for projection purposes, however, because the heat of the arc lamp expels the plasticizers and causes it to become excessively brittle. "Pitch" of Sprocket Teeth The shrinkage of release positive stock it an important factor in the design of projector sprockets, especially the intermittent sprocket. The perforations of freshly made 35-mm positive raw stock are spaced 0.187 inch from the edge of one sprocket hole to the BRIGHTER LIGHT LONGER BURNING SHARPER PICTURE free Test Samples wwv n»»MM»»» owaMSwL&wfottA CARBONS INC., BOONTON, N. J. STEADIER LIGHT LESS ATTENTION PERFECT COLOR BALANCE Free Carbon Chart 20 International Projectionist July 1960