International projectionist (Jan-Dec 1950)

lower nitrates of cellulose (chiefly cellulose tetranitrate) mixed with plasticizers such as camphor, butyl phthalate, tricresyl phosphate, and castor oil. A mixture of more highly nitrated celluloses, such as cellulose hexanitrate, is familiar as "nitrocotton" or "guncotton." Although guncotton is capable of exploding when struck sharply, as with a hammer, the pyroxylin-type mixture of lower nitrocotton in nitrate film base does not explode in the true sense of the word. In fact, nitrate film base cannot explode even by ignition (intense heating) except under phenomenally high pressures (3,000 or more atmospheres) . The so-called film explosions are really explosions of the gas produced by burning nitrate film. Ignition Level — 'Flash Point' Nitrate film base, like any other material containing large proportions of nitrocellulose, burns fiercely because it is a very unstable chemical compound which contains within itself sufficient oxygen to maintain vigorous combustion. As a matter of fact, a blazing reel of film continues to burn even when dropped into a tank of water, as the accompanying photograph shows. The ignition temperature, or "flash point," of nitrate film varies somewhat with age, new film igniting at about 180° C. and old film at about 120° £. A statement made to this effect in -an earlier contribution by the writer1 was " criticized by a spokesman for a wellknown film research laboratory in the following words:2 "The article also points out that 'old film has lost some of its camphor and is therefore more dangerously inflammable than new film.' Although it is often true that very old film may be more dangerous than new film, this increased hazard would be almost entirely due to increased likelihood of breaks in the film rather than loss of camphor." Ignoring the likelihood of breaks in old film, which is true but irrelevant as regards intrinsic inflammability, it may be stated as the opinion of authorities that aged nitrate film which has suffered incipient decomposition — principally through loss of plasticizing materials — burns more violently than the factory 1 "The Anatomy of Nitrocellulose Film: Its Import to the Projectionist" by Robert A. Mitchell; IP for February 1948, p. 5. 2 "Nitrate Film Symposium"; IP for April 1948, p. 10. TABLE A. COMPARISON OF NITRATE AND TRIACETATE FILM BASES Property Nitrate Triacetate (1) TENACITY: (a] Tensile strength (b) Tear strength 100 100 95 70 (2) PLIANCY: (a| Flexibility (b) Elasticity (c) Cold flow 100 100 100 95 75 115 (3) PERMANENCE TOWARD WATER: (a} Tendency to swell (b) Time of shrinking 100 100 100 100 (4) PERMANENCE TOWARD HEAT: (&) Tendency to emboss (bj N to become brittle (c) " to shrink Slight Great Moderate Very slight Negligible Slight (5) FLAMMABILITY Explosively inflammable Difficultly inflammable (6) PERMANENCE TOWARD TIME: (a} Tendency to shrink (bj " to become brittle (c) " to discolor Great Moderate Slight Very slight Negligible Negligible (7) TRANSPARENCE 100 103 1 ■imHi 1 | | ft «»-' m. Wf i * FIG. 1. Nitrate film burning under water. fresh product.3 Further, Max Scharnberg, Cine Engineer and Instructor at the Copenhagen (Denmark) Institute of Technology states:* "Films soften and melt at about 90° C. Old film may ignite at 120° C, and new film at approximately 180° C." Table C gives the reader a rough idea of the temperatures of various incandescent solid and liquid bodies. But it must be kept in mind that the table is based on "black-body" radiation, hence it is not valid for incandescent gases or luminescence produced by ionization. Table C reveals that the glowing tip of a cigarette (dull red heat) has a temperature of about 1,000° F., and is capable of igniting nitrate film almost instantly. , Causes of Film Fires Serious film fires usually start in the projector, but a large number of other causes is known. It has been determined by experiment that when the douser is opened to allow the full beam of the arc lamp to impinge upon film threaded up in a motionless cold projector, nothing more serious than the burning of a hole through one frame of the film ordinarily occurs. This is because the metal gate in which the film is enclosed conducts heat away so rapidly that the film is cooled below its ignition temperature. Only when the gate is rather hot (as is often the case) will the flame strike up to the upper film loop or burn down past the intermittent sprocket and ignite the lower loop. And the principal factor here appears to be the distance by which 3 "Aged celluloid which has lost a portion of its camphor burns more rapidly than that recently manufactured." "Celluloid," Masselon, Roberts, and Cillard, p. 183. * From the official and authoritative textbook, "Films-og Tonefilmsteknik" ("Motion Picture and Soundfilm Technique") by Max Scharnberg, p. 37. INTERNATIONAL PROJECTIONIST February 1950