It is important to differentiate between one form of motion picture film and another, especially before deterioration begins. Nitrate negatives are difficult to distinguish from acetate and polyester film, although there are some useful clues. Cellulose acetate film generally has the word "safety" stamped on the edge of the film, and polyester film often has the word "ESTAR." The date of the film is also an important clue. In addition, two basic tests can be conducted: the flotation test and the burn test.
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The flotation test measures the relative densities of the three main types of film using a small sample cut or punched out of a transparent area of the film. One of two solutions is mixed in a test tube: 43 centiliters (cc) of trichloroethylene and 25 cc of trichloroethane, or 10 cc of Kodak Film Cleaner and 40 cc of trichloroethylene. The small samples of film are placed in the solution, and the test tube is sealed and inverted three to four times. The sample of modern acetate film will float to the surface within 10 seconds. Polyester film will remain suspended in the solution, neither floating to the surface nor sinking to the bottom. Nitrate film will immediately sink to the bottom. Sometimes older acetate film will behave like polyester and remain suspended. Because the test uses volatile solvents, it should be performed in a well-ventilated area. The burn test must be conducted in a well-ventilated area completely separate from the film storage area and in a carefully controlled environment. A small cut sample of the film is ignited. Nitrate film will immediately ignite and cannot be extinguished until it is completely burned away. Cellulose acetate film will ignite and burn with great difficulty and will not flame up. Polyester film will similarly be very difficult to ignite and does not burn easily. |
