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28 III. 16-MM FILM AND ITS CHARACTERISTICS
Physical Characteristics Shrinkage
The continued evaporation of solvents and of moisture tends to shrink processed and raw film with storage time. This evaporation is accelerated by heat and low humidity; the heat from a projection lamp is a typical source of accelerated drying. Usually, film will reabsorb moisture if it is rehumidified, but cannot reabsorb the solvents that have evaporated, leaving a brittle product. When film absorbs moisture, it swells and becomes enlarged ; as it dries, it shrinks. Shrinkage is usually expressed as the percentage difference between the measured length of a 100-sprocket hole interval and its nominal length of 30.00 in. (The nominal distance between two adjacent sprocket holes is 0.300 in.)
With such films as sound-recording positive (Eastman Kodak 5372) an ultimate shrinkage of 0.5% is the maximum to be expected. Somewhat less than this shrinkage should be found if the film is stored in an airconditioned laboratory where such negatives are printed. Raw film (when it is removed from the can) should not exceed 0.20r/ ; more common values lie between 0.05 and 0.15r/< • If 16-mm raw film used for sound recording should exceed 0.20% , it should be looked upon with suspicion. With some release fine-grain positive films (DuPont 605 A) a shrinkage of 0.9% is the maximum to be expected. Somewhat less than this (approximately 0.6% ) should be the maximum in one-year-old films in film libraries and similar services.
Within the next few years — as soon as manufacturing conditions permit— all 16-mm base material will be of low-shrinkage types. This is but one example of the upward trend of raw film quality that has been going on without fanfare for almost a decade.
The shrinkage of reversal film and of color film will also lie in the same range. Most investigations of shrinkage have shown that the emulsion of a film as manufactured is quite elastic ; the measured shrinkage appears to be controlled almost entirely by the characteristics of the base. For a number of years it was tacitly assumed that the physical characteristics of safety base would be inherently inferior to those of nitrate base, especially with regard to toughness, flexibility, and shrinkage. Very important improvements have been made within the last few years; continued improvement seems highly probable because of the tremendous increase in demand for safety base for 16-mm film. The improvement in performance has now reached the point where the American film manufacturers