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264 J. M. CALHOUN Vol 43, No. 4
The greater change in curl observed in Table 6 at the lower relative humidities is believed to be due to plastic flow produced by transverse flattening of the film. At the low relative humidities the film has a higher curl at the time of winding which increases the widthwise stress on the film. The rather surprising fact that the film wound emulsion-in at the higher relative humidities actually becomes more positive in curl can be explained by plastic flow produced by the longitudinal winding. It may be noted that the effects of shrinkage of the base and plastic flow due to longitudinal winding operate in opposite directions when the film is wound emulsion-in, but in the same direction when the film is wound emulsion-out. This explains the much larger negative drift in curl when film is wound emulsion-out.
From a practical point of view Table 6 demonstrates that where it is desired to retain a small positive curl, film should be wound emulsion-in at moderate relative humidities (preferably 40 per cent to 60 per cent); and stored at low temperatures (50 F to 60 F) in taped cans. These conditions are more easily fulfilled in the case of raw stock. Processed film which is normally wound emulsion-out is in a somewhat less favorable condition from the point of view of curl changes, although these are generally not serious except where the film is reeled when very dry and then stored at high temperatures. A film having too high a positive curl may be improved by winding emulsion-out, while film having too negative a curl may be improved by winding emulsion-in.
CONCLUSION
The proper performance of motion picture film in practice depends to a marked degree on the physical properties of the film, and on how these properties are affected by relative humidity, temperature, and other factors. The importance of proper protection of film against moisture changes during storage, and the exercise of a certain amount of control over relative humidity in most operations in the motion picture laboratory cannot be overemphasized.
Safety base motion picture film is now substantially improved in its physical properties compared with that manufactured 6 or 7 years ago, although it is still inferior to nitrate film in some respects.
It is our hope that this discussion of the physical properties of motion picture film will clarify some of the peculiarities in film behavior, and prove of practical value to the motion picture industry.