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Oct., 1944
PROPERTIES AND BEHAVIOR OF FILM
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factor in determining the moisture content of photographic film as well as paper, gelatin, and related materials, is the relative humidity of the air and not the absolute humidity. This means that at 50 F and 60 per cent RH, for example, film will hold more moisture than at 80 F and 40 per cent RH even though the actual moisture content of the air (absolute humidity) is approximately 32 grains per Ib of dry air in the first case, and 62 grains in the second case. However, the moisture content of cellulosic materials is not entirely independent of the dry bulb temperature at constant relative humidity. Various investigations have shown that cotton,6
60
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0 20 4C
RELATIVE HUMIDITY °/o
FIG. 3. The equilibrium moisture content of Eastman Safety and Nitrate Motion Picture Positive film base at various relative humidities at 70 F.
paper,7 and cellulose acetate film8 hold slightly more moisture at lower temperatures when the relative humidity is constant. Nevertheless, in practice, the effect of a change in the dry bulb temperature at constant relative humidity on the equilibrium moisture content of photographic film is negligible, compared with the effect of a change in relative humidity at constant temperature.
The Equilibrium Moisture Content of Film.— Fig. 2 illustrates the relation between equilibrium moisture content and relative humidity for several common photographic materials— gelatin, positive emulsion, paper, and nitrate film base. The difference in the mois
20 40 60 80 100
RELATIVE HUMIDITY °/o
FIG. 4. The equilibrium moisture content of Eastman Safety and Nitrate Motion Picture Positive emulsion coated film (1943) at various relative humidities at 70 F.