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Oct., 1944 PROPERTIES AND BEHAVIOR OF FILM 237
bulb temperature must not be too high while the film is still wet or the emulsion may be softened undesirably. Very rapid drying under extreme conditions may lead to various film troubles, which are discussed in a later section.
The Rate of Conditioning of Film. — The rate at which a film attains moisture equilibrium, or the rate of conditioning, is an entirely different problem from the rate of drying of a wet film. In Fig. 8 we have plotted the change in moisture content by weight (expressed as a per cent of the total change) for motion picture positive film initially in equilibrium with air at 78 per cent RH against the time exposed to an atmosphere at 21 per cent RH. It will be observed that the change in moisture content is very rapid at first, the rate of change decreasing as equilibrium is approached. This graph also shows the relative rates of conditioning of stripped emulsion and uncoated nitrate base which are particularly interesting. The stripped emulsion conditions much more rapidly than the base, reaching 90 per cent of equilibrium in a minute or two as compared with about 40 min for the base under the conditions of the experiment. This difference is due to the greater thickness of the base as well as to the differences in chemical constitution and physical structure between the 2 materials.
A number of extrinsic factors affect the rate of conditioning of film — the air velocity, the manner of air application, the temperature, the relative humidity difference, and so forth. Attempts have been made to develop an empirical formula which would relate these variables and enable one to compute the rate of conditioning of film under various circumstances. However, no simple way of doing this has yet been found so that it is necessary to determine by trial the optimum manner for conditioning film with a given piece of equipment.
The Processing of Motion Picture Film. — The application of some of the principles just described may be illustrated in the processing of motion picture film. In the continuous machine the film is generally immersed in the developer, fixing bath, and wash water for a total of 20 to 35 min, depending on the type of film and the processing conditions employed. By the time the film leaves the wash water, the emulsion is completely swollen, but the base only partly swollen. This is due to the difference in rates of swelling between the base and emulsion (cf. Figs. 6 and 7). Surface water is then removed from both sides of the film by a squeegee.
In the first stages of drying, moisture from the wet emulsion