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June, 1946 USE OF DESICCANTS 501
cated and sealed in a moisture-proof container to avoid growth of mold, fungus, or other damage.
Special care should be given to motion picture film, once the manufacturer's seal is broken, since lack of adequate equipment will often delay processing for many days or weeks under some field conditions. The raw film should be kept as cool as possible, although there is little practical benefit to be gained at temperatures below 50 F unless film is to be stored for many months.
If the film has been exposed to high humidities, the protection of desiccation should be given unless it is to be developed immediately. In the event that the exposed film must be kept more than a week or two and that the storage temperature may exceed 85 to 90 F, it is advisable to dehydrate the film even if it has not been subjected to high humidity conditions. Any film which has been subjected to desiccating procedures should be rehumidified before handling to avoid markings caused by static electrical discharge.
Practical Use of Refrigeration. — Refrigeration equipment can be used effectively for desiccating film if conditions are set up so that the moisture given up by the film to the atmosphere in contact with it is removed therefrom as dew or frost on the cold surface. In order that the moisture content of the film shall be brought to the 40 per cent RH level which is recommended by one manufacturer the temperature of the cold surface should be 25 to 30 F lower than that of the film which is assumed to be in the range of temperatures above 60 F. In order to ensure drying within about a day, the air should be in rapid circulation and should have access to all of the film surface. With rapid circulation it will be necessary that the air leaving the cold surface be reheated (to ambient temperature) so that it will not cool the film and thus lower the vapor pressure of the moisture in it. If the operation is carried on more slowly without circulation in an uninsulated container, the film probably will receive enough heat from the surroundings to maintain its temperature. The usual insulated domestic refrigerator, in which the cold surface is small and at a temperature below the frost point while the stored material more or less surrounds it and is at a temperature 15 or 20 F higher, can be used but will be slower. On the other hand, the film will have the protective effect of the lowered temperature during dehydration. There is no need for an insulated container for the drying operation. It is necessary only that an airtight (and probably light-tight) box enclose the film and cool surface and that provision be made for