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dry the film is proportionately longer. If the developer is maintained at 65° F. and if the hardening properties of the fixing bath are maintained, abnormal swelling of the film will be prevented.
2. The rate of change of the air in contact with the surface of the film. If the layer of air in contact with the gelatine surface of the film remains stationary it soon becomes more or less saturated with water vapor and the rate of evaporation of water from the film diminishes as the air becomes saturated. It is important therefore to continually renew the air in contact with the film so that drying is not retarded. The drying reels should be revolved at the rate of about two turns per second for a 10 foot reel and the air supply should be such that there is a complete change of air in the room every 10 minutes.
3. The relative humidity of the drying air. At any temperature air is capable of absorbing a certain maximum quantity of water when it is said to be saturated. The quantity of water which air contains compared with the quantity of water which it would contain if it were saturated at that temperature is termed "the relative humidity" of the air. If the air is saturated it will not absorb any more moisture. Therefore moistened film will not dry in a saturated atmosphere so that the rate of drying is inversely proportional to the degree of saturation or the relative humidity of the air. With rise of temperature the capacity of air for absorbing moisture increases; that is, its relative humidity is lowered. Thus, in very cold weather even though the outside air may be almost saturated, on heating to room temperature the relative humidity may be as low as 20 to 30%. In cold weather therefore it is invariably necessary to humidify the air for drying purposes.
The Best Air Conditions for Drying Film
Unless the film is superhardened the temperature of the drying air should never exceed 75 to 80° F.; otherwise the gelatine will tend to soften.
If film is dried in air having a very low relative humidity and which is therefore very dry the rapid drying of the film will cause water marks wherever any drops of water remain on the film, while when thoroughly dry the film will be relatively brittle. Dry gelatine normally contains from 10 to 15% of water known as hygroscopic moisture. Gelatine containing only 10% water is brittle, while with 15% water it is pliable. In order to maintain the wearing qualities of the film it is important therefore that the gelatine should contain moisture in order to retain its flexibility and this can only be assured by drying the film in an atmosphere which contains sufficient moisture so that when the film is left in the atmosphere indefinitely it will not become brittle; in other words, the gelatine should be in equilibrium with an atmosphere having a certain critical relative humidity. It has been found that to maintain these conditions the relative humidity of the drying air should be from 75 to 80% at a temperature of 70° to 75° F. The same effect could be secured by rapid drying and then conditioning the film in an atmosphere at the above humidity but rapid drying should be avoided in order
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