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230
J. M. CALHOUN
Vol 43, No. 4
MOISTURE RELATIONSHIPS IN PHOTOGRAPHIC FILMS
One of the most important physical characteristics of a photographic film is its susceptibility to moisture. A film which is too moist is weaker, is more easily stretched and distorted, and may be tacky. A film which is too dry may be brittle and more easily cracked or torn. Moisture has a pronounced effect on shrinkage, curl, and virtually every physical property of a photographic film. Therefore, it is important that the influence of moisture on film be
clearly understood.
We frequently find that the amateur photographer, and sometimes even the professional, believes that a photographic film is either wet or dry and that there is no intermediate state. As long as the emulsion does not actually stick to the fingers, the film is often considered dry and this is responsible for some of the difficulties with green film. In actual fact, cellulose nitrate and acetate, as well as gelatin, in common with many other materials, exchange moisture vapor with their environment at every opportunity. Film in contact with dry air or wrapped in dry paper loses moisture ; film in contact with damp air gains moisture.
The physical chemical mechanism involved in this interchange of moisture is rather complex and need not be discussed here. It is only necessary to say that from a practical point of view, the moisture content of film is determined almost solely by the relative humidity of the air with which it is in equilibrium. Theoretically, the equilibrium relative humidity is the relative humidity of an atmosphere in which the material in question may be exposed for an infinite period of time without any change in moisture content. A knowledge of the rate of attainment of moisture equilibrium in a photographic film is of practical importance in many ways.
It is worth while to emphasize that the most important controlling
20 40 60 80 100 RELATIVE HUMIDITY °/o
FIG. 2. The equilibrium moisture content of gelatin, stripped positive emulsion, paper and nitrate film base at various relative humidities at 70 F.