International projectionist (Jan 1961-Dec 1962)

restore moisture to the acetate film base ,but nevertheless damage the emulsion irreparably by excessive absorption of moisture. Relative Humidity Important' Motion-picture raw stock is "stabilized" at relative humidities of 50%— 60% at the factory and sealed in cans with tape to prevent the entrance or escape of air. The raw film is stored before shipment at a temperature of about 50°F., and may safely be opened in laboratory rooms maintained at 70°— 80° F. if the cans are first allowed to warm up to the dew-point of the laboratory air to prevent the condensation of moisture and consequent spotting and mottling of the film. "Relative humidity" is a very important term in film storage and handling. It signifies the ratio of the quantity of water vapor present in the atmosphere to the quantity which would saturate the air at the prevailing temperature. Because warm air can hold more water vapor than cold air, relative humidity is not the same as absolute humidity, a measure of the intrinsic water-vapor content of air. There is thus more water vapor in the air at a temperature of 80°F. when the air is saturated (100% R.H. ) than at 30 °F. When warm moist air cools, the excess moisture condenses out as dew, but the relative humidity may remain at the same value. Emulsion Damaged by Moisture As stated above, the gelatin emulsion of the film releases and absorbs moisture much more readily than the film base of cellulose triacetate I Fig. 2 ) . The first effect of excessively humid air on film, therefore, is swelling and softening of the photographic emulsion. With prolonged exposure to high relative humidities the emulsion may become sticky and develop glossy spots called "ferrotype mottle." If the film is in a roll, the layers may adhere. Softened emulsion also "sticks" in the heated gate of the projector, and is easily scratched. At comparatively high temperatures moist emulsion frills and reticulates, utterly ruining the film for future use. Long-term storage of film at high relative humidities may cause the dye images of color prints to fade and the emulsion to become irreparably damaged by mildew or mold, a fungus growth which decomposes the gelatin. Custodians in charge of film vaults are therefore warned against the danger of relative humidities higher than 60%. Rapid "steaming" of film and short-term storage of the reels over tanks of water both fail dismally to restore moisture to the film base — that is, to restore the normal equilibrium-moisture the film originally had at 50% — 60% R.H. when shipped from the factory. It is easy to see why. A single strand of 35-mm acetate film requires about an hour of continuous exposure to moist air to attain normal moisture content from the filmdessicating R.H. of only 20% at 70°F. More startling, 35-mm acetate film wound in a roll requires a full month of continuous exposure to moist air for similar conditioning! Few theatres retain prints for so long a period of undisturbed storage. Damp Air Warps Film Film is physically deformed by sudden and prolonged exposure to atmospheres of excessively low or high relative humidity. Film which has been warped by improper storage conditions may flutter in and out of focus during projection and give unsteady images on the screen. Warped negatives give difficulties in printing. If reels of film in equilibrium with the moisture of the air at 50% R.H. are stored at 20% R.H., for example, Dry air t A I A k A -THTTjJ; Film absorbs moisture from the air... Film £ives up moisture to the air. FIG. 1 — The absorption of moisture from excessively humid air by motion-picture film is accompanied by softening and possible deterioration of the gelatin emulsion and shrinkage of the acetate base material. When film loses moisture to excessively dry air, it becomes brittle and curly. The natural moisture content of film is in equilibrium with the water vapor of the air at relative humidities between 50 percent and 60 percent. the edges lose moisture faster than the interior of the roll, resulting in buckled film. On the other hand, if reels of film at 50% R.H. are stored at 80'/ R.H., the edges of the roll swell, which may cause fluted or wavyedge film. Aside from being an impractical expedient which is harmful to the emulsion and possibly deforming to the base of the film, humidification of film in the projection room usually has other bad effects, even when it succeeds in restoring the normal moisture content of acetate or nitrate base. The spontaneous chemical decomposition of nitrate film is greatly accelerated by high relative humidities. As a matter of fact, the combination of moist air and high temperatures is very dangerous in the long-term storage of old ( and therefore unstable ) nitrate prints. Disastrous film fires have occurred in nitrate-film storage vaults where these simple facts were not appreciated and common-sense precautions not taken to retard the spontaneous deterioration of inflammable film. Even though nitrate film stock has not been manufactured in the United States since 1951, and is no longer used for projection prints, there are uncounted Per cent moisture content roco-t^oi C--JCOVO _ ru co V 2L w ft o (D O T o C * 3 ° Cu o i & V >v V &> V "3 SV Itf AS? YO ~-"~ q FIG. 2 — The gelatin emulsion of film is much more sensitive to changes in the relative humidity of the air than is the base material, absorbing and losing moisture more readily. International Projectionist May 1962