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Crahtree and Ives — Static Markings on Film 79
Methods of humidifying the air supply have been fully described in a previous communication.* Since the air in the printing room is at a higher relative humidity than that in any other room, it is necessary to boost the humidity of the air supply locally, and this can be readily accomplished either by means of water spray jets or steam jets. A series of water spray jets operated by compressed air and inserted in the air line serve to immediately change the relative humidity and have the advantage of cooling the air in hot weather. In winter both steam and water sprays are often necessary.
4. By humidifying the negative previous to printing. One contributing factor in the production of printer static is the friction between the gelatine surface of the negative and the emulsion side of the positive film in the gate, and especially during the pull-down period with old, dried out, shrunken negative. This can be largely overcome by humidifying the negative previous to printing by rewinding slowly two or three times in an atmosphere of 80% relative humidity, or by treating the emulsion side of the film with a solution of grain alcohol containing 10% to 20% water. Treatment of the film with this solution would insure that it would not attract dust in the cutting room, while it would assist in the prevention of static markings on positive film in the printer.
Dangers of Over-Humidifying Motion Picture Film
Too much humidification of film is worse than none at all for the following reasons :
a. Moisture spots are liable to be produced if drops of water condense on the emulsion.^
b. On winding moist film, the convolutions may adhere locally causing ferrotyping of the emulsion surface by virtue of being in contact with the polished base. On rewinding, the local adhesion of the film may cause more static markings than if the film had not been humidified in the first place.
c. Moistened film is more susceptible to thumb prints and abrasion marks than dry film.
d. Film which is too moist is apt to stick in the printer and may cause a stoppage, tearing of the perforations, or unsteadiness of the picture on the screen. Moist film is also apt to buckle causing lack of contact in the printer with resulting loss of definition.
* "The Development of M. P. Film by the Reel and Tank Systems," by J. I. Crabtree, Trans. Soc. M. P. Eng., Vol. 16, p. 163.