American cinematographer (Jan-Dec 1927)

May, 1927 AMERICAN CINEMATOGRAPHER Nine Cleaning Liquids for Motion Picture Film By J. I. Crabtree and H. C. Carlton First Installment of Treatise on How to Remove Foreign Substance from Cine Stock (Communication No. 306, from the Research Laboratory of the Eastman Kodak Company, and appearing in Transactions of Society of Motion Picture Engineers.) T IS necessary to clean motion picture film at various stages in its progress from the laboratory to the theatre to remove : I 1. Dirt on the base side of negative or positive film. 2. Dirt or grease which may accumulate on negative film during printing. 3. Dirt and oil which accumulates on positive film during projection. 1. When proceeding in the laboratory by the reel and tank system, if all excess water is not removed from the film previous to drying, any dissolved salts present in the water supply remain on the film after evaporation of the water. The residual salts are usually only visible on the base side of the film because on the emulsion side they have an opportunity to diffuse within the gelatin coating during drying. It is necessary to clean the back of the dried film either by wiping with a damp chamois while on the dying reels or by passing the film through a cleaning machine. Such treatment is unnecessary in the case of positive film if all excess water is removed previous to placing on the drying reel by thorough wiping or squeegeeing. In the case of negative film it is customary to wind it with the emulsion side downward onto a wooden drum covered with cloth when the base side may be cleaned without danger of injuring the image. The cloth should be removed from the drum at frequent intervals for cleaning. A suitable cleaning liquid for the above purpose should possess the following properties : (a) It should be capable of dissolving traces of inorganic salts and should also dissolve or emulsify grease and mineral oil. (b) It should be sufficiently volatile and should not cause the gelatin side of the film to swell in a period of several seconds if it accidentally has access to it. (c) The liquid should not affect the physical properties of film with safety or nitrate base or remove the color from film with tinted base. A suitable mixture fulfilling the above conditions is the following: Metric Avoir. Ammonia (Cone.) 5 cc. 2-3 oz. Water 95 cc. 12 oz. Denatured alcohol (see below) to make 1000 cc. 1 gallon The ammonia serves to emulsify any traces of grease or oil, while the mixture contains sufficient alcohol to prevent dangerous swelling of the gelatin if any of the mixture reaches the emulsion side of the film. A choice of several alcohols for preparing the above liquid is available as follows: Grain alcohol (ethyl alcohol) — This is the most satisfactory for the purpose since it has a minimum effect on the film base. Denatured alcohol — Ethyl alcohol is available containing a variety of denaturants. The most common denaturant is wood alcohol, which dissolves nitrate film base so that this should be avoided if possible. The most commonly available denatured alcohol is motor alcohol. The "Pyro" brand of the Industrial Alcohol Company is prepared according to the following formula, No. 5 of the U. S. Internal Revenue Bureau : Ethyl alcohol 100 volumes Wood alcohol 2 volumes Pyridin bases 0.25 volumes Kerosene 0.5 volumes On diluting this with water the alcohol turns milky owing to the kerosene coming out of solution. Kerosene has no effect on the film base or gelatin coating and serves to dissolve grease. Although pyridin and wood alcohol attack the film base when pure, in the above concentration and when diluted with water in the above formula they have no harmful effect on the film base during the time required for cleaning. The above cleaning liquid prepared with "Pyro" motor alcohol had only a slight tendency to produce curl on film with nitrate or acetate base after complete immersion for 24 hours at 70° F. Isopropyl alcohol — This is now available commercially and the "practical" grade is satisfactory for the purpose. It does not turn milky on mixing with water and has little or no curling effect on film with either nitrate or acetate bases, even on immersion for several hours. It is non-poisonous,1 is not decomposed on exposure to light and when used in the above mixtures does: not attack the silver image or the gelatin coating. Tertiary butyl alcohol is also available commercially and has properties similar to those of isopropyl alcohol. Its odor, however, is somewhat objectionable. All the above alcohols tend to remove more or less of the tint from nitrate or safety tinted base film but the water present in the above cleaning liquid greatly retards this action. The precise effect of cleaners prepared with the various alcohols on the tinted base is shown in the following table. Samples of film were immersed in the cleaners and the times required for visible signs of removal of the color were observed. Effect of Film Cleaning Liquids on Tinted Base Film