American cinematographer (Jan-Dec 1927)

May, 1927 AMERICAN CINEM A T O G R A P H E R Twenty-three Since the presence of 3 to 5 mg. per liter of carbon tetrachloride imparts a strong odor to the air, there is no excuse in practice for the concentration approaching the danger point, which is 10 times this concentration. The Suitability of Carbon Tetrachloride for Cleaning Motion Picture Film The above experiments indicate that carbon tetrachloride when pure is quite satisfactory for cleaning motion picture film. It is a good solvent for oils and fats, evaporates readily, is non-combustible and is readily available at a reasonable price. It does not affect the image even on prolonged contact and has a minimum tendency to decompose on exposure to light in the presence of moisture. Although toxic when impure, the, pure compound is no more toxic than benzene and if reasonable ventilation is provided, it may be used with relative safety. Tests also showed that carbon tetrachloride has no curling effect on film with nitrate or safety base after two days and it does not remove the color from either nitrate or safety film with tinted base. Manufacturers such as the Dow Chemical Co. and the Eastman Kokalc Co. supply sulphur-free carbon tetrachloride which is satisfactory for cleaning film. A few years ago many commercial samples of tetrachloride contained sulphur which was formed as a by-product in its manufacture by the action of chlorine on carbon disulphide. On exposure to the aid in the presence of moisture sulphur chloride deposits sulphur which is capable of combining with the silver image to form yellow silver sulphide. Such samples of carbon tetrachloride containing sulphur chloride when left in contact with motion picture film attacks the image, especially in the presence of moisture, and bleached it out to a faint yellowish-white image of silver sulphide. No such commercially impure samples of carbon tetrachloride have been encountered within the past two years. Mixtures of Carbon Tetrachloride ivith Inflammable Solvents In some laboratories and exchanges a mixture of carbon tetrachloride with high-test gasoline is used for film cleaning. This mixture evaporates less readily than pure tetrachloride, which may be an advantage in some cases. Its adoption in the past was a result of the toxicity of impure samples of tetrachloride, a 50 per cent mixture by volume with gasoline reducing this considerably. This mixture burns with great difficulty and is satisfactory from a fire hazard standpoint, although the proportion of the two liquids necessary to give a non-inflammable mixture depends on the nature of the gasoline. It is considered that pure carbon tetrachloride is to be preferred to such a mixture for general purposes. Film Moistening Liquids In addition to accumulating oil during projection, both the film base and gelatin coating lose moisture and tend to become brittle, owing to the excessive heat to which the film is subjected. If the film were allowed to cool to room temperature between successive projections, little trouble would be encountered, but in practice the film does not cool off sufficiently between successive projections and the resulting baking process drives out the moisture, which results in brittleness. If film which has been rendered brittle in this manner is exposed to a moist atmosphere even for only a relatively short time it tends to regain its flexibility. It is not possible to do this by placing the tightly wound reels of film in a humidor or a vessel containing water because the moisture penetrates the convolutions of film very slowly. It would be possible to humidify the film satisfactorily by passing it continuously through a humid chamber or by winding the film in contact with a damp strip of paper or other absorbent ribbon. Such a system, however, is inconvenient in the theatre or exchange. A satisfactory method of moistening film is to immerse it in a mixture of water and a water miscible volatile liquid such as grain alcohol. The percentage of water to be used in the mixture depends on the degree of brittleness of the film and the time which elapses betwen application and vaporation of the liquid. If an application machine of the Dworsky2 type is used, this depends on the rate of passage of the film through the machine. During this short period little or no swelling of the gelatin coating occurs, but sufficient moisture is absorbed to restore the flexibility of the dried out gelatin coating. Moreover, when the film is wound up in a roll, the dried out film base can also absorb moisture by virture of being in contact with the moistened emulsion. Film base absorbs moisture relatively slowly so that little or none is absorbed by it in the period of application of the moistening liquid. At the outset a survey was made of possible watermiscible volatile liquids which could be used for the purpose. The requirements of such a liquid are identical with those for the film base cleaning liquid outlined on page 2. A choice of the following liquids is possible : Grain alcohol, denatured alcohol, isopropyl alcohol, and tertiary butyl alcohol. The exact quantity of water to be added to the alcohol must be determined by trial. From 15 to 25 per cent water is usually satisfactory and this proportion holds in the case of all the alcohols named above. The condition of the film after treatment will indicate any necessary changes in the proportion of water to be added. If it is too tacky, less water should be used and if too dry and brittle the quantity should be increased. A mixture of either of the above alcohols with water has little or no solvent action on mineral oil which may be present in film after projection. However, in practice the rubber squeegees in the Dworsky1 machine tend to emulsify and remove traces of oil. If much oil and dirt is present on the film a moistening liquid which is also capable of dissolving oil must be used. 1. H. C. Fuller, Chem. and Met. Eng. 29, 538. 1923. 2. Faulkner, Trans, S. W. P. E. 25, 117. 1926. 3. K. B. Lehmann, Arch, fur Hyg. 75, 1. 1912. 4. "Industrial Poisoning in the U. S.,, by A. Hamilton (MacMillan.) 5. Haggard, J. Pharmacol and Exp. Theraup. 16, 401. 1920. 6. H. B. Lehmann. Arch, fur Hyg. 24, 1, 1911. (To be continued next month)