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540 Transactions of S.M.P.E., Vol. XI, No. 31, 1927
out film samples was noticed. Apparently the wax coating on the film surface is so thin that it does not appreciably retard the rate of evaporation of moisture from the film.
Summary and Practical Recommendations
The projection life of motion picture film can be prolonged considerably by coating either the edges in the region of the perforations or the entire gelatin surface of the film with a thin film of wax. Edge waxing as now practiced by use of the Eastman edge waxing machine is an efficient means of lubrication providing it is done correctly, but if the molten wax is not heated sufficiently during application, there is a tendency to apply too much wax to the film.
This causes an excess of wax to accumulate in the projector gate so that if the freshly waxed film is threaded in a warm projector which is then allowed to cool, the wax solidifies and holds the film so tightly that on starting the projector the film remains stationary in the gate and, in the case of most projectors, then catches fire.
An excess of wax on the film also causes the convolutions of the film to adhere together when the film roll cools after projection, and particles of wax torn from the film during rewinding tend to settle on the picture area causing spots and unevenness on the screen. Edge waxing is also impossible in the case of film with an edge sound record.
By coating the entire surface of the film with an extremely thin coating of a suitable wax, or mixture of waxes and then buffing or polishing many objections to edge waxing are overcome. This may be done efficiently by applying a 2% solution of a mixture of carnauba wax and paraffin wax dissolved in carbon tetrachloride, by means of a suitable machine, which buffs the film surface to a high gloss after application of the wax. The exact proportion of carnauba and paraffin waxes is a matter of choice. A high proportion of carnauba gives a hard highly polished coating, while a high proportion of paraffin gives a softer coating with less gloss but with a greater lubricating value. The following formula containing equal parts of carnauba and paraffin gives a sufficiently hard coating with satisfactory lubricating qualities :
Metric Avoir.
Carnauba wax (M. P. 175°-185°F.) ' 10 grams 150 grains
Hard paraffin wax (M. P. 130°-140°F.) 10 grams 150 grains
Carbon tetrachloride to 1000 cc. 32 oz.