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March, 1930] SURFACE TREATMENT OF SOUND FlUM 289
rated by passing the film through a short narrow tube through which a current of air at 120° F. is blown.
An alternative method of lubrication is to apply a 1.0 per cent solution of light motor oil in carbon tetrachloride to the entire film surface and then buff in a manner as described previously.2
In addition to providing satisfactory lubrication, it is desirable to treat sound record prints in such a manner that the film will have during handling a minimum tendency to accumulate scratches, dirt, dust, and finger marks which in turn cause ground noise. Several suitable methods of treatment have been evolved, the most satisfactory of which consists in applying a one per cent solution of cantol wax to the entire emulsion surface of the film, buffing, and edge-waxing as described above. The cantol wax provides a hard smooth surface which in itself has poor lubricating properties but the edgewaxing supplies the necessary lubrication. A film treated in this manner eventually becomes scratched but the scratches are usually confined to the wax coating and do not reach down to the silver image so that by removing the wax coating at intervals by cleaning with carbon tetrachloride and re-waxing, the image is maintained clean and free from scratches.
Three machines are necessary for the above treatment, namely: (a) the waxing and buffing machine, (&) the edge-waxing machine, and (c) the cleaning machine. Machine (b) can be attached to the end of machine (a) but it would be inefficient to attach machine (c) to machine (a) plus (b) because the former can be run at a speed of 200 feet per minute while it would not be desirable to run machine (a) at a speed greater than 30 feet per minute unless more buffing wheels were attached.
Although in some of the experiments outlined the treated film was cleaned and retreated after projecting 10 times, in practice this routine is usually not necessary, the treated film requiring cleaning only when indicated by the presence of visible dirt, oil spots, or excessive ground noise. In the case of the tests with the cantol buffed and edgewaxed sample, after projecting 130 times without further treatment of any kind the magnitude of the ground noise was only slightly greater than at the start. The projectors were kept in a very clean condition throughout the tests. This test demonstrates that it is possible to keep the projector sufficiently clean and the rewinder free from dust so that with film treated in the manner outlined no excessive ground noise is produced after 130 runs.