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528 Transactions of S.M.P.E., Vol. XI, No. 31, 1927
a drop of water sizzled on the metal, a satisfactory degree of burnishing was effected although this treatment did not materially prolong the life of the film on projection.
2. By Edge Lubrication. Before the advent of more refined machinery for the purpose, it was customary to apply a layer of wax to the edge of the film during rewinding by passing the film face downwards over two blocks or candles of hard wax separated by a distance equal to the width of a picture frame. Although effective from a lubrication standpoint, with such an apparatus it is difficult to control the quantity of wax applied because this depends on the temperature of the wax, the pressure applied, and the rate of travel of the film. Usually the tendency is to apply too much wax, which then encroaches on the picture area and causes dark spots or patches on the screen. An excess of wax is also apt to cause projector trouble as explained below.
A suitable machine for applying a thin line of wax along each edge of the film surface and between the perforations has been described by J. G. Jones. ^ This consists essentially of two parallel thin steel discs separated by a distance of 1-3/32 '^ rotating in a vertical plane. The discs dip into a bath of molten paraffin wax and apply the wax to the film at their upper edge. The quantity of wax applied is controlled by the thickness of the discs, the temperature of the molten wax, and the rate of travel of the film.
Precautions to be Observed when Edge Waxing
The above method of lubrication is entirely satisfactory providing the wax is applied correctly, and no better lubricant than paraffin wax is known to date. However, if the temperature of the molten wax is not sufficiently high during application, too much wax is applied by the discs, and this does not solidify sufficiently before the film is rewound. This causes the wax to cement the edges of the film convolutions so that on rewinding, particles of wax are torn away from the film and these tend to encroach on the picture area, causing spots and blotches on the screen. This is harmful, particularly in the case of film with an edge sound record.
Another very serious danger resulting from the application of an excess of wax arises if the projector is threaded while hot with
1 "A Film Waxing Machine" by J. G. Jones, Trans. Soc. M. P. Eng., No. 15,251 (1922).