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PROCESSING MOTION PICTURE FILM 109
scum. This is usually the product of the fixing salts brought by the film into the washing water and some colloidal materials in the water itself. Spray washing installations are, therefore, free from these defects. In those machines using tank washing it is possible to arrange for the wash water to overflow from the tank, through holes or unions to a pipe system beyond the machine, and thus to carry the scum-forming products with it.
sludge in developer. This is most severe with developers containing borax, and consists mainly of dissolved gelatin and silver. Under normal operating conditions the sludge should not become great enough to cause staining before the useful life of the developer has been exhausted.
sludge in fixer. When this occurs on mixing the solution, or a very short time after mixing it, this will be due to bad mixing and the bath should be analysed. Such deposits are immediately recognisable as differing from the normal accumulation expected in a partially exhausted bath of some age.
spots. (Drying). Drying spots or marks are usually caused by inefficient squeegees or air blowers combined with the use of very cold washwater. Even when the squeegees are operating satisfactorily very cold water may harden the film and so make it impossible for the squeegee to evenly distribute the very thin layer of moisture it must leave on the emulsion surface. Wash-water should not fall below 50°F (10°C).
staining. Seldom produced as local stains over small areas, but general staining may be caused if the solutions are used after they become exhausted. Because of this it is more usual to find positive rather than negative films stained since the solution used to process positive copies is more likely to remain in use after it has ceased to function satisfactorily. If the washing, or stop bath, between the developing and fixing solutions is not adequate local staining may occur over those areas where developing solution has remained after the film has passed into the fixing solution.
static marks. These are exposures made on raw stock, usually during unwinding, by an electrical discharge (often a clearly visible spark) between the convolutions of the film. It may be caused by very rapid unwinding or by excessive tension. The more common it becomes the greater is the indication that the prevailing humidity is incorrect. All film-handling apparatus should be housed in rooms maintained between 60 and 70% relative humidity.
streaks. These may be caused if the solutions are not agitated sufficiently. It may be overcome by circulating the developer at a rate high enough to eliminate the streaks but not sufficient to cause foaming. A similar cure can be partially effected, where possible, by increasing the speed at which the film travels through the machine.
sulphide fog. Bacteria or fungi present in a developing solution will cause the sodium sulphite to be reduced to sulphide and so produce fogging. This will not normally occur when a developer is in constant use and, indeed, may be cured from one which has been standing for