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PRINCIPLES OF CINEMATOGRAPHY
using developer which is partially exhausted and, in consequence, is inclined to produce a foam at the surface. Several 'anti-foam' solutions are available to prevent this happening before the end of the useful life of the developer is reached. Such solutions may not be wholly effective if excessive agitation is obtained by an aerating system. Should the film become contaminated with grease or oil development over those areas affected will also be retarded and a similar result will be produced.
blistering. If the gelatin enters the fixing bath in a very soft condition it is possible that the sodium carbonate of the developer, as it is neutralised by the acid in the fixer, will produce sufficient carbon dioxide gas to cause blisters, or minute depressions. This can also occur when the fixer is too acid or when the rinsing water is insufficient between development and fixation. The trouble is most likely when the hardening properties of the bath are poor and the atmospheric conditions are unduly hot.
broken splices. If the operation of the splicing mechanism and also the processing machine 'tracking' have been checked, it is still possible that splices will tend to break more readily if the film encounters severe temperature changes on passing through the successive solutions and drying compartments.
cinch marks. These intermittent, abrupt markings which may occur both on the emulsion and base-side of the film are caused when friction is produced between neighbouring convolutions of film as it is wound onto or away from the parent roll. The defect may occur in picture or sound recording cameras, processing machines, printers, rewind benches, editing machines, etc. Marks across the film may also occur, particularly when single-sided rewind plates are used and the film is permitted to wind up in an uneven manner.
developer stain. This usually occurs when partially exhausted developer or obviously discoloured developer is retained in use. It may also be expected when the sulphite content becomes too low.
directional effect. This more common defect is shown in Figure 49 and is almost entirely confined to continuous processing machinery. When film enters the machine the leading edge will come into contact with relatively fresh solution, whereas the reaction products caused by developing this leading portion will pass to the film which is to follow it thereby restraining the development over succeeding regions. Because
Fig. 49. — Directional effect a processing defect