British Kinematography (1948)

u Fog The figures for fog levels will not be presented in this paper. They will appear in detail in the complete data to be published elsewhere. It will be sufficient to state, that in most cases, the fog was not extreme at the critical gamma. Oxidation by Silver Bromide Since both Elon and hydroquinone are concerned in the reduction of silver bromide and need to be maintained at working concentration by replenishment, it is of great interest to know the influence of the composition of the developer on their relative rates of consumption. The ranges of developers referred to in the sensitometric work above were also tested to find the relative rates of exhaustion of Elon and hydroquinone as* each developer was progressively oxidised, using motion picture positive film. The experimental method used was as follows : 100 ml. of the developer under test was run into a stoppered measuring cylinder and a weighed amount of perforation hole punchings obtained in the perforating of motion picture positive film was run into the cylinder and was allowed to develop for 10 minutes at 20° C. Suitable precautions were taken to maintain an atmosphere of nitrogen above the developer at all stages and to maintain the temperatures and degree of agitation constant. After the perforation punchings had been developed for 10 minutes, the developer was analysed to find the concentration of Elon and hydroquinone remaining and the amount of silver formed in the development was also determined. In this way it was possible to draw curves relating the consumption of Elon and hydroquinone to the quantity of silver produced by development. These detailed graphs are too numerous to give here, but they are summarised in Figs. 6 and 7, which show the quantities of Elon and hydroquinone consumed in the production of a given amount of silver. These figures may be interpreted as showing the relative initial rates of removal of Elon and hydroquinone from the developers under test. It will be seen that in the low pH range, 8.7 to 9.0, a considerable proportion of hydroquinone is consumed together with the Elon, although at this level of pH a hydroquinone developer would be quite inert in all practical times of development. The fact that hydroquinone has been consumed may be explained by the mechanism of the development which was given above, viz., some of the oxidised Elon has been reduced by the hydroquinone, so that the hydroquinone becomes used up. Economics of Elon-Hydroquinone Ratio This phenomenon is most interesting to the laboratory manager from an economic point of view while Elon remains more expensive than hydroquinone. It has been suggested in some quarters that since hydroquinone is inert photographically at these low pH values and since an Elon developer will give the same general characteristics as the Elon hydroquinone developer, then the hydroquinone is best omitted to avoid its becoming aerially oxidised. We now see this to be a somewhat expensive mode of procedure, because in D76, for example, whereas a certain amount of Elon would be used if it were an all Elon developer, only half this amount of Elon is used in its present form. This, it will be seen, is much more economical in view of the fact that Elon is about three times as expensive as an equivalent weight of hydroquinone. Another fact which emerged on carrying out this work, was that a considerable quantity of developing agent is removed from the developing