British Kinematography (1949)

80 lOOg. per litre, to halve the rate of loss of developing agents, would only saVe money if the hydroquinone/sulphite price ratio is 40 : 1 or more. At a ratio of 30 : 1 the most economical sulphite concentration would be reached at 80g. per litre. At a ratio of. 20 : 1 the most economical sulphite concentration in the bath would be 60g. per litre. At a ratio of 10 : 1 the most economical concentration of sulphite would be 45g. per litre. Although the numerical values in the above example were chosen to illustrate the case of the Iy-R Spray machine, the shape of the curves and the general conclusion is probably valid for any case involving the aerial oxidation of an Elon-hydroquinone developer at pK 10 and at 20° C. when the hydroquinone/Klon ratio is sufficiently high for the hydroquinone to be predominantly lost. IV. CONCLUSION Practical experience by the motion picture processing laboratories shows that a spray or jet-applied developer can be maintained over a considerable period by suitable replenishment. The present work indicates that the cost of the chemicals wasted by aerial oxidation depends upon the volume of developer used in the installation. If the minimum volume of developer is used, e.g., about l/20th of the volume that would be required in a normal total immersion system of the same production capacity, then it is cheaper to use a spray system with a negative developer at pH 9. This arises from the fact, now presented, that the autoxidation rate in a developer of this type reaches a maximum value at a comparatively low degree of aeration. When the volume of the developer in a spray system is about 1/6 of the volume in an equivalent total immersion machine, having a fair degree of aeration, the two systems will cost about the same to replenish. At equal volumes, aerial oxidation in the spray system will waste 3 to 4 times as much developing agent and sulphite as in any normal system. At the other end of the range, at pH 10.5, a spray developing machine will cost at least twice as much in developing agent and sulphite wastage as any normal machine, and, at equal volumes, it can cost over six times as much. An interesting point emerging from this work is the very small, or negligible, rate of aerial oxidation in a system where, in spite of a vigorous stirring of the solution, no bubbles of air were carried below the surface. The figures quoted so far relate to the relative wastages by aerial oxidation. Put in another way, it may be said that in the case of negative developers the use of a small-reservoir spray system may result in some saving of chemicals, while a large-reservoir spray would just about double the total cost of developing agent consumed. In the case of positive developers at pH 10 the use of a small-reservoir system might increase the total cost of developing agent consumption by about 10%, whereas the large-reservoir system might increase the cost by about 70%. Thus the possibility of making economical use of spray processing would seem to depend upon the extent to which the volume of circulating developer can be reduced without losing control over the rate of change of the composition of the bath. The theoretically ideal condition would be that in which the volume of circulating developer is reduced to zero, for in this case the " replenisher " itself (presumably of constant composition) is sprayed on to the film and then runs to waste. This advance awaits only a suitable application technique. In the case of negative developers, there is little to be gained by using nitrogen or an air-tight system, unless the oxygen content of the air can be brought much lower than 4%. If nitrogen were used, the cost of doing this would be prohibitive. Although reducing the oxygen content of the air, especially in the case of positive developers, could be achieved by making the