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PROCESSING MOTION PICTURE FILM 111
to be a continuous operation and, once the average rate is known, small changes from this quantity can be made according to the amount of work handled by the machine and as indicated by periodic tests put through the bath with the normal work.
Developing Solutions
Before leaving the question of developer action it is necessary to establish the basic requirements which any developer must meet and to mention the chemicals most usually employed for this purpose. The main function of any developer is that of reacting with the exposed silver bromide grains in such a manner that metallic silver is formed. The 'reducing agent' used for this purpose is usually a coal-tar derivative such as Metol (also known as Elon) but since this agent would, of itself, take a very long time to complete the reaction, it is necessary to add a second chemical to accelerate reduction. The chemical used for this purpose is usually an alkali such as sodium carbonate or caustic soda. It is important to remember that, whilst the speed of the reaction is largely controlled by the strength of this accelerator, so also is the final grain size of the processed film. Since a considerable surface area of solution is usually exposed to the surrounding air, oxidation will quickly take place unless some preservative is included in the bath and, for this purpose, Sodium sulphite is usually added. Finally, it is usual to control the reaction by adding a slight restrainer, such as potassium bromide, the actual proportion of this addition being controlled by the type of film for which the developer is designed and the results which are required from such film.
Developer Circulation
As we have already discovered, several defects such as air-bells, directional effect, etc., are usually attributed to poor developer circulation. For this reason most processing equipment includes either a developer circulating pump or some form of compressed air system with which to agitate the solution. For similar reasons it is necessary to maintain the solutions at a constant temperature either by thermostatically controlled immersion heaters actually within the solution tanks themselves or similar devices situated within the re-circulating system.
Unless carefully controlled, a developer circulating or agitating system may cause more trouble than that which it is designed to overcome. Mention has been made previously of certain 'antifoam' solutions which are available to prevent foaming at the