Practical Aspects of Silver Dye-Bleach Processing

Silver dye-bleach systems are mainly used for positive-positive colour printing. Materials of different types are on the market, which allow either prints or transparencies to be made. The processing is dependent on the structure and the components of these materials. ere are three main processing steps: Silver development, dye-bleaching and removing of silver. The transparencies used for printing need a low contrast of the printing system, therefore, silver halide emulsions of low contrast are needed. During processing, the gradation can be controlled with the silver developer and/or the dye-bleach bath. Reducing the activity of the silver developer reduces printing speed. New “aterials allow the use of ordinary hydroquinone-phenidone developers, therefore only the dye-bleach bath is the controlling step.The dye-bleach bath, which has a low pH-value, contains a silver complexing substance (e.g., thiourea or iodide) and a bleach catalyst 9.g., substituted quinoxaline). As thiourea forms soluble silver complexes which influence the sensitometric properties, iodide is often preferred. The bleach catalyst must be adapted to the dyestuffs used, and it must fulfil a number of conditions. Such dye-bleach baths are degraded by small amounts of thiosulphate used for fixing the silver image and need then a high regeneration rate. It is therefore preferred to use no stop-six bath before the dye-bleach step.If the removing of silver is made in two steps (oxidation, fixing), the processing sequence needs four photographic solutions. All the silver can be recovered from the fixing bath. Such a process compares favourably with chromogenic reversal processes.