Excerpt from The Photographic Emulsion: The Mechanism of Hypersensitization
Knowledge of the mechanism of hypersensitization serves to point out the inherent limitations of the process rather than to suggest improvements in procedure. Increase in sensitivity is necessarily ga... ined at the expense of stability; the optimum excess of silver, which may be controlled by the ammonia concentration, depends on the use to be made of the hypersensitized material. In general, the more rapid the drying the higher concentration of ammonia may be used. Addition of alcohol to the bath aids in drying; it decreases the effect of a given concentration of ammonia by decreasing the dissociation, so that it is generally better to use aqueous solutions, followed by soaking the plate in 95 per cent alcohol to accelerate drying. At tempts to improve the stability of bathed plates by adding bromide to the dye bath must necessarily do so at the expense of sensitivity, since the effects of the ammonia and the bromide are opposed to each other. As soluble bromide is shown to accumulate in hypersensitizing baths or dye baths, it is obvious that their efiectiveness will rapidly fall off with use, even though the ammonia concentration remains practically the same. Combination of Silver salts with ammonia is little different from the use of higher concentrations of ammonia, although it may be preferable if it desired to avoid high alkalinity; difficultl soluble silver salts, such as'the chloride or tungstate, are preferab e to silver nitrate, since the maximum silver ion concentra tion which they can produce is limited by their solubility and there is less danger of fog from use of an excess.
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