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Abstract
Image-wise silver sulphide stains were produced on pure, silver bromide emulsions during reversal processing with a dichromate bleach when the first developer contained a small quantity of sodium thiosulphate. The role of the thiosulphate in promoting the stain was investigated. The thiosulphate was found to be strongly adsorbed to the negative-image silver and could not be removed even by strong alkaline hydrogen peroxide. The adsorbed thiosulphate was decomposed in the bleach bath and silver sulphide was formed. Sulphite ion would inhibit the stain only in the presence of silver bromide. Iodide ion acted as a stain inhibitor with or without residual silver bromide and probably competed with thiosulphate in the adsorbed layer.