Process for the manufacture of photographic silver halide emulsions containing silver halide crystals of the twinned type

Abstract

A method of preparing photograhic silver iodide, containing emulsions is provided. The silver halide crystals in said emulsions are of the twinned type and the method comprises the steps of (a) forming in a colloid dispersing medium silver halide crystals containing at least 90 mole % iodide, these said crystals being predominantly of the hexagonal lattice structure and then chemically sensitizing the said silver halide crystals which contain at least 90 mole % iodide, (b) mixing in the dispersing medium containing the said silver halide crystals an aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium bromide or chloride or mixtures thereof so forming twinned silver halide crystals containing iodide and the halide or halides being added, (c) adding a silver halide solvent to the dispersing medium and so causing the growth of the twinned crystals by Ostwald ripening, optionally (d) causing the twinned crystals to increase in size by adding to the colloidal dispersion further aqueous silver salt solution and further alkali metal or ammonium halide and then finally optionally (e) removing the water-soluble salts formed and chemically sensitizing the surface of the silver halide crystals of the emulsion. The new photographic emulsions exhibit a high internal sensitivity and covering power and contrast on development.

Dependencies are inferred from the claim text.