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AssigneeCiba-Geigy Corporation
InventorTrevor J. Maternaghan
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A method of preparing photographic silver iodobromide or iodochloride emulsions the silver halide crystals of which are of the twinned octahedral or cubic type, which emulsions have improved homogeneity of iodide content and controlled size distribution of the crystal grains, comprising the following steps: (a) Forming a monosized silver iodide dispersion in gelatin; (b) Mixing in the silver iodide dispersion aqueous solutions of silver nitrate and alkali metal or ammonium bromides or chlorides in order to form twinned crystals; (c) Performing Ostwald ripening in the presence of ammonia solution and a controlled silver ion concentration to increase the size of the twinned crystals and dissolve any untwinned crystals; (d) Optionally increasing the size of the twinned crystals by the further addition of silver and halide ions; and (e) Chemically sensitising the emulsion. The new photographic emulsions exhibit a high covering power and contrast on development.
A method of preparing a silver halide emulsion containing silver halide crystals of the twinned type which comprises the steps of (a) forming, in a colloid dispersing medium silver, halide crystals containing 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 a 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 being added, (c) adding a silver halide solvent to the dispersing medium and so causing the growth of the twinned crystals by Ostwald ripening and optionally (d) then 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 sensitising the emulsion.
Dependencies are inferred from the claim text.