Method of preparing a monosize silver halide emulsion involving Ostwald ripening followed by a crystal growth stage

A method of preparing a monosize silver halide emulsion of which most of the silver halide crystals are of the twinned octahedral type which comprises

• a. mixing an aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium halide in an aqueous dispersing medium containing a protective colloid at such a concentration that the silver halide is nucleated,
• b. causing the silver halide nuclei to increase in size in the presence of a silver halide solvent by Ostwald ripening to produce a population of twinned octahedral seed crystals,
• c. determining the maximum rate of addition of silver salt and alkali metal or ammonium halide which is possible without renucleation occurring and determining the minimum rate of addition of silver salt and alkali metal or ammonium halide which is possible without Ostwald ripening occurring, said determination is carried out by taking samples of the crystals, adding an equal quantity of silver halide at increasing and decreasing rates, respectively, and observing on a microscope when renucleation and Ostwald ripening, respectively, occur,
• d. causing the seed crystals to grow by adding to the aqueous dispersing medium aqueous silver salt solution and aqueous alkali metal or ammonium halide solution at a rate between the predetermined maximum and minimum rates and then at intervals re-determining the maximum and minimum addition rates and adjusting the addition rates of silver and halides to ensure that at all times during the crystal growth stage neither renucleation nor Ostwald ripening occurs, the p Br of the liquid phase being maintained above 0.15 during the whole crystal growth stage.