Twinned emulsions made from silver iodide seed crystals having an aspect ratio of at least 2:1
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United States of America Patent
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May 21, 1991
Grant Date -
N/A
app pub date -
Sep 1, 1989
filing date -
Sep 13, 1988
priority date (Note) -
Expired
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Abstract
There is described a method of preparing a silver halide emulsion wherein the silver halide crystals are of the twinned type which comprises the steps of (a) forming or providing in a colloid dispersing medium silver halide crystals containing at least 90 mole % iodide, at least 90% of which are of the hexagonal lattice structure with a mean thickness of less than 0.6 microns and a mean aspect ratio of greater than 2:1 (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 of ammonium bromide or chloride (or mixtures thereof) so forming twinned silver halide crystals containing iodide and the halide being added, optionally (c) adding a silver halide solvent to this 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 silver salt solution and a further alkali metal or ammonium halide and then finally optionally (e) removing the water-soluble salts formed and chemically and spectrally sensitizing the emulsion.
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