Improvements in or relating to the preparation of photographic silver halide emulsions

828,217. Photographic emulsions. GEVAERT PHOTO-PRODUCTEN N.V. May 22, 1957 [June 7, 1956], No. 17650/56. Class 98(2) In the preparation of photographic emulsions a dispersion is first formed by precipitating silver halide in an aqueous solution of a cellulose sulphate (including mixed ether cellulose sulphates) and then an aqueous binder solution is mixed with the dispersion. Aqueous solutions of a silver salt and a halide may be run simultaneously. into an aqueous solution of the cellulose sulphate to form the dispersion. The binder, such as gelatin or polyvinyl alcohol may then be added to the dispersion and the emulsion gelled (by adding a-naphthol or gallic acid when using polyvinyl alcohol), noodled and washed in the usual manner. Alternatively, the dispersion may be ripened, precipitated by adding an organic solvent such as acetone, methanol, ethanol or methyl or ethyl acetate, or a saturated aqueous solution of sodium sulphate or sodium chloride, the precipitate then being washed' and dispersed in gelatin or polyvinyl alcohol. The cellulose sulphate may be pure sulphate (or its water-soluble salts) or it may be methyl cellulose sulphate, ethyl cellulose sulphate, ethoxy-ethyl cellulose sulphate, hydroxy-ethyl cellulose sulphate, or ethylhydroxy-ethyl cellulose sulphate. The emulsions may be chemically sensitized with allyl isothiocyanate, allylthiourea, sodium thiosulphate, potassium selenocyanide, stannous salts, imino-amino-methane sulphuric acids as in Specification 798,823, or compounds of gold, ruthenium, rhodium, palladium, iridium or platinum.