Method for chemical sensitization of silver halide for photothermographic use

A method of preparing a black-and-white photothermographic emulsion comprising:

• (A) providing a photothermographic dispersion of a preformed photosensitive silver halide and a non-photosensitive source of reducible silver ions, and performing the following steps in order:
• (B-1) providing an organic sulfur-containing compound in association with said preformed silver halide grains and said non-photosensitive source of reducible silver ions, said organic sulfur-containing compound selected from one of the two following groups of compounds:
• a. one or more sulfur-containing spectral sensitizing dyes containing a rhodanine nucleus, and
• b. one or more of the following diphenylphosphine sulfide compounds PS-1 to PS-19:
• (B-2) converting from about 0.1 to about 10 mol % of the reducible silver ions in said non-photosensitive source of reducible silver ions into photosensitive silver bromide grains by addition of a bromide salt, and then
• (C) chemically sensitizing at least said preformed silver halide grains by decomposing said organic sulfur-containing compound on or around said silver halide grains by the addition, in one or more stages, of pyridinium hydrobromide perbromide to the silver halide grains at from about 20° C. to about 30° C. for up to 60 minutes, to provide a photothermographic emulsion comprising chemically sensitized photosensitive silver bromide grains in reactive association with said non-photosensitive source of reducible silver ions comprising silver behenate.

A method of preparing a photothermographic material comprising:

• (A) providing a photothermographic dispersion of a preformed photosensitive silver halide and a non-photosensitive source of reducible silver ions, and performing steps (B-1) and (B-2) but not step (C) in either order or at the same time,
• (B-1) providing an organic sulfur-containing compound in association with said preformed silver halide and said non-photosensitive source of reducible silver ions,
• (B-2) converting some of the reducible silver ions in said non-photosensitive source of reducible silver ions into photosensitive silver halide grains, and then
• (C) chemically sensitizing at least said silver halide grains by decomposing said organic sulfur-containing compound on or around said silver halide grains in an oxidizing environment to provide a photothermographic emulsion comprising chemically sensitized photosensitive silver halide grains in reactive association with said non-photosensitive source of reducible silver ions, and
• (D) simultaneously with any of steps (A) through (C), or subsequent to (C), adding a binder to form a photothermographic emulsion formulation, and
• (E) after step (D), coating and drying said photothermographic emulsion formulation on a support to provide a photothermographic imaging material.