Dependence of fog on chemical sensitization and iodide amount in cubic AgBr
emulsions has been studied. Similar fog curve shapes were obtained from th
e iodide-covered AgBr emulsions by varying the iodide amount and the sulfur
-sensitized AgBr emulsions by varying the sulfur amount. Subsequent gold se
nsitization of the two kinds of emulsions produces two fog peaks in the cur
ves with varying the iodide or sulfur amount. The surface properties of the
emulsion grains have been discussed with reference to fog formation. The o
rigin of fog is attributed to the reactivity difference between the chemica
l site and the bulk crystal surface. The reactivity sequences of different
chemical sites were derived: Ag2S > AgI > AgBr > AgCa and Au-n > Ag-n, etc.
Direct electron transfer development (DETD) and indirect electron transfer
development (IETD) were supposed to be the two processes in development. B
ased on the proposed mechanisms, the main experimental results were success
fully explained with a distribution model of chemical impurities at the cub
ic AgBr grain surface.