This study focuses on behavior of light-induced electrons, and electrons provided by fogging agents, and the effects of silver halide solvent and gold deposition on fog centers of direct positive emulsions. When an emulsion is photofogged, fog centers form preferably in the interior of the grain, indicating that light-induced electrons can be transferred to the conduction band of the silver halide crystal directly. Whereas, when a fogging agent is used, the internal fog centers form only where a sufficient amount of fogging agent exists and only after formation of the surface fog centers. In the above two cases, the ways electrons enter the conduction band are different. The results suggest that when emulsions are fogged, the addition of gold salt might reduce the dispersity of the fog centers. Plating of gold is the main reaction when enough fogging agent exists in the emulsion, and the replacement of silver atoms by gold atoms becomes important when the fogging agent is insufficient. When a direct positive emulsion is fogged, the plating of gold atoms on the fog centers is the main reaction, because of the large quantity of fogging agent.
Yun-yong Xia, Si-yong Zhuang, Feng-shan Liu, "Fogging Treatment in Preparing Direct Positive Emulsions" in Journal of Imaging Science and Technology, 1997, pp 536 - 541, https://doi.org/10.2352/J.ImagingSci.Technol.1997.41.5.art00012