Abstract: Gold plays a very important rule in the pre-fogging process of the direct positive emulsion. In this paper the effects of gold on the formation of fog centers were studied by adding silver halide solvents after the emulsions were fogged by thiourea dioxide and thiourea dioxide plus gold, respectively. The interaction between the gold and the fog centers was studied by alternatively immersing in the gold bath and sodium cyanide bath. The results indicate that the presence of gold when fogged can make the fog center size distribution narrow. It is also shown that the deposition of gold is the main reaction when there are plenty of reduction agent in the emulsion layer and the replacement of silve atomes by gold may become important when there is short of reduction agent.When the direct positve emulsion is pre-fogged,the reduction of gold ions and the deposiotion of gold onto the fog centers is the main reaction, because there is plenty of reduction agent.

Key words: direct positive emulsion, pre-fogging, fog intensification, fog center