In the process of separating the gold from other material; he was using mercury. After the gold is suspended in the mercury, the mercury is heated to vaporize the mercury leaving the gold. Normally This process is called retorting. The mercury vapors are normally sent into something like a still so that the mercury will be reclaimed and used again. Unfortunately, Claude, Jr. did not know the process, or thought he could take a shortcut in the process and not bother reclaiming the mercury. So he heated the mercury in a pan on his cook stove and as the mercury vaporized he breathed some of the vapor which condensed back to mercury in his lungs ... causing his death. Thurmon