Mercury vapors and mercury absorbed through the skin is bad-- maybe they have a temperature control to avoid vapors, and I notice he IS wearing gloves.
The hatters sometimes went mad, but they worked with the stuff all day every day. I heard that it was because they would often lick their brushes to get a fine point on them, same way the people who would paint radium onto watch hands would get radiation poisoning.
How can he be so close to that much molten metal? That stuff must be 5 billions degrees to be so fluid. How does the cannonball not melt? The powerbills for maintaining that tank must be through the roof.