Over the past few weekends, I've been "apprenticing" under a local woodsmith/blacksmith who has been gradually introducing me to the many facets of this trade. One book he gave me to read is one I'm sure you're familiar with; Practical Blacksmithing, cataloged by M.T. Richardson.
In chapter IV, Steel and its Uses, there is an article name "To Temper Steel Very Hard." It goes on to say:
"the most effective liquid is the only liquid metal-mercury. This being a good conductor of heat, in fact the very best liquid conductor, and the only cold one, appears to be the best on for hardening steel-cutting tools. The best steel, when forged into shape and hardened in mercury, will cut almost anything. We have seen articles made from ordinary steel, which have been hardened and tempered to a deep straw color, turned with comparative ease with cutting tools from good tool steel hardened in mercury."
This seems so archaic to me, but in fact may be so for good reasons. I'm sure the dangers of mercury were known then, but they still took the good with the bad.
I was just wondering if anyone has had the opportunity to try this method, or work with steel that has been tempered in this fashion.
He finishes with a bit of a disclaimer: "Beware of inhaling the vapor while hardening."