Also, the lead bath thing goes back to Stodart (and I think he got it from somewhere).

When he was using it, like Mike Blue says, it was for precise temperature control.

Imagine the setup this way:

The lead expands as it heats, and can obviously be heated above its simple melting point. Because of the expansion, you can keep it in a crucible with level marks notched in. When the thermal expansion causes the level to rise to the notch that you know represents the best temperature for hardening the steel, you know every time the steel goes in that it's going to get uniformly heated to that amount.