Guys,
When I harden a frizzen where I do not know the material:
1. First, I try heating to the transition temperature of about 1500F and air cool. The frizzens on the Grice Brown Bess lock are often air hardening steel (really - no kidding!)
2. Usually the air cooling does not harden so next, I try heating to the transition temperature of about 1500F and oil quench, I use canola cooking oil. Check for hardness with a file.
3. If the oil quench did not harden the frizzen, I repeat number 2 but quench in brine, a gallon of water and a container of salt. Check for hardness with a file.
4. If the brine quench did not harden the frizzen, I case harden with a mix of 1/3 hardwood charcoal, 1/3 leather charcoal, 1/3 bone charcoal followed by a brine quench.
5. If this does not work, I punt! Actually, I add a frizzen sole plate, riveted onto the unhardenable part.
Jim