Not knowing what material it is made out of, and not having hardened and tempered hammers, I would heat it up to non-magnetic, and then oil quench it. Then shine it up so you can see colors run. Then heat up a drift real hot, and hold the hammer head on the drift through the eye. Watch the colors run out from the eye to the ends. When the ends are straw, quench in oil again... Someone who actually has experience making hammers can probably offer better advice or correct mine or what not..