Once its annealed, the only effective way to harden it is by
work-hardening. IE, you must distort it somehow. Hammering does the
job (as you already know), but bending or twisting also does it.
Tumbling in shot can slightly harden the skin, but its not very
effective 'cos it does nothing to the interior - only the skin is
If its ear-wires you’re concerned about, then one method is to make
them a little too long, and bend the end into an “L” shape before
soldering the unbent end to the earring. You can then grip the "L"
end and twist it back and forth until the stem is as hard as you
need- be careful not to overdo it and break it. Then cut of the bent
bit and shape the end appropriately.
I often do this for brooch pins.
Regards, Gary Wooding