copper plate
You can put a thin layer of copper on a key or other small
metal object.
In a glass jar mix one part ammonia and three
parts water. Make enough to cover the key as it hangs in the
jar. Remove the plastic insulation for 1 inch on each end of
two 1 foot long pieces of copper wire. Put one end through the
hole in the key and twist to fasten. Attach the other end of
the wire to the positive clamp of a 6 or 12 volt battery charger.
Connect one end of the other wire to the negative clamp.
Put the free end of that wire into the ammonia solution. It
must not touch the key or the wire connected to the key. Check
the key about every 1/2 hour.
The copper coating is not smooth and shiney . It can be
smoothed somewhat with fine emery cloth and polished somewhat
with steel wool.