Benoit, You might investigate the traditional Korean technique of
Kum-Boo. Tim McCreight’s “Metals Technic” gives a good description
Basically, keep heating the silver piece to tarnish it, then soaking
it in hot pickle, to remove all the copper from the surface. Then,
put the silver piece on an electric hot plate, place the gold foil on
it, and burnish the gold foil onto the silver. (Wear gloves and
periodically dunk the burnisher in cold water to protect your hands
from the heat). The gold foil will permanently fuse to the silver,
adopting the texture of the underlying silver.
It’s easier than it sounds, and a very useful and inexpensive way of
adding gold highlights to a silver piece. You can practice with
scraps of silver and gold foil. Good luck!