I love to use copper to teach soldering with. The bright silver line
of the solder gives the students an obvious clue to seeing the solder
flow, something that is much more difficult on all silver projects
and must be developed.
If you want to hide the solder on a finished copper project, (and
this is where everyone will get twitchy with my trick) hold the
with a steal tweezers in some very blue pickle solution. The steal
will set up a plating environment and coat the silver solder with a
thin layer of copper. Make sure there is no other silver in the
pickle!!! Or it will copper plate too.
The pickle will be fine again as soon as you remove the steal from
it. This only works with really blue pickle, freshly mixed pickle
does not have enough copper in suspension to make the plating work.