If you are looking for an 18k alloy, then adding 25% Cu will give
you an orangy-red, but it is very brittle. Will a 14k alloy be
acceptable? How important is the karat anyway? If you go as low as
50%Au and 50% Cu, the alloy is fairly workable and very red.
Most of the copper alloys age harden quickly, so they need to be
quenched and not allowed to air cool. They can also be somewhat
temperamental when you solder them, as they really like to oxidize.
If you haven’t already purchased a copy, " The Theory and Practice
of Goldsmithing" devotes an entire chapter to the devilish details
of working with alloys of all types. This book should be in
Red in my shop is a relative term, like “expensive.” I have an alloy
that is 14k that I use, since it is noticeably red and easy to work
with. It is 58.5% Au, 40% Cu and 1.5% Ag. I can cast it without too
many problems. Getting a solder that will not show a seam is not so
easy, so I laser mine Looks great with platinum.
Can you cast it in place over the finished platinum? Could make life
a lot easier…