Also, does 18k red gold have less malleability than 18k yellow?
18K red/rose golds are a LOT harder than the typical yellow alloys.
If correctly annealed, they are nicely workable, but are just not at
all as soft. They do have one very big trap waiting for you, though,
if you incorrectly anneal and cool them, which affects soldering too.
In 18K red gold (no silver, just gold and copper), if cooled slowly
through about 750-800 degrees F, will undergo a structural change
from the normal random solid solution of copper and gold mixed
crystals, to what’s called an ordered array. In this, the structure
becomes one of alternating layers of copper and gold atoms. The
result is dramatically different from the normal metal, in that this
structure is as brittle as glass. Dropping a piece that has undergone
this shift, can shatter it. The risk is that even mild working of a
piece that has, at least partially, undergone this shift, can
introduce microscope cracks into the piece, even if it looks just
fine. Then, even a proper annealing with the correct quench to
prevent this structural shift, won’t heal the cracks, which can
still then fail as you work the metal. So ANY time this alloy is
raised to annealing or soldering temperature, it MUST be quenched
from the 'black heat" range above about 800F. Usually, the quenching
is safest when done in alcohol, not water, so as to avoid possibly
cracking the metal from too high thermal shock. The alcohol quench
still cools the metal quickly enough to avoid this structural shift.
You can also avoid it, for the most part, by using an alloy that is
only in the 'pink" range, meaning it still has a decent amount of
silver. The typical 18K yellow alloys are considerably more silver
than copper. If you reverse it to, say, 3 parts copper to 2 parts
silver, you’ll have a considerably more rose alloy, it will be
considerably harder too, but won’t be so prone to “gotcha” traps.
And the higher chemical reactivity of copper vs silver should also
make these rose alloys easier to depletion guild as well.