I covered everything with paste flux and heated until the flux
melted plus an extra bit for good measure. The 14K now looks like
rose gold. Very, very rose gold. I pickled thoroughly and wire
brushed. The 18K still looks fine. This isn't copper plated. My
question is, should I use a different flux for gold? I assumed
that the paste would protect the gold the same way it protects the
silver wire that I usually work with.
the paste fluxes, many of them at least, are very active fluxes,
capable of actually removing small amounts of some of the more
volatile or reactive metals, such as zinc. This is especially so
since these fluxes become very fluid, and as such, though they
dissolve oxides, don’t actually totally block the oxygen in the air
from the metal, since oxygen can dissolve in the flux and pass
through, slightly. At least that’s my theory on it. I might be wrong
on the details of that, but it seems to fit my experience with paste
fluxes, which sometimes, just after they’ve done a wonderful job of
promoting solder flow, seem to then actually allow or even promote
fire scale where they were placed. But the result is that with the
14K, you removed some of the surface zinc or whatever other
deoxidizer was in the metal, so now it’s more copper rich than it
was before. Wire brushing won’t fix this, since the wire brush
removes no metal. Anything that actually buffs the metal, an actual
fine abrasive, will restore the metal color. (Try the 3M radial
bristle disks. They should restore the metal color nicely)
In future, the trick is to protect the gold with a less active
material. Long the traditional method of choice is to not use a
commercial flux at all, but to use plain boric acid powder mixed in
some denatured alcohol. The alcohol is just a carrier, of course.
You dip your work in the slurry, remove and ignite the alcohol, which
then leaves a light coating of boric acid powder on the whole thing.
This is all that is needed to protect the gold from oxidation. If
you wish to use a commercial flux, I recommend Batterns, or a similar
so-called “self pickling” flux. These work well, and protect much
better, though are less active as fluxes and won’t be as aggressive
at removing oxides. Usually with gold you don’t need the aggressive
action of the paste fluxes, and it can actually cause problems (as
you’ve found).
Also, understand that like silver, yellow golds anneal virtually
instantly upon reaching any sort of visible glow, so you don’t need,
if the metal is visibly glowing, to hold it at that temp or go any
higher… The glow is less visible than with silver, so it’s easy to
overheat it if you’re not in an area with reduced lighting. You
actually don’t need it to really glow to fully anneal. Anything over
about 900 F will do the trick just fine. At that temp, in a totally
dark room, you’d be seeing the faintest of very dark red glow, and
at that temp, the boric acid powder coating will have glazed over. If
you judge the temp only by the boric acid, then let it glaze over
and hold it at that temp for a couple seconds to be sure all parts
have reached that temperature, then quench in water (yellow golds.
with rose golds, quench in alcohol, and white golds are variable,
some doing better with a slow air cool, others doing better with a
quench)
Your color change in the 14K yellow is also not totally wrong. It
does tend to do this a bit anyway, at least with some alloys, since
some of the surface zinc or silicon deoxidizers added to castable
alloys will tend to be depleted no matter what you do. but it’s
usually not an exceptionally dramatic color change. The 18K never
does this, since 18K alloys generally contain no zinc or silicon
deoxidizers in the first place, and often are more silver rich in
any case.
Hope that helps.
Peter Rowe