ā¦I decided to coat a
stone with boric acid/alcohol then sprinkle powdered charcoal on
top. I heat everything until the glass forms then allow it to
cool a little so I can scrape the charcoal off the top of the
prongs then flux and retip. The charcoal creates a reducing
atmosphere on top of the stone and also slows down the cooling
process.
Hmm. Iād guess if the charcoal helps, its only due to an
insulation effect. A reducing atmosphere isnāt really what you
want with sapphire or ruby. These stones are aluminum oxide.
While a reducing atmosphere is good for our metal and solders, it
is NOT good for an oxide based material. An overly reducing
atmosphere can actually cause the aluminum oxide to be reduced
to aluminum. You can do that with a sufficiently reducing flame,
resulting in a weird multicolored film on the surface, that even
if it then oxidizes again, has deteriorated the quality of the
polish a bit. Not as bad as does the etching attack from boric
acid or borax or flux on the stones (aluminum oxide, just like
the other metallic oxideds you are trying to prevent on your
metal, is nicely soluable in the boric acid or flux. The etching
these materials do is not from fusing to the sapphire, itās from
dissolving their way into it a little. The resulting glaze of
both boric acid and dissolved aluminum oxide is still soluable in
pickle, so it does completely pickle off again, but leaves a now
etched surface on the sapphire that requires repolishing to
repair.
The trick(s) is this: The sapphire is not excessivly soluable
in just boric acid, at least not at lower temps. Keep the more
active fluxes off the sapphire at all costs, but a little boric
acid will be OK, IF you then donāt heat the sapphire or ruby hot
enough to actually glow (keep the stone under about a thousand
degrees) and if the time at whatever high temp is kept to an
absolute minimum. Plus, since weāre often working with at least
slightly reducing flames, a glaze of boric acid will in that case
actually protect the stone from the reducing effects of the
flame. On the other hand, it then risks etching from that boric
acid if the temp gets higher than a fairly low level.
So lightly fire coat your piece if you like. Itās your option
whether to wipe off the excess boric acid that you can reach,
from the stone, especially the visible crown areas, which are not
only most visible, but also most at risk of getting hotter.
Depends much on what sort of flame your using and where youāre
working. Work in such a way that your flame is directed at the
metal, but not at the stone. You can solder on a prong without
having to get the stone glowingā¦ A small tight neutral flame
will work with both the stone and the solder. Not oxidizing, but
not all that soft and reducing either. If you work carefully,
like this, and confidently enough so youāre not taking longer
than needed, youāll not have troubles with etching the sapphire.
Initial heating and subsequent cooling can, and should, be
gentle. Youāre trying to minimize heat shock to the sapphire
too, as well as keep it from getting any hotter than needed.
But itās important to realize that this procedure will always be
a balance between risks and costs. Always examine the stones
carefully before working on them. Ask yourself if you can afford
to have something go wrong. If the answer is no, then unset the
stone and do the job the harder, but safer way. If the stone is
within the range you feel comfortable about being responsible
for should something happen in spite of your precautions, then go
ahead and work as Iāve described. But do remember that this
whole procedure is a series of tradeoffs. Itās NOT guaranteed,
and even the old pros will occasionally have a sapphire get
accidentally friedā¦
Peter Rowe