What I've always done when soldering on or near CZs is use a boric
acid/alcohol combo for a firecoat, applied liberally (especially
on the CZ). Then, I heat the stone gradually, as heating it too
quickly can shatter the CZ. Try to avoid continual flame contact
with the CZ, as the more exposure there is to direct flame, the
quicker the firecoat will disappear. What happened when you
soldered the tube setting in place was you "burnt" the stone.
Well, actually, not quite right. CZ is zirconium dioxide. As an
oxide, it is already “burnt” zirconium metal. If you overheat a
diamond, the carbon of the diamond combines with atmospheric oxygen,
and indeed, burns, or oxidizes. But CZ does not further combine with
oxygen. You don’t need to protect hot CZ from the atmosphere to
prevent burning. Some protection might be needed, however, from the
chemistry (not just the heat) of the torch flame. Parts of your torch
flame are reducing, meaning the fuel gas is looking to combine with
any available oxygen around. It can pull oxygen away from existing
oxides, including metallic oxides (swipe a reducing flame over
lightly oxidzed copper and watch it instantly appear bright while in
the flame. Remove the flame and of course it oxidizes again,
especially since you’ve heated it. Zirconium dioxide is not excempt
from this. Strongly reducing flame chemistry can damage the surface
zirconium oxide, by reducing it, at least temporarily, to zirconium
metal. Even if it re-oxidizes almost immediately, the surface will
have been disrupted, and the polish damaged. And fire coating with
boric acid is not an absolute panacea. While it protects the stone
from the air, and the flame chemistry, it is also, all on it’s own, a
decent solvent for metallic oxides. That’s why it’s used as a melting
and soldering flux. It removes (dissolves) oxides. And if you get it
hot enough, that will also include zirconium dioxide. So you’re
damned if you do, and damned if you don’t. Both the lack of a fire
coat can allow damage, and the flux firecoat itself can cause damage.
Boric acid is less active than actual soldering flux, but there is
still a danger. Personally, I prefer to make sure I’m using a torch
flame that is neutral to very slightly oxidizing. You can get away
with this if the metal itself is well fluxed and protected so the
metal does not oxidize, but then you make sure the CZ is not fire
coated. The combination of a lack of an oxide solvent (fire coat or
flux), AND an oxidizing environment (the oxidizing flame plus
ordinary air) removes both potential causes of this surface
damage/clouding. Diamond is much easier. Simply be sure it’s
sufficiently fire coated so no oxygen gets to it, and don’t wildly
overheat it, and you’re generally safe (though not always, as in
fracture filled stones, stones under stress, highly included, or
similar situations).
This technique works much better with diamonds (and even sapphires,
if you're careful), but you can even burn those if you're not
careful.
Again, while “burning” is accurate for what you do when you damage a
diamond via overheating without firecoat, it is not what happens for
sapphires and ruby. Both sapphire and ruby (same things, just
different colors, of course) are more sensative to heat damage than
is CZ, in my experience. With an unprotected sapphire, an even
slightly reducing flame getting the stone hot for even a moment,
leaves the surface with an irridescent sheen, the result of the thin
layer of aluminum metal you’ve briefly created. And it’s even more
sensative to being attacked by fluxes and boric acid fire coat, which
once molten, attack aluminum oxide quite aggressively. The result of
that is easily seen as distinct etching into the surface of the
stone. Makes a mess requiring repolishing of the stone. You’re only
real safety net, is simply to be sure no flux or fire coat is on the
stone at all, and to be sure to avoid direct flame contact, or if
that’s unavoidable, be sure the flame is distinctly oxidizing. That
too is problematic, since that flame will be hotter, which also
increases risks. And then of course there are the risks to sapphire
and ruby just from heat, having nothing to do with surface damage.
Some will have their color altered, usually lightened. Others,
especially if natural unheated stones (more costly, don’tcha know…)
may have inclusions that themselves cannot stand heat.
Perhaps the single most costly gem I ever destroyed at the bench was
an 8+ carat beautiful heart shaped ceylon sapphire. Lovely color,
natural, unheated, etc. It was in a nice hand made platinum mounting,
but the diamond setter who’d done the pave setting in the surrounding
border had missed with a drill bit, visibly nicking one of the
supporting underwires. Now, I knew I couldn’t go near this with
platinum solder. But I thought that with a tiny flame, and great
care, I might safely flow a little easy white gold solder into the
nick to hide the blemish. This, I did, having coated the whole piece
with boric acid to protect the diamonds. The flame was kept well
clear of the sapphire, which never got even hot enough to even begin
to glaze over the boric acid coating (which I’d not removed, since I
knew I wouldn’t be getting it that hot.) Heating was gentle, and
cooling was slow as well. After it had cooled to the touch, pickling
revealed that the damn sapphire had fractured, literally splitting
the poor thing in half. Closer examination (which in hindsight I
should have done well before agreeing to try and fix the nick) showed
that I’d missed seeing an almost invisible (face up with a loupe) but
fairly substantial, fluid filled inclusion within the sapphire. The
stone hadn’t needed to get all that hot. Just hot enough to turn that
water (or whatever it was) to steam, (or whatever), thus creating
pressure and fracturing the stone. Boom. 8 grand down the drain.
That’s serious money now, but in the mid 90s, when I did this, it
was worth a good deal more. My boss at the time had a devil of a time
finding a replacement. Took a trip to the Tucson show for just
that… I doubt she’s ever forgotten the incident. I know I haven’t.
And bless her soul, though she was furious/frantic/etc, she didn’t
fire my butt… Or make me buy the replacement (which I could never
have done anyway.) I was one sheepish remorseful jeweler/gemologist,
I can tell you…
Lesson learned. Never assume ANY stone is safe to heat unless you
know you can easily afford to replace it yourself without any fuss
and nobody will mind if you have to do so. And with more costly ones,
perhaps an inspection with a loupe is NOT enough. Use, or borrow, or
have someone else examine the stone with, a microscope. Carefully.
And then, back to CZs, there are the ones where nothing at all
happens to the polish, or the surface. But the body color changes to
yellow and doesn’t come back as it cools. Who knows what the heck is
happening there. I’m guessing something to do with the particular
stabilizer used in that CZ, but who knows.
Also, make sure to air cool the piece after completing the solder
operation, as the thermal shock will shatter the stone.
That is certainly true. In spades.
Peter Rowe