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Corundum damage


#1

Hello Peter! Just wanted to add a bit to your post on surface damage
to corundum. This has happened to me, and cost me money. Always a
great teacher. The store I had done the work for was tops. There was
no loss of tempers or scolding. I had unknowingly used too high a
temperature, and for too long a duration. It was my first loss of any
value (around $300 as I recall) Not long after I had made replacement
of 3 ruby cabs, I came across an article in GIAs’ "Gems and Gemology"
summer 82’, and I quote:

“Occasionally, we see rubies or sapphires that have etched
surfaces. These damaged surfaces occur when repair work is done
on an article of jewelry without removing the corundum from the
setting. Damage results from the stone coming into contact with
the usual fluxes containing Boron, in which corundum is
soluble. Under high temperatures these fluxes, whether they are
used as protective coatings or in pickling acids, become more
active and may seriously attack and partly dissolve the surface
of the stone. To prevent etched surfaces, the safest precaution
is to remove the corundum before the mounting is repaired or
pickled.” Endquote.

This of course came as no surprise. I showed the article to a number
of contemporaries. Some had seen the problem, others hadn’t. When I
do any torchwork with corundum I prefer to remove. I will often
solder with a reducing flame to solder flow temps. up to 1350degrees.
I will make every effort at not getting any flux or boric solution on
the stone. To flux a prong requires just enough temperature to singe
my fluxbrush. Maybe 150 degrees or so. If any flux sprays on the
stone, I use a toothpick to rub it all off before putting under temp.

In closing, anyone these days, who puts corundum under temperature at
their own risk, will definitely have replacements. My advice with
stones you are unwilling to replace at your cost, should be opted to
the customer or the store your working with to alleviate your risk.
Especially those corundum with inclusions; they can change for the
worse after torchwork. Much better to remove and install a new head
or bezel than to eat a stone replacement.

Tim

PS Never had any problem with pickle and corundum, any enlightenment?


#2
   PS Never had any problem with pickle and corundum, any
enlightenment? 

Tim, A new, clean, sparex solution won’t hurt corrundum at all. but
after it’s been used for a while, it will contain the ions of the
various fluxes involved. The GIA article mentions boron. I suspect
also the fluorine ions, from fluoride fluxes, especially like those
white paste fluxes that often are high in fluorides. Think about
it… You mix fluoride ions in with an acid solution, and some of the
chemistry is gonna start looking a little like Hydrofluoric acid.
Ever noticed how some types of glazed pickle pots, like the cheap
crock pots and the like, often get their glazed finishes rather
etched away after a few months or years of use? Even pyrex glass
beakers, after enough years holding old/used pickle will get seriously
etched. It’s not the pickle itself, its the OLD pickle, containing
fuorine and boron ions that let it attack these things. And if it
can attack the silica glass and glazes, the it will also be attacking
any stones with similar chemical succeptibility to HF acids… Lots
of stones are silicates of one sort or another… So if you use lots
of fluoride containing fluxes, it may pay you to change your pickle
more often…

With corundum, while it’s not generally attacked by pickle, do
remember that these acids generally are effective on metallic oxides.
Corundum is Aluminum Oxide. I’m not a chemist, so I’m not sure what
other things in the pickle might be needed to cause it to start
attacking corundum, but I can easily imagine that it’s possible.

And a note on heating gems in general. Many stones can take some
heat, of course. We can surmise that from their origins, which often
involved heat and pressure. but many stones also have significant
rates of expansion when heated, and here’s the problem. If the
outside of a stone heats faster than the inside, the outside
litterally expands away from the inside, and it cracks. metals do the
same thing, but being elastic and dfuctile and malleable etc, they
don’t generally crack from this stress. Gentle, slow heating allows
many stones to survive temps that would break them if achieved
suddenly. so you can heat treat stones for color, in a slow and
gentle furnace, that would instantly crack with a torch anywhere
nearby. but it’s more complex than that, too. Any inclusions in the
stone that are bits of another mineral or bits of fluid, etc, will
then behave differently under heat than the parent material, which
then can also cause breakage. I’ve had sapphires in a ring break in
half from inclusions, when the sapphire never got even hot enough to
cause the boric acid powder on the ring to start to melt. I normally
don’t let flux or boric acid on the sapphire, but the one I’m thinking
of I didn’t bother to worry, since I wasn’t even heating the stone at
all. but enough heat traveled up the setting to warm this stone to
maybe a bit over the boiling point of water… See where this leads?
The inclusion was fluid filled. Common in many stones. This stone
had not been heat treated. My little bit of heat caused enough steam
pressure to split this already stressed stone right in half. That was
NOT a 300 dollar sapphire either. Add another zero… (yoikes) So
the bottom line is that any time you’re working with heat near any
valuable or potentially succeptible stone, it will pay you to be
paranoid, and take all due precautions, after carefully examining the
stone to evaluate the risks involved, Removing the stone is often far
safer, and then with no stone, it’s easy to do a more professional
job of rebuilding prongs like they should be, instead of just patching
some tips with quickly soldered on bits of metal that are all too
often still visible as an evident repair when you’re done.

And on the other hand, with some prudence, sometimes you can get away
with breaking the rules. We all know that you cannot heat diamonds
beyond a certain temp without damage, right? Diamond starts to burn
in air at temps high enough to be visibly glowing. So we all know to
coat with boric acid. But we also all know that there are limits,
and when working with platinum we know we can’t use platinum grade
solders too near a diamond, right? Well… Um. sometimes… I all
too frequently get asked to restore some of those eduardian/victorian
platinum and diamond pieces… filligree, set with zillions of tiny
mellee, holding nice large center stones in prongs that have now worn
away. You all know the styles. A couple stores we do work for are in
the habit of swapping in nice costly new center diamonds, restoring
these things, and selling the now good looking pieces for high prices
as estate jewels. Good so far as it goes, but for the jeweler, fixing
these things up so that they can safely hold those big new diamonds,
or even the one still in it, can be tricky. Often you’re asked to
replace or repair prongs on thin platinum areas right next to melee
diamonds. If you do this with white gold solders, as conventional
wisdom suggests, then you end up with the risk of weak solder joints
prone to cracking when you then try to set the stone, or later if the
prong is stressed. Sometimes, this is the best you can do, so you do
what you can. But if there’s not other gold solder already in the
area, then on some of them you can use the lowest melting platinum
solders instead. I use 1100 solder on these situations about half
the time (white gold solder on the others, depending on what’s needed
and the risks involved…) Now, 1100 solder is melting at 1100
cenegrade, or about 2000 F, which is a bright glaring yellow heat.
Way too much for diamond, right? Wrong. The diamond itself can take
high heats. Just can’t do it with oxygen. And diamond is an
extremely good conductor of heat, so you don’t get the
internal/external differences of heating that lead to heat shock
cracking, unless there are mineral inclusions in the stone doing it.
What you’ve got to do is exclude oxygen. With multiple heavy coats of
batterns flux or boric acid, each glazed on, the peice cooled and then
more added until it’s a thick goey layer completely covering the
stones, the boric/flux layer will then be able to withstand the temps
of that soldering operation long enough to protect the diamonds, so
long as you can get in, solder the thing, and get out quickly. It’s
time sensative. The flux won’t hold up long. And Don’t try this with
any other stone, or with diamonds you cannot replace if you screw it
up. but most of these are small, easily replaced melee, so the risk
is manageable, and the upside is that then the repair is much better.
It’s not perfect, of course, since even with a low melting platinum
solder, the strength of the joint isn’t great. 1100 solder doesn’t
give you great results if there isn’t much surface area to bond. But
it’s a darn sight better than what you get with white gold solders.
If you need better, then you need to be able to remove some diamonds
near the repair. Often, a couple millimeters is enough distance so
that then you can use the higher platinum solders, or even weld it for
the best results. Again, it’s a time sensative thing, both to
minimize the time the stones are hot, and to minimize the amount of
time the heat has to travel away from the joint area. Best, of
course, is to spend the 35 thousand or so that the current generation
of laser welders cost. Theh you can weld these things with no risk
to the stones at all, it’s so fast… would be nice, huh?

Cheers
Peter Rowe