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Sapphires and Casting


#1

I did a little looking in the archives but I’m not entirely sure I’m
going to find my answer…by the way, hi, I’m new. ^_^; Both to the
list and to jewelry-making in general.

I have a wax design that I’d like to cast with the stone set in
beforehand. It is a clean, ‘white’ sapphire (it has a very faint
lavender cast to it). As far as I know, sapphire is durable enough to
not break in a casting, but that wasn’t my big concern. My instructor
mentioned, though, that the heat of processing and casting may
discolor the sapphire.

The difficulty is, though, the design of the ring would probably
make post-cast stone setting a bit difficult. I can live with it if
the stone changes color to something that doesn’t clash with blue,
green, or 14k yellow (the ring is also going to have a black opal in
it). Is there any way to predict what color the stone may turn? Or is
the temperature of the molten metal low enough that it won’t change?

–Marcy Osedo (Ponygirl)


#2

Marcy, while the sapphire will withstand the temperature reached in
a gold casting, there are opportunities for problems to arise.

Please note that your instructor said “discolor”, not change the
color. What he or she is most likely referring to is for a borax
coating to form on the surface of the sapphire and case a milky
swirl look. I have had this happen while re-tipping a sapphire in a
ring. If this happens, the only recourse is to have the stone
re-polished.

Other issues with cast in place are the speed which the flask is
heated and cooled. A rule of thumb is about 50 degrees per hour for
heating, and then after the pour, place the flask back into the kiln
and let it cool back to room temperature with the kiln, over a
period of a few hours. If you quench the flask as with an all gold
or silver casting, you are sure to crack the stone (sapphire, ruby,
or diamond). One other think you might encounter is that inclusions
in the stone may “POP” the stone during the heating stage.

The cast in place process does have it’s rewards though and does
offer come creative stone setting ideas to come into life.

Perhaps others on the list provide more and tips.

Don


#3

Marcy: We are located in Montana where many sapphires are mined, cut
and heat treated. Since we repair the tools jewelers use we have
customers that do sapphire heat treating and know a little about the
process. There is no way to predict what color a heat treated
sapphire will be. Basically when they are treated it is a gamble.

If you have any more specific questions about them I can get you a
name or two for more technical info.

Ken Kotoski
MPG Repair
www.mpgrepair.com
1-877-262-2185


#4

For the most part, my experience has been that sapphires are safe to
heat. I would suggest that if you not heat unusual nor particularly
valuable stones.

Gemologists are aware of particular inclusions in corundum that
indicate a stone has never been heated. Untreated stones can command
a much higher price than treated stones. Although it has been
estimated that 90-95% of the corundum an the market has been heated,
I would suggest that you take care to be sure of what you are doing
before you attempt to cast with sapphires in place. Let me go on.

A few years ago I was given some sapphire crystals to set up into
pendants and rings. Since I owned no “free form” burs and using small
ball burs wouldn’t produce the exacting precision that I was trying
to get, I decided to take impressions by heating the stones in hard
wax and then removing the stones. Upon gently heating the first it
gave a barely detectable little “pop” and seperated into two distinct
pieces. I scratched my head. I then showed my client what had
happened. Fortunately, he understood. I then exercised all the denial
that I could have possibly mustered. I assumed that this was a freak
accident. After all, I had at this point heated thousands of
sapphires to no avail and could remember this happening to only a
single stone of about 3mm. I went back to work. The second crystal
did exactly the same thing. Thus ended my impression taking in hard
wax.

If one knows what one has invested in a sapphire, one can indeed
assume the risks of heating them. If, however you are unsure of the
value, origin or treatments of a particular stone you might want to
take a little more care with it.

Bruce Holmgrain
JACMBJ
http://www.goldwerx.com


#5
If one knows what one has invested in a sapphire, one can indeed
assume the risks of heating them. If, however you are unsure of the
value, origin or treatments of a particular stone you might want to
take a little more care with it. 

Wow, that was a strange story. Kinda spooky. o_o Sounds like it fits
with what someone else mentioned, that inclusions in the stone might
cause breakage when heated. Maybe the crystals had more inclusions
inside them than clear cut gems do?

Thanks for the tips though, I think I’m going to look at possibly
redesigning the ring a bit or try heating the stone first to see if
it’ll break. It was labeled clean, so I don’t think there’ll be
anything inside that might make it pop…but since it didn’t cost me
more than $5 and I have a couple more coming in the mail, I’m not too
worried about breaking it. Thanks for all your help. :slight_smile:


#6

having been confident at repair for many years and heating diamonds
with no problem, retipping sapphires(yellow, orange, pink) that were
"clean" was a nightmare. stones cracked internally. never heat any
stone unless you are willing to assume the consequences if there is a
problem. these days you cannot tell what, how, or if treatments
were used or if there are stresses or inclusions that may react.
extra care and caution save time and grief, and if there was a way to
find that grief, i have done it! i usually have a warning, a voice in
my says, what if such and such happens, and then it does, more often
than not. now when i think of the worst case senerio, i do grief
avoidance by chosing a method with the least chance of trouble, and i
confidently quote prices on the estimated time to do it right and
safe. i retip cz’s and let them air cool, if i crack one i can
replace it quickly and cheaply and still profit if it cracks.


#7
  " i retip cz's and let them air cool, if i crack one i can
replace it quickly and cheaply and still profit if it cracks." 

Thank you for your remarks, I agree in full as I once (ONCE being
the important word), heated a yellow sapphire, 6 carats, and to my
horror, had it lose all color and turn clear. The store owner
generously split the loss with me to replace the customer’s stone,
but it was still a very expensive lesson. The longer I am in this
business, the more cautious I become.

Please, pray tell, how do you retip cz without burning it up? I have
been turning away business thinking that czs and soldering do not
mix.

Thanks, Sharon


#8

Hi Sharon, I saw a picture in a catalogue showing CZ’s being cast in
place which surprised me. So, I took one of my loose CZ’s and
applied a torch flame to it. I heated it up to orange and let it
cool with no visible effect. I guess they are fragile in the sense
that they chip and scratch easily, but they can take heat
apparently. Any comments from the gemologists?

Marta


#9

Setting C.Z’s is quite a common thing up here North of the 49th
Parrallel (Canada) there are a few things you gotta watch out for
while casting cz’s. Don’t let the little thingees touch while in the
wax stage, why? the shrinkage will have them either touch and
splinter or just turn into dust. CZ’s do take the heat. Look into
buying the better and stronger style of stones, askthe supplier if he
would suggest a stone just for wax-gold-casting remember, DON’T LET
THEM TOUCH… AT ALL TIMES! Its the gold shrinkage that ruins your
fine setting not necessarily, the heat!!! The Aqua, or Memeory Wax
allows the stone to be inserted and the wax claws will expand and
return to their shape after the stone is set. Now then,… after the
casting, if you want to remove the fractured stone, a slight
explosion occurs and splinters of cz can be felt escaping and hitting
your eyes/face. I’ve seen this many, many times. I only wear glasses
while doing this. The sudden hitting of the hammer to release this
stone is at best, dangerous. Of course don’t water quench the hot
crucible. just a normal cooling will suffice. “Gerry, the
cyber-setter !” “www.gemzdiamondsetting.com@Gerald


#10

Hello Martia,

You can easily heat up CZ’s above the melting point of gold. The only
thing they do not like is fluxes and borax solutions, these will
attack the surface of the CZ. when heated. But you can easily repair
settings with the CZ in as long as you do not put flux on the stone.
However larger ones above 4 mm are more sensitive for heat
differences an can crack inside when not heated evenly.

Martin Niemeijer