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Firing PMC with stones


#1

Hi there I found your site very interessting. I work with PMC
(precious metal clay) and am teaching classes. I was woundering if
you could help me with a question I have. I usually fire a gemstone
together with the PMC (which is actually the easiest way to make a
setting) and I have the indication of the manufacturer that the stone
needs to have at least mohs hardness of 7. But I’ve experienced for
example that a ‘pariba topaz’ keeps it’s wonderful bluegreenish
colour wherelse a ‘london blue’ or a ‘south sea blue’ topaz turns
white (i.e.clear) during the fireing in the kiln (up to 870 C then
hold for 10 min.) Could you please tell me what other indicator
(other than mohs hardness) I have to look out for so that the stone
does not change color? Would you by ancy chance know which colors of
the topaz (besides the pariba) do not change color?

For your kind help many thanks in advance
Kind regards
Gabriela

End of forwarded message


#2

Corundum that looks like alexandrite maintains its color when set in
PMC and then fired. It may look as though it changed color when it
first comes out of the kiln, but it cools down to its original shade.

Dee


#3
 Snip, needs to have at least mohs hardness of 7. But I've
experienced for example that a 'pariba topaz' keeps it's wonderful
bluegreenish colour wherelse a 'london blue' or a 'south sea blue'
topaz turns white (i.e.clear) during the fireing in the kiln (up to
870 C then hold for 10 min.) Could you please tell me what other
indicator (other than mohs hardness) I have to look out for so that
the stone does not change color? Would you by ancy chance know
which colors of the topaz (besides the pariba) do not change color? 

Pariba is a copper bearing Tourmaline from a particular region in
Brazil. It is not topaz. Nearly all topaz on the market is topaz that
is irradiated and heat treated to obtain the blue colors most people
are familiar with. Heating will cause loss of color in most if not all
samples. Many synthetics can be successfully cast in place without
any significant change in color. Any natural gemstone that is
normally heat treated to improve color or clarity may alter in
appearance with additional heating. Mark Liccini ( www.liccini.com )
did have a very good site on gemstone treatments but it has not been
available for a while. A quick search on google for gemstone
treatments should turn up a wealth of

Thurmond Moore III
owner-lapidary@caprock-spur.com
Moderator of The Lapidary Arts Digest


#4
  But I've experienced for example that a 'pariba topaz' keeps it's
wonderful bluegreenish colour wherelse a 'london blue' or a 'south
sea blue' topaz turns white (i.e.clear) during the fireing in the
kiln (up to 870 C then hold for 10 min.) Could you please tell me
what other indicator (other than mohs hardness) I have to look out
for so that the stone does not change color? Would you by ancy
chance know which colors of the topaz (besides the pariba) do not
change color? 

I had not heard of the Moh’s rule for selecting stones that would
fire well with PMC, but it may have merit.

Stones from Paraiba, Brazil of the blue color you describe are most
likely tourmaline, not topaz. I am surprised that the color held up
in the kiln, but am happy for you that it did.

London blue and south sea blue topaz has been treated to become that
color. It doesn’t surprise me at all that the heat would tamper with
previous treatment.

Any stone that has been previously treated such as citrine, apatite,
sapphire is not a very good candidate for firing with PMC. Always
ask the dealer if the stones have been heated.

Any stone with water in it, like opal, is not going to survive the
kiln. Watch for the nH2O (n being a variable number) at the end of
the chemical formula.

Many sythetic stones are suitable for firing with PMC, although I
have heard that green stones do not always retain their color.

Watch for stones with inclusions, air or water bubbles as any of
these can crack or even explode in the kiln.

I am sure there are other things to watch for, but that is all that
comes to mind at the moment.

Good luck. Let us know of your successes and failures with other
stones you may try.

Cathy
Certified PMC Artisan


#5
       But I've experienced for example that a 'pariba topaz'
keeps it's wonderful bluegreenish colour wherelse a 'london blue'
or a 'south sea blue' topaz turns white (i.e.clear) during the
fireing in the kiln (up to 870 C then hold for 10 min.) Could you
please tell me what other indicator (other than mohs hardness) I
have to look out for so that the stone does not change color? Would
you by ancy chance know which colors of the topaz (besides the
pariba) do not change color? 

I found Pariba Topaz in Tucson a year ago - February 2001. The
color resembles Pariba tourmaline, but is not as bright. The color
is the result of a diffusion process and probably the reason that it
survives heating in the PMC.

For further heating experiments - I’d look for diffusion treated
stones. Has anyone tried heating the pretty diffusion treated
sapphire with padparasha color?

Judy Hoch, G. G.

I did earn the Graduate Gemologist certification, but like the
second meaning better - Gem Goddess. :slight_smile:


#6
 Many sythetic stones are suitable for firing with PMC, although I
have heard that green stones do not always retain their color. 

If you’re talking about the really cheap synthetic spinel and
synthetic corumdums in green, the probable problem is that many of
the brigher versions of these are not actually green, but are
triplets with a top and bottom of clear material, cemented together
with a green cement layer at the girdle of the stone… There are
several types of these, and they’re not always labeled as such. some
are actually a little difficult to detect, if they’re well done and
you’re not looking for it.

Peter


#7

Gabriela, What kind of kiln do you use for your PMC?

Tom