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
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
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.
Certified PMC Artisan