[Jewelry making - Article 00517] - [Orchid] Firing PMC with stones

[Gabriela E. Oggenfuss]

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    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
           

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