[Jewelry making - Article 00531] - Re: [Orchid] Disappearing diamonds fired in PMC

[Peter W . Rowe]

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>     The reports of the diamonds burning up in PMC were fired at 1650F
>     (900C) for 2 hours. This is the firing schedule for the origninal
>     (Standard) PMC. The question is not whether the temperature alone
>     is an issue, but the amount of time spent at temperature. How long
>     did you leave the diamond at 900C? 

    At that temperature, if there's any oxygen around, it would take
    very little time at all (seconds, not minutes) for the surface of the
    diamond to become damaged, looking frosted and no longer well
    polished. Even if it's not fired long enough for the whole thing to
    disappear, it might as well do so, at least with small stones, since
    you still have to pull it out and, if large enough to warrant the
    cost, have it repolished. Any working "repair" jeweler who's
    neglected to get his diamonds clean enough, or covered with boric
    acid firecoat before soldering on new prong tips, which is usually
    done at considerably lower temperatures than 1650F, will have had
    this happen, and most such people quickly learn that it's a risky
    idea to let the diamonds get hot enough to be incandescent
    (glowing). At 1650, a diamond is glowing rather brightly, and burning
    up equally quickly, if exposed to oxygen. 

Peter
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