[Jewelry making - Article 00286] - Re: [Orchid] Cement-like gunk on castings

[Peter W . Rowe]

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>     I am not a jeweler or a caster, but wonder if the change to
>     propane may have caused a little sulfur to get into the metal from
>     the propane flame. Acetylene does not have any, but propane, being
>     a fossil fuel is likely to contain small amounts of sulfur from the
>     crude oil, and the odorant is a mercaptan which will contain some.
>     Just speculation, I am not a chemist, nor do I play one on TV. 

    Not likely a factor. Sulphur, for one thing, is not a volatile gas,
    so most sulphur compounds in crude oil or coal stay in the solid or
    liquid mass until it's burned. If sulphur were in propane, it would
    likely be a sulphur dioxide gas, which you'd smell. 

    sPropane generally has a reputation, like natural gas, of being
    clean burning, causing fewer problems with metal oxidation or
    contamination than does acetylene. 

    And in this case, if sulphur were the problem, it much more likely
    would be from the investment itself. it's gypsum based, with is a
    sulphur compound. It's one reason why investment shouldn't generally
    be heated above 1300 or at most 1350 during burnout. Above that, it
    starts to break down the gypsum, and the released sulphur compounds
    then DO cause problems, with black and rougher castings. Doesn't
    generally cause a yellowish cement like coating. 

    Most of the time I've seen anything like that, it was from burning
    out models made from various plastics, or even rubbers, instead of
    wax. 

Peter
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