[Jewelry making - Article 00063] - Re: [Orchid] Asbestos sheet

[andy cooperman]

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>       When I took casting class, we used asbestos sheet to line
>     flasks, not crucibles.  Is that what you were referring to? Now, I
>     believe, wax sheets replace the asbestos in this application. 

    After several years as an aspiring jeweler, I began a second career
    as a dental lab technician running a one man lab in a dentist's
    office.  The dentist taught me everything that he had learned and I
    figured out much of it myself, w/ a lot of time spent on the phone to
    Ney, Jelrus, Heareas, etc. 

    We used to line the flasks. (As a cushion to compensate for the
    expansion of the investment-- the so called quartz inversion-- which
    would compensate for the innate shrinkage of the cast crown.  The
    investment expands at a certain temp. and, if the flask is lined,
    expands into the soft liner which expands the mold chamber at about
    the same % that the casting would shrink, resulting in a perfect fit
    of the crown in the mouth.  I don't know if this is still done.) 

    We also used to line the crucibles.  I'm not sure why, but I suspect
    that it cut way down on any flux that was present on the metal. 
    Minute traces of flux might interfere w/ the bonding of porcelain to
    a cast coping (armature) in the case of a porcelain on metal crown. 
    I don't know if this made any real sense-- I just did what I was
    shown. 

    We used a non asbestos material that was made for this purpose.  It
    was made from Kaolin, or kaolite , was a fiberous material, and came
    as a strip wound in a coil.  At that time -- the early eighties-- it
    was available from dental supply houses. 

    I don't use anything, save a flux of boric acid on my clay crucibles
    now (as a metalsmith).  The aluminum oxide crucibles made by Wesco
    are self glazing when brought to a high temp. They are quite
    expensive. 

Hope that this helps,  Andy

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