[Jewelry making - Article 00007] - Re: [Orchid] Graphite pencil Leads

[Justine Wetherington]

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>     I wanted to get something cast that had tubes through which a
>     chain would pass.  Since I wanted a mold made so I could have lots
>     of them I thought sticking something in place before molding and
>     before casting would solve my problem.  The caster said they used
>     to use graphite to do that (so I knew the idea I had "invented"
>     was nutso) but that the graphite had lost quality and was sticking
>     inmetal.Well, I got an idea to ask about ceramic rods.  I wrote to
>     a few manufacturers and I am enclosing an excert from a reply that
>     I got.  I'd like ya'll to chew on this, toss it around and let
>     everybody else know what you think. 

    I have a few options for you.  Precious metals will not adhere to
    the majority of ceramic materials.  The problems you will encounter
    are thermal shock and the low thermal expansion coefficient of the
    ceramics relative to the metals.  That said, I can offer three
    materials that I believe will survive the application.  Aluminum
    oxide, mullite and boron nitride. Aluminum oxide and mullite must be
    cut with an abrasive diamond wheel.  The boron nitride can be cut
    with a knife.  Of the three, boron nitride will be the most thermal
    shock resistant, followed closely by the mullite and the alumina a
    distant third.  Fortunately for you, the geometry is small so I think
    all three would survive the application.  If you are concerned about
    reuse and mechanical strength, the alumina is strongest with mullite
    a close second.  Boron nitride is significantly weaker.  Thermal
    expansion may be a problem even with the small diameters.  Once the
    metal reaches solidus, it will shrink faster than the ceramic thus
    becoming locked into the metal ring.  Geometries may be small enough
    that this is not a problem or you can pull out the ceramic shortly
    after solidus is reached. 

    As an alternative, have you considered the use of coatings on a
    steel pin? If you have an acetylene torch about, try making a rich
    flame and depositing carbon onto a metal pin and use it.  Carbon and
    graphite are remarkably good release agents.  Acheson colloids sells
    graphite laden liquids that can be painted on also.  You might try
    your local weld shop and see if they sell a braze stop-off.  The
    stop-offs are ceramic powders in a vehicle designed to stop the
    adherence or flow of braze alloys.  They are readily available in a
    number of forms from multiple sources.  You can also obtain a boron
    nitride spray from us with outstanding release characteristics. 

    I like the fact that several options seem reasonable. 

    Even with ideas to solve the chain channel problem, the two
    commercial casters I talked to did not want to get involved with the
    project (piece too large) in fine silver. 

Enjoy - Justine
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