[Jewelry making - Article 00498] - Re: [Orchid] Bronze casting porosity

[andy cooperman]

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    Carlos, From your picture and description< I don't think that you
    have a "porosity" problem, per se.    It seems that what you have is
    very a large grain structure, which might be polishable. 

    Grains growth occurs as the casting cools.  The higher the flask
    temperature or the hotter the molten metal, the larger the grains
    will grow after casting.  Dental gold alloys are often marketed as
    "grain refined", that is, engineered to keep the grains small and
    well integrated with each other. 

    Large grain structure yields brittle castings. Think of a pebble
    beach as opposed to a white sand beach.  It is easy to trace a
    boundary-- or break-- between the large pebbles of a pebble beach. 
    The stones are not packed tightly together.  A white sand beach has
    grains that are packed very tightly together.  It is quite dense and
    it would be very difficult to draw a clean line or crack between the
    grains. 

    I would lower either the flask temperature or the casting temp. of
    the metal.  I would also consider quenching the cast flask after the
    red has gone from the button. 

    I would also try polishing one of these castings to see how bad they
    are. 

Good luck, Andy  


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