[Jewelry making - Article 01261] - Re: [Orchid] Silver casting porosity Problems

[Janet Alexander]

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    Hi, I have been casting now for over 20 years and I teach casting and
    wax carving at the Craft Guild of Dallas. Here are some pointers: 

    Several things can cause porosity. Here are some rules I go by. Go by
    the following rules for spruing, always have enough metal for a
    good-sized button, never allow the flask to drop below 900 degrees. 

    The sprue has three important parts in the casting process.  

    It holds your wax at a proper angle in the flask 

    It is the funnel for your metal to get to the model. 

    It is an important part of the process when the molten metal is
    solidifying. (This is where plays the most important part of the
    casting process to keep porosity down.) 

    As your metal is forced into the open cavity of the investment, it
    must have a smooth pour so that there is no turbulence. If the
    joints of the sprues are not smooth this can cause turbulence, or
    even some obstruction. 

    When molten metal solidifies it shrinks. When it shrinks, it will
    either pull more metal from the sprues, or if it can=92t it will pull
    in air, causing porosity. In an ideal casting, the item being cast
    should solidify first, and as it shrinks, it pulls the molten metal
    from the sprue. Then as the sprue solidifies and shrinks, it pulls
    molten metal from the button. The button solidifies and shrinks
    unable to pull molten metal from the sprue, it pulls in air. All
    porosity should be in the button. 

    As a rule the button should have the same weight in it as the rest
    of the casting therefore allowing the button to stay molten during
    the solidification process, and also adding the weight to help force
    the metal into the open cavity of the investment.

    Improper spruing can cause porosity. 

    Always attach your sprue to the thickest part of your wax, so that
    when the thick area of the model is solidifying, it can pull metal
    from the sprue. The sprue must be as large as the area you are
    attaching it to. 

    When attaching the sprue make sure the contact point is secure with
    no gaps. Create fillets or curve the joint from the sprue to the wax
    so that there are no harsh corners for the molten metal to pass
    through. 

    The sprue should be as short as possible, but no shorter than 1/2 an
    inch.  A sprue that is too long can start cooling down before it gets
    to the wax model and can solidify first, before the wax model. 

    Always place the sprue on the model so that when the metal is
    funneled into the open cavity it always is going in one direction. 
    Check this by holding the sprue attached to the wax model upside
    down; visually look at it to make sure that nothing is pointed
    upward or above the attachment area of the sprue junction to the
    model. 

    Check with the manufacture of the investment, some manufactures will
    state not to allow the flask to dip below 900 degrees after firing.
    Your flask may have been too cold. 

    Also you did not say whether the metal you were using was new casting
    grain or a mixture of used and new metal.  When casting, the metal
    must be new or a mixture of 1/2 new and 1/2 use metal (used metal
    being re-melting a button and sprues.) 

I hope this helps
Janet Alexander
www.ornettes.com

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