[Jewelry making - Article 00985] - Re: [Orchid] Rough surface on Sterling belt buckle casting

[Todd Hawkinson]

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Dear Ben & All,

    I have been casting both centrifugal and vacuum casting since 1971.
    These are just some immediate thoughts. 

    Investment has the same storage characteristics as a bag of ready
    mix cement. If you buy a bag of cement new right from the store and
    open it up you will see a uniform mix of ingredients. Now if you let
    the bag sit for a year or two open it up again and look at it. If
    will have separated into its different ingredients. These ingredients
    have different specific gravities. The heavier ones will settle to
    the bottom of the bag with only gravity to blame. 

    Here is my point. Investment in a barrel does the same thing. As an
    apprentice one of my jobs was to put my arm into the barrel of
    investment and mix the stuff up from the bottom to the top. Many
    times a new barrel will be fine until you reach the half way point.
    Then the castings start to turn bad. I would have to guess that the
    settled mix does not breathe as well during the casting process.
    Vacuum casting does not seem to be as affected. You could FEEL the
    difference from the top of the barrel to the bottom. It all looks the
    same only the feel is different. 

    In the dental casting area, they solved this concern by having the
    investment provided in smaller bags. It doesn't separate in smaller
    amounts as much. 

    Now if you combine the shipping of 100 pound barrels of investment
    overland by truck, guess what happens? This shaking and vibration
    can speed this process up. 

    I usually buy 100 pound barrels one at a time. The firm I
    apprenticed at used to by 20 at a time. Every month they used to flip
    the unused investment barrels upside down for storage until they used
    them. This helped a bit, but I still had to mix up the investment by
    hand. 

    I had been mentioned in AJM a while ago, because I store my
    investment in a cement mixer with a clamp tight cover. I mix my
    investment once a week and I have never had an investment related
    problem. 

My two cents,

Best Regards,

Todd Hawkinson
Instructor, Minneapolis Community & Technical College

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