[Jewelry making - Article 00287] - Re: [Orchid] Fire scale Prevention During Vacuum Casting

[Volpe, Constantino]

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Juan,  

    A few things you should know. Silver has a high affinity for oxygen
    when it is molten. Oxygen is soluble up to 30% which is amazing.
    However in its solid state the solubility is very low. From your
    e-mail I couldn't tell if you method was to put the flask in the
    bell jar the metal was poured. If so it is probably solidified by
    the time you reach an adequate vacuum and the damage has already
    been done.  You have to think from the point of view of where can
    the oxygen is coming from.  From the air around the melt. 

    This is the worst case as the metal will readily absorb oxygen
    through the surface. This is why you cast in a vacuum or under a
    cover gas or some other cover like charcoal if possible. Experience
    has shown me that melting the metal under high vacuum actually has
    caused severe fire scale on certain geometries that went away as
    soon as I switched to melting under partial pressure of argon. I
    can't really explain it other than theorizing that the high vacuum
    placed the metal in a reducing state, allowing it to reduce the
    silica in the investment upon casting, sucking up all the oxygen.
    That's the best I can come up with.  Speaking of investment, the
    molten metal can absorb oxygen from the air in the investment as it
    rushes in. Modern vacuum casting machine flush the investment with
    inert gas to get rid of this air. If you are casting using a vacuum
    table that is just sucking air in, this can be a source of oxygen. 
    The investment itself is made up of ceramic oxides. You will always
    get a metal mold reaction where some of it breaks down releasing
    oxygen into the very surface layer of your casting. How deep this
    layer goes depends on how hot the metal and mold are as this is the
    energy that drives the reaction, and how long the metal stays
    molten. For this reason you want to fill it and freeze it as quickly
    as possible.  Last but not least, you starting material can be
    loaded with oxygen to begin with. If this is the case then the inert
    gases in modern casting machines won't help since they will protect
    the melt from further oxygen entrapment but do nothing for what's
    already there. At least with an open melt use can use graphite
    stirrers to agitate the melt to force the oxygen to react with the
    carbon and be released as CO.  

    Summary: Start with low oxygen metal, protect the melt from air,
    flush the flask out with inert gas, and most important keep those
    system temperatures as low as you can go. The better you gate the
    pieces, the lower a temperature you can get away with.  

Tino Volpe  Metallurgist, 
Technical Manager  
Tiffany & Co.  
300 Maple Ridge Drive Cumberland, 
RI 02864-8707  
401-288-0124  
Constantino.Volpe AT Tiffany.com  

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