[Jewelry making - Article 00048] - Re: [Orchid] Large silver cups casting

[Patrick Storme]

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Hi,

    the problem with air bubbles that tend to adhere to the wax surface
    can be resolved by first "annealing" the wax by putting it for a few
    hours in slightly warm water.  Afterwards brush a light soapwater
    onto it, there exist also special (expensive) liquids for this
    purpose. As for the investment: do not mix with distilled water,
    because some brands do not react as they should.  However, some tap
    water sources are sometimes not suitable too, but if the investment
    gets hard in a normal way, there should not be a problem. 

    In my experience, vacuum casting is not my choice for very large
    pieces. I have much better results by casting as the ancients did:
    just by gravity.  You have to have sufficient vents at the right
    places, but this works just perfectly. The temperature of the mold
    should be as low as possible.  For casting pieces this large, I bake
    the molds as usual first to a 720 C, but let them cool down to 300
    C.  This can take many hours before the inside of the mold also
    reaches this temperature.  The temperature of the silver can stay
    normal, lets say about 50-100 C above melting point.  Heating too
    high causes to much attraction of oxygen and since you have a large
    amount of silver, it stays quite long on this temperature, so your
    casting can be carried out relatively calm.  Just before casting,
    putting in a small piece of zinc helps to eliminate excessive oxygen
    from the cast.  Melting below argon gas is also an option, but
    perhaps not available. 

    Cracking of the core is the most difficult problem but can be
    resolved by making a stainless steel reinforcement structure in the
    core, or by putting fine silver (alloyed silver oxidizes because of
    the copper content) through the wax model en thus holding the core. 
    I woudl recommend at least 1,5 mm diameter for each pin and would
    put at least 10 of them in place to hold. A different solution can be
    of casting upside down, the core stands on itself and you will have
    much less problems of falling down pieces from cracking cores. 
    Also, the warming up of the molds is very important.  This has to be
    done as slowly as possible, certainly up to 500B0C (higher can go
    quicker) and without opening the furnace now and then to check
    things.  These are often facts which cause cracking. 

Succes.
Patrick Storme
Conservation Department of Metals
Royal Academy of Fine Arts, Antwerp
Belgium


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