[Jewelry making - Article 00012] - Re: [Orchid] Preparing for pewter casting

[Peter W . Rowe]

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    Heather, pewter is really easy to cast. You've got several options
    on how to do it. 

    First, if you're most comfortable with the usual silver casting
    methods, you can use the same ones for casting pewter. The mold temp
    for casting, after burnout of the wax, should be all the way down to
    room temp. Just make sure it's warm enough so there won't be any
    possibility of condensation (water) in the mold. 

    Pewter can also be nicely cast in ordinary plaster of paris molds,
    instead of more costly casting investment. However, you cannot burn a
    wax model out of plaster, so this is then limited to two part molds
    made from a model you'd then remove manually, much like one might
    make an old style sand casting mold, or the plaster molds the ceramics
    folks use for slip casting.. Sand casting, by the way, also works
    great. So, in fact do molds made even of things like plywood, if you
    are making a shape you'll be doing some cleanup on, like handles for
    a coffee pot or something. The plywood will char slightly, but that
    won't hurt anything for one or two casts. 

    If you make an original wax or metal model, and then make a silicone
    rubber mold (not natural rubber, has to be the silicone rubbers), you
    can actually cast your pewter directly into the rubber mold, letting
    you do multiple castings with just the one mold, same as you'd
    normally do to create wax models. The mold is at room temperature
    when poured, and you can melt the pewter literally just in a tin can
    over the stove (make sure it's not a can that will come apart when
    heated, and use a steel one, not an aluminum one...) I've done this
    with standard shape/type rubber molds just as gravity pours (works,
    be sure to powder the mold so air can get out, and tie the mold shut
    with binding wire, or make some sort of frame, or support it in sand
    or something, so you've got hands free for pouring. I got better,
    more consistant results by making the original molds from pourable
    RTV rubber, poured into a standard steel casting flask as the mold
    frame (I'd cut a flask in half, so I could then extract the mold
    after it had set up. It's a harder shape to cut, but then you can
    place the mold after cutting back into such a flask, put it in a
    casting centrifuge, and cast it the same as you'd do with silver.
    never a partial fill that way. Be sure to use a seperate crucible
    for melting. If you're centrifuge casting, let the machine spin down
    totally on itself. Don't slow it down or stop it. Pewter's low
    melting point means the cooling rate after casting, especially in a
    good insulator like a rubber mold, is pretty slow, so the metal will
    remain molten a lot longer in the mold than silver or gold would
    before solidifying. If you stop the machine rotation too quickly, you
    might find metal running back out of the mold... How long it takes
    depends on how thick the thing is, but it can easily take a couple
    minutes to solidify fully. The main advantage to using a centrifuge,
    by the way, is that then you can get away with using smaller and/or
    fewer sprues than might be needed for a purely gravity pour. The slow
    cooling rate also allows you to get complete fills with less spruing
    than you'd need with silver or gold 

    Any time you're working in a metal with a melting point wildly
    different from the metals you normally use (pewter or lead based
    alloys melt far below silver or gold alloys, for example, just as
    silver and gold alloys melt far below platinum alloys), then there is
    a risk in using your usual tools for working with the low melting
    metal. If you work with pewter and then filings or tiny scraps or
    bits of the pewter get on your silver or gold alloys when you're
    working them, especially if you then heat them to anneal or solder,
    then the traces of pewter can melt their way deep into the higher
    melting metal, causing a deep and difficult to remove scar, or even a
    hole all the way through. It's a mess. So it's best to not only use
    different tools, but work on a different bench as well. However, most
    of us don't have that luxury, at least not for the seperate bench. So
    then be sure to pay particular attention to keeping the pewter
    filings and scraps cleaned up, so they won't later contaminate your
    precious metals. This is especially true for soldering blocks and
    tools, and for files, which are especially good at trapping tiny bits
    of metal that might then be released later where not wanted. 

Peter

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