[Jewelry making - Article 00564] - Re: [Orchid] Problems with melting furnace

[Peter W . Rowe]

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Vera,

>     To get familiar with the melter I decided to pour an ingot. I
>     heated the sterlling to 1700 deg F, added a pinch of flux, stirred
>     the silver with a graphite rod, picked up the crucible with the
>     tongs and poured. Initially nothing happened so I tipped the
>     crucible a bit more. The silver poured out all at once. Only part
>     of it made it into the ingot mold. I had noted that when I picked
>     up the crucible which was a cherry red, the top part lost its color
>     because the tongs sucked the heat out of it. I am guessing that the
>     initial silver hardened and when I tipped the crucible further the
>     melt went over the bump. 

    Two misconceptions exist with these italian furnaces. one is that
    the metal itself is at the temp indicated by the display. That temp
    is measured by a thermocouple outside the bottom of the crucible.
    Often, the crucible and metal lag behind the indicated temp. To be
    accurate, you need to allow the furnace to reach the setpoint temp,
    and stay there for at least several minutes, before you add the metal
    to be melted. The temp shown will drop a bit, then rise back up, and
    again, you should let it sit there for a couple minutes after it
    appears fully melted. This effect is greater with the italian type
    furnaces than with the electromelts, since with the electromelts there
    is a hollow cavity milled into the bottom of the crucible into which
    the thermocouple fits, so the temp measured on those more closely
    reads the current crucible temp, with somewhat less lag time. Not a
    bit difference, but worth noting. Personally, I prefer the italian
    furnaces even so, as I feel they're more robust, and the simpler
    shape crucibles last a little longer and break less easily. 

    The second thought is the temp you use. 1700 is not quite hot
    enough. You want a temp that's a significant interval higher than the
    actual stated melting temp of the metal. At least a hundred degrees,
    and for some alloys, 200 degrees higher than the melting point is
    better. Experience will be your guide here. Even if the top lip of
    the crucible weren't a bit cooler (and this will be the case with
    almost any type of crucible, even torch melting in a ceramic
    crucible), the metal is still cooling all through the pour and into
    the mold, so you have to start with a high enough temp so the metal
    stays liquid until it's fully entered the mold. I'd try 1800. If you
    get the same problem, raise the temp more. 1850 might be the best
    setting. And if you're casting thin or delicate items, you might need
    even more. This type of temp specification, by the way, is properly
    called a "superheat", and is considered an essential parameter of
    casting methods. You're not going to hurt the metal this way.
    Especially not an alloy like sterling, which doesn't have volatile or
    consumable deoxidizers added. 

    The bottom line is simply that your metal wasn't not enough. 

    Also, be aware that graphite crucibles with a furnace that covers
    the top of the crucible create a wonderfully reducing atmosphere for
    melting. There is little to any oxidation taking place, so
    accordingly, you need little if any flux. Just the tinyest bit. Any
    more, and you'll find flux inclusions on your casting or ingot
    surface, since unlike a clay crucible, the flux won't be sticking to
    the crucible and staying behind. At least, not as much. 

    Also, remember that graphite is a wonderful conductor of heat. When
    you stir the melt with a graphite rod, you chilled the heck out of
    the metal. You should not need to stir it like that. If you like, you
    can jiggle the crucible a bit to be sure it's fully liquid. The
    bottom of the crucible tends to be the hottest area, so if the top
    appears fully melted, and the temp indicator has been at the desired
    temp for a few minutes, the likelyhood that the bottom is not fully
    melted is slim. But again, waiting a little after it appears to melt
    helps this. Then don't stir the melt. Ordinary convection within the
    liquid metal should have nicely stirred the mix anyway for a uniform
    alloy. If you DO choose to stir the melt, assume you've just dropped
    the metal temp a hundred degrees or so in doing so. The indicated
    temp won't show this until the change has transferred to the
    crucible, and then changed the temp in the heating chamber. That
    time lag thing again, in reverse. So if you stir, let it sit again
    for a couple minutes to be sure it's back up to the desired temp
    before pouring. 

    Pouring these things also takes a bit of practice. The crucibles are
    "long", so it's all too easy to pour it so that the metal gets a good
    running start from the bottom of the crucible and comes out the top
    rather fast, causing you to miss your aim. Larger melts are easier to
    pour, since you're not tipping the crucible quite as far to get it
    started. 

    The other hint to offer is to note that lifespan of the crucibles is
    limited. Don't expect them to last forever. In our shop, we use these
    for melting all our white gold alloys for casting, which is several
    melts/pours per casting session, several times a week. Normally, a
    crucible lasts us four to six weeks this way. So order a spare to have
    on hand. The outsides of the crucibles especially, burn away. When it
    gets too too thin, change it out. this is normal for graphite
    crucibles. You can get a somewhat pricey spray paint to coat the
    crucibles with, which substantially slows the oxidation of the
    crucibles, extending their life. I've not used it, so I don't know if
    it's worth the cost. Only the outside is coated, so the crucibles
    still oxidize some from the inside anyway. (this is what protects the
    metal during melting, so it's not all a bad thing. It creates that
    wonderful reducing atmosphere in the metal's environment for a clean
    melt, and why you don't need much melting flux with these things. 

HTH
Peter Rowe
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