[Jewelry making - Article 00014] - Re: [Orchid] Button or No Button

[James Binnion]

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Hi Andrew,

>         I don't see how oxygen absorbed at the top of a button after
>     casting will affect a part  way down inside the flask, particularly
>     if, as you say, the metal has solidified by then. Flux helps
>     protect the metal when it's being melted, but in this case, what's
>     probably happening is an insulation effect from the added material
>     piled on top. I usually cover castings with a piece of soft
>     firebrick immediately after casting; this helps keep the button
>     fluid and enhances its usefulness as a reservoir of semi-liquid
>     metal to feed the part. 

    Because oxygen atoms are much smaller than metal atoms they are very
    mobile in the metal crystal lattice at high temperatures. Silver
    especially has an affinity for oxygen at elevated temperatures and
    can hold great volumes of it at elevated temperatures, this is the
    mecain.sim that creates firestain in sterling, the deep oxidation of
    copper by the traveling oxygen atoms. The glass of the flux keeps
    the oxygen out, not completely as it can still enter through the the
    investment but at a much lower rate. I have done experiments with
    this and can go into detail off line if you would like. 

    	As for the insulation  if we are talking about jewelry sized items
    here and not pours of pounds of metal then the models are solid
    before you have even finished pouring the mold and trying to keep
    the button /pouring cup molten via insulating material is not buying
    you anything. 


>>         Doesn't matter if there is a tree or not, most jewelry sized
>>         models are solid before the button starts to fill

>         "Solid" is a relative term, then. 

    No solid is definitely not a relative term it is a precisely defined
    temperature at which the metal undergoes a phase change from solid
    to liquid phase.

>     The metal inside the flask, which is typically cast hot, is in the
>     process of solidification, but it's still permeated with channels
>     of liquid metal, like a wet sponge. This will either draw metal
>     from the button or sprue as it shrinks, or it will draw it from the
>     part. I've seen it happen too many times to swallow this "no
>     button" theory whole. 

    You don't have to swallow it just prepare several identical flasks 
    ( there are so many variables in casting as you no doubt know that
    basing statements of fact on one or two casts is not advisable) and
    calculate your metal weights so that half are poured without the
    button and half are cast with a button. statistically you will see
    no difference between the two sets. Major manufacturers like Rio and
    Stuller  are not using buttons in their castings now because they
    just don't need them. It is not hocus pocus or something I just made
    up to create controversy it as I said something you can easily test
    in your own shop. 

>         I've done sculpture casting as well as jewelry, and I have to
>     say that I'm unimpressed by this analysis, despite the hi-tech
>     trappings. When "most" of the metal is solid is when shrinkage
>     porosity does its damage, pulling metal from the surface as
>     capillary channels collapse. The effects are more easily
>     appreciated in larger-scale castings, but once one learns to
>     recognize it, it can easily be seen in smaller ones as well. I've
>     also had the misfortune to have castings disrupted by mishaps in a
>     centrifugal casting machine, and have seen that after quite a few
>     milliseconds had gone by, the castings were still fluid enough to
>     be partially emptied after they were full. When I was using an
>     overloaded centrifuge that slowed down after its first few spins,
>     I've had the button drip out and empty itself, with predictable bad
>     results to the supposedly solidified castings that remained. 

    Yes in large scale castings the process of solidification is
    different in that is is slower. It is very much a matter of scale.
    if you are talking about belt buckles or sculpture rather than rings
    and earrings it is all about heat transfer, how large a mass of
    molten metal is in the mold cavity gate and sprue. I am not trying
    to make any assertions about larger castings as they have different
    dynamics but I will stand by my statements as far as jewelry ( ie
    rings, earrings small cross section items ) casting goes. 

>>      One of the things that is starting to be obvious in the casting
>>      research being done on jewelry casting is that the reduction of
>>      turbulence is very important to good castings (maybe the most
>>      important factor?) . This is backed up by the simulation that was
>>      presented in that if the metal is slowed down by turbulent flow it
>>      will not be able to feed the model cavity enough metal in a timely
>>      manner to produce a sound dense casting due to this rapid
>>      solidification.

>         I do agree that a reduction in turbulence can help, but I'll
>     still be casting with enough reserve metal to accomodate shrinkage.
>     Shrinkage is not a myth, it's generally accepted as fact in the
>     casting trade, and I'm not the only one who believes it - see
>     http://kerrlab.com/JewelryProducts/technical/Glossary.htm or
>     http://www.sternleach.com/technical/articles/silvcast.php under
>     "shrinkage porosity". 

    Shrinkage is a fact of life in castings no mater what scale. Metal
    volume changes significantly during cooling from the molten state.
    Best practice is to provide sprues and gates that will allow for
    rapid filling of the mold cavity so that a maximum amount of metal
    will be present when the casting solidifies. The point that I am
    trying to make is that trying to feed a casting that has already
    solidified with a button is a waste of time and money. And the
    research and practice  is showing that a button in this scale
    casting is not required. 

    Papers about this are available in the published proceedings of the
    Santa Fe Symposium, for the most part this work is done by
    metallurgists working in the jewelry industry. If you are interested
    I can provide titles of some of the relevant papers for you to read. 

Regards,

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
http://www.mokume-gane.com
jbin AT mokume-gane.com
Member of the Better Business Bureau

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