[Jewelry making - Article 01063] - Re: [Orchid] Crystal Size in Metal

[Peter W . Rowe]

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>>             Does each alloy have a specific melting temperature versus
>>     flask temperature to asuure the "best" casting? 

>        Yes, the super heat (temperature above the liquidus the molten
>     metal is raised to) and flask temperature needed will vary
>     depending on the cross section of the items being cast and the
>     fluidity of the alloy at a given temperature. 

    There are a number of other factors as well. The degree/speed at
    which a given metal radiates it's heat away, which is in part a
    factor just of it's melting point, but also, I think, of the metal's
    density, plus the insulating ability of the casting investement
    used, both also affect the speed at which a given molten metal will
    solidify in the mold.  And the fluidity of the metal being cast will
    affect the speed at which it can fill a mold during casting.  The
    ultimate goal is to have the metal fully fill the mold cavity at the
    lowest possible temperature (both mold and metal), in order to
    acheive the lowest amount of porosity and casting flaws, as well as
    the densest casting with the smallest and most uniform crystal
    structure.  But I really don't think there are any one set of best
    temps and settings. Too many variables.  Things like the exact
    details of which type of investment you use, and how you mix it
    (changes it's density/porosity/gas permiability, and thus the speed
    at which air is removed in front of flowing metal, as well as the
    insulating properties of the investment, which changes the rate at
    which the metal will solidify and cool), the exact method you're
    using to melt and pour the metal, the exact temps and times of your
    burnouts and the exact nature of the atmosphere in your burnout
    furnace, and the number and nature of your waxes, as well as how
    they're sprued:  all of these may affect the 'ideal' temperatures at
    which to cast a given alloy. You can come up with general
    recommendations, and these can be good starting points. 

    But the simple truth i've found is that ideals and recommended temps
    and times and all that, are very much a "your mileage may vary" sort
    of thing.  Each caster has to play with those starting points and
    recommendations to fine tune their own process.  Keep good notes on
    what you did, what you did differently, and how it affects the
    results, and you'll be able to zero in on your own best settings for
    given types of castings.  What seems to work best for you may not do
    so for someone else, nor may their own seeingly ideal settings
    always lead to success when you duplicate them. 

Peter

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