[Jewelry making - Article 00509] - Re: [Orchid] Bronze casting porosity

[Andrew Werby]

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>         I am having a real issue with porosity. I am casting in
>     bronze. Here is my process. I use a vacuum casting machine to
>     de-bubble my flasks and for the casting as well. I use a electro
>     melt furnace to melt my metal. 

	Burnout cycle:
	2 hours at 300
	2 hours at 700
	3 hours at 1350
	1 hour at casting temp( I have varied this from
	700-800-900 degrees)

    It seems like that final burnout temperature is a tad high. (This is
    Farenheit, right?) I've never needed to go above 1200F. When I've
    inadvertantly let the temperature get to 1350 or so, I've noticed
    some investment breakdown. You might also try a cooler flask
    temperature. If the mold is relatively cool, the metal will chill
    against the surface instead of staying liquid.

    melting temp of the metal is 950 degrees, I am casting it at just
    under 1000 C. I am using 100% new metal. 

    What kind of bronze are you using? Most people around here use
    silicon bronze (hercaloy or everdur) because it melts cleanly without
    flux and welds well too.

>         I usually give it about 15 minutes and I quench the flask in
>     water. 

    That might be a little quick- try giving them 1/2 an hour.

    Several of the parts I cast look like the surface has a type of
    crystalisation. That is when I look at the surface under a magifying
    glass it appears like many lines that have spaces between them
    overlapping each other. Kind of like fiberglass. I hope I am clear
    what I am trying to explain. And the porosity is strongest in the
    center of the object. I have a link to a couple photos below so you
    can see what I am trying to describe: 

    http://www.geocities.com/vovinit/porosity.htm 

    These pics are worst case scenarios. But I still get it to some
    degree in all my casts. But not alway's this bad. 

    Sorry for the poor pics but I can only get so close to the object
    with my digital camera before it losses focus. 

    Most porosity is due to inadequate gating: when there isn't enough
    liquid metal to feed the piece as it shrinks, then the piece will
    draw metal from its own semisolid areas toward the surface, resulting
    in a shriveled surface with sub-surface porosity. From the look of
    the pieces you show, this seems to be at least part of the problem.
    Is that long skinny gate part of the piece? If it's necessary, point
    that toward the top of the mold, and use a shorter and thicker gate
    from the button to the thickest part of the piece. And just after
    casting, put a piece of soft firebrick on top of the mold to keep the
    heat in the button, which encourages that reservoir of melted metal
    to stay liquid longer.

Andrew Werby
www.unitedartworks.com


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