[Jewelry making - Article 00444] - Re: [Orchid] 2 Hours Casting

[Eddie Bell]

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>     Most casting protocols are cookbook-like and don't explain
>     what is going on. The objective of burnout is simply to get rid of
>     the wax in the pores of the investment to make it porous and
>     strengthen the mold so that it is more ceramic like than simply
>     hard plaster. As you pointed out, when the pouring cup is chalky
>     white instead of grey, all of the wax is gone. We can get to about
>     1350 F as fast as possible if the oven is already at that
>     temperature instead of starting out at room temperature or 300 F as
>     often suggested. The free water in the pores of the investment will
>     keep the mold temperature from rising above 212 F (the boiling
>     point) until all of the water has turned to steam. Then the
>     temperature rises rapidly to almost the oven temperature to
>     complete the burnout. I have placed a thermocouple in the center of
>     a 2 x 2-1/2 inch flask and showed that the core temperature stayed
>     at 212 F for about 20 minutes and then the temperature rose to
>     almost 1350 F in about another 40 minutes. 

    I read the post from Fred Sias with a lot of interest and I agree
    completely with the main theme of his teaching. I would like to
    respectfully add a few little details and some context for people who
    might be casting larger flasks. Fred and I had the same teacher if he
    is talking about the late Dr. Carl Schwartz. It is true that keeping
    the outer surface of the investment from drying before the interior
    will help prevent cracks when the investment is heated rapidly.
    However, you need to keep in mind that as the diameter of the flask
    is increased the speed that it can be heated slows down
    exponentially. In part this is because the investment is a good
    insulator and like all insulators the thicker it is the better it
    works. 

    The Santa Fe Symposium has published many papers on investment and
    burnout over the last 20 years starting with one by Schwartz and
    followed by Ott, Schneller, Ingo, McKeer, McClosky Carter, Horton,
    Loewen and me (These come to mind, I hope I didn't leave anyone out).
    The heating rate of various diameter flasks has been measured and the
    results published in SFS '88 and '89 and they show that the first
    inch of investment thickness next to the flask wall can be heated
    fairly quickly when there is a large temperature differential between
    the oven air and the flask as Fred points out. This is proven daily
    by the hundreds who cast small 2-inch diameter flasks. By the time
    the flask diameter reaches 4 inches (100mm) it takes a minimum of 3
    hours to get the center of the flask dry. If you hope to cast into an
    investment mold that is not spalled or cracked plan on about 5 hours.
    On first heating, the watered investment is quite conductive, but as
    the water is driven off it becomes isolative. Once the first inch of
    investment is dry it insulates that which is further inside.
    Interestingly, a four inch flask reaches temperature equilibrium (the
    same temperature from the flask wall to the center) in about the same
    number of hours wither it is heated as fast as possible, or with a
    reasonably slow ramp rate. The bottom line is, it you cast larger
    diameter flasks, don't try to burnout in two hours, you need between
    12 and 16 depending on the equipment and the load in it. 

    Set investment when mixed with water at a ratio of 40:100 is about
    50% water by volume because the investment powder has a higher
    density than and the binder already contains about 6% water. When the
    water is driven off the space it took is empty and the investment is
    quite permeable or porous. This is also why it becomes such a good
    insulator. 

    The reason the wax must be completely removed is the residues of wax
    is carbon and if carbon is present in the investment when the metal
    is cast a chemical reaction rapidly decomposes the gypsum used to
    bond the investment. The result of the decomposition is the release
    of gases that are detrimental to casting and the general health of
    your metal. 

    When investment first sets it is quite fragile. Slowly the dissolved
    plaster precipitates and gypsum crystals grow. Over a two hour span
    what is called green strength increases until it is safe to handle
    the flasks to remove the sprue base and start the burnout cycle. The
    green strength will go up to 350-400psi compressive strength. During
    the burnout most of the strength is lost and the typical invested
    mold is only about 20% as strong when it is cast as it was when the
    burnout started. The biggest factor causing this loss of strength is
    the loss of the water. Then depending on how high you take your peak
    burnout temperature, some of the binder is also lost starting at
    about 1200 F. Still if everything is done correctly, the investment
    is plenty strong to get good smooth as-cast surfaces and reproduction
    of detail. Investment manufactures could make the investment stronger
    but the tradeoff is, stronger investment is more difficult to remove.

Eddie Bell

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