Curing Altitude Sickness
HOT WATER INVESTING RECIPE
By Marc Robinson
In Albuquerque, which is nearly a mile high, you could only draw 24 to 25 inches
maximum. This is in comparison to a full 29.9 inches at sea level. As a
result, this company simply could not draw enough vacuum to effectively
deal with all the bubbles clinging to the waxes inside the flasks.
This is strictly a matter of physics, and everybody knows you cannot fool with
Mother Nature ... or can you?
I reasoned that if there was a way to lower the vacuum boiling point so that
the mix would boil fully a lot sooner, we could lick the problem of vacuum
vs. altitude. And the way to do this was to raise the temperature of the water
in the mix.
So their mix had to be readjusted. What had to be done was raise the water
temperature, lengthen the mixing time, and generally thin out the investment
... relying on the fact that when you up the water temperature, the investment
sets up very, very fast. Much too fast to rely on any standard room-temperature
formula.
In my book, the faster the investment glazes over and sets up, the stronger
the plaster after burnout, and the less likely you will lose valuable detail
from water separation along the wax pieces themselves.
With all this in mind, here's the formula I devised for companies suffering
from the dreaded "altitude sickness."
For two perforated 4-inch flasks, 3.25-inches in diameter, I mix 1,020 cwt.
of powder to 700 ml. of water at 110 F.
This is then mixed at a quick speed for a full six minutes.
The bowl is then placed under vacuum for the first boil. Depending on the strength
of the vacuum machine, it can take between 30 seconds and one minute for the
mix to initially rise up and boil. I time 15 seconds of boil after the boil
commences. I then pour the flasks and vacuum again, this time allowing a full
one minute boil.
Now I pull the bell jar, top off the flasks, and set them aside to time the
glaze-over. The set-up time should average one-and-one-half minutes to three
minutes, total. This is very fast, and ensures detail and strength by preventing
the water and plaster from separating.
An advantage to the mix pouring thin is that the investment naturally flows
into detail and crevasses more completely than a thicker investment, and is
capable of mirroring much finer detail. This is coupled with the fast set-
up time, which, by reducing water separation, likewise helps maintain detail
and toughness. Also, when you use hot water, the mix will boil violently-so
much so that the bell jar will be totally covered with plaster. This "super-boil" not
only rids you of bubbles, but also serves to keep mixing the investment during
the actual vacuuming process.
The water temperature of 11 0 F at 5,000 feet is admittedly extreme. You may
only have to go up a few degrees for your area's altitude. But these principles
of hot water mixes even hold true in my California shop, which
is near sea level. Of course, if you want to experiment with this process,
I recommend practice on an empty flask, investment is cheap compared to wax
labor.
That, in effect, is how I fiddled with Mother Nature (sorry, Mom). By raising
the temperature of your water, you can induce the investment to boil sooner.
But Mom got me back ... after I left Albuquerque, I had to change my name and
appearance. It seems there were hundreds of workers in shops there who actually
made their living by pulling bubbles off castings!
Now they were all after my hide ... which brings a new meaning to the term "contract
casting."
