Here is a reprint from Marc Robinson's page at
knowing Marc, I'm sure he
Hot water investing...
Almost 20 years ago, I was doing some consulting work for a
large casting operation in Albuququerque, N.M. When I blew onto
their scene, they had a huge table of workers armed with needle
nosed pliers, diligently pulling nodules off trees that had
been caused by air bubbles in the investment. This was an actual
process within the scheme of their casting theater, created to
eliminate a problem that is normally dealt with during the
investing procedure by vacuuming.
Why so many bubbles? And why did the vacuum process fail to
The answer hinged on the company's location. All casters in high
altitudes have problems with their vacuum machines - the higher
above sea level you are, the less inches of vacuum you will be
able to draw . In Albuququerque, which is just over a mile high,
you ,could only draw , say, 24 to 25 inches maximum. This is in
comparison to a full 29.9 inches at sea level. As a result this
company 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 there was a way to lower the 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 re-adjusted. 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
With all this in mind, here's the formula I devised for
companies suffering from the dreaded "altitude sickness."
1. For two perforated 4-inch flasks, 3 1/4-inches in diameter, I
mix 1020 dwt. of powder to 700ml. of water at 110o F. This is
then mixed at a quick speed for a full 6 minutes.
2. The bowl is then placed under the vacuum for the first boil.
Depending on the strength of the vacuum, 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 them again, this time allowing a full one
3. 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
much 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
The water temperature of 110o 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 Albuquqerque, I had to change my name and appearance. It
seems there were hundreds of workers in shops 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
Thanks to Marc Robinson