Bob Santerre on tue 10 apr 07
I think this might be in the archives somewhere, but at the risk of
being redundant I'll say it anyway.
U S Gypsum makes a very handy slide rule-like gauge that is used to
calculate pounds of plaster needed for a given pounds of water. It's
actually a simple calculation, but their slide ruler guage makes it
super simple. They base their calculations on a factor they call
"consistency" which I suppose describes the ideal or peak hardness or
strength of a particular plaster. They make many kinds of plastetrs
and the consistency factor for #1 pottery plaster is 70. You determine
the amount of water (in pounds) you want, line that number up with the
consistency factor 70 and read off the pounds of plaster required. Very
simple.
There are more complicated calculations you can do with it as well. You
can start with the shape you want (cylinder, rectangle, sphere),
calculate the volume of mixed plaster, convert that to pounds of plaster
at a given consistency and finally calculate the pounds of water needed
for that amount of plaster at the given consistency. The little gauge
does everything but mix it for you. They used to give this slide rule
guage away free. Phone number on my gauge (now about 10-12 years old)
is (800) 487-4431. They call it a Volume and Mix Guide.
For example, at consistency 70, 3.5 pounds (1.7 quarts) of water
requires 5 pounds (2268 grams) of plaster. Final volume of mixed
plaster is about 3 quarts. If you pour the plaster slowly into the
water, it will do exactly what Marcia Selsor said, form a tiny conical
mound sticking above the water surface in the middle of the container.
I mix vigorously with a hand drill-attached mixer for maybe 15 to 30
seconds, until very smooth. Bang the container sharply on the table or
floor several times to get the bubbles to the top and let them burst.
With cool water from the tap (and a fresh bag of plaster) I get about
8-10 minutes work time before it sets up too stiff to pour.
Hope this helps.
Bob
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