Fred Paget on sun 30 jun 96
I have been experimenting with an old technique to make plaster slump and
hump molds for plates and shallow bowls.
If a liquid in a pan is rotated around its vertical axis at constant
speed the surface will assume a parabolic surface. This is due to the
interaction of the gravitational and centripetal forces on the liquid. The
faster you rotate the deeper the curve.
Now if the liquid is newly mixed plaster, when it sets up you have a slump
mold of plaster. Then it can be used as a master to make humps. It turns
out that the rotational speed needed is from about 25 to 100 rpm which is
just fine for an electric potter's wheel.
Here is how it works: you take your wheel outdoors in an area where you
can hose down plaster spills and place a deep pail on the wheel head .I
found that a 10 gallon plastic tub worked well. It has a diameter of 17
inches at the bottom and the splash pan of the wheel is only slightly
larger and acts to keep it on the wheel when it has plaster in it.
Then you mix up enough plaster to fill the tub a couple of inches deep. The
plaster should be thoroughly mixed with a drill and quickly poured into the
tub which is rotating slowly. The plaster wants to be liquid but not so
watery that water comes to the surface while curing. If it does you get a
rather rough rippled surface on the cured plaster. I found I could get the
best surface by using Ultracal 30. It makes a good master for making humps
from the concave slump surface. The wheel speed is carefully increased
until the plaster at the edge of the tub is at the desired height. Then
just leave it running until the plaster sets up.
Air bubbles are a problem and can be popped by flaming the surface with a
propane soldering torch while the plaster is liquid. My plaster mixer for
the drill is a 2 inch diameter disc of neoprene rubber mounted through its
center on the end of a quarter inch shaft about 18 inches long. The rubber
disc mixes plaster without beating in air if you are careful.
The first time I tried this I used a 5 gallon paint bucket that was old
and brittle and it worked fine except I had too much water in the plaster
and got a rough surface. The brittle bucket was broken to get the mold out.
Then I got big ideas and tried to use a garbage can lid fixed to the wheel
with clay. The plaster flew over the edge of the rim and was all over the
place. Lucky I was outdoors in my "potio" and could hose down. After that I
was more careful to have deep containers.
Fred Paget---Mill Valley,CA,USA
Nicole Lallande MIPANA INT'L on mon 1 jul 96
Fred:
Great info - but a question. If you use a 10 gallon paint can, do you oil
the inside or does the mold pop out easily?
Nicole
mipana@mipana.com
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