Re: Mixing Plaster of Paris (LONG)
E. Carroll Hale II (arthale@ACS.EKU.EDU)
Wed, 23 Oct 1996 19:27:55 -0400
Mr. Clayton G. Smith:
Mixing Plaster of Paris is simplicity itself.
Foremost - Always add the plaster to the water - not the reverse.
Use an amount of water slightly more than half the volume of mixed
plaster you desire.
Use a flexible, broad, container for mixing. The flexibility makes it
easy to clean (let the unused plaster set and crack it out). The
container needs to be broad to make sifting the plaster into the water
Sift the plaster (through your fingers - break up any lumps) into the
water. When the plaster is standing in little mounds on the surface of
the water, and the mix has doubled in volume, STOP.
Wait for the plaster to slake (2-4 minutes). You can tell it has slaked
by a slight color change - from a bluish white to a slightly yellowish
white - and a change in texture from sharp peaks to rounded mounds.
Briefly mix the plaster with your finger(s). If the plaster was fresh and
powdery in the bag, you're ready to pour. If your plaster was lumpy or
grainy, you may have to mix more vigorously.
The mix should have the consistency of very heavy cream. You can judge
this by the way it coats your finger. Plaster that has too much water
has little strength and takes a poor impression.
Pour a little of it into your form - just enough to cover the bottom about
one-eighth of an inch. Get down on your hands and knees and blow into
the form. This will force the plaster into the interstices of the
Pour in the rest of the plaster. Tapping on the side of the form
(gently) will level the plaster. You can insert a paper clip part way
into the plaster to serve as a hanger for wall displays.
When the plaster reaches a cheesy state, you can scratch whatever info
you like on the back.
Test for hardness: when pressing your thumbnail into the surface with
moderate pressure doesn't make an indentation, the plaster has set.
Take up the plaster and strip off the form. You may have to clean the
edges. Use a pan of water and a brush to clean of excess mud etc. . .
Be sure to clean off your hands before the plaster sets up on them.
Don't clean up in a sink. Plaster plugs pipes!
Do all of your plaster cleaning and scraping in a bucket, let the plaster
settle out, pour off the water and dispose of the plaster sludge in the
Plaster is alkaline - some few people get a form of dermatitis from it.
Barrier creams (vaseline) or rubber gloves solve the problem.
While it is true that plaster gives off heat (exotherm) while setting-up,
in small batches this is negligible. It barely gets warm. (Warmth is a
sign the plaster has set!)
Most Plaster of Paris (common moulding plaster) sets up in about thirty
minutes in an ambient temperature of 75 degrees f. Cold temps. retard
setting - hot temps. accelerate. Use cold water to extend setting time,
hot water to speed things up.
Plaster mix is usually pourable for the first 10 minutes, workable for
the next 10, and should not be disturbed for the final ten.
Tracks in clay or sand don't need a separator (Pam or green soap). The
clay releases naturally or is easily washed off and nothing is going to
keep sand from adhering to the plaster.
I usually use strips of flashing aluminum for my forms.
BTW, I cast a deer track three weeks ago with the temperature at about 38
f. It took two hours to set enough to be picked up!
Yours in Scouting,
Carroll Hale (MFA-Sculpture, Maryland Institute of Art, 1967)
SM T-118 Saint Mark RC Church Richmond, Ky.
Terry Howerton Sakima Group, Inc. SCOUTER Magazine Kansas City