John Rodgers on tue 11 feb 03
Denise, assuming there are no undercuts, a plaster on plaster casting
can be made to separate.
1) Tools - sponge, china bristle brush ( big price if bought in an art
store under the name china bristle brush - $1.98 if bought in a general
hardware store - it's a throw away brush - and the bristles are pure
hog), Rubber mallet, wooden block, short & stiff blade knife for
trimming. Hardwood wedges.
Here is a little aside regards china bristle, used in art painting, and
in mold making. I bought a very expensive French china bristle brush
once. $45 or $50 as I recall. It performed beautifully. My application
for it kept it wet with water and not long after it's first use I began
to smell something very bad. Time passed and it grew worse. I grew up in
farm country, and the smell was familiar - prime pig parlor. I keep
smelling this odor and finally I picked up my expensive French China
Bristle brush and sniffed!!! I gagged!! It was pure hog bristle -
unprocessed. So much for expensive French China Bristle brushes.!!!;)
2) Materials - a mold release (pure-lube, Murphy's Oil Soap, English
Mold Soap, Tincture of Green Soap -I prefer Tincture of Green Soap -
available by the pint or liter in most drug stores.), some 600 grit
wet/dry sandpaper to smooth rough spots. Methanol Alcohol in a pray bottle.
3) Process_
a) Follow instructions on all mold release agents. For use of Tincture
of Green Soap - available at any drug store - mix the soap 50-50 with water.
b) Sponge the soap on the surface of the model, working up a lather.
Allow to sit until nearly dry, then brush dry with the hog bristle
brush. The brush will pick up mostly water and leave soap. Repeat this
process three times. After the third application, drip water droplets on
the prepared surface. The water should bead up as if the surface was
waxed. If not, repeat the process until water readily beads.
The process of prepping the surface is called seizing the surface. The
stearate in the soap combines chemically with the calcium in the plaster
to form a thin layer of Calcium Stearate, which is waterproof.
Subsequent applications of soap to the surface will build up a
separation layer which under the heat of hydration of the plaster cast,
and the added moisture from the newly cast plaster, will soften and
allow ease of separation.
c) Make your plaster cast. For best results, use only #1 pottery
plaster. Density is important, but you will need to contact United
States Gypsum Corporation in Chicago and get the specs on their casting
plasters. Most casting for ceramics requires a density of 72 in the USG
#1 Pottery Plaster. For small casts a ratio of 1.5 lbs of plaster to 1
lb of water works. For large casts, the plaster calculator that is
available from USG is needed.Sprinkle the plaster into the water,allow
it to sit for a few minutes undisturbed, the mix thoroughly. I use a 20
inch stainless mixer in an electric drill. Bump the bucket of mixed
plaster on the floor and bubbles will rise to the surface the way they
do in a cake mix. After the bubbles rise, spritz the surface of the
plaster mix with the alcohol to break the bubbles. Gently stir
periodically and test for a strike. A strike is a streak made in the
surface of the plaster which does not fill rapidly. When there is a
strike, quickly spritz the surface of the prepared model, and make your
cast. Pour the plaster in the deepest section first, allowing the
plaster to rise and flow across the surface of the model. The alcohol
spritz witll help break any bubbles that may try to form. DO NOT over do
it on the spritz!!!!
d) Removing the cast - a heat producing chemical reaction will occur
within the cast plaster. It is know as the "Heat of Hydration" . Water
is being locked into the crystaline structure of the plaster and heat is
released. (1) When peak heat has been reached - you can touch it with
the hand and feel the heat - you may try to simply lift the cast from
the model. At this point there will be quite a bit of water on the
parting line surface between the model and the cast. The water and the
heat will have softened the soap and separation should be fairly easy.
(2)If the cast is stuck, try gently bunping the cast along the thicker
portions to see if the piece will release. (3)If this does not work,
immerse the whole thing in a bucket of hot water. The uneven densities
present between the dry model and the wet cast will result in expansions
that will cause them to separate. (4) If all else fails, as a last
resort, gently drive the hardwood wedges in at the separation line. It
should separate. (5) as an absolute last resort, carefully chip the cast
off the model and start over.
4) Cleanup - use the trimming knife to trim the sharpe edges off the
casting or mold. When trimming cut the plaster edge at a 45 degree bevel
and make it approximately 1/8" wide. That way it won't be as likely to chip.
If you are making molds for slip casting the molds parts must be "keyed"
to lock together and that is an additional process. Refer to Robert
Friths book titled "Mold Making for Ceramics", the definitive moldmaking
book available from Axner and other ceramic supply houses.
I you have anymore questions, feel free to ask.
Good Luck,
John Rodgers
Birmingham, AL
Recouping from the flu
Denise Johnson wrote:
> This may be a silly question but, can a plaster mold be used to cast
> plaster? My husband, a builder, says nope, it'll stick. I, on the other
> hand, am not so sure. Clay shrinks when cast which helps the removal,
> plaster doesn't so I assume I'll need some type of release.
>
>
>
>
>
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