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Plaster Moldmaking
Richard Burkett
DO THESE STEPS BEFORE MIXING PLASTER!
PREPARE THE MODEL:
1. Determine how many parts you will need for your mold. Look for
under-cuts and problem areas where the mold might not release from the
model. Draw mold separation lines on model.
2. If a one part mold, secure model (with clay, rubber cement, etc.)
to a smooth surface (Formica works well)
If a multi-part mold, embed model in clay up to the separation line
you have drawn, leaving exposed only the first part of the model to be
cast. Add some type of rounded mold alignment bumps (natches) to clay.
3. Surround the model with one inch thick clay slab walls, or a wooden
or metal box if the mold will be larger than 6 to 8 inches in any
dimension. Back up clay walls with old, discarded bricks. Allow one
inch clearance from the model at the sides and top for proper plaster
thickness.
4. Seal all corners with a small coil of clay.
5. Apply mold release to all non-clay surfaces. Avoid overly thick
application/puddles. See page 2 for choices of mold release.
6. Clear tools and other valuables from the area around the mold.
Occasionally accidents happen and plaster is hard to clean off.
7. Be sure that you have the plaster wash water bucket ready to wash
hands, buckets and tools before the plaster sets.
PLASTER MIXING:
Do not mix huge amounts of plaster. One-half of a 5 gallon bucket of
plaster will fill a fairly large mold. Mix just slightly more than you
will need. Use a consistency of 70, in other words, use 70 parts of
water to 100 parts of plaster by weight. Plaster should be fresh, no
more than 3 months old (packages are dated) and not lumpy. Sift plaster
into measured water. Let slake for 2 minutes. Stir, without adding lots
of air bubbles, until the plaster thickens just enough to leave a trail
when a finger is drawn across the surface. POUR AT ONCE. Pour the
plaster through the slightly open fingers of your hand to break the flow
and spread it evenly over the mold. Fill the mold only to ONE INCH over
the top of the object your are using. Tap or bump the table lightly to
dislodge air bubble in the mold. Avoid heavy vibration that might
dislodge the model or collapse your mold walls.
***CLEAN UP TOOLS AND BUCKETS IMMEDIATELY***
Do not use the sink. Use the wash water bucket you have ready. Clean up all spilled plaster as
soon as possible. Be neat.
AFTER POURING PLASTER:
1. Wait until the plaster has set. It will
become warm to the touch and be hard enough that it is difficult to
push a fingernail into the surface.
2. After the plaster is sufficiently set, carefully remove the mold
forms. Be gentle, as the plaster is still not very strong.
3. While the plaster is still fairly soft, round off the outside edges
of the mold with a fettling knife or Surform rasp. Do not round the
inside (working mold) edges of a multi-part mold. Leave no corners that
might chip off later.
4. Multi-part Molds: repeat all steps for each of the remaining
section of the mold, using the already cast sections as part of the
forms. Seal the mold boards (sides) carefully to avoid plaster leaks.
Separate parts carefully to avoid breakage.
5. After mold is complete, gently wash, with soap if necessary, to
remove all mold release. Allow mold to dry a few days before using.
MOLD RELEASE for plasterUse one of the following:
Mold soap - the best choice, harder to find. Apply two or three coats
with a damp sponge and buff when dry.
Green soap - add a few drops of rubbing alcohol to minimize foaming,
spray or brush onto mold. Avoid puddles of soap.Soft Soap - works like mold soap.
P.A.M./spray cooking oil - APPLY THINLY and wipe excess off model. Good
all around mold release-can clog molds, especially if applied to dry
plaster. This is often the favorite quick-and-dirty mold release for ceramists, but not
by any means the best.
Vaseline - apply fairly thinly, good for human body part molds. Be sure
to apply sufficient amount of Vaseline to hair covered areas to assure
release of plaster, almost sculpting longer hair such as eyebrows,
moustaches or other hair from the Vaseline. Vaseline is hard to remove
from the finished plaster mold.
NOTE: After plaster has sufficiently set wash the mold gently, with soap
if necessary, to remove any oily residues. PAM may cause black mold to
grow on the mold if not removed. Oil from P.A.M. or Vaseline left in the plaster
mold surface can drastically reduce the functionality of the mold for slip casting!
CLEAN UP ALL PLASTER TRASH !!
THROW ALL CLAY USED IN CONNECTION WITH MOLD MAKING IN THE TRASH.
READ CAREFULLY:
If even tiny chunks of plaster are allowed to get into the clay in the
studio, YOU WILL LOSE ALL THE FRIENDS YOU HAVE IN CERAMICS.
Plaster in clay causes "lime pops" - small chunks of bisqueware that
suddenly fall out of the side of your work after firing, leaving a
nasty, jagged, ugly hole.
Other Cautions:
Plaster heats up as it sets. Thick sections of setting plaster can get
hot enough to cause severe burns if you are making casts of body parts
from a live model. Please be careful. Keep plaster thickness under one
inch. Remove plaster from the model/victim as soon as it begins to get
warm. Be sure to use adequate mold release.
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