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Basic Slip Casting
Richard Burkett - SDSU 1993
1. Make sure that new plaster molds have dried sufficiently. It
will take 3-4 days at the least for them to be dry enough to use,
unless you leave them in the sun all day, or dry the molds in a
warm place. Be careful with forced drying, as plaster will start
to deteriorate above 130° F or so. Make sure the mold is clean and
free of all oil, mold release, or grease.
2. If the mold is completely dry (no dampness whatsoever) it
should be slightly dampened before slip casting. Do this by
running it quickly under water (or dip it in a bucket of clean
water) about 5-10 minutes before you want to pour the mold. If the
plaster is not slightly damp, the slip will cast with varying
density (harder on the outside where the dry plaster quickly draws
the water out/softer toward the inside) and cracking will be more
likely. Air bubbles and porous casts are another possibility when
casting in a dry mold. A damp mold will also make dusting more
effective (see step 4).
3. Dust the mold if necessary. Talc is fine for lowfire casting
slip. Silica is better for porcelain casting slip, as the talc
will melt on the surface during firing. Use a pounce made from
thin cloth to lightly dust the interior of the mold. Too much dust
and the fine details in the mold will be lost, and the casting may
come loose prematurely and collapse. This dusting allows the
casting to release from the mold earlier and more easily than it
would otherwise and is not always necessary with simple molds.
NOTE: Wear a dust mask and use the glaze spray booth for proper
ventilation when using Talc or Silica dust to avoid dust
inhalation.
4. Fasten the mold together tightly with large rubber bands,
pieces of rubber innertube, or heavy string tied, twisted, and
wedged snugly about the mold. Be sure the mold is quite securely
fastened, as the slip pressure inside even moderately-sized molds
is surprisingly great. With very large molds, be sure you will be
able to drain them when filled and heavy with slip.
5. Make sure that the casting slip is well blunged (mixed),
free of lumps, and of the proper consistency.
6. Prop the mold upright with a little clay if necessary, and
have a bit of clay ready to patch leaks. Pour the casting slip
into the mold quickly, filling the mold to the very top. As the
slip level drops in the mold, keep refilling the mold with casting
slip. If the mold has a short sprue, fixing a large funnel into
the sprue with a little clay may cut down on number of times you
have to refill the sprue with slip. Do this for 5-15 minutes. The
actual time will depend on the mold, slip thickness, and how thick
of a casting is desired. Check the thickness by pouring a small
amount of slip out of the mold and cutting through the casting
with a needle tool.
7. Drain the excess casting slip from the mold. You may have to
use a knife to open the sprue if it has cast shut. Do not to cut
into the plaster! After most of the slip has drained out of the
mold, prop the mold so that the sprue is down, and all remaining
liquid casting slip continues to drip out. A good method is to
place two sticks across the top of a bucket and prop the mold on
the sticks to that it drains into the bucket. Allow the mold to
drain upside down like this for at least 15-20 minutes. Avoid
shaking the mold! This may loosen the casting from the plaster
and cause it to collapse.
8. Allow the mold to dry until it can be disassembled without
tearing or collapsing the casting. This drying time will depend on
the wetness of the mold and the thickness of the casting. Typical
time from pouring to removal from mold is 1-4 hours, occasionally
it takes overnight.
9. Clean up!
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