HOW TO MAKE HOLLOW PRODUCTS BY
SLUSH CASTING WITH PLASTER
The slush cast technique for hollow cast
plaster lets you make lighter weight and hollow
A slush cast plaster figurine
The base of the slush cast plaster
figurine, showing the hollow interior
Slush casting with plaster is a technique you can use to
make large or small hollow products. Many types of
products such as piggy banks, lamp bases, large figurines,
and decorative columns and pedestals are made by slush
casting is used for two reasons. One is if you need a
hollow product - for instance, a piggy bank. Another is
if you are making a large product, and the product would
be too heavy if you cast it solid. An added benefit of
slush casting is that you use less plaster, which saves
money in materials.
basic technique for slush casting plaster is on page 70
of the Castcraft
Casting Guide. Pottery Plaster and Hydrocal
are the best plasters to use for slush casting.
Hydrostone will NOT give good results.
figurines found in craft stores and flea markets, and
most imported items, are made from pottery plaster.
Pottery plaster is the cheapest craft plaster you can
buy, and makes good quality, but somewhat soft castings.
Pottery plaster castings should be painted to seal the
plaster and protect against chipping and scratching.
Hydrocal makes a harder, more durable casting, and is
sometimes used to give an imitation ceramic look and
feel to products like decorative lamp bases. Hydrocal is
more expensive than pottery plaster, and may be more
difficult for you to obtain locally.
HOW TO SLUSH CAST
casting is usually done in a rubber mold. If you are
practicing the technique, you may want to get a small
latex rubber mold from a local craft store. The
technique is basically the same for a small casting or
a large casting, so you won't waste a lot of plaster
while you practice. Latex and Polyurethane molds are
best. Silicone rubber is usually too water-repellant,
and plaster is water based.
enough plaster to fill about 1/4 of the mold. The
plaster should be mixed fairly thick - almost like a
syrup. It should not be watery, or it will not stick
to the mold well.
a mold rinse in your rubber mold before you pour the
plaster. The purpose of a mold rinse is to "wet-out"
the mold so that plaster will flow over the mold
surface easily. If you don't use a mold rinse, your
finished casting may show a lot of tiny air pockets
and air bubbles. You can make a simple and effective
mold rinse from dish detergent. Just add three drops
of dish detergent such as "Dawn" to a cup of water and
stir gently so you don't create a lot of bubbles. Pour
the rinse into your mold, tip and turn the mold so
that the entire inner surface gets wet, and pour the
rinse back out. You can save the rinse and use it
again with another casting. Make your first slush cast
while the mold is still wet.
the thick plaster mix into your mold. Tip and turn the
mold so that plaster coats the entire inside. (You can
also shake, jiggle, and squeeze the mold if necessary
to make the plaster flow over the entire surface and
into all crevices). A coat of plaster will stick to
the mold walls. If you are casting more than one mold
at a time, you can pour the excess plaster into your
next mold, and repeat the process. If you are only
making one mold, pour off the excess as waste, or use
it to fill some other small mold. Keep track of how
much plaster and water you use, so that you don't use
any more than is actually needed to make the slush
the first coat of plaster set. When it sets, the
plaster will get firm and appear to start to dry out.
You can make your second slush cast as soon as the
first coat is firm - usually after 45 minutes to an
hour. The first coat of plaster does not need to set
a second batch of plaster. The second batch should be
slightly more watery than the first batch, because the
first coat of plaster will act like a sponge and draw
water out of the second slush cast. If the second mix
is thick to start with, when it loses water it will
get so thick that you won't be able to slush it around
the mold easily. Slush the second coat exactly like
the first. The second coat will build an obvious layer
on top of the first coat. Again, pour off the excess
plaster - whatever doesn't stick to the first coat.
the second coat get firm, and slush a third coat over
it, just like the second coat. This is all you need
for a small product such as a piggy bank - the wall of
the casting should be about 3/16 inch thick. If you
are making a larger product like a column or pedestal,
you might want to give one or two more slush coats,
just like the third. For these large products, you can
also add reinforcement such as chopped burlap to the
plaster or lay in pieces of burlap to the wet plaster
you wish, you can give your cast product a base, as
described in the Casting Guide on page 70. Basically,
for your last slush cast you use a little more plaster
than necessary to coat the walls of the mold, then
turn the mold over and put the base on a sheet of
rubber, and let the excess plaster run down and make a
solid base about 1/4 to 1/2 inch thick.
If you are making something like a piggy bank and want
to give people access to the hollow interior, you can
put a rubber stopper on the sheet of rubber, then turn
the mold over on top of the rubber stopper. When your
casting sets, you will have a hole in the base exactly
the size of the rubber stopper. Rubber and plastic
stoppers are available at many ceramic supply houses.
the casting set for a few hours, and remove it from
a PDF source list
Includes 26 suppliers of all mold making and
casting materials mentioned in ALL the demonstrations on
this web site, including silicone rubber, polyurethane
rubber, latex rubber, polyurethane plastic, epoxy resin,
polyester resin, molding clay, foam board, release
agents, pottery plaster, Hydrocal, metal powders, other
plastic resin fillers, and gelatin molding compound.
With company names, addresses, phone numbers, and web
sites. Downloads to your computer as a PDF file.