boards to work on
(Formica surface works best)
freezer paper
cheesecloth
masking tape
rubbing alcohol
mineral oil
petroleum jelly
brushes
spatulas
mixing bowls
push pins
tweezers
gram scale (manual or digital)
lock mold
silicone rubber
Step One:
Cutting up the original clay
Each piece will be completely
different. The mold-maker must decide where and what to cut based on
whether a one or two-sided mold is being made. Also, anything with a severe undercut
must be cut off and molded separately so
that the mold rubber can't get caught and rip or damage the wax when
releasing from the mold. Planning ahead for this stage when creating
the armature helps tremendously. One can thereby avoid having to cut
through wire and other heavy materials at this stage, which can
damage the clay. Many different tools can be used to cut the clay.
Depending on the size of the piece, anything from an Exacto knife
and wire cutters to a Sawzall and electric cutoff wheel may be used.
Here you can see the original clay after it has been cut into the
pieces necessary for the next step of the process. These nine pieces
will be made into four double-sided molds and a single one-sided
mold.
We'll start the easiest and simplest
mold of all, the one-sided mold.
This is also the quickest mold to make because it only has rubber on
one side. This is due to the other side being flat to the board.
First, a clay wall is built around the piece to be molded.
Once the wall has been built and sealed
tightly to the board, lubrication is applied to any of the board that is exposed
inside the wall. This prevents the silicone rubber from sticking to
the board. There are many lubricants that can be used, such as
premade mold lubricants in an aerosol can for spraying, but I prefer
to make my own. I find a mixture of 75% petroleum jelly and 25%
mineral oil works well for the purpose.
The instructions for mixing must be followed from the manufacturer
of the silicone rubber. This is where the gram scale is used, as it
is needed to measure out both the rubber and the catalyst.
Measuring out the proper amount of rubber for each coat requires
a little practice. The mold shown here has two coats of rubber, each
of which used about 25 grams. After painting on the first coat, it
is allowed to set until only the slightest tackiness remains.
The second coat, known as the build-up coat, is painted on
while the first is still tacky. If it's too dry, the two layers may
delaminate later.
After the build-up coat has set to the stage of slight tackiness, a
third coat is painted on. Immediately following this, a double layer
of cheesecloth is laid on top of the rubber layer and tamped down.
This allows the cheesecloth to absorb the still-wet rubber, and
eliminates any air pockets which can weaken the mold. The purpose of
the cheesecloth itself is to strengthen the rubber, provide
structural support to the mold, and prevent ripping when the rubber
is stretched.
Next, while the cheesecloth layer is still wet, a rubber lock is
applied and pinned to keep it in place while the rubber sets. This
lock will help later in securing the rubber to the plaster shell, or
mothermold. This outer shell keeps the flexible rubber from losing
its shape and form while it is being handled and used in the
wax-pulling process.
The last step in the rubber phase is the final coat, which
completely embeds the cheesecloth within the rubber layers. This
creates a smooth finish for easy release of the mothermold.
Mothermold materials can range from plaster to high-tech plastics. I
like to use plaster on smaller molds where weight doesn't matter too
much, and plastic for larger molds.
Once the rubber has completely set and the lock pins are removed, a
layer of paste wax is painted over the entire rubber surface and is
allowed to dry. Next, a 75/25% mixture of petroleum jelly and
mineral oil is applied on top of the paste wax. This
super-lubrication ensures easy release of the rubber from the
mothermold.
Next, a thin layer of plaster is spread over the entire surface of
the mold.
Wet hemp fibers are now dipped in plaster and worked into the mold
on top of the still-wet plaster layer. This provides strength for
the mothermold, so that even if cracked, it will not fall apart.
Here's the finished plaster with the hemp completely embedded
underneath.
Once the plaster has completely set, the clay wall can be removed.
The mothermold can now be removed from the rubber.
The rubber can now be removed from the clay original.
The excess rubber, plaster, and cheesecloth can now be trimmed to
leave a cleaner surface for reinsertion into the mothermold.
The final step is removing the rough edges on the plaster.
The mold is now completely finished and ready for wax.
The surface of the board is covered with a sheet of freezer paper.
Next, the original clay is laid out on clay supports to keep it
suspended above the board. This keeps the original from having as
little contact as possible with the board.
Now a layer of clay is built up until it reaches the vertical center
of the piece.
Here you can see what several different pieces look like after being "halved".
Here you can see how dividing the piece with clay can result in very
different layouts depending on the shape of the original pieces.
Here, I've added the pourspouts, vents, and locks. The funnel-shaped
wedges you can see here will be the pourspouts for the wax to be
poured into the mold. The vents are the red channels you see, and
they serve to both feed the wax throughout the mold, and allow air
to escape while the mold is being filled with wax. And finally, the
indentations and dimples you see around the pieces serve to lock the
two rubber halves together.
Once again, you can see how different the resulting layouts are
depending on the shapes of the original pieces.
Next, a wall is built around each mold to contain the rubber.
The instructions for mixing must be followed from the manufacturer
of the silicone rubber. This is where the gram scale is used, as it
is needed to measure out both the rubber and the catalyst.
Measuring out the proper amount of rubber for each coat requires
a little practice. This mold shows the first coat of rubber, using about
50 grams. After painting on the first coat, it
is allowed to set until only the slightest tackiness remains.
The second coat, known as the build-up coat, is painted on
while the first is still tacky. If it's too dry, the two layers may
delaminate later.
Here's the same mold after the build-up coat has been applied.
After the build-up coat has set to the stage of slight tackiness, a
third coat is painted on. Immediately following this, a double layer
of cheesecloth is laid on top of the rubber layer and tamped down.
This allows the cheesecloth to absorb the still-wet rubber, and
eliminates any air pockets which can weaken the mold. The purpose of
the cheesecloth itself is to strengthen the rubber, provide
structural support to the mold, and prevent ripping when the rubber
is stretched.
