All images and web design by Gary Persello and Bonnie Van Hall © 2010  All rights reserved. 

Making silicone molds for bronze casting
Materials:

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.









Step Two:
Making a one-sided mold

(or skip to two-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.










Plastering and Mothermold
Materials

20 minute plaster
hemp
paste wax
petroleum jelly
mineral oil
mixing bowls
brushes

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.














Step Three:
Making a two-sided mold

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.
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