I used to work in an ornamental bronze foundry that specialized in sculpture, everything from “trophy” size to 20-30 foot pieces. What you’re describing is smaller than what we generally dealt with so there might be a more efficient process for your purposes, but this was our general molding (from original) procedure.
We used a “lost wax” casting method. We needed to create a wax copy of what would end up in bronze. That wax copy would then be “invested,” put into a plaster casting mold which would be kiln baked and the cavity the wax created would be filled with bronze. It seems you will be casting solid pieces in a cold setting medium so that part of the process doesn’t apply to you. But the technique used for the wax mold might.
We frequently made molds from plastiline originals, however as I mentioned we worked on a larger scale than you described, so some experimentation on you part would be in order.
-Start by fixing the original to a flat base, a 1 foot square of plywood would do. Wrap the original in seran wrap.
-You need a flat “sheet” of clay[we used a soft water based clay for this]. As to thickness, we generally used 3/4 to an inch, but given your scale you might try smaller. Carefully wrap your original in the sheet of clay, don’t press hard enough to damage your piece.
-Create a “halo,” slice thin, even pieces of the remaining clay sheet, about as wide as it is thick. Now around the “sides” of the piece [you mentioned chess pieces, so fixed to the base and wrapped in clay I visualize a cylinder], up one side, over the top and down the opposite side attach a strip of that clay.
-Now you need some fine “shim stock,” Thin aluminum. For your purposes a beer can cut into inch wide strips would do. Press those strips into that clay “halo” you just built. [pardon the graphic]
-----<=[Your Piece]=>------
---- [shim stock]
<= [the clay "halo]
-Now with a smallish paint brush cover all the clay, aluminum and a few inches of the plywood base around the piece with Vaseline.
-Now you need some plaster and some rags [we used burlap, that would work for you but so would an old T-shirt]. Cut the rag into 1 inch squares and dip into the plaster. Then stick them on the Vaseline surface of the clay. Continue till you have a thickness of [just guessing for your size] a half an inch. Do both of the sides [separated by the shim stock]. Let it harden.
-After it’s dried it shouldn’t be too tough to separate the two halves of the plates “shell” mold with a flat blade screw driver [that’s why you used Vaseline]. After they’re off the original put the two plaster shell pieces back together (tape them) and drill a 1/2 inch hole in the top of them.
-Back to the original. Pull all that clay off of it, and the seran wrap as well. We had a commercially available mold release product in an aerosol can, not sure where you could find it and I have a hunch that something like Pam cooking spray just might work. Spray your piece (and the inside of the plaster mold)with it, and the base around it. Then place the two halves of the plaster mold around the original piece, leaving equal room on all sides. Tape it tight and press clay around the base.
-
Remember that hole you drilled into the plaster shell? Make a funnel of some sort that fits into that hole and mix up your latex “slush mold” material. Pour it through the funnel into that hole, fill it all the way. Let the latex cure.
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Once dry separate the plaster shell and your left with latex that fits into your plaster shell AND has your original inside it. To get the original out we had typical “box cutter” knives that we bent into a shape like
-^-. This let you cut along that “halo” leaving a trough on one side and a corresponding grove on the other. Sadly your original will be marred by the blade, but the latex, once separated will have an accurate impression of it.
-Now you have two plaster shell halves, and two latex mold halves that fit perfectly together. Places the latex into the plaster and spray the latex with the mold release. Put the them together and tape them tight. You should now be able to pour whatever you intend into the cavity, and once dry separate the mold and there should be very little “flashing” to contend with. A Dremmel tool should take care of any that occurs.
That mold should last long enough to make a few hundred copies.
That should give you the general idea, incase my addy isn’t in my profile, and should you need more help: bayktch@aol.com