With the idea of extending the small collection of natural forms I have been casting in glass, I found this unusual shaped root tossed by the waves onto the beach at Alameda, California and will show how to take a silicone mold off the shape and texture to prepare a wax duplicate for casting into glass, although you could use the same process to make a metal or ceramic object, as well.
After I found the driftwood, I washed it off taking away some of the mud that gave it so much character. Perhaps the hardest part of the operation was restoring the mud with modeling clay to return to the object its original character and make it viable for casting into glass.
Step 1: Covering the object with molding silicone
We chose a locally available product with the curious name of EZ Brush Vac Bag, from Douglas and Sturgess, www.artstuf.com, in San Francisco and Richmond, California. This is a two part silicone specifically designed for molding complex textures and shapes. It has a partial transparency so you can see the thickness of your application and a viscosity that allows it to be applied with sticks and cheap brushes, even on undersides and vertical surfaces of your model.
Following the directions, we mixed up a small amount, about a pint, in marked plastic containers so we could get the ratio very close to the prescribed 1 to 1. Mix thoroughly for about 90 seconds, being sure to scrape the sides and bottom of the container to get all the unmixed product swirled in.
Apply a thin layer all over the piece with sticks and brushes. This layer will be the one that actually gets the texture recorded. Everything else is backup. We eventually applied three layers a few hours apart to make the mold about 1/4-inch all over, give or take.
Step 2: Making a Mother Mold
Because the resulting silicone mold will be thin and floppy, we make a "mother mold" to go around the silicone and support it. On this shape we decided to make a three part plastic mold from a product called "Plasti-Paste." We chose the geometry so all the pieces could pull away from the silicone, even when it was full with the positive or eventually the wax, without damaging the soft interior wax. Here we made two side pieces and another one for the top. Each was designed so it would have a flange, a 90º ledge so each piece could fasten to the other with clamps or screws. We set up modeling clay so that each area of the model was demarcated with an area for the mold piece and flange. The silicone was covered with some pieces plastic wrap to keep the fit loose on the silicone. We used a wax release when we made the second and third pieces of the mother mold to keep them all from sticking together. Plasti-Paste is sticky.
We then mixed up the Plasti-Paste using a 3 to 1 ratio and up and applied it with sticks and brushes in two coats. It hardens pretty quickly. The second coat can be applied an hour after the first.
Step 3: Clean up the Mother Mold
Hardened Plasti-Paste can be rough and the edges a bit frayed. We like to take a hand grinder with a coarse sanding disk to smooth the edges and take the points off the Plasti-Paste surfaces. The flanges can be ground so their edges match for clamping or later screws.
Step 4: Cut Out the Positive and Clean the Mold
Cut lines in the silicone mold need to be thought out carefully. Obviously the minimum amount of cutting is best. We chose three lines extending up from the base over the "horns" of the wood. Cut with the sharpest knife; we used a new blade on a utility knife. The path of the cut should have some angles and creases zigs and zags, so it fits back together like a bit of a puzzle.
Clean and wash the silicone mold really well. Remove all embedded dirt, stones, sand, wood bits and anything that might get into the wax.
Step 5: Pour the Wax
Clamp or screw the mother mold together being sure it fits into the mother mold well with all its cuts laying flat inside. Set it upside down and braced, a pot is good, as it will catch any wax that spills.
We used microcrystalline wax for this, the traditional product for lost-wax casting. Usually a dark brown color, it needs to be melted until it is completely liquid, then allowed to cool until it skins over. The cooler the wax, the better the casting. If it's too hot the casting can show bubbling on the surface due to picking up moisture.
Step 6: Cool and Carefully Remove the Wax
After the wax has somewhat cooled and a thick skin has hardened on what is the bottom, exposed surface of the object, cut through the skin, which should be almost 1/2-inch thick, into the molten wax below in a circle leaving 1/2-inch wall in the casting. Pour out the still molten wax, leaving a thick wall for the entire was piece, which will then be hollow. Let the wax completely cool. We like to put the whole mold in the refrigerator or even the freezer, so the wax is really tough when the mold comes apart.
Some cleaning and chasing of the wax may need to be done to clean to fix any defects that appear. Our first two waxes out of the mold were filled with bits from the original, sand, bits of wood, the thinnest pieces of silicone and clay. But by the third casting, the wax looks good enough to move on to the next step of the process, which would be to cast it into glass, in the next Instructable!