To me, Silicone is the best molding material available, it picks up incredible detail and you can cast almost anything in it!
For this Project, I chose Smooth-On Max-30 condensation cure silicone.
This is real neat stuff! it has a shore of 30A (soft) and it has what they call "a knotty tear propagation", (if the rubber is torn, the tears quickly terminates in a "knot" reducing further mold damage)... Cool, Huh?

After completing the sculpture, I take one final "idiot" check (I learned this term from working in the film industry, it means "... check again, some idiot may have forgotten something...").
Silicone picks up EVERY detail, so you want to make sure your sculpture is perfect.
Always mix your components as per the manufactures specifications.
What I've learned is, it's not the speed of mixing, but the quality of the way you stir, making sure you scrape the sides and bottom while mixing. I then pour the mixed batch of silicone in a fresh container and re-stir again, this guarantees that your batch is perfectly mixed and you don't wind-up with some uncured spots.
I start by brushing in the silicone in all the "nooks and crannies", it's always best to brush in a thin coat and letting that set-up, then add several coats building up the surface. This technique is better than adding a thick mixture and getting a lot of air bubbles (photo - 9).
Once the silicone has cured, I add a clay wall. This will help me create the front half of the plaster mother mold (photo - 10).

  Photo - 9                                                     Photo - 10

The mother mold...

With the front half completed, I'm now ready to start the back section (photo - 11). What I've used to create the mother mold is Smooth-On Matrix-G. This is a specially formulated resin plaster composite. You use this stuff with fiberglass in a laminating fashion the same way you would use polyester resins and fiberglass, the only difference is that it has no toxic fumes or harsh odors, you would still use a dust mask when working with the powders.
You will notice that there are no "keys" to line-up the other part of the mother mold, the reason is that, I don't really need them. The way I did my wall, following the highest points on the mold in a curve fashion, creates a full "key" (photo - 12).

Photo - 11                                              Photo - 12

The back half of the mother mold is now completed and removed. With proper lighting, you can see Bill's face (photo - 13).
Opening the silicone mold and stretching it this way can be hazardous. The blue arrow is pointing at where I stopped cutting the silicone to remove the casting. Chances are, the cut would have continued and this is what they mean by "a knotty tear propagation", that prevents it from tearing any farther.
This is the finished casting pulled from the silicone mold (photo - 14). This piece is also made of Smooth-On Matrix-G and laminated the same as the mother mold, except I used chopped fiberglass.

  Photo - 13                                                    Photo - 14

A perfect reproduction, every detail is there (photo - 15). The only damage the clay sculpture sustained from the molding process was the ear tips (arrow).

Photo - 15

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Guy Louis-XVI SFX
2137 Hubbard Crescent,
Ottawa, Ontario.
K1J 6L3
Canada
613-748-7877