Here is a 2-piece silicon mold in progress. It uses silicon rubber to capture the details, and a hard shell to give the flexible rubber rigidity to hold its shape.
(Need to know more about What is a Mold? Read this post first!)
Here are a few shots of the process:
Making a clay wall to create the seam between the two halves of silicon (using oil based clay)
Adding registration keys to the seam so the two halves will align properly
Applying the first layer of silicon- this is very thin to make sure you capture all the detail with no air bubbles forming
After a few thicker layers of silicon-you want a nice smooth shaped surface for the mother mold to release easily from
The mother mold is a rigid shell to support the flexible silicon mold- this is made out of a product called "plasti-paste"
Here is a shot from the back showing the back of the silicon seam (clay wall has been removed)
This is one side of the finished mold- all that remains is to drill holes to bolt it together!
You then repeat those steps on the other side. The tricky part is decided where the seams should go and making sure that everything will release easily from the original piece you have inside, as well as from the future casts. The fun part is being able to multiply your sculpture!