Start by mixing the two ingredients together.
Silicone
and cornstarch
You need the paste to be the consistency of a dough. It shouldn’t stick to your fingers and should come together. Once you are able to roll it into a ball it is the right consistency.
Shape your ball into a rectangle or square a 1/2 inch thick (depending on the depth of the item you are casting)
Then simply push your casting item (shell) into the dough. If you are not happy with the impressions just roll it back into a ball and start again.
It will take 2-3 days, depending on the weather to dry.
Use this mold to press your polymer clay into. You may need a mold release such as oil to remove the polymer clay shape once it has been molded. Then press out and bake as per the usual polymer clay instructions.
This tutorial is shared from our sister site Craftbits.com
This is brilliant! I’m a dental hygienist and I have used dental impression materials for my molds. This would be so much easier and less expensive!