You will need:
Plaster of Paris
Wooden tongue depressors (or something to build up your dam with, Lego bricks works great!)
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Step 1: Making Your Mould
To begin with we need a nice smooth surface on our Plasticine. Any ridges or bumps will show in our final piece so its not worth rushing this point. While I'm working on the top of a plastic container - any flat surface will do.
Step 2: Building a Dam
Some simple wooden sticks sunk into the Plasticine and filling any gaps at the corners will allow us to make a nice deep mold block.
This is a self harm prosthetic for mental health scenarios - so I have cut some wounds into the Plasticine.
Make up the Plaster of Paris as per the instructions on the bag, and tip into the mold area. If you lift up and drop the whole piece onto a flat surface a few times (gently!), you can help the Plaster of Paris settle into the mold and not leave any bubbles. Now leave to set.
Step 4: Finished Mold!
Voila! One reusable mold!
Now you can make prosthetics from your mold using latex or silicone whenever you like.