Step 7: Pour the plaster
Plaster for casting is available at most art shops, or craft shops, or off the net. I ignore the fancy mixing formulae, two parts powder to one part water (by volume) will generally give good results every time. One cup of water to two of plaster. Start by getting a quick even layer of plaster all over the mask. If you use a brush be careful not to disturb the surface of the plasticine. I like the messy aspect of it all and get stuck in with my hands and fingers.
Once you have a base of plaster down, start slowly adding to the initial coat. If it all gets a bit out of hand, leave it for 20 minutes to set and then mix up some more plaster and add to the dried stuff. Slowly, slowly catchey monkey. You can add plaster bandage or scrim net to strengthen and support the plaster as you go. Eventually you want a mould at least 3/4" or 20mm thick.
Let the plaster dry for an hour. Another cup of tea and another smoke helps here.
After an hour you can de-mould. Start by pulling away the fence. this will leave a ledge around the upper plaster mould. Grip the edge with your fingers and gently ease the upper cast off the head. It doesn't matter if some of the plasticine comes away with the cast or stays stuck to the head.
Carefully clean away and remove ALL TRACES of the plasticine from both the head and the mould. When fitted back together this will leave a void into which will go our gelatin. Congratulations, you have just made a two part mould.