Making a plaster of paris mold is something I’ve wanted to try for a while. The idea is you take an object, encase it in plaster, then use the hollow space in the plaster to make a copy of the object with the material of your choice.
Because all the cool materials, like latex or resin, are quite expensive I opted for paper mache. As a model I used a piggy bank the kids brought me.
Step 1 – Make a box
This box is made from some scrap wood, about an 1″ or so bigger than the Piggy. Piggy was placed inside and some plasticene was used to block off the bottom half.
A mold release must be applied to the Piggy, the plasticene and the box walls to ensure the plaster does not stick. My mold release of choice was Vaseline, because its cheap and plentiful.
The idea is that the two parts of the plaster mold meet at the centre line. So the plasticene is there to form a barrier. I didn’t have enough plasticene to fill the bottom completely so I used scrunched up paper underneath. As an exercise, see if you can work out why this is not a good idea.
Step 2 – Pour Plaster
Plaster of paris is mixed by adding cold water and there is then a period where the plaster is running until it suddenly turns hard. It’s an exothermic reaction so it can get hot, too.
You will need lots of plaster. I mixed up three batches trying to fill up the top. Of course, what I didn’t realise at the time was that weight of the plaster was pushing against the paper and compressing it. As that shrunk, the plasticene moved down and caused cracks to appear. This became apparent at the next step.
Here’s the top mold. That was left for a few days to make sure it had really set.
Step 3 – Pour the Bottom Mold
What is supposed to happen is that you now turn over the mold, remove the plasticene and pour the bottom part of the mold.
Here’s the top mold as I was turning it over.
Oh dear. You can plainly see the top mold is quite a way over the middle of the piggy. At this point I worked out why using scrunched up paper was not a good idea.
At this point I should have just made the bottom pour but I decided to see the if the Piggy would just slide out. Not a chance, he was stuck solid.
This is what happens when you try really hard to get a stuck piggy out of plaster.
Never mind, I reassembled the mold around the piggy and put it back in the box. More vaseline was applied and more plaster of paris was poured all over it.
It was now put to one side so it could dry, and also I forgot about it a few days.