This is a fibre glass mould that I created for a puppet which was to be cast in silicon rubber. One thing I learnt doing this is that fibre glass sticks to chavant clay! So bear in mind that some kind of spray mould release might be an idea if you're going to try this. Alternatively, I've found that plaster does NOT stick to Chavant - I guess because Chavant is oil based - so this same technique can work if you use plaster instead of fibre glass.
First, the back of the sculpture is covered in cling film to keep it clean and the sculpture is then embedded in clay to create a wall. This will define where the edge of the front of the mould begins.
As this sculpture was done in Chavant which is an oil based clay, I used water based clay to create the wall. This is so it can be easily cleaned away with water and will not disturb the sculpture.
It's important to make sure the edges where the wall meets the sculpture are clean. A paint brush can be used to smooth things out. I've also cut keys and guidelines into the clay wall so that the two parts of the mould will line up once they're completed.