Plaster moulds are the easiest and cheapest of all moulds to make. They are also the heaviest and most fragile. There are two ways of making them. The first is to make a waterproof box around your object, pour in the plaster mix and wait for it to set. The second uses plaster bandage which is normally used to set broken limbs.
Steps in making boxed plaster moulds
Following is a step by step process in making boxed plaster moulds. They are very straight forward and require no special equipment or chemicals. All that is needed to make plaster moulds is plaster of Paris and some scrap timber to make the box section to hold the plaster mix until it sets.
Fasten your object to the middle of a smooth, flat piece of wood. Make a waterproof timber dam around it a couple of inches higher than the object. Use plasticene to fill all the undercuts on the object and any gaps between it and the base board. Take extra care doing this because it makes for a better mould. This is very important otherwise the mould will break when you remove it from the object your moulding.
When your satisfied nothing will obstruct the removal of the object from the mould you need to seal it and apply a release agent. The best sealer to use is shellac. Two or three coats should suffice. When its dry apply the release agent. You can use wax or PVA but the best of all is petroleum jelly. Nothing sticks to it and its cheap.
Now mix the plaster. This is the messy bit. Start with a bucket of water and add a little plaster and thoroughly mix it before adding a little more. Make sure its all completely dissolved before adding more. Continue adding plaster in this way until it becomes a smooth creamy paste with the consistency of oil.
Now pour the mixture over the object in a thin dribble. This minimises bubbles getting trapped in the mould. If the mould is not very thick you can lay some Hessian on top of the mould and add more plaster. This reinforces the plaster and prevents it cracking. The plaster will start going hard very quickly and will get quite hot as it sets.
Leave it for a day or so to cure and then carefully remove the baseboard. If you did everything correctly the object will plop out of the plaster and you will be left with a nice impression.
Plaster moulds are great for casting latex objects. You just pour in the liquid latex and wait for it to dry. Then they just peel out. Easy. It’s a great way for making decorative trims etc. If your casting with latex leave the mould as raw plaster. That way the latex can cure faster by ‘breathing’ through the plaster. If other casting materials are used seal plaster moulds with shellac and use a release agent.