This week at college I have been making plaster moulds. For my final project I am hoping to make some lightweight translucent hanging forms (more of those later) and a necessary step in the process has been to make some rather heavyweight moulds.
I started by making the forms I wanted out of clay, using a wooden board as a base. The forms were quite large so to cut down on the amount of clay needed I used some off cuts of wood in the centre and moulded the clay over them. Actually, I would have liked to go larger but I almost couldn’t lift a couple of them as it was so I thought I had better stop! I covered the clay loosely and left it overnight to dry out a bit, until the surface was firm but still workable. It is really important at this stage to get the surface of the clay as smooth as possible as the plaster will pick up every little dent, bump and imperfection.
The next step is to build a retaining wall around the clay form about an inch to an inch-and-a-half out from the form. This stops the plaster from going everywhere but also gives you something to judge the thickness of the plaster by when you are pouring. As you can see, the retaining wall does not need to be as tall as the form.
I am not going to go into the quantities for mixing plaster as I am really not an expert – I am sure you can find more reliable info elsewhere! I used fine casting plaster for my moulds. It takes quite a bit of mixing to get to the right consistency - Greek yoghurt (not the set stuff!) is probably a good description. It is important that the plaster is not too thin or it will overwhelm the retaining walls. But you do have to act quickly at this stage as it thickens up really quickly – with my first mould the plaster was a little too thick and I ended up with a couple of air bubbles on the lip of the mould where the plaster hadn’t flowed. This bit is down to experience – I got better at it!
So, your plaster is like Greek yoghurt. Pour the plaster over the clay form, making sure it is all covered. If you do not have enough plaster you can add another layer to thicken the mould up, but the first layer must cover the whole form. You can pat it down and spread it out as necessary; it thickens and sets quite quickly. Remove the retaining wall and use a scraper to take off any rough edges or unnecessary thickness – and leave to dry. The plaster does get surprisingly warm while it is setting!
Once the plaster has cooled down and dried out a bit you can remove the clay. Turn the mould over – mine were so heavy I had to get someone to help with one of them!
So I am now the proud owner of 4 rather large plaster moulds with lots of possibilities…