Working with Plaster Moulds

Hello everyone! This post is about a process widely used in industrial ceramics but, I believe, less common in studio pottery: making pieces with plaster moulds.

Although this method removes the individually hand-formed character of each piece, it is very practical and can help produce complex pieces with less labour.

For an example, consider the model I prepared in an earlier post:

First, we need a prototype, which can be made
of wood, plaster or even clay. I chose porcelain
because I felt it would give a good finish when sanded.

After sanding, I bisque-fired the piece to make it less fragile.

Next, it is important to consider the model’s symmetry. For this bottle, the division can be made as follows:

In this case, I need a two-part mould.

Next came making the plaster moulds into which the liquid clay, or casting slip, would be poured. In this test, I used 3 kg of plaster to 2 litres of water. The appropriate ratio should be checked for the plaster being used. I covered one side of the model with clay and used pieces of wood or plasterboard as a mould box for pouring the plaster. Preparing everything before mixing was important because of the time available before the plaster began to set.

Once the first half of the mould had dried, I removed the clay, turned the assembly over and repeated the process for the second half. In the experience described here, I used WD-40 as a separator between the halves. This records what I did; it is not a claim of compatibility with every plaster or process. I left it to dry thoroughly.

Adjusting slip before pouring it into a mould depends on its composition and consistency. Sodium silicate is a deflocculant used in this type of process. Water content and deflocculant dosage depend on the slip composition and condition; this is not a rule to never add water or to add silicate without assessing the mixture.

In my case, I waited 15 minutes after pouring the slip into the
mould. After 15 min, I turned it upside down with a
bucket underneath. Let it drain well and wait for it to dry. Once dry,
demould it. Here is the result:

Remove the piece from the mould and scrape away the seam where the two halves met. Leave it to dry, and it is ready!

I really enjoyed the process. Although making the mould takes
some work, I think it is a very efficient way of making ceramics.

Until the next post!