Casting moulds from two halves

For plaster casting you will need:
A non-absorbent board for making the mould on
Clay for building up the model and reinforcing the cottle
Boards to make the cottle
A length of strong string
Plaster and a bucket for mixing
Newspaper
Your chosen fruit or vegetable

 

1

Plaster casting 1

Before you begin, use some soft clay to fill in any areas on the vegetable that may cause problems with undercuts – smooth over the clay so that it blends in.
Run a length of masking tape around the model to mark the half-way position. You can do this with a marker pen on most other fruit or vegetables, but the colour of the aubergine prohibits this, so the tape is an easy alternative.

2

Plaster casting 2

Balance the model on a pad of soft clay so that the tape is perfectly horizontal. You can use a ruler to measure up to the marker all the way around the model if it helps – make adjustments until it’s perfectly positioned.
Next, begin to build up the bed of clay around the model with soft clay, to the top of the tape line.

3

Plaster casting 3

Continue to build up the clay bed around the model to the tape line as shown, allowing at least 3cm extra width on either side of the widest point of the model. Try to keep the bed as level as possible as you build it up.

4

Plaster casting 4

When the model has been satisfactorily embedded, level and smooth the clay with a kidney, taking great care around the model itself.

5

Plaster casting 5

Using the boards, start to build the cottle around the model. Fix one side in place and secure on the outside of the board with a thick coil of clay. Fix the second board in place and secure in the same way on the outside, then run a thin, soft coil of clay along the inner edge of the board and model, as shown, and up the joint in the corner to prevent any leakage of plaster when casting.
Continue to build the cottle around the model until all sides are secured on the outside and reinforced with clay to prevent plaster overflow around the model base.

6

Plaster casting 6

As a final precaution, to hold the cottle in place, run a length of strong string around the frame as shown and secure tightly. You are now ready to cast the first part of the mould.

7

Plaster casting 7

Cast the model in plaster. The amount required will depend on the size of fruit or vegetable being cast, but as a rough guide, and for this example, a mixture of 3lb (1.4kg) of plaster to 2pt (1.4 litres) of water was used for each half of the mould. Follow the technique for mixing plaster as explained here.
When the plaster has set, dismantle the cottle then remove the sharp edges around the edge of the mould with a surform blade.

8

Plaster casting 8

Turn the mould over and reposition it on the board.
Carefully, remove the clay that embedded the aubergine – it should come away easily, leaving the vegetable in place in the plaster.
Now remove the tape, carefully, to avoid pulling the aubergine out of place.

9

Using the rounded tip of a steel knife or similar tool, cut a ‘natch’ in each corner of the mould by rotating the knife in a circular motion to form a little cup cut-out. This will form the locaters when the two halves of the mould are fitted together.
Alternative items for cutting natches, depending on the size of the mould: a penny coin, a melon scoop, etc.

10

Plaster casting 10

Soft soap the surface of the mould at least three times, making sure that you soap inside the natches as well as the entire plaster surface. Wipe away the soft soap after each application until an obvious film has built up on the surface of the plaster.
You can apply the soap up to seven times if you want to make absolutely sure that the mould halves will separate after casting.

11

Plaster casting 11

Build the cottle around the mould for a second time, following the same procedure as before to ensure all possible escape routes for the liquid plaster are sealed with coils of soft clay.
Tie the cottle with string again to secure it thoroughly.

12

Plaster casting 12

Mix the same amount of plaster and cast the second half of the mould. When the plaster has been poured, agitate the surface for a second or two to bring any possible air bubbles to the surface.
Leave the plaster to set.

13

Plaster casting 13

Remove the cottle then surform the edges of the plaster to remove any sharpness. Carefully dispose of the plaster residue to ensure it can’t contaminate your clay later.
Open the mould – it should easily come apart if the plaster was soft-soaped sufficiently.
Carefully remove the vegetable – it should come out easily and cleanly.

14

Plaster casting 15

The two halves of the mould show how well the form has cast – put the halves together, then place the mould somewhere warm to dry out completely before using.

 

For more ‘How to’ guides, see here