Making a simple plaster press mould
Plaster moulds are incredibly useful for reproducing a shape quickly and exactly that can’t easily be made by hand-building or throwing. Don’t be daunted by the prospect of mixing plaster! If you weigh and measure your ingredients properly, and follow these simple steps closely, you should have no problems. Cottles are retaining walls built up around the mould, so hold the plaster in place while it sets.
1 ½ lb (675g) plaster
1 pint (575ml) water
This will give a strong enough mix for most pottery uses.
Here you see a selection of simple shapes that make ideal models. It’s important that the shape flares outward; if they close in at the rim, this will cause what is known as an undercut, which will encase the model and make it impossible to release.
TIP: Scour car boot sales and charity shops for stainless steel bowls – you’ll be amazed how many shapes and sizes you will find, for very little cost. You can also use ceramic or wooden bowls providing they’re the right shape.
Before you begin, run a thin coil of clay around the rim of the bowl then turn it over onto a board. Secure by pressing down until the bowl is flat on the surface then remove any clay that may have squeezed out around the outer rim as you pressed down. You should find the bowl is quite firmly held now.
Any surface which is absorbent must be sealed with soap ‘size’ prior to casting in plaster, otherwise they can never be separated. However, for beginners who may be unsure, it’s advisable to size all surfaces, whether absorbent or not. The size forms a skin, or barrier, between the various surfaces and is vitally important for success.
TIP: mould makers’ size or soft soap: available from your pottery supplier and usually comes in a solidified or thick form and needs diluting with boiling water 50:50 soap to water. Brush the soft soap over the entire surface of the bowl and the board if it’s wooden or another absorbent material. (Formica type boards don’t need sizing).
Using a natural sponge and water, carefully sponge the soap size off the surface of both the bowl and the board until it looks smooth and bubble-free.
Now repeat the process of applying the soap size and wiping off again at least twice more – three coatings are the minimum required. Each application builds up a film on the surface, and the more times you repeat the process, the less risk of the model sticking to the plaster – some mould-makers repeat up to seven times. The finished model should look smooth and clean, as seen here.
Place the board and model on a non-absorbent board if possible, then position a length of lino around the perimeter as shown and secure with tape so that all joining edges are sealed, to form a ‘cottle’. Run the tape around the cottle several times to make sure it can’t break open when the plaster is poured in. The lino should be deep enough to contain the model with at least 12cm spare at the top.
COTTLES FOR MOULD-MAKING
A ‘cottle’ is basically a retaining wall that’s built up around a model to contain the plaster. The material the cottle is made from is dictated by the shape of model it surrounds. You can choose from the following options:
Lino: A cheap and flexible cottle material that can easily be cut to size, is reusable and doesn’t require soap sizing. You can often buy small sections of cheap offcuts – or even get it for nothing from some suppliers.
Soft clay: Generally used for small moulding walls ie sprig moulds, knobs, handles, etc. Does not require soaping.
Wooden boards: Melamine or Formica boards are good if available, but any old boards will do providing they are deep enough to contain the model with enough space above for a good thickness of plaster at what will become the base of the mould. For easy releasing, soap these before use.
Note* Most pottery suppliers have adjustable wooden frames for mould-making, but it’s not worth splashing out on one of these unless you intend to make a lot of moulds!
Plaster bats: Useful because they can be cut to size but perhaps one of the above options would be easier for beginners.
Materials for securing cottles
Masking or gaffer tape: Can be used instead of string.
Clothes pegs: Useful for holding lino cottles together before securing with string.
String: Varying lengths of strong string, preferably nylon with a knot at one end and a loop at the other for easy tying.
Soft clay: For sealing seams on the outside of cottles to prevent plaster escaping.
Wooden wedges: Help to keep string taut when using wooden boards as a cottle. See ‘Wooden boards’ above.
With a thick coil of soft clay, carefully seal the base of the lino to the board as shown. This is important to prevent the plaster pouring out at this point. Be as meticulous as possible in securing your cottle, whatever it’s made from – seal all possible places where plaster could escape from – soft clay is the best material for this.
You’ll have to judge the amount of plaster mix you need because it’s entirely dependent on the size of the model used and the spacing of the cottle around it. But as a general rule, for larger items you’ll probably need more than you think. Allow a space of about 5cm around the model to position the cottle for a sturdy mould. Measure the water required into a bucket, then weigh out the corresponding weight of plaster. Carefully add the plaster to the water until it peaks at the surface then gently shake the bucket until all the plaster seeps down below the surface.
Stir the mixture gently with your hand to remove any lumps, and every so often wiggle your hand at the bottom of the mixture to release trapped air bubbles. This shouldn’t affect your skin but if susceptible to allergies or otherwise in doubt, wear rubber or latex gloves.
Carefully scoop off the bubbles that collect at the top of the mixture and transfer to a plastic bowl lined with newspaper. This will allow you to dispose of the waste easily. Keep the container at hand to dispose of any leftover mixture that you can’t use after making your mould. The mixture is ready when it no longer runs off your fingers and is obviously thickening. This can happen relatively quickly or sometimes can take several minutes, but once it starts to thicken you need to move to pouring quite quickly.
TIPS and safety notes
• Always add the plaster to water – never the other way around.
• It’s better to overestimate an amount of plaster than have too little, but if you don’t want to be wasteful, prepare the sprig models in Project 4 to use up the surplus. Alternatively, you could use the excess to make a batt for re-claiming clay on.
• Plaster is contaminating – never allow plaster bits to get into your clay because it will cause damage to your pots in firing. Try to keep a separate area in your workshop for plaster work and be meticulous when cleaning up after making moulds.
NEVER pour excess plaster down the sink; it will set and block the pipes!
Clean excess plaster from your hands with newspaper before washing them, again to avoid blockages.
Use newspaper to clean out the plaster bucket immediately after use.
Wear a face mask when mixing plaster if you are concerned about inhaling dust.
Carefully pour the plaster mixture directly over the model. It’s important to cover the model completely first, because if you’ve underestimated the amount of plaster required you can then add a second batch, but if the model isn’t covered first go, the join lines will show in the mould and any subsequent clay work made in it.
Troubleshooting: What to do if you’ve underestimated the amount of plaster
Don’t panic if you’ve underestimated the amount of plaster mix required – it will be fine providing the actual model is covered with plaster. Simply score the surface with a sharp tool, as shown, to form a key for the next batch of plaster. DO NOT score too deeply as you may damage the inner surface.
Mix an additional amount of plaster to fill the cottle then pour over the scored surface. When all the mixture is has been poured, agitate the surface with a hand to release trapped air bubbles. Alternatively, if the mould isn’t too heavy you can slightly lift the board it’s on and tap it back onto the work surface several times – this is generally easier with smaller moulds.
As the plaster sets it will give off heat – this is normal, so don’t panic. Now you can remove the cottle from around the mould. Using a Surform carefully remove all sharp edges from the plaster on the underside – rounding off the perimeter in the process. Turn the mould over and repeat the process on the upper perimeter so that the edge is soft.
The model should lift cleanly and easily out of the mould. Place the mould somewhere warm to dry out completely before using; this can take some time if it’s large, so patience is required. The top of the kiln, an airing cupboard or range cooker are good places, if there’s room.