A basic globe-shaped mould is probably one of the most useful the potter can have because it can be used in so many ways. From slip casting to press moulding, you will be able to produce items as diverse as light shades, vases, kitchenware or garden ornaments to name but a few, and all you need to make the mould, is a cheap plastic ball
You will need:
• A plastic beach ball – available in several sizes, look for ones that don’t have textured surfaces (like footballs)
The ball used here is slightly too big – when used for slipcasting it created a vacuum when the slip was poured out, pulling the clay away from the side. Fixing this will be the subject of a future project, so in the meantime, choose a smaller ball if you want to slipcast. However, it makes a perfect press mould
• A large cork stopper – 4cm diameter (to form the pouring hole for the mould)
• Potter’s plaster – the amount will depend on the size of your ball
• Scales for weighing plaster
• Measuring jug
• Large buckets for mixing plaster
• Plaster clay – (clay reserved exclusively for mould making – remember, once it’s been used for plaster work, clay can’t then be used for any other purpose)
• Flexible material to make a cottle – lino or more clay
• Mould maker’s size (soft soap)
Before you begin:
Prepare your workspace so that everything is to hand and all other materials (like clay) are well out of the way, to avoid plaster contamination
Line a bowl with newspaper, to collect debris
Tie a length of string around the centre of the ball to establish a centre line.
Following the line of the string, carefully draw in the centre line with a marker pen.
Remove the string when finished.
Put some double-sided tape on the underside of the cork then fix it onto the ball, equally spaced over the central line, as shown.
The cork won’t fit flush to the surface of the ball, so fill in the small space between the two surfaces with a thin coil of soft clay. Smooth over the coil with a finger to ensure the gap is sealed.
Using the marker pen, continue the central line up the sides of the cork.
Place three small wads of clay on a non-absorbent board, then position the ball on top so that the line is as horizontal as possible.
Measure the height to the central line and make small adjustments until it’s equal all the way around the ball.
Begin to build up a clay wall clay around the ball to completely contain it to the central line. Aim to extend the wall 25-35mm out from the ball.
You’ll find this task easier and the outcome far more accurate if you build up the side of the ball in small stages.
Refine the level of the clay at the centre line by filling in, where necessary, with thin coils of soft clay. Use a flat-sided rib to smooth over the surface once the level is correct to the line.
The clay wall must extend from the central line on the ball to include the cork plug (or ‘spare’ as it’s more accurately called) to the same level.
Smooth over the surface with a finger to establish the level is correct, and once again micro-check that any small irregularities in the surface are filled in with soft clay and smoothed over.
Roughly smooth and level the sides of the clay wall to vertical, so that the cottle will fit flush to it when it’s fixed in place.
Position your lino, or clay cottle around the model, (Note, it must extend at least 10cm above the top of the ball) and secure it with tape at the base, middle and top.
TIP: masking tape is perfectly acceptable for this job providing you wind it around the lino several times.
Run a thin coil of soft clay around the inner edge of the cottle to seal the space between the clay wall and lino. Smooth the coil in carefully with a finger – it may be a little tricky to get your hand into the space, so take care.
Seal around the outside base of the cottle with a thick coil of soft clay – it’s important to ensure there are no possible escape routes for the liquid plaster when working in this way – everything must be very secure!
Working on the recipe of 2 pints water to 3lb of plaster, estimate the number of pints it will take to cover the model in the cottle including a 10mm excess at the top.
The measurement will vary according to the size of ball used to make the model so specifics can’t be given, but for the model cast here, 7 pints of water were used – it is quite a large ball!
Measure the water and transfer it to your bucket.
Weigh out the correct amount of plaster and sprinkle it into the water until it’s all absorbed.
Stir the plaster with your hand (WARNING – wear a rubber glove if you have any allergies).
As you stir the plaster, bubbles will rise to the surface. Scoop these out with your hand and deposit them in the paper-lined bowl.
When there are no more lumps in the mixture, all bubbles have been removed, and it begins to thicken, quickly pour it over the model.
You may find it useful to have someone to help you if you think you’ll struggle to lift and pour the plaster.
A chemical reaction in the plaster will cause it to heat up as it sets. Once this has happened, it’s safe to remove the cottle.
If the top of the plaster is uneven, level it off with the side of a metal ruler, as shown. Surform the edges to remove sharp burrs which would otherwise break off and possibly contaminate your clay when using the mould.
You now need to turn the whole thing over to remove the clay from around the model without pulling it out of the plaster. If you find this difficult because the clay has sealed onto the work surface, simply underwire it, as shown, to release it before turning it over.
Remove the clay carefully to avoid disturbing the model and the cork.
Store the clay in a clearly marked bag – CLAY FOR PLASTER – and don’t use it for anything else.
Wipe away all traces of clay from the ball and surface of the plaster.
Shave away any sharp areas around the edge of the plaster with a surform before continuing.
Make three ‘natches’ in the plaster, equally spaced around the model, using something like the rounded tip of a knife, a penny or melon-ball cutter.
The natches ensure the two halves locate together properly, and hold the mould in place when casting.
Thin down a couple of spoonfuls of mould maker’s size (soft soap) with boiling water, to a brushable consistency. Once thinned to the correct consistency, store it in a lidded plastic container for future use.
Brush the size over the plaster surface, up to the ball and slightly over the edge where the cottle will fit back onto later. Be sure to include the natches – this is very important.
Wipe back the size with a damp sponge then repeat the whole process at least twice more.
Note – the more times you soft soap the surface, the lower the risk of the plaster surfaces sealing together when you cast the second half. Some makers repeat the process up to seven times.
Position the cottle back around the model as you did for the first half, making sure to tape it several times to secure it safely.
Seal the space between the cottle and edge of the plaster with another thin coil of soft clay, then smooth over it with a finger.
Remember to seal around the outside base of the cottle with a thick coil of soft clay.
Mix exactly the same amount of plaster-to-water as you made for the first half, skimming the bubbles off the surface as required, before pouring the mixture over the model when it reaches the correct consistency.
When the plaster has heated up and started to cool again, remove the cottle and clay seal between the two halves of the mould.
Surform the edges of the mould again to remove sharp areas which could otherwise break off and contaminate your clay later.
Open up the mould carefully – it should come apart easily if you soft-soaped it sufficiently, but if it resists, put a towel over the seam and give the whole thing a tap with the end of a rolling pin – with luck it should then ping open!
Wipe away all remaining traces of clay from the mould with a damp sponge.
Close the two halves together again and put the mould somewhere warm to dry out. Try the airing cupboard, or on top of a range cooker like an Aga if you have one. Be aware; it will take several weeks to dry out thoroughly – you will have to be patient.
This project first featured in issue 28, which can be bought here
For more projects, click here