How to use pre-existing items to make sprig moulds, which is an easy first step for beginners
Any number of items we all tend to have lying around the house can be used, and part of the fun is looking for something suitable. For a seaside theme, the items chosen here are a seahorse brooch and a selection of shells, but you could use buttons, all sorts of old jewellery (providing it’s not too intricate), small fossils like ammonites – anything small that might look good as a raised detail.
Old tiles are a perfect size for making sprigs if you have any sitting around
You will need:
Roll a thick slab of clay and cut to a size suitable for your model, then place on a small board, or tile as shown here.
Press the model carefully down into the surface with the detail you want to show facing upwards – it shouldn’t be too raised above the level of the clay but also not too submerged.
Using a wooden modelling tool, carefully work around the edge of the model to ensure the clay fits to the surface so that plaster can’t seep down the sides.
As you do this, check there are no areas of undercut on the model and if you find some, fill with soft clay, blending and smoothing the surface level again as you work. Warning – undercuts will trap your model in the plaster and must be eliminated before casting.
Soap size the surface of your model using a soft bristle brush to apply the soap. This is crucially important, especially if your model is very detailed.
Sponge off the soft soap, using a natural sponge if possible, and repeat the process several more times to build up a good film on the surface of your model.
It’s always useful to make several sprig moulds in one go if you have a particular theme in mind, so here you see the shells positioned in the clay in the same way as the seahorse brooch being soap sized – again repeating the process several times.
Roll out a thick slab of clay and cut sections to build a wall around your models as shown. The wall should extend 4-5 cm above the surface of the model.
Now seal the bases with a coil of soft clay to ensure the plaster can’t escape when poured in.
The models are now ready for casting into plaster. If making more than one mould it helps to have them positioned close together, to minimise mess when pouring the
Using the plaster to water ratio and the mixing process given here, make a small amount of plaster to fill your models. The amount will depend on how many sprigs you’re making, but in this example 1 pint was used. One sprig would require no more than half a pint. Pour the plaster over the models quickly, making sure the actual models are covered first and gently tap the tiles up and down on the work surface a few times to release trapped air bubbles.
When the plaster starts to heat up, it’s safe to remove the clay walls. Now surform the underside edges of the plaster to round them off and remove any sharp bits which, if left, could easily get knocked off and later contaminate your clay.
Carefully lift the clay off your sprig moulds – you’ll find that some models have stayed in place in the clay and the lifting is relatively easy, others may have stayed in the plaster.
If your model is still in place in the plaster, carefully lift it out – it shouldn’t be difficult, providing you can grip it easily. Brooches are good because the clasp acts as a purchase for your fingers or a tool, but if the surface was soap sized sufficiently nothing should be too stuck; unless there were undercuts that you missed.
Avoid digging into the plaster with sharp tools – it will spoil the mould.
Finish off the moulds by surforming the upper sharp edges as you did on the undersides. Put the moulds somewhere warm to dry out completely before using.
Clay used for making plaster moulds can’t be used again to make pots because bits of plaster trapped in the clay will cause explosions in firing. Keep your mould-making clay in a separate bag clearly marked ‘PLASTER CLAY’ and it can be used again for the same purpose
This project first appeared in issue 2