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Saint Gobain Formula RM1042 Casting Plaster (Plaster of Paris) From £6.00
Fine Casting Plaster is a very useful material when working with clay. Its principal advantage is that it absorbs water and picks up texture and detail from almost any surface.
Types of plaster.
Builder’s plaster is not suitable for use in mould-making. You should find fine casting plaster suitable for most jobs, as it absorbs water from any clay pressed into it, thus allowing it to release easily from the mould. It casts relatively fast and is strong enough for most clay work. When making a large mould you may want to mix some scrim with the plaster to strengthen it further while keeping the mould light enough to work with. However, if you want to make a plaster stamp or other article which you intend to last for a long time you might want to use Herculite plaster. This is much stronger but much less absorbent, so that it is unsuitable for moulds but excellent for reproducing textures. A mixture of 80% FCP and 20% Herculite is ideal for plaster batts and wedging slabs.
Plaster must be stored in a clean, dry place. If it is allowed to become damp it will ‘go off’, that is, it will harden in the bag and become unusable. Always use dry scoops and measures. Plaster can also contaminate clay, and if this happens your work can blow up during firing or shortly afterwards. For this reason you should keep your plaster workspace as far away as possible from your clay.
Mixing Fine Casting Plaster.
The correct ratio for mixing fine casting plaster is 1kg plaster to 700ml water. If too much plaster is added the plaster will not be absorbent enough; not enough plaster will result in a soft, crumbly set which may contaminate your clay. Measure the correct amount of cold water out into a clean, flexible container. Sprinkle the plaster evenly over the surface of the water, a handful at a time. The plaster should form peaks above the water surface towards the end of this process. When all the plaster has been added, place your hand in the mixture so that your wrist is a few inches below the surface and your hand is at a right angle to your forearm. Mix the plaster without allowing your hand to break the surface. This is to prevent air bubbles being introduced to the mixture. Keep mixing until the plaster seems to thicken slightly. It is now becoming ready to use. You only have around 2-3 minutes to work, so try not to make up more plaster than you need. After you have made your mould, wait until waste plaster has set and then flex the container to break it cleanly away. Plaster must never be washed down a sink as it will block it, so wait until the plaster on your hands has dried and then place all waste plaster in a bin. Clean up scrupulously afterwards.
Health & Safety.
Do not breathe dust. Wear suitable face mask and gloves when mixing. Plaster heats up when setting do not attempt to cast hands or any other parts of the body using Plaster.