First things first – get yourself a clean bucket – if the bucket is a bit gritty or dirty then this will leave little marks or dimples in your plaster.
- Work out how much water/plaster you’ll require. (Ratio of water:plaster is 1.3 water:1 plaster – for example, 5kg of water will require 6.5 kg of plaster)
- Weigh your water in the bucket and tare the scales with the bucket first – 1 litre = 1 kg.
- Weigh your plaster
- Gradually add plaster to the water in the bucket (never add water to plaster – always plaster to water to prevent lumps) allowing the plaster to slake down – this will only take a few minutes
- Immerse your hand in the bucket mix and stir gently – any rapid stirring will create bubbles and air into your plaster mix. You can also use a long spoon to mix however using your hand is better so that you can feel and squash any lumps in the process.
- Continue to stir until your mixture begins to resemble thickened cream – this will take another few minutes.
- Rotate your bucket backwards and forwards to bring any air bubbles to the surface and tap the sides of the bucket too if necessary.
- Spray the surface of your plaster mix with a 50/50 mix of methylated spirits and water to remove any surface air bubbles – or use a spoon to lift any bubbles off the top.
To make a plaster mold
- Prepare your item to be cast by placing a mold frame around your item. Paint any plastic surfaces, eg if the mold frame is plastic, the bench top, a plastic model etc, with a 80:20 Water to dish detergent mix (this will prevent the plaster sticking)
- Plug any areas around your mold frame with clay to prevent any plaster running out from underneath or through the sides of your frame.
- Pour the plaster mix slowly over your model and into the frame ensuring your model is covered by at least 2.5cm
- Stir the plaster with a soft brush or narrow wooden tool, eg a chopstick or wooden spoon to ensure the plaster fills every nook and cranny
- Tape the sides of your mold gently to bring any air bubbles to the surface and spray with your methylated spirits/water mix
- Clean up by washing out your bucket and any tools immediately to prevent the plaster setting
- Leave your mold until the plaster sets – you’ll be able to gently touch the surface and feel it get a bit warmer as it sets after which you can remove the mold
- Turn your mold over and gently remove any clay and then the mold frame/base board
- Clean any edges of your newly created plaster mold with wet and dry sandpaper or a rasp
- Allow the mold to try completely – this can take a number of weeks depending on your climate. Your mold will be ready to use when it is completely dry.