How to mix plaster and make a mold

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.

1. 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)
2. Weigh your water in the bucket and tare the scales with the bucket first – 1 litre = 1 kg.
3. Weigh your plaster
4. 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
5. 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.
6. Continue to stir until your mixture begins to resemble thickened cream – this will take another few minutes.
7. Rotate your bucket backwards and forwards to bring any air bubbles to the surface and tap the sides of the bucket too if necessary.
8. 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

1. 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)
2. Plug any areas around your mold frame with clay to prevent any plaster running out from underneath or through the sides of your frame.
3. Pour the plaster mix slowly over your model and into the frame ensuring your model is covered by at least 2.5cm
4. 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
5. Tape the sides of your mold gently to bring any air bubbles to the surface and spray with your methylated spirits/water mix
6. Clean up by washing out your bucket and any tools immediately to prevent the plaster setting
7. 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
8. Turn your mold over and gently remove any clay and then the mold frame/base board
9. Clean any edges of your newly created plaster mold with wet and dry sandpaper or a rasp
10. 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.

Please share: