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Mixing Plaster of Paris.

A very important point is to find the saturation point of your plaster.  If you don’t have enough plaster in your mould it will be too soft and wear away too fast.  To much and it will be to dense and not cast well.

Because the saturation point varies from plaster to plaster, you need to ask your supplier.  They might not always be able to tell you, then follow this method.

You need to estimate between half and three quarters of the volume of the cavity that you want to fill in water.  Because as you add the plaster the volume will increase.  Too much water and you waste plaster.

Add the plaster into the water rapidly sprinkling it over the surface of the water until it forms a small island that does not dissipate into the water.

Put a finger through the island to allow the water to penetrate the centre of it.  Leave the mixture to stand for a few seconds to allow all the plaster to get wet.  This will prevent lumps in the mixing.

Mix by stirring vigorously.  Again let the mixture stand for a while.  Stir every now and again until teh mixture thickens to a consistency of runny cream.

Pour into your mould and allow to set.  Once it warms up it is sufficently set that you can split your mould.  If your mould has more than one piece you need to paint a release agent on the next part.

The manufacturer of the plaster-of-paris that we sell recommend the following ratios of plaster:water –

between 1,66:1 & 1.428:1

or 100 parts plaster: 60-70 parts of water (by weight per 100 parts of plaster)

1 litre of water = 1kg in weight

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