Getting the measurements right when mixing plaster of Paris is crucial to the success of the project no matter how large or small the project is. To little or too much water or plaster will ruin the mix. Plaster begins to set immediately once it is stirred and cannot be repaired. The amount of plaster and water being used depends on the capacity of the mould it’s being poured into. The correct measure for plaster of Paris is always one part water and two parts plaster to make a perfect mix.
Fill the mould with water all the way to the top edge. Pour this water into the mixing container. Have one or two smaller moulds available so extra plaster mix, if any, won’t be wasted.
Sprinkle in the plaster very slowly. Do not stir the plaster until there is absolutely not even a drop of water left at the top of the container. Continue adding plaster until all the water is absorbed.
Stir the plaster mixture until there are no lumps and it has the consistency of a slightly thickened sauce or gravy. Once you stir the plaster it begins to heat and set, so work quickly.
Pour the mixed plaster into the mould. Carefully shake the mould just briefly from side to side to release any air bubbles.
Wait 20 minutes and remove the casting from the mould. If there are any rough edges, you can probably smooth them by hand. Once it dries, you can use fine sandpaper to smooth the edges.
Using colder water to mix the plaster will give you more time to pour it, as it will take longer for it to heat.
Do not pour any of the plaster or plaster water down drains or other plumbing fixtures because it will build up and clog the pipes.