Step 3: Mix and pour the plaster
Many different types of plaster are available for casting. They are mostly based around Plaster of Paris and some have quite confusing mixing formulations. I have been using these gypsum based plasters for many years now and have come to the conclusion that the following mix will work correctly for these small moulds perfectly well.
2 parts plaster powder (by volume)
1 part water (by volume)
This could not be simpler. Take three standard plastic drinking cups and fill one with water to the brim. Fill the other two with powder to the brim. Pour the water into a plastic mixing jug, then add the powder little by little stirring all the time. You should end up with a mixture the consistency of double cream. Try and make sure that all the powder has been evenly distributed through the water and that there are no lumps. Now tap the jug a few times to bring any bubbles to the surface.
Because of the melamine, the oil based clay and the Lego we shouldn't really need a mould release agent for this job. However, if you have some mould release spray there's no harm in giving it a light coating. I tend to use silicone release spray for everything, mostly because that's all I usually have handy.
Start to pour the paster mix into the mould at one edge, this should allow the plaster to flow out evenly into the mould. Don't rush it or pour too rapidly, you want the mixture to flow into all the details without trapping any air bubbles (that would be bad). Pour the entire contents out into the mould until we have a depth of about an inch. Slightly more is no problem but if the plaster is too thin it may crack when we de-mould.
Now I would leave it alone for 3 or 4 hours to give it a chance to set off well.