Plaster casting

Plaster casting is usually done using plaster of paris, but can also be done with fine domestic plaster (which dries harder and with a shinier finish).

The main things to bear in mind are:
 * 1) Clean the mould thoroughly before starting, remove all traces of old paster
 * 2) Mix small quantities at a time, plaster rapidly starts to go off though it takes much longer to set
 * 3) Add plaster to water, stirring thoroughly, never the other way round
 * Mix the plaster according to the instructions; this results in plaster of a creamy consistency, it should be possible to make a small peak in the surface but if it is anywhere close to solid then it is usually much too dry
 * As a rule of thumb the mixed plaster will usually be about 50% greater in volume than the water, so 100ml of water mixed up into plaster will be about 150ml (so don't fill your mixing pot more than half full!)


 * 1) Dampen the mould with wet water before adding the plaster
 * 2) Add plaster gently to the mould, using a spoon or some other implement to tap the inner face of the mould and release any air bubbles
 * 3) Continue tapping the mould as you add plaster, look for air bubbles rising to the surface
 * 4) Smooth off the top of the moulded shape using a metal or plastic rule - a piece of wood will tend to stick to the plaster
 * 5) For Linka World type moulds you can spray the surface with water and use a finger or spatula to check that the plaster is pressed firmly in place, this avoids tiny missed corners kin these very flat moulds

If the plaster is too dry, or you don't tap the mould, you will most likely lose detail and also get air bubbles on the surface. Using too dry a mix also results in a weaker moulding. Where you need the finished product to be very hard (e.g. if you will need to file and cut it) consider using builders' plaster instead of plaster of paris - small patching bags of fine finishing plaster are not expensive. Don't buy "patching plaster", it is coarser than plaster of paris.