How To Cast Into Latex Plaster Moulds
PREPARING PLASTER MOULDS FOR CASTING
Rinse the mould out with soapy water to lower the surface tension and shake dry before securing ready for casting.
SUPPORTING THE PLASTER MOULDS FOR CASTING
Cut out an opening in a corrugated cardboard box using scissors or a bread knife. The plaster moulds can then be inserted into these openings and secured with pins. The flange that surrounds the base of most latex plaster moulds will support the mould during casting. Use a few pins through the base into the cardboard if considered necessary to secure the mould.
MIXING THE PLASTER
Fill all plaster moulds with water and then tip it into the mixing bowl to find out exactly how much water is needed for your plaster mix. To ensure the plaster doesn't set too quickly use normal tap water at room temperature. Warm water will speed up the setting time. Only fill the bowl or bucket half full to allow easy mixing.
Slowly sift the plaster into the water. IT IS IMPORTANT TO REMEMBER TO ALWAYS ADD PLASTER TO THE WATER AND NEVER WATER TO THE PLASTER. Keep sifting the plaster over the water until it has completely filled the water and a dry mound of about 2 to 3 cm has formed above the water line.
Leave the mixture for 2 to 3 minutes so that the plaster can absorb all the water then start mixing with a spatula. To avoid beating air into the plaster mix slowly. To obtain a smooth creamy mix 30 seconds of mixing is all that is needed. Mixing it for too long can cause premature setting of the plaster and not give you a chance to pour it into the mould.
CASTING THE PLASTER MOULDS
Pour the plaster into the mould until 1/3 full. Give the mould a series of taps and light squeezes to ensure that all the difficult areas such as undercuts, areas of fine detail, faces,hands etc are filled with the liquid plaster.
This will ensure that no important detail in the mould will come out uncast and remove the chance of airbubbles.
Now fill the plaster mould until it reaches the top, giving a final series of taps to level the base and ensure that all air bubbles have risen to the surface.
The setting time for the plaster should be around 30 minutes and will generate a lot of heat. To check if the plaster moulds are ready for stripping from the plaster castings run your finger nail across the base. It should feel firm and dry.
REMOVAL FROM THE MOULD
When the castings have set take the plaster moulds out of the carboard box support and wipe the outsides of the moulds with liquid soap or dishwashing liquid.
Now you can peel the rubber mould off the casting carefully and firmly. It should peel off quite easily with the aid of the liquid soap. If you pull against the figure you could break off an arm or leg etc. So choose the path of least resistance.
Finish the casting off by trimming with a knife and sanding the base level. Any imperfections such as airbubbles and pinholes can be filled in with a bit more plaster mix smoothing it over with a little water.
The casting should be allowed to dry for several days in the sun. If you require more rapid drying a heater can be used to speed up drying to a day. Don't overheat the casting as this could case damage.
The article is now ready for decorating.