Pottery Plaster

Pottery Plaster can be poured into moulds, used like Modroc when muslin is dipped into it, carved and applied to surfaces to create a textured finish. When it is mixed with water it will change from a pourable consistency into a hard solid when dry. During this change the plaster heats up. Various materials can be added to the plaster to modify its behaviour, e.g., salt to speed setting, wallpaper paste to slow setting. Aggregates can also be added to make the mixture occupy a greater volume, e.g., sawdust, sand, vermiculite.

To work out how much plaster you need:
1. Determine the volume of the mould you are using by pouring water from a mesuring jug, keeping track of the amount it takes.
2. Use the following formula to work out how much pottery plaster is required: volume in litres x 1.18 = amount of pottery plaster in kg (e.g., if the volume of a mould is 0.4 litres, you will need approximately 0.5kg of pottery plaster: 0.4 litres x 1.18 = 0.472 kg).
Please note that the weight refers to pottery plaster in dry powder form, not pouring form (i.e., before mixing with water).

This dry white powder must be stored in such a manner that water cannot come into contact with it.  

Always wear a dust mask when handling dry powder. You may choose to wear gloves. 

 Method: Using Pottery Plaster

Step One
Dry powder is always added to the water.  Always keep one hand dry. It is best to work on top of newspaper or a plastic covered surface.

Step Two
Put the required amount of water into a plastic bucket or ice cream container. Make sure there is enough room in the container to add the plaster as the mixture will increase in volume. Warm water will speed up the setting time, which is an advantage when sand casts are being made.
Step Three
Sprinkle the powder onto the surface of the water. DO NOT STIR YET.

Step Four
Keep adding powder until the “hill” which forms in the centre of the water ceases to sink under the surface. This is the correct proportion of water to plaster. Wait until this entire “hill” is dampened by the water seeping up. 
Step Five
Stir the mixture with one hand, carefully in such a way that air bubbles are not created. Upon removing your hand, it should be coated with the mixture.
Step Six
When the mixture is ready, lift the container and gently bang it down onto the bench or floor a few times to force any air bubbles upwards. It should now be ready to pour for whatever purpose is intended.
Working time: 10 minutes until it starts to knife-set harden.
Step Seven
Drying time depends on weather and thickness of plaster. Will dry in approximately 3 hours in ideal conditions. Leave plaster to set for at least 24 hours before removing from mould.

There are many different ways you can create a mould to pour your plaster mixture into. A cardboard box with its joins sealed with strong tape is an inexpensive option.

WARNING: DO NOT attempt to make an enclosed cast of any body parts such as hands or fingers. This product gives off heat when mixed with water for setting, which may result in serious skin irritations if used for casting body parts.

Modroc Substitute
If you are working on a large scale project, Hessian strips can be dipped in your mixture and applied to an armature like Modroc.

Textured Surfaces
Before plaster sets it reaches a whip cream stage, where it is easily spread over surfaces. If you would like to create texture on a Modroc piece you can apply the mixture at this stage. 

Plaster that has been cast can then be carved into. Chisels, rasps, rifflers and sandpapers can be used to modify the form. Wear a dusk mask while carving. If you keep the plaster wrapped up in plastic, it will stay moist and the shavings will not be airborne. When it dries, the dust created will be finer and you can use sandpaper to refine your form.

Cleaning up
WARNING: Do not allow unset plaster to be washed down any drain; never allow children to wash their hands at the sink, an accumulation of plaster sediment will block drains very quickly.  Allow children to wash their hands and equipment in a plastic bucket. The sediment will settle and harden or gel, then the water may be poured off, and the plaster residue disposed of as solid waste. Plastic mixing containers can be cleaned by allowing the plaster to set hard.  It falls out easily when the container is tapped vigorously. Plastic Sheeting may be reused and cleaned in the same way.

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