How to Mix Plaster for Sculpture

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Mixing plaster with a "rule of thumb" method doesn't require precise measurements. Sculptors typically use this method to mix small to moderate amounts of plaster (less than five gallons/22 liters) for molding and casting. The same instructions apply to all standard gypsum plaster products, such as Plaster of Paris, Hydrocal, Densite, etc.

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Steps

1. Estimate the amount of mixed plaster and silica your project requires. Remember it is 1/3 plaster, 1/3 silica flour and 1/3 water. Experience is the best guide here, so as a beginner you will just have to make your best guess, then mix extra to be sure you have enough. Use commonsense.
2. Premix plaster and silica flour.
3. Pour clean, lukewarm water into an empty, flexible mixing container. A a standard two-gallon (7.5 liter) plastic bucket is a good container. The amount of water should be approximately one third the total amount of mixed plaster/silica you estimated in Step 1.
4. Add dry plaster and silica to the water. Gradually pick up handfuls and sift the powder through your fingers. This will break up any clumps, letting the powder fall into the water. Work quickly, but avoid dumping the plaster into the water. Don't stir or mix the combined water and plaster/silica.
5. Continue sifting plaster/silica into the water. Watch for it to start sinking slowly. Eventually some of the powder will stay on top of the water. As you add more plaster, distribute it to areas that still have water on top.
6. Stop adding plaster/silica when there is no more standing water in the bucket. The surface of the combined water and plaster/silica should be mostly grayish in color, with some areas of white dry powder. Don't mix it yet!
7. Let the bucket sit for a few minutes. Let it stand during the time that you make any final preparations for your project. If you are molding or casting with the plaster, this is a good time to double-check that you have applied the proper release agent to your pattern or mold.
8. NEVER mix the plaster with your hands. Plaster reaches a high temperature as it reacts chemically with the water and can cause serious burns!. Use a wooden spoon or similar instrument as one would use an egg beater: Reach down to the bottom of the container and use a side-to-side movement like an exaggerated "hello" wave.
9. Try to eliminate any lumps and break them up. When thoroughly mixed, the plaster is ready to use in your project.

Video

Tips

• The water temperature makes a difference. Hot water will speed up the setting of the plaster; cold water will slow it down. Depending on your project, you may want to use either hotter water for faster turnaround when pouring molds or cooler water for a slower process.
• Plaster tends to leach moisture from skin. You may want to use hand lotion after mixing. Almond oil is excellent, as are your normal moisturizers.
• The easiest way to clean up unused plaster is to let it harden in the mixing container. It can then be popped out into a trash can easily by turning it upside down and hitting the bottom and sides of the container with your hand (hence the importance of using a flexible bucket).
• You can use any movement you like for mixing the water and plaster, but avoid whipping air bubbles into the mix. Bubbles can be detrimental to the surface of your mold or casting.

Warnings

• Always wear a dust mask as the inhaled plaster dust particles will mix with fluid in your lungs and harden there. This is very dangerous and easily avoided.
• NEVER use hands to mix plaster or apply plaster to set directly on any body parts or skin- serious burns have occurred resulting in amputation of fingers and limbs.
• Never pour plaster down a sink or other drain. It can solidify and ruin the plumbing. Dispose of unused plaster in the trash. Rinse wet plaster off hands in a bucket of water before washing them in the sink.
• Avoid getting plaster on clothing or other items of value. It can be very difficult to remove from fabric and other porous surfaces. However, if there is an accident and it spills on fabric, it's worth trying to remove it. Don't automatically deem any soiled item ruined.

Things You'll Need

• Plaster (dry powder)
• Source of clean water
• Plastic bucket or other flexible mixing container
• Project needing mixed plaster

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