Plaster can be mixed by the less precise volume or “volcano method” or the more precise mass method.

It is recommended to use the mass method unless you do not have access to a balance or scale. Using the mass method gives a much more consistent plaster mix than the volume method.

Personal Protective Equipment (PPE)

• Do not breath or expose yourself to plater dust or other airborne particulate matter.
• Use proper ventilation and a minimum of a N95 respirator when mixing and handling dry plaster.
• Place your buck of water on top of the scale inside the ventilated hood.
• Place the bag of dry plaster next the the ventilated hood and directly under the ventilated snorkel. This helps capture the plaster dust directly at the source.
• Keep the ventilation on and your N95 respirator on while dry plaster is being used in the room. Even if you are not using the dry plaster, if another person is, or if some has just recently used dry plaster, you should keep your N95 respirator on. Keep it on until all plater particulates have been removed from the area.

Process

1. Calculate the volume of the space to fill with plaster.
2. Use a plaster calculator to determine the amount of water and plaster needed for the calculated volume. A chart for common amounts of plaster is included below.
3. Record the plaster mix ratio and amounts in your notes so you can recall them in the future. It is important for all parts of the mold to have the same density / mix of plaster.
4. Place a scale inside the ventilated hood.
5. Place your bucket or other suitable container. on the scale.
6. Tare the scale to zero to take into account the mass of the bucket.
7. Add the correct amount of water to a bucket
8. Make sure the water is not cold or hot. It should be lukewarm or room temperature or approximately 70°F or 21°C.
9. Plaster is always added to water. Do not add water to plaster or you will have a terrible mix with lots of inconsistent lumps of plaster that will likely ruin your cast form.
10. Slowly sift the plaster into the water. Do not dump the plaster. The goal is to have each particle of plaster to individually come into contact with the water to begin the hydration process. If large clumps of plaster are dumped into the water too quickly, then the plaster on the inside of the clump will not be fully hydrated and the mix will be bad.
11. Allow the plaster to slake in the water for 5-15 minutes depending on the water temperature.
12. Mix the plaster by hand for 3-5 minutes. You can also use a mechanical mixer attached to a corded drill but this will cause the plaster to set up faster by introducing more energy into the plaster.
13. Once the plaster is mixed, it is ready to pour into a mold or for its intended use.

Clean Up

Make sure to clean up plaster containers, tools, and work surfaces before the plaster sets up.

• Dispose of excess plaster in cardboard boxes. Allow the plaster to set up and then dispose of the cardboard box in an appropriate trash can.
• Scrape out all plaster into the cardboard box before it sets up.
• Rinse buckets and tools in large tubs of water to remove remaining plaster after scraping of big parts. This allows the plaster to settle in the bottom of the tub of water rather than settling in the plumbing pipes.
• After rinsing tools clean in the waste water tubs, then do a final rinse in the slop sink in the studio.
• Leave tools and buckets out to dry. Do not stack buckets before drying.
• Wipe down all surfaces and clean up all plaster debris.

Troubleshooting

• Standing water on top of your plaster cast after pouring. This is a telltale sign of not fully mixed or hydrated plaster. It could also be an indication fo an incorrect plaster to water ratio.
• Bubbles or voids on the surface of the cast next to the cast object. Pouring the plaster too quickly can trap air so pour slowly. Be careful mixing the plaster to not introduce air. Pouring the plaster after it starts to set up will cause air pockets and voids