How to Make Extra Strong Plaster of Paris for Casting

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Plaster casting is a useful art form used for all types of crafts, sculptures or modeling, but a basic plaster of Paris mixture is quite fragile; strengthening it with glue creates a plaster that withstands the test of time. Plaster of Paris is created from fine ground gypsum that has been heated to 160 degrees. The heating process removes the water and creates a powder that has been used by artists and builders for their projects for centuries.

Basic Plaster and Glue Recipe

This basic plaster and glue recipe is for small plaster casting projects. This mixture is durable enough to be sanded with fine grit sandpaper. Use this basic recipe as a foundation for larger projects. Make sure to keep the ingredient ratios the same.

Things You'll Need

  • Newsprint or old newspapers
  • 8-ounce measuring cup
  • 1/4-cup measuring cup
  • Plastic mixing bowl
  • Water
  • White glue
  • Wooden spoon 
  • Plaster
  • Face mask
  • Gloves

Mixing Plaster

Step 1: Prepare the Work Area

Cover you work area with a layer of newsprint to prevent the plaster from sticking to important surfaces.

Step 2: Mix Water and Glue

Place 1 1/4-cup water in a plastic mixing bowl. Add 1/4-cup white glue to the water. Mix the water and glue until the glue is thoroughly incorporated into the water. The water should appear milky and have a creamy consistency.

Step 3: Add the Plaster

Slowly pour 2 cups of plaster of Paris into the water. Add a small amount of plaster at a time. Sprinkle the plaster evenly across the surface of the water and glue mixture. Allow the plaster to settle to the bottom, and do not stir. Stirring at this point may cause excessive air bubbles that will weaken the plaster. Continue to add plaster of Paris until you’ve used all 2 cups.

Step 4: Rest and Mix

Let the plaster rest for about five minutes before mixing. Use the plaster mixture immediately.

Tip

  • Using more plaster in your plaster mix creates harder denser molds. Using less plaster creates more porous and brittle molds.

    Wear a face mask if the fine dust from Plaster of Paris affects you. Wear vinyl gloves to protect your hands.

Adjusting Drying Times

  • Speed up the drying time by adding a small amount of pure gypsum to the mix.
  • Add a couple of drops of lemon juice to slow the drying time.
  • Use warm water to speed up the setting time.

Clean Up

Cleaning as you go makes the process that much easier. Wash mixing utensils and bowls with warm water and soap. Never pour leftover plaster of Paris down the drain. This can clog pipes and become a costly mistake once it hardens. If plaster hardens on a surface, carefully break or chip the hardened plaster and clean the area with soap and water.

Warning

  • Mixing plaster of Paris with water causes an exothermic reaction. Never place plaster of Paris onto skin. It can cause serious burns. If it gets onto your skin wipe the wet plaster away with a dry paper towel and clean the area with a wet paper towel and soap.

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