Casting/Moldmaking with Plaster

 

Reasons to cast:

  • Exact copy
  • Multiples
  • Change of material

Terms to know:

  1. One piece mold
  2. Two piece mold
  3. Multiple piece
  4. Undercut
  5. Release agent-tincture of Green soap (Murphy’s oil soap)
  6. Keyway
  7. Splash coat

Mixing plaster

Plaster hardens by a chemical reaction with water. (Hardened plaster cannot be dissolved or softened by soaking it in water.) Once you start the chemical reaction by mixing you cannot add more plaster to the mix. It is important to get the right ratio of water to plaster. This is a weight ratio, but artists have come up with The Island/Continent Method which might be a helpful way of approaching it for you:

1) Estimate how much water it will take to fill your mold.  You actually only need 2/3 of that amount, but mold makers always mix a little extra.  Plaster is relatively inexpensive and the problems caused by running short are not worth the few cents saved.

2) Put the amount of cold water that you have so diligently estimated you need into your mixing bucket. Never put the plaster into the bucket first; always put the water in the bucket first, and then add plaster to the water.

3) Scoop up plaster with your hand. Wiggle your fingers and shake your hand to make the plaster fall through your fingers into the water in the bucket below.  This process prevents large clumps of plaster from falling into the water. Keep sifting the dry plaster into the water with your hand.  Work quickly and spread the plaster all around the surface, evenly.  Plaster should fall like gentle rain on the water surface, and then sink out of sight. You don’t want a lumpy plaster mix. Do not mix the wet plaster or disturb it in any way while sifting it in or this system of measurement will not work.  Keep sprinkling plaster into the undisturbed water.  Eventually you will notice little islands of plaster forming above the water surface.

4) Keep sifting the plaster in until you end up with one large continent (with no lakes or volcanoes) that does not sink. You now have the proper amount of plaster in the water and are ready to start mixing

Steps to make a two piece mold:

  1. Take 3/8” x 3/4”   strip of clay and apply completely around the object so as not to create an undercut in the mold. Apply the 3/8” edge to the object. This is the clay wall.
  2. Support clay wall with balls of clay.
  3. Put in keyways
  4. Apply release agent
  5. Mix plaster
  6. Apply splash coat, driving all air bubbles from surface while plaster is the consistency of pancake batter
  7. Wait 2-3 minutes until plaster sets up to whipped cream consistency and apply thick coat evenly over object half using clay wall as a guide for thickness (3/4”).
  8. Wait 10 minutes until plaster hardens and flip object over.
  9. Remove clay wall/ balls of clay and clean surface to be cast. Do not remove object from mold!
  10. Repeat above starting with #4
  11. Carefully pry mold halves apart