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Plaster and Gypsum Cement Products, Properties, and Mixing Instructions.

•     U.S. Gypsum Company offers several products useful to Ceramic artists:
  •   Duramold™ Pottery Plaster – A long-lasting, durable gravity slip casting mold.
  •   Puritan® Pottery Plaster – A hard, wear-resistant plaster mold material, recommended for jiggering.
  •   No. 1 Casting Plaster – Similar to White Art Plaster, except it mixes at a lower consistency for greater strength.
  •   No. 1 Pottery Plaster – Used for dinnerware slip casting ceramic molds.
  •   Moulding Plaster – A general purpose casting plaster used when expansion, hardness and strength are not important.
  •   White Art Plaster – Similar to Moulding Plaster, features a surface hardening agent.
  •   Pottery Plaster – A general purpose product recommended for more absorbent, slip casting pottery molds.
  •   Hydro-Stone Gypsum Cement - One of the hardest and strongest gypsum cements.  Resistant to water absorption.
  •   Statuary Hydrocal Gypsum Cement - Harder and stronger than standard Hydrocal White gypsum cement.
  •   Ceramical Gypsum Cement - A low-consistency, smooth-wearing, low absorption mold material, suitable for RAM processes.
  •   Ultracal 30 Gypsum Cement - A high-density, high-strength, and low absorption gypsum cement made for close tolerance tooling.
  •   Hydrocal White Gypsum Cement - A neutral, high-strength gypsum cement.  Can be carved or added to.

•   MIXING INSTRUCTIONS
  •   Consistency is the number of pounds of water combined with 100 pounds of material.
  •   Weigh clean, room temperature water in a clean container. Allow space for addition of material.
  •   Weigh correct corresponding amount of plaster or gypsum cement.
  •   Sprinkle the plaster or gypsum into the water.  Allow material to sink to avoid adding lumps of material.
  •   After adding all the plaster or gypsum, allow several minutes while the material soaks up water.
  •   Mix using motions that do not introduce air into mixture, either manually or with an electric drill attachment at slow RPMs.
  •   Final mix consistency should be like heavy dairy cream when poured.
•   CLEAN-UP
  •   Cleaning material from tools and buckets is easiest if material has not hardened.  Power spray with water.
  •   Absolutely, do not allow any material to enter plumbing drains.
  •   Do not allow material to contaminate clay.  Plaster absorbs water and can cause ware to explode during firing.
•   CONVERSIONS:
•   1 cubic foot = 7.481 gallons
•   1 gallon of water = 8.345 pounds
•   1 pound of water = 0.12 gallons = 0.016 cubic feet

NOTE: Calculating the amount of necessary material and water on the basis of volume is tricky.  Hydrocal White will yield about 50% more volume than the water after the addition of the material.  I like to mix more than I will need, and typically will have an extra mould available (such as a plaster batt or plastic storage container) for the excess.

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