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FAQ

Plaster

Understanding Plaster
Plaster comes from the mineral gypsum. Technically, calcium sulfate with water molecules locked into its crystal structure. Gypsum is mined, then purified and pulverized until it's as fine as flour. It's then calcined (baked) at about 240 F to drive off some of the water molecules. From that moment on, the gypsum tries to regain the water it lost by absorbing it from humidity in the air.
Plaster can be combined with other ingredients by the manufacturer to produce substances with differing degrees of strength. These gypsum cements have names such as hydrostone, ceramical, and ultracal. There are about 30 different formulations available to meet a variety of needs. For absorbent molds, potters generally use No.1 Pottery Plaster; for more durable (and less absorbent) forms, the gypsum cements are preferable. Each of these materials requires a different plaster-to-water ratio (e.g. pottery plaster uses 70% water, while ceramical uses 40%) Use the following method to mix and pour pottery plaster. This procedure differs for the gypsum cements. Follow the manufacturer's directions and specifications for best results.

Directions for mixing No. 1 Pottery Plaster:

Step 1. Adding plaster to water
Always add plaster to water, not the other way around. Water/Plaster ratios: By weight: 7 to 10. Add ten pounds of plaster to seven pounds of water. Calculate water needed for other amounts by multiplying plaster weight by 0.7. By volume: Add 2-3/4 lbs of plaster to a quart of water.
Use cool or room temperature plaster and water. Colder water may improve the strength of the plaster. It also slows the setting process. Never use hot water.

Step 2. Slaking, or resting stage
Let the plaster sit undisturbed in the water for one to two minutes. (For less than 5 pounds of plaster, a minute may be enough; for larger amounts, wait two minutes.)

Step 3. Mixing
Mix the plaster by hand or with a power mixer. Mix smaller amounts (under 10lbs of plaster) for exactly two minutes. Mix larger amounts for a longer time (e.g. 50lbs. for 4 minutes). Run your mixer at the highest speed that doesn't trap air.

Step 4. Wait until the plaster is ready to pour
Don't pour the plaster right away. Take a minute to gently stir the plaster and bump the bucket on the floor, or slap it vigorously on the side to encourage air bubbles to rise to the surface. Carefully feel the resistance to stirring, and watch the shiny surface of the plaster. In a minute or two (sometimes more with very fresh plaster) you will detect a slight increase in the viscosity of the plaster and a lessening of the surface shine. Now is the moment to pour. Gently slap the sides of the mold to settle the plaster, or shake the table from side to side a few minutes.

Step 5. Setting up
The plaster will gradually harden, release heat, and expand slightly (about 1%). In 25 to 35 minutes the plaster will feel very warm and hard. Remove the plaster from the mold or forms at this point.

Step 6. Drying the mold
Don't use your plaster mold until it is completely dry. This will take a week or more, depending on the thickness. You may speed up the process by setting the plaster in an airy, warm spot. Note: Do not put plaster in an oven or kiln to dry it. Temperatures above 125 F will damage the plaster.

For more information on mold making, Try "The Clay Lover's Guide to Making Molds: Designing,Making, Using" by Lark Books. 128pgs PB. Available by special order only. Specify item code BK-CLMM