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%)
Follow the manufacturer's directions and specifications for best results.
Plasters are sold in full-bag sizes only. Please enter quantities in multiples of 50. (Example one bag = 50, two bags = 100)
How to figure out the amount of plaster needed for a pour...
Divide the cubic inches by 80, this will give you how many quarts of water needed. Each quart of water needs 2lbs 14oz of plaster to get a consistency of 70. A consistency of 70 is the normal standard for #1 pottery plaster.
Click here for general mixing directions and here for a USG Product Comparison Chart