Foundries bake plaster to make molds.
Plaster is usually baked in foundries in order to make molds for soft metals like aluminum. It is similar to sand casting except that plaster is used as the aggregate. The process isn't suitable for ferrous metals because the plaster starts to fall apart at about 2200 degrees Fahrenheit. The plaster used is a special mix called "casting plaster" that is about 50 percent sand, 42 percent plaster and 8 percent talc, magnesium oxide and glass fibers.
Instructions
1. Mix two parts "casting plaster" with one part water in a large bucket or other suitable container. Allow the mixture to sit for one minute.
2. Mix the plaster and water for three to five minutes. Mix small batches of plaster by hand. Large batches of plaster can be mixed in the same equipment used to mix dough in commercial bakeries.
3. Pour the plaster into a mold or mold box immediately after mixing. Allow the plaster to set for 25 minutes.
4. Remove the plaster from the mold or mold box and allow the plaster to air dry for no less than two hours. Set the plaster on a clean, dry surface.
5. Bake, or "burnout", the plaster in an oven at 350 degrees Fahrenheit for 16 hours.
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