Plaster: Making a Blank

If you've not worked with plaster much, figuring out how much water and plaster to use to pour a particular piece or item is more or less guesswork. Given that plaster isn't as cheap as it used to be (at this writing, Jan 2012, I'm shelling out 30¢/lb...that can add up), having a few rules of thumb can minimize wastage (when you mix up more than you need) and effort (when you don't mix enough) for the task at hand.

In this project, I want to pour a plaster blank that is going to be used in the design and fabrication of a 15" diameter stepping stone mold.

→ USEFUL & REQUIRED ITEMS: Here is a list of items I have sitting around that fall somewhere between useful and indispensable when the time comes to pour a plaster blank you intend to use in a modeling project for producing a silicon mold: [ + ]

• Clean water.
• Plaster.
• Calculator.
• Some items for creating a crude form:
  • Flat, impervious surface (e.g. masonite, formica covered board, etc.)
  • Strips of aluminum flashing (useful for creating round or curved forms). You can also employ various sizes of wood slats (not shown here).
  • Mold release (e.g. petroleum jelly, vegetable oil, etc.).
  • Moist clay: this is really useful for holding pieces in place as well as creating unusual shapes.
  • I like to use "binder clips" a lot too (e.g. for holding two ends of aluminum flashing together). Some type of tape would also work. I use a lot of cheap masking tape.
• Container(s) for mixing plaster (e.g. old butter containers, etc.). I've used cottage cheese containers, old milk jugs (with a portion of the top cut away) and for really large pieces a 5-gallon bucket is sometimes called for.
• Some type of "stirrer" (e.g. spoon, table knife, etc.).
• Measuring cup (or graduated cylinder). Strictly speaking, you could also use inexpensive beverage cups which come in various sizes (e.g. 3 oz., 5 oz, 9 oz, etc.) and can be picked up at any local big box store. You just need some way to measure out a specific volume of water (NOTE: this is what the calculator is going to be used for...i.e. calculating the amount of mixing water you'll need).