I did this at TechShop Menlo Park. http://www.techshop.com
To save yourself a few dollars, you can re-use old plaster molds to make fresh plaster powder.
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Step 1: Find Cardboard And/or Thin Plastic Sheetss
Dig for whatever you can use to wrap the old plaster chunks and prevent the dust you'll be creating from flying around.
Step 2: Load Plaster Chunks
Load your set plaster pieces into your cardboard or plastic sheets.
Step 3: Make a Wrap
Wrap up plaster chunks into a nice secure wrap.
Step 4: Roll and Smash!
Anything big and heavy will work. I found a huge steal pipe and used it as a rolling pin.
Step 5: Banging Technique
Keep smashing as hard as you can. Watch your toes!
Step 6: Plaster Dust
Open your package to find plaster dust with some chunks. You can repeat steps to get it to its original powder form but you must mix it with fresh plaster mixture for mold making anyways and so the chunkiness won't matter.
This is a great way to get some anger out while simultaneously creating something useful and saving some money for your future art supplies.