Picture of How to pulverize set plaster for re-use
I did this at TechShop Menlo Park.

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

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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

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Load your set plaster pieces into your cardboard or plastic sheets.

Step 3: Make a wrap

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Wrap up plaster chunks into a nice secure wrap.

Step 4: Roll and smash!

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Anything big and heavy will work. I found a huge steal pipe and used it as a rolling pin.

Step 5: Banging technique

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Keep smashing as hard as you can. Watch your toes!

Step 6: Plaster dust

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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.

Landfill sites refuse drywall and plaster because as it decomposes, hydrogen sulfide gas is released. So, re-use it by crushing it and reheating it in a 325 degree F oven overnight, to drive out the hydrated water.

how long does the plaster have to be in the oven for?

Hours and hours. 6 to 8 of them. It's a reminder why cements and plasters are environmentally costly.

I built a solar oven to do the job. One small batch of plaster at a time, but no energy cost.

sethcim2 years ago
It will also accelerate the set of fresh plaster, which may be convenient or not.
n1cod3mus2 years ago
I have just had a look at your other instructables and I see you have made a furnace for lost wax casting, you might be intrested in what I want the brick grog for, I'm doing an instructable for making your own crucibles might be useful for you ;-)
n1cod3mus2 years ago
further to my earlier comment, I took an old shirt and ripped it so it was square wrapped up some old pieces of garden pot and smashed away with my club hammer and it worked great.

so the answer is yes this can be used to make your own grog, hard work with a club hammer though, but it works.
n1cod3mus2 years ago
this seems like a good method for making your own grog for ceramics from brick or old garden pots.

once its all smashed up you could pop it in to a ball mill for a finer powder
Mr. E Meat2 years ago
Don't you need to heat it to reactivate it? If I recall, it's to about 350F...
yes, to make it back into usable plaster you would need to heat it to extreme temperature to drive out all the water, even that chemically bonded to the gypsum. I don't know the temp. What he is doing here is adding it to new plaster, basically as an inert filler.
scoochmaroo2 years ago
Brilliant! I never knew you could do that.