Plaster of Paris slow cure.

I need to reduce or slow the cure or set time of Plaster of Paris for some hobby work. I have heard that instead of using water, a mixture of water and glycerin (glycerol) may be used. Is this correct? What ratio should be used? If glycerin is not the best cure retardant what should be used?
Peter.
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In Home Depot there is a product in the drywall section designed to retard the set of patching type plaster. This should work but I don't know how much it effects other characteristics, strength etc. they also have a product to speed the set.

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PVR writes:

Carbohydrates like sugar and corn syrup, and citric acid, are common retarders. And gypsum (calcium sulfate hydrated), potassium bitartrate, and (quick) lime (calcium oxide). The dose is a few percent, but why not experiment to find what meets your goals?
USG's retarder product contains hydrolyzed keratin (sounds like gelatin) about 85 percent, hydrated lime 10 percent, and sodium hydroxide (lye) 5 percent.
See:
http://www.usg.com/navigate.do?resource=/USG_Marketing_Content/usg.com/web_files/products/prod_details/USG_Retarders.htm http://www.plastermaster.com/usg/plasterfundamentals.htm
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Depends how how much you need to reduce/slow of course.
I never use Plaster of Paris but have you tried chilled water that's been in the frig overnight? Seems to slow the set time of floor patch and leveler. Just a WAG.
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