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Need Info On Using Old Plaster

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Is there a way to mix a small amount of plaster to see if it is still good to use for molds ? I have a bag that has been wrapped in plastic, unopened, for 2 years. All I know about plaster involves a complex formula to make a large amount. Seems like someone on this board said something to the effect of.." I'm always mixing up a bit in a cup to make a small sprig or something."

Was that Benzine?

 

I would appreciate advice on how to tell if the old, but unused bag is still good, it has been kept in heavy plastic bags since it was purchased, and also, HOW TO MIX a small amount to use a little bit now and then, not a big day's project.

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There are formulas for mixing on U.S. Gypsum website. Go there, look for the type of plaster you have and scale down the recipe to a small amount. I keep plaster in this muggy environment for a year or three. It is wrapped in a plastic bad and stored in a closed plastic container.

Marcia

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or do it the old way, put some water in a container, add plaster, slowly sprinkling it into the water until it forms an island when the plaster stops soaking into the water. 

 

you will be told that plaster over 6 months old is no good.  you may choose to believe it................or not.

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I've used plaster that's over five years old and the only difference I found was that it took longer to cure.  I keep it in it's original bag in a galvanized garbage can in a room away from moisture (I live in a fairly dry climate).

 

If the powder is lumpy or anything coarser than flour, I'd dump it, as it has absorbed moisture to some extent and in all probability will not cure (set up).

 

I'm with oldlady on how to mix.  Use about the amount of water equal to how much finished plaster you want, and sift the dry plaster onto the water until an island of plaster is showing.  Then--with your hand--gently mix the plaster into the water.  Try to avoid making bubbles as this weakens the end result.  NEVER use a whisk or briskly agitate the mixture.  If you are checking to see if the plaster is still usable, I'd start the mix with one cup of water and go from there.  If it hasn't pretty well set up in 15 minutes it might not be usable.  You should be able to feel heat from the plaster--if there's no heat, the plaster should be thrown away and you need to  buy some new.  

 

I know many people on the forums use pottery plaster, or dental plaster  and consider those the only ones fit for their work.  Unless I am trying to replicate something with really fine detail, I buy my plaster at Lowe's or Home Depot and it works just fine.  Even if I need detail, I might put the first two thin coats of plaster using the expensive stuff, but then use cheese cloth and my industrial strength plaster to finish out a mold.  No problems so far.

 

Just my thoughts,    Shirley

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Thanks for all your input.  I will try what Shirley describes and see what I get.  The plaster in the bag is pretty much like flour, with some small rice grain size bits that break up when I squeeze them between my fingers. If that amount seems viable, then I will use the ratios given on the US Gypsum site.

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