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The recommended shelf life for Plaster of Paris varies from 3 months to a year. I have some old stock that has been stored well-sealed, but contains a lot of clumps. The clumps can be easily crushed into powder with slight squeezing with your fingers. The powder still hardens when mixed with water, but I assume it won't have the same strength as fresh plaster since some percentage of it has already rehydrated to some degree. There are a number of options:

  • Toss it and buy fresh plaster.
  • Sift it. Use what is still powder and toss the clumps.
  • Crush the clumps, sift to ensure a fine powder, and use it all.
  • Re-calcine it in an oven (with crushing and sifting). This will return it to "fresh" Plaster of Paris (people recycle old castings this way). However, "casting plaster" typically has various additives. If those are affected by the heat, the resulting plaster may be more brittle or suffer from other shortcomings of "pure" PoP.
  • After sifting; crushing and sifting; or calcining, crushing, and sifting; mix it as filler with fresh plaster. Any shortcomings will degrade the fresh plaster a little, but there should be a reasonable mixing ratio where any degradation wouldn't be noticeable.

Using old or recycled plaster for cruder requirements, like making a support shell, is not a problem. As long as it hardens and doesn't break it's fine. Using it for fine castings is more demanding. I'm hoping for advice from people who routinely work with plaster who have tried some of these techniques. How necessary are they (how much, and what kinds of, degradation do you notice using old plaster in this condition), and do these techniques make a noticeable improvement in performance?

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Plaster of Paris is gypsum. It hardens by absorbing water to form a hydrate. Once that happens, crushing etc. has no effect. If you were able to seal it exceptionally well (not in a cardboard box), it should be good. A couple of websites say it can be dehydrated by heating above 350° F. That should restore original properties (by dehydrating it ).

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    Thanks. I can't tell whether your answer is to just not overthink it, or you might be missing some nuances. Let me probe a couple of points. 1. No matter how well it's sealed, it absorbs humidity every time the container is opened. Eventually, there's enough moisture for partial hardening of some of the material. If you break up those clumps, there's a mixture of hydrated, partially hydrated, and unhydrated material. The hydrated material acts like inert filler and the partially hydrated performs poorly. The mix will still harden, but won't be as good as fresh PoP. (cont'd)
    – fixer1234 ♦
    Nov 8, 2021 at 18:16
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    At the point where the clumps are still fragile and easily crushed (not solid rocks), is the material still close enough to fresh to be equivalent for practical purposes? 2. You can calcine fully hardened plaster back to PoP in an oven, so that will also work for partially hydrated material. However, casting plaster isn't pure PoP, it has additives to make it less brittle, control hardening speed, improve flow, etc. Will calcining affect or restore the additives?
    – fixer1234 ♦
    Nov 8, 2021 at 18:16
  • Yes, it picks up moisture every time the container is opened ; that hydrates some so the bulk is not quite as strong. So it will make a slightly weaker mix if there were lumps; it could still be satisfactory for some uses. Calcining should restore original properties; if it is pure gypsum with no additives . I expect additives could make a significant difference depending on what they are.
    – blacksmith37
    Nov 9, 2021 at 19:04
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    We're on the same page. The points in your comment are basically the premise of the question. I was hoping for more specific advice based on the observed state of the material and knowledge of the common additives, or experience routinely dealing with the issue.
    – fixer1234 ♦
    Nov 9, 2021 at 19:26

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