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?