Baking Plaster of Paris

[Gil]

Recently I dug out a five pound box of Plaster of Paris (PoP) that's been stored for over a year for a quick mold making session. The PoP failed to set right and ended up soft and soggy. I realized that water vapor had penetrated the material even though it was stored in a warm dry place.

The solution is fairly simple, just bake the PoP at 15O deg.C (~300 deg.F) for an hour to an hour and a half stirring every 30 minutes to aid the release of internal moisture. Sift the cooled product back into an airtight container.

This process is called calcining and it literally boils or cooks off the water content (hydrate) of the Calcium Sulfate (PoP). This is exactly the same process used on the mined mineral gypsum to make Plaster of Paris.

The cardboard boxes that PoP comes in at hardware stores have variable shelf lives depending on humidity levels. This can lead to unreliable results form brand new boxes. Baking a brand new box is not a bad idea before storing it in an airtight container...,

+Gil

[rmks2000]

Great tip! I usually store my PoP in my basement and it can get damp there in the Springtime. This will ensure that it stays fresh.