Plaster Consistency - Suppy Chain issues

[Jeff Longtin]

Just to give everyone a heads up: over the last two years plaster product consistency has really been eratic. At my day job I pour 2-4 Ram press molds a day. I use USG Ceramical. I order 400 lbs. every 4-5 weeks. This latest batch, dated 11-5-2022 (bagging date) is inconsistent. It mixes really thick. It's set time is somewhat normal, 15-20 minutes, but it mixes thick. (I use a scale to measure out my water and powder and a timer to time the process.)

The last few batches of Ceramical, before this, were normal. About a year ago, however, I went through several batches that set up much more quickly than I expected. 

In my own studio, where I mostly pour pottery plaster, GP K60, I've had the same experience. I don't order as often, 200 lbs. every 4-6months, but the same issues have occured. New bags that should be smooth and mix easily, are full of lumps and set quickly. It's been frustrating but I'm just happy I can actually get product these days as supply chain issues have sometimes delayed production.

[neilestrick]

@Jeff Longtin Thanks for the heads up! Are there any issues with the finished molds that come with these differences in thickness and setting times?

[oldlady]

i do not use plaster often.  i just found a bucket that has plaster in it.  dumb  question, can plaster be dried out sufficiently to work again if it is heated to remove water?  i have a few dixie cups that i carved into stamps from years ago.  can i make more or is it a waste of time?

 

 i have no plans to use this very old plaster except in the yard.   any suggestions for that use?

[Mark C.]

Plaster was hard to get last year around here as well.

[Jeff Longtin]

17 hours ago, neilestrick said:

Are there any issues with the finished molds that come with these differences in thickness and setting times?

Surprisingly, no. One mold (medallion mold) had one small bubble, in one small letter, so I remade it, but otherwise the molds themselves behave as normal. 

[Jeff Longtin]

8 hours ago, oldlady said:

Can plaster be dried out sufficiently to work again if it is heated to remove water?  

I haven't heated plaster, to revive it, but I have tried to calcine kaolin and ended up with a hard lump of kaolin. (THEN I found out it was easily available in powder form.)

[PeterH]

10 hours ago, oldlady said:

dumb  question, can plaster be dried out sufficiently to work again if it is heated to remove water? 

Personally I doubt it's practical, but this at least tells you not to overheat it (see the last sentence in the quote).
Might be worth a try in a domestic oven.
https://en.wikipedia.org/wiki/Plaster

Chemistry

See also: Calcium sulfate § Hydration and dehydration reactions

Gypsum plaster, gypsum powder, or plaster of Paris, is produced by heating gypsum to about 120–180 °C (248–356 °F) in a kiln:^[18][13]

 

CaSO 4 ⋅ 2 H 2 O ⟶ heat CaSO 4 ⋅ 1 2 H 2 O + 1 1 2 H 2 O ↑ {\displaystyle {\ce {CaSO4.2H2O {\overset {heat}{{}->{}}}{CaSO4.1/2H2O}+ 1\!1/2 H2O ^}}} (released as steam).

Plaster of Paris has a remarkable property of setting into a hard mass on wetting with water.

CaSO4⋅12H2O+112H2O⟶CaSO4⋅2H2O{\displaystyle {\ce {CaSO4.1/2H2O + 1 1/2H2O -> CaSO4.2H2O}}}

Plaster of Paris is stored in moisture-proof containers, because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration, which will make it useless after some time.^[19]

When the dry plaster powder is mixed with water, it rehydrates over time into gypsum. The setting of plaster slurry starts about 10 minutes after mixing and is complete in about 45 minutes. The setting of plaster of Paris is accompanied by a slight expansion of volume. It is used in making casts for statues, toys, and more.^[19] The initial matrix consists mostly of orthorhombic crystals: the kinetic product. Over the next 72 hours, the rhombic crystals give way to an interlocking mass of monoclinic crystal needles, and the plaster increases in hardness and strength.^[20] If plaster or gypsum is heated to between 130 °C (266 °F) and 180 °C (350°F), hemihydrate is formed, which will also re-form as gypsum if mixed with water.^[21][22]

On heating to 180 °C (350°F), the nearly water-free form, called γ-anhydrite (CaSO₄·nH₂O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C (480°F), the completely anhydrous form called β-anhydrite or dead burned plaster is formed.^[19][22]