Tom Buck on sun 31 mar 96
Ron Roy's query about reclaiming old (degraded?) plaster touched
off a data search that I was planning to do since I have some US #1
industrial grade plaster that needs to be proven as good; and I was
doing this when Louis Katz posted a reply to Ron's query.
Louis Katz says aged Plaster of Paris can be reclaimed by
calcining the material at 350 F. This is a bit high, especially if
one leaves it in the "oven" (kiln?) too long. To make this clear,
here is a bit of industrial chemistry:
There are three forms of Calcium Sulfate (Sulphate)
commerically available. The anhydrous (water-free) form, CaSO4,
does occur in nature and is called "anhydrite". Another natural
rock is CaSO4.2H20, calcium sulfate dihydrate, more often called
"gypsum". The third form is 2CaSO4.H2O, or calcium sulfate
half-hydrate (hemi-hydrate), a manufactured material more commonly
called Plaster of Paris, the plaster familiar to most potters.
Both gypsum and anhydrite are mined in many countries, usually
from many local deposts. The output of gypsum is many times that of
anhydrite. All Plaster of Paris is produced from gypsum, as is some
anhydrite. Plaster of Paris (2CaSO4.H2O) is used chiefly to produce
plaster wallboard.
To make Plaster of Paris, producers mine gypsum rock, grind
and pulversize it to 100 mesh (70-90% through) and then heat it to
128 deg C + (260-300 F). As long as the powder itself doesn't reach
163 C (325 F) the product will be Plaster of Paris (hemi-hydrate).
If the temperature goes above that, the product will be anhydrite,
a form that does not undergo hardening into a solid when mixed with
water.
US Gypsum's No.1 grade industrial plaster, as Louis Katz
noted, may have one or two additives. One is there to adsorb
moisture (coming from the air) and thusly slow the unwanted setting
of plaster on the shelf. Such dessicants may not regenerate on
heating. A 2nd additive may be there to reduce the rate of plaster
set to give the user more time to work the piece; and this material
may degrade on being heated.
So, with this in mind, Ron Roy's query can be answered. Yes,
it is possible to drive off some trapped moisture from plaster too
long on the shelf. But the result may not behave like plaster fresh
from the manufacturer. And besides, the energy costs might not
warrant it ... a new bag of plaster costs perhaps US$ 15-20 per 50
lb (22.6 kg). Cheers. Tom.Buck
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