iandol on sat 29 sep 01
Dear Tim Pozza,
No, a deflocculant would not work with Calcium Sulphate Plaster.
You need to understand the Chemistry of Calcium Sulphate. This is mined =
as two distinct minerals, Gypsum and Anhydrite. Gypsum has two molecules =
of water of crystallisation in its composition, Anhydrite has none. =
Plaster is made by heating gypsum to remove part of the water of =
crystallisation, about 3/4s, making it a Hemi-hydrate. It is then ground =
to a powder.
Any contact with moisture causes the reverse chemical reaction to take =
place and if there is an excess of water recrystallisation starts and =
interlocking crystals grow. This is the setting process.
Allow your buckets to dry and crack the excess off, or immediately put =
water into them after you have poured your plaster and let them stand. =
The set plaster should flake away. discard in the garden (great soil =
conditioner, breaks down heavy clay), not down the sink.
For a better treatment of Plater of Paris, read Jack C. Rich, "The =
Materials and Methods of Sculpture". Oxford Books. 1973. Two and a half =
columns in the index.
Best regards,
Ivor Lewis. Redhill. South Australia.
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