Plaster is made from the mineral gypsum, which is called Calcium Sulphate
by chemists (Gypsum in the ground is calcium sulphate dihydrate, after
calcining, it becomes calcium sulphate hemihydrate, also known as hydrous
calcium sulphate.) It is often called plaster of Paris, because of the
large deposits of pure gypsum underlying the French capitol, which were
utilized early on by local artisans. When heated, the mineral loses some
water which is chemically bound into it and gives it its hardness. After
it is cooked ("calcined"), it is easily reduced to a powder, but
the addition of water will reconstitute its original hardness, once the
material sets.
This remarkable material is stored as a powder, and is mixed with ordinary
water into a liquid that gets gradually thicker and thicker until it
becomes plastic, then a paste, then a cheese-bodied mass; finally turning
rock hard in about an hour. Unlike practically any other compound, when
plaster turns from liquid to solid it does not shrink, rather, it EXPANDS
ever so slightly as it forms crystals. It can be carved, sanded, drilled,
cut, textured, added to, reinforced and remain incredibly strong
throughout. Because it expands, plaster castings do not lose any detail,
and a mold with a glass smooth surface will result in a glass smooth
plaster casting.
Plaster absorbs water and can therefore be
used to make molds for casting porcelain and ceramic slipware as well as
latex forms such as those aliens and monsters used in the Star Wars
movies. Plaster is used extensively in the manufacture of pottery, and
plaster is used to cover the walls in quality homes. Plaster can be "screeded"
with a template to form straight and curved patterns with any
cross-sectional contour. At one time, virtually all decorative moldings in
houses were of plaster and, of course, plaster is the principal material
of pattern making and foundry processes. In moldmaking, plaster is used to
make mold casings (mother molds), waste molds, casting and retouching
masters.
While plaster can be cast into plaster molds if
they are sealed and well-soaped, it works best in flexible molds. These
were originally made from animal gelatin, which was superseded by vinyl
hot-pour, which in its turn was made obsolete by the modern synthetic
rubber molding materials based on silicone and urethane. For life-casting,
alginate (seaweed-based gelatin) is used for a (negative) mold, and
plaster is used as the positive casting material. Plaster molds can also
be used to cast wax, if the mold is well-soaked beforehand.
Plaster can also be alloyed with plastic resins, such as melamine, to form
"Modified Gypsum" cements, which combine plaster's ease of use
with plastic's durability and water resistance. Forton MG is one of these
alloys, and there are others as well. It can also be mixed with Portland
cement, to produce a product with qualities intermediate between the two
materials. Mixed with glue and an inert filler, it becomes
"composition", which found early use as a material for picture
frames and other small decorative items.
Mixed with sand or another refractory aggregate, it is used as
"investment" for lost-wax casting, and will hang together just
enough, after being brought to more than 1000 degrees F, to hold molten
bronze and impart shape and fine detail , while still being friable enough
to remove easily from the casting. (Premixed investment plaster is still
generally used by the jewelry industry, while its use in the art
bronze-casting industry has been largely superseded by ceramic shell). |