Preserving Collectibles column from Nov. 30, 2015, issue of Coin World:
Many numismatists have plaster objects in their collections, whether
they be a cast of a coin, an artist’s rendition or a
limited edition reproduction of an artist’s original work.
Plaster is a form of calcium sulfate. It is made by heating gypsum
rock to remove its water, which creates calcium sulfate hemihydrate.
This is then ground into a powder, commonly known as plaster of paris.
The powder is then mixed with water so it can be sculptured or cast in
a mold. Plaster sets up — or dries — through a chemical reaction. This
reaction is exothermic, meaning it gives off heat as it takes place.
Depending on the volume of plaster being used, the amount of heat
generated can be considerable.
Artists commonly use plaster as a medium because
it can replicate fine detail when cast and because it shrinks very
little when it dries, unlike clay and other materials used in
modeling. Its low cost also has made it appealing. Small plaster
casts, such as those found in numismatic collections, are likely
solid. Although plaster is commonly left “raw,” it can also be sealed
with a clear sealant or decorated with paint and pigments.
Plaster is brittle and fragile. It will break if dropped or knocked
hard enough. Plaster pieces should be displayed so that they are
safely out of harm’s way. Handle plaster pieces with great care. If
packing for shipping, make sure that the object is well padded and
packed so that it cannot move around inside the box. Plaster is also
fairly soft and will scratch easily.
Humidity/moisture is a big concern with plaster. Plaster is slightly
soluble in water and prolonged exposure to high humidity will cause
the plaster to disintegrate.
Plaster is sensitive to acids. As such, any materials in intimate
contact with it should be acid-free and inert, similar to what is
recommended for metal coins tokens or medals.
Plaster is porous and, if unsealed, will soil easily, absorbing dirt
and grime. Once soiled, it is very difficult to clean without
adversely affecting the surface of the plaster. Preventing soiling is
the best approach.
Handle plaster objects with clean dry hands. Store in an
archival-quality box. Exhibiting plaster objects in a display case or
behind glass will help keep them from getting soiled. If a plaster
objects becomes dusty, remove the dust carefully with a soft brush or
a clean dusting cloth. Do not use water to clean plaster objects.
Water can dissolve the plaster and will drive any dirt into the
plaster, making it even harder to clean. Abrasive techniques, such as
erasers, are not recommended as they can cause the surface of the
plaster to become burnished, changing its appearance.