Susan Maltby, a well-known preservation expert from
Toronto, recommends holders made of either Mylar,
polyethylene or polypropylene. All three are tough, stable
compounds — and unlike PVC, they won’t break down readily
into their chemical components.

As for PVC, Sue Maltby has a three-word recommendation:
“No, no, no!”

PVC “flips” gained wide popularity during the coin
market’s boom years of the mid- and late 1970s because they
are exceptionally clear and flexible. This seemed to make
them an ideal way to store and display rare coins. In time,
it became apparent that these relatively minor conveniences
came at a very high cost — for when heat and light act upon
PVC, it breaks down chemically and hydrochloric acid is
released. This, in turn, can cause chemical damage to the
surface of the coins that the holders are supposed to be
protecting.

Often, plasticizers are used to enhance the chemical
properties of PVC holders, and these can ooze out and form an
oily film upon the coins, greasing the skids for still
further damage.

The perils of PVC were publicized extensively in the
early 1980s, and since then there has been a noticeable
decline in the use of such holders. Obviously, though, not
everyone has gotten the message: Recently, the Numismatic
Guaranty Corporation of America (NGC) issued a press release
reporting that it has received a substantial number of coins
with PVC damage.

Those coins were submitted to NGC by dealers, collectors
and investors seeking to have them certified and encapsulated
by the company, which is one of the nation’s leading coin-
grading services. Instead, they got their coins back
uncertified, since NGC and other grading services make it a
policy not to grade damaged coins.

Mark Salzberg, NGC’s president, suggests that many of
the damaged coins now entering the marketplace may have been
set aside years ago, before the risks of PVC were fully
understood.

“Collectors and investors should check any coins they
have put away — in safe deposit boxes, for example — to
make sure the holders are chemically inert and the coins have
not been damaged while in storage,” Salzberg said.

Those who suspect that their coins may have been exposed
to PVC should remove them from their holders immediately, he
said, and take or mail them to a dealer familiar with how to
neutralize the chemical and, if possible, remedy any damage.

“We’re alerting NGC-authorized dealers to the problem
and advising them what can be done to deal with it,” he said.

Of course, PVC isn’t the only source of potential damage
to your rare coins. Other caustic chemicals, such as
compounds containing sulfur, can harm them, too. Even you
yourself could be the source of damage: If you chanced to
touch a coin with a perspiration-soaked thumb, for example,
that coin could end up with a thumbprint permanently etched
in its surface.