The American Numismatic Association (ANA) recommends storing coins in acid-free envelopes or “flips,” which are mylar cases that should not contain PVC, or polyvinyl chloride. Paper money can be stored in containers made of Polyethylene, polypropylene and polyester, Coin World contributor Susan L. Maltby wrote in a 2011 column. 

"I prefer storing numismatic materials in see-through holders rather than paper envelopes because you do not have to remove the item from the holder to examine it,” Maltby wrote. "If you do choose paper envelopes, make sure they are acid-free and lignin-free."

A dry, temperature-controlled environment is crucial to long-term preservation of coin and paper currency, according to the ANA.

Coin World contributor Q. David Bowers, chairman emeritus at Stack’s Bowers Galleries, detailed in a 2013 column how his storage method for a silver medal changed the item, possibly for the better.