Storage enclosures for books and flat paper should be made of materials which are durable (strong) and chemically stable (permanent). They should fit the item(s) properly and provide good support.
When choosing storage enclosures, "archival" and "acid-free" are two terms that you will frequently encounter. There are no standards governing the use of these terms, and they are sometimes misused, so read suppliers' catalogs carefully. If there are questions about a product, ask the supplier for details. If that information is not forthcoming, find another supplier.
The term "pH" is used to express the acidity or alkalinity of paper-based materials. It does not apply to plastics. The pH scale runs from zero to 14, with 7 being neutral, below 7 being acidic, and above 7 being alkaline.
Lignin-free, buffered enclosures (pH 8.5 or above) actively reduce the sources of acid damage and are preferred for most paper-based materials.
The Photographic Activity Test (PAT) was developed by the Image Permanence Institute (IPI) to evaluate the suitability of enclosures for photographic materials. The PAT can be performed on paper or plastic enclosures. It indicates whether the enclosures contain harmful chemicals that will cause image fading or staining. Enclosures used for photographic materials must pass the PAT (this should be noted in the supplier catalog); those that pass would be appropriate for other types of archival materials as well.
Plastics used for storage enclosures vary greatly in chemical stability. Those made of polyester, polypropylene, and polyethylene are generally safe to use for storing paper and photograph collections. Plastic sleeves can allow both sides of a paper to be viewed without touching the original, and they provide additional support during handling. Evidence has shown, however, that storage in plastic can accelerate deterioration by trapping acidic byproducts of decay. The choice of paper vs. plastic enclosures may ultimately be a compromise between protection against handling and protection against chemical damage. Plastic enclosures should be free of plasticizers, surface coatings, UV inhibitors, and other materials that may interact with collections.
Never use plastic enclosures if the media is flaking or friable, as with charcoal or soft pencil.