Polyester Encapsulation is a technique developed by the Library of Congress for the long term preservation of rare and valuable singular dheet documents. When preserved in this manner, the document can be easily read on both sides by virtue of the polyester's transparency. This method involves creating an acid- free enclosure with a special double-sided tape adhering two sheets of archival polyester plastic. Thus, the item is protected from harmful acids and remains functional as a reference tool and for photocopying purposes.