Photocopy technology emerged from machines developed for business purposes and taken up by librarians to duplicate missing, scarce, and deteriorating materials. Librarians photocopy materials to replace missing pages from books and missing issues from periodicals, and to increase the copies of items in high demand. The latter practice is, of course, illegal unless the materials are out of copyright. Photocopying to replace brittle books, however, is regarded as fair use if the library owns the titles involved and makes only one copy to replace the compromised original.

Preservation photocopy is copy made with a high-quality machine that is maintained in good enough condition to produce well-fused images (images that will not smudge or be picked up when a piece of pressure-sensitive tape is touched to the surface). The paper must be permanent/durable (acid free and buffered), and the image must be the same size as the original, and on both sides of the leaf with good registration. When replicating a brittle book, most expert copiers will construct a paper frame to obscure the black lines that would otherwise show around the original page edges and that will center the image on the copy paper. Generally, the photocopy facsimile will be bound using double-fan adhesive techniques with an inside gutter margin of no less than 3.7 centimeters.

Once they are discovered after circulation, books with brittle paper are usually replaced by preservation photocopy unless there are other copies available (for example, large sets or serials on microfilm, or reprints or new editions). Few guidelines have been developed for preservation photocopy beyond ensuring that the paper will last, the image is stable, and the final product replicates the original volume as closely as possible.

Photocopy machines are now available that can produce accurate color copies and excellent black and white copies. A color copier can also produce a quality gray-scale image, rendering an old black and white photograph extremely accurately. For this reason, quality photocopy is used increasingly to produce surrogate facsimile copies of original documents and photographs for readers and for exhibition. This is important when the original might be damaged through too much handling or is too light sensitive to be placed on display.

Older photocopied materials may have badly deteriorated because of unstable chemicals used in image production or poor-quality paper. Materials produced by old processes such as mimeograph and Ozalid wet process are undoubtedly faded and should be re-photocopied with modern equipment on good paper. Information on these processes may be found in the following:

• The Dead Media Project,

• The Early Office Museum(tm),

• Peter Graham, "Mimeograph and Its Competitors,"

• Kevin Laurence, "The Exciting History of Carbon Paper!" 1995,

• Melbourne Museum of Printing, Glossary of Printing and Typography,