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Paper is an organic material composed of cellulose fibers from plants. Papers made from the Twelfth Century to the middle of the Nineteenth Century were strong and durable, as they were made by hand, generally from cotton or flax (linen) plant fibers, which are low-acid. Hand processing produced longer fibers and thicker, stronger paper. Many books and documents published prior to 1850 are still in excellent condition.
Modern paper is generally made from wood fibers which have been mechanically ground to a pulp for newsprint, and then chemically purified for books and writing papers. The fibers are very short. Alum-rosin compounds used as sizing agents generate sulfuric acid under humid conditions, further breaking down the cellulose fibers.
Inexpensive modern papers, such as those used in paperback books, have an expected useful life of less than 30 years. Better quality modern papers may contain recycled cotton or linen fibers ("Rag Content"), making them more durable. Fortunately, the publishing industry has become aware the impermanency of modern papers, and quality books are now frequently printed on "acid-free" stock.