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Acid-free Paper
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A classification of paper in which the paper is free of acidic ingredients. The paper has a neutral base or an alkaline base, meaning that its pH is at, or above, 7. Paper manufactured with an acid-base (a pH below 7) deteriorates in a relatively short period of time, which is why old newspapers appear yellow and brittle. Acid-free paper became more available in the last half of the 20th Century and is now quite common. Since the 1970's, most of the paper used for book publishing and other printed materials in which permanence is of importance, has been produced with acid-free paper because it is much more stable than acid-based paper and is better suited for long-term preservation of printed documents.
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