Acid-free paper

From Wikipedia, the free encyclopedia

Acid-free paper is paper that has a neutral or basic pH (7 or greater), although paper having a pH between 6 and 7 is often also considered acid-free. During production, it is treated with a mild base (usually calcium or magnesium bicarbonate) to neutralize the natural acids occurring in wood pulp, and it may also be buffered to prevent the formation of additional acids (as may develop from the application of sizing).

The bicarbonate is added in excess, to supply the paper with an alkaline reserve to provide protection from further attack by acids remaining in the paper or supplied by the environment (eg. atmospheric sulfur dioxide). The bicarbonate during drying loses carbon dioxide and water and is converted to calcium carbonate or magnesium carbonate. Paper that should withstand for very long periods (eg. 500 years) should contain approximately 3 weight percents of the carbonate.

Acid-free paper is often used in books and other paper products intended to last for a long time. Paper that has not had its acidity neutralized yellows and deteriorates.

Acid–free (alkaline) paper that additionally is uncoated and meets certain standards for folding and tearing is authorised by the American National Standards Institute (ANSI) to carry the following notice, which must include the circled infinity symbol.^[1]

"The paper used in this publication meets the minimum requirements of the American National Standards Institute for Information Sciences—Permanence of Paper for Printed Library Materials, ANSI Z39.48-1992."