Paper that has a neutral or basic pH (7 or slightly greater). It can be made from any cellulose fiber as long as the active acid pulp is eliminated during processing. It is also lignin and sulfur free. Acid-free paper addresses the problem of preserving documents for long periods.
Paper made from wood-based pulp that has not had its lignin removed goes yellow and deteriorates over time. When exposed to light and/or heat, the molecules in the acidic paper will break down even faster. It was only in the 1930s that the effects of wood-pulp paper became known, when William Barrow (a librarian) published a report about the deterioration of acidic paper in the libraries. For fear of the gradual disintegration of written materials, measures have been taken to improve the quality of paper.