Acid-free paper has a neutral or basic pH (7 or greater), offering longer preservation of documents. Paper made of wood pulp that retains its naturally occurring lignin yellows over time -- a process accelerated by light and heat. Hence, acid-free paper may have its lignin removed.
Material mixed with pulp during paper manufacturing also extends paper life. Bicarbonate, chalk and acid-free sizing additives are combined to produce papers lasting hundreds of years. Up to 100 percent cotton may be included to enhance the strength of paper. Cotton also increases cost.
Archival paper is an especially permanent, durable acid-free paper. Archival paper is meant to be used for publications of high legal, historical, or significant value. In the USA, such paper must also be approved by the ANSI standards. The international standard for "permanent" paper is ISO 9706.