n. ~ Paper with a pH of 7.0 or greater when manufactured.
Acid-free papers are distinguished from papers that contain a residue of the acids used to break up wood fibers during manufacture. The residual acid continues to attack the paper fibers, making the paper brittle over time. Archival papers are typically made from alpha cellulose, are lignin free, and often contain an alkaline buffer to counter any trace of acids used in processing or environmental acids.
†(Roberts and Etherington 1982) [acid-free paper] In principle, papers which contain no free acid and have a pH value of 7.0 or greater. In practice, papermakers consider a paper having a pH value of 6.0 or greater to be acid free. Such papers may be produced from cotton fibers, rags, esparto, jute, chemical wood pulps, or virtually any other fiber, with special precautions being taken during manufacture to eliminate any active acid that might be present in the paper pulp. However free of acid the paper may be immediately after manufacture, the presence of residual chlorine from bleaching operations, aluminum sulfate (alum) from sizing, or sulfur dioxide in the atmosphere, may lead to the formation of hydrochloric or sulfuric acid unless the paper has been buffered with a substance capable of neutralizing acids.