Editing Paper (section)

==Paper stability==
Much of the early paper made from wood pulp contained significant amounts of alum, a variety of aluminium sulfate salts that is significantly acidic. Alum was added to paper to assist in sizing,<ref name="bier">{{Cite book|url=https://www.worldcat.org/oclc/813399142|title=Essentials of pulping and papermaking|last=Biermann|first=Christopher J/|date=1993|publisher=Academic Press|location=San Diego|isbn=012097360X|oclc=813399142}}</ref> making it somewhat water resistant so that inks did not "run" or spread uncontrollably. Early papermakers did not realize that the alum they added liberally to cure almost every problem encountered in making their product would eventually be detrimental.<ref name="clark">{{cite book |last=Clark |first=James d'A. |authorlink= |title=Pulp Technology and Treatment for Paper |edition=2nd |date=1985 |publisher=Miller Freeman Publications |location=San Francisco |isbn=0-87930-164-3 }}</ref> The cellulose fibres that make up paper are hydrolyzed by acid, and the presence of alum would eventually degrade the fibres until the paper disintegrated in a process that has come to be known as "slow fire". Documents written on rag paper were significantly more stable. The use of non-acidic additives to make paper is becoming more prevalent, and the stability of these papers is less of an issue.

Paper made from mechanical pulp contains significant amounts of lignin, a major component in wood. In the presence of light and oxygen, lignin reacts to give yellow materials,<ref>{{cite journal | last1 = Fabbri | first1 = Claudia | last2 = Bietti | first2 = Massimo | last3 = Lanzalunga | first3 = Osvaldo | year = | title = Generation and Reactivity of Ketyl Radicals with Lignin Related Structures. On the Importance of the Ketyl Pathway in the Photoyellowing of Lignin Containing Pulps and Papers | url = | journal = J. Org. Chem | volume = 2005 | issue = 70| pages = 2720–2728 | doi = 10.1021/jo047826u }}</ref> which is why newsprint and other mechanical paper yellows with age. Paper made from bleached kraft or sulfite pulps does not contain significant amounts of lignin and is therefore better suited for books, documents and other applications where whiteness of the paper is essential.

Paper made from wood pulp is not necessarily less durable than a rag paper. The aging behavior of a paper is determined by its manufacture, not the original source of the fibers.<ref>{{cite journal | last1 = Erhardt | first1 = D. | last2 = Tumosa | first2 = C. | year = 2005 | title = Chemical Degradation of Cellulose in Paper over 500 years | url = | journal = Restaurator: International Journal for the Preservation of Library and Archival Material | volume = 26 | issue = | page = 155 | doi=10.1515/rest.2005.26.3.151}}</ref> Furthermore, tests sponsored by the Library of Congress prove that all paper is at risk of acid decay, because cellulose itself produces formic, acetic, lactic and oxalic acids.<ref>
  {{cite web
    | title = The Deterioration and Preservation of Paper: Some Essential Facts
    | url = http://www.loc.gov/preservation/care/deterioratebrochure.html
    | accessdate = 7 January 2015
    | publisher = Library of Congress
    | quote = Research by the Library of Congress has demonstrated that cellulose itself generates acids as it ages, including formic, acetic, lactic, and oxalic acids
    | ref = refDeteriorationOfPaper
  }}
</ref>

Mechanical pulping yields almost a tonne of pulp per tonne of dry wood used, which is why mechanical pulps are sometimes referred to as "high yield" pulps. With almost twice the yield as chemical pulping, mechanical pulps is often cheaper. Mass-market paperback books and newspapers tend to use mechanical papers. Book publishers tend to use acid-free paper, made from fully bleached chemical pulps for hardback and trade paperback books.