Acid-free paper
Encyclopedia
Paper
Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....
that has a neutral or basic pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
(7 or slightly greater). It can be made from any cellulose fiber
Cellulose fiber
Cellulose fibers are fibres of cellulose from any source, either natural or manufactured.-Textile:In the textile industry regenerated cellulose is used as fibers such as rayon, . Cellulose fibers are manufactured from dissolving pulp...
as long as the active acid pulp is eliminated during processing. It is also lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
free. Acid-free paper addresses the problem of preserving
Art conservation and restoration
Conservation-restoration, also referred to as conservation, is a profession devoted to the preservation of cultural heritage for the future. Conservation activities include examination, documentation, treatment, and preventive care...
documents for long periods.
Overview
Paper made from wood-based pulp that has not had its ligninLignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
removed goes yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...
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.
During production, acid-free paper may be treated with a mild base (usually calcium or magnesium bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
) to neutralize the natural acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s occurring in wood pulp
Wood pulp
Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...
, and it may also be buffered to prevent the formation of additional acids (as may develop from the application of sizing
Sizing
Sizing or size is any one of numerous specific substances that is applied to or incorporated in other material, especially papers and textiles, to act as a protecting filler or glaze....
).
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 (e.g. atmospheric sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
). The bicarbonate during drying loses carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and water and is converted to calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
or magnesium carbonate
Magnesium carbonate
Magnesium carbonate, MgCO3, is a white solid that occurs in nature as a mineral. Several hydrated and basic forms of magnesium carbonate also exist as minerals...
. In order for paper to last at least 100 years it must have an alkaline reserve of 2% or more.
Today, much of the commercially produced paper is acid-free, but this is largely the result of a shift from china clay to (cheaper) chalk
Chalk
Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....
as the main filler material in the pulp: chalk reacts with acids, and therefore requires the pulp to be chemically neutral or alkaline. The sizing
Sizing
Sizing or size is any one of numerous specific substances that is applied to or incorporated in other material, especially papers and textiles, to act as a protecting filler or glaze....
additives mixed into the pulp and/or applied to the surface of the paper must also be acid-free.
Alkaline paper has a life expectancy of over 1,000 years for the best paper and 500 years for average grades. The making of alkaline paper has several other advantages in addition to the preservation benefits afforded to the publications and documents printed on it. Because there are fewer corrosive chemicals used in making alkaline paper, the process is much easier on the machinery, reducing downtime and maintenance, and extending the machinery's useful life. The process is also significantly more environmentally friendly. Waste water and byproducts of the papermaking process can be recycled; energy can be saved in the drying and refining process; and alkaline paper can be more easily recycled.
Standards
The company Hercules Incorporated developed the first alkaline size in the 1950s that made acid-free paper possible. Despite the advances in paper making and the identification of and concern around the brittle book problem, it took decades before the adoption of ANSIAnsi
Ansi is a village in Kaarma Parish, Saare County, on the island of Saaremaa, Estonia....
NISO
Niso
Niso is a genus of very small parasitic sea snails, marine gastropod mollusks or micromollusks in the family Eulimidae. -Species:According to the World Register of Marine Species the following species with accepted names are included within the genus Niso * Niso aeglees Bush, 1885* Niso albida...
Standard Z39.48-1984 - Permanence of Paper for Publications and Documents in Libraries in 1984. This voluntary standard covered pH value, tear resistance, alkaline reserve, and lignin thresholds for paper to last thousands of years and was developed to encourage the use of acid-free paper in library materials. The development of the initial standard was a result of the work of the Council on Library Resources, which effectively lobbied ANSI to adopt the guidelines.
In 1986, Standards Committee II of NISO was established to expand Z39.48-1984 to develop standards for coated paper, and was again called upon in 1988 to review and revise the standards for uncoated paper.
There are various standards for "acid-free" paper, with differing requirements. In some quarters, paper having a pH between 6 and 7 is often also considered acid-free. 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
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
) to carry the following notice:
"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."
The objective of ANSI Z39.48-1992 "is to establish criteria for coated and uncoated paper to last several hundred years" under optimal conditions in libraries and archives. The desired outcome of the standard is to reduce future preservation problems.
The scope of the standard is to cover publications and documents bought and maintained by libraries and archives. Such works include scholarly journals, periodicals, monographs, government documents, original documents, and significant works in fiction and non-fiction.
An equivalent international standard, ISO 9706, was published in 1994.
Manufacturers of acid-free paper can indicate the compliance of their product with the test requirements of the ISO 9706 or ANSI Z39.48-1992 standards using a circled infinity symbol (Unicode codepoint 267E, ♾).
Archival paper
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 in accordance with the ANSI standards. The international standard for "permanent" paper is ISO 9706 and for "archival" paper, the standard is ISO 11108.Often, cotton rag
Cotton paper
Cotton paper is made from cotton linters or cotton from used cloths were the primary material source, hence the name rag paper. Cotton paper is superior in both strength and durability to wood pulp-based paper, which may contain high concentrations of acids.-Properties:Certain cotton fibre paper...
paper is used for archival purposes, as it is not made from wood-based pulp. Thus, archival paper is sometimes broken down into two categories:
- Conservation-grade — acid-free, buffered paper made from wood-based pulp.
- Archival-grade (also Museum-grade) — cotton rag paper made from cotton pulp.
Durabook
Another form of "archival paper" is being made from recycled plastic in the form of Durabook. This is a patented material and process developed by Melcher Media, a New York publisher. Plastic paper does not have ligninLignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
to cause acid problems or cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
fibers that can break from folding. It also will not be damaged if soaked in water and, if the book gets dirty, can be washed. The only issues to consider are what type of plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
is being recycled and if those plastics will break down in the future.
External links
- Acid-Free Paper
- ASTM D1994-07 Standard Test Method for Determination of Acid Numbers of Hot-Melt Adhesives
- National Archives of Australia: descriptions of permanent and archival paper; archival quality trademark and register of certified products