Historically, most of society’s writings and visual images have been recorded on paper. However, paper is an organic material and is subject to deterioration caused by chemical, physical and biological agents. While documents on paper several hundreds of years old have endured, most paper manufactured in the last two hundred years has a limited storage life due primarily to acidity induced as part of paper manufacturing processes used during that time period. Exposure to acidic air pollutants also contributes to paper degradation.
Chemically this degradation is a result of acid hydrolysis of the cellulose polymers which are themselves responsible for the intrinsic strength found in paper sheets. The hydrolysis reaction are also autocatalytic since they produce additional acid products which themselves further increase paper acidity and accelerate degradation occurring.
Since untreated paper is too absorbent for the application of inks, hydrophobic fillers also called sizings are added to paper surfaces as part of the manufacturing process which can also have an impact on paper permanence. Sizing impacts absorption of liquids to prevent feathering of inks and dyes and to provide a crisp image.
Paper made before the 19th century was often made by hand from linen and cotton rag materials which are excellent sources of high cellulose, long fibers. Gelatin, from animal hides, was used to size such papers and because the resulting papers were neutral to only slightly alkaline they had very good storage properties. The paper machine appeared at the end of the 18th century, and as the demand for paper outpaced sources of available gelatin, cotton rags and linen, wood fiber took the place of cotton and linen. However, wood has shorter fibers and lower cellulose content along with lignin so that chemical methods had to be developed to free fibers from wood and other plant matter to supply the increasing amounts of paper furnish needed. Mechanical action [“beating”] to soften and bleaching to whiten these new materials yielded pulps that could be laid down by machine as a sheet with subsequent drying to form paper.
Gelatin was replaced in the early 1800s with rosin which was mainly abietic acid isolated from pine resin. Alum (papermaker’s alum or aluminum sulfate) was used to precipitate the rosin onto paper forming an alum/rosin sizing. Aluminum sulfate reacts with water - and produces sulfuric acid – and production of paper on the paper machine demanded additional alum, so papermakers often used the salt in excess. The industrial revolution added outside factors such as air pollution and acidic oxides of nitrogen or sulfur and ozone to the environment. Early bleaching processes using chlorine and hypochlorites, strong oxidizers, also degraded the cellulose and reduced its strength. All of these led to diminution of paper properties. Fortunately, bleaching processes have totally significantly over the past few decades and the chlorination and hypochlorite steps are no longer used resulting in less damage is done to resulting fiber.
The strength of paper results from a combination of factors. The most important is the condition of cellulose, in particular chain length. Cellulose is made up of repeating units of glucose monomers and the number of glucose units present provides a measure of degree of polymerization (DP). In native cellulose from wood DP is about 10,000. Depending on the nature of the pulping and bleaching, the DP of processed cellulose falls to a range of 600 – 1100.
After this reduction the paper fibers are still quite strong, but fiber strength is highly dependent upon retaining this DP and acids break cellulose bonds randomly often cutting the cellulose polymer in central regions. These attacks drastically reduce the DP of cellulose and quickly weaken the fibers. Alkalis also attack cellulose, but by a different mechanism, in which only end units are removed (end peeling) so that alkaline degradation has much less effect on reducing DP (Bristow and Kolseth, 1986)
Some low quality papers such as groundwood for newsprint also contain high amounts of lignin.
Lignin contains phenolic entities sensitive to light which develop chromophores or conjugated double bonds in the chemical structure which can absorb light in the visible range. This makes paper containing high amounts of lignin very prone to discoloration or yellowing. While there is no doubt that lignin in paper contributes to its discoloration, there is little evidence to show that paper containing lignin loses strength faster than paper of similar quality without lignin (Luner, 1988). However, standard specifications for durable paper allow no more than 1% lignin, for papers used in archives, libraries, and other permanent records.
In addition to the acid nature inherent in paper and the exposure to acidic air pollutants, it is now known that exposure of stable or acid free paper to acidic books and papers – as in a library or archive – will lead to deterioration of such seemingly stable documents stored within the same collections (Smith, 1999). Other threats to permanence include those from water and fire damage, theft, and vandalism or biological agents. These are problems while not inherently present with paper materials have been addressed in depth by the Northeast Document Conservation Center (NDCC, 1999).
Because of its importance to society, the permanence of paper is addressed in a significant and growing literature much of which has been generated by the community of archivists and librarians.
About the Author:
Doctor Joe Zicherman is a well known authority in paper preservation and has lectured on the subject for many years. He endorses several products to help amateur scrap-bookers and professional archivists deacidify paper and significantly increase the life of important document, photos and other papers. http://www.paper-preservation.com is a crucial step in preserving paper goods. For more information on paper preservation and deacidification products please visit: http://www.paper-preservation.com.
Permanence of Paper
