Permanence of Paper

-- End Ad Box --->cellulose and reduced its strength. All of these led
Historically, most of society’s writings andto diminution of paper properties. Fortunately,
visual images have been recorded on paper.bleaching processes have totally significantly over
However, paper is an organic material and isthe past few decades and the chlorination and
subject to deterioration caused by chemical,hypochlorite steps are no longer used resulting in
physical and biological agents. While documents onless damage is done to resulting fiber.
paper several hundreds of years old haveThe strength of paper results from a combination
endured, most paper manufactured in the lastof factors. The most important is the condition of
two hundred years has a limited storage life duecellulose, in particular chain length. Cellulose is made
primarily to acidity induced as part of paperup of repeating units of glucose monomers and
manufacturing processes used during that timethe number of glucose units present provides a
period. Exposure to acidic air pollutants alsomeasure of degree of polymerization (DP). In
contributes to paper degradation.native cellulose from wood DP is about 10,000.
Chemically this degradation is a result of acidDepending on the nature of the pulping and
hydrolysis of the cellulose polymers which arebleaching, the DP of processed cellulose falls to a
themselves responsible for the intrinsic strengthrange of 600 — 1100.
found in paper sheets. The hydrolysis reaction areAfter this reduction the paper fibers are still quite
also autocatalytic since they produce additionalstrong, but fiber strength is highly dependent upon
acid products which themselves further increaseretaining this DP and acids break cellulose bonds
paper acidity and accelerate degradation occurring.randomly often cutting the cellulose polymer in
Since untreated paper is too absorbent for thecentral regions. These attacks drastically reduce
application of inks, hydrophobic fillers also calledthe DP of cellulose and quickly weaken the fibers.
sizings are added to paper surfaces as part ofAlkalis also attack cellulose, but by a different
the manufacturing process which can also have anmechanism, in which only end units are removed
impact on paper permanence. Sizing impacts(end peeling) so that alkaline degradation has much
absorption of liquids to prevent feathering of inksless effect on reducing DP (Bristow and Kolseth,
and dyes and to provide a crisp image.1986)
Paper made before the 19th century was oftenSome low quality papers such as groundwood for
made by hand from linen and cotton rag materialsnewsprint also contain high amounts of lignin.
which are excellent sources of high cellulose, longLignin contains phenolic entities sensitive to light
fibers. Gelatin, from animal hides, was used to sizewhich develop chromophores or conjugated
such papers and because the resulting papersdouble bonds in the chemical structure which can
were neutral to only slightly alkaline they had veryabsorb light in the visible range. This makes paper
good storage properties. The paper machinecontaining high amounts of lignin very prone to
appeared at the end of the 18th century, and asdiscoloration or yellowing. While there is no doubt
the demand for paper outpaced sources ofthat lignin in paper contributes to its discoloration,
available gelatin, cotton rags and linen, wood fiberthere is little evidence to show that paper
took the place of cotton and linen. However,containing lignin loses strength faster than paper
wood has shorter fibers and lower celluloseof similar quality without lignin (Luner, 1988).
content along with lignin so that chemical methodsHowever, standard specifications for durable
had to be developed to free fibers from woodpaper allow no more than 1% lignin, for papers
and other plant matter to supply the increasingused in archives, libraries, and other permanent
amounts of paper furnish needed. Mechanicalrecords.
action [“beating”] to soften and bleaching toIn addition to the acid nature inherent in paper and
whiten these new materials yielded pulps thatthe exposure to acidic air pollutants, it is now
could be laid down by machine as a sheet withknown that exposure of stable or acid free paper
subsequent drying to form paper.to acidic books and papers — as in a library
Gelatin was replaced in the early 1800s with rosinor archive — will lead to deterioration of
which was mainly abietic acid isolated from pinesuch seemingly stable documents stored within
resin. Alum (papermaker’s alum orthe same collections (Smith, 1999). Other threats
aluminum sulfate) was used to precipitate theto permanence include those from water and fire
rosin onto paper forming an alum/rosin sizing.damage, theft, and vandalism or biological agents.
Aluminum sulfate reacts with water - andThese are problems while not inherently present
produces sulfuric acid — and production ofwith paper materials have been addressed in
paper on the paper machine demanded additionaldepth by the Northeast Document Conservation
alum, so papermakers often used the salt inCenter (NDCC, 1999).
excess. The industrial revolution added outsideBecause of its importance to society, the
factors such as air pollution and acidic oxides ofpermanence of paper is addressed in a significant
nitrogen or sulfur and ozone to the environment.and growing literature much of which has been
Early bleaching processes using chlorine andgenerated by the community of archivists and
hypochlorites, strong oxidizers, also degraded thelibrarians.