 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
|
 |
|
|
 |
|
|
APER
HAS BEEN AN ESSENTIAL PART of our civilisation for the last two thousand
years. Despite its ubiquity, paper is a complex material, a web of cellulosic
fibres and sizing. Fillers, coatings, pigments may also be present. Fibres
render the material its physical properties and they consist mainly of cellulose,
although other constituents may also be present, depending on the origin
and type of fabrication, such as lignin and hemicelluloses. Degradation
of physical properties of paper, so important for the user, is predominantly
a question of cellulose stability due to its important role. While the cause
for paper degradation may be mechanical, biological or chemical, it is the
latter that needs the most sophisticated conservation approach. ¶
Chemically, cellulose is a macromolecule, synthesized by plants, some microorganisms
and bacteria. It is a linear polymer consisting of glucose monomer units.
As many organic materials, it is inherently instable, however, the mode
of degradation depends very much on the macromolecule's chemical environment.
This depends primarily on the technology of paper production, but also on
subsequent application of inks and pigments. ¶
Degradation reactions of cellulose ultimately lead to bond scission. Fibre
structure becomes more ordered, the average length of macromolecules decreases
and so does the physical strength of fibre and consequently of paper.
A typically fragile acidic |
¶ In the course of production of certain papers (in use from 1807 until present time), acids were introduced in to paper during sizing. In acidic media, the glucosidic bond, linking two glucose monomers, is prone to acid-catalysed hydrolysis. This type of degradation leads to extremely rapid loss of properties and is a problem of catastrophic proportions in many modern libraries and archives.
An illuminated medieval manuscript on a relatively stable rag paper. |
¶
Although the sources of fibres (and consequently the quality) varied,
the technology of production prior to 1800 was such that the resulting
paper was chemically neutral to moderately alkaline. Oxidation of cellulose
with atmospheric oxygen (autoxidation) and a series of elimination
reactions leading to bond scission (alkaline degradation) are the
predominant mechanisms of degradation of such papers. The rates of these
processes are variable, and in the absence of catalysts, such papers are
quite stable, and many medieval books can still be safely used. This is
recognised in the standard for permanent paper (ASTM D3290), and modern
paper produced according to the specifications carries the sign 
.
Hand writing in iron-gall ink with the inked parts heavily corroded. |
¶
Autoxidation is a radical chain reaction accelerated by the presence of
catalysts such as transitional metals iron and copper. While in the bulk
of paper, the concentrations of these metals are usually not significant,
they can be present in large amounts in inks and pigments and can cause
localised corrosion of paper, the so-called ink-corrosion and copper-corrosion.
Since iron and copper containing ink (iron-gall ink), which was
in addition also acidic, was the predominant Western ink from medieval
times till 20th century, degradation of such documents leads to rapid
decomposition of written history. This problem is addressed by another
5th Framework Programme Project, the InkCor
Project.