Paper 39
Uneven dimensional changes have serious consequences for the paper con-
servator. If a piece of paper expands on one edge while an adjacent area remains
unchanged, then there will be a connecting area where there is stress. In areas
of sufficiently high stress, the paper may undergo irreversible deformation with
resulting loss of flatness. The production of such resulting wavy surfaces is
called “cockling”. The phenomenon is often seen on damp-damaged books
where water has gained local access and the paper cannot expand freely as it
is confined by other pages and the binding. Aqueous conservation treatments
are difficult to perform on bound books because of the dimensional changes
in the paper. The paper conservator has only to join two pieces of paper
together to generate the potential for cockling as the two pieces will have dif-
fering hygroexpansivities and one or both may have dried-in-stress. The con-
servator may take care to use similar papers that have been stored for some
time, align the MDs and avoid getting the paper too wet if using a water-
based adhesive.
4 Deterioration of Paper
Paper objects may suffer from damage by vermin, insect and microbial attack,
theft, fire and flood. However, from a chemist’s point of view there are two
important mechanisms by which paper deteriorates, both of which may change
its hue and make it weaker; these mechanisms are acid-catalysed hydrolysis
and oxidation. Papers vary a lot in their stability and the worst can become so
brittle that they become unusable. Papers made from rags using traditional
methods are usually extremely stable, but old papers made from poorly processed
wood pulp are often very weak.
4.1 Acid-Catalysed Hydrolysis of Cellulose
The first comprehensive and systematic work on the reasons for deterioration
of paper was performed by W.J. Barrow in the 1960s who firmly established
the link between the acidity of paper and its permanence. Although we talk
about the acidity of a sheet of dry paper, the acidity of paper, expressed as pH
is only measured in an aqueous extract, generally produced by macerating 1 g
of paper in 50 mL of pure water.
The pH is defined as pHlog 10 [H] where [H] is the hydrogen ion
concentration. On this scale, pH 7 is neutral, pH 1 is extremely acid and pH 14
extremely alkaline. Each pH decrease of 1.0 represents a 10 times increase in
hydrogen ion concentration. For example, there is 10 times more active acid
at pH 4 than pH 5.
Barrow measured the pH of many old pieces of paper and also measured
their strength and found that alkaline papers were generally strong but acid