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Why does paper degrade? |
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How to prevent degradation |
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Paper stability estimation |
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What is chemiluminescence |
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CL and the cultural heritage |
Paper stability is a term very much dependent on the user's expectations. For documents of historic importance, we expect the material to last for several hundreds of years and for the user, physical stability is the most important parameter.
CIENTIFIC
STUDIES have shown that physical stability correlates with the length of
cellulosic macromolecules, i.e. with the number of monomers in a polymeric
chain, the so-called degree of polymerisation. It is almost inevitable for
this parameter to decrease with time, even in the most favourable conditions,
and the rate can be determined by following the change in the degree of
polymerisation with time.
Climatic chamber for accelerated thermal ageing at defined temperature and relative humidity. |
¶ At room conditions, the rate
of degradation is very slow. In order to be able to measure it at all,
researchers often increase the temperature during stability evaluation
experiments (accelerated ageing in climatic chambers) and use mathematical
models (Arrhenius’s equation) in order to extrapolate their
findings to room temperature. However, experiments at several temperatures
should be done in order to perform an extrapolation, and very long experimental
times are necessary, up to half a year. Besides, cellulose degradation
is not a simple reaction, but rather involves a whole set of reactions
and their relative acceleration with temperature increase may not be uniform.
Any extrapolation is therefore prone to an error, especially if the temperature
to which the degradation rate is extrapolated is very different to the
lowest experimental temperature. If it is 70 °C, and stability at
25 °C is predicted, a chemist with average experience is only able
to say that a moderately stable paper will last between 30 and 22,000
years!
In addition, stability of cellulose during conditions of photo-ageing
is a matter of high interest for artefacts exposed in galleries and is
also addressed by the Papylum team.
Chambers for accelerated photo-ageing. |
¶ There is clearly room for improvement. We need a method that will allow shorter experimental times at lower experimental temperatures. It is hoped that measurements of chemiluminescence will fill the gap. If successful, Papylum researchers will be able to predict durability of deacidified papers in a few hours and more reliably and quick evaluation of conservation treatments will finally be possible.