48 Chapter 3
8 Accelerated Ageing Tests
When conservation treatments are being developed it is desirable to repro-
duce the effects of subsequent ageing to see if the treatment has stabilised or
destabilised the treated material. Similarly, conservation materials, e.g.adhe-
sives, packaging, etc., need to be aged to see if they will develop undesirable
characteristics on ageing, e.g.discolouration, brittleness, shrinkage, acidity, etc.
In order to find out how papers will respond to ageing, several techniques
have been developed. The method used should reflect the conditions to which
the paper will be subjected. If a paper is subjected to a great deal of light,
light ageing should be used, however, if the paper is stored in the dark or low
lighting conditions, a heat ageing regime is more appropriate.
The vast majority of paper in libraries, archives and museums is kept in
conditions of total darkness or in low incident light that contains little or no
ultraviolet light. In such conditions, light degradation is of no or little import-
ance. However, objects on display or which form part of a historical room
may receive sufficient light to cause serious concerns about degradation. A
few dyes and pigments are extremely sensitive to light and may need to have
a ban on being exhibited at all.
In this section, we will concentrate on heat ageing which is intended to
reproduce the effects of ageing in the dark and in the absence of air pollu-
tants. Chemical reactions in general, and those which cause deterioration of
paper in particular, slow down when the temperature is decreased and accel-
erate when the temperature is increased. In response to this fact, some libraries
and archives have built stores that can maintain a low temperature to prolong
the life of their collections. There is an equation that was defined by Arrhenius,
which can be used to predict the effects of temperature on the rate of chemi-
cal reactions/ageing, here it is:
where kis the rate of reaction, Aand Rare constants, Eais an experimentally
derived value called the activation energy and Tis the absolute temperature
(add 273 to the temperature in degrees Celsius to get this). Only a few paper
scientists need to perform calculations using this equation, but its importance
to most conservators and paper conservation scientists is that it describes the
way that the rate of reactions accelerate by two or three times for every
increase in temperature by 10°C.
Barrow (see above) used ageing in an oven for 72 hours at 100°C to replicate
the effects of natural ageing of paper for 25 years. Nowadays, this assumption
is still used, but regarded as simplistic and not applicable to all types of paper.
Many laboratories prefer to perform their paper ageing in an oven at a controlled
relative humidity and at a lower temperature. There is no consensus, but a
kA eERTa/