typical set of conditions is 80°C and 65% RH for 4 weeks. Usually, there is
no attempt to offer an equivalent time for ambient ageing. Recent research
from several independent laboratories has shown that the degradation products
from paper ageing appear to accelerate the ageing. Accordingly, some scientists
recommend ageing paper in sealed glass tubes which retain degradation prod-
ucts and keep the water content of the paper approximately constant. As a
general rule, the closer the conditions of accelerated ageing are to ambient
storage conditions, the better the results will be. The only drawback in ageing
at lower temperatures is that the time needed for accelerated ageing is increased.
Accelerated ageing using light should only be used to anticipate the effects
of light on paper and not for the effects of ageing generally. The energy of
photons in light transfers energy to paper in a different way and intensity to
the thermal energy of heat ageing and the results of one type of testing may
be entirely different from the other. A paper that yellows in thermal ageing
may bleach in light ageing. White light contains light of different colours; the
red end of the spectrum contains light of low energy, and the packets of light
(the photons) are not capable of damaging paper. At the blue end of the spec-
trum, the photons contain more energy. Some illuminants contain ultraviolet
light which is even more damaging to the paper. Accelerated ageing should
be performed with light with a spectral distribution as close as to that in the
intended storage conditions.
Fluctuating relative humidity can be bad for paper because of the dimen-
sional stability of the paper, i.e.production of cockling. However, recent work
has shown that large losses of tensile strength can result from a regime of
wide changes in relative humidity. This promises to be an interesting field of
research in the future. Elevated temperature combined with fluctuating RH
has been used in ageing iron gall ink on paper.
9 Safe Environments for Paper
The safest conditions for storage of paper may be easy to define, but are diffi-
cult to achieve in practice. Oxidation may be eliminated by storage in nitrogen
but this is not practical in all but a few instances. Recommended conditions
are a compromise between what is ideal and realistic.
At RH greater than about 65%, fungal growth may occur, so the RH should
be below this. Air circulation is an additional advantage. At low RH, deterior-
ation reactions that need water to proceed will be slowed down but paper and
glue become more brittle, so 40% is the effective lower limit for general storage.
Chemical reactions slow down with decreasing temperature, so ideally, stor-
age should be at as low a temperature as possible, but scholars do not like to
work in cold conditions and, anyway, the depressed temperatures are expensive
to maintain. International standards give the temperature for storage as about
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