Paper Conservation: Decisions & Compromises

pling is required, tide-lines are created due to the
application of water. A micro-electrode was used
in order to minimize the amount of water needed
and consequently, related damage induced by
the measurement (Strlic 2005). The electrode is
so far not commercially available.
Considering the damage which is induced using
the above described methods, a non-destructive
method which does not require addition of
water would be desirable. Recently, a good correlation
(R = 0.9573) between measured and predicted
pH values were obtained using a scientific
bench-top spectrometer described earlier. Using
a portable Ocean Optics spectrometer and the
above mentioned software, prediction model for
all kinds of independent paper samples is characterised
by R = 0.8802 and RMSEP = 0.73.
Although in use for decades, the standard
methodology involving pH determination after
1hr of cold extraction does not provide information
on whether gelatine sized rag paper is acidic
or alkaline, when equilibrium pH is reached. Our
study of 150 historical rag samples showed that
pH of most of the rag papers with acidic pH, as
determined using the standard method, stabilised
in alkaline region. This was observed also in
our earlier study (Strlic 2004). Determination of
the equilibrium pH value would thus be useful
in the assessment of the stability of paper. The
latter is difficult to determine, as several weeks
of measurements may be needed for paper pH to
reach equilibrium, while the recently proposed
method where the sample in water is stirred at
250 r.p.m. (Strlic 2004) is difficult to implement.
Although not as accurate as the determination
using a pH meter, NIR spectroscopy may be useful
also in this case offering a prediction for the
equilibrium pH with R = 0.8123 and RMSEP =
0.76. Prediction is based on all kinds of paper.
Conclusions
Inherent features of cultural heritage require
a specific approach to its characterisation. It is
imperative that the data is collected non-destructively
or at worst, micro-destructively. Also, the
instrumentation should be portable in order to
minimize risks to the heritage due to the transport.
The method employed should be characterised
by high sensitivity and low detection limits,
which reveal the condition, enable determination
of provenance, or point to possible future
problems and thus allow preservation actions
to be taken in time. It is also preferable that the
methods are simple, reliable and accessible.
While we know what we want, none of the
existing methods for determination of pH and
the condition of paper addresses all of the above
mentioned requirements. DP determination
is sensitive, affordable and reproducible, yet
destructive. Size exclusion chromatography of
carbanilated cellulose leaves no visible damage
to the artefact, but is instrumentally demanding
and expensive. Near infra-red spectroscopy with
accompanying software can be simple, fast, portable,
relatively affordable, but is associated with
a larger uncertainty. The choice of the analytical
method will remain subject to the nature of the
artefact and the conservation problem, among
others. While size exclusion chromatography,
when available, may be the method of choice in
case of a precious artefact or a limited number of
samples to be analysed, NIR spectroscopy may be
more appropriate for characterisation of larger
sets of artefacts, as well as for rapid characterisation
of the artefacts during daily work in conservation
departments.
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