Paper Conservation: Decisions & Compromises

Fig 2. Partial least square prediction of DP of rag paper. RMSEP is root
mean square error of prediction. It is defined as the root of the average
of the squared differences between the predicted and measured values
on the validation objects. R is the correlation coefficient, describing
the strength of a linear dependence between two variables. N is the
number of data points.
(Zou 1996), giving rise to extensive use of viscometric
determinations of degree of polymerisation
(average number of monomer units in cellulose,
DP) in characterisation of paper. Relative
uncertainty is low, around 1% and a few tenths of
mg of sample are needed. Although much lower
than in the case of mechanical properties, this is
an ample amount which prevents the use of the
technique on historic materials. Alternatively,
molar mass of cellulose may be determined using
size exclusion chromatography, either using
non derivatised cellulose (e.g. Henniges 2008) or
derivatised (e.g. Lojewski 2010). A good correlation
is observed between the viscometrically determined
DP and the molar mass of carbanilated
cellulose (Fig. 1).
It had been shown that in the case when cellulose
is derivatised using phenyl isocyanate, a few
fibres suffice for characterisation (Stôl 2002). Despite
the destructive nature of the method, the
amount of the sample is small and the method
results in no visible damage to the artefact. It
also enables evaluation of different stabilisation
processes on historical samples containing iron
gall ink (Kolar 2012). However, the uncertainty
related to the method is larger than in case of
viscometry, often exceeding 5%. This is partly
due to inhomogeneity of paper, the effect of
which is more prominent in smaller samples.
It is also an expensive method requiring highly
skilled personnel.
Near- and mid-FT-IR reflectance spectroscopy
was used to model DP values obtained using
viscometry (Trafela 2007). Quality of prediction
is often described with a correlation coefficient
R. The closer it is to +1 or -1, the more closely the
two variables are related. A good correlation (R
= 0.9658) between actual and predicted values
were obtained using a scientific bench-top spectrometer
Perkin-Elmer Spectrum GX (Waltham,
MA) which collects spectra between 714 nm ~
5,000 nm. The same instrument was used in
another study, where mechanical properties
such as tensile strength after folding (R = 0.8845)
and tensile strength (R = 0.7607) were predicted
(Lichtblau 2008). The instrument is not portable
and the otherwise non-destructive spectrometric
measurements are limited by the small size of
the sample which can be used for otherwise nondestructive
analysis. Based on the promising predictions
a commercial software was developed by
MORANA RTD, which enables determination of a
range of properties of various papers, such as DP,
Mw, pH, alkaline reserve using a reasonably affordable,
portable spectrometer which allows for
non-destructive data collection using an optical
fibre. The software is, were possible, adapted to
the requirements of the conservation community,
which is often particularly interested in more
degraded papers. Thus in addition to the prediction
of DP of all rag papers, a separate prediction
is made for more degraded papers, decreasing
the error associated with the method (Fig. 2).
2. pH
Given the importance of pH on stability of paper,
pH determination is likely the most often used
analytical method in paper conservation studies.
Standard method involves immersing 1 g
of paper into 70 mL of distilled water and pH is
determined after 1 hour (ISO 6588-1:2012). Since
destructive sampling is required, researchers
have minimized the sample requirements to 30-
50 μg by using a micro electrode. Miniaturisation
of the procedure decreases the repeatability with
acceptable values of 1.0 pH unit (Strlic 2004),
which was ascribed to the inhomogeneity of paper.
In the alternative standard (TAPPI T 529 om-09)
determination of the surface pH is made up to
30 min after application of a water droplet to the
surface of paper. Although non-destructive sam-
ICOM-CC Graphic Documents Working Group Interim Meeting | Vienna 17 – 19 April 2013
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