(Zou 1996), giving rise to extensive use of visco-metric determinations of degree of polymerisa-tion (average number of monomer units in cel-lulose, 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 us-ing size exclusion chromatography, either usus-ing non derivatised cellulose (e.g. Henniges 2008) or derivatised (e.g. Lojewski 2010). A good correla-tion is observed between the viscometrically de-termined DP and the molar mass of carbanilated cellulose (Fig. 1).
It had been shown that in the case when cellu-lose is derivatised using phenyl isocyanate, a few fibres suffice for characterisation (Stôl 2002). De-spite 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 spec-trometer 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 non-destructive analysis. Based on the promising pre-dictions 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 af-fordable, 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 communi-ty, which is often particularly interested in more degraded papers. Thus in addition to the predic-tion of DP of all rag papers, a separate predicpredic-tion 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 stud-ies. 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 pa-per.
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-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.
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 need-ed and consequently, relatneed-ed damage inducneed-ed by the measurement (Strlic 2005). The electrode is so far not commercially available.
Considering the damage which is induced us-ing the above described methods, a non-destruc-tive method which does not require addition of water would be desirable. Recently, a good cor-relation (R = 0.9573) between measured and pre-dicted 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 charac-terised 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 informa-tion 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, stabi-lised 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 use-ful 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-destruc-tively or at worst, micro-destrucnon-destruc-tively. Also, the instrumentation should be portable in order to minimize risks to the heritage due to the trans-port. The method employed should be character-ised by high sensitivity and low detection limits, which reveal the condition, enable determina-tion 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, port-able, relatively affordport-able, 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 characterisa-tion of the artefacts during daily work in conser-vation departments.
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Acknowledgement
The support by the Slovenian Research Agency (grant L1-2401), Nationaal Archief, NL and FP7 project Nano for Art, ref. no.
282816 is acknowledged.
Author’s
Jana Kolar, Morana RTD, Oslica 1b, 1295 Ivancna Gorica, Slovenia,
fax: +38617876334
jana.kolar@morana-rtd.com info@morana-rtd.com
Gerrit de Bruin, Natonaal Archief, Prins Willem-Alexanderhof 20, 2595 BE Haagse Hout, The Netherlands
Vilma Sustar, Univerza v Mariboru, Facul-ty of Agriculture and Life Sciences, Pivola 10, 2311 Hoce, Slovenia