SurveNIR project: non-destructive characterisation of historical paper

In the SurveNIR project, part-funded by the European Commission, 6th Framework Programme, a consortium of research institutions and end-users have built a dedicated near-infrared (NIR) spectroscopic instrument that enables the user to determine a variety of chemical and mechanical properties of paper, including naturally aged paper. Degradation of paper-based collections is a consequence of a variety of factors, from endogenous (paper acidity, lignin content etc.) to exogenous (pollutants, humidity, etc.). In any case, long-term monitoring of large collections is needed to assess the influence of the storage environment and of the inherent material properties on the ageing behaviour of a collection. For such a task, a simple instrument is needed that would allow us to survey a collection in a non-destructive, non-invasive and chemical-free manner. The approach will be validated in several European collections in The British Library (London), Victoria and Albert Museum (London), National Archives (The Hague), National Archives (Stockholm), National Museum of Denmark (Copenhagen), National and University Library (Ljubljana) and State Archives of Dubrovnik.

Paper-based documents have long been, and still are, the most important witness to human activity. Fortunately, paper is a long-lived material provided that the production technology favours its stability and provided that it is stored in a favourable environment. However, most of the paper produced between 1850 and 1990 is not likely to survive more than a century or two due to the inherent acidity auto-catalysing the degradation of paper. Cellulose is the most important structural element of paper and it is well-known that the rate of its degradation depends on its environment.¹ Traditionally, the condition of a paper-based object or a whole collection is assessed visually, and simple physical and chemical tests are performed, such as the folding test² or determination of pH of paper using pH-indicator pens. Due to the fact that the folding test is performed in such a way that a paper corner is actually torn away and the pens leave some of the dye used as a pH indicator on the object, neither of the two tests can be described as non- or micro-destructive. Even determination of paper pH using flat surface electrode is destructive as an area of paper has to be wetted in order that the measurement can take place at all and after drying, degradation is likely to proceed faster along the wet-dry boundary.³ Surveying methods are also highly individual,⁴  however, surveys are necessary in order to reveal the condition of a collection, the general conservation needs and in order to plan preservation activities.

Mid-infrared (mid-IR) spectroscopy is widely used in cultural heritage material diagnostics. However, aged paper is a complex system and interpretation of mid-IR spectra is often difficult. On the other hand, while near-IR spectra often exhibit fewer specific features than mid-IR and Raman spectra, they are characterised by overtones and combination vibrations, especially of NH, CH and OH functional groups, and are thus potentially more information-rich. In general, near-IR spectroscopy is gaining in importance in material studies.⁵

In order to extract complex spectral information, chemometric analysis of data is the commonly accepted approach.⁶  This means that the whole spectrum (or part of it) is compared with chemical information obtained with the same set of samples. In order to develop reliable methods, we need large sample sets; in SurveNIR, we collected more than one thousand historical paper samples that can be subjected to traditional analytical methods in order to compare the results with NIR spectra. The sample set includes the paper type that is the most often encountered in library, archival and museum collections and excludes papers evidently degraded by high temperature, fungi, water, etc. A method must be carefully validated and the user must be aware of its limitations - it can only be used for analysis of the same types of objects that were used for calibration. For example, a method developed for rag papers will most probably give misleading results for contemporary papers.

Using a Partial Least Squares analysis approach, we were able to satisfactorily relate NIR spectral information and determinations of mechanical properties⁷ and pH⁸ of a variety of historical papers. This enables us to propose the methodology for rapid determination of the most important information on historical paper needed by conservators and collection managers alike. In addition to this, we also developed methods for determination of ash content, aluminium content, carbonyl group content, lignin content⁸ and tensile strength after folding,⁷  - all from a single spectrum taken in less than a second.

The intention of the SurveNIR project is to provide museums, libraries and archival collections with a tool that would provide more in-depth information than the traditional methods but would also be user-friendly and would not require extensive technical knowledge on the part of the surveyor.⁹
As part of the dedicated SurveNIR instrument, software has been developed that incorporates the chemometric data evaluation. The concept of the software allows the user to survey whole collections in view of chemical and mechanical information of paper and thus undertake actions needed for its optimal preservation.

Case studies in seven collections from European countries in three different types of paper-based collections - museum, library and archive - will be performed to validate the approach.

Acknowledgement

The authors gratefully acknowledge the support of the European Community, 6th Framework Energy, Environment and Sustainable Development Programme, contract no. SSPI-006594 (SurveNIR). The work is the sole responsibility of authors and does not represent the opinion of the Community. The Community is not responsible for any use that might be made of the data appearing herein.

References

1. Strlic, M., Kolar, J., eds. Ageing and Stabilisation of Paper , National and University Library, Ljubljana (Slovenia, 2005)
2. Buchanan, S., Coleman, S., 'Deterioration survey on the Stanford University Libraries Green Library stack collection', College and Research Libraries (1987), 48, pp.102-147
3. Eusman, E., 'Tideline Formation in Paper Objects: Cellulose Degradation at the Wet-Dry Boundary', Studies in the History of Art 51, Monograph Series II (1995), National Gallery of Art, Washington, pp.11-27
4. Taylor, J., Stevenson, S., 'Investigating subjectivity within collection condition surveys', Museum Management and Curatorship (1999), 18, pp.19-42
5. Siesler, Ozaki et al, eds. Near-Infrared Spectroscopy. Principles, Instruments, Applications ,(Weinheim, 2002), Wiley-VCH
6. Blanco, M., Villarroya, I., 'NIR spectroscopy: a rapid-response analytical tool', TRAC 21 (2002), pp.240-250
7. Lichtblau, Strlic, et al. 'Determination of mechanical properties of historical paper based on NIR spectroscopy and chemometrics - a new instrument', Appl. Phys. A (2007), submitted
8. Trafela, Strlic, et al. 'Nondestructive analysis and dating of historical paper based on IR spectroscopy and chemometric data evaluation', Anal. Chem . (2007), 79, pp.6319-6323
9. SurveNIR webpage, http://www.science4heritage.org/survenir/