Paper Conservation: Decisions & Compromises

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Characterisation of Historical <strong>Paper</strong> - Possibilities and Limitations Jana Kolar | Dusan Kolesa | Gerrit de Bruin | Vilma Sustar Morana RTD, Slovenia, Nationaal Archief, The Netherlands Univerza v Mariboru, Faculty of Agriculture and Life Sciences, Slovenia Fig. 1: Correlation between molar mass of carbanilated cellulose and degree of polymerisation of cellulose. R 2 is the coefficient of determination. It describes how well a regression line fits a set of data. N is the number of data points. Introduction <strong>Conservation</strong> and preservation decisions often rely on the assessment of the condition of the artefact or the collection. Due to the numerous factors such as destructiveness of the methods, high price and specialised expertise needed, the use of analytical methods is still rather limited. Novel applications of analytical methods targeting cultural heritage materials, such as size exclusion chromatography and Near-Infrared spectroscopy address some of the drawbacks of the traditional chemical characterisation of such materials. This paper discusses advantages and limitations of some characterisation techniques used to assess the condition of historical paper. Experimental The pH of paper was evaluated using traditional cold extraction method (Tappi529 om-11) using a combined glass electrode. Equilibrium pH was determined by repeating the measurement until a constant pH reading was obtained. Size exclusion chromatography of cellulose derivatised using phenyl isocyanate derivatives was used as described previously (Kolar 2012). Viscometric determinations of the degree of polymerisation (DP) were performed according to the standard procedure ISO 5351/1. MORANA NIR 1.0 commercial software application was used to predict paper properties using Labspec NIR256-2.5 Near-Infrared Spectrometer by Ocean Optics. It has a spectral range 900-2500 nm and a sampling interval between 6 and 7 nm. It contains a temperature-regulated InGaAs detector array. The light source is an external tungsten halogen light (HL-2000-FHSA). Data acquisition was performed using a 600 μ diameter premium grade bifurcated optical cable. A single ply of the paper to be analysed was placed on several plies of Whatman paper. The spectra were collected using an optical probe. Independent sample sets were used for calibration and validation. Partial least square analysis was used to model the paper properties. Correlations were optimised using different pretreatments of the spectra and by selection of wavelengths. Discussion and Results 1. Condition of paper Condition of paper is an important information affecting conservation choices. The arsenal of methods includes determination of mechanical properties, viscometric determination of DP, chromatographic determination of molar masses and the use of Near-Infrared spectroscopy. The use of mechanical properties, such as folding endurance, tearing resistance, and bursting strength are limited due to their destructive nature and a requirement for a large paper sample, often exceeding 10 g. In addition, relative uncertainties of most of these methods are rather high. It had been demonstrated that mechanical properties correlate with molar mass of cellulose ICOM-CC Graphic Documents Working Group Interim Meeting | Vienna 17 – 19 April 2013 42
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 43
Page 1 and 2: Paper Conservation: Decisions & Com
Page 3 and 4: Content Lieve Watteeuw Introduction
Page 5: Michael Wheeler | Nicholas Barnard
Page 8 and 9: New Trends in Preservation in the D
Page 10 and 11: and monitoring against adverse envi
Page 12 and 13: Steve Hobaica, Patrick Loughney, Ma
Page 14 and 15: The Albums of Duke Charles de Croÿ
Page 16 and 17: Fig. 4: turning the supporting pape
Page 18 and 19: Fig. 2: Close-up of some of the los
Page 20 and 21: manuscript paper, the binding would
Page 22 and 23: Figure 2 partial sewing: the blue d
Page 24 and 25: Books in Exhibitions: History and A
Page 26 and 27: Fig. 4 Museum in London and the pre
Page 28 and 29: Risk and Safety of Illuminated Manu
Page 30 and 31: Fig. 2 Atlas, the Crown papers are
Page 32 and 33: Acknowledgments The authors would l
Page 34 and 35: Verdigris I: Compromises in Conserv
Page 36 and 37: Verdigris 2: Wet Chemical Treatment
Page 38 and 39: Integrated Modelling: The Demograph
Page 40 and 41: Fig. 3: The consequence of 50 insta
Page 44 and 45: pling is required, tide-lines are c
Page 46 and 47: Study of Phytate Chelating Treatmen
Page 48 and 49: Evaluation of stochiometric effects
Page 50 and 51: Practice and Progress in the Conser
Page 52 and 53: Fig. 1 Fig. 2 plex picture and prov
Page 54 and 55: Fig. 2: Coloured paper strips place
Page 56 and 57: the fire (on the other hand paper,
Page 58 and 59: inform decisions on conservation tr
Page 60 and 61: This paper summarizes what is known
Page 62 and 63: 2. Methods An interdisciplinary met
Page 64 and 65: Fig. 4: Dyed endpaper pasted at the
Page 66 and 67: Preservation of Architectural Drawi
Page 68 and 69: Notes 1 According Salvador Muñoz V
Page 70 and 71: course of time under the influence
Page 72 and 73: the curators in review of the treat
Page 74 and 75: cellulose powders with cellulose et
Page 76 and 77: Authors Xing Kung Liao Conservator
Page 78 and 79: Fig. 2: Our Railroad Workers and th
Page 80 and 81: The Restoration of Cartoons at the
Page 82 and 83: tation presumes that the works are
Page 84 and 85: To Remove or Retain? - Extensive In
Page 86 and 87: Fig. 4 repeated applications until
Page 88 and 89: The Migration of Hydroxy Propyl Cel
Page 90: tion issue allowed a visualisation
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Ethical Considerations Concerning t
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emoved with damp cotton swab. Then
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Conservation of a Book of Hours fro
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Fig. 4: Pigments movement and conse
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Notes 1 Brazilwood lake, lapis lazu
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pages, which would easily have torn
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The Microflora Inhabiting Leonardo
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Acknowledgments The authors would l
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Microorganisms in Books - First Res
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hyaline white mycelia on and in the
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Deconstructing the Reconstruction E
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Fig. 3: Poster after 2012 conservat
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Conservators’ Investigation of Ch
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had been used, may be false-positiv
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function will provide a holistic pi
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Fig. 2: Patriarchs of Chan Buddhism
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ing techniques, which build upon fu
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Applications of Image Processing So
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Fig. 3 Fig. 4 images in a single wi
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Analysing Deterioration Artifacts i
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Fig. 3 similarity maps. Similarity
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Strategy in the Case of a Wrecked P
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without preliminary consolidation.
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Fiber Optic Reflectance Spectroscop
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Fig. 3: 18 th century paper seal, i
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Fig. 3 Fig. 4 of responsibilities r
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Characterising the Origin of Carbon
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Fig. 3: Sampling depth for ATR spec
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Fig. 2: Filmoplast® tape attached
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Pluchart, F. 1971. Pop Art Et Cie.
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Fig. 3: The reverse of left screen
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porting the reverse of severe creas
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Paper Conservation: Decisions & Compromises

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