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Journal of Cultural Heritage 14S (2013) e161–e164

Available online at

www.sciencedirect.com

ong-term hygromechanical monitoring of Wooden Objects of Art (WOA): A toolor preventive conservation

ttaviano Allegretti a, Matteo De Vincenzib, Luca Uzielli c, Paolo Dionisi-Vicid,∗

IVALSA-CNR, S.-Michele All’Adige, ItalyIBIMET-CNR, Sassari, ItalyDEISTAF-UNIFI, Firenze, ItalyThe Metropolitan Museum of Art, New York, USA

a r t i c l e i n f o

rticle history:eceived 24 October 2012ccepted 27 October 2012vailable online 28 February 2013

eywords:ong term monitoringensitivityooden objects of artuseum climate

ariance analysisCA

a b s t r a c t

Case studies monitoring wooden objects have been conducted for many years. In some studies the mon-itoring was limited, for longer or shorter periods, to air Temperature and Relative Humidity logging,which can show if extreme values and rates of variation occur. In other cases mechanical monitoringwas combined with microclimatic logging, which provides quantitative information directly related tothe microclimate; these data are useful to validate mathematical models that eventually may predictthe long-term behaviour of the objects. Although the quality of the information obtainable by combinedmechanical-hygrothermal monitoring is more directly usable, due to the actual response to the micro-climate, using simply logged microclimatic data it is possible to formulate a statistical analysis aimed atdefining microclimate variance. Museums all over the world are engaged in lively discussions regardingthe long-term conservation of works of art created using hygroscopic materials, which are sensitive tomicroclimate fluctuations. The climate fluctuations can have both temporary and permanent effects onthe hygroscopic objects and they are a potential cause of damage. The current preventive approach isbased on a compromise between the technical limitations of the museums’ air conditioning plants andthe presumed needs of the objects, as determined by conservators and conservation scientists. The pri-mary goal is to keep the climate as stable as possible around standard values, with strict fluctuationranges usually defined as 20 ◦C ± 2 and 50% RH ± 5. There is considerable pressure in favor of wideningthe allowable ranges, based on the need of a lighter carbon footprint as well as to facilitate the loan ofartifacts between institutions. Although we have long-term evidence of the generally positive effects ofa microclimate within the standard range of allowable fluctuations, we lack experimental data regardingthe effects under broader ranges. Wooden works of art are useful in representing the complexity ofpossible reactions. Because of the mechanical response caused by thermo-hygrometric conditions, themonitoring of Wooden Objects of Art (WOAs) in their exhibition and storage environment is importantin order to protect them from potential physical/mechanical degradation. Due to the specificity of eachartwork, both from its structural point of view and from its previous microclimatic history (for the mostpart totally unknown), the analysis of an artifact’s response to short- and long-term variations can supplyuseful information about its “individual” sensitivity to the exhibition microclimate, suggesting the adop-tion of more or less rigid parameters. Case studies monitoring wooden objects have been conducted formany years. In some studies, the monitoring was limited, for longer or shorter periods, to air T/RH logging,which can verify if extreme values and types of variation occur. In other cases, mechanical monitoringwas combined with microclimatic logging, which provides quantitative information directly related tothe microclimate; these data are useful to validate mathematical models that eventually may predictthe long-term behavior of the objects. Although the quality of the information obtainable by combinedmechanical-hygrothermal monitoring is of higher magnitude, using simply logged microclimatic data

makes possible to formulate a statistical analysis aimed at defining microclimate variance, obtaining avery efficient schematization olower or higher sensitivity of

how delicate the widening of

while the eventual choices of

response of the objects.

∗ Corresponding author.E-mail addresses: allegretti@ivalsa.cnr.it (O. Allegretti), m.devincenzi@ibimet.cnr.it (M

uca.uzielli@unifi.it (L. Uzielli), paolo.dionisivici@metmuseum.org (P. Dionisi-Vici).

296-2074/$ – see front matter © 2013 Elsevier Masson SAS. All rights reserved.ttp://dx.doi.org/10.1016/j.culher.2012.10.022

f the greater or lesser stability of the climate according to the presumed

the artifact under consideration. The aim of this paper is to demonstrateallowable ranges is and how misleading a generalized approach can be,standards relaxation must be based on careful analysis of the long-term

© 2013 Elsevier Masson SAS. All rights reserved.

. De Vincenzi),

e ltural Heritage 14S (2013) e161–e164

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The result of the filtering operation is shown in the Fig. 1.In order to clarify what the methodology allows to obtain (the

“climatic distance between different microclimates” of the paper’stitle [6]), the Figs. 2 and 3 are shown:

162 O. Allegretti et al. / Journal of Cu

. Introduction

The microclimate fluctuations are considered a potential causef stress and damage for hygroscopic materials in general and forooden Objects of Art (WOA) in particular.The stratification of the mechanical effects of fluctuations during

enturies is a known issue on panel paintings [1]. The permanentupping can be, in the less harmful case, a simple even unrecover-ble distortion of the visual experience of the painted surface but,ue to the complexity of the mechanical and technological charac-eristics of the WOA, the mechanical stress can induce cracks in theooden structure and color fall requiring delicate interventions to

estore the painted surface [2].The debate about the standards for the stability of climate in

xhibition conditions is becoming very lively because of aspectshat are not tightly related to the objects response to the width ofhe acceptable microclimatic ranges but to economic and sustaina-ility reasons [3].

The criteria proposed in previous standardization procedures4] are based on the attempt to find a compromise between theeed to save economical resources and the preservation needs ofhe objects: the specific preservation criteria for the objects arextrapolated by data that are mostly based on dummies and, whenbtained, on real objects [5], they are still inadequate due to thetructural and physical complexity of the objects population.

On the other hand, the conservators’ community is aware of theeculiar sensitivities that can make the response of different objectsot easily traceable to the standards.

The best criteria to get to the relaxation of the present stan-ards (probably too strict and certainly too heavy to be supportedconomically and ecologically) could be the widening of the exper-mental support to the choices to be adopted.

In this frame, in a recent paper on Studies in Conservation [6],n algorithm designed to give a synthetic index of the reactivity ofooden objects had been proposed to the scientific community.

The so defined sensitivity, which doesn’t necessarily involve theoncept of damage, is a synthetic descriptor that includes physical-echanical characteristics that can influence the response of the

bject to the microclimate conditions.The concept of sensitivity of the objects and the deriving poten-

ial damage has been used also on a fundamental reference book,he 2007 ASHRAE handbook [7], and the modification that is underiscussion can be synthetically described as a change from the cur-ently adopted AA class of control to the A/B class (the details of theay this change should happen are currently under discussion).

The aim of this paper is to show how the same microclimate cane “felt” by hypothetical objects with different sensitivities usinghe parameter developed in the cited methodology.

The simulation has been carried out using a Finite Elementethod (FEM) model where some physical-mechanical parameters

ave been specifically tuned for the simulation.

. Materials and methods

The data we used are part of a larger monitoring campaignroject carried out in the North-East of Italy, in the Provinciautonoma di Trento, in a storeroom of the Superintendency ofrento where an important painting by Jakob Seisenegger is storedaiting to be restored and in a church in an alpine region.

The data sets used in this paper are composed of the microcli-atic series (T and RH) and, for the storeroom case-study, of the

ygromechanical log in parallel to the first ones.The hygromechanical monitoring has been carried out accord-

ng to a specific geometric approach called Deformometric Kit, aelf-powered system able to measure for long periods the local

Fig. 1. EMC of air vs time, unfiltered and filtered.

deformation in plane and out of plane of a wooden object [8].Moreover, a high sensitivity mass variation system was designedand used to measure the mass variation of the panel painting dueto sorption phenomena [9]. The hygromechanical data obtaineddirectly from the panel painting are important as a reference for thetuning and validation of the physical model, as already describedin other papers [10,11] but they will not be used in this paper. Themicroclimatic series used as an applicative example is relative toa period of 120 days with a one hour sample rate in the periodbetween autumn and winter characterised by daily fluctuations andone seasonal abrupt climatic change due to the switching on of theheating.

2.1. Microclimatic analysis

The approach that has been used consists in analysing a real cli-matic series of a monitored object using the methodology proposedin the SIC paper [6]: after the transformation of the Temperatureand Relative Humidity in Equilibrium Moisture Content (EMC) [12]combining T and RH in one value, the climatic series is filtered by thealgorithm using different ı values, where ı is the supposed fluctu-ation range that doesn’t induce significant responses of the object.This means that a bigger ı value represents a less sensitive object.

Fig. 2. The climatic distance among different climatic series using ı = 0.5.

O. Allegretti et al. / Journal of Cultural Heritage 14S (2013) e161–e164 e163

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Fig. 4. Simulated average volume MC of the board vs time under different climate.

Fig. 5. Simulated cupping deformation of the board due to anisotropy vs time underdifferent climate.

ig. 3. The climatic distance among different locations using ı = 1 (FDI: Seiseneggertoreroom; MDC A: alpine location year A; MDC B: alpine location year B).

in these charts, the displayed ellipses represent the statisticalsynthesis of the whole climatic series putting in relation the EMCvalues obtained by the climatic series with the duration in hoursof the stable periods (plateaus) obtained by the filtering opera-tion;the differences of the duration of the stable periods show howdifferently is “felt” the same microclimatic series by objects withdifferent sensitivities.

The charts are obtained using the Principal Component AnalysisPCA) statistical technique [13].

.2. Hygromechanical analysis

The second phase of the analysis of the supposed objectehaviour is based on the simulated physical and mechani-al response of a virtual wooden sample under different EMCime series, i.e., unfiltered and filtered with ı = 0.5 and ı = 1. Theygromechanical model has been developed using a FEM com-ercial software (COMSOL 4.2a) The parameters used have been

xperimentally determined in laboratory [14] and tuned by reverseethod to fit the experimental data.The 2D, time-dependent model calculates the mass transport

f water in solid wood governed by diffusion and the stress-strainnder linear elastic mechanic. Geometry of the domain is a rect-ngle, representing a transversal section of the panel 13 mm thick,ying in a cylindrical coordinate system with the centre (the pithf the virtual log) at a distance of 50 mm from the geometric cen-re of the rectangle (representing a sub-tangential board). Detailsf the model can be found in references [10,11,14]. This represen-ation, albeit under some simplified assumptions, gives importantnformation on the behaviour of the object, although modeling ofeal WOA is often very complicated [15,16]; in the latter case thenalysis of the behaviour becomes more delicate.

The following data sets have been post processed and reportedn Figs. 4–7:

volume average moisture content (MC);cupping deformation of the board due to the anisotropy of thesubtangential board;core G: the ratio of MC between the core and the surface of theboard;superficial G: the ratio of MC between the surface and the sur-

rounding EMC of air.

G is positive when desorption phenomena occur and vice versa.he magnitude of the absolute value of G is an indicator of the

Fig. 6. Simulated core G vs time under different climate.

level of the internal stresses due to unsteady MC gradient along thethickness of the board. This parameter is commonly used for thecontrol of the kiln wood drying processing where normally dryingoccurs setting G in the range of about 1.2÷3.

e164 O. Allegretti et al. / Journal of Cultural

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[ical analysis with application to wood structures of cultural heritage, Int J NumMethods Engin 88–3 (2011) 228–256.

[16] J. Colmars, Hygromécanique du matériau bois appliquée à la conservation dupatrimoine culturel: étude sur la courbure des panneaux peints, PhD thesis,LMGC Université Montpellier2, France, 2011.

Fig. 7. Simulated superficial G vs time under different climate.

. Discussion

The trends of average MC (Fig. 2) and cupping deformationFig. 3) of the virtual board under different climatic conditions doot differ in a significant way. In particular, it can be observed thathe filtered climate and in particular ı = 1 dampens the small shorteriod variation while the trend during the big seasonal variation

s quite the same.The same can be observed for core G (Fig. 6). Here, a high

ositive peak shows up during the seasonal variation due to thee-humidification effect provided by heating. In correspondencef this event, a high stress level with tension at the surface andompression at the core is predicted.

On the contrary, a different behaviour can be observed for theuperficial G (Fig. 7) of the board at different climatic conditions.he unfiltered EMC produces small but high frequency (daily) MCradients close to the surface. The filtered EMC reduces the fre-uency but it amplifies the magnitude of such gradients. Thismplification effect is proportional to the delta used.

. Future developments

The sensitivity index adopted in the analysis of microclimaticeries is at present a pure number, though based on physicalarameters. This approach makes it very easily implementable inutomated monitoring systems, like control units of HVAC systems,nd it can be used as a tool to compare different climatic situations,n case of evaluating the opportunity of a loan.

The index is currently being related to the mechanical responsesbtained in monitoring case studies in order to give to it a strongeruantitative flavor.

. Conclusions

The proposed analytical approach suggests how different can behe response of objects with different sensitivities. Though the sen-

Heritage 14S (2013) e161–e164

sitivity index doesn’t necessarily involve damage-risk information,the tool allows highlighting the individual (lato sensu) response inopposition with the standardized approach.

This approach can be useful for moving towards a moreinformed choice, because, by adopting a generic approach, the mostsensitive objects should necessarily have a guiding role that wouldnot allow the decision-makers to widen the range as much as pos-sible. On the other hand, once the actual distribution of the variousclasses of stability has been evaluated, specific decisions on themost sensitive objects can be adopted without interfering with thegeneral standard relaxation.

References

[1] R.D. Buck, Some applications of rheology to the treatment of panel paintings,Studies Conserv 17 (1) (1972) 1–11.

[2] P. Dionisi-Vici, M. Formosa, J. Schiro, L. Uzielli, Local deformation reactivity ofpanel paintings in an environment with random microclimate variations: theMaltese Maestro Alberto’s Nativity case-study, in: J. Gril (Ed.), Wood Science forConservation of Cultural Heritage, Braga 2008: Proceedings of the InternationalConference held by COST Action IE0601, Braga, Portugal, 5–7 November 2008,FUP, Firenze, 2010, pp. 180–185.

[3] Rethinking the Museum Climate, Boston, MFA, April 12–13 2010,http://blog.conservation-us.org/blogpost.cfm?threadid=2227&catid=175

[4] prEN 15999-1 CEN-TC346 Conservation of cultural property – Guidelines formanagement of environmental conditions - Recommendation for showcasesused for exhibition and preservation of cultural heritage – Part 1: Generalrequirements expected in 2014.

[5] S. Jakiela, L. Bratasz, R. Kozlowski, Numerical modelling of moisture movementand related stress field in lime wood subjected to changing climate conditions,Wood Sci Technol 42–1 (2008) 21–37.

[6] P. Dionisi-Vici, M. De Vincenzi, L. Uzielli, An analytical method for the deter-mination of the climatic distance between different microclimates for theconservation of Wooden Cultural Heritage Objects, Studies Conserv 56–1(2011) 41–57.

[7] 2007 ASHRAE Handbook – HVAC applications (SI), pp. 21.13, 2007, ISBN 978-1933742151.

[8] P. Dionisi Vici, J. Colmars, L. Uzielli, Instrumentations pour le contrôl continudes panneaux peints en bois, Techne 29 (2009) 21–27, Paris.

[9] P. Dionisi-Vici, O. Allegretti, Metodologie di analisi deformativa e di variazionedi massa su manufatti lignei posti in ambienti a clima variabile: rilevanzadiagnostica e problematiche legate alla misurazione in situ, in: PRIN2003: LaDiagnostica e la Conservazione di Manufatti Lignei, Nardini Editore, Firenze,2007.

10] O. Allegretti, F. Raffaelli, WCHO’s behavior during seasonal RH fluctuation inheated churches in the Alps., 1st workshop of COST Action IE0601, 2007, Ter-vuren, Belgium.

11] O. Allegretti, F. Raffaelli, P. Dionisi-Vici, The case-study of “The daughters ofthe Emperor Ferdinand I” by Jakob Seisenegger, in Trento (Italy): analyticalhygro-mechanical results for preventive conservation and as a support in riskassessment for technical interventions, in: Proceedings of the 5th InternationalCongress on “Science And Technology For The Safeguard Of In The Mediter-ranean Basin”, Istanbul, 2011.

12] A.J. Hailwood, S. Horrobin, Absorption of water by polymers: analysis in termsof a simple model, Trans Faraday Soc 42B (1946) 84–102.

13] K. McGarigal, S. Cushman, S. Stafford, Multivariate statistics for wildlife andecology research, Springer, New York, 2000.

14] O. Allegretti, F. Raffaelli, Barrier effect to water vapour of early european paint-ing materials on wood panels, Studies Conserv 53–3 (2008) 187–197.

15] D. Dureisseix, B. Marcon, A partitioning strategy for the coupled hygromechan-