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Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral
G Model CULHER-3258; No. of Pages 5
ARTICLE IN PRESS Journal of Cultural Heritage xxx (2017) xxx–xxx
Available online at
ScienceDirect www.sciencedirect.com
Case study
Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral Lien De Backer a , Arnold Janssens a,∗ , Marijke Steeman a , Michel De Paepe b a Research Group Building Physics, Construction and Services, Department of Architecture and Urban Planning, Faculty of Engineering and Architecture, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium b Research Group Applied thermodynamics & heat transfer, Department of Flow, heat and combustion mechanics, Faculty of Engineering and Architecture, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium
a r t i c l e
i n f o
Article history: Received 12 January 2017 Accepted 9 August 2017 Available online xxx Keywords: Relative humidity Preservation Indoor climate Panel painting HVAC
a b s t r a c t Due to an uncontrolled indoor climate or a poorly designed climate system, the environmental conditions in historical buildings are often suboptimal for the preservation of works of art. This is also the case for Jan and Hubert Van Eyck’s Ghent altarpiece, which is located in one of the chapels of the Saint Bavo Cathedral in Ghent, Belgium. Years of poor conservation conditions have led to an urgent conservation treatment in 2010 and a conservation and restoration campaign that started in 2012 and will continue through 2019. In order to contribute to a better understanding of the state of preservation of the altarpiece and the display conditions and to assess damage risks related to the current location, this paper presents the results of a two-year monitoring campaign of the climate conditions in the glass cage in the Saint Bavo Cathedral in which the altarpiece is displayed. Based on the results of the first year, measures were taken to improve the indoor climate, including the installation of a local heating and humidification system. These new conditions were monitored during the second year of the measurement campaign and are representative for the display conditions today. The results of the second year showed that exposure to high humidity’s was effectively reduced but conditions with large short-term humidity variations still occurred. However, given a correct management of the new heating and humidification systems, risks for mechanical damage may be largely eliminated. © 2017 Elsevier Masson SAS. All rights reserved.
1. Introduction The panel painting “The Mystic Lamb”, painted by Jan and Hubert Van Eyck in 1432 is generally accepted to be among the most important surviving artworks in the world. The so-called Ghent altarpiece is exhibited in its historic location, the Saint Bavo Cathedral in Ghent, Belgium. A multi-year preservation project was set up when a thorough examination in 2008 raised concerns about the state of conservation of the altarpiece and the inadequate display conditions. After an urgent conservation treatment, an assessment of the condition of the altarpiece established the need for a full restoration treatment [1]. The restoration and conservation campaign of the Ghent Altarpiece started in 2012, whereby one-third of the panels is consecutively treated in a restoration studio in the Museum of Fine Arts. Although there are plans to rehouse the
∗ Corresponding author. Tel.: +32 (0)9 264 32 88; fax: +32 (0)9 264 35 90. E-mail addresses: [email protected] (L. De Backer), [email protected] (A. Janssens), [email protected] (M. Steeman), [email protected] (M. De Paepe).
altarpiece and create optimal display conditions when a new visitor centre will be constructed in the cathedral [1,2], until that time the panels are still exhibited in the glass cage where they were housed since the 1980s, also the ones that have already been restored. Therefore, in 2011, some modifications to the exhibition chapel were made in order to reduce the most problematic variations in indoor climate. In order to document the display conditions in relation to the state of preservation of the altarpiece, and to assess damage risks related to the current location, this paper presents the results and evaluation of an indoor climate monitoring campaign of the current exhibition space. The indoor climate was assessed using different criteria for conservation relating to the main cause of damage initiation observed in the altarpiece: mechanical response to climate fluctuations, with as result that the ground and paint layers of a panel painting tend to crack or delaminate with age [3,4]. ASHRAE’s classes of control of indoor climate in museums are well known [5], not only as a prescription for avoiding various risks in new HVAC-designs, but also as an assessment of risks, given existing levels of climate control [6]. The classes of control
http://dx.doi.org/10.1016/j.culher.2017.08.002 1296-2074/© 2017 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
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CULHER-3258; No. of Pages 5
L. De Backer et al. / Journal of Cultural Heritage xxx (2017) xxx–xxx
2 Table 1 ASHRAE specifications for collections [5]. Set Point or Annual Average
50% RH or historical annual average
Class
Temperature
AA A
As A
Temperature set between 15 and 25 ◦ C
Relative humidity
Risks
Tshort
Tseasonal
RHshort
RHseasonal
± 2 ◦C
± 5 ◦C
± 5%
No changes
± 2 ◦C
+5 ◦ C −10 ◦ C +5 ◦ C −10 ◦ C +10 ◦ C max. 30 ◦ C
± 5%
± 10%
± 10%
No change
± 10%
± 10%
◦
±2 C
B
± 5 ◦C
C D
< 25 ◦ C (< 30 ◦ C) –
25–75% < 75%
define ranges of acceptable fluctuations of temperature and relative humidity (RH) for collections, as a function of the damage risk to the collection, including mechanical damage, based on a review of deterioration science (Table 1) [6]. To prevent risk of mechanical damage to most artefacts, seasonal changes or short-term fluctuations of RH should be at maximum ± 10% RH. To come to a more specific assessment of microclimate fluctuations, the monitored data are further compared to criteria, which are derived from strain limits in the pictorial layers of the panels. Uzielli et al. [7] and Hunt et al. [8] emphasize that due to the specificity of each art work, in terms of geometry, structural characteristics and history, these criteria can only be defined in a reliable way by means of the simultaneous mechanical and microclimatic monitoring of the original panel paintings. However, since this data does not exist for the Ghent altarpiece, criteria available in scientific literature are used here, with the purpose to document the climate variations to which the panels have been exposed at different time scales in comparison to limits described in literature. Lukomski [4,9] defined the yield point of the gesso ground layer as the safe strain level for the pictorial layer in a panel painting. Using experimental results of fatigue damage in gesso coated wood and numerical simulations, he showed that a sinusoidal RHcycle duration of 14 days represents the most critical duration for 10 mm thick panels, while a duration of 90 days is the most critical for 30 mm thick panels. These thicknesses are representative for the side and central panels in the Ghent altarpiece respectively. Fracture of the gesso layer may not occur as long as the amplitude of the RH-cycle remains below 15% RH and 13% RH respectively. Further, Bratasz et al. [10,11] established a method to calculate a climatological risk index for panel paintings based on the rain flow counting method [12], a method used in fatigue analysis. Using this method, they reduced simulated time series of relative humidity into simple RH-cycles of known damage impact. The definition of the risk index was based on 30-day averaged RH-values, a time span that exceeds the response time of 10 mm thick panels, and the range of 12% RH (or amplitude of 6% RH) was assumed as a very conservative threshold value for fracture of the pictorial layer. Finally, EN 15757 [13] presents a methodology to specify acceptable temperature and RH fluctuations to limit climate-induced mechanical damage of organic hygroscopic materials, based on an analysis of the particular historical microclimate. When this microclimate has been proved not to be harmful, it should be maintained, except for improvements that exclude 14% of the largest historical fluctuations. However, since the historical display conditions had been shown to be inadequate [1], the EN methodology is not applied in this paper.
No risk of mechanical damage to most paintings Small risk of mechanical damage to high-vulnerability artefacts
Moderate risk to high-vulnerability artefacts, tiny risk to most paintings High risk of mech. damage Avoid mould and high-humidity deformations
2. Research aim It is the aim of this study to document the display conditions of the Ghent altarpiece in Saint Bavo Cathedral and contribute to a better understanding of its state of preservation, both in the original situation without climate control and in the modified situation which is representative for the display conditions today. It is the aim to assess potential risks related to the current location, using available criteria for conservation relating to the main cause of damage initiation observed in the altarpiece: mechanical response to climate fluctuations. 3. Monitoring campaign 3.1. Description of the case study Since the 1980s, the Ghent altarpiece has been located in the baptistery chapel (Fig. 1, location B) situated on the ground floor in the western tower of the Saint Bavo cathedral. The altarpiece is housed in a bullet-proof glass chamber, called “shrine”, constructed of 34 mm thick glass panels, mounted in a steel frame. The frame also contains the lighting system for the polyptych. Access by the public is only allowed during opening hours in the day, with sometimes up to 800 visitors on a daily basis. Because of its negative impact on the display conditions, the original lighting system had been shut down and replaced by a temporary energy-efficient lighting system already in 2009. In February 2011, additional modifications to the baptistery were made in order to reduce the observed variations in indoor climate (Fig. 1). Between the baptistery and the cathedral, an airlock was installed to reduce draught. The air tightness of the shrine was improved by sealing seams between the glass panes and installing a climate barrier in the ceiling of the shrine. Radiant heating panels (6 × 1700 W) were placed at the ceiling of the baptistery in order to control temperature and reduce relative humidity during cold winter conditions. The heating system controls consisted of a room thermostat, with a set-point temperature of about 14 ◦ C, aiming at tempering extreme conditions in winter. To avoid too low relative humidities at times of heating, a portable humidifier was installed in the shrine, with a set-point relative humidity of 40%. Until the moment that these modifications were made, the baptistery and shrine had never been heated nor conditioned. 3.2. Measurement campaign The monitoring campaign in the cathedral started in February 2010 and lasted for two years. Data loggers of the Hobo H8 Pro Series were used for registering temperature and relative humidity with an interval of 10 minutes. The measuring accuracy of these
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
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Fig. 1. Floor plan of the Saint Bavo Cathedral with location of the altarpiece (B), floor plan and cross section of the baptistery and the shrine. Location of the loggers are indicated on the cross section.
sensors was ± 0.25 ◦ C and ± 3% RH. The sensors were installed at five different locations in the building (Fig. 1-cross section). The temperatures measured at different positions in the shrine were nearly identical and were therefore averaged in the analysis. The analysis compares the indoor climate during the first year of the measurements, with uncontrolled conditions, to the climate during the second year of the measurements, when the systems described above were in operation.
4. Evaluation of display conditions 4.1. ASHRAE classes of control To evaluate the indoor climate according to the ASHRAE control classes, a similar approach was used as proposed by Huijbegts et al. [14]. The daily minimum and maximum values of the temperatures and relative humidities measured in the shrine were compared to the target values necessary to achieve one of the control classes.
Since the altarpiece has been located in the current location in the cathedral for decades, the target values for the allowable seasonal fluctuations were based on the annual average. The target values for short-term variations were based on the measured running average (mean value over 3 months) and were compared to the measured daily extremes. Fig. 2 illustrates the approach for the evaluation of the indoor climate against the ASHRAE class A. Table 2 lists the results of the assessment of the indoor climate in both years of the monitoring campaign for all ASHRAE control classes. Results are expressed as the percentage of days per year that target values for temperature, relative humidity or the combination of temperature and relative humidity were met during the monitoring campaign. As a result of the installation of the heating system and the control to a set point of 14 ◦ C, the occurrence of high relative humidity’s observed during the first year was avoided in the second year. Consequently, high-risk extremes were prevented, as the indoor climate during the second year corresponded to the targets of control class D at all times. Also, the targets of control class C were met, with the exception of a short dry period in the beginning of
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
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Fig. 2. Comparison of measurements and target values of ASHRAE class A of control: daily minimum and maximum values of temperature (left) and relative humidity (right) for the first year (a, top) and the second year (b, bottom). Table 2 Percentage of days per year that measured time series of temperature, relative humidity or the combination of temperature and relative humidity meet the criteria of the ASHRAE classes: comparison between first and second monitoring year. Year 1
Year 2
Class
T (%)
RH (%)
T & RH (%)
T (%)
RH (%)
T & RH (%)
AA As A B C D
64 69 69 100 100 100
15 15 52 52 81 81
9 9 36 52 81 81
81 81 81 100 100 100
19 19 67 67 97 100
17 17 56 67 97 100
February 2012 (Fig. 2). This problem was caused by an inadequate control and follow-up of the heating and humidification system during a week with cold and dry weather, which should be avoided with better training of the churchwardens in managing the systems. Looking at the short-term relative humidity fluctuations in the shrine during the second year, only a limited improvement was seen. As the results for class A and B in Table 2 show, the humidity variations still exceeded ± 10% RH in 33% of the days per year, compared to 48% in the first year. Although humidity measurements showed that daily variations in temperature and humidity in the shrine were reduced compared to the conditions in the baptistery (Appendix, Fig. S1), the reduction was insufficient to keep variations within the target values at all times. Carbon dioxide measurements in the shrine and the baptistery revealed that the measures taken to seal the shrine had been unsuccessful (Appendix, Fig. S2). A detailed discussion of the indoor climate evaluation according to the ASHRAE control classes is included in the appendix.
4.2. Exposure to RH-variations at longer time scales To complement the ASHRAE-evaluation, the indoor climate was further assessed according to the rain flow counting method [12], following the procedure used by Bratasz et al. [11] to estimate the accumulated fatigue damage of the pictorial layer. The measured RH time-series were reduced to sequences of 14-daily and 30-daily average RH-values and used to count the relative humidity cycles. The evaluation of the measured RH-cycles over the corresponding periods give a qualitative indication of the improvement of the climate brought about by the modifications to the chapel to reduce fatigue damage risk of the pictorial layer. Fig. 3 presents the cycle counts for different relative humidity amplitudes for the 14-daily and 30-daily averages. Looking at the 30-daily averages, both in the first and in the second year a half cycle with RH-amplitude larger than 6% occurs. As this is the threshold value considered by Bratasz et al. [11] for fracture of the pictorial layer, the comparison shows that the damage risk of the pictorial layer has not been sufficiently eliminated by the modifications to the shrine and chapel. However, this assessment needs to be put into perspective: the threshold corresponds to the limit where 100 years of diurnal elongation cycles of the pictorial layer can be tolerated [11]. To cause damage in a single cycle RH-amplitudes should be larger than 12–15% as the study of Lukomski [4] showed and lead to a full dimensional response of an unrestrained panel. Both in the first and in the second year a half cycle with RH-amplitude larger than 12% occurs when the 14 daily-averages are considered, but the cause of these fluctuations is different in both years. While in the first year the large half cycle corresponded to the yearly RHcycle in the unheated baptistery, which had been repeated every year, the large half cycle in the second year is related to the single
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
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These measures may include HVAC-systems, microclimate boxes and visitor management, depending on the location of the new visitor centre. It must be pointed out that these conclusions are based on the comparison between the monitored indoor climate and the ranges of acceptable climate fluctuations reported in literature. Since the mechanical response of each artwork to climate variations is different, depending on its geometry, structural characteristics and history, it is impossible to accurately define specific limits to prevent strain-related damage in the restored panels of the Ghent altarpiece. To improve the understanding of the climate induced risks in the panels, it is therefore recommended to setup a mechanical-hygrothermal measuring campaign as proposed by Uzielli et al. [7]. Acknowledgements Fig. 3. Histogram showing the number of counted cycles with various relative humidity amplitudes, resulting from the rain flow counting method applied on time series of 14-day relative humidity and 30-day relative humidity averages for the two different years.
event with inadequate management of the humidification system during a cold spell. If this event is avoided during the couple of years that the restored panels are exhibited in the current location awaiting the construction of a new visitor centre with optimized control, the exposure to harmful RH-variations at longer time scales may be reduced to a minimum, even though the climate is suboptimal according to the ASHRAE control classification. To achieve this, a correct training of churchwardens and follow-up of the current systems is essential to avoid critical RH-variations when the restored panels are relocated in the shrine. Since 2015, some additional measures have therefore been taken to optimize and manage the display conditions of the altarpiece and are guarded by a permanent monitoring system for easy verification of system operation. 5. Conclusion In order to contribute to a better understanding of the state of preservation of the Ghent altarpiece in relation to the display conditions, and to assess potential risks related to the current location, this paper presented the results of a two-year monitoring campaign of the climate conditions in the glass cage in the Saint Bavo Cathedral in which the altarpiece is displayed. The indoor climate was assessed using different evaluation methods and criteria relating to the main cause of damage initiation observed in the altarpiece: mechanical response to climate fluctuations. Based on the results of the first year, measures were taken to improve the indoor climate, including the installation of a local heating and humidification system. These new conditions were monitored during the second year of the measurement campaign and are representative for the display conditions today. During the restoration campaign that is currently underway, the already restored panels are displayed again in the glass cage, awaiting the future construction of a new visitor centre with more optimized control. The results of the second year showed that exposure to high humidity’s was effectively reduced but conditions with large short-term humidity variations that may cause fatigue damage risk of the panel paintings still occurred. However, given a correct management of new heating and humidification systems, exposure to the largest humidity variations may be eliminated during the couple of years that the restored panels are exhibited in the current location. The incorporated measures thus provide an acceptable solution in the short-term, but in the medium term, it is necessary to define more robust measures able to maintain the indoor climate within the specifications of ASHRAE [5] or EN 15757 [13].
This work was supported by Research Foundation Flanders (FWO-project G.0448.10N). The authors are grateful to Anne van Grevenstein (KKSB, UvA) and Anne-Catherine Olbrechts (MOV, POV) for their support and share of information. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. culher.2017.08.002. References [1] A. Van Grevenstein, R. Spronk, Lasting support: an interdisciplinary research project to assess the structural condition of the Ghent altarpiece, in: Final project report, 2011 [downloaded March 2015] http://closertovaneyck.kikirpa.be/#home/sub=documents. [2] Royal Institute for Cultural Heritage, Het adembenemende resultaat van de eerste fase van de restauratie van het Lam Gods (the breath taking result of the first phase of restauration of the Mystic Lambin Dutch), Press conference map, 2016 [downloaded December 2016] http://www.kikirpa.be/uploads/files/12-10-2016 Persmap Lam Gods.pdf. [3] B. Cornelis, T. Ruzic, E. Gezels, A. Dooms, A. Pizurica, L. Platisa, J. Cornelis, M. Martens, M. De Mey, I. Daubechies, Crack detection and inpainting for virtual restoration of paintings: the case of the Ghent Altarpiece, Sign. Process. 93 (2013) 605–619. [4] M. Łukomski, Painted wood. What makes the paint crack? J. Cultur. Herit. 13S (2012) S90–S93. [5] ASHRAE, Chapter 23: museums, galleries, archives and libraries, in: ASHRAE handbook: heating, ventilating and air-conditioning applications, SI edition, American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2011, pp. 23.21–23.22. [6] S. Michalski, The ideal climate, risk management, the ASHRAE chapter, proofed fluctuations, and toward a full risk analysis model, in: Experts’ roundtable on sustainable climate management strategies, The Getty Conservation Institute, Tenerife, Spain, 2007 http://www.getty. edu/conservation/our projects/science/climate/paper michalski.pdf (Downloaded January 2017). [7] L. Uzielli, L. Cocchi, P. Mazzanti, M. Togni, D. Jullien, P. Dionisi-Vici, The deformometric kit: a method and an apparatus for monitoring the deformation of wooden panels, J. Cultur. Herit. 13S (2012) S94–S101. [8] D. Hunt, L. Uzzielli, P. Mazzanti, Strains in gesso on painted wood panels during humidity changes and cupping, J. Cultur. Herit. 25 (2017) 163–169. [9] M. Łukomski, Addendum to “painted wood. What makes the paint crack?”, J. Cultur. Herit. 15 (2014) e1. [10] L. Bratasz, R. Kozlowski, L. Lasyk, M. Lukomski, B. Rachwal, in: J. Bridgland (Ed.), Allowable microclimatic variations for painted wood: numerical modelling and direct tracing of the fatigue damage’, ICOM Committee for Conservation, 16th Triennial Conference Preprints, Lisbon, 2011. [11] L. Bratasz, I. Harris, L. Lasyk, M. Lukomski, R. Kozlowski, Future climate-induced pressures on painted wood, J. Cultur. Herit. 13 (2012) 365–370. [12] ASTM, Standard E 1049-85 standard practices for cycle counting in fatigue analysis, 2005. [13] CEN/TC 346, EN 15757: 2010, Conservation of Cultural Property - specifications for temperature and relative humidity to limit climate-induced mechanical damage in organic hygroscopic materials, 2010. [14] Z. Huijbregts, R.P. Kramer, M.H.J. Martens, A.W.M. van Schindel, H.L. Schellen, A proposed method to assess the damage risk of future climate change to museum objects in historic buildings, Build. Environ. 55 (2012) 43–56.
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
ARTICLE IN PRESS Journal of Cultural Heritage xxx (2017) xxx–xxx
Available online at
ScienceDirect www.sciencedirect.com
Case study
Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral Lien De Backer a , Arnold Janssens a,∗ , Marijke Steeman a , Michel De Paepe b a Research Group Building Physics, Construction and Services, Department of Architecture and Urban Planning, Faculty of Engineering and Architecture, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium b Research Group Applied thermodynamics & heat transfer, Department of Flow, heat and combustion mechanics, Faculty of Engineering and Architecture, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium
a r t i c l e
i n f o
Article history: Received 12 January 2017 Accepted 9 August 2017 Available online xxx Keywords: Relative humidity Preservation Indoor climate Panel painting HVAC
a b s t r a c t Due to an uncontrolled indoor climate or a poorly designed climate system, the environmental conditions in historical buildings are often suboptimal for the preservation of works of art. This is also the case for Jan and Hubert Van Eyck’s Ghent altarpiece, which is located in one of the chapels of the Saint Bavo Cathedral in Ghent, Belgium. Years of poor conservation conditions have led to an urgent conservation treatment in 2010 and a conservation and restoration campaign that started in 2012 and will continue through 2019. In order to contribute to a better understanding of the state of preservation of the altarpiece and the display conditions and to assess damage risks related to the current location, this paper presents the results of a two-year monitoring campaign of the climate conditions in the glass cage in the Saint Bavo Cathedral in which the altarpiece is displayed. Based on the results of the first year, measures were taken to improve the indoor climate, including the installation of a local heating and humidification system. These new conditions were monitored during the second year of the measurement campaign and are representative for the display conditions today. The results of the second year showed that exposure to high humidity’s was effectively reduced but conditions with large short-term humidity variations still occurred. However, given a correct management of the new heating and humidification systems, risks for mechanical damage may be largely eliminated. © 2017 Elsevier Masson SAS. All rights reserved.
1. Introduction The panel painting “The Mystic Lamb”, painted by Jan and Hubert Van Eyck in 1432 is generally accepted to be among the most important surviving artworks in the world. The so-called Ghent altarpiece is exhibited in its historic location, the Saint Bavo Cathedral in Ghent, Belgium. A multi-year preservation project was set up when a thorough examination in 2008 raised concerns about the state of conservation of the altarpiece and the inadequate display conditions. After an urgent conservation treatment, an assessment of the condition of the altarpiece established the need for a full restoration treatment [1]. The restoration and conservation campaign of the Ghent Altarpiece started in 2012, whereby one-third of the panels is consecutively treated in a restoration studio in the Museum of Fine Arts. Although there are plans to rehouse the
∗ Corresponding author. Tel.: +32 (0)9 264 32 88; fax: +32 (0)9 264 35 90. E-mail addresses: [email protected] (L. De Backer), [email protected] (A. Janssens), [email protected] (M. Steeman), [email protected] (M. De Paepe).
altarpiece and create optimal display conditions when a new visitor centre will be constructed in the cathedral [1,2], until that time the panels are still exhibited in the glass cage where they were housed since the 1980s, also the ones that have already been restored. Therefore, in 2011, some modifications to the exhibition chapel were made in order to reduce the most problematic variations in indoor climate. In order to document the display conditions in relation to the state of preservation of the altarpiece, and to assess damage risks related to the current location, this paper presents the results and evaluation of an indoor climate monitoring campaign of the current exhibition space. The indoor climate was assessed using different criteria for conservation relating to the main cause of damage initiation observed in the altarpiece: mechanical response to climate fluctuations, with as result that the ground and paint layers of a panel painting tend to crack or delaminate with age [3,4]. ASHRAE’s classes of control of indoor climate in museums are well known [5], not only as a prescription for avoiding various risks in new HVAC-designs, but also as an assessment of risks, given existing levels of climate control [6]. The classes of control
http://dx.doi.org/10.1016/j.culher.2017.08.002 1296-2074/© 2017 Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
G Model
ARTICLE IN PRESS
CULHER-3258; No. of Pages 5
L. De Backer et al. / Journal of Cultural Heritage xxx (2017) xxx–xxx
2 Table 1 ASHRAE specifications for collections [5]. Set Point or Annual Average
50% RH or historical annual average
Class
Temperature
AA A
As A
Temperature set between 15 and 25 ◦ C
Relative humidity
Risks
Tshort
Tseasonal
RHshort
RHseasonal
± 2 ◦C
± 5 ◦C
± 5%
No changes
± 2 ◦C
+5 ◦ C −10 ◦ C +5 ◦ C −10 ◦ C +10 ◦ C max. 30 ◦ C
± 5%
± 10%
± 10%
No change
± 10%
± 10%
◦
±2 C
B
± 5 ◦C
C D
< 25 ◦ C (< 30 ◦ C) –
25–75% < 75%
define ranges of acceptable fluctuations of temperature and relative humidity (RH) for collections, as a function of the damage risk to the collection, including mechanical damage, based on a review of deterioration science (Table 1) [6]. To prevent risk of mechanical damage to most artefacts, seasonal changes or short-term fluctuations of RH should be at maximum ± 10% RH. To come to a more specific assessment of microclimate fluctuations, the monitored data are further compared to criteria, which are derived from strain limits in the pictorial layers of the panels. Uzielli et al. [7] and Hunt et al. [8] emphasize that due to the specificity of each art work, in terms of geometry, structural characteristics and history, these criteria can only be defined in a reliable way by means of the simultaneous mechanical and microclimatic monitoring of the original panel paintings. However, since this data does not exist for the Ghent altarpiece, criteria available in scientific literature are used here, with the purpose to document the climate variations to which the panels have been exposed at different time scales in comparison to limits described in literature. Lukomski [4,9] defined the yield point of the gesso ground layer as the safe strain level for the pictorial layer in a panel painting. Using experimental results of fatigue damage in gesso coated wood and numerical simulations, he showed that a sinusoidal RHcycle duration of 14 days represents the most critical duration for 10 mm thick panels, while a duration of 90 days is the most critical for 30 mm thick panels. These thicknesses are representative for the side and central panels in the Ghent altarpiece respectively. Fracture of the gesso layer may not occur as long as the amplitude of the RH-cycle remains below 15% RH and 13% RH respectively. Further, Bratasz et al. [10,11] established a method to calculate a climatological risk index for panel paintings based on the rain flow counting method [12], a method used in fatigue analysis. Using this method, they reduced simulated time series of relative humidity into simple RH-cycles of known damage impact. The definition of the risk index was based on 30-day averaged RH-values, a time span that exceeds the response time of 10 mm thick panels, and the range of 12% RH (or amplitude of 6% RH) was assumed as a very conservative threshold value for fracture of the pictorial layer. Finally, EN 15757 [13] presents a methodology to specify acceptable temperature and RH fluctuations to limit climate-induced mechanical damage of organic hygroscopic materials, based on an analysis of the particular historical microclimate. When this microclimate has been proved not to be harmful, it should be maintained, except for improvements that exclude 14% of the largest historical fluctuations. However, since the historical display conditions had been shown to be inadequate [1], the EN methodology is not applied in this paper.
No risk of mechanical damage to most paintings Small risk of mechanical damage to high-vulnerability artefacts
Moderate risk to high-vulnerability artefacts, tiny risk to most paintings High risk of mech. damage Avoid mould and high-humidity deformations
2. Research aim It is the aim of this study to document the display conditions of the Ghent altarpiece in Saint Bavo Cathedral and contribute to a better understanding of its state of preservation, both in the original situation without climate control and in the modified situation which is representative for the display conditions today. It is the aim to assess potential risks related to the current location, using available criteria for conservation relating to the main cause of damage initiation observed in the altarpiece: mechanical response to climate fluctuations. 3. Monitoring campaign 3.1. Description of the case study Since the 1980s, the Ghent altarpiece has been located in the baptistery chapel (Fig. 1, location B) situated on the ground floor in the western tower of the Saint Bavo cathedral. The altarpiece is housed in a bullet-proof glass chamber, called “shrine”, constructed of 34 mm thick glass panels, mounted in a steel frame. The frame also contains the lighting system for the polyptych. Access by the public is only allowed during opening hours in the day, with sometimes up to 800 visitors on a daily basis. Because of its negative impact on the display conditions, the original lighting system had been shut down and replaced by a temporary energy-efficient lighting system already in 2009. In February 2011, additional modifications to the baptistery were made in order to reduce the observed variations in indoor climate (Fig. 1). Between the baptistery and the cathedral, an airlock was installed to reduce draught. The air tightness of the shrine was improved by sealing seams between the glass panes and installing a climate barrier in the ceiling of the shrine. Radiant heating panels (6 × 1700 W) were placed at the ceiling of the baptistery in order to control temperature and reduce relative humidity during cold winter conditions. The heating system controls consisted of a room thermostat, with a set-point temperature of about 14 ◦ C, aiming at tempering extreme conditions in winter. To avoid too low relative humidities at times of heating, a portable humidifier was installed in the shrine, with a set-point relative humidity of 40%. Until the moment that these modifications were made, the baptistery and shrine had never been heated nor conditioned. 3.2. Measurement campaign The monitoring campaign in the cathedral started in February 2010 and lasted for two years. Data loggers of the Hobo H8 Pro Series were used for registering temperature and relative humidity with an interval of 10 minutes. The measuring accuracy of these
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
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Fig. 1. Floor plan of the Saint Bavo Cathedral with location of the altarpiece (B), floor plan and cross section of the baptistery and the shrine. Location of the loggers are indicated on the cross section.
sensors was ± 0.25 ◦ C and ± 3% RH. The sensors were installed at five different locations in the building (Fig. 1-cross section). The temperatures measured at different positions in the shrine were nearly identical and were therefore averaged in the analysis. The analysis compares the indoor climate during the first year of the measurements, with uncontrolled conditions, to the climate during the second year of the measurements, when the systems described above were in operation.
4. Evaluation of display conditions 4.1. ASHRAE classes of control To evaluate the indoor climate according to the ASHRAE control classes, a similar approach was used as proposed by Huijbegts et al. [14]. The daily minimum and maximum values of the temperatures and relative humidities measured in the shrine were compared to the target values necessary to achieve one of the control classes.
Since the altarpiece has been located in the current location in the cathedral for decades, the target values for the allowable seasonal fluctuations were based on the annual average. The target values for short-term variations were based on the measured running average (mean value over 3 months) and were compared to the measured daily extremes. Fig. 2 illustrates the approach for the evaluation of the indoor climate against the ASHRAE class A. Table 2 lists the results of the assessment of the indoor climate in both years of the monitoring campaign for all ASHRAE control classes. Results are expressed as the percentage of days per year that target values for temperature, relative humidity or the combination of temperature and relative humidity were met during the monitoring campaign. As a result of the installation of the heating system and the control to a set point of 14 ◦ C, the occurrence of high relative humidity’s observed during the first year was avoided in the second year. Consequently, high-risk extremes were prevented, as the indoor climate during the second year corresponded to the targets of control class D at all times. Also, the targets of control class C were met, with the exception of a short dry period in the beginning of
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Fig. 2. Comparison of measurements and target values of ASHRAE class A of control: daily minimum and maximum values of temperature (left) and relative humidity (right) for the first year (a, top) and the second year (b, bottom). Table 2 Percentage of days per year that measured time series of temperature, relative humidity or the combination of temperature and relative humidity meet the criteria of the ASHRAE classes: comparison between first and second monitoring year. Year 1
Year 2
Class
T (%)
RH (%)
T & RH (%)
T (%)
RH (%)
T & RH (%)
AA As A B C D
64 69 69 100 100 100
15 15 52 52 81 81
9 9 36 52 81 81
81 81 81 100 100 100
19 19 67 67 97 100
17 17 56 67 97 100
February 2012 (Fig. 2). This problem was caused by an inadequate control and follow-up of the heating and humidification system during a week with cold and dry weather, which should be avoided with better training of the churchwardens in managing the systems. Looking at the short-term relative humidity fluctuations in the shrine during the second year, only a limited improvement was seen. As the results for class A and B in Table 2 show, the humidity variations still exceeded ± 10% RH in 33% of the days per year, compared to 48% in the first year. Although humidity measurements showed that daily variations in temperature and humidity in the shrine were reduced compared to the conditions in the baptistery (Appendix, Fig. S1), the reduction was insufficient to keep variations within the target values at all times. Carbon dioxide measurements in the shrine and the baptistery revealed that the measures taken to seal the shrine had been unsuccessful (Appendix, Fig. S2). A detailed discussion of the indoor climate evaluation according to the ASHRAE control classes is included in the appendix.
4.2. Exposure to RH-variations at longer time scales To complement the ASHRAE-evaluation, the indoor climate was further assessed according to the rain flow counting method [12], following the procedure used by Bratasz et al. [11] to estimate the accumulated fatigue damage of the pictorial layer. The measured RH time-series were reduced to sequences of 14-daily and 30-daily average RH-values and used to count the relative humidity cycles. The evaluation of the measured RH-cycles over the corresponding periods give a qualitative indication of the improvement of the climate brought about by the modifications to the chapel to reduce fatigue damage risk of the pictorial layer. Fig. 3 presents the cycle counts for different relative humidity amplitudes for the 14-daily and 30-daily averages. Looking at the 30-daily averages, both in the first and in the second year a half cycle with RH-amplitude larger than 6% occurs. As this is the threshold value considered by Bratasz et al. [11] for fracture of the pictorial layer, the comparison shows that the damage risk of the pictorial layer has not been sufficiently eliminated by the modifications to the shrine and chapel. However, this assessment needs to be put into perspective: the threshold corresponds to the limit where 100 years of diurnal elongation cycles of the pictorial layer can be tolerated [11]. To cause damage in a single cycle RH-amplitudes should be larger than 12–15% as the study of Lukomski [4] showed and lead to a full dimensional response of an unrestrained panel. Both in the first and in the second year a half cycle with RH-amplitude larger than 12% occurs when the 14 daily-averages are considered, but the cause of these fluctuations is different in both years. While in the first year the large half cycle corresponded to the yearly RHcycle in the unheated baptistery, which had been repeated every year, the large half cycle in the second year is related to the single
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002
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These measures may include HVAC-systems, microclimate boxes and visitor management, depending on the location of the new visitor centre. It must be pointed out that these conclusions are based on the comparison between the monitored indoor climate and the ranges of acceptable climate fluctuations reported in literature. Since the mechanical response of each artwork to climate variations is different, depending on its geometry, structural characteristics and history, it is impossible to accurately define specific limits to prevent strain-related damage in the restored panels of the Ghent altarpiece. To improve the understanding of the climate induced risks in the panels, it is therefore recommended to setup a mechanical-hygrothermal measuring campaign as proposed by Uzielli et al. [7]. Acknowledgements Fig. 3. Histogram showing the number of counted cycles with various relative humidity amplitudes, resulting from the rain flow counting method applied on time series of 14-day relative humidity and 30-day relative humidity averages for the two different years.
event with inadequate management of the humidification system during a cold spell. If this event is avoided during the couple of years that the restored panels are exhibited in the current location awaiting the construction of a new visitor centre with optimized control, the exposure to harmful RH-variations at longer time scales may be reduced to a minimum, even though the climate is suboptimal according to the ASHRAE control classification. To achieve this, a correct training of churchwardens and follow-up of the current systems is essential to avoid critical RH-variations when the restored panels are relocated in the shrine. Since 2015, some additional measures have therefore been taken to optimize and manage the display conditions of the altarpiece and are guarded by a permanent monitoring system for easy verification of system operation. 5. Conclusion In order to contribute to a better understanding of the state of preservation of the Ghent altarpiece in relation to the display conditions, and to assess potential risks related to the current location, this paper presented the results of a two-year monitoring campaign of the climate conditions in the glass cage in the Saint Bavo Cathedral in which the altarpiece is displayed. The indoor climate was assessed using different evaluation methods and criteria relating to the main cause of damage initiation observed in the altarpiece: mechanical response to climate fluctuations. Based on the results of the first year, measures were taken to improve the indoor climate, including the installation of a local heating and humidification system. These new conditions were monitored during the second year of the measurement campaign and are representative for the display conditions today. During the restoration campaign that is currently underway, the already restored panels are displayed again in the glass cage, awaiting the future construction of a new visitor centre with more optimized control. The results of the second year showed that exposure to high humidity’s was effectively reduced but conditions with large short-term humidity variations that may cause fatigue damage risk of the panel paintings still occurred. However, given a correct management of new heating and humidification systems, exposure to the largest humidity variations may be eliminated during the couple of years that the restored panels are exhibited in the current location. The incorporated measures thus provide an acceptable solution in the short-term, but in the medium term, it is necessary to define more robust measures able to maintain the indoor climate within the specifications of ASHRAE [5] or EN 15757 [13].
This work was supported by Research Foundation Flanders (FWO-project G.0448.10N). The authors are grateful to Anne van Grevenstein (KKSB, UvA) and Anne-Catherine Olbrechts (MOV, POV) for their support and share of information. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. culher.2017.08.002. References [1] A. Van Grevenstein, R. Spronk, Lasting support: an interdisciplinary research project to assess the structural condition of the Ghent altarpiece, in: Final project report, 2011 [downloaded March 2015] http://closertovaneyck.kikirpa.be/#home/sub=documents. [2] Royal Institute for Cultural Heritage, Het adembenemende resultaat van de eerste fase van de restauratie van het Lam Gods (the breath taking result of the first phase of restauration of the Mystic Lambin Dutch), Press conference map, 2016 [downloaded December 2016] http://www.kikirpa.be/uploads/files/12-10-2016 Persmap Lam Gods.pdf. [3] B. Cornelis, T. Ruzic, E. Gezels, A. Dooms, A. Pizurica, L. Platisa, J. Cornelis, M. Martens, M. De Mey, I. Daubechies, Crack detection and inpainting for virtual restoration of paintings: the case of the Ghent Altarpiece, Sign. Process. 93 (2013) 605–619. [4] M. Łukomski, Painted wood. What makes the paint crack? J. Cultur. Herit. 13S (2012) S90–S93. [5] ASHRAE, Chapter 23: museums, galleries, archives and libraries, in: ASHRAE handbook: heating, ventilating and air-conditioning applications, SI edition, American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2011, pp. 23.21–23.22. [6] S. Michalski, The ideal climate, risk management, the ASHRAE chapter, proofed fluctuations, and toward a full risk analysis model, in: Experts’ roundtable on sustainable climate management strategies, The Getty Conservation Institute, Tenerife, Spain, 2007 http://www.getty. edu/conservation/our projects/science/climate/paper michalski.pdf (Downloaded January 2017). [7] L. Uzielli, L. Cocchi, P. Mazzanti, M. Togni, D. Jullien, P. Dionisi-Vici, The deformometric kit: a method and an apparatus for monitoring the deformation of wooden panels, J. Cultur. Herit. 13S (2012) S94–S101. [8] D. Hunt, L. Uzzielli, P. Mazzanti, Strains in gesso on painted wood panels during humidity changes and cupping, J. Cultur. Herit. 25 (2017) 163–169. [9] M. Łukomski, Addendum to “painted wood. What makes the paint crack?”, J. Cultur. Herit. 15 (2014) e1. [10] L. Bratasz, R. Kozlowski, L. Lasyk, M. Lukomski, B. Rachwal, in: J. Bridgland (Ed.), Allowable microclimatic variations for painted wood: numerical modelling and direct tracing of the fatigue damage’, ICOM Committee for Conservation, 16th Triennial Conference Preprints, Lisbon, 2011. [11] L. Bratasz, I. Harris, L. Lasyk, M. Lukomski, R. Kozlowski, Future climate-induced pressures on painted wood, J. Cultur. Herit. 13 (2012) 365–370. [12] ASTM, Standard E 1049-85 standard practices for cycle counting in fatigue analysis, 2005. [13] CEN/TC 346, EN 15757: 2010, Conservation of Cultural Property - specifications for temperature and relative humidity to limit climate-induced mechanical damage in organic hygroscopic materials, 2010. [14] Z. Huijbregts, R.P. Kramer, M.H.J. Martens, A.W.M. van Schindel, H.L. Schellen, A proposed method to assess the damage risk of future climate change to museum objects in historic buildings, Build. Environ. 55 (2012) 43–56.
Please cite this article in press as: L. De Backer, et al., Evaluation of display conditions of the Ghent altarpiece at St. Bavo Cathedral, Journal of Cultural Heritage (2017), http://dx.doi.org/10.1016/j.culher.2017.08.002