Content uploaded by Lukasz Bratasz
Author content
All content in this area was uploaded by Lukasz Bratasz on Jan 15, 2019
Content may be subject to copyright.
Content uploaded by Michał Łukomski
Author content
All content in this area was uploaded by Michał Łukomski on Oct 18, 2018
Content may be subject to copyright.
Published in:
Studies in Conservation 2018, VOL. 63, NO. S1, S151-S155
https://doi.org/10.1080/00393630.2018.1504447
HERIe - a web-based decision-supporting tool for assessing risk of
physical damage using various failure criteria
Arkadiusz Kupczak1, Mariusz Jędrychowski2, Marcin Strojecki1, Leszek
Krzemień1, Łukasz Bratasz3, Michał Łukomski4, Roman Kozłowski1
1 Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,
Krakow, Poland
2 The National Museum in Krakow, Poland
3 Institute for the Preservation of Cultural Heritage, Yale University, New Haven, USA
4 The Getty Conservation Institute, Los Angeles, USA
Contact: Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of
Sciences, Niezapominajek 8, PL-30239 Kraków, Poland, nckupcza@cyf-kr.edu.pl
ABSTRACT:
HERIe is a web-based decision-supporting software tool to facilitate the management of
collection environments by precise assessment of climate-induced risk of physical
damage to vulnerable objects. The software translates the relative humidity and
temperature data recorded in the environment of the analysed object into a strain history
experienced by it, and estimates the risk of physical damage using selected failure
criteria. As all information is pre-calculated for the user, no engagement in complex and
time-consuming numerical simulations is required. HERIe is available for testing at
herie.mnk.pl. Detailed information on the methodologies used by the software is
available at the above website which also contains tutorial climates. The current work
aims at refining HERIe by selecting the damage criteria not only from laboratory studies
but also from direct monitoring damage accumulation in objects, especially using the
acoustic emission method. The software has been expanded to include moisture-induced
response of parchment to support managing environments in libraries and archives.
Keywords: damage; risk-assessment; software; decision making; indoor climate;
acoustic emission; painted wood; parchment
Introduction
To ensure appropriate indoor environmental conditions in buildings housing collections
to maintain high standards of collection care while reducing energy use is currently in
the spotlight of cultural heritage institutions worldwide (Staniforth 2014). To reduce
energy consumption would mean ‘greening’ memory institutions so that they preserve
cultural heritage while respecting the natural environment and resources. This attitude
has been succinctly summarized in the IIC/ICOM-CC Declaration on environmental
Guidelines (2014):
Care of collections should be achieved in a way that does not assume air conditioning
(HVAC). Passive methods, simple technology that is easy to maintain, air circulation
and lower energy solutions should be considered. Risk management should be
embedded in museum management processes.
Climate-induced damage of materials sensitive to relative humidity (RH) is an
important type of risk in most museum and library collections as such materials undergo
dimensional change when they lose or gain moisture. The constraint from free
movement or the mismatch in the response of individual materials induces tensile or
compressive stresses in the objects, which can cause deformation, cracking and
delamination of the materials. In the 1990s, the dimensional response of the objects to
changes in temperature and RH (termed ‘strain’ from here on) and the critical strain
levels at which materials begin to deform or fail were systematically examined by
Mecklenburg, Tumosa, and Erhardt (1998). The yield stress/strain of materials was
proposed as a conservative ‘failure criterion’, that is, moisture-induced strains in
materials should stay at all times in the elastic (reversible) region. Consequently,
rational and science-based specifications and standards for the control of climate in
museums and historic buildings were formulated (Bratasz 2013).
However, universal standards can be just a starting point for specifications of
environmental conditions addressing individual risks in storage, display and loans of
collections, or in preservation of decoration, fittings and finishes in historic buildings, of
varying climatic sensitivity and acclimatized to a particular local environment. Moving
to variable guidelines requires more evidence-based decision-making, which is time
consuming and often requires engagement in highly specialised modelling and
monitoring of response of objects to microclimatic variations. To overcome the barrier,
HERIe – a decision-supporting software tool for a quantitative assessment of risk of
physical damage for various categories of cultural objects vulnerable to real-world
climates – has been developed by a collaborative effort of several institutions.
The software examines the impact of one-year (or multi-year) temperature and RH
microclimate data measured in a given room, or simulated for various climate-control
scenarios. Thermal expansion or contraction have a minor effect on the overall
dimensional changes of sensitive historical materials as compared to their responses to
moisture. Therefore, only yearly or multi-year RH variations are decomposed, in the
first step of the HERIe algorithm, into a set of elementary sinusoidal RH cycles of
variable amplitude and duration using Fourier transformation. The elementary RH
cycles are then translated into elementary strain cycles of the selected object, using the
pre-calculated elementary strain cycle database, which constitutes the core of HERIe. In
the next step, the full strain-time history of an object is obtained by summing up the
elementary strain cycles. It has been verified that the superposition of the elementary
strain cycles performed by HERIe is in agreement with the outcome of the full
numerical simulation of response of the object to moisture. In the final step of the
analysis, the damage criterion is selected by the user and the environmental risk index
for the object stored and exhibited in the analysed environment is calculated.
Managing collection environments by precise risk assessment
The HERIe environmental data analysis tool facilitates quantitative assessment of risk
of physical damage induced by real-world microclimates, measured or simulated. The
software application to managing the collection environments will be illustrated by risk
of physical failure of the gesso layer on an unrestrained wooden panel subjected to RH
variations recorded in one of the galleries in the National Museum in Krakow (NMK),
Poland, in 2013 (Figure 1). Strain at break of 0.2% was assumed for stiff and brittle
gessoes at the RH low- and mid-range, considered the worst case conditions for the
gesso’s vulnerability to fracture (Rachwał et al. 2012).
Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov 1
20
25
30
35
40
45
50
55
60
65
70
75
80
RH (%)
Date
yearly average RH 45%
Jan 1 Mar 1 May 1 Jul 1 Sep 1 Nov 1
-0.2
-0.1
0.0
0.1
0.2
Strain (%)
Figure 1. Strain versus time history (right) experienced by a 0.4 mm thick gesso layer
laid on a 10 mm, tangentially-cut wooden panel open to a water vapor flow just through
the bare wood surface opposite to the painted face, exposed to RH variations (left) in a
gallery in the National Museum in Krakow in 2013. It was assumed in the calculations
of strain that no internal stresses were present in the object at the long-term average RH
of 45%. The ranges of RH and strain, recommended as safe, are marked with orange
lines.
The RH variations in the gallery stayed most of the time within the range of 35–55%
which can be interpreted as safe for objects containing hygroscopic material
(IIC/ICOM-CC 2014), however, with one exception. In March, two repeated falls in
indoor RH – to below 25% – were recorded due to insufficient humidification by the
climate-control system. However, the calculated strain which would be experienced by
a gesso layer laid on a wooden panel exposed to such RH variations just stayed below
the critical strain of 0.2% during the panel shrinkage induced by the episode and,
therefore, the variations were assumed not to involve any risk of gesso fracture for the
type of object analysed. The risk index was derived from the strain histories by the
HERIe software. When absolute maximum magnitude of strain experienced by the
gesso layer did not exceed the critical strain of 0.2%, the risk index was 0. When
absolute maximum magnitude of strain experienced by the gesso layer exceeded 0.4%,
or double the strain at break, the risk index was 1. The risk index increased linearly
between 0 and 1 for maximum strain magnitudes of between 0.2% and 0.4%.
The assessment of the risk of damage to the gesso layer on a wooden panel can be used
in analysis of RH variations simulated for what-if scenarios of interest to a museum or
other institution. Quick reaction to a failure of a climate control system in a gallery can
reduce the damage risk as the RH variations are only damaging to painted wood when
they last longer than the response time of the given panel. Figure 2 shows again the
shrinkage experienced by the gesso during the first episode of indoor RH in March,
2013 but the actual dimensional change profile is compared with those simulated for
prolonged duration of the low RH condition. During the actual episode, RH stayed
below 30% between March 15 and 18, and reached its minimal value of below 25% on
March 17 at 8 pm. The duration of the episode in the RH data was numerically
prolonged by maintaining RH on this minimal level for an increasing number of days.
The plots demonstrate that increasing duration of the fall in RH gradually augmented
damaging impact as the panel came closer to its full shrinkage in equilibrium with the
new dry condition in the environment. The risk index reflecting this damaging impact
increased from 0 to approximately 0.3 and allowed those caring for the collection to
assess quantitatively the relative risk related to time interval between the failure of the
climate-control system and the reaction of the staff dealing with the failure. The
response time of objects, specific for each object category, is an important factor for the
reduction of the risk of mechanical damage created by the climate control failures.
Detailed knowledge of the response on the time of exposure facilitates planning the
collection environment management process.
-0.3
-0.2
-0.1
0.0
0.1
Strain (%)
as recorded
1 day
2 days
4 days
7 days
Feb 27 Mar 6 Mar 13 Mar 20 Mar 27
Date
Figure 2. Strain versus time history as in Fig. 1, induced by an episode in RH fall in
March 2013 and simulated for prolonged duration of the low RH condition by one to
seven days.
Refining the damage criteria
As discussed above, critical strains of materials, derived from laboratory studies of
moisture-related and mechanical properties of wood and gesso, have been used so far in
the software to quantify the risk of damage. However, laboratory analyses are
predominantly carried out for new materials which do not necessarily reflect the
material properties and vulnerability to damage of historical objects which have
acclimatized to a particular indoor environment. Additionally, mechanical studies of art
and artists’ materials often yield discrete values of thresholds of the environmental
variations, whereas in real-world conditions the failure of historical objects visible to
those caring for the collection is preceded by the progressive evolution of micro-
damage. Therefore, the assessment of risk of damage of objects acclimatized to their
long-term storage or display conditions has been increasingly supported by recording
climate-induced damage directly in a continuous way or at a specified time interval. The
acoustic emission (AE) method, which is based on monitoring the energy released as
sound waves during fracture processes in materials, has been particularly successful in
monitoring the fracturing intensity in wood (Łukomski et al. 2017).
Figure 3 shows magnitudes of crack propagation in wooden elements of the eighteenth-
century pieces of furniture monitored for more than one year in the NMK (Strojecki et
al. 2014) and the Victoria & Albert Museum in London (Pretzel 2014) as a function of
amplitude for the episodes of restrained shrinkage induced by decreases in the indoor
RH. The plot demonstrates that no fracturing occurs for shrinkage strains below 0.2%
but also at higher strains the crack propagation is minute for any practical assessment of
damage, and could be recorded only owing to the amazing sensitivity and
reproducibility of the AE sensors. Studies of fracturing intensity in several wooden
objects exposed to larger RH variations in the laboratory further confirmed the
observations that climate-induced cracking of wood was minute – that is in the range of
mm2 – in historical objects which experienced uncontrolled environments in its past
(Łukomski et al. 2018).
The data collected so far can be interpreted by the ‘acclimatization’ of wood objects
monitored to presumably large RH fluctuations in the past which caused fracturing. The
existing cracks open and close reducing the stress and the risk of physical damage
beyond that already accumulated in the past. Conservation treatments can erase safety
margins of objects achieved by their acclimatization to unstable past climates. If cracks
in polychrome sculpture, furniture, or panel paintings act as expansion joints relieving
stress in the objects, their consolidation may make the objects more vulnerable to
climate fluctuations (Michalski 2009).
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45
-0.5
0.0
0.5
1.0
1.5
2.0
Crack propagation (mm)
Strain (%)
Figure 3. Crack propagation as a function of shrinkage strain experienced by wooden
elements in furniture (left). The data were obtained from the AE monitoring using
sensors located close to the tips of the existing cracks (right).
Managing environments in libraries and archives – the case of parchment
Research into moisture response and mechanics of art and artists’ materials has so far
predominantly focused on objects of fine and decorative art. This has been an important
drawback when it comes to historical hygroscopic materials relevant to library and
archival collections – paper, board, parchment and leather. For example, leather over
wooden covers for incunabula or attached with animal glue to boards for book bindings
may experience deformation or physical failure because of restraint on its moisture-
induced dimensional response. As a further stage of HERIe software development,
parchment was encompassed as a library material particularly vulnerable to moisture
which can be regarded as the worst case concern guiding requirements for safe
environmental conditions for the library and archival collections.
010 20 30 40 50 60 70 80 90
0
1
2
3
4
5
6
7
Dimensional change (%)
RH (%)
Figure 4. The experimental data for moisture-induced swelling of historical parchment
specimens are compared with the average curve calculated from the least-square
regression of the data to the second order polynomial.
In Figure 4, the experimental data for moisture related swelling of four samples of
historical parchment are compared with average strain vs. RH relationship for
parchment obtained by fitting the entire set of data to the second order polynomial of
the form: strain in % = 1.0335×10-1·RH–3.796×10-4·RH2. The measurements were
done in two perpendicular directions in each sample to take into account the possible
effect of collagen fibre preferred orientation, and the larger response was considered.
The yield strain of parchment – that is to say the strain beyond which parchment
undergoes permanent deformation – was determined as the upper limit of the elastic
range in the stress-strain curves recorded in the laboratory. The deformation coincided
with the point where the curve began to deviate from the straight line. The average
value of 1.9% was obtained from the measurements in the RH range between 20% and
90% which encompasses conditions of interest for the preservation of library collections
indoors, as at low RH parchment dehydrates and becomes brittle and at high RH risk of
mould growth increases (Wess and Orgel 2000). The rate of water vapour uptake or
release by parchment is high when compared to the duration of RH fluctuations – the
new equilibrium moisture content in the material is established on an RH change in less
than one hour. Therefore, RH variations were translated into the strain vs. time history
by assuming that parchment reaches its full dimensional response at each instant of the
variation. The strain of parchment calculated for the RH variations illustrated in Figure
1 very slightly exceeded the critical strain of 1.9% during the shrinkage induced by the
drying episodes in March 2013. During these episodes, therefore, parchment sheets
restrained in their movement by mounting into a rigid protective system or attachment
to boards of a book binding may undergo excessive tensile strain and deformation
(Figure 5).
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Strain (%)
Date
Figure 5. Strain versus time history experienced by parchment exposed to RH variations
as in Fig.1. The strain range, recommended as safe, is marked with orange lines.
Conclusions
A web-based decision-supporting software tool HERIe aims at gathering growing
experimental evidence on the environmental response of historic materials and objects
so that precise assessment of risk of climate induced physical damage is facilitated for
conservation and museum community. The HERIe software is an open-source tool –
with progress in research, the data base of the cultural heritage objects analysed will be
enlarged and the modelling algorithms refined. Further damage monitoring on freshly
treated/consolidated objects is necessary to refine the damage criteria available in the
software as such objects maybe particularly vulnerable to RH variations. In particular,
scarce data on critical strains are available for library and archival objects. Therefore,
monitoring of moisture-induced deformation of historical parchments is necessary to
refine the damage criteria used by the software. HERIe is available for testing at
herie.mnk.pl. Detailed information on the methodologies used by the software is
available at that website which also contains tutorial climates. Conservation
professionals, researchers, scientists and decisions-makers are encouraged to test the
tool. The feedback gained from the users is vital for its continuous development of the
tool.
Funding
The research was supported by Grant PBS2/A9/24/2013 from the Polish National
Centre for Research and Development and the Getty Conservation Institute’s Managing
Collection Environments Initiative.
References
Bratasz, Ł. 2013. Allowable microclimatic variations for painted wood. Studies in
Conservation, 58: 65-79.
IIC. 2014. IIC – ICOM CC Declaration on Environmental Guidelines. International
Institute of Conservation [accessed 6 October 2017]. Available at
<http://www.iiconservation.org/node/5168>
Lukomski, M., Strojecki, M., Pretzel, B., Blades, N., Beltran, V. L. & Freeman, A.
2017. Acoustic emission monitoring of micro-damage in wooden art objects to
assess climate management strategies. Insight, 59: 256-264.
Lukomski, M., Beltran, V. L., Boersma, F., Druzik, J., Freeman, A., Strojecki, M.,
Learner, T. 2018. Monitoring acoustic emission in an epidemiological pilot study
of a wooden object collection. In Preventive Conservation: The State of the Art,
edited by A. Nevin, J. H. Townsend, M. Strlič, N. Blades, D. Thickett, and J.
Kirby Atkinson, S181–S186. London: IIC.
Mecklenburg, M. F., Tumosa, C. S. & Erhardt, D. 1998. Structural response of painted
wood surfaces to changes in ambient relative humidity. In: V. Dorge & F. C.
Howlett, eds. Painted Wood: History and Conservation, Los Angeles: The Getty
Conservation Institute, pp. 464-83.
Michalski, S. 2009. The ideal climate, risk management, the ASHRAE chapter, proofed
fluctuations, and towards a full risk analysis model. In: F. Boersma, ed.
Proceedings of Experts’ Roundtable on Sustainable Climate Management
Strategies, Tenerife, 2007. Los Angeles: Getty Conservation Institute [accessed 6
October 2017]. Available at:
<http://www.getty.edu/conservation/science/climate/climate_expertsroundtable.ht
ml>
Pretzel, B. 2014. [Un]Reasonable – Broadening acceptable climate parameters for
furniture on open display. In J. Bridgland, ed. ICOM-CC 17th Triennial
Conference Preprints, Melbourne, 15–19 September 2014, Paris: International
Council of Museums, art. 1507, 10 pp.
Rachwał, B., Bratasz, Ł., Krzemień, L., Łukomski, M. & Kozłowski, R. 2012. Fatigue
damage of the gesso layer in panel paintings subjected to changing climate
conditions. Strain, 48: 474-81.
Staniforth, S. 2014. Environmental conditions for the safeguarding of collections:
Future trends. Studies in Conservation, 59: 213-17.
Strojecki, M., Łukomski, M., Krzemień, L., Sobczyk, J. & Bratasz, Ł. 2014. Acoustic
emission monitoring of an eighteenth century wardrobe to support a strategy for
indoor climate management. Studies in Conservation, 59: 225-32.
Wess, T. J. & Orgel J. P. 2000. Changes in collagen structure: drying, dehydrothermal
treatment and relation to long term deterioration. Thermochimica Acta, 365: 119-128.
