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ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based
decision-making for loan agreements
Joel Taylor*
Getty Conservation Institute
Los Angeles CA, USA
taylor_joel@rocketmail.com
Michał Łukomski
Getty Conservation Institute
Los Angeles CA, USA
mlukomski@getty.edu
Łukasz Bratasz
Jerzy Haber Institute of Catalysis and Surface
Chemistry
Polish Academy of Sciences
Krakow, Poland
ncbratas@cyf-kr.edu.pl
*Author for correspondence
Keywords
strain, risk, index, loans, standards, climate
Abstract
This contribution considers the potential im-
pact of using a strain index as a metric for vari-
ous conservation applications. Using new ap-
proaches to model strain calculations, the paper
describes the benefit of introducing a metric as
an alternative to targets or ranges of relative hu-
midity. Three examples of its value in analyzing
and communicating risks are presented. The pa-
per also draws parallels with other metrics and
standards that have provided insight into com-
plex practical matters, such as human comfort.
INTRODUCTION
Discussion of museum climates and loan agreements has often focused on
target ranges being “tight” or “relaxed.” Such pre-defined targets do not
necessarily account for the potential impact of moving from one climate
to another. The environmental history of an object is rarely the focus of
climate specifications for loans, which tend to use ranges or specific targets.
Consequently, the risk of damage from climate-induced deformation or
cracking (strain) is not always clear. This partly comes from the difficulty
of using strain to express risk in a way that is understood by all parties and
also from the difficulty of calculating it. Introducing ways to determine
strain risk has benefits for both the analysis and communication of a matter
that affects all institutions in a variety of ways.
This paper describes how the application of emerging tools and metrics can
contribute to decision-making for conservators, conservation scientists, and
registrars. HERIe is a web-based software tool that recalculates climate
histories into strain histories. It is moderated by an object’s properties,
such as thickness, species, and coatings that prevent vapor diffusion, such
as varnish (Kupczak et al. 2018).
A STRAIN INDEX FOR CLIMATE RISK
Considering the strain allows discussion to move beyond relative humidity
(RH) to focus on the issue that is of actual concern for loans of hygroscopic
material: the risk of damage from fluctuating RH. Strain is a measure of
relative deformation, such as change of length divided by the original
length of the material. The results from calculating the strain are not always
intuitive, but this demonstrates the need for greater exposure within the
conservation field and better calculation tools.
Loan specifications
Climate specifications for loans often consist of specific ranges that are
either based on general information about materials or a theoretically
ideal climate. Neither of these approaches consider the specific climates
in question (borrower and lender). There are many examples of objects
moving to tight, median climates where the actual risk is increased because
the original climate was not accounted for. Moving objects from dry or
humid climates to temperate ones can hold just as much risk as moving
outside of median ranges. Set target ranges can also inadvertently favor
2ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based decision-making for
loan agreements
Figure 1. (top) Comfort standard based
on tests that determine an ideal climate for
people; (bottom) adaptive comfort between
indoor and outdoor temperatures (Wikipedia,
based on ASHRAE 2013)
geographically temperate locations, since they correspond to theoretical
ideals (Taylor and Boersma 2018).
Instead of looking at pre-defined targets for one location (the borrowing
institution), a meaningful comparison of the climates at both the lending
and borrowing institutions can be undertaken, presenting more scientifically
valid guidance for assessing and managing risk.
A shift in standards
The use of strain rather than RH to measure risk holds parallels with the
human comfort standard 55:2013 (ASHRAE 2013), despite its focus on
temperature. The standard has two approaches to human comfort: meeting
a defined set point, regardless of geographical location or season, and
an adaptive approach. The adaptive approach is based on the difference
between environments (interior and exterior) rather than measured responses
to a single location. This approach affords increased flexibility in climate
management and reduced energy consumption. The paradigm shift from
measuring a single location (with the aim of keeping its climate stable) to
viewing comfort as an operation between two related locations focuses on
the most important aspect for human comfort. The calculation of strain,
especially when moving between two climates, presents the same shift
in approach. The diagrams below illustrate the approach (Figure 1). The
ASHRAE 55 adaptive standard cuts through a lot of the uncertainty and
inaccuracy by focusing on the actual phenomenon (comfort) with metrics
that everyone can easily feel and gauge.
The strain index presents a single representative number (maximal strain
vs. critical strain) for a material rather than a predefined, static target
based on some of the contributing factors.
The difference, however, is the ease of calculating the strain index for
objects. Like the ASHRAE adaptive comfort standard, the concepts are
often understood by the parties involved, but their calculation is much
more difficult. Loan agreements involve a large number of people, so
time-consuming and complicated calculations are not always possible.
Like the ASHRAE adaptive comfort approach, decision-makers are provided
with access to pertinent concepts that seem intuitive or difficult to express,
just as the advent of permanence calculations in conservation practice
helped inform decisions about storage and chemical deterioration. Like
permanence calculations, the strain index is easy to calculate, therefore
facilitating its practical application.
Strain index
There is a body of scientific research that adheres to the view that the
elastic strain experienced by a material is a useful parameter to determine
the safety of vulnerable objects exposed to variations in temperature and
RH (Bratasz 2013). Although the strain at the point of failure depends on
RH, the yield strain (from which irreversible deformation starts) does not.
The risk of physical damage from strain can be evaluated by calculating
strain-versus-time histories, like those engendered in specific objects in
3ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based decision-making for
loan agreements
real-world situations, and comparing them to critical stress levels that are
characteristic for the materials being analyzed.
In this paper, calculations were done using HERIe––a web-based decision-
support software tool (Kupczak et al. 2018). The risk of physical damage
is assessed for a selected category of objects (described by material type
and dimensions) for one year (or more) of climate data. Like the other
metrics discussed, HERIe does not create a direct simulation of specific
items but demonstrates the degree of risk from RH fluctuation for broad
categories of objects, which markedly increases the evidence base for a
decision. It is a decision aide rather than a one-to-one representation of
a specific object.
The data is broken down into a set of simpler sinusoidal wave fluctuations
for RH using the Fourier transform method. Each elementary sinusoidal
RH fluctuation is then translated into an elementary strain fluctuation
that is experienced by the object, using the pre-calculated database. The
complete strain-versus-time history of the object is then calculated by
the superposition of all of the elementary strain fluctuations. In the final
step, the risk of damage is indicated with an index that is related to the
material properties of the object (the damage criterion).
A damage criterion is the critical level at which the restrained dimensional
response of the object leads to damage. These criteria relate to the point
at which reversible (elastic) strain becomes irreversible (plastic) strain,
which leads to cracking and permanent change. The yield strain of wood
and the strain at failure in a tangential direction are generally around 0.005
and 0.02, respectively, at the RH mid-range (Mecklenburg et al. 1998).
The criteria used in HERIe are deliberately conservative in order to avoid
false negative results (i.e., making conditions appear safer than they are),
so can be considered a worst-case scenario (Kupczak et al. 2018).
The current version of the software (freely available at http://herie.mnk.pl )
analyses the influence of variations in RH and temperature on freely
responding wooden panels covered with a gesso layer, as a model system
imitating panel paintings, and on wooden elements restrained in movement,
as a model of wooden constructions such as sides of cabinets, doors, or
panel paintings restrained by rigid auxiliary support systems. Currently,
three types of wood are available for selection: lime, poplar, and oak; and
two types of gesso: soft and stiff.
EXAMPLE 1: RISK TO AN ART OBJECT TRANSFERRED FROM AN
UNSTABLE TO “IDEAL” CLIMATE
To illustrate the risk related with transferring a wooden art object from
unstable to “ideal” conditions, combined climatic data was created for
both (Figure 2). The first part of the data represents the RH measured in
a historic church with no climate control; the second part represents the
“ideal” conditions in a museum (50% ± 5%). Both data sets were recorded
in real buildings: the first in a wooden church in Europe and the second in
a purpose-built museum. Calculations were performed for a hypothetical
wooden object, such as a 2 cm-thick oak door panel, open to moisture
4ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based decision-making for
loan agreements
Figure 2. Climatic data during transfer of the
object from an unstable to an “ideal” climate.
The first part of the data represents climatic
conditions in a historic church and the second
in a museum
Figure 3. The strain experienced by a 2 cm
oak panel under restraint during transition
from church to museum. The orange lines
indicate critical strain levels beyond which the
object is at risk of physical damage
diffusion on both sides and fully restrained in its movement (as a panel
painting might be).
On November 22, 2018, the object was transferred (virtually) from the
church, with an annual average RH of approximately 71% and a seasonally
variable temperature of −4°C to 25°C, to a purpose-built museum with an
annual average RH of 50% and a temperature of 21°C. The risk of physical
damage related to this transfer was evaluated using the HERIe tool.
Since objects made with organic hygroscopic materials acclimatize to the
environment to which they were exposed in the past, the zero strain level
for the panel corresponds to average RH in the church, i.e., 71%. Between
April and November 2018, the strain level for the analyzed panel varied
between −0.003 and 0.002. This is much lower than the critical strain level
for wood, indicated by the orange horizontal lines in Figure 3. It means
that the risk of physical damage for this particular object, when it was in
the church, was negligible.
When the panel was transferred to the “ideal” museum environment, the
strain increased gradually, reaching 0.0066, considerably higher than the
critical strain level for wood (0.005). This means that transferring an object
from an unstable climate to a very precisely controlled environment may
be dangerous when there is a large difference in the average humidity
levels between two locations.
The results still seem surprising, since even in the church, the RH sometimes
drops to a level below 50% without resulting in significant risk to the
object. Thorough analysis of the hygrothermal response of the wood,
however, showed that the drop in RH in the church was too brief for the
2 cm-thick panel to fully respond.
This hypothetical situation echoes a real event, described by Padfield
(1994), in which an altarpiece was moved from a rural church to a purpose-
built museum with “ideal” conditions, resulting in significant cracking.
Using a strain index provides the opportunity to analyze and quantify the
risk and communicate the risk to decision-makers in advance. Moving an
object to a “better environment” might seem compelling to many, and is
occasionally regarded as an incentive to loan, so using methods to analyze
and illustrate the real risks can benefit all parties.
This leads to two observations:
a) The difference between average RH levels at the points of origin and
destination dominate the risk of physical damage for objects during
transfer.
b) The risk of damage generated by a specific climate depends on the
response of the object rather than pre-defined levels of RH.
The second observation means that the same climate poses a different
risk for objects of different sizes constructed with different materials. The
notion of “a safe climate” is really defined by the situation and the object
under consideration. It cannot be chosen without broadly understanding
the nature of the collection. By focusing on target levels of RH alone,
such as loan agreements or environmental classifications, emphasis can
5ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based decision-making for
loan agreements
Figure 4. (top) Temperature and RH inside a
brick church in Northern Europe over one year;
(bottom) strain experienced by a 1 cm and a
2 cm-thick oak panel, varnished (asymmetric
water vapor diffusion), and a 1 cm-thick oak
panel, unvarnished (symmetrical water vapor
diffusion)
Figure 5. RH in a historic house in Italy
(recorded) and inside microclimatic frames
with various AERs (simulated)
be drawn away from the nature of the collection (including its climatic
history). Using a strain index allows that imbalance to be redressed.
EXAMPLE 2: DIFFERENT ART OBJECTS IN THE SAME
ENVIRONMENTAL CONDITIONS
As mentioned, the different qualities of objects will influence the degree
of strain risk, so their index levels will differ. Simulation of strain-versus-
time histories for three wooden panels under restraint, in a brick church
without climate control in Northern Europe, are presented in Figure 4.
The strain experienced for a varnished 2 cm-thick panel was within the
safe limits defined by a positive and negative strain of 0.005 (horizontal
orange lines in Figure 4).
This means that there is very limited risk of mechanical damage for that
panel when inside the church. Calculations for a similar, but thinner
(1 cm), panel showed a quicker response time than the 2 cm panel. The
thinner panel swells and shrinks more than the thicker one in response to
the same RH variations. As a consequence, the strain experienced by the
thinner panel is greater and actually crosses a critical threshold three times
during the year, which indicates risk of physical damage. The calculated
risk is even higher for a third panel of the same thickness as the second
panel (1 cm) but which is unvarnished and therefore exposed to water
vapor diffusion on both sides. This results in an even shorter response
time and, as a consequence, a more pronounced dimensional response
to climatic fluctuations. Although the calculations are not intended to
represent all objects or specific items in a collection, they demonstrate the
broad issues that can lead to risks for objects that are similar but different
when exposed to the same climate. Loan agreements, for example, can
involve the exhibition of objects that might be classified in the same way
but respond quite differently to the climates in which they are housed.
EXAMPLE 3: NON-MECHANICAL APPROACHES TAILORED TO
OBJECT NEEDS
Creating a space that is isolated from the conditions prevailing in the
exhibition room or a museum storeroom, such as a vitrine/display case, can
be an effective way to reduce risk of physical damage posed by unstable
temperature and RH conditions.
While ordinary display cases are meant to protect their contents from theft
and vandalism, the purpose of a microclimatic frame is to create a barrier
for incoming/outgoing moisture. With a well-sealed construction for the
microclimatic frame, the influence of short-term humidity fluctuations in a
museum gallery inside the frame will be greatly reduced. The effectiveness
of the protection depends on how airtight the frame is and is typically
characterized by the number of air exchanges per hour, or air exchange
rate (AER).
As an example, a historic building in Italy is presented where the reduction
in climate fluctuations varies between microclimatic frames with three
different AERs. Humidity data presented in Figure 5 (30%–80%) is outside
any defined control levels, such as the broadest classification in the ASHRAE
6ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based decision-making for
loan agreements
Figure 6. Strain history experienced by the
0.4 mm-thick gesso layer painted on a 2 cm
poplar panel open on both sides (symmetric
water vapor diffusion) for the RH variations
shown in Figure 5
classification system (ASHRAE 2019). The microclimates inside the frame
were calculated using mass balance differential equations, assuming that
the AER between room and frame was equal to one change every two,
seven, and 30 days, respectively.
The buffering effect of microclimate frames is readily visible (Figure 5). The
RH in the room fluctuates within a range of 30%–80%, but this narrows to
35%–73% (1 AER per 2 days), 40%–70% (1 AER per 7 days), and 49%–66%
(1 AER per 30 days), respectively. Consequently, the climate in the frames
with an AER of 1/2 days and 1/7 days complies with broad environmental
classifications, such as ASHRAE control level C (25%–75% RH), whereas
the microclimate in the frame with an AER of 1/30 days complies with
the requirements of class A2 (annual average ± 10% RH).
The reduction in humidity variations inside microclimatic frames results
in a decreasing risk of physical damage to objects. Calculations for a
2 cm-thick panel painting made of poplar wood were performed using
HERIe. Simulations were focused on evaluating the risk of cracking for
a decorative layer, represented by a 0.4 mm layer of gesso. The gesso had
a defined limit of elasticity (3.75 GPa at 50% RH) and a critical strain of
0.002 (Mecklenburg et al. 1998, Rachwał et al. 2012), which served as
the damage criterion. The results are presented in Figure 6.
The strains in the gesso layer varied considerably, depending on RH
fluctuations. A reduction in the maximum strain was observed for the
gesso on the panel placed inside a microclimatic frame with decreasing
AER values. For the frame with an AER equal to 1/30 days, the strain in
the gesso layer was never higher than 0.002, which means that the risk
of cracking was negligible, given the conservative nature of the damage
criterion. This type of analysis enables the required AER to be defined
for panel paintings in particular environments.
Analysis of this kind allows decision-makers to ask “what if” questions about
how they can manage a loan (or set of loans with different specifications)
when gallery climates may not be suitable. The opportunities to discuss and
communicate solutions can be assessed for different cases and different kinds
of object, and insight provided into the requirements for a solution (such
as microclimatic frames). This has implications for energy consumption,
as approaches to environmental control can be compared with the costs of
altering a set point or climatic range in a gallery space or building, leading
to options that are more environmentally sustainable and cost-effective.
CONSIDERATIONS FOR MUSEUM PRACTICE
Using the strain index can also provide insight into optimal times for loan
movements in terms of season. If two institutions have climates that reflect
their geographical location, finding periods where climates are closest can
help reduce any strain involved in the transition. This can contribute to
decision-making around programming and transport in order to facilitate
activities that might hold some risk if not managed.
The strain index could even offer a viable alternative approach to climate
specifications for loans, instead of a target range for RH, just as the
7ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based decision-making for
loan agreements
ASHRAE comfort standard offers two viable approaches to the same
concept. Pretzel (2005) used permanence calculations to argue for conditions
outside a specified range that improved chemical preservation. The strain
index could provide a meaningful way to negotiate specifications. Instead
of only specifying a range and requesting data, lenders can make quick
comparisons of their objects’ current environments. A low strain value
would be a better indicator of most risks posed by RH than specifications
about fluctuation or thresholds. This would not involve divulging climate
data to any potential borrower. A quick comparison of data from a potential
borrower’s institution with their own, however, would mean that lending
institutions did not put their collection in jeopardy by requesting a potentially
damaging ideal that they do not meet.
Developments for a strain index
The means to calculate strain are currently based on the models and
information available, but there are uncertainties involved. These include
the simplification of strain calculations, available material information
not reflecting a specific item or the range reflecting a whole collection,
and models basing damage criteria on the average of material properties
(albeit with a conservative threshold for the damage criteria). The aim
of the approach, however, is to provide insight that was not accessible
before.
More damage criteria and materials can be introduced as scientific evidence
grows. Damage criteria can be defined by the user, derived from laboratory
studies, gauged by monitoring specific objects, or systematically gathered
from observations. As more systematic evidence of climate-induced change
is collected, the greater the possibilities provided by the model. Tests
on materials that already possess stress-releasing cracks could also help
refine the model.
CONCLUSION
Technology is changing at a rate where decision-makers can be empowered
with metrics that reflect their specific matters of interest. Embracing
methods that provide insight into the risks related to future activities can
help communicate the hidden risks of “ideal” conditions, the possibilities
that arise when conditions seem problematic, and even how “relaxing” or
“tightening” standards does not address the problem.
Developing tools that make the technical considerations for loans more
accessible is one aspect of a complicated process, and there is more to
loan agreements than acceptance of technical parameters. Making the
appropriate metrics more intellectually accessible, however, empowers
decision-makers to apply information that relates to the context, not an
abstract ideal. Focusing on the key issues can help reduce both damage
and energy consumption.
ACKNOWLEDGMENTS
The authors would like to acknowledge the support of the Getty
Conservation Institute’s Managing Collections Environments Initiative
8ICOM-CC
19th Triennial Conference
2021 Beijing
SCIENTIFIC RESEARCH
Increasing evidence-based decision-making for
loan agreements
Copyright © 2021 by ICOM-CC and The J. Paul
Getty Trust. All rights reserved.
To cite this article:
Taylor, J., M. Łukomski, and Ł. Bratasz. 2021.
Increasing evidence-based decision-making for
loan agreements. In Transcending Boundaries:
Integrated Approaches to Conservation. ICOM-CC
19th Triennial Conference Preprints, Beijing, 17–21
May 2021, ed. J. Bridgland. Paris: International
Council of Museums.
and the Polish National Agency for Academic Exchange (grant no. PPN/
PPO/2018/1/00004/U/00001).
REFERENCES
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). 2013.
ANSI/ASHRAE Standard 55: Thermal environmental conditions for human occupancy.
Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). 2019.
Museums, galleries, archives and libraries (A24). In ASHRAE applications handbook.
Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
Bratasz, Ł. 2013. Allowable microclimatic variations for painted wood. Studies in
Conservation 58(2): 65–79.
Kupczak, A., M. Jędrychowski, M. Strojecki, L. Krzemień, Ł. Bratasz, M. Łukomski, and R.
Kozłowski. 2018. HERIe: A web-based decision-supporting tool for assessing risk of physical
damage using various failure criteria. Studies in Conservation 63(sup. 1): 151–55.
Mecklenburg, M.F., C.S. Tumosa, and W.D. Erhardt. 1998. Structural response of painted wood
surfaces to changes in ambient relative humidity. In Painted wood: History and conservation,
eds. V. Dorge and F.C. Howlett, 464–83. Los Angeles, CA: Getty Conservation Institute.
Padfield, T. 1994. The role of standards and guidelines: Are they a substitute for understanding
a problem or a protection against the consequences of ignorance? In Durability and change:
Science, responsibility, and cost of sustaining cultural heritage, eds. W.E. Krumbein, P.
Brimblecombe, D.E. Cosgrove, and S. Staniforth, 191–99. Chichester: Wiley.
Rachwał, B., Ł. Bratasz, L. Krzemień, M. Łukomski, and R. Kozłowski. 2012. Fatigue damage
of the gesso layer in panel paintings subjected to changing climate conditions. Strain
48(6): 474–81.
Taylor, J. and F. Boersma. 2018. Managing environments for collections: The impact of
international loans on sustainable climate strategies. Studies in Conservation 63(sup 1):
257–61.
