Built Heritage 2013 Monitoring Conservation Management
Research project on the conservation of Hagia Sophia, Istanbul results of environmental monitoring
Juni Sasaki1; Keigo Koizumi2; Daisuke Ogura3; Takeshi Ishizaki4; Kenichiro
Hidaka5
1
JSPS Research Fellow in National Research Institute for Cultural Properties,
Tokyo, Tokyo, Japan; 2 Osaka University, Graduate School of Engineering,
Osaka, Japan; 3 Kyoto University, Graduate School of Engineering, Kyoto,
Japan; 4 National Research Institute for Cultural Properties, Tokyo, Deputy
Director General, Tokyo, Japan; 5 Tokyo University of the Arts, Tokyo, Japan
1. Introduction
Hagia Sophia is one of the world’s most famous cultural heritage sites because of its irreplaceable structural system and significance to multiple religions.
Hagia Sophia was erected in 532 in Constantinople (now Istanbul) and dedicated in 537. It is the largest and most beautiful Byzantine structure in the
world. In the 15th century, this building was converted to a mosque. In 1934,
Hagia Sophia became a museum and now greets about 300 million visitors
a year.
The great church’s history is intimately bound with seismic phenomena: the
main dome partially collapsed due to earthquakes and was rebuilt in the 6th,
10th, and 14th centuries. This historic building has therefore been the subject
of study and publications on structural behavior and antiseismic performance
[Van Nice, 1965; Mainstone, 1988]. We, the Hagia Sophia Surveying Project
by Team Japan, launched a survey in 1990, drawing a series of accurate
contour maps of the upper structures of the building and conducting architectural–structural analyses of the stability of the supporting system, which
has repeatedly suffered from earthquakes but essentially survived with partial
reconstructions and reinforcements [Hidaka, 2003].
Although structural studies have proceeded aggressively, there are no complete environmental studies, such as environmental monitoring of this enormous space and assessment of the structure deterioration. This is due to the
difficulty involved in installing sensors and wiring as well as a complex combination of structural, physical, biological, artificial, and environmental factors.
Even though the present-day Hagia Sophia has changed since we first saw it
some years ago, we cannot take sufficient measures to conserve and control
its appearance. What can we do to ensure that this piece of heritage is preserved for posterity? For the future conservation of Hagia Sophia, which faces
complex problems, a multidisciplinary approach such as ours is necessary.
In 2009, we launched a comprehensive effort to advance our survey in four
directions: 1) more detailed studies on Hagia Sophia itself, involving a detailed architectural survey and measurements, 2) dynamic structural analysis,
3) museological survey, and 4) environmental research. So far, our developing studies provide new insight into the conservation of complex heritage
sites as follows. We have precisely documented the present situation using a
three-dimensional laser scanner to prepare for future restorations that may be
necessitated by disasters. The results of dynamic structural analysis indicate
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that the upper part of the north tympanum and the great double arch should
be carefully observed and examined if there persist symptoms of water penetration or decay caused by water; this is because the microtremor data tacitly
provides the possibility of structural problems in the north tympanum, and
more probably, in the upper part of the great arch bordering the hemisphere
[Yamaoka et al., 2011]. The museological survey suggests the necessity of
estimating the probability of occurrence, duration, and extent of damage for
risks such as earthquakes, fire, power failures, and disorderly behavior by
visitors1.
In our presentation, we will report the results of our multidisciplinary survey,
in particular those of our environmental research in Hagia Sophia. We have
obtained environmental data from a new wireless real-time monitoring system
installed in 2010 and other important data including building material characteristics, deterioration products, and water regime in the walls, which have not
been researched sufficiently.
2. Method
Long-term monitoring of historic buildings has long been recognized as a necessity for preventive conservation. The same holds true for Hagia Sophia;
however, such monitoring has not yet been fully achieved, as already mentioned, due to the difficulties involved in the installation of sensors and wiring.
Even so, to implement effective conservation measures, conditions inside Hagia Sophia (i.e., precise temperatures and changes in humidity) must first be
assessed. For this purpose, we installed 21 sensors as part of a wireless system; 15 data loggers; and weather stations to measure temperature, humidity, rainfall, wind speed, wind direction, and solar radiation outside the building.
In developing the monitoring system, we introduced the system by a mesh
network method, which is simpler than the conventional ZigBee system in
terms of power supply and data communication. The base station of our realtime environmental monitoring system is composed of a mobile PC and a
receiver module. The sensor node, which measures 80 × 120 × 40 mm, is
composed of a small wireless communication device, size D battery, and sensor board with temperature and relative humidity sensors. The temperature
and humidity data is sent to the computer station periodically2.
The 21 sensors of the wireless system were located in places such as the
dome cornice, second cornice, and gallery (places that visitors cannot access). These were set up on lines of symmetries in the plane view and section
view of the dome construction (Fig.1).
We also obtained samples of the building materials (brick and mortar) and the
deterioration products (crystallized salt and flaking mortar). In addition, we
measured the volumetric water content distribution inside the walls using a
time domain reflectometer (TDR) surface probe.
3. Results and discussion
3.1. Interior conditions
The environmetal data indicates that the interior of Hagia Sophia is under
uncontrolled conditions. This is because the indoor temperature and absolute
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Fig.1 - Location of the sensor nodes
humidity are affected by the outdoor factors3, in general, the influences of
solar radiation [Ogura et al., 2012; Güleç, 1996]. Below we describe our observations about the interior conditions on the basis of the data obtained from
2010 to 2011:
1) Figures 2 and 3 show the vertical differences in temperature and relative
humidity. Throughout the year, the temperatures of the dome (W13) and second cornice (W09) were higher than those of the gallery (W05) and ground
level (W01). In summer, the temperature of the dome was the highest, but
in winter, the temperature of the second cornice was higher than that of the
dome. In contrast, the relative humidity of the gallery (W05) and ground level
(W01) were higher than those of other locations in summer; in addition, a little
higher relative humidity (up to 80%) was observed at lower positions (W13,
W01) in winter. As can be seen, in summer the humidity was the highest at the
gallery and ground level, while it was lower at the upper level. These results
show that condensation may not occur at the upper level, as our previous
research concluded [Takayama et al., 2001].
2) Relative humidity is high in the north aisle and in the west and east staircases on the ground level. Particularly in the east staircase, both relative
humidity (in summer reaching >90%) and absolute humidity are the highest.
Condensation might occur on the walls in these areas. The reason for high
humidity in the northern area is that the temperature in this area is lower than
those elsewhere. In addition, groundwater infiltration seems to be a cause of
high absolute humidity.
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Fig.2 - Annual variation in temperature (vertical direction). W01, ground; W05, gallery; W09, second cornice; W13, dome; Fig.3 - Annual variation in relative humidity
Due to rainwater penetration into the interior walls and the occasional condensation of water on the wall surfaces, the plaster walls are deteriorating. Also,
crystallization occurs by water movement in the porous materials. To clarify
this deterioration process, the distribution of water content inside the walls has
been measured and the characteristics of salt and wall materials have been
studied [Charola, 2000].
3.2 Water content distribution in the inner wall
To determine the water regime in the walls of Hagia Sophia, we investigated the results of visual survey and measured the volumetric distribution of
water content inside the walls with a TDR surface probe (TRIME-FM, IMKO
MICROMODUL-TECHNIK GmBH) in September 2011. The measurement of
moisture content and visual survey were performed on the interior and the
exterior walls of the second cornice, except the northeastern wall [Ishizaki et
al., 2012].
At the northeast exedra, the moisture content was high at locations 107–108,
low at 105–107, and high at 105. The moisture content profile from 1 to 6 at
the southwest exedra was similar to that at the northwest exedra. Compared
with the average moisture content in each exedra, the moisture content of the
northwest exedra was the highest (15.5 vol%), as shown in Fig.3.
To clarify the relationship between moisture content and deterioration, the
moisture content profile was compared to the degree of deterioration by observation. In general, the moisture content of the nondeteriorated area was
lower than that of the deteriorated area. Also, a comparison between moisture
content and degree of deterioration showed that the area with the highest
moisture content was seriously deteriorated.
Similarly, moisture content was closely correlated with the outside conditions.
An area on the northwestern exterior wall with many gaps and brick exfoliation
had high moisture content. Hence, bridging gaps between the wall and roof
seems to be effective in preventing rainwater penetration.
It should be noted that the moisture content of the walls is obviously different in the northwestern and southwestern walls. This is possibly due to the
presence of plaster on the exterior wall. In 2006, the style of the exterior wall
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caused controversy, i.e., controversy regarding the removal of covered plaster
from the exterior wall surface. As a result of two years of debate, the plaster
on the western exterior wall was removed in 2008, and the masonry (brick
and mortar) structure was exposed to the atmosphere because it was from the
Byzantine period. This exposure has accelerated water penetration into the
mortar layer, although it cannot definitively conclude on the basis of the pros
and cons of this daring intervention.
In conclusion, a major cause for the high moisture content in the wall is possibly rainwater penetration through gaps and occasional construction efforts on
the exposed wall4. In 2012, we suggested that the western exterior wall should
be recovered during the rainless period, from August to October. We believe
that this would successfully prevent the deterioration of Hagia Sophia. But of
course, we will continue further studies to fully clarify all routes of rainwater
penetration.
3.3. Salt crystallization
The interior walls of Hagia Sophia are suffering from salt crystallization and
flaking, the surface paints are losing color, and the structure is undergoing
fragmentation because of physical and chemical pressures at the boundary
surface, especially at the northwestern part of the building. At the upper level
(from the second cornice to the dome cornice), the interior wall surface at the
Fig.3 - Deterioration and moisture content distribution of interior wall (adapted with permission
from Sasaki et al. 2012)
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northwestern part has significantly deteriorated in just 4 years (from 2006 to
2010). Also, on the gallery wall surface, salt crystallization and flaking are
progressing quickly and widely. Therefore, we can conclude that salt crystallization at the northwestern part is quite serious5.
The elements contained in the salt samples were detected by X-ray fluorescence spectroscopy, and the crystallized molecules were identified by X-ray
diffraction analysis. The crystallized salts were identified as sodium sulfate,
magnesium sulfate, and sodium nitrate (Table 1). At the northwestern part of
the building (northwest exedra), sodium sulfate was the major salt; magnesium sulfate and sodium nitrate were found at the western and northern parts
of the exedra, where salt crystallization was in progress.
4. Conclusion
On the basis of the current study data, the conclusions can be summarized
as follows:
1. Our monitoring system provides not only fundamental information for the
conservation of Hagia Sophia but also data on microscale movements of water from the outside to the inside and vice versa.
2. The interior wall of the building is suffering from flaking of the mortar and
salt crystallization, and the exterior wall is also in a weakened condition. A simulation of moisture movement in the walls will be conducted, which is impor-
Table 1. Types of crystallized salts in Hagia Sophia
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tant for evaluating the causes of deterioration and for developing protective
measures.
2. Relative humidity in the north aisle and staircase on the ground level was
high, particularly in summer, reaching >90%. Condensation might occur on
the walls in these areas. We are using obtained data and simulation techniques to analyze the conditions of inside temperature and humidity.
3. Moisture contents of both the northwestern as well as southeastern areas
were high.
4. Significant deterioration of the second cornice was observed at the northwest exedra, where exfoliation of the repaired plaster was confirmed. The
southeast exedra was also deteriorated, but the southwestern deterioration
was lower than elsewhere.
5. Many gaps were observed at the top and bottom of the exterior wall of the
northwest exedra. Gaps were also observed at the southeast exedra. Rainwater has possibly penetrated into the wall at these locations.
6. A major cause of high moisture content in the walls is possibly rainwater
penetration. In particular, sealing gaps between the wall and roof seems to be
effective in preventing moisture penetration at the exterior walls. Condensation on the walls is also considered to be another cause. However, the relative
humidity at the upper level was lower than that at the lower level; so condensation may not occur at the upper level. Therefore, there is a high probability
that rainwater penetration is the main cause of high moisture content in the
wall of the second cornice.
As the next step in developing the abovementioned protective measures, it is
necessary to undertake a simulation study of moisture movement in the walls
due to rainwater penetration on the basis of the physical properties of the
building materials, such as bricks and lime plaster. This work is considered
important in evaluating the causes of deterioration and developing protective
measures.
Acknowledgements
We are grateful for the cooperation from the authorities of Ayasofya Museum and the
Ministry of Culture and Tourism, Republic of Turkey.
This study is supported by the scientific research fund-Grant-in-Aid for Scientific Research S, Japan Society for the Promotion of Science (Head: Prof. Kenichiro Hidaka,
University of Tsukuba, No. 21226014).
Notes
1
The museological survey was conducted as part of the preparatory survey for future
risk management planning of Ayasofya Museum. The term “risk management” in the
survey report consists of the evaluation and analysis of various risks, including manmade and economic risks, in construction and other industries, along with systematic
measures to minimize such risks (Mizushima E., 2013, Planning of Risk Management
for a Historical Building, Tokiwa University Community Development Studies, 16, 1-29).
2
Data is observed on screen displays as values, time series graphs, and distribution
maps. To monitor the data, remote access through the Internet is used. In this case, an
ID password and access code are demanded. Monitoring has been conducted since
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September 27, 2010. The sampling interval is 30 min. The data collection rates by this
monitoring system are approximately 80% to 90%. There has been no voltage reduction for more than a year (Koizumi K., Ishizaki T., Ogura D., Sasaki J., Hidaka K., 2012,
Development of a Real-time Environmental Monitoring System by a Wireless Sensor
Network in Hagia Sophia, «Science for Conservation», No. 51, 293-302).
3
The temperature and absolute humidity indoors is higher than those outdoors; the
annual mean temperature and absolute humidity are 17.5°C and 9.3 g/kg at the ground
level and 14.9°C and 8.1 g/kg outdoors, respectively.
4
This observation is consistent with the result of numerical simulation, i.e., the moisture content of the interior wall surface increases because of the penetration of rainwater
on the exterior wall surface.
5
Crystallized salt in Hagia Sophia was analyzed to identify the salt type, to clarify the
origin of the salt and the movement of water (the main cause of salt crystallization),
and to formulate a suggestion for the future conservation program, including salt and
water control (Sasaki J., Yoshida N., Ogura D., Ishizaki T., Hidaka K., 2012, Study of
Salt Crystallization on the Inner Wall of Hagia Sophia, Istanbul, Turkey, «Science for
Conservation», No. 51, 303-312).
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