Structural Detection and Restoration Proposal for the Mardin Şahkulubey Cupola

Abstract

Background: Mardin is a unique city in terms of linguistic, religious, and cultural diversity, within an atmosphere of tolerance, while various historical artifacts reflect these to the present day. This ancient city hosted numerous civilizations and experienced peak economic and political stability in the 12th century during the Artukid period. Many of the works that have endured to this day reflect that period. The Şahkulubey Cupola is one of the rare works that have survived to the present day. In this study, an analysis of the existing materials is performed to shed light on the restoration works related to the Şahkulubey Cupola. Samples were taken from the east, west, north, and south facade walls, which were deteriorated structurally. Scanning Electron Microscopy (S.E.M.) and Energy Distribution Spectroscopy (E.D.S.) analyses were conducted, and the samples’ related material properties were tested. In addition, the samples’ density, water absorption, and compressive strength properties were measured. As a result, it will be possible to conduct the restoration works with materials compatible with the existing structure of the Şahkulubey Cupola.

1. Introduction

The basic principle of cultural heritage is that it is invaluable and integral to our future. The built heritage is generally seen as a material element whose historical, documentary, and aesthetic information must be preserved for future generations. The obligation to protect and make it accessible is necessary [1]. Architectural heritage is generally built with natural stone from quarries located in the geographical vicinity. It should be preserved over time, as it is an essential element of people’s cultural heritage [2]. This is particularly important in the context of historical architecture, such as palaces, castles, or cathedrals [3].

The conservation of stone heritage is always a delicate and complex challenge. Several variables must be considered to identify the problems, define the necessary conservation measures, and select the materials and processes to use. The factors to be analyzed include the intrinsic stone properties, conservation status, degradation mechanisms, and environmental factors [1,3].

An in-depth knowledge of building materials is essential to preserve them [3]. A typical restoration practice represents cleaning, repairing, or even replacing the damaged portion of buildings with similar stones. However, the restoration work is only sometimes appropriately performed, either because the repair is poorly conducted, causing further damage, or because the natural stone used for the replacement is poorly chosen. The only way to deal professionally with replacement work is to determine the original stone and use it as a replacement to avoid contrasts in the building, or if the stone is found unsuitable under current conditions, it should be replaced with a suitable replacement after thorough technical assessment [2,4].

Although Mardin has experienced various historical eras with many linguistic, religious, and cultural differences, it exhibits excellent unity and solidarity, especially in Mesopotamia and Egypt. Artuqids settled in Mardin and established a small kingdom in this city [5]. While under the rule of the Artuqids, Mardin became a central node where caravans from Iran, Azerbaijan, the Caucasus, and Ahlat visited, along with those who set out for pilgrimage [6]. Dunyesir, established as a trade city during the Artuqid period in the south of Mardin, emerged as an essential center [7]. Today, this place is called Kızıltepe.

The cupola is a structure found in all regions where Turks reside, from East Turkestan to Anatolia. One is the Şahkulubey Cupola, located in Kızıltepe, a district of Mardin province, Turkey. The cupola, an important historical, cultural, and spiritual monument, was built by the Artuqid Principality in the 12th century [8].

It is obligatory to preserve and sustain the quality of the buildings within the scope of the Immovable Cultural Heritage through appropriate restoration and intervention [9]. Re-functioning is an essential issue in the use of historical buildings today and prevents them from being abandoned and deteriorating over time [10,11,12,13].

In a study titled “Nonlinear Structural Performance of a Historical Brick Masonry Inverted Dome”, the authors determined the average density of the Mardin limestone material to be 2200 kg/m³ and the compressive strength to be 23 MPa. In the study, the brick’s average density and compressive strength were chosen to be 1800 kg/m³ and 4 MPa, respectively. The average compressive strength of the Cas mortar was taken as 2.5 MPa [14].

In a study titled “Investigation of engineering characteristics of Mardin stone used in eco-building”, the authors found that the majority of the Mardin stone, with a ratio of 95%, is composed of micritic and crystallized calcite. The specific mass of Mardin stone is 2680 kg/m³, its water absorption capacity is 13.72%, and its porosity rate is 22.16%. The compressive strength of the stone is 20 MPa, and the tensile strength is 3.73 MPa [15].

Research on the historical Ankara Castle contributed significantly to the restoration by concluding that granite was used on the eastern facade of the fortress walls, andesite on the southern facade, and baked clay products on the northern and western facades [16]. During the restoration works for the Kütahya Castle, it was observed that the stones used in the walls of the Kütahya Castle had different compressive strength values. Thus, it is suggested that the section to be restored should account for differences in the quality and aging of the stones during the integration and strengthening techniques. As a result of the S.E.M. analysis, the images obtained at ×50, ×100, ×250, ×500, ×1000, and ×2000 magnifications were consistent with the results of the physical experiments. In the E.D.S. analysis, it was determined that the samples showed calcium oxide (CaO), that is, limestone and calcium aluminate (CaAl₂O₃) properties, according to the information given in the geological maps of the region [17].

In masonry structures, the mortar that binds the material is as important as the carrier material. In this context, in a study on Roman, Byzantine, and Seljuk period mortars, the mortars’ physical, chemical, mechanical, and microstructural properties were investigated. Although the structures were built in different periods, the material properties (physical, chemical, and mechanical) obtained as a result of the analyses showed similarity when compared across periods, since the mortar material was obtained from the sources in the region and rocks with similar characteristics [18].

In a study on the Emir Saltuk Cupola, a photogrammetric survey was prepared for the Emir Saltuk Vault, one of the three vaults in Erzurum. In addition, three-dimensional point data were created by measuring control points using a total station. Surveys were prepared to serve as a basis for the restoration projects [19].

In a study on the Kemah Grave Monuments, nine mausoleums (vaults and tombs) located in the Kemah district of Erzincan province were examined in terms of plan, architecture, and decoration. In addition, the place and importance of the works in Anatolian Turkish–Islamic architecture were emphasized [20].

In the research on the restoration of the Akseki Buttoned Houses, as a result of S.E.M. analysis, the images obtained at ×50, ×100, ×250, ×500, ×1000, and ×2000 magnifications were compatible with the results of the physical experiments. In the E.D.S. analysis, it was determined that the samples showed calcium oxide (CaO), that is, limestone and calcium aluminate (CaAl₂O₃) properties, according to the information given in the geological maps of the region [21].

No previous studies in the literature focused on the construction technique of the Şahkulubey Cupola. There are only limited observations that are not in-depth, and these observations indicate that there was no floor in the crypt, the dome was built on an octagonal structure, and the material utilized was cut stone.

This study determined the characterization of the materials used in the construction of the Şahkulubey Cupola. It aimed to obtain essential findings that will shed light on the selection of materials for restoration works.

The Mardin stone examined within the scope of this study is frequently seen and used in the Mardin–Kızıltepe–Ömerli–Midyat regions. Mardin stone was deposited during the Lower Eocene–Lower Oligocene reef-origin Hoya Formation. The average thickness of the formation is between 50 and 600 m. The lithologies that form the formation are chalky limestones, biomicrite, dolomitic limestones, clayey limestones, and fossiliferous limestones. The colors of our rocks are yellow, pink, red, white, off-white, gray. The formation was formed on the shallow sea-shelf edge and occasionally presents products with reef characteristics. The distribution of the formation is in the Mardin–Diyarbakır–Siirt–Adıyaman (Hoya Formation) and the Kilis–Gaziantep regions, and it is called Havra stone (Gaziantep Formation) [22].

Mardin stone is cut into 19 × 20 × 30 cm and special sizes and is frequently used in structures and works of art in the region. The highly porous Mardin stone is generally used in works of art that require rough workmanship. After the products that will require fine workmanship are extracted, they are processed in the shade and given the desired shape and brought to light. The fact that these types of limestones offer chalky properties, have a fine grain structure and features that allow them to be easily shaped when extracted increases their appeal for fine workmanship.

2. Geographical and Climatic Features of the Region

2.1. Geographical Features

Mardin City is in the Tigris and Euphrates Basins in the southeastern Anatolia Region. It is surrounded by Nusaybin and Midyat in the east, Kızıltepe in the west, Ömerli and Yeşilli districts in the north, and Syrian territory in the south. The surface area of Mardin province is 8858 km². In terms of its geographical location, it is located between 36.444–38.123 northern latitudes and 39.762–42.131 east longitudes, in the southeastern Anatolia Region (Figure 1 and Figure 2). Mardin is on the highway and railway route that connects Turkey with Syria and Iraq. It has the only border gate opening to the Middle East [23].

2.2. Climate Characteristics

Similar to other cities in southeastern Anatolia, Mardin City is also influenced by tropical and polar air masses. Along with this influence, the altitude, the direction of the mountains and their slope exposure, and the degree of terrestrial effects are the factors that determine the climate of Mardin. The elevation from the sea level of Mardin City is 1.083 m. The high mountains in the northern region impact the environment of Mardin. The city has the characteristics of the terrestrial climate. Summers are arid and hot, and winters are rainy and cold. In the plain section, which forms the southern part of Mardin city center, summers are sweltering, and winters are mild and rainy. In this part, there is little and non-permanent snowfall. Since the city is situated on the slope of the mountain and is high, summers are relatively cooler than those in the Mardin Plain, which forms the south, and winters have heavy winds, heavy rain, and snowfall [24].

The average temperature in Mardin is 15.8 °C. With an average temperature of 35 °C, July is the hottest month. The temperature in January is 0.6 °C, and it is the month with the lowest temperature of the year. The average number of rainy days per year is 79; the highest number is observed in March, with an average of 12.33 days. There is much more rainfall in winter and spring than in summer and autumn. With 2.3 mm of precipitation, August is the driest month of the year. The highest precipitation can be observed in January, with 117.2 mm of rainfall (

Table 1. Meteorological analysis of Mardin province (between 1941 and 2021) [25].

Mardin	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Annual
	Measurement Period (1927–2021)
Average Temperature (°C)	3	4.2	8	13.4	19.5	25.6	29.8	29.6	25.3	18.6	11.1	5.4	15.8
Average Maximum Temperature (°C)	5.8	7.3	11.6	17.3	24	30.6	35	34.7	30.1	22.9	14.5	8.1	19.8
Average Lowest Temperature (°C)	0.6	1.4	4.7	9.7	15.1	20.3	24.5	24.7	20.8	14.7	8.1	2.9	11.8
Average Sunbathing Time (hours)	4.5	5.1	6	7.3	9.7	12.1	12.4	11.4	10.3	7.7	5.9	4.4	7.7
Average Number of Rainy Days	10.25	8.92	12.33	11.58	10.08	2.25	2.17	0.83	0.92	5.42	8.25	11.67	79
Average Monthly Total Rainfall (mm)	117	104	97.4	81.9	47.2	6.6	3.2	2.3	4	33.9	71.6	109.9	647
Highest Temperature (°C)	19.4	19.5	27.5	33.6	36.1	40	42.5	42	39.3	35.6	26.1	24.1	42
Lowest Temperature (°C)	−13.4	−14	−11.7	−5.3	2.6	0.6	11.8	12.8	8	−2.5	−9.5	−11.9	−14
Daily Total Highest Rainfall				Daily Fastest Wind							Highest Snow
03.02.1982 145.9 mm				01.11.2008 47.8 m/sn							06.03.1959 93 cm

, Figure 3).

3. The History of Şahkulubey Cupola

Cupolas are monumental tombs with unique structures built during the era of the Seljuk Dynasty. The structures built on the places where Muslims bury their dead are called cupolas or mausoleums. In general, the cupola is a mausoleum with a polygonal base and an architectural style, in the form of a pointed (cone) top, unlike [26]. Pre-restoration photographs (Figure 4 and Figure 5) and models (Figure 6) of the restored Şahkulubey cupola are below.

4. Experimental Study

Because the examined Şahkulubey Cupola was repaired at different times throughout history, detailed research was carried out to get representative samples from the appropriate parts by determining the original sites of the construction period of the structure above. The samples were taken from the north, south, east, and west facades of the outer wall of the Şahkulubey Cupola, which was affected by environmental effects and climatic parameters (temperature, humidity, wind, precipitation, and frost) (Figure 7). The samples taken from different facades were prepared by cutting them using the experimental processes.

Tests related to mechanical tests were carried out according to the principles specified in Turkish Standard (T.S.) EN 1926 [27]. In addition, the material characteristics of the samples were revealed using an S.E.M. and E.D.S.

4.1. Mechanical and Physical Experiments

Compressive strength, density, and water absorption tests were performed on the samples taken from the east, west, north, and south facades of Şahkulubey Cupola. In addition, the average values obtained from the porosity and compaction tests are shown in bar graphs (Figure 8 and Figure 9). The experiments can be carried out on cubes with sides of (50 ± 5) mm or on right circular cylinders with a diameter and height of (50 ± 5) mm. This study was carried out using cube samples. Three samples were used for all experiments. Since cube samples conforming to standards were used, no correction factor was used [27,28,29]. It has been observed that on windy facades, the stone wears out more with rain. On windless facades, the stone maintains its strength as it did on the first day.

When the compressive strength results are examined, it is seen that different results are obtained in each direction. The highest compressive strength value was found in the south direction sample, with 22.71 MPa, and the lowest in the east direction sample, with 9.15 MPa. The value in the west direction sample was 15.18 MPa, and the value in the north direction sample was determined as 20.77 MPa. No samples were taken from the building using core drilling or any other method. The pieces are all different sizes and cannot be standardized. For this reason, if the conversion coefficient is used according to the dimensions, it is possible to obtain close values. For density, it was understood that the samples taken from the western facade had the highest value (2.42 g/cm³), while those taken from the northern facade had the lowest (2.37 g/cm³) value. According to the water absorption test (Figure 9), it was found that the samples taken from the northern facade had the lowest (8.01%) average value. The samples taken from the south facade had the highest (9.69%) average value. When the porosity test is examined, it is seen that the samples taken from the east and west facades have the lowest (0.19) porosity, and the samples taken from the south facade have the highest (0.21) porosity. According to the compactness test, it was understood that the samples taken from the east and west facades had the highest (0.81) average value. In contrast, the samples taken from the south facade had the lowest (0.79) average value. When thermal conductivity is examined, it was found that the samples taken from the eastern front had the highest mean value, and the samples taken from the west side had the lowest mean value. It is seen that the granite used in the construction of the eastern facade has significantly higher values than the materials used in the construction of the other facades in terms of all the examined properties.

In this study, the Young’s modulus of the material (19.8 GPa) and the Poisson ratio (0.21) were determined. Similar results were obtained in other studies on Mardin stone [30,31].

4.2. S.E.M. and E.D.S. Analysis

Morphological, structural, and qualitative elemental analysis was performed on the materials used via the high-resolution images taken by scanning electron microscopy and the information obtained from E.D.S. The results of the S.E.M. and E.D.S. analyses performed are shown in Figure 10. The E.D.S. results are given in Appendix A.

For the scanning electron microscopy (S.E.M.) analysis, test samples were prepared in dimensions of 1 cm × 1 cm × 1 cm, and their surfaces were covered with gold. Photographs were taken using secondary electron (S.E.), recoiled electron (B.S.E.), or mixed (SE + BSE) signal images from the sample surface under high vacuum conditions. A non-standard quantitative point analysis was performed from specific locations.

For imaging and E.D.S. analysis with Scanning Electron Microscopy (S.E.M.), test specimens (1 × 1 × 1 cm) were prepared, and their surfaces were covered with gold. Photographs were taken by taking secondary electron (S.E.), backscattered electron (B.S.E.), or mixed (SE + BSE) signal images from the surface of the samples when the pressure was reduced to a minimum. Non-standard quantitative point analysis was performed from the determined locations. In the S.E.M. images made on the sample taken from the South side of the Şahkulubey Cupola, a rectangular, compact structure with sharp corners was observed. In the S.E.M. images made on the sample taken from the north of Şahkulubey Cupola, a compact structure with sharp corners, as well as rectangular and other different geometric shapes, was observed.

5. Discussion

5.1. Mechanical and Physical Experiments

It was observed that the internal structures of the samples on the east and south facades were generally homogeneous, the particles were in a tight form, and there were no excessive voids. Still, the internal structures of the western and northern facades samples were heterogeneous. There were regional voids in the internal structure.

When the physical analysis results were examined, it was determined that the compressive strength was the highest in the samples taken from the south and the lowest in the samples taken from the east. In the density experiment, the samples from all directions showed similar characteristics. The water absorption test determined that the average water absorption values of the samples taken from the south direction were the highest, and the average water absorption values of the samples taken from the north direction were the lowest. In the porosity test, it was observed that the samples taken from the east have the lowest average porosity, and the samples taken from the south had the highest average porosity. In the composite results, it was observed that the samples taken from the east had the highest average density, and the samples taken from the south had the lowest average.

It was observed that the facades with low compressive strength and high porosity were the facades exposed to both the dominant wind direction and excessive sun.

5.2. S.E.M. and E.D.S. Analysis

When the chemical test results were examined, the S.E.M. imaging determined that the material generally has sharp corners and a rectangular and compact structure. The E.D.S. analysis revealed that the material was mainly composed of Ca, Mg, Si, and Fe. As a result of this study, it was observed that the material reflects the characteristics of the limestone used in the Mardin region, and it is registered geographically as “Mardin Stone”. These stones stand out because they are easy to cut and, therefore, easy to process. For this reason, it is possible to come across rich ornaments on the stone. It is a stone that has been used since it was discovered.

According to the results of the microchemical E.D.S. analysis,

• In the southern EDS-1 region: Ca, Mg, O;
• In the southern EDS-2 region: Si, Mg, O;
• In the southern EDS-3 region: Fe, Si, O;
• In the southern EDS-4 region: Mg, Ca, O;
• In the southern EDS-5 region: Si, N, O;
• In the northern EDS-1 region: Fe, Si, O;
• In the northern EDS-2 region: Ca, Mg, O;
• In the northern EDS-3 region: Si, Ca, O;
• In the northern EDS-4 region: Ca, Mg, O;
• In the northern EDS-5 region: a-Si, Ca, O.

The predominant structure was detected.

In the study titled “Investigation of Mardin Limestone as a Building Stone”, the chemical properties of the limestones taken from the quarry in the Mardin Babol region, the Kasımiye Madrasa, and Abdullatif Mansion are compared with the samples taken from the Şahkulubey Cupola. The chemical component of the samples in the study consists of calcite crystals and contains iron and magnesium in its structure. The chemical composition of the samples taken from the Şahkulubey Cupola is similar to that of the samples in the study [32]. In contrast, in the SEM-EDS analyses of the Kümbet samples, a silicate mineral was encountered in the sections with fibrous structure or in sections where the structure was changed from a tight structure to a fibrous structure.

6. Conclusions

The ancient city of Mardin, one of the oldest cities in Mesopotamia, located between the Tigris and the Euphrates rivers, has hosted many civilizations throughout history. Perhaps one of the most important was the Artukoğulları, who remained in power for about 300 years and brought innumerable works to the city. One of these rare works is the Şahkulubey Cupola in the Kızıltepe district.

Historical artifacts have been exposed to many natural or human-made destructions from the past to the present. Şahkulubey Cupola is among the structures affected by this destruction. It is of great importance for countries to protect the cultural heritage structures and pass them on to the next generations. Moreover, the preservation of Mardin, which can represent the cultural richness at the forefront of Turkey’s cultural heritage, and, therefore, its artifacts, is of even greater importance.

Due to its structural damage, the dome is currently closed to visitors and may pause a life-threatening risk; as a result, it has remained derelict. The restoration of the Şahkulubey Cupola, which is on the verge of collapse, is essential for determining the current damage, conducting surveys, performing physical and chemical tests, and carrying out subsequent investigation. Bringing such structures and similar structures back to life should be the duty of every country and nation to protect its past.

It has been seen that the Şahkulubey Cupola needs restoration. During the studies that started in 2019 and have continued until today, consultations have been held with the General Directorate of Foundations, the relevant provincial and district municipalities, and the Şahkulubey family. At the end of 2021, the Governor’s Office awarded the tender for the restoration and landscaping of the Şahkulubey Cupola.

Within the scope of this study, the characterization of the materials used in the construction of the Şahkulubey Cupola was carried out, and significant findings were obtained, which will shed light on the selection of materials that can be used in the restoration works. Considering these data, the Şahkulubey Cupola renovation works should be evaluated, and the appropriate materials should be used.

By analyzing the Şahkulubey Tomb samples, their physical, chemical, and internal structural properties were determined. It was observed that the stone has a heterogeneity that does not contain too many hard sections. This situation is also seen in its physical properties. This is particularly the reason for the high water absorption values. The effect of the hard sections is reflected in the mechanical properties. The best example to explain this situation is the differences in some compressive strength values in mechanical tests and their high values. It was seen that the samples consist of calcite crystals and contain magnesium and iron in their structure. The aim is to use Mardin limestone, whose physical and mechanical properties and internal structure analyses are given above, more appropriately as a building material and in environments where environmental conditions are considered. Thus, the usage efficiency of this material is increased, and its use in conditions suitable for its properties will also be ensured.

The compilation and analyses of all the results presented in this paper constitute solid support for decision-making in conservation and restoration actions, namely in developing compatible restoration products to repair these materials, which are so common in Turkish architecture.