Material Analysis for Restoration Application: A case study of The World's First University Mor Yakup Church

Abstract

Introduction

It is known that the lack of information about the compatibility of materials during the selection of restoration materials in the past years has caused serious damage to historical textures and structures in many countries. In this context, it is important to determine and apply a repair material that is close to the original historical material and can replace the original material in restoration works. This situation requires a comprehensive and detailed research that includes the collection and analysis of original material samples [1]. In various studies investigating restoration practices, it is emphasized that integrated studies on the analysis of existing materials and the design of restoration materials should be included in the restoration programs of monuments [2–4]. The results of the scientific analysis of the original materials are accepted as a reference point in restoration research and this is considered as a mandatory step in the conservation process [5]. The purpose of the analysis of historical materials, which is considered a mandatory step in restoration applications, is to obtain information about the physicochemical composition of the materials and the raw materials used, as well as the production technology [6]. After this knowledge, a detailed study can be carried out on the synthesis of compatible materials in order to proceed with the restoration of the historical building [7–10].

To preserve the cultural heritage intact, it is necessary to obtain detailed information about the petrographic and petrophysical properties of the rocks and to carry out a comprehensive diagnosis of in situ stone decay and environmental conditions [11]. Petrographic analysis is an important tool for the characterization of materials and the study of the effects of deterioration on stone, cementitious materials and ceramics. Petrographic analyzes provide information about the nature and distribution of the mineralogical phases and elements within the stone sample and the stone interface that contribute to the identification of essential minerals and the stone degradation process [12]. Petrophysical and petrographic properties determine the amount and degree of weathering of stone monuments, and weathering processes and damage analyzes can be easily explained by examining these properties in the material [13–18]. However, in order to understand the material behavior in detail, it is necessary to conduct an integrated systematic study from different aspects such as examining the in-situ conditions of the building, archaeometric research, investigating the previous phases of the building [19–20].

It is stated that from the microstructural analysis results obtained by the experimental analysis of the material, the area’s most susceptible to weathering and deterioration, the mechanism of stone deterioration and the causes of deterioration can be determined in detail, and a guideline can be formed for the selection of the most suitable materials for the restoration of the structure [21–28]. In these studies, it has been proven that doing mineralogical, petrographic and chemical researches together can provide useful data on the nature of the building materials used, ancient production technologies and can help select compatible materials for restoration work. It is emphasized that such diagnostic studies are necessary for the development of an appropriate orientation for the preservation of structures in situ [29–38].

Based on this requirement, it was aimed to examine the material properties of the Mor Yakup Church, located in Mardin Province Nusaybin District, in order to create a laboratory guideline that reveals the material analysis to be used in the restorations to be made at the Mardin regional scale. The city of Mardin has a unique history that is reflected in the different stages of construction, the use of materials and the application of techniques. The selected study area and other monuments around it were also built with limestone. Limestone is a porous sedimentary rock widely used in many historical buildings and monuments around the world. Exposure to environmental factors causes significant changes in the physical and mineralogical properties of limestone [39]. As a result of the decrease in the durability of the limestone material over time, restoration works become a necessity in the buildings built with this material. The stone material seen in the structure examined within the scope of the study carries signs of deterioration in other structures in the vicinity. In this context, the detailed documentation, analysis, laboratory and microstructural analyzes used in this research can be used as a prototype for the restoration phases of other similar monuments in the vicinity. This study will help analyze the damage to stone material in a meaningful way and will be helpful for experts to take appropriate conservation strategies. This practice has not yet been established at the regional scale of Mardin and such initiatives are thought to support the contemporary trend towards the preservation of cultural heritage.

Study Area

The Mor Yakup Church, located in the Nusaybin District of Mardin Province, represents one of the most important and oldest Christian Medieval monuments that have survived in the region. Built by the Bishop of Nusaybin, Mar Yakup, in 311 AD, the Church has a three-nave basilica (Fig. 1). While it was a pagan structure, it was converted into a church and school in 326 AD. The fact that the building was converted into a baptistery in 359 in the Early Christian period and then into a church in the 8th century makes the building important and different in terms of Christian architectural history. The building, whose history is based on pre-Christianity and defined as a "special structure", is evaluated as a baptistery structure connected to the cathedral and monastery complex with the spread of Christianity in the region, and today it is referred to as the Mor Yakup Church and the building is associated with the Syriac Orthodox church [40] (Fig. 2).

Nusaybin, the district where the building was built, was known as a center of science and culture before Christ. The poet philosopher Vifa who lived long before Jesus and II. Philosopher Mor Ibn Serabyon, who lived in the middle of the century, was one of the Paganist Assyrians who were educated in Nusaybin schools. The first Christian-Assyrian Academy was opened in Nusaybin under the name of Nisibis Academy by Mor Yakup from Nusaybin, one of the founders of the Antakya Academy, in 200 AD. Excavations in the Mor Yakup Monastery, which was once the world's first educational university where philosophy, logic, literature, geometry, astronomy, medicine and law were taught, are still continuing. The fact that the oldest Syriac inscriptions were found here as a result of the excavations strengthens the argument of the Nysibis school as the oldest university. Serious inscriptions are emerging that the Nisibis Academy, which was found in the archaeological excavations in the Mor Yakup Monastery in Mardin Nusaybin, was the first university in the world. Important finds were also obtained in terms of Syriac history. The inscriptions unearthed here shed light on Syriac history. It is also important for Syriac literature. Writers and poets such as Mor Efrem were trained. These saints wrote the hymns and texts that are read in churches today.

Nusaybin historically known as Nisibis or Nesbin, is a city in Mardin Province, Turkey. The city of Nisibis is one of the oldest settlements in Mesopotamia. The fact that it was established on the meeting point of historical trade routes has carried the city forward in science, science and economy. The fact that the oldest Syriac inscriptions were found here as a result of the excavations strengthens the argument of the Nysibis school as the oldest university. Especially the closure of the Edessa School has turned it into an important education center. The church was once the world's first educational center where philosophy, logic, literature, geometry, astronomy, medicine, and law were taught. Important finds in terms of Syriac history were also obtained during the excavations. The oldest inscriptions shed light on Syriac history. It is also important in terms of Syriac literature. Writers and poets such as Mor Efrem were trained. These saints wrote the hymns and texts that are read in churches today.

As a result of the excavations, a cathedral dated to the 4th century was found in the northwest of the building, and there are building remains designed for different uses in the east and south directions. The building, known as the church, is referred to as the first university of education in the world, which has survived to the present day [42]. The building reflects the characteristics of the Late Roman - Early Byzantine period with its architecture and high relief stone decorations. The building, which has reached today with the repairs and changes it has undergone in various periods and partially preserved its architectural integrity, is still in need of maintenance and repair. After the excavations carried out in recent years, sections of different qualities have emerged in terms of materials, construction techniques and architectural elements on the northern, southern and eastern facades of the building. The southern part of the building is divided into two separate parts by two opposing buttresses. To the east, there is a square-planned space with a width and length of 7 m. There are two door openings on the north and south walls of the place. An apse is visible on the eastern wall. There is an arch opening to the second section in the west. The most important feature of this place is the wall decorations. There are door openings on the north and south walls of the western space. When the position of the buttresses in between and their relationship with other architectural elements are evaluated, it is understood that they were added later. When the eastern wall is examined from the outside, it is seen that the apse part may have been added later. The decorations inside the space are considered to be a deeper and earlier example of the ornamentation that was common in Northern Mesopotamia in the 5-6th centuries and is dated to the 4th century (Fig. 3–7).

Method And Material

The building was built in stone masonry technique and its facades were covered with cut stone (Fig. 3,4). Light beige-colored limestones were used in the original pavements of the first phase of the building. Limestones, which are one of the main rock groups suitable for the geological formation of the region, exhibit properties that are suitable for deterioration in unsuitable weather and environmental conditions with their porous and soft texture. Plaster application indicating different periods of use was found on the interior walls and superstructure elements of the building (Fig. 5). The upper floor spaces were plastered again during the recent repairs (Fig. 6, 7).

Material

Laboratory analyzes were carried out by taking 3 mortars, 3 plasters and 2 stone samples (Fig. 5) from the Mor Yakup Church. Protein and oil properties of the samples by spot tests, conductivity with the analysis of water-soluble salts, strength properties of the samples by physical tests (unit volume weight, water absorption capacity, porosity, Schmidt hammer hardness and ultrasonic velocity measurement tests), total moisture, organic and carbonate (CO₃²⁻ ) content with calcination analysis at 105°C, 550°C and 1050°C loss, general texture, mineral content, quality and proportions of acid-treated sample aggregates by stereo microscope analysis were determined by petrographic analysis.

2.1. Obtaining Samples

2.2. Protein And Fat Analysis With Water-soluble Salts

The salt content of the materials that make up the structures provides information about the physical conditions of the structures. Salts found naturally in different building materials or dissolved in water and then transported to the surface or pores of the materials by water as a result of capillary effect provide information about the chemical changes that can occur both in the material itself and in the structures of other materials with which it is associated. To be able to determine the qualities and amounts of water-soluble salts (chloride (Cl-), sulfate (SO₄ ²⁻ ), carbonate (CO₃ ²⁻ ), and nitrate (NO₃ ⁻ ) salts) in the contents of the samples, and in order to determine whether additives such as saponifiable fat, protein are added or not, analyzes related to spot tests were made and the results are given in (Table 1). In addition, oil and protein analyzes were performed to determine the contents of the layers defined as the protective layer on the surface of some samples (Table 2).

2.3. Physical Tests

The physical conditions of the materials (unit volume weight, water absorption capacity, porosity, Schmidt hammer hardness and ultrasonic velocity) were determined by the basic physical tests performed on the samples (Table 3).

2.4. Loss Of Ignition, Acid Treatment And Sieve Analysis

The results of the calcination (heat loss) analysis of the mortar and plaster samples at 105 ± 5 o C, 550 ± 5 o C and 1050 ± 5 o C, the proportion of silicate aggregates that did not react as a result of acid treatment, and the size distribution of these aggregates are given in Table 4.

2.5. Petrographic Properties

The textural and aggregate/binding properties of the samples prepared for thin-section optical microscope analysis were examined under a stereo microscope, and the mineral contents and ratios of thin sections were examined under a polarizing microscope and the results are shown in Table 5.

Results

According to the results of the archaeometry analyzes performed on 3 mortars, 3 plaster and 2 stone samples taken from the Mor Yakup Church, the binder, filler and additive types and proportions (by weight) of the mortar and plaster samples and the physical and petrographic qualities of the stone samples were obtained.

In the spot anion tests (Cl⁻, SO₄ ²⁻, CO₃ ²⁻, NO₃ ⁻) applied on mortar, plaster and stone samples, sulphate (SO₄ ²⁻ ) salinization was detected in low amounts in mortars and plasters, but not in stones (Table 1). There is a very high rate of chloride (Cl⁻) type salinization in all of the samples. The most basic situation reflecting this situation should be the cement-containing mortars used in the repair. A very high amount of nitrate (NO₃⁻) type salinization was determined in all of the samples. The source of this type of salinization is environmental effects (especially from the exhaust gas effect). Nitrate has a role in intense black layering, which can be observed visually, especially on stone surfaces. It is interesting that the samples did not contain dissolved carbonate (except for Example 4). This indicates that the structural segregation is low in these samples. Organic additives (plant, straw, tow, etc.) were not determined in the aggregate structure of the mortar samples. In plaster samples, on the other hand, it was determined at a very low rate in Sample 4 and Sample 5. While protein and oil content were not found in the mortar samples, the oil amount was determined in Example 4 of the plaster samples. The oil content determined in this example is due to the effect of the black layer. The black layer has also caused carbonation towards deterioration and decomposition in the same sample (Table 1,2). In support of this situation, when the black layer samples on the plaster were analyzed separately in terms of protein and oil contents, protein was determined in the samples (Table 2).

The total water-soluble salt content of the mortar and plaster samples is quite high as expected since they have lime-type binders. The total salt content in mortars varies between 6.5–18.8% (average 12.07%) and in plasters between 11.1–14.8% (average 12.93%) (Table 1). The total salt contents of the limestone (Sample 7) and travertine (Sample 8) stone samples were 3.1% and 3.5%, respectively. The rates determined by considering the porous structure of the stones are quite high and at a level that will create a destructive effect (Table 1,3,5c). The source of the determined salinization is the decomposed joint and rubble filling mortars and plasters.

The physical properties of the structural samples were determined by the basic physical tests (BHA: unit volume weight, SEK: water absorption capacity, P: porosity, SH: Schmidt Hammer hardness and SV: ultrasonic velocity measurement) (Table 3). The saturated/dry unit weights of the mortars (Examples 1–3) are respectively 1.92–2.15 g/cm3 (average 2.05 g/cm³) / 1.54–2.05 g/cm³ (average 1.79 g) /cm³), the saturated/dry unit volume weights of the plasters (Examples 4–6) are respectively 1.80–1.99 g/cm3 (average 1.90 g/cm³) / 1.58–1.94 g/cm³ (average 1.76 g/cm³). The unit volume weights of the mortar samples are higher than the plasters. The water absorption capacity and porosity of the mortar samples are between 2.19–16.62% (average 7.52%) / 4.50-25.67% (average 12.30%) respectively, and the water absorption capacity and porosity of the plaster samples are also they vary between 1.17–10.90% (mean 4.54%) / 2.28%-17.17% (mean 7.39%), respectively (Table 3). Samples with high unit weight and low porosity are expected to be more durable. Mortar samples have a higher unit volume weight than plasters on average values. Sample 1 from the mortar samples and Sample 4 from the plaster samples are similarly the samples with the lowest strength. When the physical properties of the stone samples are evaluated, the limestone (Sample 7) and travertine (Sample 8) samples have similar physical properties (Table 3). However, although the limestone sample seems to be slightly more durable than the travertine sample in terms of its basic physical properties, it has been determined that the travertine sample exhibits a more homogeneous rock structure than the limestone sample with the ultrasonic velocity measurement value (Table 3).

Mortar samples contain moisture content varying between 18.38–18.94%. Plaster samples (Samples 4–6; average 11.10%) are in a more humid environment than mortar samples (Samples 1–3; average 4.47%). Organic carbon content was determined between 4.44–5.52% (on average 5.51%) in the structure of the mortar samples, and between 9.29–16.67% (on average 11.99%) in the structure of the plaster samples. Organic tow (plant, straw, etc.) added to the structure of the plaster samples in order to increase the adhesion to the surface can be observed in the plasters in a way that differs from the mortars (Table 4).

The total carbonate content of the acidic treated mortar samples varies between 96.67–97.40% (average 97.12%), and in plasters between 93.01–99.51% (average 97.13%). Both the loss of ignition and the carbonate content determined in mortars and plasters by acidic treatment are quite high (Table 4). The total carbonate content of the samples is directly related to the lime content. The lime/carbonate dense binder content of the joint mortar and plaster samples of the building does not show compatibility with the binder aggregate (1:2 and 1:3) content seen in traditional applications.

The size of the aggregates obtained after the acidic treatment applied to the mortar and plaster samples was evaluated by sieve analysis. For this, sieves ranging from 125 µm to 6300 µm were used (Table 4). While there is no aggregate over 1000 µm in mortar (Samples 1–3) and plaster (Examples 4–6) samples, coarse aggregate and a more homogeneous distribution are observed in plasters at a lower rate than mortars.

Mortar and plaster samples were examined in terms of matrix (binder) and aggregate properties (mineral/rock type, structure, distribution, size, orientation) by thin-section optical microscope analysis. Mortar samples are of different petrographic characteristics. 3 mortar samples belonging to the building were classified in 3 different groups and 3 plaster samples were classified in 2 different groups (Table 5a,5b). Optical microscope examinations revealed that the plaster samples have a two-layered structure. A high amount of binder was determined in the petrographic structures of the mortar and plaster samples, supporting the loss of ignition and calcination analysis (Table 5a). The matrix total binder (%TB) content of the mortar samples varies between 87–91% and in plasters between 90–97% (Table 5a). The upper layers of the plaster samples contain higher aggregates than the lower layers (Table 5a). Lime constitutes the binding structure of the mortar and plaster samples, all of which have unique characteristics (Table 5b). There are brick fragments at the rate of 1% of the total aggregate in the aggregate structure of the upper layers of Sample 3 from the mortar samples, and Sample 4 and Sample 5 from the plaster samples. In addition, organic additive parts (at the rate of 1% of the total aggregate) were determined in the structure of the lower layers of the same plaster samples (Table 5b). Mortar samples are richer in aggregate content than plaster samples. The rounded structure of the aggregates in the mortar samples indicates that the original application was made using aggregates from the stream bed. On the other hand, rounded and broken/angular aggregates are found together in plaster samples. Aggregate dimensions are indicated on thin-section optical microscope microphotographs with a measuring stick (1000 µm).

The rock structures were determined by petrographic examinations of the stone samples (Sample 7.8) sampled from the building (Table 5c). Accordingly, Example 7 has limestone (sparitic texture) and Example 8 has travertine type ski structure. Travertine in the rocks presents a more porous structure than the sample limestone (Table 5c).

Discussion

In this study, it was aimed to examine the material properties and problems of the Mardin Province Nusaybin District Mor Yakup Church to be used in restorations to be made at the regional scale of Mardin, in order to create a laboratory guideline that reveals material analysis. As a result of the study, it is seen that the analysis studies provide information about the nature and distribution of the mineralogical phases, elements in the stone sample, which contributes to the identification of basic minerals and the behavior responsible for stone deterioration.

The first finding that should be emphasized is that petrographic analyzes appear to be an important tool for the characterization of materials and for examining the degradation and deterioration effects on stone and cementitious materials. This finding supports that the petrographic analyzes obtained in various studies in the literature provide information about the nature and distribution of the mineralogical phases and elements within the stone sample and the stone interface, which contributes to the identification of essential minerals and the behavior responsible for stone degradation. In addition, petrophysical and petrographic properties determine the amount and degree of weathering of stone monuments and support the fact that weathering processes and damage analyzes can be explained by examining these properties in the material [12–18].

The second finding obtained from this study was that petrographic analyzes alone would not be sufficient for a detailed understanding of the harmonious material behavior to be used in the restoration phase, and that an integrated systematic study should be carried out from different aspects such as examining the in-situ conditions of the building, archaeometry research, and investigating the previous phases of the building. It has been proven that the combination of integrated analysis studies and mineralogical, petrographic and chemical researches in the study provides useful data on the nature of the building materials used, old production technologies and can help select compatible materials for restoration work. This finding supports the fact that with the results of microstructural analysis obtained from the combination of experimental analyzes of the material obtained in various studies in the literature, the area’s most susceptible to weathering and deterioration, the mechanism of stone deterioration and the causes of deterioration can be determined in detail and a guideline can be created for the selection of the most suitable materials for the restoration of the structure [21–28].

The third finding obtained from this study is that there is a very high rate of chloride (Cl⁻) type salinization in all of the samples. The most basic situation reflecting this situation should be the cement-containing mortars used in the repair. A very high amount of nitrate (NO₃⁻) type salinization was determined in all of the samples. The source of this type of salinization is environmental effects, especially from the exhaust gas effect. Nitrate has a role in intense black layering, which can be observed visually, especially on stone surfaces. All these results show that the limestone deteriorates due to the cement-containing mortars used in the building and environmental effects, especially the exhaust gas effect. It has been determined that nitrate salt has a role in the intense black layering, which can be observed visually, especially on the stone surfaces. This result supports the fact that different types of material deterioration can be seen in buildings due to environmental sources that are effective in geographical contexts, and that such analysis studies are necessary in order to identify the most common material problems in that geographical context and to develop an orientation suitable for the in-situ preservation of structures [29–38].

Conclusion

In this study, mortar, plaster and stone samples were taken from the Mor Yakup Church masonry, and various archaeometry methods were used and the samples were defined. The analyzes show that the deterioration process has already begun inside the building materials. In this context, basic suggestions are stated that will help the building to be repaired in accordance with the relevant principles and ethics.

The stone/rocks sampled from the masonry and rubble fillings of the building are limestone and travertine. For stone repairs, it is recommended to use limestone and travertine type rocks, which can be obtained from local sources and will show textural and formation harmony with the original material.

It is recommended to use 25% by weight of lime, 30% toothed/fractured, sieved, washed, distribution compatible with the original mortars, without carbonate content, aggregate with local stream bed material, 40% limestone powder, 5% broken brick lime mortar containing in the homogeneously mixed binder in the joint mortar repairs of the building. Brick fracture additive is recommended both for compatibility with the original material and for reducing the effects of dampening and salinization in these parts.

It is recommended to use lime mortar containing 30% by weight lime, 20% sieved, washed, without carbonate content local stream bed material aggregate, 35% crushed, sieved limestone powder/crack, 10% volcanic rock dust, 5% broken brick in order to protect the plastered surfaces of the building. The organic additive determined in the original plasters was not recommended due to the original material supply and application difficulties.

Lime mortar consisting primarily of slaked and rested lime is recommended for mortar and plaster repairs. Lime mortars have more successful strength properties for structures in the medium and long term in terms of strength. It is also possible to use special hydraulic lime produced for restoration in a ready-to-use condition in mortar repairs.

There is a very high rate of chloride (Cl-) type salinization in all of the samples. The most basic situation reflecting this situation should be the cement-containing mortars used in the repair. A very high amount of nitrate (NO3-) type salinization was determined in all of the samples. The source of this type of salinization is environmental effects (especially from the exhaust gas effect). Nitrate has a role in intense black layering, which can be observed visually, especially on stone surfaces. In this context, cement-containing materials should not be used at any stage of the repair mortar and plaster. During the repair phase, it is also recommended to test the proposed mortar contents and analyze them to determine their compatibility with the proposed material. Properly performing the repairs under the supervision of experts in the context of the relevant national/international regulations and ethics, monitoring them over time, and renewing them in the future will reflect the entire procedural nature of the repairs.

Declarations

Declarations

Conflict of interest

There is no conflict of interest

Funding:

This research has received no funding.

Author contributions:

L.K. Literature review, writing the manuscript, A.A. Writing and editing. M.Y. supervision,editing

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