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Razani M, Velayati R, Haghi P. (2025). Characterization of Ancient Mortars of Ojan Castle in Bostanabad County Belong to Sajuk-Ilkhanid Era. Parseh J. Archaeol. Stud.. 8(30), 151-178. doi:10.61882/PJAS.8.30.151
URL: http://journal.richt.ir/mbp/article-1-920-en.html

Abstract
This article discusses the structural analysis of ancient mortars used in Ojai’s Castle, which belongs to the Ilkhanid period in Bostanabad County, East Azerbaijan Province. Various methods, including field studies for documentation, sampling, and laboratory analysis using X-ray diffraction (XRD) and X-ray fluorescence (XRF), were utilized to investigate the structural and physicochemical properties of the ancient mortars. For this purpose, ten mortar samples were collected from different sections of the outer walls of the castle, which were excavated in the past five years, and their dominant phases and compositions were identified using XRD and XRF. The results show that the composition of the mortars used in the castle is significantly similar. Contrary to existing assumptions based on the use of lime-based mortars in cold regions and stone structures, the mortars in Ojan Castle are composed of gypsum and quartz-based high-purity gypsum mortars. These mortars were used for structural reinforcement and, especially, for enhancing their compressive strength. Based on these findings, this study can serve as a reference for future research on the structural analysis of ancient mortars to restore the castle.
Keywords: Characterization, Seljuks-Ilkhanid, Ojan Castle, Mortar, XRD, XRF.

Introduction
Historical architecture exemplifies the dynamic progression and development of knowledge, structure, recognition, and production of diverse materials. Mortars, as significant architectural elements, play a crucial role in this evolution. Remarkably, numerous mortars have retained their structural attributes for centuries. These historical mortars represent cultural accomplishments, exhibiting distinct properties influenced by raw materials, technological methods, and their manufacturing and utilization processes. The analysis of historical mortar’s composition serves as a valuable tool in understanding the construction techniques and technological advancements employed in the past. By studying the structure, composition, and processing methods of traditional mortars, we can make informed decisions regarding their conservation, restoration, reproduction, and identification of potentially damaging factors in historical buildings. Furthermore, mineralogical and chemical analysis of mortars provides essential insights into their setting characteristics, mineral grading, and compound quantities, offering a comprehensive understanding of their properties. Concerning the specific topic of this research, which is related to the historical and cultural mortars of the Seljuk-Ilkhanid era we can say undoubtedly, the Ilkhanid era can be considered one of the brilliant periods in the history of urban planning in Iran and another beginning in the emergence of a new style in creating and developing urban spaces. This style, which is known as the “Azeri style”, hastened the creation of royal cities such as Ghazaniyeh and Soltanieh and the strategic mother city of Tabriz, which is the manifestation of the ideals and urban planning ideas of the Mughal Ilkhanid. It has remained unstudied until today. In this research, the structural analysis and identification of the mortar composition of the historical castle of Ojan (figure1-3), related to the Seljuk-Ilkhanate period, will be examined and studied. Until now, five seasons of scientific archeological excavations have been carried out in the historical castle of Ojan from 2018 to 2022, and the results show remarkable stone architecture and architectural decoration such as Muqarnas made by gypsum, tiles, shaped stones, and various objects. Belongings include copper coins, pieces of pottery from the Seljuk-Ilkhanate period, glass, metal nails, etc. (figure4-7 & table 1) 
The most important research questions regarding the mortars used in Ojan Castle are: 1. What types of mortars are present in Ojan Ilkhanate castle? 2. What are the fundamental characteristics of the mortar used in Ojan Castle? The research methodology for this study consists of three main components:  1. Gathering documentary information through library studies. 2. Conducted field studies, including sampling from the historic Ojan Castle and documenting the current condition of the site to understand the nature of the building under investigation. 3. Performed laboratory studies, specifically analyzing samples taken from the outer wall of the castle. This analysis includes structural analysis and qualitative examination of mortar materials.

Materials and Methods
In this study, after examining the explored sections of the Ojan castle, mapping of the structure was carried out. Based on the current condition of the walls and the materials used in the construction (Fig. 8), 10 random representative samples were taken from different sections of the castle, including the outer parts of the main walls and the areas between the main materials. The samples were selected from different parts of the walls to ensure sufficient dispersion. The sampling method involved separating layers from both intact and deteriorated sections. Fig. 9 and indicates the locations where the samples were taken. Based on field observations, the mortar layers, especially in shallower and more exposed areas, had weak and powdery structural conditions. However, at depths greater than 20mm from the wall surface, the mortar exhibited better strength compared to the surface layers. The color of the mortar was mostly white, with some areas having a slight grayish tint, and no traces of plant remains or other components were observed. In the next stage, images and visual information related to each sample were recorded on-site. Descriptions of the visual characteristics of the samples, along with images and sampling locations, were collected and documented in Table 2. All samples were powdered and passed through a 200-mesh sieve. They were then barcoded and sent to the laboratory for XRF (table 3, 4, figure10) and XRD (table 5, Fig. 11) analysis. The samples were barcoded using the following format: “Ojan Castle, 2020 (OC20), Tranche number (T), Sample code (S)”. For example, a sample would be labeled OC20-T16S01.

Discussion 
With the aim of structural analysis of the mortar used in the stone walls of Ojan Castle belonging to the Seljuk-Ilkhanate historical period in Bostanabad city, 10 samples of mortar were selected from different parts of the outer walls. Based on the results of the experiments (XRD, XRF), the use of mortar with the leading and dominant phase of gypsum is used in all samples, and also all types of phases with silicate structure and phases in the soil can be seen in all samples. To construct this mortar, high-purity gypsum and sand were used. Approximately %8 of the mortar composition consisted of other components, mainly soil elements such as Muscovite, Albite, Salts, etc. The presence of these materials and phases is natural due to the inherent porosity of gypsum mortar and the fact that this architectural structure has been buried in the soil for several centuries.

Conclusion 
The use of semi-fired, semi-pounded gypsum, with the addition of sand as an intermediate mortar between stones, as well as for pointing in various sections of the outer walls of Ojan Castle, indicates the use of processed gypsum mortar in cold regions such as Bostanabad in the northwest of the country. As we know, in the field of conservation and restoration of historical architecture, the use of authentic materials that correspond to the original substance is crucial for preserving the authenticity, historical value, and cultural significance of ancient artifacts.
Based on this principle, the use of traditionally processed gypsum for constructing traditional mortar in the conservation of this ancient site can be considered significant in terms of maintaining authenticity. Considering recent seasons of excavation and access inside the castle, conducting similar tests on samples of interior plaster can reveal the differences between the mortars used on the outer and inner surfaces. This information can provide insights for proposing restoration and conservation procedures for the site.