Abstract
Zahhak Castle in Hashtrood, one of the prominent archaeological sites in northwestern Iran (East Azerbaijan Province), houses valuable stucco decorations. A portion of the artifacts recovered from excavations is preserved at the Zahhak Castle site museum, another portion in the storage of the Azerbaijan Museum in Tabriz, and a limited number in the museum’s main gallery. These artifacts include animal, human, floral, and geometric motifs. The present study aimed to evaluate the crystalline structure, mineralogical composition, and the extent of structural degradation of the excavated stucco decorations, employing laboratory techniques including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), ion chromatography (IC), proximity, and monitoring of environmental conditions (temperature and humidity). To compare structural damage, samples from both the site museum and storage artifacts were selected and analyzed. Phase analysis results indicate that the main phases of the stucco decorations are gypsum (CaSO₄·2H₂O) and quartz (SiO₂), with minor amounts of calcite (CaCO₃), albite (NaAlSi₃O₈), anorthoclase (O₈(Si₃Al)(Na,K)), and anorthite (CaAl₂Si₂O₈) also identified. According to IC results, the presence of harmful chloride and nitrate ions was detected at low and non-destructive levels. Crystallographic data suggest that structural alterations mainly occurred prior to excavation and during artifact burial. However, the high humidity conditions at the site museum and the lack of adequate conservation standards may accelerate deterioration processes. For optimal preservation, it is recommended that environmental temperature and humidity be continuously monitored and that standard display cases, along with periodic monitoring, be employed as effective conservation measures.
Keywords: Qaleh Zahhak Hashtrud, Stucco Decorations, XRD, Fe-SEM, Heritage Conservation.

Introduction
Excavations at the Zahhak Castle site in Hashtrood were conducted on a limited scale by Ghandgar and Rahmatpour between 2000 and 2007. During the excavations in the hall of the site (which is now the Zahhak Castle site-museum), a number of stucco decorations with various motifs—including plant, human, animal, and geometric designs, as well as pieces related to half-columns—were discovered. The stucco decorations obtained from the Zahhak Castle excavations are currently preserved in two collections: The Azerbaijan Museum in Tabriz and the Hashtrood Museum. These decorations include 47 pieces housed in the Azerbaijan Museum and a considerable number of stucco fragments of various sizes that remain at the site-museum. Of the 47 stucco pieces in the Azerbaijan Museum, five are displayed in the main galleries, while the remaining pieces are kept in the museum storage under relatively stable temperature and humidity conditions. The stucco works at the Zahhak Castle site are currently placed on the ground on a plastic sheet covered with excavation soil, and some pieces are kept in glass display cases and niches within the site-museum.
Considering the thermodynamic properties and crystal solubility of gypsum, along with the specific temperature and humidity conditions at Zahhak Castle during different seasons, it is not unexpected that these conditions may lead to particular changes in the structure of the excavated stucco decorations. This study aims to investigate these changes by comparing the microstructure of the stucco decorations at the Zahhak Castle site with those housed in the Azerbaijan Museum. The research examines these changes by analyzing the environmental parameters to which the pieces have been exposed, as well as considering the chemical composition of the gypsum used in the decorations and the ions present in the site environment. The processes that have caused—or may cause—the deterioration of these works will be evaluated. Ultimately, the study seeks to propose appropriate environmental conditions that would lead to the more sustainable preservation of these artifacts.
Therefore, the research questions are as follows: What types of damage have affected the stucco decorations at Zahhak Castle, and what are the causes of this damage? Additionally, what temperature and humidity conditions are most suitable for the proper preservation of the stucco decorations at the site?

Discussion
Site-museums encompass historical sites that have been excavated during various archaeological seasons. The primary purpose of creating such spaces is to present important archaeological findings and data derived from excavations. During excavations, some artifacts are preserved in situ by providing necessary conditions and facilities, and are made accessible to the public so that visitors can establish a direct connection with the context of the artifacts’ creation and formation.
In site-museums, visitors can simultaneously observe both the artifacts and the historical site, while also witnessing ongoing archaeological activities as well as conservation and restoration operations (Hassanzadeh, 2012: 378). The quality of the museum’s interior space is enhanced through the arrangement, organization, and structuring of its elements and forms, and it is designed with the aim of creating an attractive environment for tourists (Azarbouye Dinki et al., 2018).
Accordingly, the display cases at the Zahhak Castle site-museum (Fig. 1) are not suitable for the proper preservation of the stucco artifacts under study. These display cases should not only ensure protective security but also enable better exhibition of the artifacts. Currently, the artifacts are kept in glass display cases that allow visitors to access the items by sliding the glass and, in some instances, even remove them from the cases.
The overall arrangement of visual elements is designed to prepare the viewer’s mind to receive information. The artifacts inside the display cases are arranged in two rows, and visitors must sit to view the items on the lower level, which sometimes discourages them from observing these pieces. Additionally, the artifacts are presented without any information regarding their condition, year of discovery, or function, which generally renders them meaningless and of little cognitive value to visitors.
For designing an appropriate enclosure for the Zahhak Castle artifacts, it is recommended to consider the display case and communication spaces, the visual requirements of the environment, climatic conditions, lighting, materials and construction techniques, and security and protective systems (Fig. 2). In this regard, it is suggested that stucco artifacts of similar types be grouped in the same enclosure, accompanied by explanations regarding the excavation period, historical era, and probable function of each piece. Furthermore, the pieces should be categorized according to their design and motif and displayed in separate enclosures.
The primary factors for physical and chemical preservation in museums include the monitoring of temperature, relative humidity (RH), and fine particulate pollutants, and careful supervision of environmental conditions is of high importance (Mansourzadeh & Hatami, 2017: 289–302; Rahmani-Garmi, 2011: 62). To control the environmental conditions at the site where excavated artifacts are housed, monitoring tools such as thermometers and hygrometers should be used. Additionally, the implementation of air conditioning, protective measures, and museum security systems is essential.
In the case of the Zahhak Castle site-museum, comprehensive preservation of the artifacts is particularly important because these items are part of a valuable historical heritage, and proper care and maintenance are necessary.
The main factors in the preservation of artifacts in museums include both physical preservation and protection against chemical agents (Mansourzadeh & Hatami, 2017: 289–302). Careful monitoring of environmental conditions, including temperature, relative humidity (RH), and fine particulate pollutants, is one of the most important measures for safeguarding collections (Rahmani-Garmi, 2011: 62). To control the environmental conditions were excavated artifacts are housed, the use of monitoring tools for temperature and relative humidity, such as hygrometers and thermometers, is essential.
Furthermore, the implementation of ventilation systems and comprehensive protective and security measures in the museum, especially at the Zahhak Castle site-museum, must be thoroughly considered, as the excavated artifacts are part of a valuable historical heritage and their preservation is essential.
Regarding the Zahhak Castle site-museum, since the site’s temperature rarely reaches 40 °C—which typically does not cause phase transformations in gypsum—there is likely no significant concern about the deterioration of the stucco artifacts due to heat. Similarly, considering the low concentrations of chloride and nitrate ions in the stucco samples, the likelihood of phase transformations in the artifacts is also reduced. However, it is still recommended to maintain the temperature and relative humidity at approximately 20–25 °C and 40–50%, respectively, to prevent potential damage to these works.
To improve the storage conditions of the stucco artifacts in the Azerbaijan Museum’s repository, it is necessary to use an automatic air ventilation system that operates several times a day at predetermined intervals to remove excess humidity from the storage environment. Therefore, to monitor the temperature and humidity in the repository, the use of temperature and humidity data loggers is essential for recording and surveying environmental conditions, allowing precise control over storage parameters. Seasonal analysis of the data collected from the temperature and humidity data loggers, as well as air treatment systems, can help prevent damage to the stucco artifacts and minimize structural deterioration of the pieces.

Conclusion
This study demonstrates that the degradation processes have not significantly altered the structure of the stucco decorations at Zahhak Castle within the site-museum. Examination of the crystalline structure of these artifacts, both at the site-museum and at the East Azerbaijan Archaeology Museum, indicates that structural changes in the stucco were likely limited to the pre-excavation period, during the burial of the artifacts in soil, and that no substantial changes have occurred from the time of excavation (2003) to the present (2024).
Harmful anions, such as chloride and nitrate, are present only in minor amounts in the Zahhak Castle stucco, which slows down the processes of dissolution and recrystallization of the gypsum artifacts. Despite the anticipated lack of significant changes in the artifacts at the site-museum, it is recommended that these pieces be placed in standard display enclosures and maintained under stable temperature and humidity conditions to prolong their lifespan.
It is noteworthy that the stucco decorations at the site-museum generally lack distinctive designs or motifs, and most intact samples have been transferred to the repository of the East Azerbaijan Archaeology Museum. The primary reason for this transfer was the absence of a secure storage environment at the site-museum. It is expected that the installation of temperature and humidity data loggers, security equipment, and the use of standard display cases will improve the protective conditions of the site-museum.
Furthermore, the absence of specialized staff at the site-museum leads to confusion among visitors and the loss of opportunities to convey valuable information about the artifacts. Access to Zahhak Castle is only possible via challenging routes, which, combined with the lack of expert personnel, may reduce visitor motivation. It is therefore recommended that the artifacts be preserved near their original locations, with suitable conditions for display and protection. Similar artifacts should be kept in designated, separate enclosures from other types and displayed on shelves accompanied by complete information regarding their condition at the time of excavation and their probable function.