Evaluation of stone weathering in urban environments using tombstones (case study: Yazd city)

 ABSTRACT
Urban weathering refers to the processes of loosening, decay, and eventual deterioration of materials used in various urban constructions. This study focuses on assessing the degree of weathering in gravestones from the Joy-e-Horhor and Khold-e-Barin cemeteries in Yazd. A combination of petrographic analyses and longitudinal monitoring of Schmidt hammer rebound values for hundreds of gravestones was employed to achieve this. The findings indicate that in addition to the petrographic characteristics of the stones, such as mineralogical composition, mineral diversity, and rock texture and fabric, local climatic conditions significantly influence the weathering and degradation of these materials. Key processes contributing to the loss of stone durability include temperature fluctuations leading to thermal expansion and contraction, the albedo effect of the stone, wet-dry cycling, and the crystallization and dissolution of secondary minerals like calcite and gypsum. Gravestones made of travertine and marble, characterized by a predominance of calcite minerals and light-colored surfaces, exhibit higher resistance to weathering compared to other lithologies, provided they are not exposed to excessive moisture or frequent washing. In contrast, low-grade metamorphic rocks such as slate and phyllite are the least suitable for gravestones due to their high density of fractures and cleavage planes. Similarly, dark-colored igneous rocks are prone to rapid durability loss, as the differential thermal expansion and contraction of their constituent minerals in response to temperature changes accelerate their weathering processes
Extended Abstract
Introduction
Beyond their economic and social significance, stone buildings hold substantial cultural and national heritage value. However, preserving these structures has been an ongoing challenge. In addition to human factors, weathering processes, including physical, chemical, and biochemical effects, are key contributors to the deterioration of these invaluable cultural assets. Weathering in urban environments occurs with increased intensity and speed due to extreme fluctuations in weather conditions, making it a significant environmental hazard with considerable risk. Moreover, urban settings can be viewed as museums of diverse types of stone, providing an opportunity to study the processes of change and evolution affecting each type. Cemeteries serve as valuable urban sites for studying stone changes, given the variety of stones used for commemorating, protecting, and decorating burial sites. Conveniently, many stones have their installation dates inscribed, which aids in examining urban stone weathering. This enables researchers to assess the degree and extent of weathering among different stone types exposed to the same microclimatic conditions over defined, shorter periods within cities. Urban gravestones have historical, religious, cultural, and emotional significance, yet this importance is often overlooked due to their gradual deterioration. An essential approach to protecting tombstones from weathering involves understanding how various stone types endure under different climatic conditions. This study addresses this objective by examining the weathering rates of stone materials over the last century in two cemeteries in Yazd, a city known for its warm, dry summers and cold, dry winters.
 
Methodology
Following field studies of various urban structures, gravestones from tombs were chosen and assessed as part of a broader field community. In Yazd, there are two main cemeteries, the older Joy-e-Horhor and the newer Khold-e-Barin, featuring a variety of tombstone styles. A Schmidt hammer was used to achieve a precise, quantitative evaluation of gravestone weathering. After the initial identification, the gravestones were examined with the Schmidt hammer for six working days. When the hammer is pressed against the rock surface, it delivers impact energy through a plunger, after which a rebound number indicating the surface hardness is displayed directly on the hammer. To assess the weathering and deterioration of the gravestone, the L-type Schmidt hammer (measured in N/mm²), as per ASTM D5873, was utilized. This hammer type is particularly sensitive in the lower measurement range, providing more accurate results when testing weak, porous, or weathered stone surfaces. Following this, thin sections were prepared for a petrographic analysis of the samples using a polarized microscope. Since tombstones are typically installed shortly after an individual's death, the engraved date on each tombstone served as the time reference for this study.
 
Results and discussion
In the cemeteries of Yazd, many stones have shown significant weathering within just a few decades, though the rate of weathering varies across different stone types. Field and laboratory analyses identified the primary stones used in these cemeteries, including travertine, sedimentary limestone (such as Lashotor limestone from Isfahan and Taft limestone), white marble (including the variety known as Javid), metamorphic rocks like slate-phyllite and schist, and mafic igneous rocks, commonly marketed as granite. Correlation graphs of Schmidt hammer rebound values over time showed that newer stone types generally had the highest rebound values. Over time, different stone types displayed weak to strong negative correlations, indicating decreased stone strength. The irregular rebound values may be due to non-uniform and inconsistent conditions during the foundation and installation of the gravestones. Travertine and limestone types showed the greatest resilience to weathering in Yazd, with a strong correlation between their Schmidt hammer rebound values and time. Despite being rich in calcite, two distinct erosion patterns emerged among the limestone types. Taft limestone exhibited more severe erosion over time than Lashotor limestone. The observed differences in weathering can be mainly attributed to varying sedimentation conditions and clay mineral content within the stones. With its higher clay mineral content, Taft limestone is more prone to weathering. Metamorphic rocks used in Yazd cemeteries fall into two categories as those with weak and moderate metamorphism. Factors like lithology, metamorphic features (such as cleavage and mineral composition), and pressure and temperature conditions have impacted their Schmidt hammer rebound values in low-grade metamorphic rocks. Rocks that have undergone low-temperature metamorphism, containing minerals like clay and chlorite, are notably more brittle and sensitive to climate effects. As a result, slate-phyllite metamorphic rocks showed lower Schmidt hammer rebound values, while cordierite-schist displayed higher values and a strong correlation with time. Some metamorphic rocks' fragility and significant weathering are due to their distinct foliation and structural discontinuities. The main mafic igneous stones used were primarily olivine gabbro and gabbro-diorite. These igneous rocks exhibited a negative correlation between Schmidt hammer rebound values and time, likely due to rapid spalling from lithostatic pressure release, as well as their coarse-grained, polymineralic texture. This texture makes them vulnerable to thermoclastic and cryoclastic weathering processes in Yazd’s warm-dry and cold-dry climate, resulting in severe weathering within a short time. The dark color of these igneous rocks has also increased temperature fluctuations between day and night and across seasons, further intensifying these weathering processes.
 
Conclusion
Studies and analyses indicate that gravestone weathering rates in urban areas differ from those in natural environments. This difference is due to moisture sources in urban settings, such as cemetery moisture, gravestone washing, and watering of plants, which contribute to both physical weathering and stone dissolution. Additionally, while stone selection in the past depended heavily on proximity to the site, improved transportation now allows stones to be sourced from nearby and distant locations. Therefore, it is crucial to consider lithological properties and the stones' responses to varying climates to ensure long-term stability and durability beyond visual qualities like cost, color, and aesthetics.  Factors including texture, mineral content type and diversity, color, porosity, surface weaknesses, and quarry extraction depth all play a significant role in a gravestone's resistance to weathering. The final results of this study indicate that the primary natural factors influencing the weathering of tombstones in Yazd include temperature-driven expansion and contraction, repeated wet-dry cycles, secondary mineral crystallization and dissolution, and albedo characteristics.
 
Funding
There is no funding support.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
We are grateful to all the scientific consultants of this paper.