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Moradi Y. (2023). Archaeological Excavations at Bisoutn Bridge: Stone, Brick and Lime Workshops. Parseh J Archaeol Stud. 7(25), 105-136. doi:10.30699/PJAS.7.25.105
URL: http://journal.richt.ir/mbp/article-1-759-en.html

Abstract
The Bisotun bridge was built across the Dinavar Ab River, flowing on the eastern outskirts of the present-day town of Bisotun. The construction of the bridge’s substructure was initiated in the late Sasanian period, but the program was abruptly terminated, as is the case with other Sasanian projects in Bisotun. Subsequently, the Ḥasanwayhids, a local Kurdish dynasty in western Iran, completed the construction of the bridge. High traffic and natural hazards such as floods and earthquakes have inflicted damage upon the bridge in subsequent periods, i.e., from the Saljuq era to the first Pahlavi period. Consequently, governments made considerable efforts to restore or reconstruct various parts of the bridge. Workshops were established at a short distance northeast of the bridge to provide construction materials for the restoration and reconstruction of the damaged parts. To the northeast of the bridge, there exists a low mound measuring 50 m in length and 40 m in width. The mound was excavated in 2002 under the direction of the present author, revealing four distinct archaeological strata. The earliest layer (I) includes a seasonal settlement from the Qajar period. Layer II contains a cemetery from the same period. Layer III encompasses several brick and lime kilns, dating back to the Ilkhanid and Qajar periods. Layer IV yielded parts of a stone-cutting workshop from the late Sasanian/ Ḥasanwayhid period. This article aims to provide the first comprehensive description of the findings within each archaeological stratum, using historical and descriptive-analytical research methods. We will also propose a chronological framework for the excavated materials based on archaeological evidence and thermoluminescence dating analysis. Furthermore, the article will delve into the production processes of brick and lime produced in the excavated kilns. Moreover, we will provide insights into the process by which the stone blocks were produced in the stone-cutting workshop.
Keywords: Bisotun Bridge, Stone Workshop, Brick, Lime, Kiln, Cemetery.

Introduction
The Bisotun Bridge is located on the eastern outskirts of the present-day town of Bisotun, on the “Great Khorasan” highway to Baghdad, spanning the Dinavar Ab River (Fig. 1). The bridge is 145 m long and consists of six spans. The bridge piers are constructed of well-dressed stone blocks, while the walls, buttresses, and vaults are made of bricks. Archaeological excavations and architectural studies have revealed that the bridge has been constructed, reconstructed, and repaired in eight distinct stages, occurring at different times ranging from the late Sasanian period to the first Pahlavi era.
On the northeast side of the bridge, there was a low-laying mound measuring 50 m in length and 40 m in width (Figs. 2–3). This mound was covered with a considerable amount of kiln slag, fragments of baked bricks in various sizes, and stone blocks. These findings indicated the presence of workshops related to the production of building materials for the initial construction and/or repair of the bridge. In 2002, the mound was partially excavated under the direction of the present author. The archaeological deposits of the mound had an approximate thickness of 3.50 m. The excavation was carried out with two objectives. First, to identify the potential workshops involved in the production of building materials used in the construction of the bridge. Second, to shed light on the processes by which the building materials were produced in these workshops.
This article aims to first provide a detailed description of the findings from each archaeological stratum, employing historical and descriptive-analytical research methods. Subsequently, by utilizing archaeological evidence and thermoluminescence dating analysis, we will determine the dates of the architectural structures under investigation. Furthermore, we will endeavor to explain the processes and techniques employed in the production of brick and lime production based on the layout and architectural characteristics of each kiln. Moreover, we will discuss the process by which finely cut stone blocks were produced in the stone-cutting workshop. We intend to address  the following questions, drawing on historical documents and archaeological evidence:
1. During which period were the excavated workshops actively in operation?
2. What were the production processes of building materials in these workshops?
3. Can we establish a chronological framework for the excavated cemetery?
4. Are the graves associated with one religious group, or do different groups of graves represent diverse religious affiliations?

The site
During the excavation of the mound, a stone-cutting workshop from the late Sasanian or Hasanwayhid period was discovered. In this workshop, stone blocks for the construction of the bridge piers were skillfully dressed (Fig. 4). The workshop suffered damage due to the subsequent construction of brick and lime kilns (Figs. 5, 7, and 17). Nonethless, twelve cut stone blocks of varying sizes and shapes were found in the excavated area. Furthermore, the excavation revealed three kilns (Figs. 5-6) engaged in the production of bricks and lime. 

Kiln 1 
This kiln is oriented in a north-south direction, and only its furnace has survived. The furnace represents three construction phases. In Phase I, it had a rectangular plan with external dimensions of 7.50 m in length and 4.80 m in width (Figs. 7–8). The relatively modest size of the furnace indicates that it was likely not intended for extensive brick production but rather for the supply of bricks for the reconstruction and repair of the bridge during the Ilkhanid period. In Phase II, the furnace’s plan turned to a circular shape, with a diameter of 3.30 m and a height of 2.10 m (Figs. 7 and 16). In Phase III, another air flue was built upon the one from Phase II. During Phases II and III, this kiln was used for lime production. Originally functioned as a brick-manufacturing kiln during the Ilkhanid period, it was transformed into a lime kiln in the Qajar period.

Kiln 2
This kiln has an approximately circular plan (Figs. 17–18). The inner diameter of the kiln ranges from 2.50 to 2.60 m, while the remaining height of its walls stands at 3.30 m (Figs. 19–21). Within the kiln, there is a channel oriented in a north-south direction, allowing the inflow of air for the combustion of fire inside the kiln. The presence of substantial quantities of lime inside the kiln strongly suggests its use in the production of lime for the repair of bridge piers.

Kiln 3
This kiln has a circular plan with a varying inner diameter. The kiln’s dimensions include a diameter of 1.80 m from the floor up to a height of 55 cm, a diameter of 2.40 m from there to a height of 1.20 m, and a diameter of 2.70 m from that point to the highest part of the wall (Figs. 24–27). The channel on the floor of the kiln is designed to facilitate air circulation. Three additional channels, sharing similar characteristics, have been built on top of this primary channel. Similar to kiln 2, the presence of substantial quantities of lime inside the kiln indicates that it was used to produce lime for the repair of bridge piers.

Conclusion
The excavation of the mound has yielded four distinct archaeological strata. Layer I represents a seasonal nomadic settlement dating back to the Qajar period. Layer II consists of a graveyard from the Qajar period. Layer III contains three kilns used for brickmaking and lime production, which can be dated back to the Ilkhanid and Qajar periods. Layer IV reveals a stone-cutting workshop, likely dating back to the Sasanian or Ḥasanwayhid period. The proximity of these kilns and the stone-cutting workshop to the bridge indicates that these workshops were established nearby to facilitate the construction process and ensure a readily available supply of building materials.
The brick-manufacturing kiln discovered in the excavation is an extensive open kiln with fixed bricks and a stationary fire. It appears to lack an upper chamber or enclosed walls. In this kiln, the brick-making process involved stacking mud bricks on top of the kiln’s furnace to form cylindrical, square, or polygonal brick towers. These bricks were arranged with gaps between them to allow for the circulation of air, hot gases, and flames. Openings in the lower rows connected to the kiln’s fire vents, facilitating heat transfer. After the initial firing, the baked bricks were removed, and fresh bricks were placed for the next firing. The design and operation of this kiln closely resemble those from the Sasanian period, suggesting little change in brick production technology over time.
The lime kilns are of the pit type with a circular layout. These kilns relied on limestone and a fixed fire, resulting in uneven temperature distribution and variable lime quality. Often, stones remained either uncalcined or partially burned. Lime production involved two methods: arranging limestone around the kiln’s circumference, with heat generated in the central empty space, or stacking alternating layers of charcoal and limestone inside the kiln. The top of the kiln was sealed with mud mortar. These kilns were used during the Qajar period to produce lime for bridge pier repairs.
Within the stone-cutting workshop, stone blocks were meticulously shaped for use in constructing bridge piers. Initially, stone cutters extracted stones from the quarry and cut them into geometric shapes and approximate sizes. Subsequently, the blocks were transported to the workshop, where they underwent further cutting, shaping, and surface polishing. One block bears a stone-cutting mark on its smooth surface, indicating that these stone-cutting marks were made in the workshop.

Acknowledgments
I am indebted to the Iranian Centre for Archaeological Research (ICAR) for generously granting the excavation permits, and to the Bisotun World Heritage Base for its financial support, unfailing administrative cooperation, and logistical assistance.
I would also like to express my sincere appreciation to the following individuals for their invaluable contributions. Zeinab Valizadeh for proofreading the text; Hamed Rezae for producing the architectural plans and cross sections; Sara Mahbobi for redrawing and revising the plans and cross sections; Dariush Afkari for supplying the countor map of the site; and Elham Afkari for her photography of the coin and seal. 

Conflict of Interest
No potential conflict of interest was reported by the Author(s).