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J. Conserv. Sci > Volume 40(5); 2024 > Article
Lee, Chun, and Kim: Scientific Conservation Treatment of the Gilt-bronze Yoni Excavated from Hong Nang Sida Temple in Lao PDR

ABSTRACT

The gilt-bronze yoni excavated from the Hong Nang Sida Temple in Lao PDR is the first gilt-bronze artifact of its kind to be discovered in Las PDR and is thus recognized for its significant historical value. At the time of its discovery, the bottom of the gilt-bronze yoni had already been damaged and dirt had firmly adhered to its entire body. Therefore, conservation treatment involving surface cleaning, stabilization treatment, reinforcement treatment, attachment of fragments, and restoration of the missing parts was carried out in order to conserve the artifact for a long time. To identify the manufacturing method of the gilt-bronze yoni, observation under an electron microscope and component analysis were carried out, confirming that the exterior was produced by casting using bronze, and then gold plated by amalgam using mercury. Also, based on a digital image analysis that showed extensive corrosion in the upper part of the yoni, it was judged highly likely that the artifact had been directly used in Hindu religious ceremonies. Furthermore, as a result of measuring the radiocarbon age of the molding sand remaining inside the gilt-bronze yoni, the production period was estimated to be around 1,099 土 84 years BP, meaning that the yoni was manufactured around the eleventh or twelfth century, which is similar to the construction period of the Hong Nang Sida Temple.

1. INTRODUCTION

In February 2019, during the restoration process of the Hong Nang Sida Temple in Laos, led by the Korea Heritage Agent as part of an Official Development Assistance (ODA) project, a gilt-bronze yoni was discovered for the first time in Laos. A complete gilt-bronze artifact was discovered in a Khmer temple, where artifacts are rarely preserved due to cultural heritage looting and war damage. This rare finding drew significant interest from cultural heritage experts and the people of Laos. In Hinduism, the yoni is a sacred representation of the goddess, symbolizing the female form and is usually enshrined within the main sanctum of Hindu temples (Yu et al., 2023). The Hong Nang Sida Temple, where the gilt-bronze yoni was found, translates to the “Temple of Princess Sida” and is presumed to be a Hindu temple established in the 12th century under the Khmer Empire. This temple, located at Vat Phou, marks the starting point of the ‘Ancient Khmer Road,’ making the gilt-bronze yoni, a holy artifact of Sadashiva worship, a crucial relic for understanding the ancient cultural exchange between Laos and Cambodia (Park, 2024). As the gilt-bronze Yoni is a key artifact for studying ancient Khmer history, it is crucial to implement conservation measures that fully maintain its form. However, due to the high susceptibility of its metal material to corrosion, urgent conservation treatment was required. Consequently, in April 2019, scientific conservation treatment was conducted on-site at the Hong Nang Sida Temple in Laos. Since the site lacked facilities for conserving metal artifacts and the treatment had to be completed within a short period, meticulous pre-planning was essential. Furthermore, scientific investigation of the artifact could only be conducted on-site, necessitating scientific analysis and digital documentation. This study scientifically analyzes the manufacturing techniques and production period of the gilt-bronze yoni based on the conservation process and methods applied to the artifact.

2. RESEARCH SUBJECT

Hong Nang Sida Temple is a Hindu temple whose name means “Room of Princess Sida.” It is part of the “Vat Phou and Associated Ancient Settlements within the Champasak Cultural Landscape,” a UNESCO World Heritage Site designated in 2001 as a World Heritage in Laos. This temple was constructed in the 12th century by the Khmer people, who dominated the Southeast Asian region from ancient times through the medieval period (Korea Heritage Agency, 2020). However, the Hong Nang Sida ruins had been left in a collapsed state for many years, necessitating urgent restoration. The Korea Heritage Agency has been leading an international cooperation project to restore the temple since 2013 (Figure 1a).
In the Khmer Hindu ruins, there is a yoni in the center of the sanctuary that symbolizes Shakti, a goddess and the consort of the god Shiva. A yoni is paired with a linga, a symbol of Shiva, and is usually made of stone, although ones made of bronze have also been found. They oni found at Hong Nang Sida, in particular, was the first gilt-bronze artifact discovered in Laos.
The gilt-bronze yoni was unearthed in a lying position on February 13, 2019 at a point between the platform pillar stone and floor stone during a dismantling survey of the Hong Nang Sida platform(Figure 1b). At the time of the excavation, hard soil was stuck to the entire surfece of the gilt-bronze yoni, but a gold-plated layer could be discerned in some areas. The lower part of the gilt-bronze yoni had been damaged, and some of the fragments were also recovered during excavation(Figure 1c). As there are no previously reported cases of a gilt-bronze yoni with information on its production method and period, it was subjected to a scientific analysis and conservation treatment in order to preserve it stably for a long time.

3. CONSERVATION TREATMENT OF THE GILT-BRONZE YONI

3.1. Investigation of the Gilt-Bronze Yoni’s Condition before Conservation Treatment

The gilt-bronze yoni unearthed from Hong Nang Sida Temple was inspected visually to check its condition prior to conservation treatment. Five 3.5-mm holes were drilled into the upper part, revealing green-colored bronze corrosion between the surface and soil stuck to it. Soil was firmly stuck on the surface of the sides of the yoni, but a gold-plated layer could still be seen in some areas. The lower part was excavated in a broken state, and it was found that one of the broken fragments had a smooth cross section and could be joined to the body.
In order to accurately record the state of the yoni before the conservation treatment, pictures were taken from various angles with a digital camera and a textured 3D model was produced using a photogrammetry program. Also, 3D modeling of the gilt-bronze yoni was performed using a 3D scanner, and drawings were produced based on the actual measurements (Figure 2).
Digital image analysis was carried out to quantitatively determine the degree of corrosion of the gilt-bronze yoni. The image data used for the analysis were obtained by taking photographs from various angles, and photogrammetry modeling was performed using a 3D program, after which the 2D images extracted for each orientation from the 3D data were used (Figure 2). Then, using an image classification program, Rol (RGB value ofthe damage pattern), was set for the plating and corrosion layer, and statistical analysis was carried out (Park and Lee, 2024).
For image classification with respect to the damage pattern, a method of classification using the maximum likelihood, which corresponds to supervised classification, was used. This method involves performing classification according to a statistical probability distribution under the assumption that the RO I for each class of each band basically follows a normal distribution. Because it considers many variables, it has the highest classification accuracy of all the various supervised classification methods.
The digital image analysis showed that the gold plating remaining on the gilt-bronze yoni covered 54.3% of its entire surface. To be more specific, in terms of orientation, the amount of remaining gold plating covered 71.6% of the front, 50.4% of the back, 59.6% of the right side, and 36.1% of the le左 side of the yoni, which meant that the plating peeling rate was relatively high on the left side. When the yoni was divided into upper and lower parts, the upper part was found to be 33.9% gold plating, 62.7% black corrosion, and 2.9% green corrosion, while the lower part was 65.5% gold plating, 18.2% black corrosion, and 0.2% green corrosion. In sum, the plating had peeled off more extensively on the left side and upper part of the yoni than on the other areas (Figure 3). Therefore, it was confirmed that the degree of corrosion in each direction varied substantially, even in the case of such a small artifact measuring about 6 cm. The gilt-bronze yoni was discovered with its left side facing the ground and its lower part in a broken state. Considering that the broken fragments were unearthed together at the same site, it is believed that the yoni was damaged by physical or chemical weathering after being buried in the ground.

3.2. Surface Cleaning and Plating Exposure

The soil was firmly stuck on the surface of the sides of the yoni, but a gold-plated layer could still be seen in some areas, on the surface of the gilt-bronze yoni was firmly stuck to the plating and corrosion layer. To effectively remove it, a surface cleaning test was conducted on the recovered fragments of the gilt-bronze yoni prior to wet cleaning, using a wet dressing method and a dipping method.
The wet dressing method involved soaking a wet cloth with a mixture of ethyl alcohol and deionized water at a ratio of 50:50, attaching it to the surface of the artifact, and then removing the soil (Figure 4a). However, it was difficult to remove the soil using this method. The dipping method was carried out by immersing the entire specimen in ethyl alcohol and removing the soil. It also proved difficult to completely remove the soil using this method (Figure 4b).
Data from previous research were examined to find a suitable method of removing corrosion from gilt-bronze relics. It was found that acids such as formic acid and H2SO4 had been used in the past. The use of acids had the advantage of removing corrosion from surfaces cleanly in a short amount of time, but the downside was that the plating would turn red and the acid solution would flow into the metal and cause corrosion again. In recent times, rust has been removed by applying physical methods using microscopes and small tools without the use of any chemical agents (National Research Institute of Cultural Heritage, 2009).
Therefore, the process of removing foreign matter from the surface of the gilt-bronze yoni and the surface of the plating began by removing as much dirt from the surface as possible with ethyl alcohol, and the remaining dirt was removed little by little with a surgical scalpel under a magnifying glass and a stereomicroscope (Figure 4c). As for the fragile plated layer with a high risk of detachment, a Paraloid B-72 10% (in acetone) solution was applied with a brush to strengthen the surface, and then the dirt adhering to the surface was removed.

3.3. Stabilization and Reinforcement Treatment

To ensure the long-term stability of artifacts, stabilization treatment to prevent metal corrosion is essential. The stabilization treatment of the gilt-bronze yoni was carried out by removing foreign matter from its surface and then immersing the yoni, with the plating exposed, in ethyl alcohol to allow the moisture inside it to be replaced with alcohol.
Then, the ethyl alcohol was removed by drying the yoni in a dryer at 70°C for about 60 minutes (Figure 5a). And a Benzotriazole (BTA) 3% (in ethyl alcoho1) solution was applied to the surface of the yoni to form a Cu-BTA. film (Figure 5b), and then the white BTA powder remaining on the surface after the BTA. treatment for stabilization was removed using ethyl alcohol. Afterwards, the gilt-bronze yoni was dried at a temperature of 70°C for about 60 minutes to volatilize the ethyl alcohol remaining inside.
To strengthen the gilt-bronze yoni, which had become weak and vulnerable after being buried for a long time, it was subjected to reinforcement treatment, which consisted in immersing it in a solution with 10% Paraloid B-72, an acrylic synthetic resin, in acetone. At this time, primary reinforcement treatment was performed by decompressing the yoni in a vacuum impregnator for 1 hour to allow the Paraloid B-72 resin to penetrate into the bronze yoni. Afterwards, the gilt-bronze yoni was dried at 40°C for about 10 minutes using a dryer and then subjected to the reinforcement treatment again in a Paraloid B-72 10% (in acetone) solution under the impregnation condition for a shorter time than the first treatment (Figure 5c).

3.4. Bonding and Restoration of Missing Parts

In order to restore the gilt-bronze yoni to its original appearance, it was necessary to bond the broken fragments from the lower part and reinforce the missing parts. It is common to use a reversible method to join together the fragments of metal artifacts so that conservation treatment can be performed again in the future. Therefore, the broken fragments from the lower part of the gilt-bronze yoni were bonded with a cyanoacrylate-based adhesive.
However, it was noted that it would be difficult for the gilt-bronze yoni to stand on its own even if the fragments of the lower part were joined because there were missing parts. Accordingly, to ensure the stability of the gilt-bronze yoni, the missing parts were restored. After making a model from the remaining part of the main body with tin foil, restoration pieces were made for the missing parts with quick-drying epoxy resin, which were then attached to the relevant areas (Figure 6a). The restoration pieces were individually made using epoxy resin for each missing part so that they would be easy to remove (Figure 6b). Each restored piece was polished with sandpaper to resemble the surface texture of the gilt-bronze yoni, color-matched using gold leaf and acrylic paint, and then bonded to the body (Figure 6c).

3.5. Results of the Conservation Treatment

Regarding metal artifacts that have undergone conservation treatment, accurate records are required for their ongoing management, such as additional conservation treatment in the case of re-corrosion, inspection of the restored area, and monitoring. Accordingly, drawings of the gilt-bronze yoni were prepared by marking the restored areas, crack joints, etc. on drawings created by 3D scanning. Each part of the yoni was photographed after completing the conservation treatment to clearly apprehend its state (Table 1).
A storage box was created to keep the gilt-bronze yoni safe and secure after the conservation treatment. The box was made in the actual shape of the yoni so that the artifact would not move during transportation, and PE foam was added to enhance shock absorption. Also, a desiccant (silica gel) was placed inside the storage box so as to control the moisture level in consideration of the fact that it is hot and humid in Laos.

4. SCIENTIFIC ANALYSIS OF THE GILT-BRONZE YONI

4.1. Analysis methods

To analyze the material and surface gilding layer of the gilt-bronze Yoni, non-destructive P-XRF (Portable X-Ray Fluorescence; BRUKER, S1 TITAN, DEU) was used. The P-XRF analysis conditions were set to 6-50 kV, 5-200 μA, with an Rh target. For corrosion product analysis, an X-ray Diffractometer (PANalytical EMPYREAN, NED) was utilized. The analysis was conducted using a Cu target at 40 kV and 40 mA, with the powder sample placed on a spinner stage and scanned at 0.026°/sec over a range of 5°-70°.
To observe the microstructure, the sample was mounted in epoxy resin and polished down to a 1 μm diamond suspension. The polished specimen surface was etched with a solution of 120 mL ethanol, 30 mL hydrochloric acid, and 10g of iron (III) oxide, and then observed using a metallurgical microscope (Leica DMRBE, DEU). For fine microstructures or gilded layers that were difficult to observe with the optical microscope, the specimen was carbon-coated and observed with a Scanning Electron Microscope (SEM, JEOL IT300, JPN) while analyzing with an Energy Dispersive Spectrometer (EDS, OXFORD, UK). Additionally, to determine the production period of the gilt-bronze yoni, remaining casting sand inside was sampled and subjected to radiocarbon dating.

4.2. Composition analysis

As the gilt-bronze yoni excavated from Hong Nang Sida Temple was the first such artifact to be discovered in Laos, there were no data or information on scientific analysis available locally at the time of its discovery. Accordingly, to understand the production techniques used to make the gilt-bronze yoni, an analysis of its components was carried out using a portable XRF analyzer on site after performing surface cleaning, so as to prevent foreign matter on the yoni from being analyzed as well.
An analysis of the plating, which remained relatively intact in the lower part of the gilt-bronze yoni, showed its composition to be 43.80 wt% gold (Au), 18.12 wt% copper (Cu), 29.88 wt% tin (Sn), and 3.17 wt% mercury (Hg), the detection of which was atypical. On the other hand, an analysis of an area that no longer had any plating due to corrosion showed its composition to be 30.43% copper, 56.44% tin, and 3.65% lead, without any detection of mercury (Table 2).
It was confirmed that the base metal of the gilt-bronze yoni had been significantly corroded. X-ray diffraction analysis was performed to determine the type of corrosion on the surface, and diffraction patterns of copper oxide [tenorite (CuO)] and tin oxide [cassiterite (SnO2)] were observed (Figure 7).

4.3. Microstructure of Metallurgical analysis

In order to take a closer at the components and microstructure of the gilt-bronze yoni, scanning electron microscopy (SEM) and EDS analysis were conducted on 4 mm fragments that could not be joined or restored during the conservation treatment process.
The results of examining the cross-section of the plating showed the thickness of the plating to be relatively uniform at about 3 to 9㎛ (Figure 8a), and it was found that a gold grain state was maintained in areas where the finishing (polishing) was difficult (Figure 8b). These characteristics are evidence of the use of gold powder as the plating material (Jeon et al., 2010). Figure 8c of the gilded layer by SEM-EDS analysis showed that it was 83.29∼83.73% gold, 1.25∼1.37% silver, and 11.78∼12.38% mercury (Table 3). The fact that a high mercury content of around 12% was detected in the plating indicates that the gilt-bronze yoni was plated by the mercury amalgam method (Yang and Kim, 2012). The mercury amalgam gilding method involves mixing mercury with gold and applying it to the surface, then heating it to a high temperature to evaporate the mercury, leaving only the gold on the surface (Han et al., 2002). The non-corroded microstructure appeared to be in the shape of the δ phase(Figure 8d). The components of the δ phase were 64.82% copper and 34.36% tin, based on which it was judged that the main body of the gilt-bronze yoni was made of a copper-tin binary alloy. Moreover, the components of inclusions in the base metal were analyzed to be 75.67% copper and 22.15% sulfur, suggesting that chalcopyrite-based copper was used in the making of the gilt-bronze yoni (Table 3).

4.4. Pattern-like areas on the surface

Upon examining the left-side surface of the gilt-bronze Yoni with a digital microscope, we observed what appeared to be scratches, revealing patterns engraved in a fine size (Figure 9a). This section shows that each dot engraved by a tool is connected in a consistent directional pattern (Figure 9b). It is unclear whether these are symbols or decorative motifs . However, the pattern appears to have been created using a pointed tool, with small dots overlapping in one direction. Additionally, some carved areas show evidence of gilding, suggesting that the triangular dotted pattern was engraved on the surface before the gilding process.
Upon observing the five holes drilled in the center of the upper part with a microscope, the inner surfaces of the holes appear clean (Figure 10a), while finishing marks are visible on the outer surfaces (Figure 10b). This suggests that the holes were drilled from the inside out. Additionally, the area around the holes on the outer upper surface shows no signs of gilding. When magnified under a microscope, a clear boundary of the gilding layer can be seen, suggesting that this area was intentionally left ungilded (Figure 10c).

4.5. Radiocarbon Dating

An examination of the microstructure of the gilt-bronze yoni using a scanning electron microscope showed it had been produced by casting. Also, during the cleaning process, earth-colored carbon was found attached to the inside of the yoni, along with molding sand (Figure 11a). Based on these findings, it was judged that the molding sand and carbon found inside the yoni had been used at the time of its production.
To determine the production period of the gilt-bronze yoni, radiocarbon dating was performed on a small amount of carbon recovered from its inside, the results of which showed that it was produced 952 ± 38 years before the present (BP) (Figure 11b). For the calculation of BP years, A.D. 1950 is used as the reference year. However, considering that the concentration of radiocarbon 14C in the atmosphere has changed continuously over the past 50,000 years due to natural phenomena, as well as to the Industrial Revolution that began in the mid-eighteenth century and the atomic bomb experiments that were mainly carried out from the mid-1950s to the late 1960s, there is a need to make corrections in order to calculate an accurate calendar age (Kim et al., 2018).
As a result of correcting the radiocarbon age using the globally applied IntCal 13 calibration curve (Reimer et al., 2013), the production period of the gilt-bronze yoni was calculated as 1,099 ± 84 years BP. Therefore, the gilt-bronze yoni is believed to have been manufactured around the eleventh or twelfth century, which is similar to the construction period of Hong Nang Sida.

5. CONCLUSION

The gilt-bronze Yoni excavated from the Hong Nang Sida site in Laos is the first gilt-bronze artifact discovered in the country, marking it as a culturally significant heritage item with immense historical value. At the time of discovery, the artifact’s lower part was damaged, and its surface was covered with firmly attached soil. Korean experts conducted a range of conservation treatments to ensure the long-term conservation of the gilt-bronze yoni. These treatments included cleaning, stabilization, reinforcement, fragment reattachment, and restoration of missing parts, with the goal of maintaining the artifact’s original form as closely as possible.
Microscopic examination and compositional analysis revealed that the yoni was cast in bronze and then gilded using the mercury amalgam method. Additionally, the significant corrosion observed on the upper portion of the yoni suggests a high probability that it was directly used in Hindu religious ceremonies. Radiocarbon dating places the artifact’s production around the 11th to 12th century, coinciding with the construction period of the Hong Nang Sida Temple, thereby offering valuable insights into the religious and cultural context of that time.
The conservation of this gilt-bronze yoni is highly significant as it represents the first instance of applying Korean conservation techniques directly to a cultural heritage artifact overseas. In Hinduism, the yoni is a sacred object paired with the Linga, used in rituals to invoke fertility and prosperity through holy water ceremonies. The corrosion concentrated on the upper portion of the yoni supports the idea that it was actively used in such ceremonies. This study provides foundational data for researching the history and religious practices of the ancient Khmer Empire and is regarded as an important example of collaboration between ancient Southeast Asian culture and Korean conservation technology.

ACKNOWLEDGEMENTS

This paper has been revised and supplemented with a published ‘Investigation and Research of Hong Nang Sida Temple’ by Korea Heritage Agency(KHA). And this study was conducted with the support of the “Restoration and Capacity Building for the Conservation of the Vat Phou-Hong Nang Sida World Heritage Site in Lao PDR” project, carried out by the Korea Heritage Agency (KHA) and was also part of the R&D research project of the National Research Institute of Cultural Heritage (NRICH).

Figure 1.
Current status of excavation of the gilt-bronze yoni; a) Hong Nang Sida Temple restoration site, b) Location of excavation, c) State of yoni at the time of excavation.
JCS-2024-40-5-10f1.jpg
Figure 2.
Digital image extractions of the gilt-bronze yoni.
JCS-2024-40-5-10f2.jpg
Figure 3.
Digital image analysis modeling of the gilt-bronze yoni.
JCS-2024-40-5-10f3.jpg
Figure 4.
Process of removing contaminants from the surface of the gilt-bronze yoni; a) Wet dressing test, b) Dipping in Ethyl alcohol, c) Surface cleaning using a magnifying glass and microscope.
JCS-2024-40-5-10f4.jpg
Figure 5.
Stabilization and consolidation treatment of the gilt-bronze yoni; a) Drying, b) BTA stabilization treatment, c) Using vacuum impregnation equipment.
JCS-2024-40-5-10f5.jpg
Figure 6.
Restoration process of missing parts of the gilt-bronze yoni; a) Fabrication of pieces to replace the missing parts, b) Attachment of restoration parts, c) Color matching using gold leaf.
JCS-2024-40-5-10f6.jpg
Figure 7.
Results of X-ray diffraction analysis of the surface of the gilt-bronze yoni.
JCS-2024-40-5-10f7.jpg
Figure 8.
Microstructure of the gilt-bronze yoni; a) Optical micrograph of cross section of gilded layer, b) State of corrosive chemicals on the surface by SEM, c) SEM image of magnified gilded layer of Figure 8a, d) SEM image of magnified base metal of Figure 8a.
JCS-2024-40-5-10f8.jpg
Figure 9.
Carving evidence of gilt-bronze yoni observed; a) Carving area(upper left side), b) Optical micrograph of Magnified carving area.
JCS-2024-40-5-10f9.jpg
Figure 10.
Hole around center of the gilt-bronze yoni; a) Punching and drilling evidence of inside, b) Punching and drilling evidence of outside, c) Gilded area around center hole.
JCS-2024-40-5-10f10.jpg
Figure 11.
Radiocarbon dating; a) Inside of the gilt-bronze yoni, b) Results of radiocarbon dating the gilt-bronze yoni.
JCS-2024-40-5-10f11.jpg
Table 1.
Photographs and drawing taken before and after the conservation treatment of the gilt-bronze yoni
JCS-2024-40-5-10i1.jpg
Table 2.
Results of analysis of the gilded layer and base metal by P-XRF
Position Element content (wt%)
Cu Sn Pb Au Ag Hg Fe Si P Total
Gold gilded layer 18.12 29.88 0.22 43.80 0.69 3.17 1.01 1.07 - 97.96
Base metal 30.43 56.44 3.65 - 0.47 - 4.63 2.03 0.63 98.28
Table 3.
Results of SEM-EDS analysis of the gilt-bronze yoni
Position Element content (wt%)
Cu Sn Pb Au Ag Hg Fe Ni S Total
Surface area of a gilded layer 3.12 - - 83.73 1.37 11.78 - - - 100.00
Cross section area of a gilded layer 3.08 - - 83.29 1.25 12.38 - - - 100.00
δ phase of a base metal 64.82 34.36 - - - - 0.26 0.56 - 100.00
Inclusion of a base metal 75.67 - - - 2.18 - - - 22.15 100.00

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