was shown to be fourth grade (highly weathered – HW). Finally, by examining petrographic homogeneity with granitic rocks in the surrounding area, it was found that the rocks around Mireukri in Chungju and the quarry in Mungyeong are highly similar with those of the study subject.
Keywords: Mireukri temple site, Deterioration, Creep, Nondestructive diagnosis, Petrographic homogeneity
Introduction
The Stone Standing Buddha and the Stone Chamber, that is the main Buddha and Geumdang (the main temple) of the Mireukdaewonji, are located in 204Mireukri, Chungju City, Korea. The Mireukdaewonji is one of the important Buddhist temples in the Goryeo Dynasty (AD 918 to AD 1392) of ancient Korea. Within this cultural heritage zone, various stone cultural heritages such as the Five-story Stone Pagoda in Mireukri (Treasure No. 95) are widely distributed.
The Stone Standing Buddha is composed of six stones and is surrounded by masonry on three sides: east, south, and west. It is presumed that there was a wooden structure on the upper part of the Stone Chamber, but is no longer present (Ryoo, 2018). There is no exact historical record of when it was destroyed.
The temple’s history and name were revealed by conducting five excavation surveys from 1977 to 1991 (Kim, 2008). Since then, various art historical and archaeological studies have been carried out on the stone chamber structure, the only Stone Standing Buddha facing north in Korea, the form of the Bogae (a kind of stone hat), and the hand gestures holding a flower bud-shaped object (Jeong, 2008). On the other hand, conservation-related scientific investigation was not sufficient, and only visual observation of the physical, chemical and biological factors and surroundings was conducted.
With obvious structural deformation occurring in the western wall of the Stone Chamber, dismantling and repair work is currently underway (Fig. 1). In addition to this, to prepare the basic data for conservation and maintenance of the Stone Standing Buddha and Stone Chamber, a need for the scientific investigation has emerged. It is also required to examine the substitute stone for damaged parts of the Stone Chamber. Therefore, we expect that the results of this study will provide the basic data for the establishment of the long-term conservation management plan and the provenance interpretation for stone heritages.
Figure 1: Photographs showing the dismantling and repair work of the Stone Standing Buddha and Stone Chamber in the Mireukri temple site. (A) Before 2015, (B) March 2018, (C) March 2019.
205Research Methods
This study conducted not only prior research including historical and archeological studies but also an overall conservation scientific survey of the Stone Standing Buddha and Stone Chamber.
We analyzed the petrological characteristics of the constituent rocks by observing the microscope and measuring the magnetic susceptibility and so on. Samples collected from the dismantled stone walls and the surrounding sites were examined using stereoscopic and polarizing microscopes, and X-ray diffraction analysis, and the geochemical properties were also examined using ICP-AES, ICP-MS, and INAA. The chromaticity was also measured to check the quantitative color of the samples.
A deterioration map was prepared to evaluate the conservation status of the Stone Standing Buddha, and infrared thermography analysis was performed to detect the damaged zones that are not easily detectable with the naked eyes (Jo and Lee, 2014), and the P-XRF was performed to analyze surface contaminants. Besides, to evaluate the material properties, the degree of weathering and internal defects of the Stone Standing Buddha and Stone Chamber, ultrasonic velocity was conducted (Lee et al., 2017). By comparing P-wave pulse velocities measured by the indirect method in the masonry before dismantling and those of the Stone Standing Buddha, the degree of weathering in physical properties both were considered. Quantitative stages of weathering were calculated based on the coefficient of weathering (K) proposed by Iliev (1966). K value is defined as the rate of change in the ultrasonic velocity of fresh and weathered rock.
Results and Discussion
According to the results of the analysis of the material characteristics, the rocks that make up the Stone Standing Buddha and Stone Chamber are pinkish medium-grained biotite granite, milky medium-grained biotite granite, porphyritic granite, and aplite. The host rock is pinkish medium-grained biotite granite that accounts for more than 90 percent of the total, while other types of rocks are used as stones to fill the inside or between the walls (Fig. 2). Particularly, the Urna on the forehead of the Stone Standing Buddha and one Stone Seated Buddha enshrined inside the chamber consist of limestone.
Figure 2: Diagram showing the proportion for constituent rocks of the Stone Chamber in the Mireukri temple site.
The pinkish medium-grained biotite granite, a representative rock, is about 1 to 5 mm in grain size and composed mainly of quartz, alkali feldspar, plagioclase, and biotite. The magnetic susceptibility is measured in the range of 0.08 to 2.04 (× 10–3 SI unit) in the Stone Standing Buddha and 0.01 to 2065.24 (× 10–3 SI unit) in Stone Chamber. Generally, it is distributed below 2.00 × 10–3 SI unit and the mean values are similar.
As a result of on-site investigation, damage types in the Stone Standing Buddha are mainly observed for physical damage such as crack, scaling, scale-off, and break-out, while chemical deterioration is found to be black, brown, white discoloration and believed to be red pigment in the lips. No biological damage is identified as the cleaning treatment was completed. As each damage type of the Stone Standing Buddha occupies below 6 % of the whole, by and large, the Stone Standing Buddha was found to be of good condition in deterioration assessment. In the case of the Stone Chamber, complex cracks in each material are mostly observed and discoloration caused by oxidation of minerals like iron and manganese is partially confirmed.
Infrared thermography analysis showed local exfoliations in the entire of the Stone Standing Buddha, which covered with surface contaminants before cleaning. Exfoliation occupied less than 3 % overall but the frequency of occurrence was high. Relatively large exfoliations were identified in the left arm and the pedestal.
The results of P-XRF measurement on surface contaminants indicated that manganese, iron, calcium 207and lead acted as the main determinants of black, brown, white discoloration and red pigment, respectively. Remarkably, the content of lead on the face of the statue was measured to be 18 times higher than other parts. This allows us to estimate the possibility of using the white pigment, hydrocerussite (2PbCO3·Pb(OH)2). It is necessary, however, to carry out further research to investigate a definite cause for this phenomenon (Fig. 3).
Figure 3: Current state of the face on the Stone Standing Buddha and result by P-XRF analysis. (A) Front view of Buddha’s face, (B) Urna and heart-shaped small hole on the face, (C) Measurement points of P-XRF and 2D modeling of Pb contents.
Physical evaluation by ultrasonic velocity was performed at 571 spots on the surface of the Stone Standing Buddha. As a result of ultrasonic measurements, the mean ultrasonic velocities of 2,710 m/s in the Bogae and over 3,000 m/s in the rest of the statue were shown. By applying the absolute coefficient of weathering (K), the weathering indices of the whole were estimated to be in the moderately weathered (MW) stage except for the Bogae which was in the highly weathered stage.
