Sample
Among the excavated artifacts, seated Buddha’s robes were selected as a specimen. A pedestal which was covered by the Buddha’s robes, supporting the Buddhist image, is called as Sanghyunza in Korean. The Buddha’s robe was excavated in two sizes; photos in Figure 2 (a) to (c) are big robes and (d) to (e) are small robes. The width x height of the big and the small ones are 31.6 x 21.9 cm, 21.1 x 18.1 cm, respectively. It is indicated that it was a part of big Buddha statue or Buddha’s pedestal. Both of robes are bent in the shape of L, and there are grooves and holes that appear to be used for fixating it to somewhere. It is presumed that each part was made of cast bronze and fixed into the pedestal frame. When both of Buddha’s robes were excavated, they were covered with soils and green corrosions. After removed covered soils, surface of the Buddha’s robes was bumpy with non-uniform attachments. Gold foil existed on the front side, however, it could not be seen inside. The big Buddha’s robe had crack from left side, but it is not too unstable.
In general, Buddha statues and pedestals were made separately in unified Silla period [7]. Of the big size Buddha statues, standing Bhaisajyaguru statue of Baengnyulsa in Gyeongju is an example of separate casting. Not only the pedestal but also both hands of Bhaisajyaguru were casted separately as shown in Figure 3–(a). However, there was no example of the pedestals which were casted separately similar to excavated Buddha’s robe. In the case of ceramic pedestals, similar manufacturing methods existed. A ceramic Buddha pedestal excavated in Cheongyang Bonuiliyoji (7th) was manufactured in each parts and it has a hole to fix them. It is speculated that the Buddha’s robe similarly made with a ceramic Buddha pedestal from Bonuiliyoji (Figure 3-(b)).
Alloy composition
Table 1. Excavated Buddha’s robes XRF results (wt%)
|
|
Cu
|
Sn
|
Au
|
Pb
|
Big size robe front
|
87.7
|
10.1
|
2.0
|
-
|
Big size robe inside
|
92.6
|
5.6
|
-
|
1.8
|
Small size robe front
|
73.4
|
4.9
|
21.7
|
-
|
Small size robe inside
|
77.8
|
13.8
|
-
|
8.4
|
XRF results of specimen indicate that both of them were made using alloy of copper and tin, and gold plating was done only on the front side. Lead was detected only inside, and it was possible that lead and gold plating were related. Before gold plating was performed, casting of Buddha’s robe was grinded to make surface smooth; hence, the lead attachment on surface was eliminated at that time. In the Silla Dynasty, the method of gold coating used mainly mercury amalgam in the Korean Peninsula. In case of mercury amalgam, mercury with gold was applied to a bronze surface, followed by heating to a temperature at which mercury vaporized (638 K). The vaporization temperature of mercury was higher than the melting point of lead, therefore, any remaining leads of front surface would have melted. The mercury amalgam was easy to use for large and complex artifacts, therefore, it was mainly used as a gold plating method for the Silla Dynasty’s bronze [8].
Microstructure
Figurer 4-(a) shows that microstructure of specimen was confirmed to be recrystallized polygon, with twins and impurities. According to the EDS analyses as shown in figurer 4–(c), the weight % of sample was Cu – 2.89 wt% Sn. The size of polygonal structure medially increased. Polygonal structure indicated that it was annealed; twins indicate that it had been heated to the copper’s recrystallization temperature (573 K). The polygonal structure and twins indicated that the final heating of Buddha’s robe was probably performed to vaporize the mercury to coating gold. Mercury (Hg) with gold was detected on the surface of other excavated artifacts at the same time, and it was speculated that mercury amalgam would also be used for the Buddha’s robe.
Impurities optical microscope image is shown in Figure 4–(b). ⓐ was base metal and ⓑ were impurities. The microstructure of specimen contained polygonal impurities which were grey and 5~50 μm in size. The results of the EDS analysis as shown in Figure 4–(d) was Cu – 36.47 mol% S – 5.72 mol% Fe – 0.43 mol% Se. The molar ratio of Cu : S was 2 : 1. A trace element Se was both detected and not detected depending on the impurities. Selenium (Se) was discovered in 1817 by Jons Jacob Berzelius [9], and was mainly found in sulfuric copper ore, lead and nickel ore. It was obtained from the anode of copper refineries as electrolytic metal refining byproduct [10]. In the Silla Dynasty’s bronze, Se would be contained in Cu2S because there was no electrolytic copper refining method.
A boundary between ⓐ and ⓑ was cut with FIB and studied using TEM-EDS. As shown in Figure 5, Cu and Sn were detected in ⓐ, Cu, S and Fe were detected in ⓑ Cu–S grain. Sulfur was mainly detected but Sn was not detected in Cu–S grain. EDS mapping shows that Fe was isolated and presented in a size of 1 mm or less in the Cu–S grain. The high Fe particles could not be confirmed using SEM and OM.
The magnified TEM image of Cu–S grains is shown in Figure 6–(a), indicating two different crystals ① and ②. EDS of the Cu–S grain inside point shows that ① was Cu – 34.01 mol% S – 1.13 mol% Fe and ② was Cu – 37.85 mol% S – 12.32 mol% Fe. ①’s and ②’s electron diffraction patterns are showed in Figure 6–(b) and 6–(c), respectively. ①’s electron diffraction pattern corroborated with that of Cu2S (JCPDF 33-0490) and that of ②’s corroborated with Fe3O4 (JCPDF 19-0629). Two or more crystals were mixed in ② particle, generating a structure of bright pattern, similar to magnetite (Fe3O4). These were intermediate products in the copper refining process, and it was speculated that a part of the matte was remaining. However, usually magnetite was not helpful in making copper, hence, all the iron was removed into the matte by oxidation [11].
1.1 Surface corrosion layer
A layer of specimen surface was unlike any other bronze corrosion layer. Although there were multiple possibilities depending on the burial environment, generally, bronze surface layer of excavated artifacts contained CuO, Cu2O, and greenish copper corrosion matters, tin oxide and lead oxide [12, 13]. However, as shown in specimen surface, it was consisted as a thick lead layer (200 mm) as shown in Figure 7. The thick lead layer contained long needle shape crystals near surface and elliptical crystals near αCu. To confirm the components of each structure, it was analyzed using EDS mapping and the results are shown in Figure 7 (b) – (h). Pb, Si and Fe were detected using EDS mapping in the thick lead layer, and Cu was highly detected in grey and yellow elliptical grain. Because Si, Al and Fe are present in large amounts in the earth’s crust, it may indicate that these elements were related with bronze corrosion environment. In contrast, Si, Al and Fe could also be slags, which were stabilizing substances, to remove Fe from chalcopyrite.
According to the optical microscope image as shown in Figure 8, copper grains used to exist in high lead corrosion layer in large amounts. These grains were maximum 9 mm size and were shown in ① yellow color inside, ② grey color outside and ③ black color of lead layer. The composition of these layers are listed in Table 2. Layer ① contained 93.20 mol% Cu, layer ② contained 55.47 mol% Cu, 42.31 mol% O and layer ③ contained 52.19 mol% O, 24.42 mol% Pb, 6.55 mol% S, 3.42 mol% Fe, 2.10 mol% Cu. Layer ② contained a lower copper and a higher oxygen than layer ①. Layer ③ contained a higher lead, iron and oxygen and a lower copper than layer ① and ②. To analyze the copper grain, a boundary between brown and grey was cut with FIB studied using TEM, and the results are shown in Figure 9.
Table 2. The EDS results of corrosion layers
|
mol%
|
O
|
Si
|
S
|
Fe
|
Cu
|
Pb
|
①
|
4.14
|
-
|
1.91
|
0.58
|
93.20
|
0.18
|
②
|
42.31
|
-
|
1.58
|
0.34
|
55.47
|
0.18
|
③
|
52.19
|
11.31
|
6.55
|
3.42
|
2.10
|
24.42
|
Figure 9-(a) shows three layers’ in TEM image; layer ① yellow copper grain on the left, layer ② grey edge in the center, layer ③ black background on the right. The boundaries between the three layers were distinguished, however, as shown in Figure 9-(b), upper part of layer ① and ② breaks the boundary was confirmed. In contrast, the boundary of layer ② and ③ was clearly separated in TEM image of Figure 9-(a), hence, ③ layer seemed not to be related with layer ① and ②.
To find out a crystal structure in each layer, we analyzed electron diffraction pattern and the results are shown in Figure 10. The electron diffraction pattern of layers ① and ② corroborated with that of Cu (JCPDF 04-0836) and layer and CuO (JCPDF 48-1548), respectively. Layer ③’s electron diffraction pattern showed rings, indicating that it was polycrystalline. In general, tenorite (CuO) is not kinetically favored and is usually found in burned burial environments or slowly heated in air [14]. It was possible that the mercury amalgam method gilded the Buddha's robes and heated to a temperature at which mercury vaporized (638 K). Hence, it seemed that layer ① and ② existed since Buddha’s robe was created.