## 3 Results and Discussion
### Bronze Mirror Excavated from Pit Tomb Ga-2 (No. 1)
#### 3.1.2 Patina  
Enlarging the corrosion layers of the bronze mirror in DF photography revealed thick, localized layers of red and green corrosion products (Figure 4a). Alternating layers of corrosion material indicated an irregular series of changes in the burial environment. According to the bronze artifact corrosion layer classification system proposed by [PERSON] et al. [9], this mirror's corrosion was classified under the type II corrosion category; type II corrosion is identified when corrosion products, such as malachite and cuprite, are present. Micro-Raman spectroscopy was then used to identify these corrosion products. Here, red corrosion products (a) were identified as cuprite (Cu\({}_{2}\)O), dark green corrosion products (c) were malachite ((Cu\({}_{2}\)CO\({}_{3}\)(OH)\({}_{2}\)), and bright green corrosion product (b and d) were cassiterite (SnO\({}_{2}\)). Notably, cassiterite is the mineral form of SnO\({}_{2}\), and the Raman shift observed in the bronze mirror had significant differences from the reference value (Figure 4b) [5]. In this study, the major Raman shift of cassiterite was moderately observed at a range of 545 cm\({}^{-1}\) to 568 cm\({}^{-1}\). Meanwhile, the Raman shift of natural tin oxide cassiterite, the reference, generally shows a Raman shift ranging from 633 cm\({}^{-1}\) to 775 cm\({}^{-1}\) (Figure 4c). Raman researchers reported that in nanocrystalline SnO\({}_{2}\), a wide band that appears below 580 cm\({}^{-1}\) was observe (Figure 4c) [10]. Based on existing research, nanocrystalline SnO\({}_{2}\) Raman shifts were observed at 486 cm\({}^{-1}\), 568 cm\({}^{-1}\), and 706 cm\({}^{-1}\)[11; 12; 13]. Therefore, the cassiterite found in the bronze mirror is nanocrystalline SnO\({}_{2}\). Since cassiterite is generated during long-term soil burial, the possibility of uniform detection is low, and a Raman shift error occurred. The possibility of substitution due to various environmental changes during soil burial also cannot be excluded. More importantly, the presence of nanocrystalline SnO\({}_{2}\) can be attributed to long-term bronze corrosion in a burial environment.  
EPMA analysis found that Sn content increased significantly toward the edges' corrosion layers, which contained SnO\({}_{2}\) (Figure 5). Excluding highly reducing environments with low pH, SnO\({}_{2}\) could be found in a wide zone between the PO\({}_{2}\) = 1 and PH\({}_{2}\) = 1 bars in a stable state, as seen in the Cu-Sn-Cl-H\({}_{2}\)O Eh-pH diagram. Moreover, the Gibbs energies of SnO\({}_{2}\) and Cu\({}_{2}\)O are \(-\)519 kJ/mol\({}^{-1}\) and \(-\)416 kJ/mol\({}^{-1}\), respectively, at a temperature of 24.85 \({}^{\circ}\)C. These findings, along with the insolubility of SnO\({}_{2}\), explain the high levels of Sn in the artifact's corrosion layers [7].  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Position**} & \multicolumn{8}{c}{**Element (wt\%)**} \\ \cline{3-10}  & & **O** & **S** & **Cu** & **Sn** & **Pb** & **Fe** & **Cl** \\ \hline \multirow{5}{*}{\(1\)} & 1 & 0.83 & \(-\) & 69.58 & 28.02 & 1.57 & \(-\) & \(-\) \\  & 2 & \(-\) & 21.97 & 74.71 & \(-\) & 3.32 & \(-\) & \(-\) \\  & 3 & 0.42 & \(-\) & 65.98 & 33.60 & \(-\) & \(-\) & \(-\) \\  & 4 & \(-\) & \(-\) & 73.15 & 26.85 & \(-\) & \(-\) & \(-\) \\  & 5 & \(-\) & \(-\) & 67.54 & 30.36 & 2.10 & \(-\) & \(-\) \\  & 6 & \(-\) & 1.41 & 86.89 & 2.84 & 5.78 & 0.65 & 2.44 \\  & 7 & 11.35 & \(-\) & 33.07 & 51.34 & 3.44 & \(-\) & 0.81 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Ga-2 (No. 1) microstructure energy dispersive spectrometer (EDS) results.  
Figure 4: **(a)** Ga-2 (No. 1) corrosion layer darkfield (DF) images, (**b**) Raman spectrum, and (**c**) Raman spectrum of nanocrystalline SnO\({}_{2}\).  
Figure 5: Ga-2 (No. 1) electron probe microanalyzer (EPMA) mapping image.