# Formation Mechanism of Crystal Spots in Jian Kiln Oil-Spot Glaze Revealed by Simulation Experiments
## 3 Results and Discussion
### Microstructure and Phase Composition
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Point** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **TiO\({}_{2}\)** & **K\({}_{2}\)O** & **Na\({}_{2}\)O** & **CaO** & **MgO** \\ \hline
1 & 12.33 & 5.79 & 68.91 & 4.20 & 0.98 & 2.21 & 1.81 & 3.76 \\
2 & 11.43 & 7.81 & 65.09 & 4.88 & 1.28 & 3.27 & 1.86 & 4.39 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical composition from EDS analysis of marked points 1 and 2 in the crystalline spot region on the glaze surface of sample RC-2 from Figure 3a,d (wt%). Figure 5a,b depicts the scanning electron microscope (SEM) images of the crystalline spot region of sample RC-5, where Figure 5b is a magnified image of area 1 from Figure 5a. From Figure 5a,b, it can be observed that after reduction, the glaze surface exhibits a layer of nanocrystals of varying sizes, predominantly consisting of nanocrystals ranging from 50 to 100 nm, with some measuring between 200 and 300 nm. According to the Raman spectrum of the crystalline spot region (Figure 5c), a Raman peak corresponding to Fe\({}_{3}\)O\({}_{4}\) is detected at 680 cm\({}^{-1}\), indicating that the nanocrystals are primarily composed of Fe\({}_{3}\)O\({}_{4}\) crystals. Furthermore, the Raman peaks at 480 cm\({}^{-1}\) and 983 cm\({}^{-1}\) indicate that the crystalline layer is relatively thin and still contains a certain amount of glass phase. As the size of the grains is generally less than 100 nm, which meets the conditions for Rayleigh scattering [19], the crystalline spots exhibit a pale blue color. The significant differences in the glaze surface characteristics between samples RC-4 and RC-5 indicate a pronounced effect of the high-temperature reduction atmosphere on the glaze. To further elucidate the impact of the reduction atmosphere on the oxidation state of iron ions in the glaze melt, X-ray photoelectron spectroscopy was employed to analyze the ratio of Fe\({}^{2+}\) to Fe\({}^{3+}\) in the glaze layers of samples RC-4 and RC-5 before and after reduction. Figure 6a,b presents the fitted peak spectra of the Fe 2p orbital for samples RC-4 and RC-5, where the peak area ratios indicate that the ratios of Fe\({}^{2+}\) to Fe\({}^{3+}\) are 71.17/28.83 for sample RC-4 and 51.92/48.08 for sample RC-5. These results indicate that after reduction treatment, the concentration of Fe\({}^{2+}\) in the glaze melt significantly increased, suggesting that the reduction atmosphere altered the oxidation state of the iron ions. Additionally, the Fe 2p3/2 orbital peak of sample RC-5 exhibited significant broadening and shifted towards lower binding energy (with Fe\({}^{2+}\) having a lower binding energy than Fe\({}^{3+}\)), further indicating an increase in the concentration of Fe\({}^{2+}\). The increase in Fe\({}^{2+}\) concentration leads to a reduction in the viscosity of the glaze melt, facilitating the migration of bubbles generated during decomposition from the glaze layer to the  
Figure 4: (**a**) shows the microstructure of the glaze surface of sample RC-4; (**b**) presents the Raman spectrum of the glaze surface of sample RC-3; (**c**) illustrates the microstructure of the glaze surface of sample RC-4; (**d**) displays the Raman spectrum of the glaze surface of sample RC-4. glaze surface [20; 21]. Furthermore, in an oxygen-deficient environment, the decomposition reaction of Fe\({}_{2}\)O\({}_{3}\) intensifies, resulting in the production of a large number of bubbles. When the bubbles burst, the oversaturation of iron leads to the precipitation of Fe\({}_{3}\)O\({}_{4}\) crystals. In a reducing atmosphere, the surface tension of the glaze melt is approximately 20% higher than that in an oxidizing atmosphere, resulting in contraction of the glaze surface [22; 23]. The smaller crystalline spots on the glaze surface coalesce under the influence of surface tension, ultimately forming larger crystal spots.