# Formation Mechanism of Crystal Spots in Jian Kiln Oil-Spot Glaze Revealed by Simulation Experiments
## 3 Results and Discussion
### Microstructure and Phase Composition  
To investigate the evolution patterns and mechanisms of the microstructure during the formation of the crystalline spots, we selected samples RC-2, RC-3, RC-4, RC-5, and RC-7 for microstructural analysis based on their visual characteristics. The silver spot region on the glaze surface of sample RC-2 is formed by irregular granules measuring 2-5 \(\upmu\)m that are interconnected (Figure 3a), with EDS analysis of spot 1 indicating an Fe\({}_{2}\)O\({}_{3}\) content of 68.91 wt% (Table 3). The Raman spectrum of the spot region (Figure 3b) exhibits multiple Raman peaks at 225, 294, 405, 500, 606, 668, and 1315 cm\({}^{-1}\). The peak at 225 cm\({}^{-1}\) corresponds to the A\({}_{1g}\) vibration mode of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\), while the peaks at 243, 294, 405, and 606 cm\({}^{-1}\) correspond to the E\({}_{g}\) vibration modes of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). The peak at 668 cm\({}^{-1}\) is attributed to the longitudinal optical (L\({}_{\rm o}\)) E\({}_{\rm u}\) mode of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\), and the vibration band at 1315 cm\({}^{-1}\) is a broadened version of the 668 cm\({}^{-1}\) band [14; 15; 16]. Therefore, the crystalline spots are composed of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). The cross-section of the glaze layer in sample RC-2 (Figure 3c) reveals numerous bubbles, indicating a vigorous reaction during the decomposition of Fe\({}_{2}\)O\({}_{3}\), which serves as direct evidence of the decomposition process. In the enlarged rectangular area around the bubbles in Figure 3c (Figure 3d), irregular granule aggregates measuring 1 to 3 \(\upmu\)m are observed within the bubble walls and the glaze layer, with EDS analysis of spot 2 indicating an Fe\({}_{2}\)O\({}_{3}\) content of 65.09 wt% (Table 3), indicating the formation of localized iron-rich regions within the glaze layer. The glaze melt undergoes decomposition in an oxidizing atmosphere at approximately 1220 \({}^{\circ}\)C, and the generation of bubbles facilitates the migration of iron-rich particles to the glaze surface, leading to the initial crystallization on the glaze surface. Figure 2: Optical images of samples RC-1 to RC-9. Figure 4a shows the scanning electron microscope (SEM) image of the glaze surface of sample RC-3, where no distinct crystalline particles are observed. The Raman spectrum displays only two broad peaks at 485 cm\({}^{-1}\) and 980 cm\({}^{-1}\) (Figure 4b), corresponding to the bending and asymmetric stretching vibrational modes of the [SiO\({}_{4}\)]\({}^{4-}\) tetrahedra in the glass matrix [17; 18]. This indicates that the glaze phase is predominantly glassy, with no crystalline phase detected, suggesting that the crystals precipitated during heating have re-melted into the glass phase as the temperature increased. Figure 4c presents the SEM image of the glaze surface of sample RC-4, revealing the precipitation of crystalline particles ranging from tens to hundreds of nanometers on the glaze surface. Figure 4d shows the Raman spectrum of the glaze surface of sample RC-4, which exhibits two Raman peaks at 334 cm\({}^{-1}\) and 680 cm\({}^{-1}\), corresponding to the T\({}_{\rm 2g}\) vibrational band (334 cm\({}^{-1}\)) and A\({}_{\rm 1g}\) vibrational band (680 cm\({}^{-1}\)) of Fe\({}_{3}\)O\({}_{4}\)[14; 15]. Therefore, the precipitated nanocrystals are primarily composed of Fe\({}_{3}\)O\({}_{4}\) crystals. The iron oxide in the glaze continues to decompose at high temperatures, facilitating the migration of iron-rich particles to the glaze surface, thus achieving a saturated state. Once super-cooling conditions are present, crystallization phenomena will occur. Figure 3: **(a)** Micromorphology of the crystalline spot region on the glaze surface of sample RC-2; **(b)** Raman spectrum of the crystalline spot region; **(c)** micromorphology of the cross-section of the glaze layer in sample RC-2; **(d)** enlarged morphology of the rectangular region in the glaze layer from Figure **(c)**.