Micro-Raman Study of Chinese Iron-Based Brown Wares Produced During Song Dynasty (960–1279 CE): Probing Crystals in the Glazes

Abstract

Brown glazed wares, as some of the famous Song wares, attract significant attention nowadays due to the glaze containing a large amount of metastable ε-Fe₂O₃, a promising multiple-functional electromagnetic material. In this work, typical fragments were systematically analyzed by micro-Raman spectroscopy combined with scanning electron microscopy as well as X-ray fluorescence. Abundant ε-Fe₂O₃ crystals were observed in the glaze surfaces, with the exception of numerous hematite crystals detected in the surfaces of fragments excavated in Hunyuan kilns (Shanxi province). The correlative analyses of Raman and XRF data indicate that relatively high SiO₂ and low CaO contents in the system may benefit ε-Fe₂O₃ precipitation, and the crystallization temperature may range from 1150 to 1200 °C. In addition, various crystals were detected in the glazes, including magnetite, magnesioferrite, zircon, anatase, pseudobrookite, rutile, cordierite, cristobalite, quartz, and mullite.

1. Introduction

Brown glazed wares, as precious types of ancient ceramics in China [1,2], have attracted significant attention in recent years due to the presence of a large amount of metastable ε-Fe₂O₃ crystals in their glazes. ε-Fe₂O₃ is a multifunctional electromagnetic material with great application potential [3,4]. Unlike the nano- or submicron-sized ε-Fe₂O₃ particles obtained through modern wet chemical synthesis [5,6], the ε-Fe₂O₃ crystals precipitated in ancient brown glazes are larger, typically reaching several micrometers to tens of micrometers in size [4]. This phenomenon has been observed in products from numerous kilns in both northern and southern China, such as the Ding kilns [7], Jingxing kilns [8], Yaozhou kilns [8], Dangyangyu kilns [8], Xin’an kilns [8], Lushan kilns [8], Deqing kilns [9], Jingdezhen kilns [9], Qilizhen kilns [8], and Jian kilns [10,11,12,13,14,15,16]. It is noteworthy that, with the exception of the Dangyangyu kiln samples where the ε-Fe₂O₃ phase was detected only in small brown areas [8], most brown glazes are primarily composed of relatively uniform, large dendritic ε-Fe₂O₃ crystals [8]. These widespread discoveries suggest that as early as the Song Dynasty, ancient potters may have mastered the controlled synthesis technology of micron-sized ε-Fe₂O₃ crystals.

To elucidate the mechanisms enabling the stable existence of micron-sized ε-Fe₂O₃ crystals in brown glazes, scholars have conducted in-depth research from various perspectives, including firing techniques [16,17], ion substitution behavior within the crystals [10,11], and the structure of the vitreous glaze layer [8]. Simulation firing experiments on sherds from the Yaozhou kilns [8] and Jian kilns [12] indicate that a strongly reducing atmosphere and an appropriate crystallization temperature (1160–1190 °C) are key conditions for the stable formation of ε-Fe₂O₃, a conclusion also supported by studies on Japanese Hidasuki pottery [12]. Further morphological and structural analysis of ε-Fe₂O₃ crystals from Yaozhou and Jian Kiln samples revealed that, besides iron and oxygen, the crystals contain elements such as aluminum, magnesium, and titanium [8]. These impurity ions can partially substitute for iron sites, reducing the crystallographic order and thereby enhancing the stability of the crystals. Theoretical calculations have also confirmed that appropriate aluminum doping can strengthen the Fe-O bonds by compressing the Fe₂O₃ lattice, thus enhancing the structural stability of the ε-phase [11]. Moreover, the presence of mullite crystals was detected in the Jian glazes to enhance further the Al-substitution effect as well as to provide nucleation sites for ε-Fe₂O₃ crystals, allowing ε-Fe₂O₃ sizes up to tens of microns [15]. In addition, the study of the glassy matrix surrounding the ε-Fe₂O₃ crystals in black wares of Deqing kilns [7] showed that it contains slightly higher SiO₂ and lower CaO than the global glaze phase, indicating that the crystallization of ε-Fe₂O₃ may require relatively high SiO₂ and low CaO contents in the system. Systematic analysis of the black-glazed and brown-glazed porcelains from the Qingliang Temple kiln site [18] dated to the Yuan and Ming dynasties reveals that their coloration is predominantly due to the crystallization of ε-Fe₂O₃ and the enhancing effect of the underlying black glaze. The distinctive regions of different kilns patently cause the differences in raw materials used by the local potters. However, the common characteristics concerning raw materials of these brown wares from the different kilns still remain unclear and should be further investigated.

Micro-Raman spectroscopy (μ-RS) is a powerful technique to identify microcrystalline phases in the glassy matrix of ancient ceramics due to the microscale of the probe and the high sensitivity [19,20]. On the one hand, it not only enables the discrimination of the types of iron oxide particles, including ε-Fe₂O₃, hematite, magnetite, maghemite, etc., but also the detection of the structure variations of crystals caused by sizes or ion substitutions [10]. In addition, combined with SEM observations, the micro-Raman spectrum could precisely analyze the nature of a single particle at the micron scale. Moreover, such micro-scale technique is also nondestructive, making it suitable to investigate precious ancient artifacts.

In this work, typical fragments excavated from seven Chinese northern and southern kilns and an archaeological site were selected. Micro-Raman spectroscopy combined with scanning electron microscopy as well as X-ray fluorescence (XRF) were exploited to analyze the chemical composition and nature and distributions of crystals in the glazes. Based on the results, the common characteristics concerning raw materials and preparing technology are discussed.

2. Experimental Details

2.1. Archaeological Descriptions

Typical brown fragments produced during the Song Dynasty were selected. Among them, 6 fragments were excavated from Hunyuan kilns, Datong city, Shanxi province, labeled as HYY-01~06; 3 fragments were excavated from Jingxing kilns, Shijiazhuang city, Hebei province, labeled as JXY-01~03; 37 fragments were excavated from Dangyangyu kilns (Jiaozuo city, DYY-01~06), Ganquancun kilns (Luoyang city, GQC-01~08), Chengguancun kilns (Luoyang city, CGY-01~11) and Lushan kilns (Pingdingshan city, LSDD-01~12) respectively, Henan province; 19 fragments were excavated from Yaozhou kilns (Tongchuan city, labeled as YZY01, 05~12) and Yingou archaeological site (Weinan city, labeled as YG-01~10), respectively, Shaanxi province; and 5 fragments (labeled as LHP-01~05) were excavated from Luhuaping kilns, Nanping city, Fujian province. Figure 1 presents the representative fragments.

2.2. Analytical Techniques

2.2.1. Imaging Techniques

The morphology of the glaze surfaces was first observed by a KEYENCE optical microscope equipped with a surface depth observation system (VHX-7000, KEYENCE, Osaka, Japan). The morphological observation in high magnification was carried out using SEM (Flex1000, Hitachi, Tokyo, Japan). Prior to analyses, the samples were deposited with a thin layer of gold (with thickness around tens of nanometers) to enhance their conductivity.

2.2.2. X-Ray Fluorescence

The chemical composition of the glaze surfaces was analyzed using an energy-dispersive X-ray fluorescence spectrometer (XGT-7200V, HORIBA, Kyoto, Japan). The beam spot is around 1.2 mm, with an energy power of 30 kV and acquisition time of 120 s for each point under vacuum. At least three points were recorded on each sample to assess the homogeneity.

2.2.3. Micro-Raman Spectroscopy

The crystals in the glaze surfaces were investigated by confocal micro-raman spectrometer (inVia, Renishaw, Wotton-under-Edge, UK) with beam size of 2–3 μm and excitation by a cw 532 nm solid-state laser. The analyses of crystals in the glaze surfaces could lead to a larger spot size. The power of 1.31 mW at 100× magnification was selected to optimize the signal-to-noise ratio to avoid any thermal effect. The Origin (2021) software was applied to process the data. Baseline subtraction and minor smoothing were carried out during the data processing. The final spectra were compared by the RRuFF database (https://rruff.info/).

3. Results

3.1. Optical and SEM Observations

Under coaxial illumination optical microscopy, the glaze surfaces of the investigated ceramic samples exhibit distinct and highly informative morphological features, as systematically presented in Figure 2. Analysis reveals that the glaze surface of the HYY ceramic samples (Figure 2a) is primarily composed of small crystals on the scale of several micrometers, which exhibit an iridescent luster under light. In contrast, the glaze structures of the other ceramic samples (Figure 2b–i) demonstrate significantly greater complexity. They are composed of a multi-scale crystalline assemblage, incorporating not only a background matrix of micron-sized crystals similar to those observed in HYY but also featuring prominent, well-developed larger crystals with dimensions ranging from tens to hundreds of micrometers. These larger crystals display distinctive dendritic or palm-frond-like morphologies and show iridescent or silvery reflections under coaxial illumination. To gain deeper insight into the microscopic morphology of the crystals within the glaze layer, we conducted high-resolution observations on the samples using SEM, with the results shown in Figure 3. The SEM analysis confirms and elucidates the significant differences in crystal morphology and size among the different ceramic samples. Irregular crystals several micrometers in size were observed in the HYY glaze (Figure 3a), while numerous dendritic or palm-leaf-like crystals ranging from tens to hundreds of micrometers in size were found in the other glazes shown in Figure 3b–i. It is noteworthy that such dendritic or palm-leaf-like crystals with pronounced fractal characteristics are not isolated phenomena limited to these specific samples. Similar crystal morphologies have been reported in products from several historically renowned kilns. For instance, entirely analogous crystal forms have been consistently documented in wares from the Qilizhen kilns [8], the Yaozhou kilns [8], and the celebrated Jian kilns [10,11,12,13,14,15]. Previous research [17] has provided a fundamental explanation for the remarkable optical properties of these micro-crystals. These seemingly monolithic crystals are, in fact, hierarchical structures composed of nano-scale sheet-like subunits. This intricate nano-layered architecture can interact with incident light through coherent scattering, interference, and diffraction mechanisms. It is this physical mechanism that macroscopically gives the glaze the iridescent or silvery metallic luster effects observed in Figure 2b–i.

3.2. Raman Analyses of the Glaze Surfaces

3.2.1. Abundant ε-Fe₂O₃ Crystals

To identify the phase composition of the crystals in the glaze, we performed micro-Raman spectroscopy analysis on the samples. The Raman spectrum of the crystals measured from the HYY ware (Figure 4A) exhibits six distinct characteristic peaks located at approximately 231 cm⁻¹, 294 cm⁻¹, 411 cm⁻¹, 506 cm⁻¹, 606 cm⁻¹, 673 cm⁻¹, and 1336 cm⁻¹. The positions and relative intensities of these peaks correspond well with the standard Raman characteristics of hematite (α-Fe₂O₃) reported in the literature [5]. It is noteworthy that the Raman peaks near 673 cm⁻¹ and 1336 cm⁻¹ may have their positions or intensities influenced by cation substitutions within the crystals. In contrast, the spectra of the crystals from other wares (Figure 4B–I) show similar Raman features, with a total of 14 peaks located near 130 cm⁻¹, 156 cm⁻¹, 177 cm⁻¹, 241 cm⁻¹, 306 cm⁻¹, 360 cm⁻¹, 384 cm⁻¹, 427 cm⁻¹, 449 cm⁻¹, 503 cm⁻¹, 576 cm⁻¹, 690 cm⁻¹, 753 cm⁻¹, and 1375 cm⁻¹, which are entirely consistent with the Raman spectrum of the metastable ε-Fe₂O₃ [5].

Further detailed comparison uncovered systematic variations in both peak positions (spectral shifts) and full width at half maximum (FWHM, representing peak broadening) among the characteristic Raman peaks of ε-Fe₂O₃ crystals originating from different kiln sites (Figure 4B–I). To quantitatively investigate these spectral discrepancies, we systematically selected 11 representative characteristic peaks, designated as P1 through P11 in Figure 4B, for rigorous statistical analysis. Principal component analysis (PCA) was subsequently employed to process the extracted datasets of peak positions and FWHM values, enabling dimensional reduction and pattern recognition within the multivariate data. The distributions of both the peak positions (Figure 5a) and FWHM values (Figure 5b) of the crystals in Hebei wares (Jingxing kilns), Shaanxi wares (Yaozhou kilns and Yingou archaeological site), and Fujian wares (LHP kilns) are relatively gathered. Conversely, the points of Henan wares (Dangyangyu kilns, Xin’an kilns, and Duandian kilns) are dispersed.

3.2.2. Rare Crystals

Further analysis of the glaze layers also revealed the presence of multiple crystals besides ε-Fe₂O₃, with their typical morphologies shown in Figure 6 and Figure S1. In Figure 6a–d, crystals with various morphologies such as polygonal, paramecium-like, palm-leaf-like, and platy, with sizes of several tens of micrometers, can be clearly observed in samples GQC-05, LSDD-09, CGY-10, and LHP-05, respectively. To accurately identify the phases of these crystals, we collected their micro-Raman spectra, with the corresponding results shown as spectra pt02, pt03, and pt08 in Figure 7. The Raman spectrum (Figure 7, pt02) obtained from the polygonal crystal in Figure 6a shows six distinct Raman peaks at approximately 140 cm⁻¹, 217 cm⁻¹, 347 cm⁻¹, 502 cm⁻¹, 657 cm⁻¹, and 723 cm⁻¹. This spectral signature matches the typical characteristics of magnesioferrite spinel (MgFe₂O₄) [21]. Among them, the four peaks at 140 cm⁻¹, 347 cm⁻¹, 502 cm⁻¹, and 723 cm⁻¹ can be assigned to the five first-order Raman-active modes (A_1g + E_g + 3T_2g) of the spinel structure. Particularly noteworthy is the additional A1g symmetry peak at 657 cm⁻¹, whose appearance is generally attributed to cation exchange between the octahedral and tetrahedral sites in the spinel structure. The Raman spectrum corresponding to the paramecium-like crystal in Figure 6b (Figure 7, pt03) presents distinctly different features, with nine sharp Raman bands observed at 202 cm⁻¹, 215 cm⁻¹, 225 cm⁻¹, 357 cm⁻¹, 402 cm⁻¹, 440 cm⁻¹, 808 cm⁻¹, 976 cm⁻¹, and 1008 cm⁻¹. These are consistent with the Raman characteristics of zircon [14]. Furthermore, micro-Raman analysis performed on the palm-leaf-like and platy crystals shown in Figure 6c and Figure 7 (pt08) clearly displays the characteristic spectral features of hematite (α-Fe₂O₃). This spectrum includes Γ-point phonon vibration modes below 620 cm⁻¹, specifically with peaks at 232 cm⁻¹ [A_1g], 249 cm⁻¹ [E_g], 297 cm⁻¹ [E_g], 412 cm⁻¹ [E_g], 506 cm⁻¹ [A_1g], and 617 cm⁻¹ [E_g]. Additionally, characteristic bands attributed to first-order and second-order longitudinal optical (LO) phonons were observed at 673 (1LO) and 1319 cm⁻¹ (2LO), respectively [22].

Figure S1 reveals the presence of four distinct types of submicron-sized crystals identified in samples DYY-05, GQC-05, and HY-06, underscoring the mineralogical diversity within the glaze microstructures. The corresponding micro-Raman spectra for these crystals are presented as curves pt01, pt04, pt05, pt06, and pt07 in Figure 7, providing definitive phase identification. The spectrum in Figure 7, curve pt01, exhibits four distinct Raman bands at approximately 143 cm⁻¹, 235 cm⁻¹, 448 cm⁻¹, and 608 cm⁻¹. This signature is consistent with the Raman signal of titanium oxides. Specifically, the first band at 143 cm⁻¹ can be attributed to anatase [23], while the remaining three bands align with the characteristic features of rutile [24]. The spectrum in Figure 7, curve pt04, shows seven Raman scattering bands that can be unambiguously assigned to pseudobrookite [25]. Compared to the standard Raman spectrum of pseudobrookite, the observed peak shifts and increased spectral broadening are likely caused by the partial substitution of iron (Fe) and/or titanium (Ti) sites in the crystal structure by other cations. This ion substitution effect often induces micro-strain in the lattice and alters phonon scattering behavior [25]. The spectrum in Figure 7, curve pt05, presents more complex Raman features. One set of bands located at 225 cm⁻¹ [A_1g], 413 cm⁻¹ [A_1g], 781 cm⁻¹ [E_g], and 1079 cm⁻¹ [A_1g] corresponds to the typical vibration modes of cristobalite [26]. Another set of bands at 129 cm⁻¹ [E_g], 205 cm⁻¹ [A_1g], 465 cm⁻¹ [A_1g], and 678 cm⁻¹ [E_g] matches the characteristic Raman peaks of quartz [26]. This indicates that the analyzed micro-region consists of coexisting cristobalite and quartz. The spectrum in Figure 7, curve pt06, exhibits the characteristic Raman-active modes of magnetite spinel. The five first-order Raman peaks are located at 156 cm⁻¹ (T_2g), 238 cm⁻¹ (E_g), 361 cm⁻¹ (T_2g), 554 cm⁻¹ (T_2g), and 682 cm⁻¹ (A_1g). The spectrum in Figure 7, curve pt07, is highly similar to the portion attributed to quartz in spectrum pt05. Its main peak positions match the standard Raman spectrum of quartz [27], leading to the identification of this crystal as quartz. It is important to note that, limited by the micron-scale spot size of μ-RS, this study could not achieve a complete statistical detection of all submicron crystals within the glaze. Nonetheless, the work endeavored to identify and document as many observable crystal types as possible within the detectable range. These have been summarized in

Table 1. Crystals detected in the glazes of the samples using micro-Raman spectroscopy. Hem: hematite; Magn: magnetite; Mafe: magnesioferrite; Zir: zircon; Ana: anatase; PS: pseudobrookite; Rut: rutile; Cor: cordierite; Crs: cristobalite; Qtz: quartz; Mul: mullite. The mullite crystals were detected by SEM-EDS in Refs. [11,14].

Samples	ε-Fe2O3	Hem	Magn	Mafe	Zir	Ana	PS	Rut	Cor	Crs	Qtz	Mul
HYY-01~06	×	×	×	×	×	×	×				×
JXY-01~03	×	×			×			×
DYY-01~06	×	×			×	×	×	×		×	×
GQC-01~08	×	×	×	× [21]	×		×				×
CGY-01~11	×	×	×	×	×	×	×	×			×
YZY-01,05~12	×	×		×	×	×		×	× [11]		×	× [11]
YG-01~10	×	×	×		×		×
LSDD-01~12	×	×	×	×	×		×				×
LHP-01~05	×	×		×							×	× [14]

to systematically reflect the diversity of crystal composition in the glaze.

4. Discussion

Abundant ε-Fe₂O₃ crystals were widely identified in the glaze surfaces of the majority of the investigated samples, indicating that the formation of this metastable phase was a reproducible phenomenon achieved across various northern and southern kilns in ancient China. However, a notable exception was observed in the case of the HYY fragments, where the predominant crystal detected was hematite rather than ε-Fe₂O₃. This divergence in crystalline phase assembly suggests a unique local chemical environment or specific firing practice employed at the HYY kiln site. The relatively elevated contents of CaO and MgO in the HYY glazes, as indicated in Table S1, likely played a critical role in this phase selection. It is proposed that these specific oxide concentrations modified the physical properties of the glaze melt—such as its viscosity and undercooling behavior—under the given firing conditions. These modifications appear to have thermodynamically suppressed the formation of metastable ε-Fe₂O₃, while simultaneously favoring the crystallization of the more stable hematite.

Furthermore, a significant morphological difference was noted: the ε-Fe₂O₃ crystals observed in LHP glazes are notably larger in size than those found in glazes from other kiln sites. This disparity can reasonably be attributed to the higher overall iron content present in the LHP samples (8–10 wt%, Table S1), which provides a greater supply of solute for crystal growth, compared to the range of 4–9 wt% measured in fragments from other kilns (Table S1). It is also noteworthy that although the ε-Fe₂O₃ crystals extracted from shards originating in Hebei, Henan, and Shaanxi provinces exhibit broadly comparable grain sizes, they display subtle but discernible variations in crystal morphology, such as the branching density of dendritic forms or the outline of palm-leaf-like crystals. These morphological nuances may reflect minor differences in local raw material compositions or specific cooling schedules employed by the different kiln traditions. Compared to the pure phase, the significant peak shifts and increased line broadening observed in the ε-Fe₂O₃ crystals within the glass have been demonstrated to originate from the substitution of Fe ions by Al ions [10,11]. Such ionic substitution not only induces minor changes in lattice constants and lattice distortion, thereby altering vibrational frequencies (leading to peak shifts) and increasing structural disorder (resulting in broadening), but more importantly, it can enhance the structural stability of the metastable ε-Fe₂O₃ phase by modulating the lattice energy [11].

To rationalize the crystallization conditions of ε-Fe₂O₃ from a thermodynamic perspective, we referred to the CaO-FeO-SiO₂ ternary system phase diagram [28] (Figure 8a). Figure 8b shows an enlarged view of the region relevant to iron oxide crystallization. Based on replication experiments simulating the production of Jian wares [12], the potential crystallization temperature range for ε-Fe₂O₃ crystals was estimated to be between 1160 and 1190 °C; this critical temperature window is highlighted in brown in Figure 8b. The phase diagram analysis further indicates that a glass matrix composition characterized by relatively high SiO₂ and low CaO content—falling within or near the highlighted brown area—is particularly favorable for the crystallization of ε-Fe₂O₃. This conclusion, derived from phase equilibrium considerations, is consistent with previous empirical findings from studies on Deqing wares [7].

Table 1 summarizes the types of crystals identified in the glazes of all investigated samples. The following crystals are referred to by their abbreviations: hematite (Hem), magnetite (Magn), magnesioferrite (Mafe), zircon (Zir), anatase (Ana), pseudobrookite (PS), rutile (Rut), cordierite (Cor), cristobalite (Crs), quartz (Qtz), and mullite (Mul). ε-Fe₂O₃ is the major phase in each glaze, with Hem present as a minor phase. Zir was detected in every glaze except for LHP wares. Qtz was detected in every glaze except for JXY and YG glazes. In addition, HYY glazes contain Magn, Mafe, Ana and PS. JXY glazes contain Rut and DYY wares contain Ana, PS, Rut, and Crs. CQC, CGY, and LSDD glazes contain Magn, Mafe, and PS, but Ana and Rut were also detected in the latter. Although both the Yaozhou kilns and the Yingou archaeological site are located in the Guanzhong Plain, China, YG glazes contain Magn and PS. In contrast, YZY glazes contain Mafe, Ana, Rut, Cor, and Mul. Mafe and Mul were also detected in LHP glazes. Based on Raman analyses combined with SEM observations, significant large ε-Fe₂O₃ crystals were observed in the surroundings of Zir (Figure 6b), Mafe (Figure 6a), and Mul [10] particles. Such a phenomenon was found for each particle of the three types. Interestingly, Zir, Mafe, Mul and ε-Fe₂O₃ have similar crystalline structure. The unit cell structure of Zir, Mafe, Mul and ε-Fe₂O₃ is composed of octahedra ([ZrO₆], [FeO₆], [AlO₆] and [FeO₆]) and tetrahedra ([SiO₄], [MgO₄], [SiO₄], and [FeO₄]), respectively. The former three types of particles might possibly serve as substrates to promote growth of ε-Fe₂O₃ crystals.

Furthermore, the crystals observed within the glaze layer in this study are likely related to the raw materials utilized locally. It should be noted, however, that the formation of microcrystalline structures is not only governed by the chemical composition of the raw materials but is also significantly constrained by the firing conditions, such as temperature, atmosphere, and cooling rate. Consequently, information on these crystalline phases may offer potential clues for investigating the technological traditions of ceramics from different production areas. That said, it must be clearly emphasized that, given the multifactorial nature of microcrystal formation and the limited sample size in the current analysis, these crystalline characteristics are not yet sufficient to serve as unique or definitive “fingerprints” for provenance attribution. Future work should integrate multidimensional evidence, including major and trace element analysis of the body and glaze, simulated firing experiments, and systematic studies with larger sample sizes, to reliably evaluate the practical value of these crystalline characteristics in provenance discrimination.

5. Conclusions

Micro-Raman spectroscopy was exploited to non-destructively analyze the fragments of iron-based brown glazed wares, which were excavated from seven Chinese northern and southern kilns and an archaeological site. Abundant ε-Fe₂O₃ crystals were observed in the glaze surfaces of all wares except for HYY glazes, which mainly contain hematite crystals. The Raman spectra of these ε-Fe₂O₃ crystals were analyzed, and the PCA method was exploited to investigate the positions and FWHM of the characteristic Raman peaks. Both the positions and FWHM of Hebei, Shaanxi, and Fujian wares are relatively gathered, whereas the points of Henan wares are dispersed. The CaO-FeO-SiO₂ phase diagram and previous imitative experiments indicate that relatively high SiO₂ and low CaO contents in the system benefit ε-Fe₂O₃ precipitation. In addition, various crystals were detected in the glazes, including magnetite, magnesioferrite, zircon, anatase, pseudobrookite, rutile, cordierite, cristobalite, quartz, and mullite.