3.1 Chemical composition
As shown in Table 1, the main constituent of Hare’s fur glaze area was probably 62.20 wt.% SiO2
, 16.43 wt.% Al2
, 8.35 wt.% Fe2
, 3.02 wt.% K2
O and 6.97 wt.% CaO, and the black glaze area mainly consisted of 77.07 wt.% SiO2
, 10.61 wt.% Al2
, 4.10 wt.% Fe2
, 2.06 wt.% K2
O and 3.85 wt.% CaO. The body contained approximately 66.32 wt.% SiO2
, 18.50 wt.% Al2
, 10.04 wt.% Fe2
, 1.41 wt.% Na2
O, and 2.49 wt.% K2
O. The results demonstrated that SiO2
were major components of the HF sample, in accordance with a aluminum-deficient and silicon-rich trait of black-glazed porcelain. Minor elements such as sodium, potassium and calcium existed as flux. Jian glazes were classified as high temperature calcia-iron oxide-aluminosilicate glazes, and iron oxide functioned as both flux and phase-separation accelerator during firing
. Previous studies indicated that[28,29]
, there were two kinds of microstructural forming mechanics: (1) local phase separation in glaze surface neighbouring area followed by crystallization of iron oxide, (2) crystallization of anorthite accompanied by inter-crystal phase separation and the subsequent crystallization of iron oxide.
On purpose of making further confirmation on the oxidation valence state of iron in crystalline area, the synchrotron XANES technique was employed to measure the cleaned surface. Before the experiment, spectra of iron foil (Fe), hematite (Fe2O3) and magnetite (Fe3O4) were collected as standard references, which HF sample spectra can be compared accurately. As transition metal element, iron atom has the electron configuration of 3d6 4s2, while Fe3+ and Fe2+ correspond to 3d5 and 3d6, respectively. Fe3+ exhibits good stability when compared to Fe2+ due to electrons partially occupy the d orbit. The XANES results were illustrated in Fig. 2.
For iron foil in which the absorption K-edge was observed at 7131.2 eV, the spectrum was characterized by a small shoulder together with one inapparent crest. In post-edge region, a series of peaks turned up in succession. These characters were distinguished well between the hematite and magnetite. There was a striking pre-edge shoulder in divalent or trivalent iron, which contributed to the 1s to 3d transition. Seen from Fig. 2(b), the spectra of hematite and magnetite crystals behaved differently in the post-edge peak position. Fe3O4 reference showed three evident divisive crests at 7184.2, 7229.1 and 7271.8 eV, respectively. While the corresponding position in Fe2O3 spectrum seemed not obviously. In the HF sample result (Fig. 2a), the absorption edge peak appeared at 7133.6 eV, followed by a small shoulder at 7147.3 eV. The iron occured positive trivalent, which were consistent with the reported studies. In addition, divalent iron ions contributed to deepening the blue green of glaze surface while trivalent iron ions presented yellow. It indicated that Fe3+ was the color mechanism of the sample.
Major chemical compositions of glaze and body by XRF analysis (wt.%)
|Hare’s fur glaze
3.2 Microstructure analysis by OMFigure 3 presented the surface morphology of HF sample revealed by optical microscopes. Low magnification image of the glaze surface (Fig. 3a) showed the typical characteristic of brown streak-like or silk-like hare’s fur pattern. Seen from the Fig. 3b-3d, there were three types of crystal clusters in the glaze surface, one displayed a fan-shaped structure consisted of plentiful needle-like crystals; one embraced a grid structure tightly packed with plentiful spherical or irregular-shaped particles, and some white and translucent crystals were also clearly visible, which may be attributed to the residual unmelted quartz particles; the last was flower-like cluster ranging from several hundred micrometer to more than one millimeter in size. Some large-scale flower-like or fan-shaped crystals mainly aggregated at the edge of the bowl. These crystals on the surface all appeared to be brownish, which was in accordance with the appearance of hematite crystals. The cross-sectional observation showed that the glaze contained significant amounts of bubbles and crystals in the glaze-body interface and body. In addition, a brown-yellow devitrified layer appeared at the top zone of HF glaze, which may be concentrated by numerous small Fe-rich crystals.
3.3 Phase analysis by XRD and RS
With the aim of studying the crystallization behavior, the XRD pattern was performed on the glaze surface to trace the mineral composition and the relevant result was displayed in Fig. 4. Compared to the standard PDF card, the main crystalline phase was hematite (ε-Fe2
) with a small amount of quartz (α-SiO2
) on the glaze. Both the thermodynamic qualification and equilibrium thermodynamic conditions for the crystallization during the firing process were conducive to the precipitation of crystals from the glaze. The porous and loose structure of porcelain body increased the concentration of dissolved oxygen and allowed the oxidation of the Fe-rich phase into hematite. The presence of unmelted quartz in the molten glaze came from the residual mineral raw materials.
To unambiguously confirm the iron oxide phases, Raman spectra carried out at different spots on the outer surface of HF glaze were shown in Fig. 5. As can be seen, the spectra taken at flower-like crystal clusters (spectrum A corresponding to Fig. 3d), fan-shaped crystal clusters (spectrum B corresponding to Fig. 3b) and honeycomb crystal clusters (spectrum C corresponding to Fig. 3c) were all identified to belong to the epsilon-hematite (ε-Fe2O3). ε-Fe2O3 phase, a rare and metastable Fe2O3 polymorph, has an orthorhombic crystal structure with the Pna21 space group (a=5.0810 Å, b=8.7411 Å, c=9.4083 Å). It is defined as an intermediate between hematite (α-Fe2O3) and maghemite (γ-Fe2O3). The collected Raman peaks here shared similar peak positions at 122, 155, 234, 353, 448, 563, 689, and 1366 cm−1, which were corresponding to first-order phonon vibrational modes of ε-Fe2O3. Majority of the detected Raman peak positions at the spectrum A, B and C matched well with each other, while few of them shifted slightly due to the faceted aspect of crystal lattice originated from the laser source (λ = 532 nm) and/or phonon confinement effect in our samples. This conclusion was consistent with the XRD observation. Beside these, we measured the phase constituent of the underlying black glaze (Fig. 5b), no other diffraction signal can be found, which implied that the black glaze was of a glassy nature.
3.4 Microstructure analysis by SEM-EDS
Figure 6 showed the morphological features of Hare’s fur glaze. Based on surface observation, the boundaries between the yellow-brown Hare’s fur area and black glaze area were distinct, and the chromogenic crystals on the yellow-brown Hare’s fur area were uniformly distributed, as revealed in Fig. 6a. The crystals in the yellow-brown Hare’s fur area can be divided into three types (hereafter, types A, B, C) on account of their sizes and microsructure, which were represented in Figure 6(b-d). With type A, the crystals inhomogenous dispersed on the streak embraced flower-shaped structure, characterized by many branches radiating from its center, petals growing along the branches and needles on both sides of the petals. This type of crystal was usually small in size. With type B, the crystals were organized orderly in dendritic-like or leaf-like manners at micrometers scales. This structure was generally accompanied by large main branches, and the secondary branches tightly arranged on the two sides of the main branch and were parallel to each other. Under the higher magnification in Fig. 6g, some large-scale dendrites were even approximately hundreds of microns and the high-order subbranches were covered by numerous intensive smaller twigs. Type C was featured as honeycomb structure closely packed with plentiful spherical or irregular-shaped crystal clusters. These crystals mainly existed in the junction zones between Hare’s fur area and black glaze area.
Figure 6(e, f) presented the cross-section morphology of the HF sample. The glaze was about 1 mm thick. In comparison with the porcelain body, the glaze was rather less impure. The layer near the glaze surface contained some irregular and disordered circles or spots with higher average atomic contrast, indicated that most of the impurities was the single crystal or crystal clusters precipitated from the glaze layer, combined with a low coverage rate. Fig. 6h displayed a detailed image of the upper glaze layer. Large number of small-scale flower-shaped or feather-like crystals distributed discretely and showed a gradient variation tendency on the shape and arrangement from the glaze surface to inside.
EDS analyses performed on the glaze were shown in Fig. 7 and Fig. 8. From Fig. 7, it was manifested that the whole glaze (both the Hare’s fur area and black glaze area) were mainly composed of the following elements: Al, Si, Fe, K, Ca and O. Comparatively, the element contents in the yellowish-brown Hare’s fur area differed a lot from those in the black area, indicating that the phase separation have taken place in the surface glaze. Al, Fe and Ca were enriched in the yellowish-brown Hare’s fur area while Si in the black glaze area. For Fe element, it had a higher concentration than that of other elements in the Hare’s fur area, confirming that the chromogenic crystals in crystallization zone were ε-Fe2O3, which was in accordance with previous studies. From the elemental distribution map examined on the cross sections with the depth of about 45 µm inside the glaze surface, we observed that the crystals in upper layer was much larger than that of the following part, and the content of Fe-rich crystals varied unevenly along the thickness direction. The ferric oxides gathered around the bubbles were carried to the glaze surface, causing an iron enrichment area on or beneath the surface.
Figure 9 showed the secondary electron images and corresponding magnified view of parts in the well-defined dendritic-like and leaf-like crystals. As discussed above, the dendritic crystals had a typical hierarchical structure with large main branches and parallel secondary branches on the two sides arranged symmetrically in rows. In detail, some small-scale feather-like or leafy crystal clusters aggregating at the Hare’s fur streaks were mainly constitutive of acicular rods growing out the glaze surface with different lengths, as portrayed in the Figure 9(b, c). It can be observed that both ε-Fe2O3 crystals in the yellowish-brown Hare’s fur area were all matured over the underlying black glaze layer, corroborating the previous XRD and RS results. Herein, this highly differentiated and well-bedded structure in distribution provided some useful guidance for the forming mechanism of Hare’s fur pattern.