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Unveiling the science behind the tea bowls from the Jizhou kiln. Part II. Microstructures and the coloring mechanism
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Unveiling the science behind the tea bowls from the Jizhou kiln. Part II. Microstructures and the coloring mechanism Changsong Xua,b,c,1, Weidong Lia,b, Jingkun Guoa
⁎,1
, Xiaoke Lua,b, Wenjiang Zhangd, Hongjie Luoe,
a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Key Scientific Research Base of Ancient Ceramics, State Administration for Cultural Heritage, Shanghai 200050, China c University of Chinese Academy of Sciences, Beijing 100049, China d Jiangxi Provincial Institute of Cultural Relics and Archaeology, Nanchang 330008, China e Shanghai University, Shanghai 200444, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Jizhou kiln Black-glazed tea bowl Microstructure Coloring mechanism Thermal history
In this part of the study, optical microscope, field emission scanning electron microscope-energy dispersive spectrometer, transmission electron microscope-selected area diffraction-energy dispersive spectrometer, microRaman and reflectance spectrum were applied to further study the microstructures of the characteristic areas of the glazes and illustrate the coloring mechanism of the “hare's fur” tea bowls with blue or bluish violet patterns. The results show that due to the diffusion between the cover glaze and ground glaze caused by compositional difference, as well as the “boiling effect” of the ground glaze, local chemical composition changes to form the different microstructures in different regions. Large sized interconnected phase separation structure with approximately 350 nm characteristic size forms in the unreacted portion of the cover glaze, leading to the scattering of all wavelengths of the incident visible light. With the contribution of the ground glaze which has low immiscibility tendency, small sized interconnected phase separation structure with approximately 170 nm characteristic size forms at the edge of the white region, leading to the scattering of the blue light range of the incident visible light. The fluctuation of thermal history gives various appearance to the Jizhou “hare's fur” tea bowls, although they share the same coloring mechanism. In general, chemical composition, microstructure and firing schedule cooperate with each other to create the changeful appearance of the Jizhou tea bowls.
1. Introduction The Jizhou kiln, located in present-day Ji’an county of Jiangxi Province, is one of the largest folk kilns in ancient China [1]. It started firing porcelain in the Tang dynasty (618–907 A.D.), became prosperous in the Southern Song dynasty (1127–1279 A.D.) and closed down at the end of the Yuan dynasty (1271–1368 A.D) [2]. The blackglazed tea bowls produced for tea tasting and competition in the Southern Song dynasty are among the most attractive porcelain wares produced [1]. The potters applied the two-layer glazing technique to bring about the “partridge feather”, “hare's fur”, “tortoise shell” and “tiger stripe”. Among them, “hare's fur” making full use of the flowing of the glaze in firing process forms the colorful and changeful patterns naturally, which is called “Yao Bian” in part I of the study. According to historical documents referred in part I of the study, the wares produced
by the same recipe and glazing technique created the diverse patterns naturally on surface is called “Yao Bian” in ancient times. However, in modern times, “Yao Bian” is referred to the wares with blue opalescence appearance like Jun wares. The original meaning of “Yao Bian” is used in this study, but more attention will be paid to the samples with blue streaks. In recent decades, a few researchers have extended their studies to examine these wares on a micro scale. Chen et al. [3] found the phase separation structures by using TEM on the opaque glaze area of the Jizhou tea bowls for the first time. The author inferred that the phase separation structures result from the physical and chemical reactions between the cover glaze and the ground glaze. Zhang [1] also reported on the interconnected phase separation structure of the Jizhou tea bowls without further discussion. Because of the limited testing technology, previous studies on the
⁎
Corresponding author at: Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. E-mail address: [email protected] (W. Li). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ceramint.2018.07.183 Received 18 February 2018; Received in revised form 20 July 2018; Accepted 20 July 2018 0272-8842/ © 2018 Published by Elsevier Ltd.
Please cite this article as: Xu, C., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.07.183
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Fig. 1. No.19 Jizhou “hare's fur” tea bowl.
Fig. 2. OM images of No.19 Jizhou “hare's fur” tea bowl (a) Surface of the sample; (b) Cross section of the sample; (c) Cross section of the white region marked in (a) (400 ×); (d) Cross section of the bluish violet region marked in (a)(500 ×).
2. Experimental
glaze microstructure were preliminary. The microstructure difference of the characteristic areas and the coloring mechanism are still unknown. Therefore, this part of the study will focus on the microstructures of the characteristic areas of the Jizhou tea bowl samples and the influence of the microstructure upon the coloring in order to illustrate the coloring mechanism of the “hare's fur”.
To further study the microstructures of the characteristic areas of the “hare's fur” tea bowls, 7 typical samples were selected for testing. The characteristic part of the sample was cut from the sherd by diamond saw and the cross section was polished by diamond powder. All
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The reflectance spectra, which describes the various colors of the glazes, were measured by a spectrum measuring system equipped with the metallographic optical microscope (Leica DM6000). The xenon lamp and halogen lamp were used in this measuring system and the diameter of the light spot was 2 µm. Nonlinear curve fit in Origin software was used to find the peak position of the spectra. The microstructures of the glaze surfaces and cross sections were studied using a field emission electron microscope (FEI Magellan 400) equipped with EDS. Samples were etched with 1% HF for 60 s. SEM images were processed by a 2D-Fourier transform programmed in MATLAB software to obtain the characteristic size of the phase separation structures. Since the defects on the natural surface of the glaze can largely influence the 2D-Fourier transform results, only the SEM images of the cross sections were used to calculate the characteristic size of the phase separation structures. A field emission transmission electron microscope (JEOL JEM2100F) was used to analyze the microstructures and the micro-areas of
Table 1 The thickness of each layer in the glaze of No.19 Jizhou “hare's fur” tea bowl. Layer
Thickness In white region
unreacted portion of the cover glaze bluish violet layer on surface light brown reaction layer ground glaze layer
In bluish violet region
~ 28 µm ~ 120 µm ~ 45 µm
~ 13 µm ~108 µm ~ 69 µm
samples were ultrasonically cleaned by water and ethanol, 3–5 times for 15 min. After that, all samples were dried in drying oven for 12 h. The photos of samples were taken by digital camera. The macroscale morphology of the glaze surfaces and cross sections were observed with an optical microscope (Keyence VHX-2000). The surface crystallization was analyzed by micro-Raman Spectrum (Horiba XploRA one 532 nm).
Table 2 The FESEM analysis results for the cross section of No.19 Jizhou “hare's fur” tea bowl.
In white region
In bluish violet region
Position (distance below the surface)
Corresponding layer
Microstructure
Characteristic size of the phase separation structure
20 µm (A) 60 µm (B) 220 µm (C) 5 µm (D) 50 µm (E) 200 µm (F)
unreacted portion of the cover glaze reaction zone ground glaze bluish violet region reaction zone ground glaze
Fig. Fig. Fig. Fig. Fig. Fig.
325 nm 301 nm
3a 3b 5a 3c 3d 5b
162 nm 178 nm
Fig. 3. FESEM images of the cross section of No.19 Jizhou “hare's fur” tea bowl (a) Unreacted portion of the cover glaze (region A in Fig. 2c) (20000 ×); (b) Reaction zone below the unreacted portion of the cover glaze (region B in Fig. 2c) (20000 ×); (c) Bluish violet region (region D in Fig. 2d) (20000 ×); (d) Reaction zone below the bluish violet region (region E in Fig. 2d) (20000 ×).
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a
glaze at a high temperature to achieve the natural and unique patterns. To demonstrate the formation regularity and the coloring mechanism of the patterns, we use 3 “hare's fur” samples (No. 10, No.11 and No. 19) as examples. More information of samples can be found in part I of the study. Sample No.19 is a sherd of the Jizhou “hare's fur” tea bowl, as shown in Fig. 1. A single ground glaze was applied to the outside of the bowl, and two glazes were applied to the inside of the bowl. In Fig. 2a, the OM images corresponding to the area in the red box in Fig. 1a shows the white/bluish violet interlaced “hare's fur” formed on the inner surface, and blue or bluish violet colors were created at the edges of the white streaks. According to Fig. 2b, the dark gray coarse body has pores, large quartz particles and impurity inclusions. The cross section of the white region marked in Fig. 2a is shown in Fig. 2c. The very thin white layer at the top of the cover glaze does not react with the ground glaze. The light brown opaque region (referred to as reaction zone), which results from the interaction and diffusion between the cover glaze and the ground glaze during firing, accounts for the largest proportion of the cross section. The reaction zone is irregular and wavy. A blue or bluish violet color also appears at the boundary between the reaction zone and the ground glaze. According to the cross section of the bluish violet region, as shown in Fig. 2d, the bluish violet layer on the surface is much thinner than the unreacted portion of the cover glaze, and the reaction zone is also wavy. The bluish violet layer is observed again between the reaction zone and the ground glaze. Based on the observation of the cross sections, the white region and the bluish violet region on the glaze surface are quite thin. The reaction zone and the ground glaze occupy the largest area of the cross section of the glaze. A blue or bluish violet color usually appears between the reaction zone and the ground glaze. Like the glaze surface, the cross section of the glaze is also full of color and changes. The diverse colors in the glaze indicate the variations of the local chemical composition, which leads to the differences in microstructure. The thickness of each layer is summarized in Table 1. The same piece of the sample was analyzed by FESEM to study the microstructures of the cross sections. The result of the FESEM analysis is summarized in Table 2. According to the FESEM analysis, the unreacted portion of the cover glaze is vitreous, with no unmelted inclusions. As discussed in part I of the article, the cover glaze has a strong tendency towards phase separation, and it has large sized interconnected phase separation structures, as shown in Fig. 3a. All wavelengths of the incident visible light are scattered by the structure, as shown in Fig. 4b, resulting in the opacification of the cover glaze. With a low Fe2O3 content, the unreacted portion of the cover glaze appears matt and white.
b
Fig. 4. OM image and the corresponding reflection spectra of the different regions of the surface of No.19 Jizhou “hare's fur” tea bowl.
the chemical compositions. Powders were scraped from the different color areas of the glazes for the TEM samples. 3. Results and discussion Jizhou “hare's fur” tea bowls make full use of the flowing of the
Fig. 5. FESEM images of the cross section of No.19 Jizhou “hare's fur” tea bowl (a) Ground glaze below the unreacted portion of the cover glaze (region C in Fig. 2c) (100000 ×); (b) Ground glaze below the bluish violet region (region F in Fig. 2d) (100000 ×).
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Fig. 6. No.10 Jizhou “hare's fur” tea bowl.
Fig. 7. OM images of No.10 Jizhou “hare's fur” tea bowl (a) White/bluish violet/brown interlaced glaze(100 ×); (b) Big flower-like crystal which precipitates on the glaze surface(900 ×).
dramatically due to the influence of the ground glaze. Similarly, when the cover glaze is thin enough, after interacting with the ground glaze, the local phase separation tendency dramatically decreases to form the bluish violet region on the surface. However, the influence of the high content of Fe2O3 in the ground glaze on the reaction zone is prominent. Iron ions diffuse into the reaction zone during firing to the direction of the concentration gradient, leading to the partial absorption of the incident visible light and making the reaction zone appear light brown. According to Fig. 3d, with a smaller phase separation structure size, the opacification degree of the reaction zone beneath the bluish violet region is lower than that of the region beneath the unreacted portion of the cover glaze. Sample No.10 is a sherd of the Jizhou “hare's fur” tea bowl, with a ring foot and part of the belly, as shown in Fig. 6. A high-iron ground glaze was applied to the outer wall of the bowl. Inside, the ground glaze and the cover glaze were applied on the light gray body. During firing, the white/bluish violet/brown interlaced “hare's fur” pattern with natural variations was created on surface. Irregular brown spots are distributed between the hare's furs. The closer to the bottom, the smaller the brown spots. Using the optical microscope, it is observed that the unreacted portion of the cover glaze is matt, white and opaque,
As discussed in Part I of the study, the tendency towards immiscibility of the ground glaze is smaller than that of the cover glaze. Small worm-like phase separation structures form in the ground glaze, as shown in Fig. 5. However, the influence of this structure on the coloring can be ignored. The high content of Fe2O3, which absorbs most of the incident visible light, as shown in Fig. 4b, leads to the dark color of the ground glaze. The bluish violet region is the result of the interaction and diffusion between the ground glaze and the cover glaze during firing. Small size of the interconnected phase separation structure (Fig. 3c) in this region forms the amorphous photonic structure [4], which leads to the selective scattering of the blue light range of the incident visible light, as shown in Fig. 4b. With no cobalt in the glaze, it is confirmed that the bluish violet in the sample is a special structural color. An interconnected phase separation structure (Fig. 3b) forms in the light brown reaction zone, with a similar size to that of the unreacted portion of the cover glaze. This phenomenon indicates that the ground glaze has little influence on the microstructure formation of the reaction zone. While the bluish violet region, which has small sized phase separation structures, appears between the reaction zone and the ground glaze, where the phase separation tendency decreases
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wavelengths of the incident visible light, as shown in Fig. 8b, resulting in the opacification of the glaze. With a low Fe2O3 content, the unreacted portion of the cover glaze is matte, white and opaque. However, at a high temperature, the viscosity of the cover glaze decreases; the cover glaze flows and forms a thin area. Due to the diffusion with the ground glaze, the phase separation tendency of the thin cover glaze region is reduced, and a small phase separation structure is formed. The structure selectively scatters the incident visible light, as shown in Fig. 8b, leading to the bluish violet color on surface. Since both the thin cover glaze and the proper firing schedule are necessary to form the bluish violet streaks, this kind of sample is not common. According to the TEM analysis of the unreacted portion of the cover glaze and the bluish violet region, as shown in Fig. 10a and b, the interconnected 3D network phase separation structures with different sizes are observed visually, which is consistent with the result of the FESEM analysis. The brown spots on the glaze surface are attributed to the “boiling effect” of the ground glaze. The relatively high content of Fe2O3 in the ground glaze pyrolyzes to generate bubbles above 1230 °C during firing [5]. The bubbles rise and drag the ground glaze to the glaze surface. Releasing of the bubbles leaves brown spots on the glaze surface between the white/bluish violet interlaced “hare's fur.” The high content of Fe2O3 in this region absorbs most of the incident visible light, as shown in Fig. 8b, resulting in the low reflectivity and the dark color. In this region, a droplet phase separation structure is observed by TEM analysis, as shown in Fig. 10c. The result of SAD is a dispersed diffraction ring, indicating the vitreous state of the brown spot region. According to the EDS analysis, as shown in Table 3, the droplet is rich in CaO and the matrix is rich in SiO2. Based on the glass chemistry, the strong immiscibility between SiO2 and CaO results in the phase separation in the glaze. Other modifiers such as Fe, Mg, P, Ti ions locate preferably in the phase enriched by CaO, consequently favoring phase separation. A droplet phase separation structure is also observed in the ground glaze, as shown in Fig. 10d. However, the size of the phase separation structure is much smaller than that in the brown spot region because of the difference in chemical composition. As shown in Table 4, the droplet is rich in CaO and the matrix is rich in SiO2, similar to the above result. In the white unreacted portion of the cover glaze region, big flowerlike crystals consisting of outward radial growth dendritic crystals precipitate on the surface, as shown in Fig. 9d. Based on the EDS analysis (Fig. 9e) and the micro-Raman spectrum (Fig. 11), the flowerlike crystal is diopside. However, according to the Part I of the study, MgO content in the cover glaze is not high enough to cause diopside crystallization, which indicates that MgO is heterogeneous in the cover glaze. According to EDS analysis, Mg and Ca are enriched in the same phase. Therefore, it is inferred that the phase separation in the cover glaze promotes the nucleation and the crystallization of diopside. Diopside crystals precipitate from the Ca (Mg) -enriched phase, consuming CaO and MgO, which leads to a significant decrease in the local phase separation tendency around the precipitation position. To study the body and the glaze thoroughly, FESEM analysis was applied to examine the cross section of the sample. Large diopside crystals can be found on the glaze surface, as shown in Fig. 12a. Between the body and the glaze, the crystals are too scarce to form the interlayer, as shown in Fig. 12b. The A-C regions in Fig. 12b are magnified for further study, as shown in Fig. 12c-e. In region A (Fig. 12c), the unreacted portion of the cover glaze with an interconnected phase separation structure forms. The characteristic size of the phase separation structure is 355 nm. All the incident visible light wavelengths are scattered by the structure, resulting in the opacification of the glaze. Region B is the bluish violet layer between the reaction zone and the
Fig. 8. OM image and the corresponding reflection spectra of the different regions of the surface of No.10 Jizhou “hare's fur” tea bowl.
while the bluish violet region is located at the edges of the white streaks, as shown in Fig. 7a. In some places, big flower-like crystals precipitate on the glaze surface. The diameter of the flower-like crystal in Fig. 7b is approximately 140 µm. Fig. 8 shows the reflectance spectra of the characteristic regions. Regions 1, 2, 3 and 4 correspond to the white region, two bluish violet regions, and the brown spot region, respectively. As shown in Fig. 8b, the reflectivity in white region is high in all wavelengths of the incident visible light. According to the peak of the spectra, which is 497 nm, the white region is in the blue hue but can't be detected by the naked eyes. The main reflection peaks of region 2 and 3 are 434 nm and 416 nm, corresponding to the blue light range. In the brown spot region, the reflectivity is low in all wavelengths of the incident visible light. To further study the various color regions, FESEM and TEM analysis were applied to the natural surface of the sample, as shown in Figs. 9, 10, respectively. Both the unreacted portion of the cover glaze and the bluish violet region are vitreous and phase-separated, showing phase separation structures with different sizes (Fig. 9b and c). It is obvious that the unreacted portion of the cover glaze has larger phase separation structures than does the bluish violet region. Based on the FESEM analysis, the unreacted portion of the cover glaze is found to be vitreous with no unmelted inclusions. The large size of the interconnected phase separation structure scatters all the
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(e)
Fig. 9. OM and FESEM images of the surface of No.10 Jizhou “hare's fur” tea bowl (a) OM image of the glaze(100 ×); (b) Interconnected phase separation structure in the unreacted portion of the cover glaze(10000 ×); (c) Interconnected phase separation structure in the bluish violet region(10000 ×); (d) Flower-like crystal on the surface(500 ×); (e) EDS spectrum of the crystal on the surface.
The formation temperature of the secondary mullite is approximately 1200 °C [6], indicating that the firing temperature of this sample is above 1200 °C. Sample No.11 is a sherd from the Jizhou “hare's fur” tea bowl with a ring foot and part of the belly, as shown in Fig. 13. A high-iron ground glaze was applied to the outside of the bowl. Inside, both the ground glaze and the cover glaze were applied to the gray body. This causes a white/bluish violet/yellow/brown interlaced “hare's fur” patter that is totally different from the “hare's fur” pattern of sample No.10 and sample No.19. An optical microscope was used to view the natural surface of
ground glaze region. An interconnected phase separation structure forms in region B (Fig. 12d) and the characteristic size is approximately 170 nm. This structure belongs to the amorphous photonic structure, leading to the selective scattering of incident visible light in the wavelengths of the blue light range. Region C (Fig. 12e) is the ground glaze region, which is close to the body. In this region, a small droplet phase separation structure forms, which indicates a low tendency towards immiscibility. As shown in Fig. 12f, large unmelted quartz particles and a large amount of the rod-like crystals are found in the body. The EDS analysis shows that the rod-like crystals are secondary mullites (3Al2O3:2SiO2).
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Fig. 10. TEM images of No.10 Jizhou “hare's fur” tea bowl. (a) Unreacted portion of the cover glaze(10000 ×); (b) Bluish violet region(10000 ×); (c) Brown spot region(12000 ×); (d) Ground glaze(25000 ×).
crystal is augite. FESEM results indicate that the white unreacted portion of the cover glaze and the bluish violet region are both vitreous and phase-separated and have phase separation structures of different sizes, as shown in Fig. 15b and c. In the unreacted portion of the cover glaze, a large interconnected phase separation structure forms. All wavelengths of the incident visible light are scattered by the structure, as shown in Fig. 16c, resulting in the opacification of the cover glaze. A small interconnected phase separation structure is found in the bluish violet region. The structure belongs to the amorphous photonic structure, leading to the selective scattering of the incident visible light in the wavelengths of the blue light range, as shown in Fig. 16c. To obtain the structural information about the entire glaze, FESEM analysis was applied to the cross section of the sample. As shown in Fig. 17a, no crystals occur within the glaze layer. Magnifying the A-C regions at different depths of the glaze, as shown in Fig. 17b-d, it is found that from the surface to the interior, the size of the phase separation structure is decreasing. As discussed in Part I of the study, the cover glaze has a strong phase separation tendency, while the ground glaze has smaller phase separation tendency due to the difference in chemical compositions. Diffusion between them causes variations in the local chemical compositions at different depths of the glaze, leading to a decrease of phase separation tendency from the surface to the interior. In Fig. 17e, the interlayer between the body and the glaze consisting of large flower-like crystals are well developed. A phase separation
Table 3 TEM-EDS results for the brown spot region of No.10 Jizhou “hare's fur” tea bowl. wt%
droplet matrix
MgO
Al2O3
SiO2
P2O5
K2O
CaO
TiO2
MnO
FeO
5.36 1.30
12.40 14.46
58.79 75.01
3.87
0.95 0.88
9.88 4.48
1.13
1.72 0.77
5.89 3.10
Table 4 TEM-EDS results for the ground glaze of No.10 Jizhou “hare's fur” tea bowl. wt%
droplet matrix
MgO
Al2O3
SiO2
P2O5
K2O
CaO
TiO2
MnO
FeO
6.86 1.63
11.52 13.00
44.51 76.13
4.25
1.3 1.86
14.37 4.44
2.09
2.78
12.32 2.93
sample No.11. In Fig. 14a, white/bluish violet/brown interlaced streaks are observed on the glaze surface, which result from the melting and diffusion between the cover glaze and the ground glaze. The yellow region is the surface crystallization region, as shown in Fig. 14b. Based on the micro-Raman analysis (Fig. 14c), the big yellow flower-like
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red circle, is proven to have the phase separation structure, as is shown in Fig. 19 [7]. This reminds us that apart from thermodynamics, kinetics is also important to the formation of phase separation structures. A firing circle includes heating, holding and cooling process. At a high temperature, the viscosity of the glaze decreases and the microstructure develops. The lower limit of this temperature refers to the glass transition temperature Tg. If the cooling speed is slow and the glaze is held above Tg long enough for the development of the microstructure, the phase separation structure can be formed in the glaze with a chemical composition outside the phase separation region. In that case, the real phase separation region is extended. As illustrated in Part I of the study, the recipe and the raw materials of the glaze are relatively consistent within a certain region during a short period of time. The cover glaze and the ground glaze are both calcium-alkali glazes, while the cover glaze has a higher CaO content and the ground glaze has a higher Fe2O3 content. The interaction and diffusion between the cover glaze and the ground glaze due to the compositional difference during firing result in the formation of the white unreacted portion of the cover glaze, the light brown reaction zone, the bluish violet structural color region and the brown spot region in almost all of the “hare's fur” samples, sharing the similar coloring mechanism. However, the long and hill-climbing dragon kiln widely used at the Jizhou kiln site has big differences in temperature and
Fig. 11. Micro-Raman spectrum of the flower-like crystal which precipitates on the surface of No.10 Jizhou “ hare's fur” tea bowl.
structure can be found within the interspaces of the crystals of the bodyglaze interaction layer, as shown in Fig. 17f. As discussed in Part I of the study, the cover glaze has a stronger phase separation tendency and easily forms the phase separation structure. As shown in Fig. 18, some of the ground glazes have phase separation tendency. However, the others are far from the phase separation region, and it is difficult to form the phase separation structure in thermodynamics. Interestingly, according to FESEM analysis, the farthest ground glaze from the phase separation region, marked with a
Fig. 12. FESEM images of the cross section of No.10 Jizhou “hare's fur” tea bowl (a) Flower-like crystals on the surface(300 ×); (b) Entire cross section of the glaze and part of the body(400 ×); (c) Magnification of region A(10000 ×); (d) Magnification of region B(25000 ×); (e) Magnification of region C(25000 ×); (f) Large unmelted quartz particles and a large amount of rod-like secondary mullites in the body(5000 ×).
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Fig. 13. No.11 Jizhou “hare's fur” tea bowl.
(c)
Fig. 14. OM images and micro-Raman spectrum of the surface of No.11 Jizhou “hare's fur” tea bowl (a) OM image of the glaze surface(100 ×); (b) Yellow flower-like crystals which precipitate on the surface(200 ×); (c) Micro-Raman spectrum of the yellow flower-like crystal.
atmosphere at different positions in the kiln. It is the fluctuation of thermal history that influence the flowing, diffusion and interaction of the glazes and bring about the changeful appearance of Jizhou “hare's fur” tea bowls, as shown in Fig. 1, Fig. 6 and Fig. 13.
compositional difference vary the local chemical composition, leading to the rich and changeable microstructures of the glaze, which is vital to glaze coloring. The liquid-liquid phase separation structure resulting from the immiscibility between CaO and SiO2 is widely observed in all regions of the Jizhou “hare's fur” tea bowls—the cover glaze region, reaction zone and ground glaze region—but the characteristic sizes of the phase separation structure are different in different regions. A large-scale interconnected phase separation structure is formed in
4. Conclusions Diffusion between the cover glaze and ground glaze due to the
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Fig. 15. OM and FESEM images of the surface of No.11 Jizhou “hare's fur” tea bowl (a) OM image of the glaze(100 ×); (b) Interconnected phase separation structure in the unreacted portion of the cover glaze(10000 ×); (c) Interconnected phase separation structure in the bluish violet region(10000 ×).
(c)
Fig. 16. OM images and the corresponding reflection spectra of the different regions of the surface of No.11 Jizhou “hare's fur” tea bowl. 11
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Fig. 17. FESEM images of the cross section of No.11 Jizhou “hare's fur” tea bowl (a) Entire cross section of the glaze and part of the body(250 ×); (b) Magnification of region A(3000 ×); (c) Magnification of region B(3000 ×); (d) Magnification of region C(3000 ×); (e) Well developed interlayer(3000 ×); (f) Phase separation structure within the interspaces of the crystals(20000 ×).
the unreacted portion of the cover glaze, and the characteristic size of the phase separation structure is approximately 350 nm. All wavelengths of the incident visible light are scattered by the structure, resulting in the opacification effect. In the ground glaze, small-scale droplets or worm-like phase separation structures are formed. The ground glaze has a dark color since it has a high content of Fe2O3, which absorbs most of the incident visible light. The cover glaze forms a thin area due to flowing. A small-sized interconnected phase separation structure is formed under the influence of the ground glaze, with a characteristic size of approximately 170 nm. The blue light range of the incident visible light is selectively scattered by the structure, causing the special bluish violet structural color on the glaze surface. The light brown region is created by the interaction between the cover glaze and the ground glaze, and the brown spots on surface arise from the “boiling effect” of the ground glaze.
Fig. 18. The glazes’ compositions on the (73.5 ± 3.5) mol% SiO2 plane of the K2O-CaO-Al2O3-SiO2 system [7].
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Acknowledgment This work was supported by the National Natural Science Foundation of China [General program, Grant no.51672302; Key program, Grant no. 51232008] References [1] F.K. Zhang, A study on Jizhou Temmoku, J. Jingdezhen Ceram. Inst. 7 (1986) 109–114. [2] J.Z. Li, A History of Science and Technology in China, Science Press, Beijing, 1998. [3] X.Q. Chen, R.F. Huang, S.P. Chen, Structural nature of Jizhou Temmoku, Hebei Ceram. (1985) 2–8. [4] H.W. Yin, Study on the Coloring Mechanism and Preparation of Structural Color (Doctoral dissertation), Fudan university, 2008. [5] W.D. Li, H.J. Luo, J.A. Li, J.Z. Li, J.K. Guo, Studies on the microstructure of the blackglazed bowl sherds excavated from the Jian kiln site of ancient China, Ceram. Int. 34 (2008) 1473–1480. [6] H.K. Schuller, Reactions between mullite and glassy phase in porcelains, Trans. Br. Ceram. Soc. 63 (1964) 103–117. [7] W.D. Kingery, P.B. Vandiver, I.W. Huang, Y.M. Chiang, Liquid-liquid immiscibility and phase separation in the quaternary systems K2O-Al2O3-CaO-SiO2 and Na2OAl2O3-CaO-SiO2, J. Non-Cryst. Solids 54 (1983) 163–171.
Fig. 19. Phase separation structure of the ground glaze of No.12 Jizhou “hare's fur” tea bowl (20000 ×).
Apart from thermodynamics, kinetics are also important to the formation of the phase separation structure and consequently influence the coloring. Due to the fluctuation of thermal history, the Jizhou “hare's fur” tea bowls produced by the same recipe and glazing technique shows the changeful patterns on surface.
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Unveiling the science behind the tea bowls from the Jizhou kiln. Part II. Microstructures and the coloring mechanism Changsong Xua,b,c,1, Weidong Lia,b, Jingkun Guoa
⁎,1
, Xiaoke Lua,b, Wenjiang Zhangd, Hongjie Luoe,
a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Key Scientific Research Base of Ancient Ceramics, State Administration for Cultural Heritage, Shanghai 200050, China c University of Chinese Academy of Sciences, Beijing 100049, China d Jiangxi Provincial Institute of Cultural Relics and Archaeology, Nanchang 330008, China e Shanghai University, Shanghai 200444, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Jizhou kiln Black-glazed tea bowl Microstructure Coloring mechanism Thermal history
In this part of the study, optical microscope, field emission scanning electron microscope-energy dispersive spectrometer, transmission electron microscope-selected area diffraction-energy dispersive spectrometer, microRaman and reflectance spectrum were applied to further study the microstructures of the characteristic areas of the glazes and illustrate the coloring mechanism of the “hare's fur” tea bowls with blue or bluish violet patterns. The results show that due to the diffusion between the cover glaze and ground glaze caused by compositional difference, as well as the “boiling effect” of the ground glaze, local chemical composition changes to form the different microstructures in different regions. Large sized interconnected phase separation structure with approximately 350 nm characteristic size forms in the unreacted portion of the cover glaze, leading to the scattering of all wavelengths of the incident visible light. With the contribution of the ground glaze which has low immiscibility tendency, small sized interconnected phase separation structure with approximately 170 nm characteristic size forms at the edge of the white region, leading to the scattering of the blue light range of the incident visible light. The fluctuation of thermal history gives various appearance to the Jizhou “hare's fur” tea bowls, although they share the same coloring mechanism. In general, chemical composition, microstructure and firing schedule cooperate with each other to create the changeful appearance of the Jizhou tea bowls.
1. Introduction The Jizhou kiln, located in present-day Ji’an county of Jiangxi Province, is one of the largest folk kilns in ancient China [1]. It started firing porcelain in the Tang dynasty (618–907 A.D.), became prosperous in the Southern Song dynasty (1127–1279 A.D.) and closed down at the end of the Yuan dynasty (1271–1368 A.D) [2]. The blackglazed tea bowls produced for tea tasting and competition in the Southern Song dynasty are among the most attractive porcelain wares produced [1]. The potters applied the two-layer glazing technique to bring about the “partridge feather”, “hare's fur”, “tortoise shell” and “tiger stripe”. Among them, “hare's fur” making full use of the flowing of the glaze in firing process forms the colorful and changeful patterns naturally, which is called “Yao Bian” in part I of the study. According to historical documents referred in part I of the study, the wares produced
by the same recipe and glazing technique created the diverse patterns naturally on surface is called “Yao Bian” in ancient times. However, in modern times, “Yao Bian” is referred to the wares with blue opalescence appearance like Jun wares. The original meaning of “Yao Bian” is used in this study, but more attention will be paid to the samples with blue streaks. In recent decades, a few researchers have extended their studies to examine these wares on a micro scale. Chen et al. [3] found the phase separation structures by using TEM on the opaque glaze area of the Jizhou tea bowls for the first time. The author inferred that the phase separation structures result from the physical and chemical reactions between the cover glaze and the ground glaze. Zhang [1] also reported on the interconnected phase separation structure of the Jizhou tea bowls without further discussion. Because of the limited testing technology, previous studies on the
⁎
Corresponding author at: Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. E-mail address: [email protected] (W. Li). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ceramint.2018.07.183 Received 18 February 2018; Received in revised form 20 July 2018; Accepted 20 July 2018 0272-8842/ © 2018 Published by Elsevier Ltd.
Please cite this article as: Xu, C., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.07.183
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Fig. 1. No.19 Jizhou “hare's fur” tea bowl.
Fig. 2. OM images of No.19 Jizhou “hare's fur” tea bowl (a) Surface of the sample; (b) Cross section of the sample; (c) Cross section of the white region marked in (a) (400 ×); (d) Cross section of the bluish violet region marked in (a)(500 ×).
2. Experimental
glaze microstructure were preliminary. The microstructure difference of the characteristic areas and the coloring mechanism are still unknown. Therefore, this part of the study will focus on the microstructures of the characteristic areas of the Jizhou tea bowl samples and the influence of the microstructure upon the coloring in order to illustrate the coloring mechanism of the “hare's fur”.
To further study the microstructures of the characteristic areas of the “hare's fur” tea bowls, 7 typical samples were selected for testing. The characteristic part of the sample was cut from the sherd by diamond saw and the cross section was polished by diamond powder. All
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The reflectance spectra, which describes the various colors of the glazes, were measured by a spectrum measuring system equipped with the metallographic optical microscope (Leica DM6000). The xenon lamp and halogen lamp were used in this measuring system and the diameter of the light spot was 2 µm. Nonlinear curve fit in Origin software was used to find the peak position of the spectra. The microstructures of the glaze surfaces and cross sections were studied using a field emission electron microscope (FEI Magellan 400) equipped with EDS. Samples were etched with 1% HF for 60 s. SEM images were processed by a 2D-Fourier transform programmed in MATLAB software to obtain the characteristic size of the phase separation structures. Since the defects on the natural surface of the glaze can largely influence the 2D-Fourier transform results, only the SEM images of the cross sections were used to calculate the characteristic size of the phase separation structures. A field emission transmission electron microscope (JEOL JEM2100F) was used to analyze the microstructures and the micro-areas of
Table 1 The thickness of each layer in the glaze of No.19 Jizhou “hare's fur” tea bowl. Layer
Thickness In white region
unreacted portion of the cover glaze bluish violet layer on surface light brown reaction layer ground glaze layer
In bluish violet region
~ 28 µm ~ 120 µm ~ 45 µm
~ 13 µm ~108 µm ~ 69 µm
samples were ultrasonically cleaned by water and ethanol, 3–5 times for 15 min. After that, all samples were dried in drying oven for 12 h. The photos of samples were taken by digital camera. The macroscale morphology of the glaze surfaces and cross sections were observed with an optical microscope (Keyence VHX-2000). The surface crystallization was analyzed by micro-Raman Spectrum (Horiba XploRA one 532 nm).
Table 2 The FESEM analysis results for the cross section of No.19 Jizhou “hare's fur” tea bowl.
In white region
In bluish violet region
Position (distance below the surface)
Corresponding layer
Microstructure
Characteristic size of the phase separation structure
20 µm (A) 60 µm (B) 220 µm (C) 5 µm (D) 50 µm (E) 200 µm (F)
unreacted portion of the cover glaze reaction zone ground glaze bluish violet region reaction zone ground glaze
Fig. Fig. Fig. Fig. Fig. Fig.
325 nm 301 nm
3a 3b 5a 3c 3d 5b
162 nm 178 nm
Fig. 3. FESEM images of the cross section of No.19 Jizhou “hare's fur” tea bowl (a) Unreacted portion of the cover glaze (region A in Fig. 2c) (20000 ×); (b) Reaction zone below the unreacted portion of the cover glaze (region B in Fig. 2c) (20000 ×); (c) Bluish violet region (region D in Fig. 2d) (20000 ×); (d) Reaction zone below the bluish violet region (region E in Fig. 2d) (20000 ×).
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a
glaze at a high temperature to achieve the natural and unique patterns. To demonstrate the formation regularity and the coloring mechanism of the patterns, we use 3 “hare's fur” samples (No. 10, No.11 and No. 19) as examples. More information of samples can be found in part I of the study. Sample No.19 is a sherd of the Jizhou “hare's fur” tea bowl, as shown in Fig. 1. A single ground glaze was applied to the outside of the bowl, and two glazes were applied to the inside of the bowl. In Fig. 2a, the OM images corresponding to the area in the red box in Fig. 1a shows the white/bluish violet interlaced “hare's fur” formed on the inner surface, and blue or bluish violet colors were created at the edges of the white streaks. According to Fig. 2b, the dark gray coarse body has pores, large quartz particles and impurity inclusions. The cross section of the white region marked in Fig. 2a is shown in Fig. 2c. The very thin white layer at the top of the cover glaze does not react with the ground glaze. The light brown opaque region (referred to as reaction zone), which results from the interaction and diffusion between the cover glaze and the ground glaze during firing, accounts for the largest proportion of the cross section. The reaction zone is irregular and wavy. A blue or bluish violet color also appears at the boundary between the reaction zone and the ground glaze. According to the cross section of the bluish violet region, as shown in Fig. 2d, the bluish violet layer on the surface is much thinner than the unreacted portion of the cover glaze, and the reaction zone is also wavy. The bluish violet layer is observed again between the reaction zone and the ground glaze. Based on the observation of the cross sections, the white region and the bluish violet region on the glaze surface are quite thin. The reaction zone and the ground glaze occupy the largest area of the cross section of the glaze. A blue or bluish violet color usually appears between the reaction zone and the ground glaze. Like the glaze surface, the cross section of the glaze is also full of color and changes. The diverse colors in the glaze indicate the variations of the local chemical composition, which leads to the differences in microstructure. The thickness of each layer is summarized in Table 1. The same piece of the sample was analyzed by FESEM to study the microstructures of the cross sections. The result of the FESEM analysis is summarized in Table 2. According to the FESEM analysis, the unreacted portion of the cover glaze is vitreous, with no unmelted inclusions. As discussed in part I of the article, the cover glaze has a strong tendency towards phase separation, and it has large sized interconnected phase separation structures, as shown in Fig. 3a. All wavelengths of the incident visible light are scattered by the structure, as shown in Fig. 4b, resulting in the opacification of the cover glaze. With a low Fe2O3 content, the unreacted portion of the cover glaze appears matt and white.
b
Fig. 4. OM image and the corresponding reflection spectra of the different regions of the surface of No.19 Jizhou “hare's fur” tea bowl.
the chemical compositions. Powders were scraped from the different color areas of the glazes for the TEM samples. 3. Results and discussion Jizhou “hare's fur” tea bowls make full use of the flowing of the
Fig. 5. FESEM images of the cross section of No.19 Jizhou “hare's fur” tea bowl (a) Ground glaze below the unreacted portion of the cover glaze (region C in Fig. 2c) (100000 ×); (b) Ground glaze below the bluish violet region (region F in Fig. 2d) (100000 ×).
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Fig. 6. No.10 Jizhou “hare's fur” tea bowl.
Fig. 7. OM images of No.10 Jizhou “hare's fur” tea bowl (a) White/bluish violet/brown interlaced glaze(100 ×); (b) Big flower-like crystal which precipitates on the glaze surface(900 ×).
dramatically due to the influence of the ground glaze. Similarly, when the cover glaze is thin enough, after interacting with the ground glaze, the local phase separation tendency dramatically decreases to form the bluish violet region on the surface. However, the influence of the high content of Fe2O3 in the ground glaze on the reaction zone is prominent. Iron ions diffuse into the reaction zone during firing to the direction of the concentration gradient, leading to the partial absorption of the incident visible light and making the reaction zone appear light brown. According to Fig. 3d, with a smaller phase separation structure size, the opacification degree of the reaction zone beneath the bluish violet region is lower than that of the region beneath the unreacted portion of the cover glaze. Sample No.10 is a sherd of the Jizhou “hare's fur” tea bowl, with a ring foot and part of the belly, as shown in Fig. 6. A high-iron ground glaze was applied to the outer wall of the bowl. Inside, the ground glaze and the cover glaze were applied on the light gray body. During firing, the white/bluish violet/brown interlaced “hare's fur” pattern with natural variations was created on surface. Irregular brown spots are distributed between the hare's furs. The closer to the bottom, the smaller the brown spots. Using the optical microscope, it is observed that the unreacted portion of the cover glaze is matt, white and opaque,
As discussed in Part I of the study, the tendency towards immiscibility of the ground glaze is smaller than that of the cover glaze. Small worm-like phase separation structures form in the ground glaze, as shown in Fig. 5. However, the influence of this structure on the coloring can be ignored. The high content of Fe2O3, which absorbs most of the incident visible light, as shown in Fig. 4b, leads to the dark color of the ground glaze. The bluish violet region is the result of the interaction and diffusion between the ground glaze and the cover glaze during firing. Small size of the interconnected phase separation structure (Fig. 3c) in this region forms the amorphous photonic structure [4], which leads to the selective scattering of the blue light range of the incident visible light, as shown in Fig. 4b. With no cobalt in the glaze, it is confirmed that the bluish violet in the sample is a special structural color. An interconnected phase separation structure (Fig. 3b) forms in the light brown reaction zone, with a similar size to that of the unreacted portion of the cover glaze. This phenomenon indicates that the ground glaze has little influence on the microstructure formation of the reaction zone. While the bluish violet region, which has small sized phase separation structures, appears between the reaction zone and the ground glaze, where the phase separation tendency decreases
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wavelengths of the incident visible light, as shown in Fig. 8b, resulting in the opacification of the glaze. With a low Fe2O3 content, the unreacted portion of the cover glaze is matte, white and opaque. However, at a high temperature, the viscosity of the cover glaze decreases; the cover glaze flows and forms a thin area. Due to the diffusion with the ground glaze, the phase separation tendency of the thin cover glaze region is reduced, and a small phase separation structure is formed. The structure selectively scatters the incident visible light, as shown in Fig. 8b, leading to the bluish violet color on surface. Since both the thin cover glaze and the proper firing schedule are necessary to form the bluish violet streaks, this kind of sample is not common. According to the TEM analysis of the unreacted portion of the cover glaze and the bluish violet region, as shown in Fig. 10a and b, the interconnected 3D network phase separation structures with different sizes are observed visually, which is consistent with the result of the FESEM analysis. The brown spots on the glaze surface are attributed to the “boiling effect” of the ground glaze. The relatively high content of Fe2O3 in the ground glaze pyrolyzes to generate bubbles above 1230 °C during firing [5]. The bubbles rise and drag the ground glaze to the glaze surface. Releasing of the bubbles leaves brown spots on the glaze surface between the white/bluish violet interlaced “hare's fur.” The high content of Fe2O3 in this region absorbs most of the incident visible light, as shown in Fig. 8b, resulting in the low reflectivity and the dark color. In this region, a droplet phase separation structure is observed by TEM analysis, as shown in Fig. 10c. The result of SAD is a dispersed diffraction ring, indicating the vitreous state of the brown spot region. According to the EDS analysis, as shown in Table 3, the droplet is rich in CaO and the matrix is rich in SiO2. Based on the glass chemistry, the strong immiscibility between SiO2 and CaO results in the phase separation in the glaze. Other modifiers such as Fe, Mg, P, Ti ions locate preferably in the phase enriched by CaO, consequently favoring phase separation. A droplet phase separation structure is also observed in the ground glaze, as shown in Fig. 10d. However, the size of the phase separation structure is much smaller than that in the brown spot region because of the difference in chemical composition. As shown in Table 4, the droplet is rich in CaO and the matrix is rich in SiO2, similar to the above result. In the white unreacted portion of the cover glaze region, big flowerlike crystals consisting of outward radial growth dendritic crystals precipitate on the surface, as shown in Fig. 9d. Based on the EDS analysis (Fig. 9e) and the micro-Raman spectrum (Fig. 11), the flowerlike crystal is diopside. However, according to the Part I of the study, MgO content in the cover glaze is not high enough to cause diopside crystallization, which indicates that MgO is heterogeneous in the cover glaze. According to EDS analysis, Mg and Ca are enriched in the same phase. Therefore, it is inferred that the phase separation in the cover glaze promotes the nucleation and the crystallization of diopside. Diopside crystals precipitate from the Ca (Mg) -enriched phase, consuming CaO and MgO, which leads to a significant decrease in the local phase separation tendency around the precipitation position. To study the body and the glaze thoroughly, FESEM analysis was applied to examine the cross section of the sample. Large diopside crystals can be found on the glaze surface, as shown in Fig. 12a. Between the body and the glaze, the crystals are too scarce to form the interlayer, as shown in Fig. 12b. The A-C regions in Fig. 12b are magnified for further study, as shown in Fig. 12c-e. In region A (Fig. 12c), the unreacted portion of the cover glaze with an interconnected phase separation structure forms. The characteristic size of the phase separation structure is 355 nm. All the incident visible light wavelengths are scattered by the structure, resulting in the opacification of the glaze. Region B is the bluish violet layer between the reaction zone and the
Fig. 8. OM image and the corresponding reflection spectra of the different regions of the surface of No.10 Jizhou “hare's fur” tea bowl.
while the bluish violet region is located at the edges of the white streaks, as shown in Fig. 7a. In some places, big flower-like crystals precipitate on the glaze surface. The diameter of the flower-like crystal in Fig. 7b is approximately 140 µm. Fig. 8 shows the reflectance spectra of the characteristic regions. Regions 1, 2, 3 and 4 correspond to the white region, two bluish violet regions, and the brown spot region, respectively. As shown in Fig. 8b, the reflectivity in white region is high in all wavelengths of the incident visible light. According to the peak of the spectra, which is 497 nm, the white region is in the blue hue but can't be detected by the naked eyes. The main reflection peaks of region 2 and 3 are 434 nm and 416 nm, corresponding to the blue light range. In the brown spot region, the reflectivity is low in all wavelengths of the incident visible light. To further study the various color regions, FESEM and TEM analysis were applied to the natural surface of the sample, as shown in Figs. 9, 10, respectively. Both the unreacted portion of the cover glaze and the bluish violet region are vitreous and phase-separated, showing phase separation structures with different sizes (Fig. 9b and c). It is obvious that the unreacted portion of the cover glaze has larger phase separation structures than does the bluish violet region. Based on the FESEM analysis, the unreacted portion of the cover glaze is found to be vitreous with no unmelted inclusions. The large size of the interconnected phase separation structure scatters all the
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Fig. 9. OM and FESEM images of the surface of No.10 Jizhou “hare's fur” tea bowl (a) OM image of the glaze(100 ×); (b) Interconnected phase separation structure in the unreacted portion of the cover glaze(10000 ×); (c) Interconnected phase separation structure in the bluish violet region(10000 ×); (d) Flower-like crystal on the surface(500 ×); (e) EDS spectrum of the crystal on the surface.
The formation temperature of the secondary mullite is approximately 1200 °C [6], indicating that the firing temperature of this sample is above 1200 °C. Sample No.11 is a sherd from the Jizhou “hare's fur” tea bowl with a ring foot and part of the belly, as shown in Fig. 13. A high-iron ground glaze was applied to the outside of the bowl. Inside, both the ground glaze and the cover glaze were applied to the gray body. This causes a white/bluish violet/yellow/brown interlaced “hare's fur” patter that is totally different from the “hare's fur” pattern of sample No.10 and sample No.19. An optical microscope was used to view the natural surface of
ground glaze region. An interconnected phase separation structure forms in region B (Fig. 12d) and the characteristic size is approximately 170 nm. This structure belongs to the amorphous photonic structure, leading to the selective scattering of incident visible light in the wavelengths of the blue light range. Region C (Fig. 12e) is the ground glaze region, which is close to the body. In this region, a small droplet phase separation structure forms, which indicates a low tendency towards immiscibility. As shown in Fig. 12f, large unmelted quartz particles and a large amount of the rod-like crystals are found in the body. The EDS analysis shows that the rod-like crystals are secondary mullites (3Al2O3:2SiO2).
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Fig. 10. TEM images of No.10 Jizhou “hare's fur” tea bowl. (a) Unreacted portion of the cover glaze(10000 ×); (b) Bluish violet region(10000 ×); (c) Brown spot region(12000 ×); (d) Ground glaze(25000 ×).
crystal is augite. FESEM results indicate that the white unreacted portion of the cover glaze and the bluish violet region are both vitreous and phase-separated and have phase separation structures of different sizes, as shown in Fig. 15b and c. In the unreacted portion of the cover glaze, a large interconnected phase separation structure forms. All wavelengths of the incident visible light are scattered by the structure, as shown in Fig. 16c, resulting in the opacification of the cover glaze. A small interconnected phase separation structure is found in the bluish violet region. The structure belongs to the amorphous photonic structure, leading to the selective scattering of the incident visible light in the wavelengths of the blue light range, as shown in Fig. 16c. To obtain the structural information about the entire glaze, FESEM analysis was applied to the cross section of the sample. As shown in Fig. 17a, no crystals occur within the glaze layer. Magnifying the A-C regions at different depths of the glaze, as shown in Fig. 17b-d, it is found that from the surface to the interior, the size of the phase separation structure is decreasing. As discussed in Part I of the study, the cover glaze has a strong phase separation tendency, while the ground glaze has smaller phase separation tendency due to the difference in chemical compositions. Diffusion between them causes variations in the local chemical compositions at different depths of the glaze, leading to a decrease of phase separation tendency from the surface to the interior. In Fig. 17e, the interlayer between the body and the glaze consisting of large flower-like crystals are well developed. A phase separation
Table 3 TEM-EDS results for the brown spot region of No.10 Jizhou “hare's fur” tea bowl. wt%
droplet matrix
MgO
Al2O3
SiO2
P2O5
K2O
CaO
TiO2
MnO
FeO
5.36 1.30
12.40 14.46
58.79 75.01
3.87
0.95 0.88
9.88 4.48
1.13
1.72 0.77
5.89 3.10
Table 4 TEM-EDS results for the ground glaze of No.10 Jizhou “hare's fur” tea bowl. wt%
droplet matrix
MgO
Al2O3
SiO2
P2O5
K2O
CaO
TiO2
MnO
FeO
6.86 1.63
11.52 13.00
44.51 76.13
4.25
1.3 1.86
14.37 4.44
2.09
2.78
12.32 2.93
sample No.11. In Fig. 14a, white/bluish violet/brown interlaced streaks are observed on the glaze surface, which result from the melting and diffusion between the cover glaze and the ground glaze. The yellow region is the surface crystallization region, as shown in Fig. 14b. Based on the micro-Raman analysis (Fig. 14c), the big yellow flower-like
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red circle, is proven to have the phase separation structure, as is shown in Fig. 19 [7]. This reminds us that apart from thermodynamics, kinetics is also important to the formation of phase separation structures. A firing circle includes heating, holding and cooling process. At a high temperature, the viscosity of the glaze decreases and the microstructure develops. The lower limit of this temperature refers to the glass transition temperature Tg. If the cooling speed is slow and the glaze is held above Tg long enough for the development of the microstructure, the phase separation structure can be formed in the glaze with a chemical composition outside the phase separation region. In that case, the real phase separation region is extended. As illustrated in Part I of the study, the recipe and the raw materials of the glaze are relatively consistent within a certain region during a short period of time. The cover glaze and the ground glaze are both calcium-alkali glazes, while the cover glaze has a higher CaO content and the ground glaze has a higher Fe2O3 content. The interaction and diffusion between the cover glaze and the ground glaze due to the compositional difference during firing result in the formation of the white unreacted portion of the cover glaze, the light brown reaction zone, the bluish violet structural color region and the brown spot region in almost all of the “hare's fur” samples, sharing the similar coloring mechanism. However, the long and hill-climbing dragon kiln widely used at the Jizhou kiln site has big differences in temperature and
Fig. 11. Micro-Raman spectrum of the flower-like crystal which precipitates on the surface of No.10 Jizhou “ hare's fur” tea bowl.
structure can be found within the interspaces of the crystals of the bodyglaze interaction layer, as shown in Fig. 17f. As discussed in Part I of the study, the cover glaze has a stronger phase separation tendency and easily forms the phase separation structure. As shown in Fig. 18, some of the ground glazes have phase separation tendency. However, the others are far from the phase separation region, and it is difficult to form the phase separation structure in thermodynamics. Interestingly, according to FESEM analysis, the farthest ground glaze from the phase separation region, marked with a
Fig. 12. FESEM images of the cross section of No.10 Jizhou “hare's fur” tea bowl (a) Flower-like crystals on the surface(300 ×); (b) Entire cross section of the glaze and part of the body(400 ×); (c) Magnification of region A(10000 ×); (d) Magnification of region B(25000 ×); (e) Magnification of region C(25000 ×); (f) Large unmelted quartz particles and a large amount of rod-like secondary mullites in the body(5000 ×).
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Fig. 13. No.11 Jizhou “hare's fur” tea bowl.
(c)
Fig. 14. OM images and micro-Raman spectrum of the surface of No.11 Jizhou “hare's fur” tea bowl (a) OM image of the glaze surface(100 ×); (b) Yellow flower-like crystals which precipitate on the surface(200 ×); (c) Micro-Raman spectrum of the yellow flower-like crystal.
atmosphere at different positions in the kiln. It is the fluctuation of thermal history that influence the flowing, diffusion and interaction of the glazes and bring about the changeful appearance of Jizhou “hare's fur” tea bowls, as shown in Fig. 1, Fig. 6 and Fig. 13.
compositional difference vary the local chemical composition, leading to the rich and changeable microstructures of the glaze, which is vital to glaze coloring. The liquid-liquid phase separation structure resulting from the immiscibility between CaO and SiO2 is widely observed in all regions of the Jizhou “hare's fur” tea bowls—the cover glaze region, reaction zone and ground glaze region—but the characteristic sizes of the phase separation structure are different in different regions. A large-scale interconnected phase separation structure is formed in
4. Conclusions Diffusion between the cover glaze and ground glaze due to the
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Fig. 15. OM and FESEM images of the surface of No.11 Jizhou “hare's fur” tea bowl (a) OM image of the glaze(100 ×); (b) Interconnected phase separation structure in the unreacted portion of the cover glaze(10000 ×); (c) Interconnected phase separation structure in the bluish violet region(10000 ×).
(c)
Fig. 16. OM images and the corresponding reflection spectra of the different regions of the surface of No.11 Jizhou “hare's fur” tea bowl. 11
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Fig. 17. FESEM images of the cross section of No.11 Jizhou “hare's fur” tea bowl (a) Entire cross section of the glaze and part of the body(250 ×); (b) Magnification of region A(3000 ×); (c) Magnification of region B(3000 ×); (d) Magnification of region C(3000 ×); (e) Well developed interlayer(3000 ×); (f) Phase separation structure within the interspaces of the crystals(20000 ×).
the unreacted portion of the cover glaze, and the characteristic size of the phase separation structure is approximately 350 nm. All wavelengths of the incident visible light are scattered by the structure, resulting in the opacification effect. In the ground glaze, small-scale droplets or worm-like phase separation structures are formed. The ground glaze has a dark color since it has a high content of Fe2O3, which absorbs most of the incident visible light. The cover glaze forms a thin area due to flowing. A small-sized interconnected phase separation structure is formed under the influence of the ground glaze, with a characteristic size of approximately 170 nm. The blue light range of the incident visible light is selectively scattered by the structure, causing the special bluish violet structural color on the glaze surface. The light brown region is created by the interaction between the cover glaze and the ground glaze, and the brown spots on surface arise from the “boiling effect” of the ground glaze.
Fig. 18. The glazes’ compositions on the (73.5 ± 3.5) mol% SiO2 plane of the K2O-CaO-Al2O3-SiO2 system [7].
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Acknowledgment This work was supported by the National Natural Science Foundation of China [General program, Grant no.51672302; Key program, Grant no. 51232008] References [1] F.K. Zhang, A study on Jizhou Temmoku, J. Jingdezhen Ceram. Inst. 7 (1986) 109–114. [2] J.Z. Li, A History of Science and Technology in China, Science Press, Beijing, 1998. [3] X.Q. Chen, R.F. Huang, S.P. Chen, Structural nature of Jizhou Temmoku, Hebei Ceram. (1985) 2–8. [4] H.W. Yin, Study on the Coloring Mechanism and Preparation of Structural Color (Doctoral dissertation), Fudan university, 2008. [5] W.D. Li, H.J. Luo, J.A. Li, J.Z. Li, J.K. Guo, Studies on the microstructure of the blackglazed bowl sherds excavated from the Jian kiln site of ancient China, Ceram. Int. 34 (2008) 1473–1480. [6] H.K. Schuller, Reactions between mullite and glassy phase in porcelains, Trans. Br. Ceram. Soc. 63 (1964) 103–117. [7] W.D. Kingery, P.B. Vandiver, I.W. Huang, Y.M. Chiang, Liquid-liquid immiscibility and phase separation in the quaternary systems K2O-Al2O3-CaO-SiO2 and Na2OAl2O3-CaO-SiO2, J. Non-Cryst. Solids 54 (1983) 163–171.
Fig. 19. Phase separation structure of the ground glaze of No.12 Jizhou “hare's fur” tea bowl (20000 ×).
Apart from thermodynamics, kinetics are also important to the formation of the phase separation structure and consequently influence the coloring. Due to the fluctuation of thermal history, the Jizhou “hare's fur” tea bowls produced by the same recipe and glazing technique shows the changeful patterns on surface.
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