Alchemy in the Art of Traditional Japanese Ceramics: Microstructure

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Alchemy in the Art of Traditional Japanese Ceramics: Microstructure and Formation Mechanism of Gold-Colored Bizen Stoneware Yoshihiro Kusano, Minoru Fukuhara, Taichi Fujino, Tatsuo Fujii, Mikio Takano, and Jun Takada Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00368 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Crystal Growth & Design

Alchemy in the Art of Traditional Japanese Ceramics: Microstructure and Formation Mechanism of Gold-Colored Bizen Stoneware Yoshihiro Kusano,1,* Minoru Fukuhara,1 Taichi Fujino,1 Tatsuo Fujii,2 Jun Takada,2 and Mikio Takano3 1

Department of Applied Chemistry and Biotechnology, Okayama University of Science, 1-1 Ridai-cho,

Kita-ku, Okayama 700-0005, Japan 2

Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku,

Okayama 700-8530, Japan 3

Research Institute for Production Development, 15 Shimogamo Morimoto-cho, Sakyo-ku, Kyoto, 606-0805

Japan *To whom correspondence should be addressed. Tel & Fax: +81 86 256 9827, E-mail: [email protected]

1

Okayama University of Science

2

Okayama University

3

Research Institute for Production Development

ABSTRACT The microstructure and formation process of the golden color on traditional Japanese Bizen stoneware was investigated through model experiments. The current compositional and structural research of pottery 1 ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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fragments has revealed that the golden color comes from Fe oxide consisting of approximately 100 nm thick agglomerates of Al-substituted hematite (α-(Fe1-xAlx)2O3, x ≈ 0.05). The color is reproducible in the laboratory by sequential heat treatments of Bizen clay pellets under oxidizing and reducing atmospheres with an amount of potassium supplied as a melting point depressant. Lustrous colors such as silver and gold in Bizen stoneware have generally been attributed to the optical interference in superficial carbon films produced by burning wood fuel. Here, we show that the golden color is caused by the formation of Alsubstituted hematite, not by the formation of carbon.

KEYWORDS: Bizen stoneware, Gold color, Hematite, Al-substitution, Microstructure

Introduction Bizen stoneware is one of the most traditional Japanese ceramics, and it has been used for over 1000 years. Colors such as red, orange, purple, yellow, black, silver, and gold appear in various forms of Bizen stoneware without the aid of artificial glazing or pigments. For this reason, Bizen stoneware has been referred to as an “art of clay and flame.” Bizen bowls and vases have been frequently used in Japanese tea ceremonies because these are subdued in appearance and thus conform to a pair of elemental Japanese aesthetic concepts: wabi (an aesthetic sense from richness and beauty of simplicity) and sabi (an aesthetic sense from rustic simplicity and loneliness). This is why the great tea master Sen no Rikyu (1522–1591) preferentially used Bizen stoneware as part of his tea ceremony. In the course of our recent work, we have succeeded in elucidating the mechanism of the red striped pattern known as hidasuki, and replicated the process.1-4 Hidasuki appears when a shaped clay is strapped with rice straw and fired in an oxidizing atmosphere such as air. The surface area in contact with the rice straw becomes molten to a typical depth of 50 µm when heated to 1250 °C due to the flux effects of potassium supplied from the rice straw. Upon cooling, colorless, platy corundum (α-Al2O3) crystals are firstly deposited, around which hematite (α-Fe2O3) crystals precipitate and grow while maintaining an 2 ACS Paragon Plus Environment

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epitaxial relationship with the mother corundum crystals.1-4 The reddish color of the hematite crystals contrasts with the light yellowish color of Fe-substituted mullite [(Al1−xFex)6Si2O13; x ≤ 0.1] that dominates the straw-free areas.1,5,6 As metallic sheen coatings on ceramics, precious metal paints such as gold, platinum and silver can be enameled on the surfaces of glazed pottery or porcelain.7 Without the use of precious metal paints, glazes that contain silver and copper can be enameled on white glazed surfaces, which appear as metallic sheen on their surfaces due to the formation of metal nanoparticles of silver and copper.8 Among the Bizen colors, gold appears almost accidentally on the surface of the stoneware when a straw-covered clay form is fired in the strongly reducing atmosphere of a wood-fired kiln. However, the resulting phases and coloring mechanisms have remained unclear for centuries. Herein, we report the results of a compositional and structural study on gold-colored fragments provided by a Bizen potter and demonstrate how this color can be reproduced in the laboratory.

Experimental Procedure The overall cross-sectional structure of the colored surface of the potter’s fragments was observed using scanning transmission electron microscopy (STEM, JEOL JEM-2800) with energy-dispersive X-ray spectroscopy (EDS) after thinning by Ar ion milling at room temperature. The crystalline phases formed were identified using powder X-ray diffraction (XRD, Bruker D2 PHASER) with Cu Kα radiation and transmission electron microscopy (TEM, JEOL JEM-2800) together with EDS. Further detailed TEM studies were conducted on crystals chemically isolated from glassy matrices by acid treatment with 47% hydrofluoric acid (HF) for 15-60 s, which were then dispersed in carbon tetrachloride (CCl4), and placed on microgrids. The clay used in the reproduction experiments was the same Bizen clay as that used since the beginning of our ongoing research.3 The chemical composition of the clay is given Table 1. Pellets with a diameter of 20 mm were formed from the dried Bizen clay powder (2.00g) and heated in an alumina crucible to 1230 °C at a 3 ACS Paragon Plus Environment


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rate of 2.5 °C/min in air, with or without laterally surrounding the pellets with powdered potassium carbonate (K2CO3 ≥ 99.5%, 0.03g). The atmosphere was then switched to a strongly reducing mixed gas of 10 vol% carbon monoxide (CO) and 90 vol% argon (Ar), in which the pellets were held at the same temperature for 6 h and subsequently cooled to 900 °C at a rate of 1.5 °C/min. The atmosphere was then switched back to oxidizing and the pellets were held at this temperature for 2 h followed by cooling to room temperature. The resulting surface colors were measured using a colorimeter (Konica Minolta, CM-2600d) with a standard illuminant (D65).

Results and Discussion Figure 1 shows a rabbit-shaped ware produced by Yoriaki Matsumoto, a contemporary master Bizen potter following the traditional technique. The gold colored surface appeared where the shaped clay was in contact with straw. The clay and rice straw was fired in a climbing kiln with red pine wood used as fuel. Firing in a strongly reducing atmosphere generated by burning fuel is essential for lustrous coloring; therefore, it is reasonable to assign the luster to optical interference within carbon films deposited on the stoneware surface. From this standpoint, Yamaguchi et al. performed carbon deposition experiments on glass substrates to clarify the relation between film thickness and color, and estimated that a carbon film thickness of 38 nm results in gold coloring.9 The phase identification of carbon was not indicated in the paper.9 However, in the present study, such carbonaceous materials were not detected. Figure 2a shows a crosssectional STEM image of a gold-colored fragment provided by the potter. The distributions of the most abundant cationic elements, Fe, Al, Si, and K, are shown in Figs. 2b–2e. The structure can be described as a 2 µm thick K- and Si-dominant glass covered with an Fe-enriched top layer and with Al-enriched solids formed at the bottom layer. The K- and Si-oxide glass is formed because the rice straw provides an amount of potassium.1,3 The stick-like crystals at the bottom layer are Fe-substituted mullite [(Al1-xFex)6Si2O13], which appears widely in traditional ceramics.1 The top Fe-enriched layer is approximately 100 nm thick and is crystalline, as revealed by the spots of electron diffraction (ED) patterns, such as that shown in the inset of 4 ACS Paragon Plus Environment

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Fig. 2a. Based on comparison with the crystallographic data for known Fe oxides, the ED pattern was assigned to the[11 2 0] zone axis of hematite. This oxide has a trigonal unit cell of the R 3 c space group with a = 0.5036 nm and c = 1.3749 nm in the hexagonal setting (PDF # 33-0664). Hematite can form a solid solution, expressed as α-(Fe1-xAlx)2O3, x ≤ 0.09) at 1100 °C.9 Crystalline hematite formed on gold colored Bizen also contains approximately 5 mol% Al, which was measured by EDS analysis (Figure S1). Alsubstituted hematite hereafter is referred to as hematite for the sake of simplicity. Figure 3 shows a planeview TEM image of a fragment of the top layer, and an ED pattern (inset of Fig. 3) taken from this location indicates that the leaf-shaped fragment is composed of platy hematite crystals agglomerated with their (0001) planes (c-planes) parallel to the leaf surface. According to master Bizen potters, covering with rice straw and cooling in a reducing atmosphere after heating in an oxidizing atmosphere of air at approximately 1230 °C are crucially important to achieve the gold coloration. Bizen clay was then heated with K2CO3, which is more easily handled than rice straw, to 1230 °C in air, held at the same temperature for 6 h in a gas mixture of 10% CO and 90% Ar, and subsequently cooled to room temperature in the same gas mixture. Consequently, a transparent bluish gray vitreous luster was observed on the sample surface. Figure 2 shows that the final ware contains iron in an oxidized state, Fe3+, rather than in a reduced state such as Fe2+ or Fe0, which reveals that thermal treatment in an oxidizing atmosphere is required for this coloring. Accordingly, the clay pellets with K2CO3 were annealed at several temperatures in air on cooling process after heating at 1230 °C in the gas mixture to obtain the gold color in the laboratory. Figure 4 shows a pair of clay pellets with different colors. Both these pellets were surrounded with K2CO3 and heated to 1230 °C in air at a rate of 2.5 °C/min, held at the same temperature for 6 h in the CO/Ar gas mixture, and subsequently cooled to 900 °C in the same atmosphere at a rate of 1.5 °C/min. The color changed depending on the final holding and cooling treatment. The sample shown in Fig. 4a (Sample A) was quenched to room temperature, whereas Sample B in Fig. 4b was annealed at 900 °C for 2 h in an oxidizing atmosphere of air and then cooled to room temperature in air. Sample A appeared lustrous and bluish

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gray, whereas Sample B was lustrous and golden (L* = 61.4, a* = 6.1, and b* = 20.1 in the CIE 1976 L*a*b* color space). The color became reddish when held at 900 °C in air for more than 2 h or at temperatures higher than 900 °C in air. Cross-sectional high-angle annular dark field–STEM (HAADF–STEM) images and EDS maps of Fe are shown in Figs. 4c and 4d for Sample A and in Figs. 4e and 4f for Sample B. For both Samples A and B, Fe was concentrated in the top layers; however, the degree of concentration was considerably higher for B than for A. The diffuse diffraction pattern shown in the inset of Fig. 4c indicated poor crystallinity, although basic structural information could be obtained by TEM measurements. The upper and lower insets in Fig. 4d show a magnified TEM image and its fast Fourier transform (FFT), respectively. The FFT pattern reveals that the particle has hexagonal symmetry and a lattice spacing of d = 0.20 nm, which is consistent with the [111] zone axis view with d = 0.203 nm for α-Fe and indicates that the top layer consists of nanoparticles of α-Fe due to the use of a strongly reducing atmosphere. On the other hand, the spotty diffraction pattern for Sample B shown in the inset of Fig. 4e indicates the [ 2 20 1 ] zone axis of hematite. In comparison with the case of the [11 2 0] zone axis in Fig. 2, the c-axis is tilted by 90° to set the c-plane perpendicular to the sample surface. This result was confirmed using Mössbauer spectroscopy (Figure S2).11 Here we discuss the difference in the crystal orientation between the hematite layers in Figs. 2a and 4e. Figure 5 shows a plane-view STEM-secondary electron image (STEM-SEI) of a fragment of the layer shown in Fig. 4e. The inset ED pattern in Fig. 5 indicates the [11 2 0] zone axis of hematite, which is the overhead view of the [ 2 20 1 ] zone axis shown as the inset in Fig. 4e. To clarify the crystal orientation, schematic illustrations of the crystal structure of hematite based on the ED pattern in Fig. 4e are shown in Figure S3. The hematite platelets in Fig. 5 were ca. 300 nm wide and ca. 19 nm thick, and grew perpendicularly from the fragment. In the liquid phase formed by the reaction between the clay and potassium, hematite precipitates as hexagonal platelets, of which the thickness corresponds to the c-axis direction, with growth in the in-plane direction.1,3 The thickness direction of these platelets is also the c-axis of hematite (Figure S4). Careful observation of the image in Fig. 4e revealed the platelets indicated by the arrowheads. The crystal 6 ACS Paragon Plus Environment

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orientation of hematite is likely to be dependent on the thickness of the liquid layer on the top surface [e.g. 2

µm in depth (Fig. 2a) and more than 5 µm for the sample in Fig. 4e)]. Therefore, we attempted to prepare a clay pellet by reaction with a small amount of K2CO3 (0.02 g). The thickness of the glass layer of the sample was estimated to be ca. 3 µm (Figure S5a). The plane-view TEM image of hematite crystals formed on the sample surface showed the same textural feature as that shown in Fig. 3, and the ED pattern indicated the same [0001] zone axis of hematite as that shown in Fig. 3 (inset in Figure S5f). Taking these results into account, it was concluded that when the liquid layer on the sample surface is thin, the c-plane of hematite is parallel to the sample surface. However, when the liquid layer is thick, the c-plane is perpendicular to the sample surface. Although the c-axis directions of hematite were different, the thicknesses of both hematite layers shown in Figs. 2a and 4e were identical (ca. 100 nm), which indicates that the gold color is dependent on the thickness of hematite and not the direction of the crystal axes. The mechanism for the formation of the gold color is summarized as follows. A liquid phase is formed by the reaction of potassium provided by rice straw (or K2CO3) with the Bizen clay. Ferric ions in the liquid phase are reduced to ferrous ions and metallic iron by the introduction of CO. The viscosity of the liquid phase at the sample surface increases upon cooling to 900 °C; therefore, oxidation upon annealing at 900 °C in air occurs only at the surface of the liquid phase, and the metallic iron and ferrous ions on the surface are oxidized and crystallized as hematite. The hematite particles formed by the oxidation of metallic iron are expected to serve as nuclei for the formation of ferric ions in the liquid phase on the surface. Hematite is a known insulator with a band gap of 2.2 eV that absorbs visible light at wavelengths shorter than λ = 560 nm. 12,13 Thus, hematite appears reddish in color. Takada reported that the absorption of visible light by polycrystalline hematite increases with particle thickness, which results in a change in color from yellowish red (particle thickness = 50 nm) to purplish red (particle thickness = 250 nm).14 Although hematite in gold-colored Bizen is approximately 100 nm thick, it appears yellowish in color. In the case of 100 nm thick hematite, it should appear as reddish in color. The hematite layer in gold-colored Bizen is comprised of single crystals along the thickness direction, so that the transmitted light is not attenuated by grain 7 ACS Paragon Plus Environment


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boundaries.15 Another reason is the substitution of Al3+ for Fe3+ in hematite, and the yellowish tone of the Alsubstituted hematite becomes stronger with the Al molar ratio.16 Consequently, the lustrous golden color of traditional Bizen stoneware can be attributed to the yellowish color of a 100 nm thick Al-substituted hematite layer and the light reflected from the glassy phase.

Summary The mechanism for the formation of gold coloration on traditional Bizen stoneware was investigated from a solid-state chemistry perspective. The crystal phases in gold-colored Bizen were identified to be Alsubstituted hematite (α-(Fe1-xAlx)2O3, x ≈ 0.05). The gold color was formed by heating Bizen clay and K2CO3 (as a rice straw analog) at 1230 °C in a mixed gas of 10 vol% CO and 90 vol% Ar for 6 h, cooling to 900 °C in the gas mixture, and subsequent annealing at 900 °C in air for 2 h, which resulted in a 100 nm thick Alsubstituted hematite layer on the sample surface. Artificial gold-colored Bizen was successfully reproduced using K2CO3 in place of rice straw. Master Bizen potters have considered that the gold color is caused by optical interference within a carbon film produced from the firewood used as fuel. However, we have shown here that the golden color is caused by the formation of a 100 nm thick Al-substituted hematite layer. We consider that chemical studies of the traditional arts can provide new inspiration to artists and help them to produce works that are beautiful.

Acknowledgments The Bizen clay was provided by Sanrokugama in Bizen-shi, Okayama. This work was supported by a Kakenhi Grant-in-Aid (No. JP15K05656) from the Japan Society for the Promotion of Science (JSPS).

Table 1. Chemical composition (wt%) of the Bizen clay. SiO2

TiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

Total

63.51

0.69

21.61

2.77

0.56

0.66

2.05

0.51

92.36


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Figures

Figure 1. Gold-colored Bizen stoneware provided by the master Bizen potter Yoriaki Matsumoto.



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Figure 2. (a) Cross-sectional STEM bright field (STEM-BF) image of the surface region of the gold-colored pattern. EDS maps of major elements; (b) Fe, (c) Al, (d) Si, and (e) K. (f) Composite EDS map of Al + Fe + Si + K. The ED pattern in (a) was assigned to the [11 2 0] zone axis of hematite.

Figure 3. Plane-view TEM image of a fragment of the top layer. The ED pattern (inset) obtained from all the


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crystals shown in Fig. 3 indicates that the hematite crystals were preferentially oriented along the (0001) plane parallel to the sample surface, which is consistent with the ED pattern shown in the inset of Fig. 2a.

Figure 4. (a,b) Photographs, (c,e) cross-sectional HAADF–STEM images, and (d,f) Fe EDS maps for samples heated with K2CO3 to 1230 °C in air, held at the same temperature in a mixed gas of 10 vol% CO and 90 vol% Ar for 6 h, and subsequently cooled to 900 °C in the same atmosphere. The sample in panel (a) (Sample A) was quenched to room temperature (a, c, and d), whereas the sample shown in panel (b) (Sample B) was annealed at 900 °C for 2 h in an oxidizing atmosphere of air and then cooled to room temperature in air (b, e, and f).



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Figure 5. Plane-view STEM-SEI of a fragment of the sample shown in Fig. 4e. The inset ED pattern indicates the [11 2 0] zone axis of hematite, which was taken from an area of approximately 600 nm in diameter. Hematite platelets ca. 300 nm wide and ca. 19 nm thick were grown from the fragment.

References (1) Kusano, Y.; Fukuhara, M.; Fujii, T.; Takada, J.; Murakami, R.; Doi, A.; Anthony, L.; Ikeda, Y.; Takano, M. Microstructure and Formation Process of the Characteristic Reddish Color Pattern Hidasuki on Bizen Stoneware: Reactions Involving Rice Straw. Chem. Mater. 2004, 16, 3641-3646. (2) Kusano, Y.; Fujii, T.; Takada, J.; Fukuhara, M.; Doi, A.; Ikeda, Y.; Takano, M. Epitaxial Growth of εFe2O3 on Mullite Found Through Studies on a Traditional Japanese Stoneware. Chem. Mater. 2008, 20, 151156. (3) Kusano, Y.; Fukuhara, M.; Takada, J.; Doi, A.; Ikeda, Y.; Takano, M. Science in the Art of the Master Bizen Potter. Acc. Chem. Res. 2010, 43, 906-915. (4) Kusano, Y.; Doi, A.; Fukuhara, M.; Nakanishi, M.; Fujii, T.; Takada, J.; Ikeda, Y.; Takano, M.; Henrist, C.; Cloots, R.; Rulmont, A.; Ausloos, M. Effects of Rice Straw on the Color and Microstructure of Bizen, a 12 ACS Paragon Plus Environment

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Traditional Japanese Stoneware, as a Function of Oxygen Partial Pressure. J. Am. Ceram. Soc. 2009, 92, 1840–1844. (5) Schneider, H. Temperature-Dependent Iron Solubility in Mullite. J. Am. Ceram. Soc. 1987, 70, C-43C-45. (6) Ronchetti, S.; Piana, M.; Delmastro, A.; Salis, M.; Mazza, D. J. Synthesis and Characterization of Fe and P Substituted 3:2 Mullite. Euro. Ceram. Soc. 2001, 21, 2509-2514. (7) Shiraki, Y. Glaze and Its Pigment; GIHODO SHUPPAN Co., Ltd.: Tokyo, 1968; pp 774-788. In Japanese. (8) Pradell, T.; Fernandes, R.; Molina, G.; Smith, A. D.; Molera, J.; Climent-Font, A.; Tite, M. S. Technology of production of Syrian lustre (11th to 13th century). J. Eur. Ceram. Soc. 2018, 38, 2716-2727. (9) Yamaguchi, K.; Nishmoto, N.; Miyake, H.; Kakitani, S.; Mitsudo, H. Formation Mechanism of “Kinsai/Ginsai.” Bulletin of Okayama University of Science. 1995, 31, 147-153. In Japanese. (10) Muan, A.; Gee, C. L. Phase Equilibrium Studies in the System Iron Oxide-Al2O3 in Air and at 1 Atm. O2 Pressure. J. Am. Ceram. Soc. 1956, 39, 207-214. (11) Fujii, T.; Takano, M.; Kakano, R.; Isozumi, Y.; Bando, Y. Spin-flip anomalies in epitaxial α-Fe2O3 films by Mössbauer spectroscopy. J. Magn. Magn. Mater. 1994, 135, 231-236. (12) Marusa, L. A.; Messier, R.; White, W. B. Optical Absorption Spectrum of Hematite, α-Fe2O3 near IR to UV, J. Phys. Chem. Solids, 1980, 41, 981-984. (13) Wang, Y.; Lopata, K.; Chambers, S. A.; Govind, N.; Sushko, P. V. Optical Absorption and Band Gap Reduction in (Fe1-xCrx)2O3 Solid Solutions: A First-Principles Study. J. Phys. Chem. C, 2013, 117, 2550425512. (14) Takada, T. Studies on Iron Red Glazes. Jpn. J. Powder Powder Metall. 1958, 4, 50-67. In Japanese. (15) Takada, T.; Kiyama, M. Considerations on the Effects of Their Particle Size and Shape on the Colour of Ferric Oxide Powders. Jpn. J. Powder Powder Metall. 1958, 4, 63-73. In Japanese. (16) Hashimoto, H.; Nakanishi, M.; Asaoka, H.; Maeda, T.; Kusano, Y.; Fujii, T.; Takada, J. Preparation 13 ACS Paragon Plus Environment


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of Yellowish-Red Al-substituted α-Fe2O3 Powders and Their Thermalstability in Color, ACS Appl. Mater.

Interfaces, 2014, 6, 20282-20289.

TOC Alchemy in the Art of Traditional Japanese Ceramics: Microstructure and Formation Mechanism of GoldColored Bizen Stoneware Yoshihiro Kusano,* Minoru Fukuhara, Taichi Fujino, Tatsuo Fujii, Jun Takada, and Mikio Takano

The lustrous golden color of traditional Japanese Bizen stoneware was attributed to the yellowish color of a 100 nm thick Al-substituted hematite layer and the light reflected from the glassy phase. The color is reproducible in the laboratory through sequential heat treatments of Bizen clay pellets under oxidizing and reducing atmospheres with potassium supplied as a melting point depressant.


Alchemy in the Art of Traditional Japanese Ceramics: Microstructure

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