Integrated analysis of a black-glazed porcelain bowl in Tushan Kiln dated back to Song Dynasty, China

Journal Pre-proof Integrated analysis of a black-glazed porcelain bowl in Tushan Kiln dated back to Song Dynasty, China

Qinglin Ma, Shuqiang Xu, Julin Wang, Jingyan Yan PII:

S0254-0584(19)31028-4

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122213

Reference:

MAC 122213

To appear in:

Materials Chemistry and Physics

Received Date:

29 June 2019

Accepted Date:

22 September 2019

Please cite this article as: Qinglin Ma, Shuqiang Xu, Julin Wang, Jingyan Yan, Integrated analysis of a black-glazed porcelain bowl in Tushan Kiln dated back to Song Dynasty, China, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122213

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof Integrated analysis of a black-glazed porcelain bowl in Tushan Kiln dated back to Song Dynasty, China Qinglin Maa, Shuqiang Xub, Julin Wangc,d*, Jingyan Yanc,d a. Institute of Cultural Heritage, Shandong University, NO.27 Shanda Nanlu, Jinan, Shandong Province, China; b. Institute of Cultural Heritage and History of Science & Technology, University of Science and Technology Beijing, Beijing 100083, China; c. Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, PR China; d. Key Research Base of State Administration of Cultural Heritage for Evaluation of Science and Technology in Cultural Relics Protection Field, Beijing 100029, China;)

*Corresponding author: E-mail: [email protected]

Tel: +8613910664281.

Abstract： Research on corrosion of archaeological porcelain glaze has rarely been carried out in the past, although corrosion damage precious heritage items continuously. A group of black-glazed porcelains from the Song Dynasty was excavated at a poor conservation site in Chongqing, and one of their fragments was investigated with optical microscope, SEM-EDX, FTIR and Raman spectroscopy. The results showed that the alteration crust of the glaze mainly consisted of silica-rich gel and contained a variety of heterogeneous phases, including the liquid phase separation structure and different crystals. The crystals mainly include hematite, magnetite, rutile, pseudobrookite and silicon dioxide. The inter-diffusion reaction between alkalis in glaze and hydrogenated species in contact solution was the main reaction, which led to the formation of silicon-rich hydrate layers. The amorphous layers eventually transformed into acicular silica crystals under long exposure to environment. Keywords：Ceramic; Glass; Amorphous structures; SEM; Raman spectroscopy

1. Introduction Tushan Kiln in Chongqing was one of the representative kilns belonging to Jian Kiln series in southwestern region of ancient China. It started in the late Northern Song Dynasty (960-1127 A.D), flourished in the Southern Song Dynasty (1127-1279 A.D.), and declined and ended in the Yuan Dynasty (1271-1368 A.D.) [1]. The porcelain produced here was famous for its amazing black color and textural patterns in glaze, such as “Hare’s Fur” (HF), “Oil Spot” (OS) and “Yohen” patterns [2-4]. To date, many studies on the chemistry and microstructure of the black glaze, represented by the products from Jian Kiln, have been carried out and good progress has been achieved. The recipe of glaze of Tushan wares, affected by Jian wares, used a mixture of local clay, limestone, and wood ashes [2]. The detailed chemical composition of the black glaze in Tushan Kiln was studied in Li’s [1] work. The glaze appearance of 1

Journal Pre-proof the Jian Kiln series was mainly affected by the iron elements in the glaze. On one hand, iron exists in the vitreous network structure in the form of Fe3+ ions and Fe2+-O-Fe2+ atomic groups [5], causing the glaze to appear brown or black. On the other hand, it precipitates from the glaze under supersaturation and forms different iron oxide crystals and liquid phase separation structure (The homogeneous glaze liquid will be separated into two different immiscible phases at a high temperature.), causing the glaze to present uneven colors and texture features [1, 2, 6]. To sum up, the black glaze is mainly composed of glass phase and other dispersed heterogeneous phases such as bubbles, crystals and the liquid phase separation structure. More detailed information can be found in literatures [1, 5-7]. However, glaze on porcelains is a kind of vitreous coating [6, 8], and due to the impact of environment, cracks, pits, crusts and other alterations will appear on its surface which will reduce the value of porcelain. Since the manufacturing techniques were lost [7, 9], it is now urgent to understand the composition of the archaeological black glaze and its alteration mechanism. So far only a few articles referring to the corrosion of ceramics could be found [10, 11]. Most of these papers discussed ceramics together with glass [12-14], as they both contain silicate glass. The study of glass corrosion dates back as early as to 1961 [15] with both the analyses of archaeological samples [16, 17] and simulated corrosion tests [18-21]. The structure of glass alteration products is related to many factors, which includes the glass composition and its surrounding aqueous solution [12, 16, 22]. The glass erosion can be distinguished as two steps: inter-diffusion ( leaching or ion-exchange ) and dissolution (or hydrolysis) [18, 23].When pH is lower than 9, the ion-exchange (H+ or H3O+ ions in the aqueous environment with glass alkalis) is the main reaction which forms a dealkalinised hydrated layer (Formula (1), (2)). As pH increases, the hydrolysis of the Si-O-Si-network structure prevails (Formula (3)) and local condensation reactions of insoluble species can occur to form a silica-rich gel. -Si-O-M+(glass) + H+(aq) ↔ -Si-OH(glass) + M+ （1） + -Si-O-M (glass) + H3O+(aq) ↔-Si-OH(glass) + H2O + M+ （2） -Si-O-Si- + OH- ↔ Si-OH + -Si-O（3） Usually, due to the reaction between the active cations released from the first two processes and the exogenous anions, a secondary crystalline phase may form on or in the alteration crust [24]. Hench [25-27] summarized six types of altered glass surfaces, which were listed in Table 1. Table 1 Leached Glass Surface Types [25-27] Type

Structure Description

Type I

Hydrated layer on surface，extremely thin and its thickness can’t be measured

Type II

Surface layer depleted in alkali 2

Journal Pre-proof Type IIIA Type IIIB

Silica-rich layer adjacent to pristine glass and cation-rich (leached from bulk) layer adjacent to solutions Multiple layers laminated by oxides, hydroxides and hydrated silicates

Type IV

Silica-rich non-protective layer; low durability

Type V

No layer formation

In this research, one black-glazed porcelain bowl the inner surface of which was covered with special alteration crust from Tushan Kiln was studied with different spectroscopic techniques. The chemical compositions and structures of the black glaze and the alteration products were analyzed referring to the research of glass corrosion products, and finally the process and mechanism of glaze alteration formation in buried environment was proposed and discussed in detail.

2. Material and methods 2.1. Samples The fragment is part of a defective porcelain bowl (Fig. 1a) from the Tushan Kiln site of Chongqing. Chongqing is located in southwestern China with a hot and humid subtropical climate and abundant forests and rivers. There are rich porcelain clay, limestone and geothermal (including mineral water) and other resources here. It was well known for its advanced development in porcelain industry in Song Dynasty. The sample bowl was discarded due to its serious defects in the firing process and buried at Xiaowan site of Tushan Kiln for several centuries. It was not found until the year 2004 in the humid and compact light yellow cinder layer of the site [4], with acid soil and a small amount of clay and laterite around. Before being excavated, the fragments were buried in such a condition that their outer surfaces were in contact with of soil and groundwater, while the inner surfaces were only in contact with water. After excavation, the sample was washed with distilled water. The three samples were named as T1 (Fig. 1a), T2 (Fig. 1b), T3 (Fig. 1c). All the samples are of grey body and black glaze. There are many opaque white alteration layers on the glaze surface, and there are many acicular crystals with the size of 1~2 cm on the alteration layers.

3

Journal Pre-proof

Fig. 1. The defective porcelain bowl of Tushan Kiln from Chongqing, a: T1; b: T2; c: T3.

2.2. Analytical methods The fragment was analyzed using several destructive and non-destructive techniques, including optical microscope (OM), scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX), Raman spectrometer and infrared spectrometer. For a comprehensive study of the undamaged glaze and alteration layers, the morphologies and compositions of outer and inner surface were first examined. Then, in order to observe the cross section of the sample, the parts with/without alteration phases were selected and cut out. They were then embedded into an epoxy resin and were polished to 0.05 μm with SiC. A German optical microscope Leica DM4000M was used to obtain the surface and cross-section morphologies. To gain the most representative view, the samples were observed in dark field. Scanning electron microscopy analysis was performed to obtain the high-resolution image of the samples and the major element mapping. Samples were coated with gold and then observed using a Hitachi S-3600N SEM equipped with an EDAX-Genesis 2000XMS EDX at an accelerating voltage of 20 kV. To fully characterize the crystalline phases in the alteration crust, the fragment was analyzed with an XploRA confocal Raman microspectrometer (by Jobin-Yvon-Horiba, France), equipped with an Olympus BX-41 microscope and with the spectral resolution being 2~3 cm-1. Laser with a wavelength of 785 nm was used in this study and the laser power ranged between 1.25 and 6.25 mW. The diameter of the analyzed laser spot was about 2 µm with its duration of acquisition being 100 s. 4

Journal Pre-proof Different crystalline phases were identified by matching specific peaks to the pre-existing database RRUFF Project website [28]. The compositions of the alteration crusts were identified using a Nicolet Nexus-670 infrared spectrometer at room temperature, with the wavelength ranging from 400 cm-1 to 4000 cm-1. The alteration crusts were mixed with dry KBr to form the measurement pellet. The spot diameter of Infrared laser was about 10 mm. The samples were scanned 32 times at a spectral resolution of 4 cm-1.

3. Results and discussion 3.1 Morphology and composition of black glaze (1) Chemical composition of glaze Average chemical compositions of glaze in three samples were shown in Table 2. It can be seen that, the results of three samples were similar. CaO concentrations was lower than 10 wt% and K2O concentrations higher than 3 wt%, indicating that the black glaze were alkali-lime glaze [29]. Comparing with the data collected in Li’s [1] paper, our samples do not contain MnO and P2O5, and the similar concentrations of CaO and K2O indicated that this sample is a late-stage product. Iron oxides functioned as a glaze colorant. Small amount of magnesium and titanium (MgO: 2.8 wt%, TiO2: 1.0~1.2 wt%) were also detected. The chemical analysis results are consistent with the recipe of black glaze. Table 2 Chemical composition of glaze in black glazed porcelain bowl of Tushan Kiln (Wt %) Sample

Na2O

MgO

Al2O3

SiO2

K2O

CaO

TiO2

Fe2O3

T1

0.3

2.8

16.2

64.1

4.0

5.2

1.0

6.4

T2

0.7

3.4

16.2

63.9

3.4

5.8

0.9

5.8

T3

0.4

3.2

14.8

65.1

3.4

5.4

1.0

6.7

(2) Morphology of glaze Since the chemical compositions of glaze of our three samples were similar, one sample (T1) was selected to be carefully studied. Fig. 2 showed the surface and cross-section morphological features of black glaze without alteration layers. There were a few cracks on the surface, and plenty of crystals in the glaze, some of which were exposed to environment (Fig. 2a and Fig. 2b). The glaze was heterogeneous comparing to ordinary glass. There were a large number of other phase components (collectively referred to as heterogeneous phase). Cross-section morphology showed that there were many different crystals in the glaze. These crystals could be roughly divided into several categories according to their 5

Journal Pre-proof color and shape: red granular crystals (point “A” in Fig. 2a, Fig. 2c and Fig. 2e), granular crystals (point “B” in Fig. 2b, Fig. 2c and Fig. 2d), cruciform crystals (point “C” in Fig. 2d) and elliptical/ canine dentate crystals (point “D” in Fig. 2e and Fig. 2f). The first three types of crystals often clustered together, while the last type of crystals generally have larger particle size and exist in glaze alone. Besides, a large number of bubbles in different diameter scattered in the glaze (Fig. 2c).

Fig. 2. Surface (a, b) and cross-section (c, d, e, f) morphologies of the glaze. The left columns are OM images, and the right columns are BSE images. Points A: red granular crystals, points B: granular crystals, points C: cruciform crystals, points D: elliptical/ canine dentate crystals.

(3) Composition of heterogeneous phase SEM-EDX analysis result showed that the crystals in the glaze were mainly oxide with Si, Fe and Ti, as shown in Fig. 3 and Table 3. The point EDX 1 corresponded to granular crystals in the glaze, the content of Fe was 74~84wt% and it was inferred to be iron oxide. EDX 2 and EDX 3 corresponded to elliptical/ canine 6

Journal Pre-proof dentate crystals, only Si element was detected and it was inferred to be quartz. EDX 4 was similar with EDX 1. Content of Fe and Ti in EDX 5 and EDX 6 were very high and it was inferred to be Fe-Ti oxide crystals. Different crystal phases were identified based on their Raman spectroscopy results, as shown in Fig. 4. It’s worth mentioning that, although some granular crystals (C5) were mainly composed of Fe, Raman spectroscopy showed a characteristic peak at 674 cm-1. It was reported that iron oxides with characteristic peaks at 674 cm-1 include magnetite and magnetic hematite (γ -Fe2O3) [30]. Meanwhile, hematite often forms homogeneous polymorphism with equiaxed maghematite, and the existence of this crystal is often found in Jian Kiln ceramics [2, 7], which is usually associated with hematite, so it is presumed that C5 crystal was maghematite. Additionally, the concentration of Fe and Ti in C7 was very high as described above. Raman spectroscopy results showed that the material is pseudobrookite, which is a kind of brookite (titanium dioxide). Ti can be replaced by Fe3+, and it is called pseudobrookite if it is rich in Fe. Different crystal types according to composition and morphology were summarized in Table 4. Compared with previous literatures, Li [1] recorded the existence of quartz crystals and pyramidal magnetite crystals in black glaze of Tushan Kiln, and the iron oxides microcrystals, discrete hematite, magnetite crystals and the mixtures of both in the black glaze of Jian Kiln. Dejoie et al. [2] also identified the presence of ε-Fe2O3, a rare metastable polymorph of Fe2O3, in HF and OS black glaze of Jian Kiln. In the past, SiO2 and Fe2O3 crystals were often found in the glaze of kiln system. In this study, for the first time, different phases of titanium dioxide crystals were found.

Fig. 3. EDX analysis of black glaze cross-section in sample T1 Table 3 Chemical composition of the glaze cross-section of sample T1（SEM-EDS data, Wt%） Spot

Mg

Al

Si

K 7

Ca

Ti

Fe

Journal Pre-proof EDX1

2.5

5.6

10.7

1.0

0.6

5.2

74.4

EDX2

-

-

100.0

-

-

-

-

EDX3

-

-

100.0

-

-

-

-

EDX4

2.1

5.5

2.4

-

-

5.9

84.1

EDX5

3.7

2.3

-

-

-

37.2

56.8

EDX6

4.9

2.5

1.7

-

-

38.9

52.0

Fig. 4. The Raman spectra of different crystals in the glaze Table 4 Different crystal types in glaze of black glazed ceramic Crystal type

Morphology

Chemical compostion

C1

Quartz

Elliptical/ Canine dentate crystal

SiO2

C2

Hematite

Red granular crystal

α-Fe2O3

C3

Hematite

Cruciform crystal

α-Fe2O3

C4

Hematite

Granular crystal

α-Fe2O3

C5

Maghemite

Granular crystal

γ-Fe2O3

C6

Rutile

Reddish brown granular crystal

TiO2

C7

Pseudobrookite

Granular crystal

8

TiO2 (Ti often replaced by Fe3+)

Journal Pre-proof 3.2 Morphology and composition of the alteration layer 3.2.1 Morphology of the alteration layer Fig. 5 showed surface and cross-section morphology of alteration layers. It can be seen that, fragile alteration layer covers the glaze and its thickness is much smaller than that of the glaze below. Under low magnification of optical microscope observation, acicular crystals with different diameters were observed (Fig. 5b). Moreover, the higher-resolution SEM images showed that these crystals might be evolved from the alteration layer. For one certain acicular substance in Fig. 5c, part of it was connected with the alteration layer and slightly protruding from the surface (area A), and the other part was simply separated with the alteration layer (area B). Fig. 5d~f indicated that there was a clear dividing line between the alteration layer and the glaze. Different from the transparent alteration section under the OM, Fig. 5d showed that the acicular crystals inside the alteration layer were milky white and the cross-section of the acicular substance in Fig. 5f had cracks between acicular crystals and the alteration layer. In general, most acicular crystals were connected to the alteration layer, and their cross-sectional shapes were mainly elliptical or polygonal. In addition, the alteration layer also contained other types of impurities, as shown in Fig. 5e, among which the most obvious two types were the red granular phase and the non-uniform brown phase. It was worth mentioning that, the thickness of the areas without impurities was about 10 μm and no more than 15 μm, and that of the areas containing the impurities was more than 20 μm. These impurities were wrapped in the alteration layer and there was no trace of embedding from the outside. There were cracks both on the alteration layer and the interface of glaze/alteration layer (Fig. 5b and Fig. 5f). The cracks on the former were perpendicular to the surface while those on the latter were parallel to the surface. In addition, cracks were also found around the heterogeneous structure of the alteration layer. The neo-formed precipitation (after manufactured) was not observed in the cracks, so it was assumed that these cracks appeared as a result of dehydration shrink caused by the change of external temperature/humidity [17, 24, 31].

9

Journal Pre-proof

Fig. 5. Surface (a, b, c) and cross-section (d, e, f) morphologies of the alteration layer. (a). alteration crust and glaze, (b). acicular crystals, (c). SE images of the alteration surface (including acicular crystals), (d). the alteration layer containing an acicular crystal, (e). the alteration layer containing multiple impurities, (f). the BSE images of alteration layer containing an acicular crystal.

3.2.2 Composition of the alteration layer The compositions and the mass percentages of the glaze and the alteration layer were determined by EDX analysis. The analyzed zones were marked as EDX 1-8 on the SEM images in Fig. 5f and Fig. 6, and the results were presented in Table 5. Since the alteration layer is porous texture and epoxy resin may penetrate into the layer, so the quantitative analyses of oxygen and carbon can not be performed. The percentages indicated were normalized into the sum of the analyzed element as 100%. The results showed that the main chemical compositions of the glaze and the alteration layer were the same (including mainly Si, Al, Fe and small amount of Mg, K, Ca, Ti). Moreover, the chemical compositions of the alteration layer without impurity was characterized by high silicon contents (92-98 wt%), as shown in Fig. 6a (EDX 1, EDX 2) and Fig. 6b (EDX 1) ). This result indicated that the alteration layer was the Si-rich gel due to the condensation of leaching layer [19], which has also been observed in other archaeological glass [24]. Infrared spectra of alteration layer (Fig. 7) showed the characteristic bands of the quartz at 461, 792 and 1084 cm-1 [32, 33]. In general, the infrared spectra of quartz, tridymite and opal are similar since they belong to quartz family minerals. However, the sediments of quartz has characteristic double peaks in the vicinity of bands at 780~790 cm-1, while tridymite and opal have a single peak. There was also a weak peak at 565 cm-1 and it was the characteristic peak of tridymite [32-34]. Besides, bands at 1640 cm-1 and 3437 cm-1 could be attributed to the adsorbed water molecules in alteration layer. The infrared spectra revealed that the alteration layer is composed 10

Journal Pre-proof of hydrous SiO2 amorphous structure (SiO2-rich gel), with a small admixture of tridymite, and the fact also proves that the alteration layer was evolved from the highly dealkalinised glaze.

Fig. 6. SEM-EDX analysis of the cross-section of alteration layer and glaze. (a). alteration layer and glaze containing different types of crystalline phases, (b). alteration layer containing multiple impurities, (c). crystalline phase in alteration layer. Table 5 Chemical compositions (EDX) of the glaze and alteration layer (wt %) Spot Fig. 5f

Fig. 6a

Fig. 6b

Fig. 6c

Mg

Al

Si

K

Ca

Ti

Fe

EDX1

-

2.8

97.2

-

-

-

-

EDX1

-

2.0

98.0

-

-

-

-

EDX2

-

3.5

96.5

-

-

-

-

EDX3

3.7

2.3

-

-

-

37.2

56.8

EDX4

4.9

2.5

1.7

-

-

38.9

52.0

EDX5

2.5

5.6

10.7

1.0

0.6

5.2

74.4

EDX6

-

-

100.0

-

-

-

-

EDX7

-

-

100.0

-

-

-

-

EDX8

2.1

5.5

2.4

-

-

5.9

84.1

EDX1

-

7.7

92.3

-

-

-

-

EDX2

1.4

6.2

19.5

-

-

-

72.9

EDX3

1.3

12.5

65.1

5.9

2.2

4.5

8.5

EDX4

1.3

11.9

30.8

1.7

-

6.1

48.2

EDX5

0.8

18.0

67.0

6.3

-

1.6

6.3

EDX6

0.8

12.8

66.0

5.9

2.4

4.7

7.4

EDX7

-

1.3

98.7

-

-

-

-

EDX8

3.1

13.6

60.8

5.8

7.2

1.5

8.0

EDX1

-

-

100.0

-

-

-

-

The results are normalized to the sum of analyzed element as 100%.

11

Journal Pre-proof

Fig. 7. Infrared spectrum of the alteration layer.

To sum up, the alteration layer mainly consisted of amorphous silica which was caused by highly dealkalinisation of glaze. The alteration gel was mainly composed of Si and small amount of Al elements, suggesting the leaching of Mg, K, Ca, Ti and Fe elements. During the burial period, the black glaze contacted with acidic aqueous media, thus inter-diffusion became the main reaction. The selective leaching of cations in glaze contributed to the fading, and note that the hydrogen ions were smaller than the leaching alkali ions, inducing the local condensation of dealkalinised hydrated layer. The white Si-rich gel layer under the macroscopic observation was finally formed. The formation of cracks was due to the dehydration contraction. Furthermore, after the alkali leaching of glaze, there was no corresponding water environment which could induce the local precipitation.

3.3 The heterogeneous phase in alteration layer The alteration layer contained a variety of impurities with different particle sizes and compositions. These impurities existed in certain shapes, and they were independent from each other. When exploring the sources of these impurities, it is noted that the heterogeneous phase of the alteration layer of glass in previous studies is generally the materials come from outside and assembled in alteration layer [24], or the neo-formed precipitation phase during the alteration stage [18]. These secondary phases were mostly insoluble salts produced by the reaction of the leaching cations in 12

Journal Pre-proof the glass with the external anions, and they could be found on the surface [23, 35-37] or in the cracks [18, 31] of alteration layer. Composition of the heterogeneous phase in alteration layer of the samples was complex. Table 5 revealed that heterogeneous phase had a chemical similarity to the glaze, thus the elemental distribution maps was performed (Fig. 8) on the representative region that shown in Fig. 5e. First of all, the severely depleted element K, Mg, Al, K, Ca and others in area A of the alteration layer confirmed it to be the amorphous silica. While the B area under OM was red, mainly consisting of Fe and Ti elements, and it was similar to the crystals in the glaze. Area C was light brown under OM, and the contents of element Al, Si, K did not show significant difference compared with the glaze, albeit a few areas had a high concentration of Ti. For area D, although the Mg and Ca contents differed, other elements displayed comparable concentrations. Area E had the highest Si concentrations. Different heterogeneous phases had different elemental compositions, and the in-depth component analyses showed that these heterogeneous phases came from the glaze before it altered.

Fig. 8. Elemental distribution maps of the cross-section of alteration layer and glaze, corresponding to Fig.5e. A, B, C, D and E are the different phases of alteration layer, respectively.

3.3.1 Crystalline phases in alteration layer Raman spectra and EDX analyses of the glaze and the altered layer showed similar crystallographic composition. Fig. 9~Fig. 11 were the Raman spectra of the three kinds of crystalline compounds, which were detected both in the glaze and the alteration layer. In the black glaze, different crystals were detected. Zones EDX 6 and EDX 7 in Fig. 6a were transparent grain crystals (as shown in Fig. 3, the following were the same) in black glaze. Only Si element was detected by EDX, and the Raman spectrum (line 1 in Fig. 9) indicated that it was quartz. Zones EDX 5 and EDX 8 in Fig. 6a were granular crystal clusters, in which the concentration of Fe was as high as 74~84 wt %, 13

Journal Pre-proof and the Raman spectrum (line 2 in Fig. 10) showed characteristic bands of hematite. In addition, Raman spectra analyses of the red granular crystals and cruciform crystals in Fig. 2 were performed, and their results were line 1 and line 3 in Fig. 10 respectively. Their bands both corresponded to the standard Raman spectra of hematite in RRUFF Database. Raman spectra of partial crystals were also identified to be magnetite crystals (not shown), although they didn’t have significantly difference in appearance. The results were consistent with the records of the crystals in Jian Kiln series black glaze [1]. Furthermore, the intense Raman feature in Fig. 11 identified polymorphs of TiO2: Zones EDX 3 and EDX 4 in Fig. 6a had a high concentration of Fe and Ti, and their Raman spectra were identified as pseudobrookite (line 1 in Fig. 11). Raman spectra of partial red crystals were also identified to be rutile (line 2 in Fig. 11). The discovery indicated that in the initial firing process, different forms of TiO2 crystals were precipitated in glaze. The same compositions were also found in the alteration layer. Zone EDX 1 in Fig. 6c was the crystalline phase in alteration layer, which was also detected by EDX as only containing element Si, and its Raman spectrum (line 2 in Fig. 9) overlapped the standard quartz bands. The Raman spectra of the crystals at zones EDX 2 and EDX 4 in Fig. 6b corresponded to curves 4 and 5 in Fig. 10, respectively, and were identified as hematite. Particularly, the Raman spectrum of some red granular crystals in the alteration layer were identified as anatase (line 3 in Fig. 11), which was one of the three polymorphs of TiO2. It was necessary to note that there were small differences of the crystal Raman bands between glaze and the alteration layer (Fig. 9 and Fig. 10, respectively): The interference peak of Fe2O3 appeared at 300 cm-1 in line 1 in Fig. 9, and the Raman spectra of the crystals (lines 1, 2 and 3) of glaze in Fig. 10 were relatively poor. Considering that the ultra small size of the crystals under analysis and the presence of Fe or Ti cations in different valence states surrounding the crystals in the glaze [1], these cations tend to interfere with the Raman Spectra. However, there were fewer unstable ions in the alteration layer, so their peak shape was better. Consequently, the heterogeneous crystalline phases in alteration layer were essentially the oxides of Fe, Ti and Si, which were the same as those found in glaze. They were different from the normal secondary precipitation in the alteration layer of glass. Generally speaking, the secondary precipitation phases usually either connected with each other through cracks and destroyed the structure of corrosion layer [18, 31], or concentrated, rather than being alone [24, 38], whereas the heterogeneous crystalline phases in this study were independent and discrete, and were more similar to the crystals in glaze when referring to their composition and distribution characteristics. In a word, these heterogeneous crystalline phases should be formed in glaze during firing process, and remained in the alteration layer when the glaze was corroded. The only difference was the acicular crystals in Fig. 5. Zone EDX 1 in Fig. 5f was the cross section of acicular substance, which had high concentration of Si 14

Journal Pre-proof elements and little Al elements (Table 5), thus it had the similar ingredients with the amorphous phase in alteration layer (e.g. zone EDX 1 in Fig. 6a). Although its Raman spectrum (line 3 in Fig. 9) identified it as quartz, the composition and morphology indicated that it was not the same quartz crystal as discussed above. Additionally, the characteristic peak at bands 565 cm-1 of the infrared spectrum (Fig. 7) revealed the existence of tridymite in the alteration layer [32-34]. Based on related research, there was a gradual evolution process between different silica phases in siliceous rock [39] and glass [40]: amorphous opal → unordered monoclinic tridymite → monoclinic tridymite → quartz, and the final shape was mostly needle-like or granular. So it was inferred that the acicular quartz were evolved from the silica-rich gel in the alteration layer after a complex process, with a higher evolution degree. And the presence of amorphous phase and tridymite indicated that there was also lower degree evolution process for amorphous opal.

Fig. 9. Raman spectra of different crystals in the glaze and alteration layer, compared with standard spectrum of quartz. Line 1: crystalline phase in glaze, corresponding to zone EDX 6 in Fig.6a; Line 2: the crystalline phase in alteration layer, corresponding to zone EDX1 in Fig.6c; Line 3: the acicular crystals in alteration layer, corresponding to zone EDX1 in Fig. 5f. The dotted line was the reference Raman spectrum of SiO2 in RRUFF database.

15

Journal Pre-proof

Fig. 10. Raman spectra of certain crystals in glaze and alteration layer, compared with standard Fe2O3. Line 1, 2 and 3 were for the crystalline phases in the glaze, corresponding to the red granular crystal (point A), the granular crystal (point B) and the cruciform crystal (point C) in Fig. 2, respectively. Line 4 and line 5 were for the crystalline phases in alteration layer, corresponding to zones EDX 2 and EDX 4 in Fig. 6b, respectively. The dotted line was the reference Raman spectrum of Fe2O3 in RRUFF database.

16

Journal Pre-proof

Fig. 11. Raman spectra of certain crystals in glaze and alteration layer, compared with standard TiO2. Line 1 and 2 were for the crystalline phases in glaze (corresponding to EDX 4 in Fig. 6a). Line 3 was for the crystalline phase in alteration layer. The dotted line was the reference Raman spectra of three polymorphs of TiO2 in RRUFF database, respectively.

3.3.2 Other heterogeneous phases in alteration layer The liquid phase separation structure was found in the alteration layer, which also strongly proved that the heterogeneous phase was the residue of the original glaze. In the lower-left corner of Fig. 6b, the area within the white frame was magnified and a large number of isolated particles were displayed. Zone EDX 3 in Fig. 6b had the same elemental composition with the glaze (zone EDX 8 in Fig. 6b), and zone EDX3 had higher concentrations of Fe and Ti elements. Although the contents of Si and Al elements in zone EDX 4 were at high levels, the Raman spectra identified them as Fe2O3 crystals (see line 5 in Fig. 10). The results revealed that a 17

Journal Pre-proof large number of small particles, which were rich in Fe and Ti elements, were distributed around the Fe2O3 crystals, and these facts indicated that the region was a common liquid phase separation structure [1, 6]. Region in EDX 6 was the same as that of zone EDX 4. Liquid phase separation structure was a kind of characteristic structure in glaze, rather than the neo-formed phases. The EDX 5 region was a heterogeneous phase with a relatively high content of Al elements, and its composition was similar to that of feldspar (may be potassium feldspar). Together with all the heterogeneous phases in the alteration layer of Fig. 6b, it was presumed that the site might initially be a crater defect in glaze. During the firing stage, the bubbles generated in glaze may burst out of the layer. If it was too late to flatten before cooling, defects would occur, and different impurities (primary minerals, etc.) might be included. The detailed information about the formation of liquid phase separation structure could be seen in reference [1], and this structure might also be present in the crater defect. When the ceramic glaze altered in the burial environment, the stability of partial phases in crater defect might be better than that of the glass and thus remained in the final alteration layer. In summary, the heterogeneous phase in alteration layer was a component of the original glaze except for the needle-like crystal. All the findings revealed that the alteration layer was not further destroyed after the formation of silica-rich gel, instead a protective layer was formed, which inhibited the external components penetrating into the alteration layer and glaze. The alteration layer was relatively stable during the long buried period and the possible explanations were as followed. First, the research on corrosion resistance of glass showed that, the chemical durability of glass-ceramics was enhanced when the glass with high Si content was corroded, via increasing the network interconnection of residual glass network [41]. Second, the characteristics of the black glaze itself should be considered, for example, the high iron contents in glaze, since some studies have proved that iron and its corrosion products have a significant impact on the alteration kinetics and alteration phase of glass [19, 42-43]. Third, the relatively stable environment the sample exposed to was also beneficial for the stability of alteration layer. The process from glaze to alteration layer only underwent a significant process of the leaching of alkali ions, during which the local condensation of silica-rich amorphous structure occurred. And after certain period, the amorphous structure finally evolved into the protective film. This kind of structure corresponded exactly to the Type II surface of corrosion glass as Hench [25, 27] summarized (silica-rich layer of Type IV was poorer in durability, glass inner alkali and silica tend to congruent dissolution), and some heterogeneous phase was also left in the alteration layer because of the chemical stability of this region.

3.4 Formation process of alteration layer The structure of alteration layer depends on the composition of glaze (SiO2 concentration, categories of alkali and alkaline earth metal elements, etc.) and the 18

Journal Pre-proof outside solution (composition, temperature, pH, and the rate of solution renewal, etc.) [18, 23-24, 44]. As the black porcelain bowl of Tushan Kiln was buried in the wet and acidic environment for centuries, the ion-exchange process was the main reaction. The formation of the alteration layer can be divided into four stages (Fig. 12): First of all, as the inner glaze surface contacted with water, the alkali or alkaline-earth ions in glaze underwent ion-exchange with the surrounding aqueous media (step 1). Since the volume of exchanged hydrogenated species was smaller than that of the alkalis in glaze, the dealkalinised hydrated layer would undergo local condensation reactions, which was the first shrinkage behavior in the alteration process [12], and hence resulted in local reorganization within the alteration layer and then the amorphous structure of SiO2 was generated. In the region where heterogeneous phases existed, including the crystals or liquid phase separation structures, less ion-exchange occurred during this period. So the magnitude of the shrinkage was smaller, resulting in the rugged alteration surface (step 2). During the long burial period, the amorphous structure of SiO2 underwent crystallization induced by the environment, resulting in the formation of acicular silica crystals (the mark in Fig. 12 only represented the occurrence of crystallization and its cross-sectional shape was not limited to the oval), which continuously generated and grew across the alteration surface (step 3). When the environment changed (out of water or unearthed), the dehydration shrinkage occurred in alteration layer (the second shrinkage behavior) and then crack appeared (step 4), and these cracks were evident at the interface of different phases. The black glaze was different from the ordinary homogeneous glass, and due to its complicated composition and structure, the alteration products was different as well. First, the thin alteration crust was a single fragile layer covering the glaze surface instead of the multiple layers laminated [18, 24, 45], as summed up by Brill [15]. The reason why it could avoid being destroyed by external substances still needs to be further studied combining the in situ environment. Secondly, the alteration layer contained different crystalline phases, and the observed phases were not secondary phase precipitation, which revealed that the leached cations largely drifted into the external environment. More specifically, the liquid phase separation structure was also observed in alteration layer which was usually found in glaze. The presence of liquid phase separation structure indicated that in the alteration process, the rate of ion-exchange would be different due to the difference of glaze phases, and the rate of the part regions with heterogeneous phase would be much slower. Finally, after the evolution of glaze into alteration crust, crystallization may occur to form acicular silica crystals, and discovery of the formation of acicular silica crystals was the first case in the study of corrosive of ceramic artifacts.

19

Journal Pre-proof

Fig. 12. Schematic diagram of formation and evolution process of the alteration in black glaze.

4. Conclusions Several pieces of black-glazed bowl fragments from the ruins of the Tushan Kiln, which date back to the Song Dynasty, had a special white alteration layer on the inner surface. Through analyzing the composition and structure of glaze and alteration crust, several conclusions were made as follows: 1. The studied samples from Tushan kiln are black brown calc-alkali glaze. In addition to the glass phase and a small number of discrete pore distribution, the glaze layer also distributes a large number of different types of crystals, mainly including hematite, magnetite, rutile, pseudobrookite and silicon dioxide. Iron oxides are supposed to be used as the colorant of the glaze. 2. The samples were exposed in acidic aqueous medium during buried period. The inter-diffusion reaction between alkalis in glaze and hydrogenated species in contact solution was the main reaction, and shrinkage occurred since the ions volume changed, thus the silicon-rich hydrate layer was formed. Since Mg, K, Ca, Ti, Fe and other elements were released in the altered part during the exposure time, the corrosion layer on the black glaze surface showed a white crust. 3. As the glaze had complex phase compositions, different phases underwent different corrosion kinetics. Some of the heterogeneous phases, such as crystals or liquid phase separation structures, had better stability, thus the inter-diffusion rate was slower in 20

Journal Pre-proof this region and part of them was left in the alteration crust. As a result, the alteration layer with uneven thickness was formed. 4. The amorphous structure of the alteration layer would crystallize under long exposure to environment and led to the formation of acicular silica crystals, which eventually distributed in the surface. This work revealed the corrosion mechanism of black glaze containing a variety of heterogeneous phases, and it illustrated the evolution process of the black porcelain artifacts of Tushan Kiln exposed to the long acidic humid underground environment. Different from the homogeneous glass, complex compositions and structures in black glaze had a special impact on the alteration layer. The study presented the changes of the environment which black-glazed porcelain exposed to and revealed relevant historical information, thus providing guidance to their conservation. At the same time, it also provides reference for the study of corrosion of other type ceramic relics.

Acknowledgements The authors would like to thank researchers Wang Chun (Chongqing China Three Gorges Museum, China) for providing samples and the archeological information. Thanks also go to Liu Jie, Hu Fengdan (Chinese Academy of Cultural Heritage, China), Dr. Wang Yingzhu (University of Science and Technology Beijing) for the assistance during the testing and writing.

Declarations The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. There were no conflicts of interest as well.

References [1] J. Li, Science and Technology History of China. Vol. Ceramics. Science Publishing Company, Beijing, 1998. [2] C. Dejoie, P. Sciau, W. Li, L. Noe, A. Mehta, K. Chen, H. Luo, M. Kunz, N. Tamura, Z. Liu, Learning from the past: rare ε-Fe2O3 in the ancient black-glazed Jian (Tenmoku) wares, Sci. Rep. 4 (2014) 4941. [3] L. Chen, Yaobian and Hongcai patterns in porcelains from Chongqing Tushan Kiln, China Academic Journal Electronic Publish House, (2010) 48-50. [4] Chongqing Cultural Heritage Research Institute, Tushan Kiln in Chongqing, Science Press, Beijing, 2006. [5] F. Zhang, The Science of Chinese Ancient Ceramics, Shanghai People's Fine Arts Publishing House, 2000. 21

Journal Pre-proof [6] W. Li, Glazes in China, (2014) 1-11. [7] W. Li, H. Luo, J. Li, J. Li, J. Guo, Studies on the microstructure of the black-glazed bowl sherds excavated from the Jian kiln site of ancient China, Ceram. Int. 34 (2008) 1473-1480. [8] Y. Shiraki, K.Liu, G.Liu, Glaze and Colorants, China Architecture & Building Press, Beijing, 1979. [9] C.Y. Chiang, H.F. Greer, R.S. Liu, W. Zhou, Formation, crystal growth and colour appearance of Mimetic Tianmu glaze, Ceramics Int. 42 (2016) 7506-7513. [10] M. Özçatal, M. Yaygıngöl, A. İssi, A. Kara, S. Turan, F. Okyar, Ş. Pfeiffer Taş, I. Nastova, O. Grupče, B. Minčeva-Šukarova, Characterization of lead glazed potteries from Smyrna (Izmir/Turkey) using multiple analytical techniques; Part I: Glaze and engobe, Ceram. Int. 40 (2014) 2143-2151. [11] H. Sadek, μ-PIXE mapping of archeological glazed pottery from Egypt, Appl. Surf. Sci. 332 (2015) 281-286. [12] R.G. Newton, Conservation of glass, Stud. Conserv. 50 (2005) 135-164. [13] J. Henderson, Ancient Glass: an interdisciplinary exploration, Cambridge Univ Pr, 2013. [14] R.A. Mccauley, Corrosion of Ceramic Materials, Third Edition, Crc Press, 2013. [15] R.H. Brill, The record of time in weathered glass, Archaeology, 14 (1961) 18-22. [16] M. Li, J. Wang, J. Wang, Q. Ma, Z. Zhang, Synthesis and Raman study on needlelike silicates in ancient Chinese Pb-Ba glass in Qin and Han dynasties, J. Raman. Spectrosc. 45 (2014) 672-676. [17] N. Attard-Montalto, A. Shortland, 17th century blue enamel on window glass from the cathedral of Christ Church, Oxford: Investigating its deterioration mechanism, J. Cult. Herit. 16 (2015) 365-371. [18] T. Lombardo, L. Gentaz, A. Verney-Carron, A. Chabas, C. Loisel, D. Neff, E. Leroy, Characterisation of complex alteration layers in medieval glasses, Corros. Sci. 72 (2013) 10-19. [19] A. Michelin, E. Burger, E. Leroy, E. Foy, D. Neff, K. Benzerara, P. Dillmann, S. Gin, Effect of iron metal and siderite on the durability of simulated archeological glassy material, Corros. Sci. 76 (2013) 403-414. [20] A. Tournie, P. Ricciardi, P. Colomban, Glass corrosion mechanisms: a multiscale analysis, Solid State Ionics 179 (2008) 2142-2154. [21] S. Gin, P. Jollivet, M. Fournier, C. Berthon, Z. Wang, A. Mitroshkov, Z. Zhu, J.V. Ryan, The fate of silicon during glass corrosion under alkaline conditions: a mechanistic and kinetic study with the international simple glass, Geochim. Cosmochim. Ac. 151 (2015) 68-85. [22] S. Davison, Conservation and Restoration of Glass, Butterworth Heinemann press, 2006. [23] L. De Ferri, P.P. Lottici, G. Vezzalini, Characterization of alteration phases on Potash–Lime–Silica glass, Corros. Sci. 80 (2014) 434-441. [24] A. Silvestri, G. Molin, G. Salviulo, Archaeological glass alteration products in marine and land-based environments: morphological, chemical and microtextural characterization, J. Non-Cryst. Sol. 351 (2005) 1338-1349. [25] L.L. Hench, D.E. Clark, Surface analysis of glasses, 199 (1982) 203-228. [26] L.L. Hench, Glass surfaces - 1982, Le Journal de Physique Colloques, 43 (1982) C9-625-C629-636. [27] L.L. Hench, Physical chemistry of glass surfaces, J. Non-Cryst. Sol. 25 (1977) 343-369. [28] Lafuente, B. Downs, R.T. Yang, H. Stone, N., 1. The power of databases: The RRUFF project : Highlights in Mineralogical Crystallography, Roczniki Akademii Rolniczej W Poznaniu Rolnictwo, (2015). [29] K. Liu, Principle of Pottery Technology, South China University of Technology Press, 1990. [30] Y. Xu, Y. Qin, F. Ding, Characterization of the rare oil spot glazed bowl excavated from the Xiao kiln site of north China, Ceram. Int. 43 (2017) 8636-8642. 22

Journal Pre-proof [31] L. Gentaz, T. Lombardo, A. Chabas, C. Loisel, D. Neff, A. Verney-Carron, Role of secondary phases in the scaling of stained glass windows exposed to rain, Corros. Sci. 109 (2016) 206-216. [32] I.I. Plyusnina, M.N. Maleyev, G.A. Yefimova, Infrared-spectroscopic investigation of cryptocrystalline varieties of silica, Int. Geol. Rev. 13 (1971) 1750-1754. [33] V.C. Farmer, Infrared Spectra of Minerals, Science Press, 1982. [34] Z. Zheng, R.J. Reeder, Experimental studies of siliceous phases in opaline silicalite-on mineral classification of opaline silicalite, Acta Mineralogica Sinica, (1986) 258-265+294. [35] G. Woisetschläger, M. Dutz, S. Paul, M. Schreiner, Weathering phenomena on naturally weathered Potash-Lime-Silica-Glass with medieval composition studied by secondary electron microscopy and energy dispersive microanalysis, Microchima. Acta. 135 (2000) 121-130. [36] L. Gentaz, T. Lombardo, A. Chabas, C. Loisel, A. Verney-Carron, Impact of neocrystallisations on the SiO2–K2O–CaO glass degradation due to atmospheric dry depositions, Atmos. Environ. 55 (2012) 459-466. [37] M. Melcher, M. Schreiner, Leaching studies on naturally weathered potash-lime–silica glasses, J. Non-Cryst. Sol. 352 (2006) 368-379. [38] G.A. Cox, B.A. Ford, The long-term corrosion of glass by ground-water, J. Mater. Sci. 28 (1993) 5637-5647. [39] H.Y. Chen, L.F. Lu, W.X. Liu, B.J. Shen, L.J. Yu, Y.F. Yang, Characteristics of pore structures during diagenesis of opal-siliceous shale, Petroleum Geology and Experiment, 39 (2017) 341-347. [40] A. Verney-Carron, S. Gin, P. Frugier, G. Libourel, Long-term modeling of alteration-transport coupling: Application to a fractured Roman glass, Geochim. Cosmochim. Ac. 74 (2010) 2291-2315. [41] W. Deng, J.S. Cheng, P.J Tian, Chemical durability and weathering resistance of canasite based glass and glass-ceramics, J. Non-Cryst. Sol. 358 (2012) 2847-2854. [42] V. Philippini, A. Naveau, H. Catalette, S. Leclercq, Sorption of silicon on magnetite and other corrosion products of iron, J. Nucl. Mater. 348 (2006) 60-69. [43] N. Jordan, N. Marmier, C. Lomenech, E. Giffaut, J.J. Ehrhardt, Sorption of silicates on goethite, hematite, and magnetite: experiments and modelling, J. Colloid. Interf. Sci. 312 (2007) 224-229. [44] L. Gentaz, T. Lombardo, C. Loisel, A. Chabas, M. Vallotto, Early stage of weathering of medieval-like potash-lime model glass: evaluation of key factors, Environ. Sci. Pollut. Res. Int. 18 (2011) 291-300. [45] M. Gulmini, M. Pace, G. Ivaldi, M.N. Ponzi, P. Mirti, Morphological and chemical characterization of weathering products on buried Sasanian glass from central Iraq, J. Non-Cryst. Sol. 355 (2009) 1613-1621.

23

Journal Pre-proof Figure captions: Fig. 1. The defective porcelain bowl of Tushan Kiln from Chongqing, a: T1; b: T2; c: T3. Fig. 2. Surface (a, b) and cross-section (c, d, e, f) morphologies of the glaze. The left columns are OM images, and the right columns are BSE images. Points A: red granular crystals, points B: granular crystals, points C: cruciform crystals, points D: elliptical/ canine dentate crystals. Fig. 3. EDX analysis of black glaze cross-section in sample T1. Fig. 4. The Raman spectra of different crystals in the glaze. Fig. 5. Surface (a, b, c) and cross-section (d, e, f) morphologies of the alteration layer. (a). alteration crust and glaze, (b). acicular crystals, (c). SE images of the alteration surface (including acicular crystals), (d). the alteration layer containing an acicular crystal, (e). the alteration layer containing multiple impurities, (f). the BSE images of alteration layer containing an acicular crystal. Fig. 6. SEM-EDX analysis of the cross-section of alteration layer and glaze. (a). alteration layer and glaze containing different types of crystalline phases, (b). alteration layer containing multiple impurities, (c). crystalline phase in alteration layer. Fig. 7. Infrared spectrum of the alteration layer. Fig. 8. Elemental distribution maps of the cross-section of alteration layer and glaze, corresponding to Fig.5e. A, B, C, D and E are the different phases of alteration layer, respectively. Fig. 9. Raman spectra of different crystals in the glaze and alteration layer, compared with standard spectrum of quartz. Line 1: crystalline phase in glaze, corresponding to zone EDX 6 in Fig.6a; Line 2: the crystalline phase in alteration layer, corresponding to zone EDX1 in Fig.6c; Line 3: the acicular crystals in alteration layer, corresponding to zone EDX1 in Fig. 5f. The dotted line was the reference Raman spectrum of SiO2 in RRUFF database. Fig. 10. Raman spectra of certain crystals in glaze and alteration layer, compared with standard Fe2O3. Line 1, 2 and 3 were for the crystalline phases in the glaze, corresponding to the red granular crystal (point A), the granular crystal (point B) and the cruciform crystal (point C) in Fig. 2, respectively. Line 4 and line 5 were for the crystalline phases in alteration layer, corresponding to zones EDX 2 and EDX 4 in Fig. 6b, respectively. The dotted line was the reference Raman spectrum of Fe2O3 in RRUFF database. Fig. 11. Raman spectra of certain crystals in glaze and alteration layer, compared with standard TiO2. Line 1 and 2 were for the crystalline phases in glaze (corresponding to EDX 4 in Fig. 6a). Line 3 was for the crystalline phase in alteration layer. The dotted line was the reference Raman spectra of three polymorphs of TiO2 in RRUFF database, respectively. Fig. 12. Schematic diagram of formation and evolution process of the alteration in black glaze.

24

Journal Pre-proof 1. Alteration in underground environment of ancient Chinese black-glazed porcelains. 2. The complex structures of black glaze have a special effect on its alteration process. 3. The first discovery of acicular silica crystals on alteration crust. 4. Refining corrosion mechanisms of underground black glaze.