Elemental Characterization by Edxrf of Imperial Longquan Celadon Porcelain Excavated from Fengdongyan Kiln, Dayao County*

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Archaeometry 57, 6 (2015) 966–976 doi: 10.1111/arcm.12149

ELEMENTAL CHARACTERIZATION BY EDXRF OF IMPERIAL LONGQUAN CELADON PORCELAIN EXCAVATED FROM FENGDONGYAN KILN, DAYAO COUNTY*

L. LI,1 L. T. YAN,1 S. L. FENG,1 Q. XU,1 L. LIU,1,2 Y. HUANG1,2 and X. Q. FENG1†

1Key Laboratory of Nuclear Radiation and Nuclear Energy Technology, Institute of High Energy Physics, Chinese Academy of Sciences, 19 Yu Quan Lu, Beijing 100049, China 2University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China

A mass of Longquan porcelain shards carved with ‘Guan’ or the dragon patterns were unearthed in the early Ming Dynasty layer of the Fengdongyan kiln site at Dayao County. These celadon shards were fired in the Hongwu and Yongle eras of the Ming Dynasty. In order to research the raw materials and firing technology of the imperial porcelain, 85 typical shards were analysed by energy-dispersive X-ray fluorescence (EDXRF). The results indicate that the

contents of TiO2 and Fe2O3 in the body vary in the Hongwu and Yongle eras. Compared with Longquan glazes in the Southern Song Dynasty, the average values of K2O, Fe2O3 and TiO2 are higher, but that of CaO is lower in early Ming imperial porcelain glazes. Principal components analysis (PCA) shows that different degrees of elutriation of the same raw materials are the main reason for this difference in the Hongwu and Yongle periods. However, the raw materials of imperial porcelain glazes show no obvious changes and have inherited the earlier tradition. The production and ﬁring technology of imperial porcelain reached a higher level and had not declined in the Early Ming Dynasty.

KEYWORDS: EDXRF, LONGQUAN KILN, IMPERIAL PORCELAIN, FENGDONGYAN KILN, PCA

INTRODUCTION Longquan celadons, with higher firing technology and greater artistic merit, play a significant role in the history of ancient Chinese celadon. According to previous archaeological evidence (Li 1998), it is well known that the Longquan kilns, located in Longquan City in Zhejiang Province, began to fire celadon in the Southern and Northern Dynasties (ad 420–589), and developed in the Southern Song (ad 1127–1279) and Yuan Dynasties (ad 1271–1368). It used to be thought that the firing technology had declined in the Ming Dynasty (ad 1368–1644), as stated in several references (Chou et al. 1973; The Light Industry Hall of Zhejiang Province 1989; Li 1998). But did the firing technology of Longquan celadon really begin to decline in the Ming Dynasty? Were the Longquan kilns still the famous kilns that made porcelain for the palace? In the past, research interest was focused on the development history and the decorative arts of the civilian porcelain of Longquan celadon (Li et al. 1984; Ye et al. 1999; Jin 2007). Due to the lack of specimens, the raw materials and firing technology of the imperial porcelain of Longquan celadon has been little investigated. But the excavation of the Fengdongyan kiln provides physical evidence. From September 2006 to January 2007, the Fengdongyan kiln, located in

Figure 1 The location map for the Dayao County kiln, amongst the Longquan kilns.

Dayao County, in Longquan City (see Fig. 1), was excavated by the Zhejiang Province Institute of Cultural Relics and Archaeology, the School of Archaeology and Museology of Peking University and the Longquan Museum. A mass of exquisite Longquan celadon shards of theYuan and Ming Dynasties were unearthed, including abundant civilian porcelain and a lot of official wares carved with ‘Guan’ or the dragon patterns in the Hongwu and Yongle eras of the Ming Dynasty (Zhejiang Institute of Cultural Relics et al. 2009). The emergence of this imperial porcelain has led to great repercussions in archaeology. Some scholars have published compari- sons between the imperial porcelain of the Ming Dynasty and the Longquan porcelain of the Southern Song Dynasty (Peng et al. 2009). However, what differences are there between the imperial porcelain of the Ming Dynasty and the civilian porcelain of the Early Ming Dynasty? At present, there is little research comparing imperial porcelain with civilian porcelain in the Ming Dynasty. In this paper, we have determined the chemical composition of imperial Longquan celadon porcelain using energy-dispersive X-ray fluorescence (EDXRF). It is well known that the contents of major, minor and trace elements of the porcelain body and glaze are dependent on its raw material and manufacturing technology (Li 1998; Kerr and Wood 2004; Artioli 2010), which can be used to indicate the age of the porcelain and its provenance. From a statistical analysis of the elemental data, the raw material and the development of porcelain firing technology are dis- cussed. The information is used to display the inheritance relationship of the raw materials used for the body and glaze in different periods.

Figure 2 Photographs of imperial porcelain samples from the HWM and YLM periods.

Table 1 Detailed information on the celadon shards from the Fengdongyan kiln site

Group Date Number of shards Exterior characteristics

HWM Hongwu era of the Ming Dynasty 32 Celadon glaze, grey body YLM Yongle era of the Ming Dynasty 53 Celadon glaze, grey body EM Early Ming Dynasty 48 Celadon glaze, grey body

SAMPLES In this work, 85 typical shards of imperial porcelain from the Fengdongyan kiln were provided by Zhejiang Province Institute of Cultural Relics and Archaeology. These imperial porcelain samples can be grouped into two cultural periods: the Hongwu (HWM group) and the Yongle (YLM group) eras of the Ming Dynasty. Since Hongwu and Yongle are Early Ming, 48 shards of civilian porcelain excavated at Fengdongyan dated to the Early Ming Dynasty (EM group) were selected for comparison with the imperial porcelain. Some photographs of imperial porcelain shards are shown in Figure 2 and detailed information about the samples is listed in Table 1.

THE EDXRF EXPERIMENT A sample measuring 30 mm × 10 mm was cut from the shard, and the cross-section was polished, and then washed three times in an ultrasonic cleaner with deionized water and dried at 105°C. The EDXRF experiments were performed on an EDAX Eagle III spectrometer at the Institute of High Energy Physics, CAS, Beijing, China. The spectrometer has a Mo tube and a 125 μmBe window with an incident beam angle of 65° and an emergence angle of 60°. The detector is a liquid-nitrogen-cooled Si(Li) crystal with a resolution of 160.3 eV at Mn–Kα. There is a vacuum chamber and the diameter of the X-ray beam spot is selected to be 1 mm. The voltage and current of the X-ray tube are 40 kV and 250 μA, respectively. The software employed for spectrum deconvolution and analysis is VISION32, which is associated with the instrument. A set of standard reference samples with known chemical compositions, which were developed by the

Table 2 The quantitative results for the ancient ceramic samples

Na2O MgO Al2O3 SiO2 K2O CaO TiO2 MnO Fe2O3 Cu Zn Rb Sr (%) (%) (%) (%) (%) (%) (%) (%) (%) (ppm) (ppm) (ppm) (ppm)

Experimental 0.65 0.62 24.34 67.54 2.27 0.54 0.88 0.022 2.77 46 54 119 109 value Certiﬁed 0.44 0.70 23.90 67.50 2.30 0.62 0.95 0.026 2.70 27 59 113 103 value

Institute of High Energy Physics, are used to calibrate the ceramic matrix in order to obtain reliable experimental data. The homogeneity of elements in these ceramic reference samples met the requirements for non-destructive quantitative analysis (Li et al. 2010).

The elemental abundances of Na2O, MgO, Al2O3, SiO2,K2O, CaO, TiO2, MnO, Fe2O3, CuO, ZnO, Rb2O and SrO are quantified by the fundamental parameter (FP) method (He and Espen 1991; Sitko 2008). This method assumes that the unknown samples have approximate compositions, and then calculates the fluorescence intensities based on the Sherman equation (Sherman 1955) and compared to the measured intensities (Lachance and Claisse 1994). Successive adjustments of the composition are carried out until the theoretical and measured intensities are consistent. The final concentrations are assumed to represent the actual compositions. In order to get a better measure of the precision and accuracy of the data in this work, ancient ceramic samples of known composition are selected to compare the expected and observed elemental concentrations measured on the ceramic reference samples used in the analysis. The results show that the experimental values are well in agreement with the certified ones (see Table 2). The average values of each elemental composition in the porcelain body and glaze are displayed below, in Tables 3 and 5, respectively. In this paper, the data for Na2O and MgO are provided as references because of the poor fluorescent yields and low counts obtained for the characteristic X-ray radiation.

DISCUSSION

The elemental characteristics of imperial porcelain with a Longquan celadon body

There are no evident differences between the average concentrations of SiO2,Al2O3 and MnO in the imperial porcelain body for the HWM and YLM groups, as shown in Table 3. It can be seen that the K2O contents in the Hongwu era of the Ming Dynasty are a little higher than those of the Yongle era of the Ming Dynasty, while the CaO contents are lower, as can be seen in Figure 3. ± ± The average TiO2 content in the HWM group is 0.21 0.03 % higher than that (0.14 0.02 %) in the YLM group. The abundance change of Fe2O3 in the imperial porcelain body is similar to that of TiO2, as shown in Figure 3. It is clearly observed that few differences exist in the major elements of the porcelain body between the HWM and YLM groups. It is well known that in the past, the raw material for making porcelain was obtained in the vicinity of the kiln sites. According to the available documents (The Light Industry Hall of Zhejiang Province 1989), the composition of Fe2O3 in the porcelain clay in the Longquan district is 0.33–1.24 %, which is less than that (1.81–2.10 %) in the imperial porcelain body; however, the Fe2O3 content in Zijin clay is greater than 8%, as shown in Table 4. Therefore, it seems that some Zijin clay was added: not

© 2014 University of Oxford, Archaeometry 57, 6 (2015) 966–976 970 04Uiest fOxford, of University 2014 © Archaeometry 57 21)966–976 (2015) 6 , Table 3 The average values of each composition in the bodies of the samples .Li L.

Group Na2O (%) MgO (%) Al2O3 (%) SiO2 (%) K2O (%) CaO (%) TiO2 (%) MnO (%) Fe2O3 (%) CuO ZnO Rb2O SrO

(ppm) (ppm) (ppm) (ppm) al et

HWM 0.56 ± 0.08 0.48 ± 0.06 20.5 ± 0.5 70.4 ± 0.7 5.36 ± 0.31 0.045 ± 0.016 0.21 ± 0.03 0.053 ± 0.006 2.10 ± 0.10 59 ± 6 101 ± 10 353 ± 20 49 ± 7 . YLM 0.65 ± 0.10 0.46 ± 0.06 20.6 ± 0.5 70.7 ± 0.8 5.20 ± 0.42 0.078 ± 0.019 0.14 ± 0.02 0.053 ± 0.008 1.81 ± 0.12 58 ± 5 107 ± 21 324 ± 22 37 ± 5 EM 0.64 ± 0.11 0.45 ± 0.06 20.8 ± 0.9 70.6 ± 1.2 5.19 ± 0.48 0.059 ± 0.021 0.13 ± 0.03 0.051 ± 0.010 1.82 ± 0.13 57 ± 6 104 ± 22 325 ± 26 34 ± 6 Characterization by EDXRF of imperial Longquan celadon porcelain 971

Figure 3 The histogram of the elemental contents in K2O, CaO, TiO2 and Fe2O3 by EDXRF.

Table 4 The chemical composition of porcelain clay and Zijin clay in Dayao County of Longquan City (The Light Industry Hall of Zhejiang Province 1989)

SiO2 (%) Al2O3 (%) Fe2O3 (%) TiO2 CaO (%) MgO (%) K2O (%) Na2O (%) (%)

Porcelain 65.41–75.75 16.00–23.00 0.33–1.24 – A few Trace amounts 2.71–5.35 0.02–1.12 clay per cent Zijin clay 55.70 25.24 8.18 0.69 1.64 A few per cent 2.61 0.82 of Dayao County only can it improve the strength of the body, but it can also increase the colour density of black or grey in the porcelain body. The average contents of the trace elements Cu, Zn, Rb and Sr were close to each other in the HWM and YLM periods. In this work, principal components analysis (PCA) was used to study the raw material used for the body in different cultural periods. The main objective of PCA is to reduce the dimen- sionality of the observations. Figure 4 shows a factorial analysis diagram from the composition of the imperial porcelain bodies made in the Hongwu and Yongle periods. The data for K2O, CaO, TiO2,Fe2O3, Rb and Sr are used and the eigenvalue sum of Factors 1 and 2 accounts for

81.12% of the total variance. Factor score 1 (F1) mainly includes the changes in TiO2 and Fe2O3, while Factor score 2 (F2) represents the K2O content. In Figure 4, the data are divided into two clusters by variation in F1, which shows that they clearly belong to the HWM and YLM groups, respectively. The samples of the YLM group are situated in the lower region, while most of samples in the HWM group are located in the upper region, and only a few overlap with the YLM samples. This also shows that the difference between the porcelain bodies of the Hongwu and Yongle periods is mainly caused by changes in the TiO2 and Fe2O3 contents in the imperial porcelain body. However, it can be seen from F2 that the samples in the

Figure 4 The distribution of the celadon body from the HWM and YLM periods by PCA: a plus sign (+) indicates the celadon body of the Hongwu era of the Ming Dynasty and a triangle (▲) represents the celadon body of the Yongle era.

HWM and YLM groups are distributed in the same area, so the concentration of K2Oisnot signiﬁcantly different according to the PCA results. The reason for this difference is the use of different raw materials or differing degrees of elutriation of the same raw materials. In view of the slightly different major element contents and the similar trace element contents, we conclude that differing degrees of elutriation of the same raw materials are the main reason for the differences between the Hongwu and Yongle periods. The EDXRF values for civilian porcelain bodies are also shown in Table 3. It can be seen that

the values for TiO2 and Fe2O3 in the HWM period are higher than those of the Early Ming Dynasty (EM period), whereas the contents of other elements are similar. However, there are no evident differences in the elemental features of the porcelain bodies in the YLM and EM groups. Figure 5 shows the PCA factor analysis based on the chemical components in the porcelain bodies of the HWM, YLM and EM groups. The same elements are selected as shown in Figure 4. The eigenvalue sum of Factors 1 and 2 accounts for 77.5% of the total variance. Most of the data in the HWM group are independently distributed in the upper region, while the samples from the YLM and EM groups are situated in the lower region. This shows that the degree of elutriation of the raw materials used for making imperial porcelain bodies in the Hongwu period was different from that for the civilian porcelain bodies. The sample plots overlap each other in the YLM and EM periods, and the data plots for the YLM period form a relatively concentrated distribution. This means that similar processing of raw materials was used in the imperial porcelain bodies of the Yongle period and the civilian porcelain bodies in the Early Ming Dynasty.

The elemental characteristics of imperial porcelain with a Longquan celadon glaze As shown in Table 5, there are no evident differences in the average concentration of each composition in the porcelain glaze of the Hongwu and Yongle periods, and the elemental

Figure 5 The analytical results of PCA on porcelain bodies of the HWM, YLM and EM periods: a plus sign (+) indicates the celadon body of the Hongwu era of the Ming Dynasty, a triangle (▲) represents the celadon body of the Yongle era and an asterisk (*) represents the civilian porcelain body of the Early Ming Dynasty. values of the porcelain glaze are close to each other in the imperial and civilian porcelain. According to the documentary evidence (Peng et al. 2009), the recipe for the glaze is made up from plant ash and porcelain stone. From the perspective of the chemical composition of the glaze, there are no obvious differences in the imperial porcelains, so the raw materials of the imperial porcelain glaze of the Hongwu and Yongle periods are unchanged and have an inheritance relationship. The quality and formula changes of the Longquan celadon glazes varied over time (Li 1998). During the Southern Song Dynasty, the quality of the celadon glaze had been significantly improved, and the very famous Fen ching (lavender grey) and Meizi ching (plum green) had been created (Li 1998). However, the colour of the Longquan imperial porcelain in the Ming Dynasty was mainly yellow–green (see Fig. 2). So, what changes in the imperial porcelain glaze had taken place as compared with that of the Southern Song Dynasty? The formation of the glaze colour is related to many factors, and the major influences are the recipe, the glazing craft and firing temperature, and so on. In this paper, the data of Fen ching and Meizi ching (Li 1998; Xiong et al. 2004) for the Longquan glazes dating from the Southern Song Dynasty are compared with the analytical results obtained by EDXRF for the imperial porcelain samples of the Ming Dynasty.

As shown in Table 6, the average K2O contents of the Fen ching and Meizi ching glazes (4.00–5.36 %) were lower than those for the imperial porcelain samples (5.78–5.83 %) in the

Ming Dynasty, while the change of concentration for CaO was opposite to that for K2O. In the ceramics, K2O and CaO are often referred to the flux, which was used to reduce the firing temperature of the body and glaze. The difference is that the fluxing ability of K2O is stronger than that of CaO. In addition, the glossiness of the glaze is closely related to the oxide content with a high refractive index, the content of the flux and the glass phase (Zhang 1998). Increasing the chemical composition of K2O is advantageous for improving the refractive index of the glaze layer, reducing the firing temperature and the high-temperature viscosity of the glaze and

Table 5 The average values of each composition in the sample glaze .Li L.

Group Na2O (%) MgO (%) Al2O3 (%) SiO2 (%) K2O (%) CaO (%) TiO2 (%) MnO (%) Fe2O3 (%) CuO ZnO Rb2O SrO (ppm)

(ppm) (ppm) (ppm) al et

HWM 0.47 ± 0.13 0.67 ± 0.16 12.3 ± 1.2 69.4 ± 3.4 5.78 ± 0.82 7.80 ± 2.05 0.18 ± 0.03 0.31 ± 0.09 2.16 ± 0.13 73 ± 8 140 ± 40 291 ± 18 547 ± 125 . YLM 0.59 ± 0.13 0.67 ± 0.11 13.0 ± 1.1 69.2 ± 1.9 5.83 ± 0.32 7.25 ± 1.79 0.18 ± 0.04 0.35 ± 0.07 2.17 ± 0.26 74 ± 8 124 ± 36 296 ± 16 509 ± 95 EM 0.50 ± 0.13 0.63 ± 0.14 12.7 ± 1.2 69.7 ± 2.6 5.84 ± 0.69 7.31 ± 2.10 0.17 ± 0.03 0.32 ± 0.08 2.06 ± 0.30 68 ± 9112± 44 286 ± 24 568 ± 144 Characterization by EDXRF of imperial Longquan celadon porcelain 975

Table 6 The data of Fen ching and Meizi ching for Longquan glaze dating from the South Song Dynasty (Li Jiazhi 1998; Xiong Yingfei 2004)

K2O (%) CaO (%) TiO2 (%) Fe2O3 (%)

Fen ching (lavender grey) 4.87 8.39 – 0.95 5.06 9.94 – 1.1 4.53 7.95 0.07 1.54 4.25 10.7 0.06 1.29 Meizi ching (plum green) 4.41 9.88 – 0.91 5.36 9.05 – 1.32 4.12 9.21 0.06 0.82 4.00 9.99 0.08 1.68

increasing liquid formation at high temperature. Compared with Fen ching and Meizi ching in the

Southern Song Dynasty, the increased K2O content in the imperial porcelain glaze of the Ming Dynasty gives a glaze with good transparency and gloss.

In Tables 5 and 6, it can be also seen that the TiO2 and Fe2O3 contents of the imperial porcelain glaze of the Ming Dynasty are higher than those of the Southern Song Dynasty. The colour of celadon glaze mainly depends on the Fe2O3 content and on the firing temperature and atmosphere. The high levels of Fe2O3 in imperial porcelain make the glaze take on a yellow–green colour, as shown in Figure 2. According to the available documents, imperial porcelain began to be fired at the Fengdongyan kiln in the Hongwu era (ad 1368–1398) of the Ming Dynasty (Zhejiang Institute of Cultural Relics et al. 2009). As is known, in ancient times the best-quality porcelain was dedicated for palace use. In this work, the results of the analysis show that the raw materials for both the body and the glaze of the imperial porcelain are similar to those of civilian porcelain. Therefore, we can conclude that the production and firing technology of Longquan celadon had reached a higher level and did not decline in the Early Ming Dynasty.

CONCLUSION In this paper, EDXRF has been applied to analyse the elemental composition of Longquan imperial porcelain that was ﬁred at the Fengdongyan kiln in the Hongwu (ad 1368–1398) and

Yongle (ad 1402–1424) eras of the Ming Dynasty. The elemental contents of TiO2 and Fe2O3 in the body vary in the different cultural periods. Compared with Longquan glaze dating from the

Southern Song Dynasty (ad 1127–1279), the average values for K2O, Fe2O3 and TiO2 are higher, but the CaO content is lower in the imperial porcelain glaze. The PCA results show that the degree of elutriation of the raw material used for making the imperial porcelain body in the Hongwu period was different from that of the civilian porcelain, but the Yongle body is similar to that of the civilian porcelain in the Early Ming Dynasty. However, the raw materials of the imperial porcelain glaze of the Hongwu and Yongle periods show no obvious change and show an inherited relationship. The production and ﬁring technology of Longquan imperial celadon had not declined in the Early Ming Dynasty.

ACKNOWLEDGEMENTS The authors greatly appreciate the Zhejiang Province Institute of Cultural Relics and Archaeol- ogy, which provided ancient Chinese celadon excavated from the Fengdongyan site of the Longquan kiln. This work was ﬁnancially supported by the National Natural Science Foundation of China (11205167, 11305183 and 11175190).

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