The chemical composition and manufacturing technology of glass beads excavated from the Hetian Bizili site, Xinjiang

Abstract

1. Introduction

The Xinjiang Uygur Autonomous Region, located in the northwest of China, has been a vital area of cultural diversity and complexity [1]. Previously, archaeologists focused mainly in the Central Plains of China. Nowadays, Xinjiang has attracted more attention as the findings there could indicate the interactions among various populations in terms of trading goods, technologies, and cultures [2]. A significant number of glass beads in various styles have been excavated in the Xinjiang area during the past decades. From the spatial and temporal distribution of the glass beads in Xinjiang, the early glass products mainly distributed along the Tianshan Mountains. During the Han dynasty when the Silk Road was started, the glass beads in southern Silk Road have increased sharply, suggesting frequent crowd activities and trade exchanges during this period

The Bizili site situates in the southeast of the Bizili Village, Shanpula Township, Lop County, in Southern Xinjiang. The Xinjiang Institute of Archaeology excavated 40 tombs during the cooperation with road construction in 2016. According to the characteristics of these tombs, the date of Bizili site was from the Han Dynasty (202 B.C.–220 A.D.) to the Wei and Jin Dynasties (220 A.D.–420 A.D.). These tombs are all pit tombs and can be divided into knife-shaped pit tombs and rectangular pit tombs. The M5 is the former type. It is large and has more burials. Ninety-seven human bones were found in the M5 filling, with more females than males. Eleven more human bones were at the bottom of the tomb. Although the tomb was stolen and burned before it was excavated, a lot of relics are still preserved, such as potteries, woodwork, pieces of iron, woolen fabrics, and beadwork. Notably, a suet jade pendant was found for the first time in Xinjiang. In ancient time, Bizili belonged to Khotan, a kingdom on the south of the Silk Road. The dry climate makes it possible to preserve historic relics [3].

This paper aimed to analyze the chemical composition and manufacturing technology of the glass beads from the Bizili site by using various analytical techniques and to reveal the spread of the ancient glass products and cultural interactions along the Silk Road.

2. Materials And Methods

In this study, twelve glass beads excavated from tomb M5 of Bizili site which belong to Western Han dynasty (202 B.C.–8 A.D.) were selected and tested (from HLB-1 to HLB-12). Table 1 lists the details of the samples, including the sizes and types. Apart from two monochrome beads, the majority of the samples were glass eye beads, a special type with an eye motif on the monochrome surface of the bead [4]. The glass eye bead first appeared during the Eighteenth Dynasty of Egypt, related to the ‘evil eye’ [5]. These unearthed glass beads were generally in good conditions with weathering in different degrees on the surfaces. Photographs of the beads are shown in Fig. 2.

The manufacturing process of glass beads are complicated and the regional characteristics are obvious. In this study, we observed typical samples under both optical microscopy and synchrotron radiation microtomography (SR-µCT) to obtain their structural information. Sample surface was cleaned by ethyl alcohol. The optical microscopy observation was carried out in School of Cultural Heritage, Northwest University. The instrument was a KH-7700 from the HIROX Company, with a MX-5040RZF lens and a metal halogen cold light source. The sample was observed on a 50 times magnification. SR-µCT is a nondestructive 3D imaging technology that can clearly reflect the internal structure of the objects and suitable to apply for archaeology study. The samples were scanned by synchrotron radiation micro X-ray fluorescence at the Shanghai Synchrotron Radiation Facility, Shanghai, China. The charge-coupled device detector has a spatial resolution of 5.2 µm. The distance between the detector and samples is 10 cm with the source energy setting of 28 keV.

Chemical compositions of the samples were analyzed by micro-energy dispersion X-ray fluorescence spectrometry (XRF) and laser ablation inductively-coupled plasma atomic emission spectroscopy (LA-ICP-AES). For better preserved monochrome beads without complex ornaments, the bulk chemistry was determined by the non-destructive micro-energy dispersion XRF at the School of Cultural Heritage, Northwest University. The instrument model is BRUKER ARTAX 400, with a primary beam of 1 mm in diameter. The X-ray energy was 30 kV, the current was 900 µA, and the counting time was 300 seconds. The carrier gas was helium and the standard samples used for calibrations were Corning Glass A, B, C and D. Other beads decorated with complex patterns were analyzed by LA-ICP-AES, which could accurately measure the chemical compositions of glass beads in different parts. The experiment was carried out in the school of Archaeology and Museology, Peking University. The UP266-MARCO laser ablation system of NEW-WAVE (USA) was connected with the Prodigy full spectrum direct-reading ICP-AES of LEEMAN-LABS (USA). The standards for calibration were Corning Glass B, C, and D, and the relative standard deviation of major elements can be less than 0.01. The laser spot diameter was 610 µm.

Raman spectroscopy is a non-destructive method for analyzing material structure by obtaining the fingerprint frequency of samples without special requirements for the samples. It was performed at the Emperor Qin Shihuang’s Mausoleum Site Museum (Xi'an, Shaanxi) at room temperature, using a 514 nm Nd:YAG laser for the spectral range from 100 to 1000 cm^− 1. Laser energy of 10 mW was employed, and the acquisition time for spectrum was about 10 s for each time and accumulated 3 times.

3. Results

1. Chemical components

The only two monochrome glass beads are blue (HLB-11) and transparent (HLB-7), respectively. Both of them were analyzed by energy-dispersive XRF, while the remaining samples were analyzed by LA-ICP-AES. The glass components of different colors were tested and the chemical compositions were given as oxides in Table 2.

Based on the chemical compositions, all the glass beads from the Bizili site are soda-lime glass but with differences in the fluxes. Soda concentrations range from 1–12% and the lime concentrations vary from 1–15%, due to weathering in different degrees [6].

Four of the opaque glass eye beads (HLB-1, HLB-2, HLB-4, and HLB-8) are similar in chemical compositions. Figure 3 suggests the contents of SnO₂ and PbO in the yellow eyeballs are significantly higher than that in the blue base beads.

2. Raman Spectroscopy Test

As shown in Fig. 1, eight out of twelve glass eye beads (HLB-1 to HLB-6, HLB-8, and HLB-9) are opaque. The Raman spectroscopy analysis was used to identify the mineral chemical characterization of their opacifiers. The results are shown in Fig. 4.

The results of Raman spectroscopy tests in the yellow eyeballs show peaks at the shift of 134 cm^− 1 and 322 cm^− 1, similar to the lead-tin yellow, a synthetic material which is widely used in glass-making to get yellow and opaque glass [7]. According to the structures and chemical compositions, the lead-tin yellow has two types as Pb₂SnO₄ and PbSn_1 − xSi_xO₃. Compared with the standard spectrum [8], the opacifiers of the yellow eyeballs are PbSn_1 − xSi_xO₃.

The four beads among the opaque beads (HLB-3, HLB-5, HLB-6, and HLB-9) show high contents of antimony in the white parts, ranging from 1.3 wt.% to 8.1 wt.%. In Fig. 5, the peaks at the shift of 232 cm^− 1 and 667 cm^− 1 are identified as calcium antimonite (Ca₂Sb₂O₆), a common opacifier [4].

3. Microscopic Observation

Although the quantities of samples from Bizili are limited, the glass beads are rich in manufacturing processes. We found three special types of glass beads, i.e. the glass eye beads, the etched beads and the glass beads with pigments. Figures 6–8 show the microscopic observation of representative glass beads from different groups. The optical microscope can reveal the machining traces on the surface of glass beads, while SR-µCT can reveal the internal structures.

The etched glass bead HLB-10 is fragmentary. Its yellow band decoration could be observed under the microscope as shown in Fig. 7a. Figure 7b and 7c show the SR-µCT of the body and the decoration, respectively.

Figure 8 shows the decoration of the glass bead with pigments under the microscope.

4. Discussion

5. Conclusion

The early glass production mainly occurred along the Tianshan Mountains in Xinjiang, related to the Mesopotamia and Western Asia. After the Western Han Dynasty, glass production along the south of the Silk Road grew explosively, and the impact of India had become manifest [32][33].

The Bizili site locates in the Hotan Prefecture, Xinjiang, which belonged to the kingdom of Khotan (a powerful country in the south of the Silk Road). This place has been important in economic, trade and cultural exchanges between East and West. This research suggests that the Bizili site was influenced by many cultures, especially the Bara site of Ancient India. Based on available archaeological materials, many sites along the south of the Silk Road had a great connection with the Bara site (a glass-making and processing workshop). The Bara site was located in the Peshawar region of northern Pakistan, ruled by Kushan (55 A.D.–425 A.D.). Most scholars suggest that people from Kushan Empire entered the southern margin of the Tarim Basin after 200 A.D. However, archaeological data suggest great connections between Xinjiang and Central Asia in culture, economy, and politics occurred as early as 400 B.C. In Shanpula county (Khotan), Xinjiang, a large number of tombs excavated in 1984 (3rd century B.C.–3rd century A.D.) contain artifacts representing distinctive features of the Ancient Rome and Ancient Greek[34]. The migration of people is often accompanied by the spread of culture, technology, and commodities, so glass products appeared in large quantities during this period. Apart from glass products, a large number of Kharosthi documents have been excavated at many sites along the Southern Silk Road from the late 19th century until present (over 1000 pieces). Kharosthi originated from ancient Gandhara, written in the Asoka period of the Indian Peacock Dynasty in the 5th century B.C. [35], and was popular in Central Asia. In 175 A.D., the officers and soldiers of the Eastern Han Dynasty withdrew from Xinjiang, resulting in a weakened connection between Khotan and the Central Plains. Meanwhile, the influence of the Kushan Empire on Khotan gradually increased. Hence, Kharosthi, the official language used by the Kushan Empire, began to prevail in Khotan. In addition, a special coin found in the south of Xinjiang has Kharosthi and Chinese engraved on its two sides, respectively [34], suggesting the influence of the Kushan Empire.

This study analyzed glass beads from the Bizili site located on the southern road of the Silk Road. The beads are all soda-lime glass and can be divided into natron glass and plant ash glass based on their fluxes. Considering the characteristics of manufacturing technology of glass beads, although the number of glass beads was small, most beads have strong foreign cultural characteristics that were believed to have originated from many regions. The influence of Ancient India is particularly obvious.

Declarations

Authors’ contributions

Dong Wang and Rui Wen designed the research described in this paper. All the experiments had been done by Dong Wang. Dong Wang and Rui Wen drafted the majority of the manuscript. Xingjun Hu and Wenying Li provided the samples in this study. All authors read and approved the final manuscript.

Acknowledgment

This study was sponsored by the National Natural Science Foundation of China (Grant No. 11575142). Special thanks go to the Shanghai Synchrotron Radiation Facility for helping with the analysis and the Xinjiang Institute of Cultural Relics and Archaeology for providing the glass beads samples in this study. Many thanks also to due to Xiaoya Zhan for suggestions of modifying the language.

Competing interests

The authors declare that there are no conflicts.

Availability of data and materials

Please contact the corresponding author upon reasonable for data requests.

Funding

Beyond the National Natural Science Foundation of China (Grant No. 11575142) support, the school of cultural heritage, Northwest University also supported this research (2019WYYCY-04).

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Author information

Affiliations

Key Laboratory for Cultural Heritage Study & Conservation (Northwest University), Ministry of Education, Xi’an, China

Dong Wang, Rui Wen

Research Center for Archaeological Science, Northwest University, Xi’an, China

Dong Wang, Rui Wen

Xinjiang Institute of Cultural Relics and Archaeology, Urumchi, China

Xingjun HU c, Wenying Li

Corresponding author

Correspondence to Rui Wen.

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