4.1 Chemical compositions
The plot of MgO versus K2O of the glass beads shows two distinctive groups. The first group (HLB-5, HLB-6, HLB-7 and HLB-9) with low MgO and K2O contents could be considered as natron glass but with low Na2O, which was possibly influenced by weathering. The natron glass is the predominant type of ancient glass in the Mediterranean and Europe from the middle of the 1st millennium B.C. until the 9th century A.D. [18][19]. Based on the research of Brill [26], these four samples are similar to the European glass in chemical composition. For example, the glass from Cosa (Roman) has MgO of 0.64%, K2O of 1.12%, and Al2O3 of 2.43%, which are almost the same with our samples. In addition, the HLB-9 black has high contents of K2O (5.45%) and Al2O3 (6.84%), possibly caused by the use of impure sand or raw materials. Further research is needed to investigate the cause. HLB-5, HLB-6, and HLB-9 use white opaque glass as the base beads with different-colored eyeballs. The Fe2O3 contents in the black eyeballs are higher than that in the white base, making them appear black. The blue eyeballs mainly use Co as a coloring agent. Notably, HLB-7 has high transparency without perforation, which is possibly a semi-finished glass product.
The second group beads (HLB-1, HLB-2, HLB-3, HLB-4, HLB-8, HLB-10, HLB-11, and HLB-12) are characterized by high levels of both MgO and K2O (over 1.5 wt.%). Brill suggested the MgO and K2O contents above 1.5 wt.% indicated the use of plant ash during manufacturing [20]. Plant ash is quite variable in chemical composition because of different species of plants, growing environment, the plant parts used, and the burning temperatures [21]. Thus, the chemical compositions of plant ash glass vary slightly from region to region. The eight plant ash glass beads from the Bizili site might be from different regions. Five samples (HLB-1, HLB-2, HLB-4, HLB-8, and Hlb-10) are vegetable soda-lime glass (v-Na-Al glass) with high Al2O3. This type of glass has high Al2O3 over 4%, and MgO and K2O higher than 1.5%. Dussubieux L [22] identified three main types of v-Na-Al glass. The first type was likely produced in Bara, Pakistan, a site dating from 200 B.C. to 200 A.D., and they are mainly glass ornaments found in North India, China (Xinjiang) and Bangladesh [23]. The other two types of v-Na-Al glass appear at younger sites dated from 900 A.D. and onward. Considering the spatial and temporal distribution of the v-Na-Al glass, our samples probably originated from Bara. The high contents of Al2O3 probably indicate the use of sand during production. Four of the five glass beads have similar appearance that comprises of blue glass base bead and yellow eyeballs with red and black glass concentric ornamentation made by different techniques. This special type of glass eye beads is found in large quantity in Bara and named as the Bara-type beads [2][10]. In terms of chemical composition, the Bara beads are soda-based and characterized by relatively high Al2O3 with CaO varying from 4.4 to 9.4%[24], similar to our samples. Collectively, the appearance, chemical compositions and distribution characteristics all suggest our glass bead samples originated from Bara.
We undertook comparative studies of other glass from literature to investigate if their compositions are representative of the glass from Central Asia, India and Mediterranean of the contemporary period. Some scholars considered potash and aluminum are very useful indicators in distinguishing plant ash glass from different origins [17]. In general, most plant ash glass production with K2O higher than 4% suggests an origin from Central Asia, otherwise it might be from Western Asia. From the research of Brill [26], a plot of Al2O3 versus K2O shows distinctive groups of beads from Mesopotamian, Indian, and Afghanistan. As shown in picture 9, both HLB-3, HLB-11 and HLB-12 fall into the Afghanistan origin, suggesting that they probably originated from Central Asia.
4.2 Opacifiers
Ancient craftsmen obtained opaque glass by adding opacifiers. Glass opacification process is often characterized by significant amounts of particles dispersed into glassy matrices. These particles include tin oxide, lead oxide, calcium antimonite, lead antimonite, etc[27]. The opaque glass beads from the Bizili site have antimony-based and tin-based opacifiers, of which the former was used earlier than the latter in glass production.
The antimony-based opacifiers were used in Egypt and the Near East in opaque glass production in the mid-second millennium B.C. [28], including lead-antimony oxide yellow and calcium antimony oxide white, until about the fourth century A.D.
Compared with antimony-based opacifiers, the tin-based opacifiers appeared later during the first to second centuries B.C.. The lead-tin oxide yellow glass appeared in Northwestern Europe, but the tin oxide white glass in this period is rare [27]. In about the fourth century, lead-tin oxide yellow glass was used in the Eastern Mediterranean and Levant and became an important part of glass making. During the early Islamic period (the eighth century), lead-tin oxide glass continued as shown in a set of glass tesserae. However, in the later Islamic period (ninth to tenth centuries), the tin oxide was more commonly used in Iraq and Iran [29-30]. The latest research tested lead stannate in a glass bead from Sardis (the eighth to the seventh century B.C.), which is the earliest known occurrence of tin-based opacifiers so far [31].
The lead-tin yellow exists as Pb2SnO4 or PbSn1-xSixO3, both with high refractive indices (> 2) [32]. During the production process of lead-tin yellow glass, the lead-tin calx, the fine powder that is left after heating a mixture of lead and tin to their melting point and beyond to temperatures above 600 °C is very important. The typical opaque yellow glass has a median Pb/Sn value of 9.1 [27]. In this study, except for HLB-4 with a Pb/Sn value of 9.07, the other three glass beads have relatively low Pb/Sn values (HLB-1=3.79; HLB-2=3.70; HLB-8=6.60).
The use of opacifiers was not a common feature of glass-making in Ancient China, because antimony-based opacifiers and tin-based opacifiers were not found in lead-barium glass, potassium glass or high-lead glass made in China. Thus, the opaque glass beads from the Bizili site were imported glass products. As the glass spread eastward, the use of opacifiers also gradually affected other areas. The glass beads produced at the Bara site were likely influenced by the West.
4.3 Manufacturing technology of glass beads
4.3.1 The glass eye bead
The Bara-type glass eye beads and other glass eye beads show obvious differences. The Bara-type glass eye beads share similar appearances and chemical compositions but are of low quality. Eyeballs are not bonded closely to the base beads and often fall off (Fig. 5b). Comparatively, other glass eye beads are well-made, and they look the same with the layered glass eye beads which were prevalent in the Mediterranean from the 6th to 3rd centruies B.C. In 1500 B.C., faience with similar appearance appeared in Egypt [33] and then gradually spread to Persia, South Russia, Central Europe and other regions [34][35]. During the East Zhou Dynasty (770 B.C.–256 B.C.), glass production and the glass-making technique have spread into China. Compared with glass vessels, the technique of making a glass bead is simpler, so the early glass products in China are mainly glass beads. Available archaeological records show a large number of similar glass beads found in southern China [36]. These glass beads are mainly lead barium glass that is uniquely made in China in addition to natron glass. The development of China's early glass industry was likely influenced by layered glass beads. The craftsmen learned the glass bead manufacturing techniques and imitated the appearance of layered glass beads with local raw materials.
The microscopic features of glass eye beads shown in Fig. 10 indicate two ways to make eyeballs. In Fig. 10a, it is clear that the decoration is formed by overlapping glass of different colors from the top to the bottom. The manufacturers firstly obtained glass wafers with concentric circles by dropping glass liquid of different colors in sequence (Fig. 10b) as shown by the traces of glass flowing, and then inserted them into the base beads. The other method used a glass rod to dip glass liquid of different colors in sequence to produce concentric circles. When it was solidified, the rod was cut into blocks of suitable sizes and embedded into the base beads (Fig. 10a).
The variation of brightness on a CT slice reflects the differences in density and chemical composition, so the base beads and eyeballs can be clearly distinguished by different brightness. In general, air bubbles in glass were standard spherical shapes during the vitrifying process [37]. In Fig. 5e, a lot of air bubbles of different shapes spread across the base beads. Between the base beads and the eyeballs where the bubbles accumulated, the air bubbles are oblate and bigger than elsewhere. It indicates while the base beads were in a molten state, the craftsmen inserted the pre-made eyeballs into the base beads. Between the base beads and the eyeballs, the air bubbles were deformed by external pressure. As shown in Fig. 5e, the arrows indicate the pressure direction of inserting eyeballs into the base beads. Many small air bubbles fused here to form a bigger bubble. This provides further evidence that the base beads and eyeballs were separately made.
We drew a three-dimensional model based on the SR-uCT data of sample HLB-1 as shown in Fig 11 to identify how the glass beads were made.
In general, the ancient craftsmen mainly made glass beads by winding and drawing [38], normally, the bubbles in glass matrices were formed into standard spherical shapes during the vitrifying process [37]. However, the slender bubbles parallel to the perforation in Fig11 B indicate that the glass bead was made by drawing. Ancient manufacturers used tools called Lada, a long hollow metal pipe, in conjunction with a mobile inner rod known as the chetak [39] to draw glass into glass tubes, and then cut it into glass beads. Slender bubbles are the main feature of glass beads made by drawing. Figures11 C and11 D are enlargement of bubbles revealing more details. The shape of bubble in Fig11 D is different from others with an ellipsoid at one end. In the glass beads making process, the shape of bubble changed a lot by the influence of pull (the arrow represents the direction of the pull).
Table.5 The information of the sites
Site
|
Age
|
Jierzankale
|
600 B.C.–400 B.C.
|
Shanpula
|
100 B.C.–400 A.D.
|
Niya
|
202 B.C.–420 A.D
|
Bizili
|
202 B.C.–8 A.D.
|
In addition to the Bizili site, the Bara-type glass beads were found in many other sites of Xinjiang, including the Shanpula site [8], the Niya site [10], and the Jierzankale site [11]. Table 5 lists the details of the archaeological sites with the Bara-type glass beads in Xinjiang. From Table 5, except for the Jierzankale site, other sites correspond to the age of the Bara site (2nd B.C. to the 2nd A.D.). In addition, these sites, except for the Jierzankale site, have also excavated bi-color glass beads made of blue ground embedded with yellow curved stripes [37], which originated from Bara as well. These suggest the Bara-type glass eye beads from the Shanpula site, the Niya site, and the Bizili site may originate from the Bara site. Archaeological dating results show the lower age limit of the Jierzankale site is obviously earlier than the age of the Bara site, indicating the glass beads of blue monochrome base decorated with multi-color eyes were not created by Bara. The Bara site was probably influenced by other neighboring glass-making sites.
4.3.2 The etched glass bead
As shown in Fig.6, though the bead HLB-10 is fragmentary, the yellow band decorations could still be observed under the microscope. Fig. 6a clearly shows the position of the decoration, while Figs. 6b and 6c show many differences between the body and decoration on the SR-μCT. In Fig. 6b, the body is dense with scattered round air bubbles. The consistent brightness reflects a uniform glass phase in chemical composition. Comparatively, contrasting brightness exists in Fig. 6c. The inner part of the glass bead is brighter and dense with few bubbles, whereas the surface of the glass bead is darker and the structure is loose and porous with a large number of bubbles. The cause for this phenomenon is likely related to the manufacturing technology that used the same method with the etched carnelian bead, which originated in the Indus Civilization. The etched carnelian beads were found at the Chanhu-Daro site of the Harappa Culture in the Indus Valley at first. The craftsmen firstly mixed a special plant juice with alkali, then etched out patterns on the base bead, and buried the beads in charcoal ash for permanent decoration. A large number of etched carnelian beads have been excavated in India and they display different patterns of decoration, including the cruciform ornament [40]. According to the SR-μCT results, the etched glass bead in this study was probably made by the same process as the etched carnelian bead was. The decoration area has a lot of bubbles because of chemical reactions between glass and alkali. The alkali solution penetrates downward, causing the corrosion layer. It is likely a new type of glass bead influenced by the etched carnelian beads. Both studies on chemical composition and the manufacturing technology indicate the etched glass beads from the Bizili site probably originated from India.
4.3.3 The glass bead with pigment
The glass beads with pigment (HLB-12) have blue base beads with eye-like decorations. According to the results of LA-ICP-AES, the base bead is plant ash soda-lime glass, though the content of SiO2 in the decoration area is very low. The black area is high in MnO2 (44.7%) and the white area contains 21.4%–28.1% of PbO, indicating that the material used on the surface was probably inorganic mineral pigments rather than glass. From optical observations, the pigment layer is very thin, and less than one millimeter with a lot of particles. In the process of making glass beads, the craftsmen fused the monochromatic glass to make the base beads and then drew different ornamentations on the base beads with pigments of different colors. To combine the base beads and the pigments, these glass beads were probably fired again. When the temperature is high, the decorations may become less regular due to the fluidity of glass.
The glass beads with pigment are rarely found in archaeological excavations except in China. Based on archaeological reports [15, 41], using pigments to decorate glass beads has already appeared in Central Plains (China) during the Warring States period. The features of chemical composition of HLB-12 correspond to Central Asia, suggesting such decorating method probably appeared in Central Asia in the Western Han dynasty. Given that the technology appeared earlier in China than in Central Asia, we don't exclude the possibility that Central Asia was influenced by China.