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.
Table 2
Principal composition of the glass beads(wt.%)
Sample ID | SiO2 | Al2O3 | Fe2O3 | MgO | CaO | Na2O | K2O | MnO2 | P2O5 | Sb2O3 | PbO | BaO | SnO2 |
HLB1-blue | 46.14 | 9.34 | 3.20 | 2.34 | 15.99 | 7.76 | 6.50 | 0.29 | 4.16 | 0.15 | 0.23 | 0.16 | 0.44 |
HLB1-yellow | 63.96 | 6.69 | 0.86 | 3.57 | 5.11 | 3.79 | 5.44 | 0.07 | 0.64 | 0.06 | 7.02 | 0.15 | 1.86 |
HLB2-blue | 66.40 | 8.12 | 1.86 | 2.65 | 6.27 | 3.76 | 4.97 | 0.15 | 2.93 | 0.06 | 0.14 | 0.24 | 0.46 |
HLB2-yellow | 52.33 | 4.89 | 1.03 | 1.69 | 4.23 | 2.48 | 3.36 | 0.05 | 2.85 | 0.07 | 20.03 | 0.41 | 5.43 |
HLB3-blue1 | 71.07 | 2.58 | 0.86 | 2.03 | 3.45 | 10.62 | 5.94 | 0.99 | 0.29 | 1.40 | 0.08 | 0.07 | 0.02 |
HLB3- blue2 | 67.44 | 4.37 | 0.57 | 3.32 | 3.43 | 12.55 | 6.20 | 0.08 | 0.49 | 0.01 | 0.00 | 0.09 | 0.04 |
HLB3-white1 | 72.54 | 4.81 | 1.77 | 2.39 | 7.51 | 2.29 | 5.30 | 0.32 | 0.29 | 1.35 | 0.21 | 0.21 | 0.07 |
HLB3-white2 | 75.49 | 5.22 | 2.25 | 2.10 | 2.38 | 1.94 | 5.63 | 0.36 | 0.92 | 2.21 | 0.22 | 0.04 | 0.09 |
HLB4-blue | 75.74 | 3.08 | 0.44 | 2.29 | 2.13 | 9.82 | 5.18 | 0.05 | 0.25 | 0.00 | 0.00 | 0.14 | 0.02 |
HLB4-yellow | 66.89 | 4.82 | 0.50 | 3.05 | 3.43 | 8.57 | 7.11 | 0.14 | 0.40 | 0.01 | 4.17 | 0.11 | 0.46 |
HLB5-black1 | 81.12 | 2.30 | 1.06 | 0.74 | 2.41 | 1.18 | 1.04 | 0.38 | 0.38 | 7.63 | 0.38 | 0.04 | 0.14 |
HLB5-black2 | 87.32 | 3.03 | 1.69 | 0.56 | 1.85 | 1.00 | 1.84 | 1.25 | 0.29 | 0.05 | 0.00 | 0.13 | 0.11 |
HLB5-white | 72.55 | 2.40 | 0.53 | 0.50 | 1.52 | 2.88 | 2.17 | 0.19 | 0.41 | 8.49 | 7.11 | 0.01 | 0.40 |
HLB6-white1 | 72.70 | 3.13 | 0.61 | 0.65 | 2.40 | 3.66 | 3.54 | 0.55 | 0.44 | 7.81 | 3.65 | 0.02 | 0.17 |
HLB6-blue | 87.32 | 2.65 | 1.03 | 0.76 | 2.54 | 1.75 | 1.98 | 0.36 | 0.34 | 0.09 | 0.06 | 0.03 | 0.13 |
HLB6-white2 | 70.20 | 2.40 | 0.48 | 0.74 | 2.55 | 5.75 | 2.75 | 0.76 | 0.35 | 8.15 | 5.40 | 0.02 | 0.08 |
HLB7 | 91.72 | 1.27 | 0.74 | 0.88 | 2.70 | 0.16 | 1.45 | 0.07 | | 0.05 | 0.07 | 0.69 | 0.05 |
HLB8-yellow | 67.60 | 4.60 | 1.06 | 2.52 | 2.83 | 8.64 | 5.95 | 0.13 | 0.36 | 0.00 | 5.00 | 0.11 | 0.76 |
HLB8-blue | 83.84 | 6.69 | 0.48 | 1.70 | 1.84 | 1.00 | 2.71 | 0.09 | 0.29 | 0.01 | 0.03 | 0.11 | 0.09 |
HLB9-black | 72.34 | 6.86 | 0.74 | 0.42 | 2.15 | 3.05 | 5.45 | 0.22 | 2.93 | 0.59 | 0.09 | 0.03 | 0.97 |
HLB9-white1 | 74.27 | 2.55 | 0.37 | 0.55 | 3.57 | 5.94 | 2.30 | 0.73 | 0.26 | 3.40 | 5.60 | 0.03 | 0.06 |
HLB-white2 | 89.66 | 1.45 | 1.02 | 0.66 | 2.64 | 1.29 | 1.20 | 1.30 | 0.10 | 0.00 | 0.00 | 0.02 | 0.02 |
HLB10-1 | 68.17 | 6.77 | 2.36 | 3.42 | 7.02 | 5.49 | 4.62 | 0.68 | 0.26 | 0.00 | 0.00 | 0.16 | 0.19 |
HLB10-2 | 72.86 | 7.19 | 1.37 | 2.16 | 3.89 | 5.85 | 4.08 | 1.25 | 0.02 | 0.11 | 0.00 | 0.13 | 0.25 |
HLB-11 | 81.24 | 4.31 | 1.30 | 1.71 | 4.23 | 0.49 | 2.80 | 0.22 | 0.02 | 0.28 | 0.02 | 0.76 | 0.13 |
HLB12-blue1 | 63.46 | 4.28 | 0.54 | 7.98 | 11.89 | 3.50 | 4.76 | 0.18 | 0.73 | 0.05 | 0.00 | 0.47 | 0.26 |
HLB12-blue2 | 69.46 | 5.33 | 0.52 | 4.37 | 2.40 | 9.33 | 6.80 | 0.09 | 0.41 | 0.00 | 0.00 | 0.08 | 0.02 |
HLB12-blue3 | 66.32 | 4.99 | 0.48 | 4.06 | 3.38 | 12.91 | 6.05 | 0.08 | 0.39 | 0.00 | 0.00 | 0.08 | 0.02 |
HLB12-black | 14.62 | 1.74 | 1.24 | 3.69 | 6.66 | 3.06 | 2.23 | 44.77 | 8.48 | 2.63 | 9.72 | 0.07 | 0.04 |
HLB12-pink | 54.56 | 4.16 | 2.97 | 12.11 | 5.54 | 2.08 | 5.22 | 0.40 | 8.02 | 0.08 | 2.79 | 0.06 | 0.09 |
HLB12-white1 | 44.02 | 3.01 | 1.87 | 2.43 | 6.40 | 2.64 | 2.69 | 5.38 | 7.69 | 1.55 | 21.46 | 0.05 | 0.07 |
HLB12-white2 | 20.50 | 2.11 | 1.41 | 2.97 | 10.20 | 2.40 | 2.71 | 9.26 | 17.18 | 1.73 | 28.14 | 0.04 | 0.06 |
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 PbSn1 − xSixO3. Compared with the standard spectrum [8], the opacifiers of the yellow eyeballs are PbSn1 − xSixO3.
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.