3.1 Lead isotope analysis
The Plots of lead isotope ratios contains 8 samples from G-1, G-2, G-3, G-4, G-5, G-6, G-8 and G-10. The lead isotope ratios characterized by 206Pb/204Pb ratios ranging from 18.125 to 18.533, 207Pb/204Pb ratios ranging from 15.652 to 15.699, and 208Pb/204Pb ratios ranging from 38.491 to 38.854 (shown in Fig. 4). By comparing the lead isotope data of Eastern Han Dynasty and the distribution patterning of the Pb isotopes in China, it is found that the lead isotope date of the Gaigongmao fall within the range of the Yangtze River basin (Hsu and Sabatini, 2019).
3.2 Ultra depth of field 3D microscope analysis
The 3D microscope images were prepared for the scratches on the surface and the casting jointed places of the samples, as shown in Fig. 4. Many scratches were observed with different depth and directions, Fig. 4 (a), especially on the spherical top surface of the Gaigongmao. It was inferred that, the scratches were left by the “Yaguang” technic in the process of the gilding technology. In ancient China, the gilded layer was applied by hand-made, to make the gilded layer firm and the color uniform, the “Yazi” which was made up by jade was invented and used for press the gilded layer, aim to make the layer tight during the gilding process (Cao, 2010).
Meanwhile, many deeper scratches were observed in Fig. 5, they were hard to see by eyes but have the same direction with small and dense features. As shown in Fig. 5 (b), the scratches on G-2 were upright with the same direction. The scratches on G-4 were uniform and shown by slanting lines, in Fig. 5 (c). On the top part and the casting joint of the samples, many deep polish lines appear with the same direction, we infer that the gilt-bronze was polished before the gilded process, and many kinds of handcraft were used for ensure the shape of the samples.
Furthermore, grooves beside many rasping can be seen in the casting joints, the rasping have the same direction and deep depth, and on the bottom of the grooves many gilded layers can be seen, so that, the grooves were produced before the gilded process, shown as Fig. 5 (d), (e), (f), (g), (h), (i). The details of the grooves in the samples of G-1, G-2, G-5, G-6, G-7, G-10, G-11 are shown in Fig. 6. The grooves by hand-made observed above the Zao, and sample G-3, G-8 show a groove blow the Zao, combine with the places fibers around, we can infer that the grooves may have the functional properties of secure a rope.
3.3 ART-FTIR analysis of the fibers
ART-FTIR has been widely tested for the investigation on fibers, we sampled the fibers around G-5, G-6, G-8. Comparison of the spectra of mineralized excavated cellulosic fibers with spectra of modern fibers, has enabled fiber identification and observation of certain peak differences attributed to mineralization.
In Fig. 7 (a), for the peak at 2900 cm− 1 ν(C-H) stretching vibration, indicative of polysaccharides. 1635 cm− 1 corresponding to vibration of water molecules absorbed in cellulose (Margariti, 2019), 1365 cm− 1 for δ(C-H) bond in cellulose, 1155 cm− 1 is the ν(C–C) ring breathing of the indicative of polysaccharides in cellulose, 1105 cm− 1 ν(C–O–C) glycosidic ether band which indicative of polysaccharides in cellulose (Ludovico Geminiani and Francesco Campione, 2022).
Table 2
The diameter of the micro-fibers from G-8
| microfiber1 | microfiber2 | microfiber3 | microfiber4 | microfiber5 | microfiber6 |
Diameter (µm) | 24 | 22 | 18 | 17 | 16 | 19 |
Average (µm) | 19.33 |
Guided by the ATR-FTIR analysis results of hemp (Alessia, 2022), 1735 cm− 1 ν(C = O) ester will appear within the fibers, but there aren’t any peak in 1735 cm− 1 appear in the ATR-FTIR spectra of the fibers from G-5, G-6 and G-8, therefore, hemp can be preliminarily ruled out. Jute is a kind of bast fiber, and jute contains much more lignin than the others (Jakes, 2007), and thus shows clearer lignin peaks at 1595 cm− 1, so the fibers around the Gaigongmao should not be jute, either.
In the detail image of the fibers of ultra-depth of 3D microscope, Fig. 7 (b), many micro-fibers can be observed in G-8, with mid-cavity and natural curling in the longitudinal fiber bundle, only little crazing shown in longitudinal of the fiber, moreover, there aren’t irregularly bamboo-like transverse segments arranged, the average diameter is 19.33µm (Table 2), which consistent with the average diameter and the characterizations of cotton fibers.
As stated above, compare with the ART-FTIR analysis results of other natural fibers (Tanil and Sevim, 2017), the fibers sampled from G-5, G-6 and G-8 should be identified as cotton by preliminary.
3.4 Wood species identification
The wood in G-5 should be identified as bamboo, Fig. 8. According to the papers, bamboo began to be the material of the Gaigong in the Han Dynasty. Usually, bamboo Gaigong was used for San type, this type of chariot parasol was ride by males with strict ride manners because of the open-air (Gu Ban and Sun, 2020; Yi Jia and Yan, 2000). On the other hand, the Peng type was ride by females and peerage, this type was composed by hard and thick woods, with an enclosed space (Huang, 2023).
The identified of bamboo Gaigong and the tiny size of the Gaigongmao are the strong evidences of the San type.
3.5 Virtual restoration of the winding patterns
The function of the Gaigongmao was to fastening the Gai and keep it open, the threads around them play a role in tied around the Gaigong and stretch the Gai round. Base on the places the grooves in and the traces the fibers around, as shown in Fig. 9 (a), we can infer that, the effect of the groove was to lock the fibers in their position.
Combine with the position where the grooves and the fibers on, we diagram the virtual restoration pictures of the winding patterns, Fig. 9 (b). The usage of the Gaigongmao may was that, at first, making a cotton thread winding though the groove below the Zao, winding though the joint place, then, making the cotton thread winding though the groove above, to tie a knot of the line on the opposite side, at last, tie the ends through the Gaigong and fixing them together.
The literature indicated that there are holes in the bottom of Gaigong in order to securing the ropes (Sun, 1985), but there isn’t any evidence about the holes in this paper, thus, it doesn't do any additional discussion here.
3.6 Scientific analysis and the gilding technology of the gilt-bronze
Base on the report on the excavation, gold and silver have been noticed by Chinese people at the late Neolithic period, a golden earring has been excavated in the Yumengou site (Weiyuanhui, 1979), Gansu province, China. The earliest gilded artefact was reported from the Spring and Autumn Period (Mou, 1984), at the Shizishan cemetery, Shaoxing city, China. Thus, it is considered that, the gilding technology was invented at the late Spring and Autumn Period, start to developed in the Warring States Period, and became popular in the Qin and Han Dynasties (Qi, 1998).
The technology of mercury gold gilding is well known as fire gilding (Ma and Scott, 2002), the ancient method about fire gilding can be summarized for three steps. Firstly, polish the surface of the bronze, dip a little diluted acid and spread it evenly (Wang, 1984), then, wipe the surface by a new cotton cloth or a clean fur until it is smooth enough, the smoothness performance of the bronze surface was directly related to the gilding result. Secondly, forge the gold and formed it into a thin gold paper, cut the gold paper into pieces, and put them into a crucible filled with mercury, the proportion was gold: mercury = 1: 7 (Jiang, 2010), cover the crucible tightly, the gold will dissolve in the mercury when the temperature reaches 400 ℃, after that, the Hg-Au amalgam can be produced (Chen, 2007). Finally, apply the Hg-Au amalgam on the surface of the bronze artefacts and spread evenly. In this step, some craftsmen would choose to brush some mercury on the surface at first, spread the amalgam evenly by a bristle brush, then, heating the object, a thin gold layer remains when the Hg evaporated by high temperature. Repeat the last step 3 ~ 4 times, the gilding technology has been finished.
It is reported that, the thickness of the gilded layers can reach 2–10 µm (Jin et al., 2016) by mercury gilding technology. In this paper, the bottom of G-5 was sampled (Fig. 10), and surveyed by element mapping and EDS.
Table 3
The bulk element compositions of the gilded layer, cross-section and the surface with none gilded layer samples obtained by EDS
| Element (wt. %) |
Au | Ag | Hg | Cu | Sn | O | C |
Gilded layer of G-2 | 47.64 | 3.94 | 4.14 | 9.26 | 11.14 | 12.87 | 11.01 |
Gilded layer of G-5 | 64.46 | 7.65 | 5.76 | 7.56 | 2.48 | 4.35 | 7.75 |
Gilded layer of G-6 | 38.04 | 2.55 | 3.56 | 13.58 | 14.90 | 18.55 | 8.82 |
Gilded layer of G-8 | 65.33 | 11.98 | 6.87 | 12.98 | 1.53 | 1.32 | - |
Cross-section of G-5 | - | 1.10 | - | 17.01 | 35.51 | 35.41 | 2.81 |
The surface with none gilded layer of G-3 | - | 0.24 | - | 4.43 | 43.41 | 37.98 | 9.34 |
The element mapping images of the gilded layer from G-5 are shown in Fig. 10, the elements Au, Ag, Hg, Cu, Sn and O were detected. The EDS results of the gilded layer (G-2, G-5, G-6, G-8), the cross-section of G-5 and the place gilded layer peeled off sampled from G-3 are reported in Table 3. Among them, Hg was commonly found in the gilded layer, which is characteristic feature of the gilding. Shown as the images of Au, Ag and Hg in Fig. 9, the shape of the peeled off gilded layer can be clearly profiled, proving the Au and Hg was only exist in the gilder layer. It’s worth noting that Ag, which was detected in the gilded layers, may due to the underdeveloped gold purification technology of ancient China, or purpose to adjusting the color of the layer by added the silver into the gilding mercury amalgam (Cai, 2019). With the time goes by, the bronze and its’ gilded layers were terrible rusted in the wet environmental conditions, the gilded layers fell off and honeycomb corrosion holes appear in the layers (Fig. 11). On the other hand, this phenomenon of honeycomb corrosion holes means the rusting and also explains why amount of O exist on the surface in element mapping images. In another case, Sn and O abound in the cross-section and the place gilded layers fell off, detected relatively less in the gilded layer, in Table 3, which consider that Sn and O come from the oxidative rust of bronze alloy. Many research about bronze mechanism of corrosion reported that, the sequence of corrosion is Pb>Cu>Sn (Julin Wang and Xu, 2004), thus, the inexistence of Pb in the surface and cross-section of the samples and the irregular distribution of Cu (in Fig. 10 and Table 3) were related to the corrosion of the bronze alloy, which led to the Pb loss first.
The thickness of the gilded layer was about 1.143µm in the measurement of the gilded layer peeled off from G-11. The SEM observation shows long and narrow indentations on the gilded layer of G-6, and the places with the indentations were more stable and attach to the surface of the bronze, shown in Fig. 10 (a), which show the scientific and reliability of the “Yaguang” technic in the gilding process.