Analysis of the wooden stick, straw and paper fibers
The fibers present in Fig. 4a, b are long and narrow with rounded parenchymal cells which suggest the wooden stick was made of moso bamboo. The fiber of sample SHZ3 is observed with coarse serrated cells and parenchymal cells of different sizes in Fig. 4c, d, indicating that wheat straw is the fiber source . Figure 4e, f show a bright yellow color of the stained fibers, suggesting a high content of lignin. Moreover, the cross helical striations with rounded parenchymal cells are seen along the fibers. All these findings are in accord with the characteristics of coniferous wood pulp. The fibers shown in Fig. 4g, h are long, narrow and stiff with thick-walled and rectangular-shaped cells. It is believed that bamboo is possible the major source for making the paper of CQY2 and CQY3, however, the exact species is hard to determine .
Analysis of the pigments
The micro-Raman spectra are provided in Fig. 5a–f and detailed results are available in Table 2. As seen from sample SHZ1, the paintings drew on the white substrate were made of red, blue and black colors. The micro-Raman analysis of the red pigment (in Fig. 5a) identified red lead as the main constituent of the pigment [9, 10]. It is a toxic substance which normally used as a mothproof agent to ensure the long-time preservation of the objects in ancient China . The identified peaks of blue pigment are similar to that of indigo (in Fig. 5b), which is a common natural dye that was widely used in ancient painting and dyeing [12, 13]. However, the black pigment cannot be determined due to significant fluorescence.
The sample CQY1 which presents a flesh color appearance was obtained from the fractured neck of a figurine. As shown in Fig. 5c, the micro-Raman result identified the pigment as red lead mixed with basic lead carbonate [14, 15]. CQY2 and CQY3 were collected on the same figurine, respectively from the green belt and the pattern of the paper-made dress. Due to the color fading, only a limited number of peaks were identified from the Raman spectrum of CQY2 (in Fig. 5d). The relevant peaks of indigo are observed. The peaks presented at 1275.96 cm−1 and 1610.77 cm−1 indicated possibly another organic yellow dye was added to obtain a green hue. CQY3 was dyed with two different blue pigments, as shown in Fig. 5e, f, the main constituents were identified respectively as ultramarine (for deep blue) and indigo (for light blue) . Since the use of artificial ultramarine blue was widespread across China by the late Qing Dynasty, PLM was employed to further characterize the particle of the pigment. As seen from Fig. 5g, the shapes of ultramarine blue particles are regular and smooth, and the grain sizes are even which suggest the artificial pigment .
Analysis of the clay fragments
XRD analysis was employed to characterize the compositions of the clay samples of SHZ1, SHZ2, CQY1, FXL1 and FXL2. As seen from Fig. 6a, b, d, the main constituents of SHZ1, SHZ2 and FXL1 were indicated as quartz (SiO2) and gypsum (CaSO4·2H2O). Refer to the relevant studies [17, 18], it is suggested that this kind of clay was widely used in the local-made clay sculptures. In addition to quartz and gypsum, calcium carbonate (CaCO3) which is the main composition of lime was found in CQY1 and FXL1 (in Figs. 6c, e). Since CQY1 and FXL1 were collected from the fractured neck of figurines, lime was probably applied to improve the smoothness and brightness of the surface.
The results of granulometric analysis presented in Fig. 7 show the particle sizes of the samples are mainly between 1 and 75 µm. The clay fragments were formed with mostly the particles of 2–5 µm in size. The results further confirmed the fine quality of the clay that used in making the figurines.
As observed from the cross-section images in Fig. 8, plenty of fibers were found inside the clay samples. Refer to the traditional sculpturing craft, natural cotton or similar plant fiber is commonly mixed with clay to prevent cracking .
Analysis of the textile fibers
The IR spectra of the textile fibers are available in Fig. 9. In Fig. 9a, c, d, the presence of a broad band from 3297 to 3279 cm−1 is commonly related to the inter-molecular hydrogen bonding of silk sericin. The fine absorption around 1636 cm−1 can be assigned to C=O stretching vibrations in amide I proteins, and peaks at about 1510 cm−1 are generally associated with the N–H in-plane bending and C–N stretching bending . The bands observed at about 1444 cm−1 and 1165 cm−1 are the fibroin bands arise from the vibrations of alanine and tyrosine side-chains . These characteristic bands identified the composition of JYG1, CQY4 and FXL3 as silk.
In Fig. 9b, the main vibration at 3340 cm−1 can be assigned to intra-molecular hydrogen bonding of cellulose. The absorption band at 1635 cm−1 is related to O–H bending of adsorbed water. The peaks presented at 1428 cm−1 and 1316 cm−1 are respectively corresponded to CH2 scissoring and CH2 rocking, while 1370 cm−1 and 1200 cm−1 are respectively assigned to C–H bending and C–O stretching. The anti-symmetrical bridge C–O–C stretching is observed at 1161 cm−1 and the band at 1057 cm−1 is related to C–O stretching. And the β-linkage of cellulose presented at 897 cm−1 further confirmed the fiber of sample JYG2 as natural cotton .