Material
Famen Monastery, located in Famen town of Fufeng county, Baoji city, west of Xi′an, Shaanxi province, China (Fig. 1a), has attracted extensive research interest due to the discovery of four miraculous Buddhist finger bone relics and a large number of delicate tributes for the true body relic [16, 17]. These diverse tributes were uncovered from the underground palace of the Tang pagoda in the 1980s (Fig. 1b), where 336 items in total were unearthed, including 118 gold and silver artifacts, 35 textiles, 20 glassware, 16 porcelains, 70 bronzes and irons, 26 wooden lacquered objects, 11 stoneware, and 40 gems [16]. According to the stone Yiwuzhang (inventory stele), these treasures were placed in the pagoda crypt in 874 CE and most of them were donated by Emperor Yizong (833–873 CE) and Xizong (862–888 CE) of Tang, while some were offered by senior monks and eunuchs [18]. Moreover, the inscriptions on the gold and silver vessels suggest a variety of provenances, e.g., the products from the imperial workshop were labeled as “Wensiyuan”, established in 854 CE in Chang′an (modern Xi′an), and the tributary gifts from regional officials were marked with the officials’ names and their jurisdiction areas [18, 19].
A delicate silver box, found in the southeast corner of the rear chamber (Fig. 1c), was analyzed in this study. This box is in a floral shape and possesses straight walls, a plain bottom, and a splayed high ring foot (Fig. 2a, b). The lid cover is decorated in high relief on the front with two lions leaping and chasing each other in the center. The lions are surrounded with scrolling passionflowers also in high relief. These motifs are reserved on a ring-matted background and enclosed by a rhombus frame of raised pearl roundel. Outside the rhombus frame, the four corners of the lid cover are filled with four pairs of raised passionflowers, also on a ring-matted background; the rim of the lid cover is decorated with raised lotus petals (Fig. 2a, c). The box walls are decorated with scrolling lotus on a ring-matted background, and the rim of the ring foot is decorated with lotus leaves (Fig. 2b). Except for the matted ground, the relief motifs on the lid cover, main motifs on the box walls, and lotus leaves on the ring foot all were gilded (Fig. 2a).
More importantly, source information was found on the box. The Chinese-inked Neiku on the front of the lid cover (Fig. 2c) indicates that the box was collected in the royal storeroom [18]. The inscription was found on the outer surface of the bottom, i.e., a gilded box with length of 6 Cun and weight of 20 Liang was presented by the official Li of Jiangnanxidao, to the Emperor of the Tang Empire for celebrating the Yanqing festival (Fig. 2d). Jiangnanxidao represents the regions around the modern Nanchang city of Jiangxi province. Yanqing festival was established in 859 CE for celebrating the birthday of Emperor Yizong [21]. In summary, this box was first made in southern China, then it was presented to Emperor Yizong and stored in Neiku, and finally, it was taken to Famen Monastery in 874 CE. Therefore, this silver box was probably produced during the second half of the ninth century CE. The length, width, height, and mass of the box are 17.3 cm, 16.8 cm, 11.2 cm, and 799 g, respectively [16].
Methods
The studied silver box is stored in the Famen Temple Museum. It was not allowed to take the box out of the museum for conventional laboratory analysis. Only portable analytical instruments with permission, such as the microscope and X-ray fluorescence (XRF) spectrometer, were permitted into the museum to conduct the non-destructive analysis.
Microscopic analysis
The structure, morphology, and tool marks of the box were investigated using a three-dimensional digital microscope (KEYENCE VHX-600, Japan) and recorded with high-resolution (1600 × 1200 pixels) images. An objective lens of VH-Z20R was used in this study, which has a magnification of 20–200 X and a depth of field ranging from 34 to 0.44 mm. The microscope consists of a 100 W halogen lamp cold light source. Therefore, the color of the image obtained using the microscope gets influenced by the yellow light. In order to obtain the original color of the gilded silver box, the white balance of the micrographs was corrected by using the Adobe Photoshop 13.0 software.
Compositional analysis
Different components of the box were analyzed using a handheld portable XRF spectrometer (p-XRF, Thermo Niton XL3t800, USA) equipped with a 2 W, 50 kV silver anode X-ray tube and high-performance silicon drift detector. The mode for alloy is divided into three modes due to certain elements. Precious Metals mode (effective testing diameter: 8 mm) is for the analysis of precious metal, including elements Ag, Au, Cu, Zn, Fe, Pb, Ir, Ti, Cr, Mn, Co, Ni, Ga, Nb, Mo, Ru, Rh, Pd, Cd, In, Sn, and Pt, where the platinum family element is included. Standard Alloy mode (effective testing diameter: 3 mm) is for the analysis of the unknown metal, including elements Ag, Au, Cu, Zn, Fe, Pb, Al, Ti, V, Cr, Mn, Co, Ni, As, Se, Zr, Nb, Mo, Ru, Pd, Cd, Sn, Sb, Hf, Ta, W, Re, and Bi, where more metallic elements are included. Electronics Alloy mode (effective testing diameter: 3 mm) is for the analysis of the metal coating, including elements Ag, Au, Cu, Zn, Fe, Pb, Hg, Al, Ti, V, Cr, Mn, Co, Ni, Se, Br, Zr, Nb, Mo, Pd, Cd, In, Sn, Sb, Ba, Hf, Ta, W, Pt, and Bi, where Hg could be measured. These different modes have little effect on the content of main elements.
In the current study, Precious Metals mode was used to analyze the base of the box, Standard Alloy mode was employed to analyze the joining area, and Electronics Alloy mode was utilized to explore the gilt layer. Moreover, the measuring head touched the tested surface during the analysis. Notably, 3–5 measurements were conducted on every component of the box and the valid data were incorporated into the final reported result. The measurement time for each spot analysis was 30 s and the elemental results detected were normalized.