Copper granulation: scientific analysis on the ornaments from the coronet of Lady Pei of the early Tang Dynasty (618-712 AD) in Xi'an, Shaanxi, China

摘要

Granulation is an ancient decorative technique for metalwork. Granulated finds are most commonly made of precious metals such as gold and silver, while copper granulation is very rare, and it has even been claimed that it was not employed in antiquity. This paper presents a scientific analysis of granulated copper ornaments from a gilded coronet belonging to Lady Pei (d. 691 A.D.) that was uncovered from an early Tang Dynasty tomb in Xi'an, China. In addition to common methods such as X-ray radiography, metallographic investigation, and scanning electron microscope observation including energy dispersive spectroscopy to examine the microstructure and elemental composition, X-ray micro-computed tomography was used to investigate the internal microstructure of granulation for the first time. The results affirm that copper granulation technique was used to decorate the coronet. Notably, further study shows that the copper granules and wires were joined to the substrate with silver-copper alloy hard solder and arrives at an estimate of the alloy mixing ratio. These observations are used to infer the manufacturing process: copper granules of two sizes were first brazed to the wires and substrate, and then mercury was used to gild the whole ornament. This work provides new insights for the study of ancient granulation techniques, especially through the scientific study of the very rare examples of copper work.