Technical characteristics and coating formation mechanism of gilded silver products unearthed from the Consort Tomb of Emperor Shengzong of the Liao dynasty

Abstract

Compared with other gold plating processes, mercury gilding was widely used in ancient China due to the less consumption of gold and excellent adhesion between the gold layer and substrate. Herein, the comprehensive analyses of the silver crown and boots unearthed from the Consort Tombs of Emperor Shengzong (圣宗萧贵妃) of the Liao (辽) Dynasty reveal the surface composition and structural characteristics and also the unique multilayered structure and cross-sectional compositional distribution of the gold layer. First, the existence of mercury on the golden surface of cultural relics, as well as the unique porous structure and granular surface morphology, indicate that the artifacts were probably processed using mercury gilding. In addition, the intermetallic compound, i.e., Au5Hg, in the gold layer indicates that the processing temperature was 388–419 ℃; thus, the formation mechanism was the transformation of solid solution in the amalgam to intermetallic compound during the heating of Au-Hg system. Moreover, the cross-sectional transition layer is mainly composed of Ag and Hg due to the interstitial diffusion of mercury atoms at room temperature and vacancy diffusion of silver atoms during the heating process, bonding the gold layer with the substrate. Obviously, the bonding mechanism of the gold layer and substrate was interstitial diffusion and vacancy diffusion within the Ag-Hg system. The current work reveals the special multilayered cross-section of silver gilding relics and shows that mercury played a crucial role in both the formation of the gold layer and bonding between the gold layer and substrate during the silver gilding process.