A study on ancient casting method through microstructures of impurities and copper found in bronze excavated from the Unified Silla Period

In Hwangnyongsa temple site in Dongdaebong mountain in Korea, a large number of gilt-bronze artifacts were excavated. Optical microscope, X-ray fluorescence, scanning electron microscope, energy dispersive X-ray spectroscopy, focused ion beam, transmission electron microscopy are used to identify the structure of the specimen and impurities. This article investigates the impurities and copper grains in bronze Buddha’s robe from Hwangnyongsa temple site to suggest ancient bronze craft technology in the unified Silla period. XRF results of specimen indicate that were made using alloy of copper and tin, and gold plating was done on the front side only. The microstructure of specimen was confirmed as a recrystallized polygonal structure, with twins and impurities. The results of impurities’ EDS indicated the molar ratio of Cu : S as 2 : 1 and electron diffraction pattern corroborated with that of Cu 2 S (JCPDF 33-0490). A layer of specimen surface consisted of high lead layer and copper grains. It is speculated that the copper grains were formed by reaction of Pb with matte (Cu 2 S) during casting. Lead with black gas would help to make high quality bronze. The copper grains used to exist in Silla, as detected in Koryo bronze artifacts. Hence, it was possible that the ancient artisans knew that lead make bronze quality good when they were casting bronze.