An experimental research on the rhombic pattern swords of Wu and Yue states during the Eastern Zhou Period, China

Bronze weaponry crafted by the Wu-Yue states represents the zenith of weapon-casting technology in China during the Eastern Zhou era. Among these relics, swords and spears with dark rhombic patterns have astonishingly retained their pristine condition after thousands of years of subterranean burial. This remarkable preservation has generated considerable scholarly interest. However, obtaining suitable specimens for in-depth study has long been a challenge. This study utilizes metallurgical microscopy, Scanning Electron Microscopy (SEM), and Energy Dispersive Spectrometer (EDS) to analyze two bronze swords with dark rhombic patterns from the Dahan Cemetery in Tengzhou City, Shandong Province. The analysis reveals that the rhombic pattern consists of a dense δ-phase layer, with some areas showing an outer ε-phase layer, indicating a hot tinning technique. Tin-plating experiments were conducted using various methods to simulate this tinning process. The results suggest that when a mixture of tin soap and tin powder is applied to the bronze surface and heated, a dark rhomboid pattern appears, similar to that found on the famous Sword of Gou Jian. A silver-white layer enriched with tin was discovered beneath the dark rhombic pattern through additional surface polishing. The residual tin soap on the bronze swords demonstrated notable resistance to corrosion; however, under acidic conditions, it tended to transform into organic acids, which could erode the α-phase of the bronze, leading to discoloration and darkening of the sword.