Analysis of ancient Pb-Sn solder in China

Four ancient solder samples were examined by using optical microscope, scanning electron microscope (SEM), X-ray energy dispersive spectrometry (EDS), and X-ray diffraction spectrometry (XRD). Among these samples, three were unearthed from the tombs of the Warring State period (the 5th-3rd centuries BC) at Gaozhuang, Huaiyin, Jiangsu Province of China and one from the tomb of the Eastern Han Dynasty (the 1st-3rd centuries AD) at Shitang, Mianyang, Sichuan Province of China. The results show that all the solders are made of Pb-Sn alloys. One of the Gaozhuang samples contains 68.1% Pb and 21.4% Sn, and its microstructure consists of α and β phases. This sample was taken from a bronze Jian vessel leg, but it seems to have not been involved in welding. The other two samples that were clearly involved in welding contain not only Pb and Sn, but also Cu, and the Cu contents are 1.6% and 3.8%, respectively. It is pointed out that Cu in these two solder samples resulted from the dissolution of Cu into the molten Pb-Sn solder. At the interface between the liquid solder and the bronze substrate, a scallop-type layer of Cu-Sn inter-metallic compound (Cu₆Sn₅) formed and grew. Some irregular pieces of Cu₆Sn₅ also formed within the bulk solder.