For a long time, experts and historians have considered the source of lead in early Yayoi bronzes to be from the Korean peninsula; and that in bronzes of Han mirrors and late Yayoi bronzes to be from northern China, and the lead from late Kofun bronzes to be from southern China.
New studies have now concluded that the lead contained in early Yayoi bronzes does not match with that found in Korean peninsula but instead has been found to be Chinese lead. Sources for Early Yayoi bronze mirrors were traceable to the Chinese sources of lead (used from the Xia/Shang to Han dynasty) of the peculiar Yunnan type lead (Sanxingdui type) and of the Hebei-Liaoning type, while lead of Late Yayoi bronze mirrors were of the northeastern Chinese type.
The lead in Western Han mirrors and late Yayoi bronzes (that used to be considered to be from Shanxi provinces and other places in northern China) are been traced to northeastern China.
After the Yayoi Period, the lead in the Kofun period in bronzes came from southern China, but also from other sources including Hebei and Liaoning provinces.
In East Asia, the technique of casting bronze mirrors with stone molds had emerged by 2,000 B.C. in present-day Gansu-Qinghai area at the upper reaches of the Yellow River. Possibly the earliest bronze mirror, a cast bronze mirror was found at the Lajia Village, in Northwest China’s Qinghai Province dating to between 3,800 -4,000 years before present. The find, belonging to the Qijia Culture (emerged and spread around the upper reaches of the Taohe, Daxia and Weihe rivers in Gansu and the Huangshui basin in the upper reaches of the Yellow River in Qinghai, during the transitional period from the Neolithic Age to the Bronze Age (2250-1900 BC)) suggests that some elements of early Chinese bronze casting may have originated in western China – and may even have been linked to the bronze casting of Central Asia and the Iranian area.
Since then, this technique diffused eastward along the zone on both sides of the later Great Wall into Northeast China, Korean Peninsula and Kyushu region in present-day Japan and formed the tradition of stone mold-casting technique of bronze mirrors. In Japan, this technique lasted to the 3rd century CE.
A later new technique of casting bronze mirrors with pottery molds emerged around the 9th century BCE (Western Zhou Dynasty) in the Shaanxi Plain, the western Henan and southern Shanxi Provinces. The technique peaked in the end of the third century BCE (the Qin and Han Dynasties) when the technique then spread rapidly to Northeast China, Korean Peninsula, and diffused into Japan Archipelago, finally ended East Asia’s tradition of stone mold-casting technique of bronze mirrors at the beginning of the third century CE.
Excavating Qijia Culture Site (China Daily 10/12/2000)