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.
History of early bronze mirror casting techniques in East Asia
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.
Reference:
Excavating Qijia Culture Site (China Daily 10/12/2000)
Heritage of Japan 
It is also pertinent here to consider the competing Bronze Culture of Ban Chiang, Southeast Asia further south of Eurasia – which recently has come to light as having possessed a sophisticated metallurgical industry from as early as 2,100 B.C. However, despite the early dates, studies have determined that the metal-working technique likely came from elsewhere because of the lack of evidence of early use of pure copper (ancient smiths preferred alloyed copper, i.e. bronze). Source: “Bronze from Ban Chiang, Thailand: A View From the Laborator” Science & Archaeology, Expedition Vol. 43 No. 2 Summer 2001 (retr. 2/2011: http://penn.museum/documents/publications/expedition/PDFs/43-2/Science.pdf)
In “The Bronze Age of Southeast Asia” Charles Higham provides a systematic and regional presentation of the current evidence. He suggests that the adoption of metallurgy in the region followed a period of growing exchange with China. Higham then traces the development of Bronze Age cultures, identifying regionality and innovation, and suggesting how and why distinct cultures developed.
This five thousand year old Bronze Age civilisation was uncovered in the 1970s. Older than any in China or Mesopotamia, this area on the Khorat Plateau is touted as a possible ‘cradle of civilisation’.
“Archeological excavations in Thailand (Spirit Cave, Non Nok Tha) and northern Vietnam (Dongson, Hoabinh) reveal a major surprise: the first Southeast Asians had agriculture and pottery at the same time as the city-states of ancient Mesopotamia. In fact, evidence now suggests that rice was grown here a long time before it was grown anywhere else, and even the pottery found here may be the world’s oldest. The most impressive discovery was made at Ban Chiang, a hill on Thailand’s Khorat Plateau, in the early 1970s; this hill covered a village that was settled continuously, for more than three thousand years. 126 skeletons were discovered intact, buried with the pottery and metal tools it was thought they would need in the afterlife. One 4,000-year-old skeleton was nicknamed “Nimrod” because he showed all the marks of a mighty hunter; he was unusually tall, and buried with deer antlers, hunting weapons, and a necklace of tiger claws. Even the oldest graves contained bronze bracelets, bells and spearheads, dating back as far as 3600 B.C.!
At this early date the Khorat smiths were doing better work than their Mesopotamian counterparts; by 3000 B.C. they figured out that the strongest bronze alloy is made by mixing 1 part of tin with nine parts of copper. They were probably helped by a geographical advantage: Southeast Asia is the world’s richest source of tin. Tin is uncommon in the West, and before the Mesopotamians discovered it, they made bronze by mixing copper and arsenic, with brittle and sometimes hazardous results.
Since little archeological excavation has been possible in Southeast Asia to date–thanks to modern problems like the Indochina wars–the discoveries made so far have caused considerable controversy. Traditionally it was believed that the Middle East is the only cradle of civilization, and distant centers of civilization like India, China and Central America somehow learned it from their elder brothers in Egypt and Iraq. The discoveries mentioned above bring this theory into question. Did the Middle East invent everything first? Did the Far East get started on its own, without help from the West? Part of the controversy stems from the fact that in Iraq we can trace the development of metalworking from its earliest stages, while the bronze works found so far in Thailand are products of a fully developed metallurgy. Pro-Thai advocates argue that we have not yet figured out where Mesopotamia first got its tin, so if there was any transfer of metals and ideas, it was from east to west, not the other way around.
At the 1600 B.C. level, archeologists came across another important discovery: iron spearheads, knives and bracelets. Normally the Hittites of ancient Turkey are credited with being the first people to forge iron, but these objects are just as old as anything the Hittites produced. Around the same time, the people went from dry cultivation to the wet-cultivation of rice in flooded fields that is still practiced today; that greatly increased the total food supply. The carabao, or water buffalo, was domesticated around this time to pull plows, and the discovery of spindles and bits of thread suggest that they already knew how to make cloth from silk (this may have been learned from the Chinese).”- Source: Charles Kimball’s Concise History of SEA http://www.guidetothailand.com/thailand-history/
Ban Chiang rice Early Period II finds include rice dated to the 3rd millenium B.C. It is considered that the earliest rice arrived in Thailand’s Bang Pakong Valley from multiple directions but that a logical source for the Early Bang Pakong Valley rice-tempered pottery tradition would be from the east, i.e. present-day Cambodia during the 3rd millenium B.C. Source: Rice in Pottery: New Evidence for Early Rice Cultivation in Thailand by Brian Vincent (retr. 2/2011 http://ejournal.anu.edu.au/index.php/bippa/article/viewFile/344/334)