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Tin sources and trade in ancient times
Tin is an essential metal in the creation of tin bronzes and its acquisition was an important part of ancient cultures from the Bronze Age onwards. Its use began in the Near East and the Balkans around 3000 BC. Tin is a relatively rare element in the Earth's crust, with about 2 parts per million (ppm), compared to iron with 50,000 ppm, copper with 70 ppm, lead with 16 ppm, arsenic with 5 ppm, silver with 0.1 ppm, and gold with 0.005 ppm . Ancient sources of tin were therefore rare, and the metal usually had to be traded over very long distances to meet demand in areas which lacked tin deposits.
Known sources of tin exploited in ancient times include: the southeastern tin belt running from Yunnan in China to the Malay Peninsula, Devon and Cornwall in England, Brittany in France, the border between Germany and the Czech Republic, Spain, Portugal, Italy, and Central and Southern Africa (; ). Other minor sources of tin have been suggested in Iran, Syria, and Egypt, but the archaeological evidence is inconclusive.
== Early use ==
Tin extraction and use can be dated to the beginnings of the Bronze Age around 3000 BCE, when it was observed that copper objects formed of polymetallic ores with different metal contents had different physical properties . The earliest bronze objects had tin or arsenic content of less than 2% and are therefore believed to be the result of unintentional alloying due to trace metal content in copper ores such as tennantite, which contains arsenic . The addition of a second metal to copper increases its hardness, lowers the melting temperature, and improves the casting process by producing a more fluid melt that cools to a denser, less spongy metal . This was an important innovation that allowed for the much more complex shapes cast in closed molds of the Bronze Age. Arsenical bronze objects appear first in the Near East where arsenic is commonly found in association with copper ore, but the health risks were quickly realized and the quest for sources of the much less hazardous tin ores began early in the Bronze Age . This created the demand for rare tin metal and formed a trade network that linked the distant sources of tin to the markets of Bronze Age cultures.
Cassiterite (SnO2), the tin oxide form of tin, was most likely the original source of tin in ancient times. Other forms of tin ores are less abundant sulfides such as stannite that require a more involved smelting process. Cassiterite often accumulates in alluvial channels as placer deposits due to the fact that it is harder, heavier, and more chemically resistant than the granite in which it typically forms . These deposits can be easily seen in river banks as cassiterite is usually black, purple or otherwise dark in colour, a feature exploited by early Bronze Age prospectors. It is likely that the earliest deposits were alluvial in nature, and perhaps exploited by the same methods used for panning gold in placer deposits.
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