The Thurrock hypothesis ― British tin trade in the 2nd century BC and later, part two

Bronze coin of Massalia (Marseille, France)

The type lasted from the later 3rd cent BC
down to about the middle of the 1st cent BC
Image courtesy of Classical Numismatic Group, Inc

Massalia was a Phocaean (Asia Minor) colony founded in about 600 BC. One of its most famous citizens was Pytheas who lived in the 4th century BC and who attempted to find a sea route to Cornwall to trade for tin. Unsuccessful in this venture, the Massalians settled for an overland route. Complicating matters, too, was the fact that Carthaginian pirates preyed on ships headed for Massalia and other trading ports in the western Mediterranean. The British tin trade had been important for a very long time and the earlier trade was dominated by the Phoenicians (who founded Carthage in the 1st millenium BC). Deposits of tin were rare, and without it, bronze could not be made.

Strangely, the classical authors of the 1st century BC are silent about the British tin trade, and after Caesar, sources in Iberia and Brittany were utilized. It is also strange, that of all ancient Greek bronze coinage, it is these coins of Massalia which have the lowest levels of tin in their alloy and it is these coins which served as a model for the British Thurrock high tin bronze (potin) coins.

My source for the metallurgy of the British high tin bronze coins which start with the Thurrock types and end with the flatter, linear style coins still based on the same design is J. P. Northover, is Materials issues in the Celtic coinage, in Celtic Coinage: Britain and Beyond, British Archaeological Reports (BAR) British Series, 222, Oxford 1992. It is a complex subject and the paper should be studied for the complete picture. Speaking of the Thurrock type, and the "analysis of an example found in an excavated context at Maiden Castle" Northover says:

"The composition is of a bronze with an important cobalt impurity, with Co>>Ni and with iron, arsenic and silver as the other significant impurities. This impurity pattern is highly characteristic of metalworking in southern Britain in the La Tène Iron Age, and can almost certainly be associated with a copper source in south-west England, that is the metal itself is British in origin." (p.261)

The metal type does not last past the time of Caesar, and is focused in the southwest, although it also circulated in the southeast. Northover mentions only two continental examples: "..some cauldrons at La Tène itself showed the same bronze type in association with iron of potentially British origin." but, most significantly, he mentions "some semi-quantitative and qualitative analyses published by Ulrich Zwicker" in which bronze coinage from Massalia showed the same profile.

Think about it: we have a British "fingerprint" showing up in Massalian coins which have the lowest levels of tin recorded for the Greek series, and these types serve as the model for the Thurrock and subsequent types. Furthermore, these "tin coins" are focused in hoards in southeast England near the closest shipping route to the continent, and Massalia had been favoring an overland route. The south Worcestershire site had surprised Dean Crawford by its containing quite a number of non-hoard Thurrock types, and it also had evidence of some metalworking in the form of differing silver alloys in pellets and tiny ingots.

My hypothesis is that the Thurrock and subsequent "potin" coins are not really coins in the sense that we understand coinage today, but were "ingots in disguise" which originally were used to supply Massalia with tin starting when Carthage was trying to monopolize western Mediterranean trade. some short time after Caesar, the supply of bronze in Britain reveals a couple of different types of continental alloys and the high cobalt to nickel British metal becomes extinct. Massalia was obtaining British tin for its local bronze coinage, but its large population meant that, even with this trade, supplies were inadequate to produce bronze with a more suitable tin level. This, in turn, led to an exploration for alternative, and closer, sources of tin and the British trade dried up.

This series will continue tomorrow with an explanation of the purpose of the southeastern high tin bronze hoards as a new hoard type classification and with other examples of that class of hoard.