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Cannonball Debate

Science and medieval Spaniards met head-on at the Institute of Geological & Nuclear Sciences, and the Spaniards came off second-best when the institute's Rafter Radiocarbon Laboratory was asked to date a 35-kg iron cannonball found in Wellington harbour.

Dredged up during reclamations at Petone in 1926, the cannonball caused speculation that it may have come from a Spanish ship visiting Wellington in the 14th or 15th century. Confirmation would mean that Europeans visited New Zealand long before Abel Tasman, the acknowledged European discoverer of the country.

Radiocarbon dating iron is not technically feasible, but cast iron does contain about 4% carbon. Before 1700, it was made by adding charcoal to the molten ore. Charcoal is made from organic material which can be radiocarbon-dated to give the time of manufacture of the object. After 1700, cast iron makers began to use coke as the supply of carbon, which contains little or no radiocarbon, and so cannot generally be radiocarbon dated.

Nuclear physicist Dr Roger Sparks took a small sample from the ball and extracted the carbon by dissolving the iron in acid. Analysis with the laboratory's accelerator mass spectrometer showed no radiocarbon, indicating iron made by the coke process, and hence manufactured after 1700. It was the first time in New Zealand that cast iron has been dated by this method.

Although the analysis ruled out the possibility that the hefty missile fell off a medieval ship, a mystery remains as to who left it in the Petone sand and when.

John Callan, GNS