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CANNONBALLS from centuries-old shipwrecks are wreaking havoc鈥攂y exploding on archaeologists鈥� desks.

Ancient lumps of metal rust quickly once they鈥檙e pulled up from the bottom of the sea. The combination of oxygen and sea salt causes the rapid oxidation that can crumble valuable artefacts to bits. But Robert Child from the National Museums and Galleries of Wales in Cardiff has noticed another frightening effect.

Back in the 1970s, Child helped to retrieve some small cannon shot 6 centimetres in diameter from the 1691 wreck of the HMS Coronation in 12 metres of water off Plymouth Sound. Back in the lab, he hammered a mass of solidified sand off the balls. After a few minutes, one cannonball had heated up to a few hundred degrees, glowed a dull red, and was burning its way through the pine table. 鈥淭here was smoke coming off the bench,鈥� he says.

Since then, Child has heard of other recovered cannonballs suddenly splitting weeks after being pulled from the sea. He says the explosions happen because the balls have developed a lattice-like structure, which creates a large surface area that reacts with oxygen to produce massive amounts of heat.

The porous structure evolves over hundreds of years. In shallow seas, the cast iron slowly rusts into iron oxide, expanding the volume of the cannonball. Eventually sand and rotting sackcloth covering the rusted ball form a concrete-like coating, sealing it off from any oxygen. In this new environment, decaying organic matter turns the iron oxides back into pure iron, leaving holes where the oxides previously took up space.

Child says that the density of his shot agrees with this hypothesis. It weighs in at 4 grams per cubic centimetre, close to the 3.7 g/cm³ you would expect if pure iron had displaced iron oxides. Pure iron has a density of 7.2 g/cm³.

Thirty years after the observation, Child and his supervisor from that time, David Rosseinsky, have submitted their work to the International Journal of Nautical Archaeology. They hope their work will help conservators determine how best to keep oxygen out of their samples, perhaps by sealing them in wax or plastic.