Old balls still scorch
Pores made shipwrecked cannon balls glow spontaneously.
6 May 2002
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Goodness gracious! Two British chemists believe they have solved the 26-year-old mystery of how shipwrecked cannonballs that were rescued from the deep spontaneously erupted into great balls of fire.
"They were glowing bright red and you could feel the heat coming off them as the desk began to smoke," recalls Bob Child, now a chemist at the National Museums and Galleries of Wales in Cardiff.
It all happened in 1976, when Child was conserving artefacts recovered from HMS Coronation. Among the haul from the 1691 wreck were several dozen iron cannonballs, encrusted in a concrete-like coating from three centuries beneath the waves.
Breaking off this airtight layer with a hammer, Child recalls being "bloody amazed". Several of the cannonballs spontaneously began to heat up, so violently that they almost set fire to the wooden desk on which they were sitting. Child estimates that they reached temperatures of 300-400 oC.
Now he and fellow chemist David Rosseinsky, a visiting fellow at the Gintic Institute in Singapore, think they know why.
After the Coronation sank, conditions on the sea bed, surrounding by salty, oxygen-rich water, allowed the iron balls to rust almost all the way through. This would have caused them to expand, making them less dense. Indeed, the recovered balls were much lighter than expected.
At the same time, the cannonballs were gradually sinking into the sand, which reacted with the metal to form an airtight seal around them. Over the decades, rotting organic matter that was entombed along with the oxidized metal converted it back into pure iron.
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Crucially, the volume remained the same - leaving pores where the iron oxide had been. When the seal was broken, air permeated these pores, with dramatic results.
Corrosion researcher Stephen Fletcher at Loughborough University, UK, explains that it is not unusual for finely divided iron to heat up rapidly when exposed to air. As iron oxidizes it releases energy, and the huge surface area of the fine filaments means that this happens so quickly that they can actually burn.
Hand-warming devices for Arctic explorers, round-the-world sailors and British holidaymakers exploit this phenomenon. "But you just don't expect a solid cannonball to be made up of finely divided and compressed iron powder," says Rosseinsky.
Recent interest in the difficulties of conserving artefacts recovered from the sea persuaded Child to re-examine the mystery in an attempt to explain what had previously been an interesting conversation-starter, he says. Other researchers will have the opportunity to test his and Rosseinsky's conclusions - if they can wait another 300 years or so.
