May 16, 2012
Sulphur And Iron Commonly Found In Old Shipwrecks
Sulphur and iron compounds have been found in shipwrecks off the coast of Sweden and in the Baltic Sea, according to a new study. The group behind the results includes scientists from the University of Gothenburg and Stockholm University. The results were presented recently in the Journal of Archaeological Science.
Scientists reported large quantities of sulphur and iron compounds in the salvaged 17th century warship Vasa a few years ago. This resulted in the development of sulphuric acid and acidic salt precipitates on the surface of the hull and loose wooden objects.
Other vessels also affected
Similar sulphur compounds have been discovered in other shipwrecks including fellow 17th century warships Kronan, Riksnyckeln and Stora Sofia, the 17th century merchant vessel in Gothenburg known as the GÃ¶ta wreck, and the Viking ships excavated at Skuldelev in Denmark.
"This is a result of natural biological and chemical processes that occur in low-oxygen water and sediments," explains Yvonne Fors from the University of Gothenburg's Department of Conservation, one of the scientists behind the study with Stockholm University.
Preventive action possible
Similar problems have previously been reported for the Dutch vessel Batavia in Australia, which was lost in 1629, and for Henry VIII's flagship Mary Rose in the UK, which sank off Portsmouth in 1545.
"Our work on the Vasa and the Mary Rose has given us a good insight into these problems," Yvonne Fors says. "With the right actions, such as new preservation procedures, we'll be better able to prevent these shipwrecks from developing such serious problems with sulphuric acid."
Toxic hydrogen sulphide reacts with wood
Bacteria can break down organic material including the wood cells in a vessel's hull, even in low-oxygen-water. Sulphates occur naturally in the water and are transformed by bacteria into toxic hydrogen sulphide. This reacts with the wood. When iron ions are present, sulphur and iron compounds form and then oxidize into sulphuric acid and acid salt precipitates in a damp environment after the vessel has been recovered.
"For some of the wrecks, such as the Skuldelev Viking ships and the GÃ¶ta wreck, the conservation treatment is already finished," says Fors. "It's then a matter of keeping an eye on the chemical developments, which requires additional resources."