Deterioration of the seventeenth-century warship Vasa by internal formation of sulphuric acid

The seventeenth-century Swedish warship, Vasa, was recovered in good condition after 333 years in the cold brackish water of Stockholm harbour. After extensive treatment to stabilize and dry the ship's timbers(1), the ship has been on display in the Vasa Museum since 1990. However, high acidity and a rapid spread of sulphate salts were recently observed on many wooden surfaces(2), which threaten the continued preservation of the Vasa. Here we show that, in addition to concentrations of sulphate mostly on the surface of oak beams, elemental sulphur has accumulated within the beams (0.2-4 per cent by mass), and also sulphur compounds of intermediate oxidation states exist. The overall quantity of elemental sulphur could produce up to 5,000 kg of sulphuric acid when fully oxidized. We suggest that the oxidation of the reduced sulphur-which probably originated from the penetration of hydrogen sulphide into the timbers as they were exposed to the anoxic water-is being catalysed by iron species released from the completely corroded original iron bolts, as well as from those inserted after salvage. Treatments to arrest acid wood hydrolysis of the Vasa and other wooden marine-archaeological artefacts should therefore focus on the removal of sulphur and iron compounds.