An outbreak of acidity and sulphate salts was observed on the hull of the 17th-century Swedish warship Vasa, following humidity variations in the Vasa Museum during the rainy summer in 2000. Analyses of total sulphur in oak cores sampled from 16 locations showed, in the outermost 2 cm., mean concentrations betwen 0.2 and 6 mass percent. Synchrotron-based sulphur K-edge X-ray absorption spectroscopy revealed that a large amount of elemental sulphur in the moist wood was being oxidised to sulphuric acid, possibly catalysed by iron compounds, with the overall rate estimated to be about 100 kg sulphuric acid annually. Associated acid catalysed hydrolysis of the cellulose threatens the future preservation of the vessel. Results of tests of proposed new treatments using chelating agents to extract iron compounds from waterlogged wood and simultaneously neutralise acid are presented.

In marine-archaeological oak timbers of the Mary Rose large amounts of reduced sulfur compounds abound in lignin-rich parts such as the middle lamella between the cell walls, mostly as thiols and disulfides, whereas iron sulfides and elemental sulfur occur in separate particles. Synchrotron-based x-ray microspectroscopy was used to reveal this environmentally significant accumulation of organosulfur compounds in waterlogged wood. The total concentration of sulfur in reduced forms is ≈1 mass % throughout the timbers, whereas iron fluctuates up to several mass %. Conservation methods are being developed aiming to control acid-forming oxidation processes by removing the reactive iron sulfides and stabilizing the organosulfur compounds.

Synchrotron-based sulfur X-ray absorption spectroscopy reveals considerable accumulation of organosulfur (e.g. thiols), pyrite and iron(II) sulfides in marine-archaeological wood preserved in seawater, e.g. for historical shipwrecks such as the Vasa and Mary Rose. In the museum, oxidation of the sulfur compounds in the presence of iron ions may cause severe acidity in the moist wood. This tutorial review discusses developments of conservation methods to remove acid and iron, and how to analyse and stabilise sulfur compounds in the wood.

Outbreaks of acidic sulfate salts have been reported in numerous areas on the wooden surfaces of the Swedish historical shipwreck Vasa (from 1628) in the Vasa Museum. Effects of ammonia vapour treatment to reduce the acidity in Vasa wood, similar to that previously applied to the Dutch East Indiaman Batavia (from 1629), have been investigated by means of solid state ¹³C-NMR and MALDI-TOF spectrometry. No major changes were found in the molecular weight distribution of the bulking agent polyethylene glycol (PEG) or in the crystallinity of pulp cellulose after mild exposure to ammonia vapour. Further studies on the long-term pH stability and the post-treatment properties of the wood using spectroscopic methods and mechanical testing are in progress.

Fresh pine wood blocks were submerged in sulfate and iron(II) containing media, inoculated with bacterial consortia isolated from seawater, aiming to simulate the seabed conditions of the Vasa shipwreck (1628). The consortia contained erosion (EB) and sulfate-reducing bacteria (SRB). Sulfur K-edge x-ray absorption near edge structure (XANES) spectroscopy and scanning x-ray spectromicroscopy images showed that organic sulfur, mainly thiols (R-SH), had accumulated in the lignin-rich middle lamella in EB-degraded parts of the wood. The sulfur content in the wood increased more than 10 times in two years. In another series with active inoculums from marine archaeological wood, the sulfur XANES spectra showed after four years anaerobic treatment considerable amounts also of inorganic iron sulfides, Fe_1-xS, which oxidized at atmospheric exposure. A sediment sample from the Vasa’s seabed was also rich in iron sulfides, including pyrite FeS₂. X-ray fluorescence mappings of sulfur and phosphorous distributions indicate that scavenging SRB penetration, producing hydrogen sulfide in situ, is restricted to EB-degraded parts of the wood structure. The sulfur isotope depletion of ³⁴S from d³⁴S = 21‰ in marine sulfate to d³⁴S = 6‰ and 1.8‰ for fractions of reduced sulfur and sulfate separated from a Vasa wood sample, respectively, suggests bacterial transformation. A fuller understanding of the routes of sulfur accumulation, as reactive iron sulfides and as organic sulfur, has important implications for improving conservation methods of marine archaeological wood. Moreover, the biogenic accumulation of organically bound sulfur specifically in lignin-rich parts of waterlogged wood has wider geochemical significance for fossil fuels of marine origin, as lignin-rich humic matter is important for the diagenetic formation of kerogens from anoxic marine sediments.