Open access publications in the SLU publication database

Abstract

Analyses of wood from the Swedish warship Vasa revealed a complex chemical composition with increased acidity and depolymerisation of hemicellulose and polyethylene glycol (PEG). Pronounced depolymerisation and acidity were found in ferriferous segments below the surface region with high iron-sulfur-ratios. Decomposition products such as low molecular acids, xylan and PEG oligomers were identified by NMR and MALDI-TOF mass spectrometry in aqueous wood extracts. The concentrations of low molecular acids (formic, glycolic and oxalic) were enhanced compared to unconserved waterlogged wood and recent oak. The highest concentration of formic acid was found at sites with PEG depolymerisation shown by changes in the molecular weight distribution (MWD). Statistical simulations of degradation and model experiments on PEG and holocellulose with Fenton's reagent support degradation initiated by hydroxyl radicals. Multi-elemental analyses were performed by scanning electron microscopy showing different patterns in relation to depth and degradation. The surface region (0-5 mm) was characterized by the presence of particles in the lumina (gypsum, elemental sulfur, iron sulfur compounds) and a background of evenly distributed sulfur and iron compounds. Below the surface, in segments with depolymerisation, crusts of iron compounds were found in the lumina, and ferriferous particles (10-100 nm) were frequently observed in the cell walls. EXAFS analysis of the iron speciation showed that iron is present as hydrated iron(II) ions and iron(III) compounds and complexes. Sulfur K-edge XANES analysis of extracted wood showed that reduced organic sulfur compounds (ROSC) and intermediate oxidized sulfur species bind to macromolecules. The conclusion of the Vasa wood analyses is that degradation processes in the wood are initiated by the presence of iron compounds in regions low in sulfur. In the presence of significant amounts of ROSC the degradation patterns are less common indicating antioxidant properties, i.e. an opposite effect with regard to iron. Full-scale iron extraction experiments with iron chelators on conserved wood artefacts were efficient but time-consuming (years). Minor effects of the extraction treatment were observed on the MWD of oak holocellulose analysed by size-exclusion chromatography.