Analysis of acids and degradation products related to iron and sulfur in the Swedish warship Vasa

Abstract

Aqueous wood extracts from the historic Swedish warship Vasa have been analyzed by 1 H-NMR spectroscopy, ion chromatography, and matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry as part of studies on the chemical degradation related to increased levels of iron and sulfur. The results show that low molecular organic acids have accumulated in the Vasa wood after the 1961 salvage. The increased acidity was found in a context of chemical degradation of the wood polymers and the conservation agent polyethylene glycol (PEG) in iron-rich parts of the timber. Formic, glycolic and oxalic acid are all possible endproducts of oxidative degradation of wood polymers, whereas hydrolysis of acetyl groups in xylan may have contributed to increased concentrations of acetic acid. MALDI-TOF spectra of PEG displaced towards lowmolecular PEG oligomers, as reported earlier, were accompanied by increased levels of formic acid, indicating oxidative degradation of PEG. PEG with a carboxylic acid end group (PEGC) was observed to a minor degree in the wood. However, analysis of stored conservation treatment solutions showed high concentrations of PEGC yielding significant contributions to the acidity during the 1960s conservation period. PEGC was probably formed as a result of microbial processes during the early conservation regime. Calculations using concentrations and well-established acidity constants show that oxalic and formic acid are the primary contributors to a low pH in the wood. The increased acidity in the interior of the wood was found in the absence of sulfur compounds but in a context of iron. The majority of the sub-samples with significant levels of sulfate in the surface region with a prospective sulfur oxidation, however, showed neither a decreased pH nor significant depolymerization. This indicates that oxidation pathways of organically bound sulfur do not necessarily produce strong acids, and thereby free protons, as the final product. These observations imply opposing effects of iron and reduced sulfur species, with iron acting as a initiator in oxidative reactions of Fenton type, whereas the reduced organic sulfur compounds may act as anti-oxidants.

Chemical characterization of archaeological wood: Softwood Vasa and hardwood Riksapplet case studies

“…The state of conservation of Vasa is a complex equilibrium between the chemical modification occurred under the waterlogged conditions and the consolidation treatments to which it has been subjected. It was recognized that the sulfuric acid, generated by oxidation of the sulfur produced by the sulfate-reducing bacteria, (ubiquitous in the near anoxic conditions on the Vasa shipwreck site) could be the main threat to the longevity of the treated Vasa timbers [16][17][18][19]. This assumption might be important with respect to the degradation of polysaccharides in the bacterially degraded surface regions, however, in the interior of the wood, other processes related to iron catalyzed degradation and increased acidity due to low molecular organic acids are possibly more important [20,21].…”

“…First of all, the maximum water content (MWC %), along with iron and sulfur concentration were estimated. These parameters are widely recognized as important features in degraded waterlogged wood characterization [19,22,23]. Moreover, the different wood components were isolated following well-known methodologies and then compared to reference samples (Abies alba and Quercus robur).…”

The influence of chemical degradation and polyethylene glycol on moisture-dependent cell wall properties of archeological wooden objects: a case study of the Vasa shipwreck

“…The significantly increased content of soluble carbohydrates in VASA-A compared to the recent oak (Table 1) shows that this sample is a typical Vasa sample in which hemicelluloses have undergone depolymerization (Almkvist and Persson2008b). This was further supported by the presence of signals of low-molecular oligomers in the MALDI-TOF MS spectra (see S1) with a mass difference of 132 l corresponding to depolymerized xylan (Almkvist and Persson 2008a).…”

Natural polymers as alternative consolidants for the preservation of waterlogged archaeological wood

“…It is important to consider the three most important issues affecting the conservation of the Mary Rose, which will determine the applicability of any alternative material. These are structural instability upon drying (Giachi et al, 2010), biological degradation (Bjordal et al, 1999;May & Jones, 2008) and, more recently discovered, the build-up of Fe 3+ from corroded iron bolts that can catalyse the production of acid in the timbers (Sandström et al, 2002;Fors & Sandström, 2006;Almkvist & Persson, 2008b, 2008c. Ultimately, the ability to address these conservation issues will determine the future of any alternative consolidant materials.…”