Formic and acetic acids in archaeological wood. A comparison between the Vasa Warship, the Bremen Cog, the Oberländer Boat and the Danish Viking Ships

Abstract

Abstract A method has been developed to analyse and quantify formic and acetic acids in archaeological and fresh wood. The method takes advantage of the fact that, in equilibrium, the gas-phase concentrations of formic and acetic acids over pulverised archaeological wood in a sulfuric acid solution are proportional to their absolute concentrations in wood. The method is based on automated solid-phase micro extraction (SPME) combined with gas chromatography-mass spectrometry (GC-MS). Good linearity of the calibration curves was observed. The content of formic and acetic acids in archaeological wood from ships conserved with polyethylene glycol (PEG) was determined. The content of formic acid is related to the intensity of conservation and, hence, the PEG content in wood. Accordingly, formic acid found in the hull of the ships may partly be a result of PEG degradation. The formic acid content in the Vasa warship is, on average, not higher than in the other ships analysed. In contrast, the acetic acid content in PEG-preserved archaeological wood is lower than in fresh wood. The acetic acid content is age-dependent and is lowest in 1000-year-old wood. The acetic acid probably originates from the wood.