Deterioration of the Hanson Logboat: chemical and imaging assessment with removal of polyethylene glycol conserving agent

Abstract

The state of preservation of wood in two samples from the Hanson Logboat, currently on display in Derby Museum and Art Gallery, was analysed using elemental analysis (EA), pyrolysis–gas chromatography/flame ionisation detection (Py-GC/FID), pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) and scanning electron microscopy (SEM). The samples were collected in 2003, after the boat had undergone conservation, and in 2011 after the condition of the boat began to deteriorate. Solvent extraction enabled removal of polyethylene glycol, with which the wood had been impregnated during conservation, allowing the degradation of the cellulose and lignin polymeric components of the woods to be assessed. Elemental compositions (C, H, N, O, S), Py–GC/FID, Py-GC/MS and SEM imaging reveal extensive degradation of the wood polymers during the eight year period since conservation.

A review of analytical methods for assessing preservation in waterlogged archaeological wood and their application in practice

“…SEM-back-scattered electron (SEM-BSE) or SEM-energy dispersive X-ray (SEM-EDX; sometimes termed SEM-EDS) analyses detect scattered electrons or X-rays emitted from the sample during SEM analysis. As these are characteristic of the atomic weight of the elements present, analysis can provide an elemental map of the surface of the sample, meaning that inorganic components and crystal formations can be characterised [11,25]. Both techniques have also been shown to provide lignin distribution maps when pre-treated with a reagent that preferentially binds to lignin [72,73], and have been used to investigate the effectiveness of conservation treatments [74].…”

“…A combustion analysis system can give a measure of the relative carbon, hydrogen, nitrogen (and sometimes sulfur) content of a sample (CHN(S)) [83]. The oxygen content can be derived by correcting for any inorganic contaminants and moisture content, or by using an alternative, less widely available combustion system [11]. As cellulose contains more hydrogen relative to carbon than lignin does, a decrease in the hydrogen content relative to the carbon content can signal loss of the carbohydrate fraction [11,17].…”

“…The oxygen content can be derived by correcting for any inorganic contaminants and moisture content, or by using an alternative, less widely available combustion system [11]. As cellulose contains more hydrogen relative to carbon than lignin does, a decrease in the hydrogen content relative to the carbon content can signal loss of the carbohydrate fraction [11,17]. Oxygen to carbon ratios also decrease with increasing degree of decay, characteristic of preferential preservation of the carbon-rich lignin component [84].…”

“…However, the data obtained is limited, with information on the different mechanisms of decay impossible to elucidate [83]. It must also be noted that the percentages can vary depending on the species of the starting material, and although conserved objects can be analysed, the method is heavily influenced by the presence of preserving agents, so caution should be used if the conservation history of an object is not known [11].…”

“…Appropriate analysis establishes a robust baseline against which any further deterioration can be tracked, for example: when a site is being monitored [3,4], when comparing material from different sites or phases of investigation [5,6], or when gathering experimental data on decay mechanisms [7][8][9]. Analysis is also important post-excavation; detecting decay over periods of storage or display can help identify when conditions are not conducive to the continued survival of an object [10,11]. Analysis during or after conservation can be a critical part of establishing when a conservation treatment has worked, or indeed is having a negative effect [12,13].…”