Chemical analyses of extremely degraded wood using analytical pyrolysis and inductively coupled plasma atomic emission spectroscopy

Alum-treated archaeological woods from the Oseberg collection, excavated and treated in the early 1900s demonstrate an extreme deterioration, only discovered in the past decade. This research was aimed at understanding the characteristics of the naturally aged material through chemical analyses of both organic and inorganic components, in order to provide information for the preservation/conservation of these precious objects. Analytical pyrolysis-gas chromatography/mass spectrometry with in situ silylation using hexamethyldisilazane (Py(HMDS)-GC/MS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) were used to investigate a set of samples of alum-treated archaeological wood and untreated archaeological wood. Sound woods of the same species were also analysed in order to compare the results. Results from Py(HMDS)-GC/MS analyses of alum-treated woods from Oseberg showed an extreme depletion of carbohydrates and a highly deteriorated lignin network. The majority of the lignin had undergone oxidation reactions, illustrated by high relative amounts of acidic pyrolysis products. In particular, p-hydroxy-benzoic acid was detected for the first time as a degradation product of archaeological wood. Results from ICP-AES showed variable concentrations of aluminium and potassium from the alum treatment, as well as iron, copper, zinc and calcium. The extent of oxidation observed by Py(HMDS)-GC/MS appeared to correlate with relative amounts of iron and calcium in the samples, which may suggest that degradation promoted by iron compounds is inhibited in the presence of calcium compounds. The results obtained for a sample of archaeological wood treated with alum salts five years ago showed that chemical deterioration of lignin had already begun, mainly involving depolymerisation reactions, though only a slight increase in oxidation was evident. In addition to increased insight into potential deterioration reactions, occurring in a complex material such as alum-treated archaeological wood, it was possible to relate the degree of chemical degradation of the Oseberg woods to their visual condition (observed with the naked eye). © 2015 Elsevier B.V