New insights into the degradation processes and influence of the conservation treatment in alum-treated wood from the Oseberg collection

Abstract

The Oseberg collection includes the most complete ensemble of wooden remains from the Viking Age. However, since many of the wooden objects were treated with alum in the early 1900s, they now suffer from dramatic conservation issues. A multi-analytical approach was adopted to investigate both the organic and the inorganic components of some selected wood fragments, with the aim of fully characterising the materials and their decomposition products. A particular focus was taken on the differences between the surface and the core of the fragments analysed, and on the correlations between the results obtained by the different techniques, in order to disclose possible interactions between the materials during degradation. In addition to differences in alum concentration and wood alteration between the surface and the core, some decomposition/transformation products of alum, such as mercallite (KHSO4), were identified by FTIR and XRD. Contextual interpretation of the results obtained by ICP-OES elemental analysis of inorganic components and Py(HMDS)-GC/MS characterisation of degraded lignocellulosic materials supported some previous observations about potential relationships between specific metals (Al, Fe, Ca) and wood degradation and enabled new correlations to be highlighted. Although similar degradation patterns were revealed in the investigated objects–depletion of holocellulose, oxidation of lignin and some transformation of alum - a notable variability at the molecular level was highlighted. This is an important factor to be taken into account for the planning of re-treatment strategies of these extremely precious artefacts

Temperature- and humidity-induced changes in alum-treated wood: a qualitative X-ray diffraction study

“…Alum-treated objects also display high acidity and lose structural integrity over time [2][3][4][5]. Today, extensive chemical characterisation of wood from the Oseberg collection has linked extreme degradation of wood polymers to alum treatment and consequent release of sulfuric acid [6][7][8]. Despite these investigations, questions remain about whether alum itself is stable in its current form in treated wood within reasonable temperature and relative humidity (RH) ranges, which are important to address in order to assess whether it is necessary to remove alum in eventual retreatment as opposed to de-acidification without alum removal.…”

Recent Advances in Analytical Pyrolysis to Investigate Organic Materials in Heritage Science

“…Analytical pyrolysis was exploited to investigate the degradation of archaeological wood in the presence of unstable inorganic components,b yi nvestigating the collection of Oseberg Viking finds. [24] Thea rtefacts were treated with alum, and contained significant amounts of iron. The amount of specific pyrolysis products had ap ositive and significant correlation index with the concentration of metals present in the archaeological wood samples,and in particular with Al and Fe,q uantified by inductively coupled plasma mass spectrometry (ICP-MS) (Figure 4).…”

Anwendung der analytischen Pyrolyse zur Untersuchung organischer Materialien in Kulturgütern

“…Die analytische Pyrolyse wurde dazu genutzt, den Einfluss der Anwesenheit von instabilen anorganischen Bestandteilen auf den Zerfall von archäologischem Holz in der Oseberg Wikinger‐Sammlung zu untersuchen . Die Artefakte wurden mit Alaunen behandelt und enthielten eine signifikante Menge an Eisen.…”