Oxidative degradation of archaeological wood and the effect of alum, iron and calcium salts

Caitlin M A Mcqueen,Martin N Mortensen,Susan Braovac,Francesco Caruso,Sara Mantellato

doi:10.1186/s40494-020-00377-0

Caitlin M A Mcqueen, Martin N Mortensen + Show 3 more

Open Access

https://doi.org/10.1186/s40494-020-00377-0

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Abstract

Oxygen consumption measurement was used to study potential oxidative degradation reactions occurring in wooden artefacts from the Viking age Oseberg collection in Norway. Model samples of fresh birch were impregnated with iron, calcium and alum salts to mimic concentrations of such compounds found in Oseberg artefacts and to assess their effect on oxygen consumption rates. The results showed that heated impregnation with alum salt (KAl(SO4)2·12H2O) significantly increased the rate of oxygen consumption, confirming a previously observed link between alum-treatment and wood oxidation. The presence of iron salts in alum-treated wood specimens, even at low concentrations, also substantially increased the oxidation rate. However, the mechanism by which this occurred appeared to be influenced by the alum-treatment. Samples treated with both iron and calcium salts were also studied, in order to investigate a proposed inhibition of iron-induced oxidation by calcium ions. However, these did not appear to consume oxygen at significantly different rates. In Oseberg samples, a large variation in oxygen consumption rates from 0.48 to an apparent 8.2 μg O2 (g wood)−1 day−1 was observed, and these values were consistently higher than those for reference fresh wood. The results demonstrated that oxygen consumption measurement is a viable method of evaluating chemical stability in this case, but is best applied to model samples with limited compositional variability.

Highlights

Ongoing degradative processes are a well-documented problem in collections of waterlogged archaeological wood, as are the challenges associated with studying them [1,2,3,4]
Fresh birch samples with iron(II) and calcium salts Figure 1 shows the results of the sets of model samples investigating the effects of iron and calcium ions
Amounts of iron absorbed by the model samples were somewhat higher than the quantities found in the Oseberg samples: the mass difference of the samples treated with FeCl2 before and after treatment reflect an Fe content in the model samples of ca. 68 mmol/100 g wood, compared to ca. 0.1 to 3.5 mmol/100 g in the Oseberg samples

Summary

Introduction

Ongoing degradative processes are a well-documented problem in collections of waterlogged archaeological wood, as are the challenges associated with studying them [1,2,3,4]. As the processes are slow, the chemical changes they produce within a reasonable timeframe can be hard to detect, and some uncertainty arises when using destructive analysis of micro-samples due to the heterogeneity of the material. Oxygen consumption measurement has been shown to be an effective means of monitoring real time chemical change in wooden objects related to degradation [5, 6]. It was shown that oxidative processes, probably autooxidation, were ongoing and measurable as oxygen consumption, despite conservation treatment [8, 9]

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