Climatically Induced Degradation Processes in Conserved Archaeological Wood Studied by Time-lapse Photography

Abstract

Samples of conserved archaeological wood of different ages, origins, and conservation histories were aged in a climate chamber for seven months, while the humidity alternated between 30% RH for 12 hours and 80% RH for 12 hours at a constant temperature of 30°C. Photographs were taken once every hour, which enabled the creation of a time-lapse movie. Some samples degraded visibly, whereas others were unaffected. Most of the samples were robust and would be able to survive well even in a very poor museum climate. Among the sensitive samples, three types of degradation were identified, namely disintegration, pyrite oxidation, and efflorescence of white crystals. Disintegration was ascribed to dimensional changes caused by the RH alternations in very fragile wood. The white efflorescence was interpreted as the recrystallization of an alum-associated substance, possibly mercallite (KHSO 4). The pyrite oxidation was observed as the efflorescence of a thick yellow, grey, and green powder. Characterization of selected samples was performed using X-ray fluorescence spectrometry, X-ray diffraction spectrometry, scanning electron microscopy with energy dispersive X-ray spectroscopy, inductively coupled plasmaoptical emission spectroscopy, Fourier transform infrared spectroscopy, ionic conductivityliquid chromatography, and pyrolysis-gas chromatography-mass spectrometry with in situ silylation using hexamethyldisilazane.

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

“…Otherwise alum appeared to remain unchanged within the temperature and RH ranges used. This is generally consistent with the previous studies; in their dynamic vapour sorption experiments Mortensen and Taube [10] observed mass loss at 0-10% RH at 30 °C for both pure potassium alum and alum-treated Oseberg wood, but otherwise noted the mass of hydrated alum was almost constant up to 95% RH. However, they comment that in the experiments performed by Häggström et al [4], the combination of 40 °C and 15% RH must be close to the dehydration point of alum, although significant mass loss was not observed.…”

“…Similar observations were made for samples 185-4 and 185-6, indicating that alum was not stable under the drying conditions, undergoing dehydration and taking years to revert back to its original fully hydrated form. Such slow rehydration was surprising, given that previous analysis of alum and alum-treated wood by dynamic vapour sorption showed rehydration of dehydrated alum within a few hours when exposed to 30-40% RH at 30 °C [10]. This suggests that the rate of this process is greatly reduced under ambient conditions, which in our lab are temperatures around 20 °C with large variations in RH.…”

“…In an earlier report, efflorescence of small, white, needleshaped crystals were observed on the surface of alumtreated fragments from object 229 after being subjected to RH cycling between 30% and 80% in 12 h intervals at 30 °C for 7 months [11]. Although the sample size of these crystals was too small for conclusive XRD analysis, elemental analyses detected potassium and sulfur and potassium bisulfate was identified in the wood fragments, suggesting that the efflorescence may be related to the potassium bisulfate component.…”

“…Some previous studies have looked into the response of alum to changing RH, finding that alum-treated wood is indeed sensitive to such changes [5,9], although this appears to be due to moisture uptake of the wood and not necessarily alum, which in its pure form does not display significant mass gain or loss for a range of RHs and temperature between 15 and 95% and 15-40 °C [4,10,11]. However, responses of salts to ambient and artificially-induced climate fluctuation, such as formation of crystalline efflorescence, have been previously observed in alum-treated wood from the collection [11,12]. In order to further explore such observations, as well as clarify appropriate RH and temperature limitations for alum-treated objects, we herein expand on the previous studies by investigating RH-and temperature-induced responses in alum-treated wood at the crystalline level using powder X-ray diffraction (XRD).…”

Navigating conservation strategies: linking material research on alum-treated wood from the Oseberg collection to conservation decisions

“…The iron content in Oseberg alum-treated wood samples analyzed by ICP-OES was 3.5 mmol/100 g or less [21,26], much lower than that found in other published finds where iron content ranged from ca. 10 to several 100 mmol per 100 g of sample (mmol/100 g) [33][34][35].…”