Cutajar, Michelle
(2023)
New Bioinspired Polymer Consolidants for the Oseberg Artefacts and Other Archaeological Wood.
PhD thesis, University of Nottingham.
Abstract
The Oseberg Viking ship burial is one of the most extensive collections of Viking wooden artefacts discovered in the world. In the early 20th century, many of these artefacts were treated with alum. Today, it is known that this alum treatment was very damaging to the wood, as a significant number of the artefacts demonstrate a high degree of deterioration. The Saving Oseberg project was set up in 2014 by the Museum of Cultural History at the University of Oslo to work towards gaining understanding of the chemical composition of the alum-treated wood and establishing novel conservation treatments. Current studies on the state of the Oseberg collection prove that the artefacts are undergoing continuous degradation.
Alum-treated wood is a highly complex substance and no truly effective treatment has yet been developed for the most degraded objects. It is therefore crucial to develop new polymers which could be used to conserve these artefacts and prevent their disintegration. Consolidants which are soluble in organic solvents are particularly needed, as these could be used to treat some of the most degraded pieces in the Oseberg collection. Additionally, bioinspired polymers represent a more sustainable alternative to some of the consolidants currently in use. To this end, the research aim was to develop novel, organic solvent-soluble, bioinspired polymers which would be suitable to use for the retreatment of the Oseberg artefacts, as well as other archaeological wood.
Terpenes were identified as a suitable feedstock for the synthesis of such compounds, as they are derived from biomass and are easily functionalised. Two monomers were subsequently synthesised, derived from α-pinene, each modified with an acrylate group. An additional, non-terpene monomer was also prepared using oleic acid. These were then used to synthesise three hydroxylated polymers named TPA5, TPA6 and TPA7. TPA5 and TPA6 were homopolymers which were entirely terpene-derived, while TPA7 was a copolymer with α-pinene and oleic acid components. The polymerisation reactions of all three molecules were optimised so that each had a small molecular weight which enables wood penetration (~ <5 kDa). All the polymers were soluble in isopropanol, a solvent which was deemed acceptable to use in archaeological wood treatment.
These polymers were extensively characterised, primarily using analytical ultracentrifugation, viscometry and other hydrodynamic techniques, in order to investigate whether they would be suitable for wood consolidation purposes. These characterisation studies confirmed that TPA5, TPA6 and TPA7 were excellent candidates for use as potential consolidants. Due to time constraints it was decided to not continue developing TPA5 beyond this point and instead focus on TPA6 and TPA7. This was because the synthesis routes of these two polymers were more efficient and less laborious than that of TPA5. The synthesis of TPA6 and TPA7 was then scaled up to be able to make enough material for the subsequent wood testing.
These preliminary wood studies were carried out using archaeological pine. The polymers were dissolved in isopropanol at an appropriate concentration and the wood specimens were immersed in the solutions. Subsequent analyses involved weight and dimensional measurements, colour and pH measurements, Fourier-transform infrared spectroscopy, scanning electron microscopy and hardness tests. These studies confirmed that both TPA6 and TPA7 successfully penetrated the wood, with the highest concentration of polymer being found on the specimen surfaces. The hardness tests showed that treatment with these polymers caused the wood surface to become more resistant to indentation.
Other research that was carried out involved characterising two existing consolidants, Butvar® B-98 and PDMS-OH, with the use of analytical ultracentrifugation. This was used to confirm their molecular weight and heterogeneity. The tests that were carried out indicated that the samples showed considerable conformational asymmetry from measurements of intrinsic viscosity, which would facilitate networking interactions as consolidants. It is anticipated that the accumulated data on these two consolidants will enable conservators to make a more informed decision when it comes to choosing which treatment to administer to archaeological artefacts.
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