Synthesis and characterisation of lignin-like oligomers as a bio-inspired consolidant for waterlogged archaeological wood

Abstract

The development of new materials for the consolidation of waterlogged archaeological wood from sustainable sources is an important area of research, as the most widely used consolidant today is petroleum based. Ideally a new consolidant will interact with the existing wood structure, ensuring maximum compatibility. Lignin is often the major component remaining in archaeological wood, as it is less susceptible to degradation than holocellulose. Therefore, in order to maximise the potential for interaction with the wood cells, lignin-like oligomers have been synthesized from isoeugenol using a water soluble copper salen catalyst at pH 10, giving a weight average Mw of 1.6 kDa. Analysis by NMR spectroscopy has shown that the oligomers have a lignin-like structure with β-O-4′, β-β′ and β-5′ connections. A 10 w/w% solution of the oligomers in ethyl acetate was found to thoroughly penetrate 1 cm3 samples of waterlogged archaeological wood (density of 0.146 g/mL, maximum water content of 620%) after 14 days impregnation, as determined by FTIR spectroscopy. No impregnation material could be seen by SEM, suggesting that it coats the cell walls upon drying. This indicates that dehydrogenated polymers penetrate waterlogged archaeological wood well and have the potential to be developed into consolidants.

Tapping into Synchrotron and Benchtop Circular Dichroism Spectroscopy for Expanding Studies of Complex Polysaccharides and their Interactions in Anoxic Archaeological Wood

“…Waterlogged archaeological wood requires preservation strategies that inhibit or minimise the effects of structural instability on drying, biological degradation including microbial attack and chemical degradation due to Fe 3+ -catalysed production of acid which erodes the structural integrity of the wood. Over the past 20 years or so, wood artefacts such as the warships Mary Rose and Swedish Vasa have been treated with aqueous polyethylene glycol containing a broad spectrum biocide with the aim of supporting the remaining wood cell walls (a significant proportion of which is likely to be lignin due to its well documented recalcitrance to microbial and chemical decay) and inhibiting biological growth ( [46,47] and reviewed in [48,49]). Similarly, from the mid-1800's until the 1950s, concentrated solutions of alum salts (KAl(SO 4 ) 2 •12H 2 O, aluminium potassium sulphate dodecahydrate) at 90 • C were used for strengthening wooden artefacts and maintaining their shape [50][51][52][53].…”

“…The alum salt replaced the remaining water in the wood cells and recrystallized immediately upon removal from the heat treatment, thereby rapidly preserving the shape of the wooden artefacts. However, there is increasing recognition that new and/or additional strategies are required for conservation of such waterlogged wooden artefacts and a variety of approaches are being investigated [53][54][55][56][57][58], including those involving the use of new consolidants that specifically interact with the longer-surviving lignin content of archaeological wood [59,60].…”

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

“…Objects which have been restored will not withstand immersion either. For such objects, solvent-based strengthening agents are being investigated in Saving Oseberg [39][40][41][42][43].…”

The Svedberg Lecture 2017. From nano to micro: the huge dynamic range of the analytical ultracentrifuge for characterising the sizes, shapes and interactions of molecules and assemblies in Biochemistry and Polymer Science

“…McHale et al ( 2016 , 2017 ) have been seeking to develop lignin replacements for the decayed wood using isoeugenols (monomer M ~ 160 Da, comparable to that of an amino-acid or carbohydrate residue) which are structurally very similar to lignins, built up from very similar monomer units (Fig. 2 c), but of lower molecular weight (low enough to be absorbed into the wood), but have to be made to polymerize in situ using a peroxidase in the presence of hydrogen peroxide H 2 O 2 .…”