A new approach to wood preservation

Wood can be a difficult, and costly, archaeological material to preserve. This is nowhere better highlighted than by the enormous efforts put into place to help conserve the Mary Rose. When the remains of Henry VIII’s warship were lifted out of the Solent in 1981 (see CA 318 and 272), little was known about how best to preserve the wreck once it had been removed from the protective anoxic conditions under the seabed. But over the past three decades, multiple techniques have been developed to keep the ship, and other wooden artefacts recovered from archaeological contexts, from degrading before our eyes. In this month’s ‘Science Notes’, we explore the latest of these preservation techniques.

One of the biggest wins in wood preservation came with the discovery of the use of nanoparticles (NPs) – particulates of matter which, due to their small size, drive different physical and chemical properties. In the case of the Mary Rose, strontium carbonate (SrCO3) NPs were found to neutralise sulphuric acid – which forms once sulphur in the wood is exposed to oxygen – thereby arresting the degradation of the wood that would have otherwise occurred through acidification. But this process can be quite time-consuming and volatile, especially for artefacts as big as the Mary Rose, as it requires complete emersion in an alcoholic solution for several hours.

Wood samples were taken from this 2nd century BC Gallo-Roman ship discovered in Lyon, France, near the River Saône.
Wood samples were taken from this 2nd century BC Gallo-Roman ship discovered in Lyon, France, near the River Saône.

A new paper, recently published in the journal Nanoparticles (www.mdpi.com/2079-4991/10/9/1744), has built on the advances made during work on the Mary Rose and outlined a new nanoparticle protocol, which, if successfully applied in other cases, would provide a cheaper and easier way to preserve wooden artefacts, particularly exceptionally large ones such as boats or bridges. The research was led by a team from the National Research Council, the University of L’Aquila, in Italy, and the Institut Laue-Langevin and ARC-Nucléart, both in Grenoble, France, using wooden samples taken from a 2nd century BC Gallo-Roman ship discovered in Lyon, near the River Saône, also in France.

What spurred the research was the team’s successful development of a magnesium hydroxide (MH) nanoparticle suspension that uses water instead of alcohol. This is significant progress, because water is not only cheaper and less volatile than alcohol, it is also more environmentally friendly, meaning that this new procedure could make wood preservation much easier to do. But the researchers had to check that this new type of suspension would actually work on archaeological specimens.

First, though, samples taken from the Lyon ship had to be prepared by soaking them in a solution of polyethylene glycol (PEG) to replace the water in the wood and prevent it from cracking once it dried. The PEG was then removed from the wood samples through immersion in deionised water, before they were dried using a lyophilisation procedure, which works by freezing the wood and then slowly reducing the pressure, while adding heat, to allow the frozen water to sublimate.

Once this was completed, the researchers could use their new aqueous NP suspension. In particular, the team wanted to know whether this procedure could be used preventatively as well as curatively. To see if it could prevent any damage, first the samples were submerged in the new MH NP solution, and then subjected to a relatively high temperature and humidity for two days to accelerate the process of acidification. To test whether it could ‘cure’ the wood from damage that had already been done, the procedure was reversed: first, they were acidified, and then treated with the new solution.

The results were extremely promising. For the samples that were tested for the preventative effects of the NPs, the samples remained at a neutral pH, even after being exposed to extreme conditions, while those that were treated in a curative fashion successfully went from an acidic pH to a neutral one. Importantly, in examining the samples using SEM-EDS technology (see CA 368, 367, and 366), the team was able to map the presence of magnesium in each sample, finding it to be homogeneously distributed throughout the entire depth of the samples. This indicates that they were successfully penetrated by the NP solution and hence the whole sample protected from acidification. Overall, it seems that the water-based MH NP treatment could be used to great effect in helping to preserve certain wooden artefacts in an easier and more sustainable way.