Moisture behaviour of PEG-treated archaeological oak

The role of a hydrogel on a degraded cellular structure documented with neutron imaging

Dominique Derome
Laboratory for Multiscale studies in building Physics, Empa, Dubendorf, Switzerland

Eleonora Piva
Portsmouth University

David  Mannes
Neutron Image, SINQ, Paul Scherrer Institute

Guylaine Desmarais
Chair of Building Physics, Swiss Federal Institute of Technology Zurich (ETHZ)

Eleanor Schofield
The Mary Rose Trust

The warship, Mary Rose, which was raised in 1982 after 437 years under the sea bed, gives a unique insight into Tudor life. To preserve this important shipwreck, archaeological wood from the shipwreck was sprayed with an aqueous solution of polyethylene glycol (PEG, first grade 200 then 2000) for 19 years in order to mechanically stabilise the remaining ship hull, and to minimise collapse and shrinkage of the wood upon drying. Once the wood consolidated, the ship is acclimatized to museum indoor conditions. Thus, in 2013, the drying of the ship was started under an environment of 19degC and 54%RH.

The structure of wood has two main scales: an anisotropic cellular structure and a nanoporous cell wall material. Over a growing season in a temperate climate, the tracheids vary from earlywood to latewood cells, resulting in a growth ring pattern. Earlywood cells possess a larger diameter, a larger lumen and thinner walls than latewood cells do. During the years under water, the ship wood polymeric components deteriorated to some extent, mainly resulting in a loss of holocellulose in the cell walls. PEG200 is used to reinforce the cell walls. In addition, the cellular and growth ring structure can still be present but most of the lumens are filled with PEG2000 as it is used to consolidate the more degraded 10 mm from the surface of a timber piece. Thus, archaeological wood has different degrees of degradation from surface inwards, which can cause different water-PEG-wood interaction mechanisms. Overall, the presence of PEG presents a clear challenge in terms of imaging with neutrons.

Monitoring the moisture content (%MC) of the wood allows us to evaluate the drying process, to predict the drying rate and to understand the resulting impact of these on the mechanical properties of wood and the movement currently taking place on the ship structure due to shrinkage. To document in detail the drying process, the moisture content distribution is imaged at high temporal, spatial and hygric resolution using neutron imaging. Water sorption and desorption experiments are performed on small samples of archaeological oak wood treated with PEG200 and PEG2000 and in recent PEG-treated oak wood, under controlled conditions. Neutron radiography is based on intensity measurements of a neutron beam transmitted through an object. The high interaction of neutrons with hydrogen allows the quantification of the water present in the samples. The measurements were performed at the Neutra beamline, SINQ, Paul Scherrer Institute, CH.

To further our understanding of the material properties and behaviour of the Mary Rose hull, a complete desorption and sorption series of selected timbers are documented with neutron imaging. We present the sorption and desorption moisture content distribution, and resulting   swelling/shrinkage, fields. From these, water diffusion coefficients are indirectly determined for PEG treated wood, compared with that of untreated archaeological wood.