Morphological Study of Bio-Based Polymers in the Consolidation of Waterlogged Wooden Objects

The removal of water from archaeological wooden objects for display or storage is of great importance to their long-term conservation. Any mechanical instability caused during drying can induce warping or cracking of the wood cells, leading to irreparable damage of the object. Drying of an object is commonly carried out in one of three ways: (i) air-drying with controlled temperature and relative humidity, (ii) drying-out of a non-aqueous solvent or (iii) freeze-drying. Recently, there has been great interest in the replacement of the standard, but limited, polyethylene glycol with biopolymers for wood conservation; however, their behaviour and action within the wood is not completely understood. Three polysaccharides-low-molar-mass (Mw) chitosan (Mw ca. 60,000 g/mol), medium-molar-mass alginate (Mw ca. 100,000 g/mol) and cellulose nanocrystals (CNCs)-are investigated in relation to their drying behaviour. The method of drying reveals a significant difference in the morphology of these biopolymers both ex situ and within the wood cells. Here, the effect these differences in structuration have on the coating of the wood cells and the biological and thermal stability of the wood are examined, as well as the role of the environment in the formation of specific structures. The role these factors play in the selection of appropriate consolidants and drying methods for the conservation of waterlogged archaeological wooden objects is also investigated. The results show that both alginate and chitosan are promising wood consolidants from a structural perspective and both improve the thermal stability of the lignin component of archaeological wood. However, further modification would be necessary to improve the biocidal activity of alginate before it could be introduced into wooden objects. CNCs did not prove to be sufficiently suitable for wood conservation as a result of the analyses performed here.