The weakened microstructure of archaeological wood (AW) objects from waterlogged environments necessitates consolidation to avoid anisotropic shrinkage upon drying. Polymer impregnation through submergence or spraying treatments is commonly applied, and for larger and thicker objects, the impregnation period can stretch over decades. Thus, for efficient treatment, continuous monitoring of the impregnation status is required. Today, such monitoring is often destructive and expensive, requiring segments for extraction and chromatographic quantification. This study proposes an in situ Raman spectroscopic method for quantification of polyethylene glycol (PEG) in waterlogged AW. A calibration model was built on standards of PEG, cellulose powder, and milled wood lignin using orthogonal partial least squares (OPLS). The OPLS model had a strong linear relationship, and the PEG content in wood of varying degrees of degradation could be determined. However, the accuracy of the model was low with a root mean square error of prediction of 11 wt%. The low accuracy was traced to the heterogeneity in the calibration and validation set samples with regard to the small probing volume of the confocal instrumental setup.

As the redox catalytic activity of iron may be detrimental for waterlogged archaeological wood, it is of importance to know what ligands bind the iron, their distribution in the wood, and what the reactivity of these compounds are. We have identified iron-tannin precipitates in archaeological oak timber from the shipwreck of the seventeenth-century warship The Sword with confocal Raman spectroscopy in combination with light-and scanning electron microscopy. Iron tannin precipitates are present as larger aggregates (10-50 µm) in the lumen and vessels, but also as contamination within the wood cell wall. As the stability constants of iron-tannin and iron-diethylenetriamine-pentaacetic acid (DTPA)a chelating agent commonly employed by conservators to extract ironare in the same range, extraction experiments were carried out. These demonstrate that iron-tannin precipitations are difficult to extract. Tannin chemistry in waterlogged oak needs to be further explored to understand the mechanism of iron accumulation in wood as well as to correctly treat and preserve cultural heritage.