Raman Spectroscopic Investigation of Iron-Tannin Precipitates in Waterlogged Archaeological Oak

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 <i>The Sword</i> 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 iron – are 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.