Hydroxide Nanoparticles for Deacidification of Archeological Wood

In the last decades, some major shipwrecks underwent extensive treatments, aimed at stabilizing the wooden structure and allowing their preservation and exposition as objects whose historical importance is fundamental for both social and economic aspects. The Vasa warship and the English Mary Rose represent two of the most famous examples, and share some common conservation features. Both these shipwrecks, in fact, have remained buried for centuries in relatively anoxic waters that slowed bacterial degradation. Following their salvages, in the second half of the 20th century, the two ships underwent massive consolidation treatment, including extensive impregnation with poly (ethylene glycol), PEG. Despite the good preservation status at the moment of their rescue, however, the two ships developed, in the last decades, similar conservation issues due to the presence of sulfur compounds. While the sulfate-rich waters of the Stockholm harbor and of the southern coast of England inhibited the action of aerobic microorganisms, on the other hand they provided the perfect environment for the metabolic cycles of sulfate reducing bacteria, resulting in the production of a large amount of reduced sulfur compounds which penetrated inside the ships’ timbers. After the recovery of the ships, exposition to oxygen started turning the reduced compounds into sulfuric acid, which degrades wood both chemically and mechanically by catalyzing the hydrolysis of cellulose. The presence of iron compounds inside wood, due to bolts and other parts, favors the process, since iron ions catalyze the oxidation of reduced sulfur, and degrade cellulose through Fenton-like reactions. Moreover, iron is involved in the degradation of PEG, producing organic acids (acetic acid, formic acid). Investigations on the Vasa, for example, mapped the pH values of wood in several degraded areas, reporting values below 2. As a matter of fact, acid hydrolysis and oxidation of cellulose threaten the conservation of these important historical objects, and represent two among the main issues to be addressed by conservation strategies aimed at preserving the ships. The ideal treatment, in fact, should focus on both these synergistic degradation pathways, limiting or eliminating them in one single step. This contribution will highlight how nanotechnology may contribute to address such severe conservation issues.