During its time in the sea bottom of Stockholm harbour, huge amounts of iron and sulfurous compounds accumulated in the wood of the royal warship Vasa. Since 2000 it has been noticed that changes are taking place in the wood, changes that threaten the stability of the ship.
At first it was believed that the conversion of sulfur to sulfuric acid was the culprit, but now it has been shown that it is the iron from the ship’s rusted bolts and cannonballs is causing the most serious deterioration of the wood. This is the subject of a dissertation by Gunnar Almkvist from the Swedish University of Agricultural Sciences.
The wood in the royal ship Vasa has been seriously affected by the biological and chemical processes that the hull was exposed to during its period under water (1628-1961), during its conservation period (1962-1989), and subsequently in its modern museum setting.
Iron and sulfurous compounds (a total of 5-10 tons) were incorporated in the wood through natural processes when the ship lay on the sea bottom of Stockholm harbour.
The sulfur comes from the degradation of Stockholm’s unpurified sewage on the oxygen-free bottom, while the iron primarily comes from the bolts that held together the hull and other iron objects onboard, such as cannonballs. In the subsequent conservation, the wood was treated with huge amounts of polyethylene glycol (PEG) and fungicides.
During the rainy summer of 2000, the humidity varied dramatically inside the museum, and during this time white and yellow precipitates were discovered on the ship. These deposits turned out to be acidic sulfur and iron compounds, and it was concluded that sulfur in the wood had been converted into sulfuric acid. Concern about what was happening to the Vasa led to a commitment to pursue research into the causes and how they could be prevented.
Gunnar Almkvist and his colleagues have now completed a thorough examination of the chemical degradation processes in the wood, and it turns out that it is the iron that is the main culprit, not the sulfur.
The most serious problem is that the wood substances have begun to be degraded at some depth in the lumber and that this degradation has also affected the conservation agent (PEG). Analyses show that the wood is extremely acidic in places where the scientists found degradation.
The low pH level is a result of the formation of organic acids such as formic acid and oxalic acid with the degradation of wood components and PEG. In these sections there are only tiny amounts of sulfur. Certain types of sulfurous compounds appear to have a protective effect when the wood is in much better condition where there is a great deal of sulfur.
Everywhere there has been degradation, on the other hand, there is plenty of iron, and this iron has proven to be in a form that is highly mobile and chemically active. Iron can vary in its oxidation levels and then can take on special properties and participate in special reactions.
In the presence of oxygen or other oxidation agents, iron ions can form so-called radicals. These are very short-lived but highly reactive and can attack most biological substances. The theory is therefore that the iron is catalyzing (hastening) the formation of radicals in the wood of the Vasa. Sulfur, on the other hand, can function as an antioxidant that captures radicals and prevents them from reacting with the wood or with PEG. The element that was originally thought to be the threat thus appears to be a safeguard.
The dissertation also reports from attempts to extract iron compounds from the Vasa wood. The results show that this is possible, though very time-consuming. In the process, other water-soluble compounds are also extracted and acids are neutralized. The method can be successfully used for smaller objects and can also be developed for the treatment and conservation of waterlogged archeological wood.
Link to the complete dissertation: The Chemistry of the Vasa - Iron, Acids and Degradation: http://diss-epsilon.slu.se/archive/00001809/
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