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Conservation in water


I am actually doing a project on the deterioration of the Vasa by internal formation of sulphuric acid, and I am requesting an information. Indeed, a few months ago I went to visit your great museum, and I read something about the fact that throwing the wast in the Stockolm bay had been actually really helpful to preserve the ship in water. Could you please remind me of how exactly the waste was helping? I think it was something about making the water anoxic but I can't remember for sure.

It would really help me for my project!

Tack så mycket!

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  • Dear Annabelle,
    I am Vasa Museum's conservator and your question has been passed on to me. What you read in the museum was actually a little provacative and needs more explanation. The waste (natural sewage) that was thrown in Stockholm harbour was high in sulfates, which coverted to hydrogen sulfide under water. Hydrogen sulfide contributed to the anoxic conditions and is also toxic to microorganisms. Consequently microbial decay of the wood was limited. However, the sulfides remained in the outer dedgraded surface of the wood and once the ship was raised and exposed to oxygen again, they were converted to acids. Importantly the reaction was catalysed by the high levels of iron ions in the wood from all the ship's bolts which had corroded under water. Before 2004, the climate system in the museum was unable to deal with the thousands of visitors and the relative humidity fluctuated greatly. The consequent wetting/drying cycle acted like a pump to bring the acids to the surface, where they precipitated as a range of iron sulfate salts. This is not the whole story, however. When we investigated deeper into the timbers, below the degraded surface, we found that conditions were also very acidic. In these areas there is no sulfur, but high levels of partly oxidised iron. It is this iron that is continuing to oxidise that is causing the acidic conditions. Here, organic (not sulfuric) acids are being produced. The wood has also lost about 50% of its strength which is a major concern for the ship and which is why we are planning a better support structure. Today, we don't  focus on the sulfuric acid so much, but the danger of the iron inside the Wood. I hope this helps. You are welcome to contact me again if you have questions.
    Emma Hocker
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