The sulfuric acid threat to the Vasa

Outbreak of sulfate salts

During the wet summer in 2000 the relative humidity on several occasions reached levels higher than 65% in the Vasa Museum. An increasing number of white and yellowish salts started to precipitate inside the ship and on artefacts in the magazine.

Salt formation on a beam below the Vasa's galley in the hold (S4, see below) and on a chest in the magazine       Photos: M. Sandström, and the Vasa Museum

In February 2001 chemists were invited to a meeting at the Vasa Museum to discuss the problems. With a Guiner-Hägg camera, we could identify crystalline salts on the surfaces. The most common precipitate inside the Vasa was found to be the yellow-coloured natrojarosite, NaFe₃(SO₄)₂(OH)₆, an iron(III) sulfate that sometimes appears greenish on moist wood. Other commonly occurring salts are small bluish-white crystals of the iron(II) sulfate melanterite, FeSO₄·7H₂O, and ordinary white gypsum (calcium sulfate, CaSO₄·2H₂O. Also, crystalline elemental sulfur (consisting of ring-shaped S₈ molecules) was found on some surfaces, mostly in the hold (Sandström 2001).

Common crystalline salts on the Vasa's surfaces:

Jarosite: NaFe3(SO4)2(OH)6	Melanterite: FeSO4·7H2O
Gypsum calcium sulfate: CaSO4·2H2O	Elemental sulfur (S8 crowns)

More information with molecules and minerals visualized in 3-D can be found in the homepage of: The Virtual Museum of Minerals and Molecules.