Acidity and Salt Precipitation on the Vasa; The Sulfur Problem

Abstract

The present major conservation problem for the Vasa is the oxidation of sulfur compounds in the wood, which after several intermediate steps results in sulfuric acid and high acidity leading to wood degradation.In February 2001 the extent of this problem was disclosed by analyzing two core samples of oak wood from a massive oak beam in the hold at the bottom of the ship, down to 80 mm depth.Electron spectroscopy for chemical analysis (ESCA or XPS) was used for quantitative measuements of all elements present in significant amounts in one core sample, and in the other the different forms of sulfur were analyzed by means of synchrotron based sulfur K-edge X-ray absorption spectroscopy (XANES).Both the ESCA and the XANES results show the total sulfur concentration to be highest at the surface, with sulfate as the dominating form, SO4 2 ".The relative amount of non-oxidized sulfur, mainly elemental sulfur, increases rapidly below the surface, but the total sulfur concentration decreases at the same time.Deep below the surface at low sulfur concentration again oxidized sulfur, mostly sulfate, dominates.The XANES spectra clearly show several intermediate oxidation states of sulfur as minor components.Thus, below the surface to a depth of about 2 cm reduced sulfur, mainly elemental sulfur, is present in significant amount, about 0.5 mass%.If the sulfur concentration in this core is representative, as also later analyzes indicate, the corresponding amount of concentrated sulfuric acid may when all sulfur is oxidized be about 0.5 kg/m 2 wooden surface, or totally more than 5000 kg for the Vasa!The ESCA analyses show a rather high and only slowly decreasing level of oxidized borate at all depths.Since borate is not naturally present in oak wood, this shows that boric acid from the solutions used in the initial conservation treatment, has penetrated deep into the wood.Iron is present in oxidized form with rapidly decreasing concentration below the surface, as also for chloride and calcium.Silicon was only found in the surface layer, while sodium occurred with constant concentration at all depths.Surface salt precipitations sampled inside the hull of the Vasa were analyzed by means of X-ray powder diffraction (XRD) and sulfur K-edge XANES spectroscopy.The XRD analyzes showed that the crystalline part of the samples contained sodium, ammonium, potassium, calcium and magnesium sulfates of various composition; in most cases gypsum CaS04-2H20, but also lecontite (K, NH 4 )NaS0 4 -2H 2 0, mascagnite (NH 4 )2S04-2H 2 0 and starkeyite MgS04-4H 2 0. Iron sulfates, especially natrojarosite NaFe3(SC>4)2(OH) 6 but also melanterite FeS04-7H20, were found and in a some places crystals of elemental ct-sulfur (brimstone Sg) but only traces of pyrite, FeS2.The XANES spectra showed in the surface samples in addition to the dominating sulfates, also some amount of elemental sulfur.The salt formation on the surface is a sign of an ongoing production of sulfuric acid, which will cause high acidity and wood degradation.The high concentration of sulfur compounds found in the wood shows that this is a severe threat to the Vasa.Lowering the relative humidity RH noticeably will only marginally slow down the reaction rate of a chemically controlled oxidation process, and gives unfavorable side effects.The presence of iron in the wood, and corroding iron bolts, gives additional problems because of the catalytic effect of iron on the oxidation process.It seems important to keep the conditions constant, the RH but also the temperature, in order to prevent water and oxygen migration in the wood, and to apply pH raising and iron removing consolidation treatments to the already acidified wood.Possible treatments are proposed.

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