diversity of bacteria identified comprised sequences representing bacterial
groups capable of sulfur oxidation (Thiobacillus prosperus, Methylobacterium
spp. and pseudomonads). They are capable of transforming reduced sulfur
compounds present in archaeological wood to sulfuric acid.
A possible treatment under investigation for sulfur contamination in Mary
Rosetimbers involves the controlled oxidation of reduced sulfur species by
microbes. Timber samples previously examined by XANES have been treated
with a chemical bio-stimulant containing a powerful blend of six specially
selected non-pathogenic strains of Bacillus, a non pathogenic chemolithotroph,
Thiobacillus denitrificansand two Gram-negative sulfur-oxidising facultative
heterotrophs of the species Starkeya novellaand Paracoccus pantotrophus.
The end product of their activity is sodium sulfate instead of sulfuric acid,
which can be easily washed out from archaeological timbers.
In addition to this study, timber samples previously examined by XANES
and infused with acid salts have been treated with various biocides to inhibit
continued bacterial oxidation of reduced sulfur species. This may also help
prevent continuing production of acid.
To prevent oxidation of reduced organosulfur in the wood cell walls, con-
servators are currently investigating antioxidants. Commercial antioxidants
(ascorbic acid, anthocyanidins, proanthocyanins, flavon-3-ols, flavonones and
flavonols) are being used to prevent oxidation of reduced species by mopping
up free radicals.
The removal of sulfur contamination from wood is not easy. To date con-
servators have tried various solvents for removing sulfur compounds, includ-
ing alcohol, acetone, and tertrachloroethylene (TCE). Hydrochloric acid has
been used to remove acid-volatile sulfur (AVS), essentially pyrrhotite. However,
after extensive extraction of hydrochloric acid-treated samples with the best
elemental sulfur solvent, TCE, S k-edge XANES of the samples found little
change in the spectrum, although there may have been some change in the
total sulfur content. Chromium(II) reducing agents have also been used to
remove elemental sulfur and pyrite, and we are currently working on ways to
measure the chromium-reducible sulfur (CRS) and AVS in samples of archaeo-
logical wood; these methods are currently in use in soil science. However,
these methods of removing sulfur compounds are not suitable for the treat-
ment of actual artefacts. It seems unlikely that any extraction process will be
possible, at least on any large scale.
However, it is worth noting that while the Swedish warship Vasais still
contaminated with sulfur and generating acid, it is now apparent that during
its treatment by spraying a considerable amount of acid was generated, as the
pH of the spray solution kept falling. This was neutralised by the addition of
borax/boric acid buffer. In the case of the Vasa, the major sulfur contaminant
appears to be elemental sulfur that is essentially insoluble in water. However,
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