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The article below describes the science of preserving Henry VIII's favorite ship, oxidation of sulfur and
formation of sulfuric acid as a result of that oxidation.
At the end of June 2009, the city of Portsmouth celebrated the 500th anniversary of the coronation of Henry VIII and
the commissioning of the Mary Rose, Henry VIII's favorite warship. As the Mary Rose museum is currently closed, mainly
pictures from the festivities are used on this page.
Discovery of the wreck
The ship's wreck was first found by accident in 1836, but its location was only known to the man who made
that discovery. A new search was started in 1965 and was successful. To some degree, the ship had been protected against
circulating seawater (unlike the Wasa, about which
my colleague Willem de Lange wrote in a
conference report).
The remnants of the hull had functioned as a
silt trap and a hard clay layer had formed. As a result of that protective layer,
about 19,000 object were recovered in good shape.
It sounds pretty straightforward, but it is not quite a easy as fishing that toy out of the pond after
your kid drops it in, delighting in the splash it makes.
First, the ship was carefully put upright in a
dry dock at a temperature of 2 to 6 °C
and a relative humidity of 95%.
In 1994, continuous spray treatment of the waterlogged remains with an aqueous solution of polyethylene glycol (PEG)
was started. The PEG takes the place of the water in the wood and prevents cracking of the wood during drying.
Tons of sulfur accumlated in the timber
After the discovery was made that the 17th-century
Swedish warship Wasa had accumulated tons of sulfur in its timbers during
the time it rested on the floor of Stockholm harvor, concern rose for the Mary Rose. This sulfur, namely,
is slowly oxidizing to sulfuric acid, which would eventually affect the stability of the timbers.
Accumulation of reduced sulfur compounds is common in wood preserved in a marine environment.
The almost anoxic conditions in which ship wrecks generally are found slows down normal degradation of wood, but
does produce hydrogen sulfide (H2S). Several factors, including the presence of iron,
influence the penetration of this H2S
into the wood and its reaction into other sulfur compounds.
Samples were taken from the Mary Rose and analysed. The total sulfur and
iron concentrations were determined by X-ray photoelectron spectroscopy,
high-resolution x-ray fluorescence line scans and element analyses. The Mary Rose turns out to contain about two
tons of sulfur (on a total of 280 tons).
Sulfur K-edge XANES was performed at Stanford's Synchotron Radiation Laboratory to determine the sulfur species in situ
and found at least six species, but four reduced species dominated: thiols, disulfides, elemental sulfur and also
some pyrite. Timber that had been treated with the spray showed almost no sulfate, but surface samples and timbers
stored in magazines did.
In addition to samples from the Mary Rose's hull, samples were also taken from a conserved gunshield that had
become impregnated with salt. X-ray powder diffraction found pyrite and traces of mackinawite
(Fe8S9), and also
rozenite (FeIISO4.4H2O), melanterite (FeIISO4.7H2O)
as well as some natrojarosite
(NaFeIII3(SO4)2(OH)6).
Two years later, all reduced sulfur had oxidized, mainly to rozenite.
Stopping the pH decrease
To keep the pH in the Mary Rose's timbers close to neutral, a large amount of sodium hydroxide has been added over
the years and the continuous spray treatment washed away a great deal of acid too.
Iron catalyzing oxidation
Notably iron - and hence the presence of nuts, bolts, nails and other steel or iron objects - plays a role. Divalent iron
competes well for sulfur with organic substances and can later catalyze oxidation, including the resulting
formation of sulfuric acid. Therefore, iron extraction with a strong chelating agent, such as the EDTA
derivative EDMA, may be a further
treatment option.
These findings may also have geochemical implications. The high sulfur concentrations were predominantly found
in lignin-rich parts and more specifically, in the middle lamellae between wood cells. This indicates that the
penetrating sulfur species reacted with active sites in the lignin, resembling early diagenesis of humic matter
and carbohydrates. This may stabilize organic matter in anoxic sediments and could be related to the
sulfur content of fossil fuels.
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