As a consequence, the presence of high concentrations of iron sulfides in
dry conserved archaeological wood results in structural damage and blooms
of sulfate crystals develop on the object surface. In addition, the sulfuric acid
produced inside ship timbers and artefacts starts to hydrolyse the wood. An
analysis of Mary Rosetimbers has shown high and variable iron concentra-
tions, up to about 8% in the surface layers. Also, iron ions catalyse oxidative
degradation of cellulose in a chain reaction, involving free radicals and molecu-
lar oxygen, and the wood loses much of its strength.
To ascertain the composition of iron/sulfur-containing particles in archaeo-
logical wood, the pre-edge features of iron and sulfur K-edge X-ray absorp-
tion near-edge spectroscopy (XANES) and X-ray diffraction has provided
valuable information on the iron compounds and its oxidative state. Iron com-
pounds commonly encountered in archaeological wood are goethite (FeOOH),
magnetite (Fe 3 O 4 ), molysite (FeCl 3 ), pyrrhotite (FeS), pyrite (FeS 2 ), macki-
nawite (Fe 9 S 8 ), rozenite (FeIISO 4 4H 2 0), melanterite (FeIISO 4 7H 2 0), and natro-
jarosite (FeIII 3 (SO 4 ) 2 (OH) 6.
It is imperative that iron compounds are removed from waterlogged archaeo-
logical wood. In order to dissolve iron compounds from archaeological wood,
special complexing agents (chelates) are used to form strong and also soluble
complexes with iron(III) ions. Ethylenediiminobis(2–hydroxy–4–methyl–
phenyl)acetic acid (EDMA) is a chelating agent used by scientists at the
Mary RoseTrust.
In conjunction with XANES and X-ray diffraction, initial investigations
involving synchrotron based X-ray microspectrometry (at ESRF) has revealed
two types of reduced sulfur compounds in the timbers of the Mary Rose.
Organosulfur compounds are found in the lignin-rich middle lamella between
wood cell walls, mostly as thiols, disulfides and elemental sulfur, while inor-
ganic iron sulfides including iron pyrite, occurs in separate particles in the cell
lumen.
Based upon these finding, new methods are now being developed to con-
trol the acid-forming oxidation processes, by removing reactive iron sulfides
and stabilising the organo-sulfur compounds. Current ideas indicate that after
removing the reactive iron sulfides, the remaining organo-sulfur compounds
could be prevented from forming acid by the addition of antioxidants to con-
servation treatments, enabling long-term stable conservation of marine archaeo-
logical wooden objects.
By using sulfur spectroscopy, synchrotron-based XANES and X-ray micro-
spectrometry it has been revealed that the acid problem originated from
microbial activity within the marine sediments (Figure 18).
SRBs are known inhabitants of marine sediments. These bacteria convert
sulfates into sulfides under anaerobic conditions. Sulfides in the form of
hydrogen sulfide penetrate the wood and react to form two categories of sulfur
300 Chapter 11