A study of the sulfur problem in Mary Rose timbers using X-ray techniques

Authors: Aaron Berko (University of Kent at Canterbury)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: October 2011
Diamond Proposal Number(s): 1205

Abstract: This thesis reports a study of the sulfur problem which is the production of sulfuric acid via the oxidation of iron sulfide in the timbers of the Mary Rose a flagship of Henry VIII 's navy which sunk in 1535 and was raised in 1982. The work has involved a range of chemical and physical techniques with particular use of synchrotron sources to measure the iron and sulfur speciation in the timbers with X-ray absorption spectroscopy (XAS) measurements. XAS measurements are almost unique in providing the speciation of atoms in a sample via the XANES. They are particularly useful for sulfur, which has a wide range of oxidation states. However, the current work has shown the need to use the bulk and microfocus XAS measurements in parallel, particularly for archaeological samples. It is clear that the iron and sulfur contents and speciation can vary widely from sample to sample of the Mary Rose timbers. In the study on the effectiveness of chelating agents in the removal of iron species from the timbers care was taken, wherever possible, to ensure that the same samples and sample positions were used for the before and after treatment measurements. The nature of the iron and su~ur species is of extreme importance because it is assumed that it is Fe" that gives rise to the production of su~uric acid. The current study has shown that in the timbers that had not been PEG treated contained iron in the surface regions that was predominantly Fe", similar to the findings of other workers. Most of the work in this thesis used samples close to the surface of the timbers. It was only in samples taken deep into the timbers experiments that there were significant concentrations of Fe". A range of sulfur species were found in the samples. The predominant species were reduced sulfur species, elemental sulfur and sulfate. Very little pyrite was found -in the timbers studied, but it should be noted that these timbers had not been PEG treated. Some pyrite was found in the cell walls. The present studies were predominantly on the surface regions of the timbers and the conclusion is that the bulk of the pyrite which may have been present had oxidised in the moist, oxygen containing environment in which they had been stored after recovery from the sea bed. A key finding of the present study is the co-location of iron and sulfate in the timbers. This had been suggested but had not been experimentally verified. The production of sulfuric acid in the timbers is thought to involve the oxidation of iron sulfides in the presence of water to produce is iron sulfate and sulfuric acid. The fact that the present experiments show iron and sulfate in the same positions i'] the XANES maps strongly supports the proposed oxidation mechanism of iron sulfides. The bulk and microfocus XAS experiments show that a large fraction of the iron in the current samples was in the form of an oxide. This is most likely to be goethite (FeO(OH)). The XANES analysis and the fitting of the EXAFS are consistent with this identification. All the four chelating agents used in this work (EDTA, DTPA, ammonium citrate and calcium phytate) were effective in removing iron from the timbers. However, the more efficient are DTPA and calcium phytate in terms of amount removed at fixed molarity. For samples that had been treated with PEG the current work showed that the chelating agents were less effective. This is presumably due to the PEG blocking the penetration of the solutions of the chelating agents into the wood.

Journal Keywords: Mary Rose; Heritage Science; Xas

Subject Areas: Archaeological and Cultural Heritage, Chemistry, Materials

Instruments: I18-Microfocus Spectroscopy

Other Facilities: Yes, Daresbury and Soleil. More than 50% Diamond

Added On: 12/12/2011 09:35

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