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Condition of the wreck
The wreck of the Vrouw Maria is almost complete and appears as very well preserved. However, more detailed interpretation of the condition of the wreck and her preservation in the prevailing conditions and assessment of the factors affecting the possible raising and subsequent conservation can be achieved only through material analyses. Currently there are two wood samples: in 2007 a pinewood sample and in 2009 an oak-sample were raised. In addition to analyzing these, knowledge of the decomposition processes can be gathered by raising and analyzing analogous samples of modern and archaeological wood placed in the site in 2002.
The main causes of deterioration of wood in marine conditions are biological organisms. Physical factors can also be significant and different substances also can and will accumulate in the wood from the environment and from adjoining materials, iron and sulfur species being perhaps the most significant ones. This is more a post-conservation problem, because what happens is that accumulated sulfur and iron compounds, if not removed during conservation, cause chemical reactions which eventually form sulfuric acid on the wood. This endangers long term preservation of conserved wood, as is demonstrated by the reactions reported for instance in the wood of the Vasa in Stockholm, Sweden. This complex issue is certainly a major topic in marine archaeological conservation at the moment.
It is also important to remember that in 18th century vessels the wooden structural parts were bound together by iron bolts and nails or wooden pegs, or a combination of these. Iron readily corrodes in marine environment and often the iron fittings are badly corroded and can make complex structures such as wrecks weaker than they appear.
A sample raised from the wreck in 2007 was thoroughly studied at the University of Helsinki. Light- and scanning electron microscopy were used to study the condition of the cell walls and to search for signs of microbial attacks. Elemental analysis was used to analyze the sulfur and iron content of wood and DNA isolation was used for mapping the microbes. The research still continues at the University of Helsinki as part of the research project “Microbial Degradation of Archaeological Wood”.
The species of the sample turned out to be Scots pine (Pinus sylvestris), not oak, which should be the principal species used for the hull. The sample appeared to be in a good condition with only a thin surface layer (3-5mm) of very soft degraded wood. As was expected, there were no borers’ cavities to be seen and fungi and bacteria were the principal causes of degradation. At the surface layers soft rot fungi had caused most of the degradation, but also some signs of bacteria were noticed there. Deeper in the wood the degradation was less pronounced and mainly caused by erosion bacteria. There were also some signs of fungi spreading deeper in the wood.
According to elemental analyses both sulfur and iron have been accumulated in the sample-wood. This is important information when considering the possible conservation, although the amounts of sulfur and iron are not as significant as in the case of the Vasa.
The sample raised in 2007.
The results and interpretations of the 2007 sample are now being complemented by analyzing a new sample brought up in July 2009. This time the species proved to be oak. Bio-film samples of the freshly raised wood were taken and a piece was cut for the researchers of the University of Helsinki to be analyzed within the research project “Microbial Degradation of Archaeological Wood”. The sample is being studied by light- and scanning electron microscope, elemental analyses are being carried out, microbe mapping is underway and mechanical properties are studied by ultrasound methods. As with the previous sample, the aim is to verify the state of degradation and identify the causes of degradation. The harmful compounds accumulated in the wood are being studied at the University of Helsinki and a sample has been sent to ARC-Nucléart in France, where it is being studied within the ongoing research project “PrévArch” focusing on characterizing and extracting of sulfur compounds from wet archaeological organic materials.
The final results of the analyses will be utilized in designing the possible condition research of the whole hull and in designing the conservation procedure suitable particularly for the Vrouw Maria wood.
The sample raised in 2009.
During the MoSS-project, analogous samples of modern and archaeological wood were placed in the vicinity of the wreck, both in aerobic and anaerobic conditions. Samples brought up so far were studied with light microscopy, scanning electron microscopy and x-ray within intervals of 3 and 12 months. The results indicate similar degradation patterns as has been observed with the samples above. It is to be highlighted that only two sets of samples have been analyzed to date while the majority of them remain in the site and can be used in future studies.
Current situation and future
Although these analyses cannot be seen as characterizing the whole wreck, the analyses reveal that for now the most efficient wood destroyers – white rot, brown rot or wood borers – are not present in the wreck site. The samples with only a few millimeters of badly degraded wood on the surface and signs of bacterial degradation deeper shows typical deterioration for such wood in this particular Baltic Sea environment. Information of great importance is also the data about accumulation of sulfur and iron inside the wood. While the amounts seem to be much smaller than in the Vasa, this is definitely something to take into consideration in the case the wreck or artifacts will ever be conserved
The conditions can be considered as relatively favorable for long term preservation of such a wreck. The components of the hull have only a thin layer of badly degraded wood and they should be structurally strong. However, it has to be noted and stressed that normal decomposition of wood proceeds slowly and the wreck will not survive forever as it is, at least not without any actions.
Further analyses – why and how?
One has to bear in mind that the vessel has been bound together by various iron fastenings. The condition evaluation of the iron parts is so far based mostly on hands-on experiences and common corrosion theory rather than on research of scientific standards. Based on these observations the smaller iron parts such as nails are assumed to be badly corroded and thus significantly weakened. This of course is bound to affect the overall structural condition of the wreck. The iron parts thus form a problem, which is very much connected to the in-situ preservation but also in the possible raising scenarios, because the corroded nails will probably have to be replaced for the raising and subsequent conservation and display. Iron parts are indeed problematic given the lessons learned from the Vasa about the problems of leaving corroding iron in the wood. Study of the iron fasteners and joints is to be one of the objectives of the 2010 fieldwork.
The samples analyzed so far give lots of information but they do not give sufficient answers to the questions related to the possible raising scenario of the wreck. At least questions regarding the condition and strength of the wreck-structure as a whole, the need and type of supports during and after the lift and the conservation challenges need to be clarified in the case a raising decision is made. Sufficient condition research requires lots of interventive sampling, so this should be initiated only after a clear decision to raise the wreck has been obtained.
Extensive condition research can be done by drilling thin samples and analyzing them in laboratory and carrying out mechanical tests. In addition, development of in-situ ultrasound scanning method, which would enable non-invasive research of physical properties of the wood, would be of great use. It is important to perform a comprehensive mapping of sulfur- and iron compounds accumulated in the wood in order to prepare for the conservation challenges. Information of the amount, distribution and speciation of the compounds is needed, as these may affect future conservation treatment protocols. In the worst case scenario, long-term chemical treatments would be required, but alternatively, the compounds may be in a form, which poses less severe problems if the wood is kept in a controlled environment. Preliminary clarification of the iron- and sulfur compounds will be gained from the samples being analyzed now, but these do not give a comprehensive enough picture of the whole of the hull.
Sources and selected literature
Gregory, D 2004. Degradation of wooden shipwrecks: threats. MoSS newsletter 2/2004. p. 4.
Kinnunen, V, 2008. Vrouw Maria hylyltä kesällä 2007 nostetun puunäytteen kunnon ja puun hajottajamikrobien tutkimus sekä puun alkuaineanalyysi. Master’s thesis, Department of Biological and Environmental Sciences, University of Helsinki. 2008.
Palma, P. 2004 Final Report for the Monitoring Theme of the MoSS Project. MoSS project Final Report. 2004. p. 8-37.
Pelanne, M., Tikkanen, S (eds.) 2007. Vrouw Maria – selvitys tutkimuksista, tuloksista ja tulevaisuuden eri vaihtoehdoista. The Maritime Archaeology Unit of the National Board of Antiquities of Finland. Erikoispaino Oy, Helsinki.
Salmi, A. Steffen, K. Eskelinen, J., Peura, M., Montonen, L., and Hæggström, E. 2009. Non-destructive evaluation of the 18th century ship wreck Vrouw Maria. IEEE UFFC2009, Rome, Italy, 23-26.9.