The structure of wood

The parallel cellulose polymers (right corner) are held together by cross-linking hemicellulose and some lignin to form fibrils, which provide mechanical strength and stability to the wood. The cell wall is a layered structure around the lumen, the empty space where the protoplasm has been located (see below), with a middle lamella (ML) and primary wall (P) in the outer part. The secondary wall is mainly for support and comprises primarily cellulose and lignin. Often one can distinguish three distinct layers, S₁, S₂ and S₃, which differ in the orientation of the cellulose fibrils. The cell wall is built up in a similar way as a tire that has a series of steel cords embedded in an amorphous matrix of rubber. In the plant cell wall, the "cords" are analogous to the cellulose fibrils and they provide the structural strength of the wall. The rubber in the tire corresponds to non-cellulosic cell wall components, see: http://employees.csbsju.edu/ssaupe/biol327/Lecture/cell-wall.htm; http://sunflower.bio.indiana.edu/~rhangart/courses/b373/lecturenotes/cellwall/cellwall.html.

The scanning electron microscope (SEM) pictures (below) show cell walls around large lumen in marine archaeological oakwood with somewhat degraded S₂ layers. The arrow indicates the lignin-rich middle lamella (ML) holding the cell walls together. In the lower picture a pyrite (FeS₂) particle is visible.

Acid in wood

The cellulose polymer consists of linked glucose rings, usually about 10 000 subunits. An oxygen atom between the carbon atoms 1 and 4 joins the rings. These so-called glycoside linkages are easily broken (hydrolysed) in an acid environment, while fairly stable under neutral and alkaline conditions. The catalytic degradation is initiated by a hydronium ion (H₃O⁺), binding to the bridging oxygen atom. This facilitates breaking of the bond between the oxygen and carbon atom 1. When a water molecule then binds to carbon atom 1, a hydronium ion is regenerated and can initiate a new hydrolysis reaction. The length of the cellulose chain will gradually decrease. Finally only small fragments of crystalline cellulose of about 200 glucose units will remain, and the tensile strength of the wood will have completely disappeared (Johansson 2000).

How fast and far the Vasa's wood deteriorates in the museum environment has yet to be investigated, however. The rate depends not only on pH but also on temperature, humidity, presence of iron compounds and possibly other factors. The presently accumulated amount of sulfuric acid in the wood is estimated to about 2 tonnes, and additionally 5-6 tonnes may form if the remaining sulfur will become fully oxidized. The acid is forming continuously, and higher acidity will increase the rate of cellulose degradation in the wood. The bicarbonate/soda treatment of the Vasa's wood surfaces has only a temporary and insufficient effect, and it is important to develop and apply better methods. The large amount of sulfur in the hidden surfaces under planking and dunnage is a difficult problem for efficient treatment.

Further research and treatment

Our discoveries about accumulation of reduced sulfur compounds in waterlogged wood showed the need for further insight in the requirements for lasting conservation treatments. However, experience shows that the long-term consequences of new conservation procedures must be carefully investigated to avoid future problems. In December 2002 the National Maritime Museums (NMM) invited applications from scientists interested in taking part in a research project on the Vasa and sulfur. The sum of eight million Swedish kronor spread over four years was put up by five funding bodies: the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, the National Heritage Board, the Bank of Sweden Tercentenary Foundation, the Swedish Foundation for Strategic Research, and the Swedish Agency for Innovation Systems. Five research teams were selected in October 2003 to work on a project to find a long-term solution to the Vasa’s sulfur problems. Professor Emeritus Lars Ivar Elding of the University of Lund coordinates scientific and economic aspects of the NMM based project. Eight million Swedish kronor will be spent over four years on wide-ranging scientific studies aimed at halting the breakdown of the ship’s timber, see: http://www.vasamuseet.se/Vasamuseet/Verksamhet%20och%20Projekt/Bred%20forskning.aspx?lang=en.

The continued work is directed toward developing scientifically based treatment methods for the Vasa, which also will be of interest for other marine archaeological artefacts.  Nowadays, there can be no doubt that the Vasa's significance historically, culturally, and also economically, is so great that all necessary efforts should be made. An interesting comment regarding the exorbitant costs to maintain and conserve artefacts and materials recovered from an underwater environment, was made in the Guidelines to the American Abandoned Shipwreck Act i US law from 1988, see : "The best example is in Sweden, where sufficient public and private funds were made available to document, raise, maintain, conserve, interpret, and exhibit the intact 17th century Swedish warship Vasa. Revenues generated annually into the Swedish economy by tourists visiting the Vasa are said to be $275 million"!

 The following questions are being investigated within the Save the Vasa project:

• what are the factors/substances that accelerate the oxidation and decomposition processes and how can these be removed or be made inactive?
• what methods can be developed to remove the acid formed and to prevent/delay continued acid formation?
• how rapidly and in what way do acid and iron compounds cause decomposition of the wood in the current circumstances?
• how can the iron compounds in the wood of the Vasa be removed or deactivated? This includes finding a suitable inert material to replace existing bolts where this is possible.
• how can the stability and decomposition of polyethylene glycol be characterised and what is the possibility of reinforcing the conservation protection? This includes investigating the interplay between wood, iron and PEG under the conditions that prevail in the Vasa and identifying the decomposition products of the polyethylene glycol.

Other important tasks connected to the preservation of the Vasa is the installation of a more efficient climate control system for the exhibition hall of the Vasa Museum, which was completed in 2004, and the construction of a special cradle that supports and distributes the Vasa's weight better. A better cradle can prevent the hull from subsiding even more, and can also facilitate the exchange of the bolts and other conservation treatments involving partial dismantling. An advanced laser positioning system, designed by the Department of Geodesy and Photogrammetry, KTH, is already installed and shows the tiniest movements in the hull of the Vasa. The system is very valuable for monitoring movements when changing bolts and dismantling parts of the hull, and will reveal subsiding if the mechanical strength of the wood gradually decreases. The coordination between the research work, the development and testing of the devised conservation procedures, and their actual application to the Vasa, will be very important and will certainly extend over a long period of time.

Final comments

Because of our scientific collaboration and the fortunate availability of new analytical techniques, the causes of the problems were discovered at an early stage, which provided time to develop cures and take appropriate actions. The robust construction of the Vasa, with massive timbers in carrying parts, ensures that the danger is not immediate. Despite the large weight of the hull, no alarming structural damage is yet apparent. Nevertheless, the sooner an appropriate treatment of the Vasa can begin, the better. The work will probably be a constant struggle against the ravages of time and acid, but the Vasa is well worth all efforts. The Vasa may be under acid attack, but she will prevail in her first real battle!