Shipwreck Wood Condition Analysis

Introduction

Shipwrecks are unique time capsules that require special consideration to ensure they are preserved for the next generation. They often look sturdy and strong; however, there are underlying deterioration factors that could be present from being buried in a coastal environment:

Salts: Chlorides from the marine environment are a known problem in the preservation of maritime archaeological materials, particularly for metals. Over time they penetrate the wood and metal components at an electron level and once bonded can be catalysts for other reactions or expand when drying causing materials to burst apart.

Organisms: Microscopic bacteria and fungi tunnel into the wood and structurally weaken the wood cells. While some deterioration is visible at the surface, this can also occur deep inside the wood. Conservators analyse the wood with high-powered microscopes to determine what condition it is in and predict what special needs it may have during drying.

Inorganic Compounds: Iron and sulphur found in the sediments that bury shipwrecks can interact negatively with the wood and cause additional unwanted reactions. This has been demonstrated on the wooden Vasa shipwreck in Sweden. Using elemental mapping, conservators can determine if these inorganic compounds are present deep inside the wood.

Analysis

One method that conservators use to examine materials is with a Scanning Electron Microscope (SEM). These are high-powered microscopes that use an electron beam to excite the material and provide highly detailed surface imagery. Many times they are fitted with an energy dispersive x-ray (EDX), which can provide elemental analysis. For example, you can identify the types of metals present or map the surface of a wood cell to determine if there is sulphur or chlorine present.

These microscopes are highly specialised and are often only found at academic institutions or research centres. We work with the microscope facilities at Victoria University Wellington Te Herenga Waka in the School of Chemical and Physical Sciences.

Conclusion

By identifying the exact deterioration mechanisms of archaeological waterlogged wood, Conservators can better understand what condition the wood is in now and what deterioration may occur in the future. Additionally, analysis of the wood will inform the best treatment methodology.

FAQ’s

How can you determine the condition of the entire vessel based on a few samples?

We can’t! BUT, we do try to sample in representative areas. It is not economically or physically feasible to sample the entire vessel, but the conservators will select areas that are likely to inform the worst-case scenarios or areas needing specific study.

Does the analysis include wood identification?

Not beyond a very basic differentiation of hardwood or softwood and possibly a genus. Wood identification using microscopy is a highly specialised skill and we are not trained in this area beyond basic measures. However, we can coordinate for this analysis to be completed.

What can I expect to see in the report?

The report will detail: where the samples were taken from on the vessel using descriptions, images and drawings; a description of the sampling methodology; the types of analysis performed; the results of the analysis; and, the interpretation of the results from a conservation perspective. Our goal is to provide information that other specialists can use as well. The raw images and data collected from the analysis will also be provided.

How is this different from dendrochronology?

Examining the wood cell structure at high magnification provides an understanding of what condition the wood is in. Dendrochronology examines the growth of a tree and can inform researchers as to the age of the wood when it was felled and possibly the area where the wood was grown. We do not provide dendrochronology advice, nor can we use the same samples, as the preparation and mounting techniques are different.

What do you actually do to perform this analysis?

When we are on site, we will document the vessel efficiently but in enough detail to provide future researchers with information on where samples were taken. This can be useful for monitoring changes or comparing wood samples after treatment. Sampling generally occurs with a forestry borer that screws into the wood and creates a pencil sized diameter hole and collects the core. The core is then carefully transported to the lab where it is sectioned into increments (these depend on how thick the wood is and how much analytical time is available). For example, we may collect a core from a plank that is 5cm thick. We could then examine the wood every 1 cm to determine how far biological decay is occurring and how deeply inorganics have penetrated. This sampling is irreversible; however, if the hole is noticeable, it can be filled for aesthetic purposes. We do try to limit the amount of sampling to only take what is needed to understand the research question. The samples are mounted onto a metal platform that has carbon tape attached (to increase conductivity from the electron beam). When necessary, samples are  coated in carbon to also increase conductivity and improve the imaging. We can only look at a certain number of samples for each platform. The microscope has a vacuum that takes about 30 minutes to pump down. The conservator then examines the sample surfaces and takes .jpeg or .tiff images and completes elemental analysis. The results are then compiled into a report. The samples are generally kept as an archive for future analysis by the client.

Isn’t there a simpler way of determining the condition of the wood?

Wood from a maritime context can become waterlogged or affected by biodeterioration (i.e. bacteria and fungi) over time. The surface of the wood can be in better or worse condition than the interior depending on the deterioration factor. So what may look like really stable wood, could be weakened. There are subjective tests, such as the ‘pin test’ or Pilodyn technique, which inserts a pin into the wood to see how easily it penetrates. These are great tools for the conservator to use, but the depth of penetration can vary based on the species of wood and location of the sampling. They are subjective tests and can be interpreted broadly. Not only do we want to see HOW decayed the wood is, we want to know WHAT the mechanism behind the decay is which is why we use scanning electron microscopy.

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