Excavating the Rooswijk … virtually!

The next blog in our British Science Week 2020 series come from MSDS Marine, a Marine and Coastal Contractor specialising in the management, execution and support of archaeological projects in the marine environment. 

The Rooswijk was a Dutch East India Company vessel which sank on the treacherous Goodwin Sands, off Kent, in January 1740. The ship was outward-bound for Batavia (modern-day Jakarta) with trade goods. The site is now protected by the Protection of Wrecks Act 1973. The ship’s remains are owned by the Dutch Government; however, the UK government is responsible for managing shipwrecks in British waters, therefore both countries work closely together to manage and protect the wreck site.

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Figure 1. Clockwise from top left: Multibeam image showing the main area of wreckage on the Rooswijk, A diver excavating in 2018. Lead project Conservator Angela Middleton examining a concreted chest from the side. A screenshot of the Rooswijk virtual trail.

A two-year archaeological excavation project was undertaken between 2017 and 2018 due to the site being at high risk of loss through environmental changes and unauthorised diving. Wrecks such as the Rooswijk are part of the shared cultural maritime heritage across Europe and it’s important that cultural heritage agencies are able to work together to ensure that sites like this are protected, researched, understood and appreciated by all. The project involves an international team led by The Cultural Heritage Agency of the Netherlands (RCE) in partnership with Historic England. MSDS Marine are the UK Project Managers for the project.

In 2019 MSDS Marine, working with ArtasMedia, created a virtual tour of the site: https://msdsmarine.com/projects/dive-trails/rooswijk-virtual-trail/. Now the projects archaeologists are working with the μ-VIS X-ray Imaging Centre at the University of Southampton to further excavate the site virtually!

A number of stacks of coins were found during the excavation. Some of these were carefully separated by the conservators from the Investigative Science Team at Historic England (Figure 2). Some could not be separated.

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Figure 2. An MSDS Marine conservator separating coins from the Rooswijk in the Historic England laboratory.

A number of stacks were then sent to the μ-VIS X-ray Imaging Centre (www.muvis.org) at the University of Southampton to be micro-CT scanned. X-ray micro-Computed Tomography (µ-CT) scanning is a volumetric scanning technique, which enables us to virtually cut open materials to look inside with micrometre spatial resolution, while preserving the condition of the object we are scanning. During the scan, the object is rotated 360 degrees as thousands of 2D X-ray projection images are acquired. These 2D images are then reconstructed into a three-dimensional volume, which is made up of cubic pixels with intensities related to the amount of x-ray energy absorbed at that point.

We used the custom walk-in scanner (the Hutch) at the µ-VIS X-ray Imaging Centre to scan the concreted coins, which were stacked in sealed tubes to prevent excessive drying during the scanning process (Figure 3).

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Figure 3. Concreted coins mounted for µ-CT scanning within the custom Nikon/X-tek 450/225 kVp Hutch at the µ-VIS X-ray Imaging Centre, University of Southampton

The digital reconstructed volumes were then sent to MSDS Marine, where myVGL software (Volume Graphics GmbH, Germany) was used to manipulate the volume data, so that the individual faces inside the stacks could be seen (Figure 4). These coin faces have not been seen since they were packed into chests for the voyage almost 280 years ago.

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Figure 4. A Rider coin from 1739 that has been virtually separated from a large coin stack.

The coin face slice images will be sent to Jan Pelsdonk, the projects numismatist, for identification and will contribute to the understanding of the wreck.

The application of scientific techniques like CT scanning and digital model processing have contributed hugely to the understanding of underwater heritage, and continue to offer new and exciting ways of investigating these important cultural sites.

Phoebe Ronn, MSDS Marine Phoebe@MSDSMarine.co.uk

www.MSDSMarine.co.uk

Katy Rankin, µ-VIS X-ray Imaging Centre, University of Southampton, k.rankin@soton.ac.uk

www.southampton.ac.uk/muvis