Forty years after having been recovered from the depths of the Solent, more than 3,000 Tudor bricks that once formed part of the galley ovens on Henry VIII’s warship the Mary Rose are now at risk of deterioration, due to elements within them reacting with oxygen in the air to produce physical and chemical conditions capable of causing irreversible damage.
The Tudor vessel sank during the Battle of the Solent in 1545, and its surviving timbers and contents have been undergoing conservation since the wreck was raised in 1982 (see CA 272). Excavated remains are now housed at the purpose-built Mary Rose Museum in Portsmouth Historic Dockyard. There, visitors can see a large portion of the ship’s hull, as well as some of the 19,000 artefacts – and a reconstructed galley oven.
Most of the oven bricks recovered from the seabed during the 1980s excavations were put into storage, having been rinsed to remove seawater and carefully dried. ‘These bricks were thought to be stable and inert. However, salt crystals started to become visible on the surface of the bricks,’ said Professor Eleanor Schofield, Deputy CEO for the Mary Rose Trust. ‘These crystals were a worrying sign, and indicative of potentially serious and continuous damage that was being caused deep within these precious historical artefacts.’
Now, working with colleagues from Landesmuseum Hannover, the University of Kent, and Diamond Light Source (the UK’s national synchrotron – a type of particle accelerator), Eleanor has co-authored a new paper examining the elemental composition of the bricks and the contaminants affecting them.
In order to make sense of the crystallisation process, which has the potential to break the bricks apart over time, the researchers needed to find out what compounds were present in the artefacts without compromising their structural integrity. The team there-fore used a variety of light microscopy, electron microscopy, and X-ray techniques at the Diamond Light Source facility in Oxfordshire (which can produce sharper and higher-resolution results than those typically achievable in traditional lab-based settings) to examine samples taken from 20 bricks with visible salt forma- tions. ‘The key to preserving these important artefacts comes from understanding the chemical and physical processes that are occurring inside the bricks,’ said co-author Dr Donna Arnold from the University of Kent’s School of Physical Sciences. ‘It was essential to use non-destructive techniques that do not damage the bricks but reveal subtle internal processes that can help us preserve them for centuries to come.’
X-ray diffraction (XRD) analysis showed that, in addition to silicon oxide (the main component of the bricks), there were iron and calcium salts present in the samples. Scanning electron micros-copy (SEM) and electron-dispersive X-ray spectroscopy (EDS) analysis of the salt crystals then revealed two crystal morpholo- gies with different compositions, which could affect how the salts grow within the bricks: long, thin needles (containing calcium sulphate) and a more compact morphology (containing iron sulphate). Testing pH, meanwhile, revealed that, as the salts dissolved, they created an acidic environment that could eventually damage the bricks.
Further XRD analysis indicated several key phases of salt crystallisation, a fuller understanding of which will be fundamental to the development of tailored treatment, storage, and display methods capable of protecting the bricks. ‘This is the first step to developing strategies to prevent any further damage to these precious historical artefacts,’ Donna said. ‘Further work still needs to be done to understand more fully the salt formation, including a more detailed analysis of the various phases present. However, we now have the knowledge that is needed to begin to devise strategies to counteract the damage from salts.’
Interestingly, Donna observed, no sodium or chlorine (components of sea salt) was detected in the bricks, despite them having spent several hundred years submerged in the Solent. Donna put this down to the efforts of early conservators, who successfully eradicated sea salt from the bricks.
The research was published in the Journal of Cultural Heritage (https://doi.org/10.1016/j.culher.2022.02.013). For more about the history of the Mary Rose and for information on visiting the museum, see https://maryrose.org and CA 280.