Scientists identify the chemicals causing the hull of Henry VIII’s warship the Mary Rose to degrade

Scientists have identified the chemical compound causing the hull of the Mary Rose, Henry VIII’s famous warship, to degrade — offering hope for its preservation.

The Mary Rose was salvaged in 1982, following some four centuries on the seabed of the Solent where she sank on July 19, 1545 in a battle against the French fleet.

Recovery of the carrack-type warship — dubbed ‘England’s Pompeii’ by historian David Starkey — has provided valuable insights into Tudor society.

Artefacts recovered from the wreck have included weapons, sailing equipment, naval supplies and varies possessions and items used by the crew.

However, the wreck’s removal from the water has allowed the formation of acidic, sulphur-based compounds that are breaking down the wood of the hull. 

To better understand this process, experts analysed wood samples taken from the vessel at different stages of its conservation.

The team found that damaging sulphur compounds can be found up to 0.47 inches into the hull’s timber — especially in the presence of iron and zinc deposits.

The findings of the study — the first ever to monitor this kind of degradation over time — could help guide future efforts to stop the acidic compounds from forming.

The results follow the staving off of a different threat to the vessel last month, when Historic England gave £25,000 to maintain conservation efforts amid COVID-19.

The Mary Rose Museum in Portsmouth was forced to close its doors on March 18 due to lockdown — with the future of the vessel beyond this year remaining uncertain.

According to historians, the Mary Rose sank north of the Isle of Wight while engaging an invasion force of French galleys. 

The vessel — which had served for 33 years — had been extensively modified over time, and it is believe that she had gained so much additional weight in the form of cannons that the warship had become unstable. 

Executing a turn — and reportedly caught in a gust of wind — the ship tilted such that water was able to pour into her gunports, sinking her, after which the warship became buried in silt on the seafloor that preserved around 40 per cent of the hull.

Having sunk and been buried, the wreck was subject to colonisation by anaerobic bacteria which release a compound called hydrogen sulphide. 

While harmless enough in the oxygen-free environment beneath the sea bed, raising the vessel has allowed this hydrogen sulphide to react with the oxygen in the atmosphere to form corrosive sulphuric acid and sulphate salts.

This issue is made worse by the fact that the Mary Rose contains a considerable amount of iron — in the form of nails, bolts, and cannons  — which catalyses the acid production.

Conservators have been considering spraying the vessel’s hull with tiny particles of strontium carbonate — a treatment which would trap the sulphur in the wood to form non-reactive compounds that would not damage the wreck further.

Any such treatment, however, requires a detailed understanding of the chemical processes taking place within the Mary Rose’s fragile timbers. 

Following the Mary Rose’s salvage on October 11, 1982, the remains of the warship’s hull was treated with various sprays intended to help ensure her preservation.

Initially, cold water was used to limited biological activity and prevent the previously-submerged wood from drying out,

Had this been allowed, the hull material would have shrunk by around 20–50 per cent while undergoing severe cracking and warping as water evaporated from the cellular structure of the previously waterlogged wood.

Beginning in 1994, conservators began treating the Mary Rose with varying molecular weights of polyethylene glycol (PEG) — initially to replace the water in the cellular structure of the wood, and subsequently to seal the timber’s surface.

Only in 2013 — more than three decades after she was raised from the seafloor — was the vessel allowed to begin to dry out in a specially controlled environment, removing residual water while minimising resulting shrinkage and warping.

In their newly-published study, Mary Rose Trust head of conservation Ellie Schofield and colleagues took core samples from six locations across the warship’s hull for analysis — both during the PEG treatment and again during the drying phase.

According to paper author Giannantonio Cibin of the Diamond Light Source, the team used a particle accelerator known as a synchrotron to perform various analyses on the hull samples taken from the Mary Rose during the conservation process.

For example, a technique known as ‘X-ray absorption near-edge spectroscopy’ allowed the team to determine the exact nature of the sulphur compounds attacking the vessel, and ‘X-ray fluorescence imaging’ to determine where these are located.

A third technique — ‘Fourier transform infrared spectroscopy’ — enabled the researchers to assess how degraded the wood itself had become.

‘This allowed the quantitative study of these complex systems and has shed vital light on the evolution of complex sulphur-based compounds lodged in Mary Rose timbers as a function of drying time,’ Dr Cibin added.

The researchers found that the damaging, oxidised sulphur can be found up to 0.47 inches (12 millimetres) into the dried timber that makes up the hull of the Mary Rose. 

Analysis also revealed that the areas exhibiting the most degradation of the timber were found where there were also iron and zinc deposits — highlighting their potential roles in the breakdown of the wood’s internal structure.

‘We now have a uniquely detailed picture of wood deterioration,’ said paper author and Mary Rose Trust head of conservation & collections care Ellie Schofield.

‘This study is the first time we have been able to show the correlation between metal deposits, sulphur speciation and degradation of Mary Rose wood samples as a function of drying time and depth into the wood.’

The full findings of the study were published in the journal ChemPlusChem.