A shot at conservation: Using the latest technology to save the Mary Rose’s cannonballs

7 mins read
Just some of the 1,248 iron cannonballs that have been recovered from Henry VIII’s flagship, the Mary Rose. (ALL IMAGES: The Mary Rose Trust, unless otherwise stated)
The arsenal of iron shot that was carried aboard Henry VIII’s flagship, the Mary Rose, may have once struck fear into the hearts of those manning the 16th-century French fleet, but today they are somewhat more delicate. Without the right conservation, these cannonballs will slowly crumble to pieces. Eleanor Schofield explains how she aims to stop the rot.

When dredgers finished making way for Britain’s new aircraft carriers, HMS Queen Elizabeth and HMS Prince of Wales, in Portsmouth Harbour last year, they had raised 3.2 million cubic metres of mud. In it, they also raised an archaeological treasure trove from the depths, reflecting Portsmouth’s long maritime history. Ranging from a German Junkers Jumo 211 engine and a British torpedo to shoes and a skull, some of these artefacts were sent to storage in the building where the greatest, most complex, and most expensive excavated object in the history of maritime archaeology – the Mary Rose – now resides.

Housed at Portsmouth Historic Dockyard, the Mary Rose is today much more than the remains of a Tudor flagship. It is a groundbreaking conservation project, aimed at protecting the surviving portion of the hull and the thousands of items that were salvaged alongside it. So it was natural that the Defence Infrastructure Organisation, which was responsible for the newly dredged items, would consult the Mary Rose conservators on how to stop these artefacts from deteriorating. Unfortunately, and perhaps surprisingly, the answer is that we simply do not know how to do it – conserving marine archaeological artefacts is a challenging endeavour, and one that continues to stretch the abilities of even seasoned experts.

The Tudor wreck was rediscovered in 1971 and raised to the surface in 1982.

Sunk while leading an attack on a French invasion fleet in 1545, under circumstances that remain enigmatic, the Mary Rose survived in Solent seawater for over 400 years because she was slowly covered with layers of silt on the seafloor through which damaging oxygen could not penetrate. The wreck was rediscovered in 1971 and raised in 1982 (see CA 272), but conserving the ship’s remains has been an onerous task. It first involved constant spraying with fresh water, and then, from 1994 to 2013, dousing the timbers with polyethylene glycol (PEG) to support them from the inside. Since that process was completed, dehumidifiers have sucked a vast amount of water from the hull – and now, with the ship almost completely dry and after a multimillion-pound revamp of the Mary Rose Museum in 2016 (see CA 280), visitors can enjoy a full, uninterrupted view of the only 16th-century warship on display in the world.

A more intimate story of the vessel and her crew is now told by the 19,000 items that have been recovered from around the wreck’s resting place, including personal and everyday items. Wooden tankards, cowhorn inkpots and a gold ring are among the objects illuminating life aboard the Mary Rose. Around half of the artefacts, though – some 10,530 – are related to blasting other ships out of the water, from beautiful bronze guns to incendiary arrows. Among the most numerous of these objects are the cannonballs, known as ‘shot’. Made of diverse material such as stone, lead, and composites, but most often iron, 1,932 of them have been excavated from the wreck site.

Today, the remains of the Mary Rose are on display at Portsmouth Historic Dockyard.

The iron missiles, numbering 1,248, represent some of the earliest examples of mass-produced cast iron in Britain. They seem to belong to a purpose-made batch, with each made using a similar method at roughly the same time, and cast into various sizes of the same spherical shape. They were innovative items, dating to shortly after the introduction of the blast furnace to England, and this arsenal not only offers a unique insight into a key period of technological development within these shores, but also provides a huge dataset with which we can study the changes that occur over time to marine archaeological iron – vital clues for conservation work.

Unfortunately, seawater and iron are not happy bedfellows: this combination causes corrosion that eats away at the metal and weakens its structure. Some of the most common marine iron-corrosion products include goethite, magnetite, and lepidocrocite, all very damaging, especially when combined with akageneite (which is rapidly formed when chlorine in the seawater salt is exposed to any humidity); all combine to mount a brutal attack on specimens of shot. Worse, sometimes damage is easy to miss, since corrosion can happen from the inside out. It may be only when the corrosion products crack and burst out of the original object that conservators become aware of it.

Iron shot in passive storage.

In an attempt to combat these effects, all the shot that ranged from eggto football-sized was originally stored at the Mary Rose Trust in a passivating solution to reduce the chemical reactivity of their surface – but while this slowed deterioration, it did not stop it. Since then, the Mary Rose team has tried a range of other ways to keep the aged ordnance in good condition. In the early 1980s, they experimented with hydrogen reduction, a method that removes all the problematic chlorine in the iron by heating and flowing hydrogen gas over it to form hydrogen chloride. Although this technique did completely remove the chlorine, heating the iron fundamentally changes its microstructure, and so it goes against modern conservation ethics of making treatments reversible.

With hydrogen reduction falling out of favour, conservators trialled two other strategies involving corrosion inhibitors – but neither was found to remove all the chlorine. Furthermore, unless an object is stored at exceptionally low relative humidity, which is impossible to achieve when it is on public display, deterioration will continue. To make matters worse, the team had no way to tell how effective these treatments were, because there is no technique that can probe far enough into the metal to see how the composition and structure of the shot has been affected.


This presented a problem. How do you figure out how to conserve these iron shot when you have no way of looking inside to see how effective the different techniques are? The obvious answer was to break into them – known as ‘destructive testing’. But this is exactly why hydrogen passivation was rejected: irreversibly changing an artefact, especially breaking it apart, flies in the face of conservation ethics. With many shot disintegrating in front of the conservation team’s eyes, though, there was no other option. After consulting with other collections and Mary Rose curators, the difficult decision was made last June to cut a segment from 12 shot – just 1% of the entire collection – in order to save the rest.

Making sure to include examples covering each of the four conservation strategies – passivating solution, hydrogen reduction, and the two different types of soaking – samples have been taken from shot at various stages of decay in a research collaboration between the Mary Rose Trust, Diamond, and UCL. This ran the full gamut of preservation, from objects where a strategy has worked really well, to ones where it really hasn’t worked at all – one cannonball was literally in pieces in a bag.

New methods are being trialled to halt corrosion and help conserve the Mary Rose shot. These photographs show a corroded iron cannonball from the wreck

These samples were then taken to Diamond Light Source, the UK’s national synchrotron science facility. Measuring half a kilometre in circumference and producing light up to 10 billion times brighter than the sun, Diamond is designed to produce very intense beams of X-ray, infrared, and ultraviolet light, which is channelled to laboratories called ‘beamlines’, where it is used to examine the smallest objects (such as molecules and atoms), objects that visible light is simply incapable of seeing. It is these small scales that the Mary Rose team needs to probe.

After taking visual observations of the segments of shot, assessing their colour and how easy they are to cut, Diamond is now being used to map the different elements that they contain – hopefully revealing where iron, chlorine, sulphur, and various different compounds are present. This will give us detailed knowledge of the type, depth, and spread of corrosion in each case, giving unprecedented insight into how effective the marine archaeological conservation treatments have been.

To complement this research, experiments are also under way in which a piece of modern iron and some corrosion products are being compared to archaeological iron by submerging them in different solutions. Over the course of six months to a year, the team will use Diamond to investigate these samples once a week to see exactly how corrosion takes hold, simulating what happened to the materials and what compounds are formed along the way. With little knowledge of how the Mary Rose shot came to be in the state they are in today, these experiments mimic the ageing process, hopefully revealing even more information on how best to conserve these precious artefacts long into the future.

Detail of a corrosion layer

Although controversial, destructive testing in this unique case seems justified. The experiments have already uncovered so much useful information that has never been known before, from the surprisingly high metal content in corroded shot, to patterns where sulphur and chlorine gather within the missile’s structure. Furthermore, 99% of the collection will remain untouched, and the shot that is used will not be completely destroyed by the experiments. Indeed, the artefacts that are featured in the study could still be displayed to help tell the ongoing Mary Rose story, explaining why and what the conservation team is doing to look after this special collection. Looking beyond the museum, the results of this work will be published and made available for everyone around the world who is struggling to conserve marine archaeological iron. Of course, some may never be convinced that this level of deliberate damage to historic objects is warranted, but it could be argued that watching these important artefacts slowly corrode to dust, or hiding them away from the public in specially adapted chambers, defeats the object of conservation. We need to use all the tools at our disposal to find ways of conserving and displaying artefacts in perpetuity. After all, what is the point of conservation if people cannot see and enjoy these wonderful objects from our past?

This feature appeared in CA 337.

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