On the cleaning of metal finds from the London and the Rooswijk

March 12, 2018March 24, 2018 ~ Serena ~ 1 Comment

I am pleased to introduce this week as my guest blogger Elisabeth Kuiper, who has just completed an internship with Historic England. She tells us about her recent experiences in the conservation of metal artefacts from two designated shipwrecks:

Most historic ships are full of iron: think of nails and bolts in all sizes, ship equipment, rigging elements, chains, anchors, iron cannons and all sorts of different tools used on the ship. This iron, in the unfortunate event of ending up on the seabed, usually grows very bulky corrosion products eventually covering the original surface of an object. Iron objects from a maritime archaeological context are thus very often found as mysterious and unrecognisable lumps, known as concretion, as they have become covered by a thick formless mass of corrosion which can incorporate sediment and shells, and also different objects in the vicinity. In order to understand the concretion and what is, or used to be, inside it, the conservator uses X-radiography. X-radiography gives the opportunity to investigate the concretion without damaging it: dense areas or voids will show up on the image and so may be able to tell what has caused the concretion.

Once it is clear that the concretion may hold something worth investigating further, the conservator will start off mechanically cleaning it. Corrosion products are taken off layer by layer until the original surface of the object is found. In the process of cleaning other artefacts which may not have been seen previously on the x-radiograph, can be found trapped in the corrosion layers, for example, pieces of glass, ceramics or small metal objects.

Unlike any other metal, in an advanced stage in the corrosion process the iron of the original object can have migrated entirely to its corrosion layers, and we are left with a void that retains the shape of the object precisely. If needed, these voids can be filled with a silicone rubber or casting resin. Once all concretion is removed the conservator is left with a perfect cast of the object that would otherwise be lost forever.

I am a Professional Doctorate student in conservation and restoration at the University of Amsterdam, specialising in metal conservation, and have been working at Historic England on an archaeological conservation work placement for the past months. My main focus during my time at Historic England was the remedial and investigative conservation of finds from the protected wreck sites of the Rooswijk and the London. The London was a Royal Navy warship that went down in the 1665 after an accidental explosion aboard the ship, and many different objects were recovered during the salvage operations between 2014 and 2016.

The Rooswijk was a Dutch East Indiaman that ran aground on the Goodwin Sands, off Kent, in the winter of 1740. The shipwreck was partly excavated and recorded in the summer of 2017, after which the finds were taken to Historic England storage facilities for assessment, analysis and conservation.

During my time at Historic England I have worked on quite a diverse range of finds from both wreck sites, but what they all had in common was the various amounts of iron corrosion on the object’s surface. As previously mentioned, this is quite typical for maritime archaeological artefacts, which (as we will see) can even be totally enveloped in iron corrosion. A few of the more straightforward objects I have worked on were from the London:

Hammer laid vertically, showing concretion at the head, with scale marker and label to left. x-ray of hammer, laid diagonally with head at top left, concretion showing up as white around the head. Hammer laid vertically showing head with concretion removed, scale rule and label to left

Figure 1. Different stages of conservation process on hammer from the London: before treatment (left), x-radiograph (middle) and after cleaning treatment (right) © Historic England

In Figure 1 above we see a hammer, with iron corrosion products covering the original surface. The hammer was cleaned using a pneumatic tool called an air-scribe, which can be seen as a small jackhammer. It is ideal for removing concrete-like iron corrosion products, with the x-radiograph was used as a guide during the cleaning. When looked at carefully, the x-radiograph clearly shows the typical lamellar structure of corroded wrought iron. Wrought iron is essentially pure iron containing less than 0.2% carbon by weight. The main compositional variation is in the presence of slag inclusions. When worked these slag inclusions are forced out in the direction of working. On the seabed not only does the metal surface corrode, but also the walls of the slag inclusions, as seawater is able to penetrate deep into the metal. As a result the metal shows a wood grain-like appearance, typical of wrought iron recovered from shipwrecks.

The same became clearly visible as corrosion was cleaned away on a rigging element called a deadeye:

Deadeye with rust-coloured concreted surface, label and scale rule to left Deadeye following removal of concretion, showing its shape and dark colour more clearly. Scale rule below the object.

Figure 2. Deadeye from the London, before (left) and and after (right) cleaning © Historic England

Up to now I have discussed corroded iron objects. Surprisingly, it’s not only objects made from iron that can become covered by a thick iron concretion crust. As we will see in the next images, copper alloy objects can also become unrecognisably changed due to maritime corrosion processes:

Pan body showing rust-coloured concretion, particularly around the edges, with scale rule and yellow archaeological tag below. Pan after cleaning, with concretion removed, showing a darker metal colour and some discolouration. Scale rule and yellow tag below.

Figure 3. Copper alloy object from the Rooswijk, before (left) and after (right) cleaning © Historic England

The artefact shown here is a copper-zinc alloy object, presumably a pan of some sort. Probably it will have had a handle that was riveted to the pan itself. These rivets were already visible on the x-radiograph, but were uncovered during cleaning.

Detail of 4 rivets on rim of pan, with centimetre scale

Figure 4. Detail of rivets on rim of pan after treatment (above) with corresponding features visible on x-radiograph prior to treatment (below) © Historic England

x ray of whole pan, with 4 rivets showing up as small white round features at top

Cleaning of maritime archaeological finds can be rewarding and satisfying work, in the sense that the disfiguring corrosion layers are slowly removed to reveal a recognisable object once more. Sometimes these objects can even be in quite a good condition. The cleaning of concreted artefacts can almost be seen as a mini-excavation. To illustrate this, I will show one last treatment on a concretion, which furthermore posed quite a challenge:

Irregular lump of concretion with shells and other material embedded, scale rule and yellow number tag at bottom

Figure 5. Concretion from the Rooswijk before cleaning treatment © Historic England

Fig 6

Figure 6. X-radiograph of concretion in Fig 5 before cleaning treatment, where rings, a coin and many beads (lighter areas) as well as different sizes of nails (darker areas) become apparent © Historic England

In this case, cleaning of the concretion was more of a challenge because of the mixture of elements and materials in it. The concretion consists of approximately 17 copper alloy rings, 1 silver coin and over 400 tiny glass beads. What was left of the iron (mostly nails and/or small bars), as explained earlier, were just voids. The concretion itself proved to be a harder material than the glass beads, which tended to shatter when the air-scribe came close. Mechanical means thus did not seem to suffice to remove the beads from the concretion, but a chemical treatment would be difficult to select, as the other metals would react to the chemicals as well as the iron concretion. As a first step, the concretion was mechanically cleaned until beads and artefacts, including voids, started appearing:

Detail of artefacts in concretion revealed after cleaning, such as rings and yellow beads

Figure 7. Reverse side of the concretion from the Rooswijk, with detail photo above left; the complete artefact below right, after initial mechanical cleaning; notice the yellow beads, copper alloy rings and coin © Historic England

The same lump as in figure 6 following initial cleaning, with rings, beads and coins now visible. Yellow tag on left, scale rule on object

Detail of obverse side of concretion, with rings visible on the left: scale rule on object on the right, yellow tag below

Figure 8. Obverse side concretion from the Rooswijk after initial mechanical cleaning; notice the voids in the shape of a nail (on the right) and small bar-like shapes © Historic England

Because initial research proved the voids to be ‘just nails’, the decision was made to record them as best as possible, but then to sacrifice them in the bigger scheme of things. This way, the concretion could be broken apart in smaller pieces that offered the opportunity to treat them separately from the coin and rings. This work is still ongoing and consists of a combination of mechanical and chemical treatments in order to gently dislodge all the different objects from the concretion for further study.

Thank you to Elisabeth for sharing the problems and processes of conserving concreted objects from the London and the Rooswijk, and which complement previous blogs by our conservators: see links below. We hope she has enjoyed her time with us and wish her all the best for the future.

For more archaeological conservation stories on the varied artefacts from the London:

The London: A conservator’s tool-kit

Conservation of artefacts from the London

How to do . . . archaeological conservation

Glossary:

lamellar:

Festival of Archaeology 2017

July 24, 2017 ~ Serena ~ 1 Comment

Conservation of Artefacts from the wreck of the London

I am pleased to welcome this month’s guest blogger Eric Nordgren of Historic England, who tells us more about conservation of artefacts excavated from a maritime context.

Eric at work in the lab. © Historic England

I have been working with Historic England as a conservation project assistant since November 2016. My main role is to carry out remedial and investigative conservation on artefacts lifted from the London protected wreck. The London was a Royal Navy warship that sank in the Thames Estuary following an explosion in 1665. A program of work to better understand this protected shipwreck has been under way since 2014, resulting in surface recovery of exposed objects and in two seasons of underwater excavation and recovery of hundreds of artefacts made of wood, leather, rope, ceramic, glass, iron, copper and lead. The London: Excavation of material at risk project is a collaboration between Historic England, the protected wreck licensee, Cotswold Archaeology, and Southend Museum Services, where the artefacts and site archive will be deposited.

The process of conserving marine archaeological material can often involve quite a bit of time and repetition: consider that 150 Apostle musket cartridge bottles have been recovered from the London, from complete examples with cap, to some that consist of just a few broken fragments. Each one has to be photographed, assessed, repackaged in soft nylon netting, wet cleaned to remove mud from the sea bottom, desalinated, treated with polyethylene glycol and freeze dried. I’ve just finished the wet cleaning stage which took 4 workdays!

At top of image, label naming the artefact with text '6901: The London 3527: wood'. Centre of image, wooden bottle, with stopper to left, body of bottle to right, underneath this is a scale marked off in centimetres. — ‘Apostle’ musket cartridge bottles. © Historic England

Jumble of black bottles, all with individually numbered white labels. — Some of the 150 bottles after wet cleaning. © Historic England

It’s not just the Apostle cartridges, all the artefacts from the London have to go through similar stages. The water in all 158 boxes of artefacts has to be changed every month in order to remove salt from the marine environment in a process called desalination. Both organic (wood, leather) and inorganic (ceramics, glass, metal) materials can be damaged if allowed to dry out while they still contain soluble salts such as sodium chloride. Artefacts are soaked in baths of distilled water which allows salts to diffuse out, allowing them to be safely dried. Desalination isn’t difficult, but it does take some time and requires knowledge of the drying behaviour of a wide variety of materials.

Though some stages of marine conservation are repetitive, there are lots of interesting moments as well. One of the most exciting things about archaeological conservation is finding out more about the artefacts during the process and especially discovering clues about who made them and the technology they used. This process is called ‘investigative conservation’ and uses a variety of tools and techniques such as microscopy, X-radiography and digital imaging.

Here is one example discovered during digital x-radiography of a pewter spoon:

Bowl of a spoon, darkened with age and contact with water, against a plain grey background. — Pewter spoon from the wreck of the London. © Historic England

Digital x-ray of the same pewter spoon from the London. Computed radiography revealed a touchmark of the letters ‘B A’. © Historic England

The letters ‘BA’ can be seen in the x-ray, just above the point where the handle meets the bowl of the spoon. Marks in this area are called ‘touch marks’ and can tell us where and when the spoon was made and who made it. Some marks on pewter made in London or Edinburgh can be identified by records on ‘touch plates’ kept by the Worshipful Company of Pewterers, but marks from the period of the London are difficult as many records from the Pewterers were destroyed in the Great Fire of London in 1666, just a year after the ship sank. Still, it may be possible to identify who made the spoon based on comparison with other examples. We are hoping to find out more about who ‘BA’ refers to.

Another type of mark was found on a leather strap during wet cleaning. A stamped letter (or letters) can be seen in this photo taken with raking light illumination:

Horizontal strip of dark leather against a white background. Just off-centre to the right of the strip is a stamped mark resembling a letter P. — Leather strap from the *London*, showing stamped mark. Could it be from the leather worker or the artefact’s owner? © Historic England

Markings like this can be added by the leatherworker who made the strap, or might indicate its function or the sailor who used it on board the ship. We will pass this information on to the experts studying leather artefacts from the London.

Sometimes we find unknown or unexpected materials on artefacts during conservation, and need to investigate them further to get a better idea of what they are made of and why they are there. I noticed a yellow material with a tar-like odour inside the layers of a leather shoe from the London. Using a technique called Fourier-Transform Infra-Red spectroscopy (FT-IR for short) I was able to determine that it was indeed an organic material with chemical bonding similar to natural resins. This material may have been applied during the shoe’s construction as an adhesive or a sealant.

Two shoe soles one below the other, against a white background, and a centimetre scale rule underneath. Annotated on the lower sole where unknown tar-like materials have been seen. — leather shoe fragment from the *London*, with location of unknown material. © Historic England

Graph marked in units of 5 from 50 to 98 on the vertical axis, and 4000 backwards to 650 on the lower axis, showing significant spike between the 3000 to 2000 mark. — Transmittance spectrum produced during FT-IR analysis of the shoe sole. © Historic England.

Conservation work on material from the London is quite rewarding as we have a chance to progress artefacts from post excavation though conservation treatment, learning more through investigative conservation along the way and preparing them for storage and display at Southend Museum.

Find out more about the London by following #LondonWreck1665

Many thanks to Eric for his fascinating blog. The thing that caught my attention particularly was the stamped leather – that such detail has survived 350 years of immersion in a hostile environment and can be recovered by archaeology is amazing.

For more on conservation of artefacts from the wreck of the London, please also have a look at an earlier post from 2015.

The Stirling Castle

August 1, 2016June 1, 2018 ~ Serena ~ 8 Comments

Eat my Hat

Sailors kept their chewing tobacco in their hats, the linings of which became soaked in sweat and tobacco juice. If they ran out of tobacco they would take out the linings of their hats and chew them. [http://www.plymouth.gov.uk/navalsayingsdh accessed: 11.07.2016]

So this famous phrase has a nautical origin! Sailors and their hats are today’s theme:

One of the perks of being an archaeological conservator is that you get close to artefacts. Really close! Even closer! So close in fact, that you can hold, smell and properly look at artefacts: back, front, sides, and all around. And we conservators like to look closely. We like to see what an artefact is made from, how it is made, what condition it is in, and what that can tell us about the people that made or used it.

I recently had the privilege to work on the collection from the Stirling Castle protected wreck owned by the Isle of Thanet Archaeological Society on a project funded by Historic England’s commissions fund and headed by the Maritime Archaeology Trust. The main aim of this project was to catalogue and consolidate the archive, to enhance access to this fantastic wreck assemblage dating to the early 18^th century.

Some of my favourite artefacts were beautiful knife handles, an ivory comb and copper alloy cauldrons. But two items stood out: leather hats. Items of clothing rarely survive in the archaeological context. But due to special preservation conditions for organic materials, such as leather, wool or linen, wreck sites play an important role in redressing the imbalance by allowing us an insight into clothing and dress, which are under-represented in collections when compared to non-organic materials, such as ceramics, for example.

Side view of crown and brim of brown leather hat, against a wide background. — Leather hat, *Stirling Castle* wreck assemblage, on loan from the Isle of Thanet Archaeological Society to the Shipwreck Museum, Hastings. Image © Historic England

The hats stood out from the rest of the Stirling Castle collection for a number of reasons:

Beaver felt and wool seem to be the prevailing materials for hats of this period both in surviving examples and in art;
The style and method of construction also appear unusual for the period;
They are very well preserved: they are better-preserved than other leather artefacts from the same collection, such as the shoes or book covers;
There are no parallel finds known to us at the time of writing

Because these hats are so unusual, we are trying to learn as much about them as possible. They are a strong contrast to the leather shoes which are regularly found in shipwreck contexts and are well-understood, e.g. Mary Rose (1545); London (1665), HMS Invincible (1758).

Component parts of black leather shoes, such as uppers, heels and small parts, laid out together against a white background, with a ruler at bottom left for scale. — Leather shoes from the *Stirling Castle* wreck assemblage. Image © Historic England.

And this is where you come in: We have embarked on a project to study and investigate these two hats from various angles. We have chosen a multidisciplinary approach combining scientific investigations with art historical research as well as citizen science.

We’ve identified some surviving hats and contemporary images of hats, but we need your help to find more. We are putting the word out there asking members of the public as well as museums and collections to look at paintings and drawings of hats, or even hats themselves, dating to around the end of the 17^th to early 18^th centuries. Our aim is to collate a database of other hats and depictions of hats, to be able to compare our two hats from the Stirling Castle with other examples.

Here is the other hat from the assemblage:

View of brown leather hat against a white background, showing that the crown of the hat is laced together with a decorative thong. — Leather hat from the Stirling Castle wreck assemblage, Ramsgate Maritime Museum, showing thong lacing at the back. Image © Historic England

To examine the hats more closely, have a look at the 3D animations of the Hastings hat and the Ramsgate hat: click and drag to rotate in any direction, and see inside the hats, use mousewheel to zoom. (Best viewed in Firefox, Chrome, Opera, Safari 10.9 and above, IE 11)

As you can see, the hats are constructed in several pieces, with one piece for the top of the crown, the main body of the crown overlaps at the front and is laced at the back with a thong, and finally a wide brim.

We are mainly focussing on maritime scenes in paintings and drawings, due to the obvious maritime connection of the hats to the wreck of the Stirling Castle, but are interested in other depictions or real-life examples of similar looking head gear from other contexts too, perhaps hats worn by working-class people such as labourers and agricultural workers. These can be of any date to help with the comparison, but late 17th to early 18th century works would be especially helpful.

Please tell us as much as you can about the images or surviving examples of hats: where they’re from, their date and context, and a brief description of the style of hat, and its construction, and send us a link or photograph if possible.

As an example of what we’re looking for, have a look at this engaging 18th century image and the caption we have written for it.

Painting in neutral shades depicting two men, one wearing a hat and white shirt, clutching a wine bottle, the other behind his outstretched arm, looking at him. This man has a patched sleeve and holds a wine glass. — unknown artist, European School, 18th century; *The Wine Seller*; Southend Museums Service; http://www.artuk.org/artworks/the-wine-seller-2514. (Creative Commons license). The wine seller wears a brown hat with a low, wide, rounded crown, and a shaped wide brim. The texture suggests it may be a felt hat, as was usual for hats of this period.

The citizen science part of the project opens today, 1st August 2016, and will run until 31st August 2016. It will be shared on the @HE_Maritime Twitter account with the hashtag #LeatherHats. Please feel free to share widely, using the same #LeatherHats hashtag. If you have any clues, ideas or images of hats you would like to share with us, please contact us by commenting on this blog, on Twitter @HE_Maritime, or by e-mail at StirlingCastleHats@HistoricEngland.org.uk

Thank you!

Angela Middleton, Archaeological Conservator, Historic England, and Serena Cant, Marine Information Officer, Historic England.

No.83: The London, No.3: A Conservator’s Tool Kit

May 11, 2015May 11, 2015 ~ Serena ~ 2 Comments

This week Angela Middleton, Archaeological Conservator at Historic England, is my guest blogger, explaining the tools of her trade in conserving some of the objects recovered from the London.

A conservator’s tool kit: air brush, hammer and chisel

As a conservator you may spend many hours peering down a microscope, using a scalpel and slowly removing layers and years of dirt or corrosion: a painstakingly slow process; just like watching paint dry or grass grow. Progress can be hard to measure and to the untrained eye is often barely noticeable.

So why bother, you may ask?

During conservation, the conservator and the object go through a couple of stages. You normally start off with an assessment, where the condition of the object is evaluated, allowing a picture of the artefact’s composition, construction and state of preservation to emerge. Following that you devise a treatment according to the artefact’s condition and its purpose.

The ultimate goal is always to stabilise the object, preserve it for the future and to understand it: and by doing this a conservator also helps others to understand and appreciate it. This is often difficult when the surface is obscured by corrosion products or discoloured due to centuries of being buried. Removing these obscuring and distracting layers will help to reveal the object.

Lately I have been working on artefacts from the London, a shipwreck which sank off Southend-on-Sea in 1665. After their initial assessment (see Heritage Calling: Looking Inside) and a lengthy programme of desalination* (remember this is like watching paint dry…), artefacts can be actively conserved, without obscuring fine surface details or allowing layers of dirt to be consolidated onto the surface.

So this is where the pressure washer comes in. I have been using an air-brush system to clean off loose surface dirt on some of the leather from the London. It works just like a conventional pressure washer, albeit on a smaller scale, with the advantage of being able to regulate the pressure down and work with a really small outlet, enabling you to focus on small areas.

The example shown below is a leather sole from one of the many shoe finds. It is contaminated with iron compounds, which are commonly found in the burial environment (iron compounds originate from naturally occurring minerals or from corroding artefacts in the vicinity). They settle on the leather surface and do not only obscure fine surface details but also discolour the artefact. If not using an air-brush system, I would be cleaning them with sponges, which can sometimes be too harsh on sensitive surfaces such as leather, which can be easily marked and damaged. The air-brush is a much more gentle method of cleaning.

Here is the mini pressure washer in action:

However, sometimes ‘gentle’ is just not good enough, especially for maritime finds. They often become covered in huge and unsightly concretions. A concretion is formed when a corroding iron object interacts with the surrounding environment, encapsulating marine organisms, surrounding sediments, corrosion products and even other artefacts in a lump. In most cases the artefact cannot be recognised at all. In order to stabilise and understand the object, these concretions have to be removed. And yes, as the name suggests: they can be as hard as concrete. There is little choice but to use a hammer and a chisel to remove them: tools you don’t often find used by an archaeological conservator.

The example below is a concretion containing a multitude of artefacts. Visible at the top was a copper alloy artefact, half embedded in the concretion. A conservator would normally take an X-ray to visualise the embedded artefact(s). However, the concretions are often so dense that X-radiography is of limited use. So in this case I used the shape of the object partly showing at the top to guide me and started chiselling the concretion away. Once again it was a slow process, but totally worth it. What I managed to reveal and finally remove from the concretion was a pair of callipers: the only one from the wreck so far and in near perfect condition. Callipers were used to check the diameter of shot. By also knowing the material and the density the weight can be calculated. In our example it looks like the diameter is engraved on one side of the scale and the weight on the opposing side. The anaerobic conditions on the seabed and inside the concretion have preserved the markings on the calliper and it showes very little corrosion.

Left to right: Concretion as found, the callipers are visible at the top; callipers after being removed from the concretion. — **Left to right:** Concretion as found, the callipers are visible at the top; callipers after being removed from the concretion.

The other example is an iron cannonball which was also covered in concretion. It was important to remove it, not only to reveal the true size of the artefact, but also to reduce treatment times. The thick layer of concretion forms a barrier and will hinder passage of water during desalination.

After the concretion had been removed the cannonball diameter could be determined as roughly 15cm, making it a 30-pounder, suitable for a demi-cannon.

Each conservation task requires a specific set of tools, depending on the job in hand and the nature of the artefact. The gadgets an archaeological conservator uses are very different to what a paintings or textile conservator would use. However, the similarity is that each conservator strives to preserve the object and enable others to study and enjoy it.

*Desalination: During burial salts from the burial environment accumulate inside artefacts. If such an artefact is simply dried, salt crystals will form, which expand in volume on drying, which can cause surface layers of the artefacts to flake off, or the whole artefact can actually fall apart. Also salts are hygroscopic, which means they attract moisture from the air. This moisture can cause further corrosion. This is especially true for metal artefacts.

During desalination artefacts are immersed in tap water, and then in de-ionised water, to remove water-soluble salts. This is achieved by regularly changing the water and measuring the chloride level or the conductivity of the storage solution. Once these readings remain sufficiently low, the artefact is considered desalinated and can be treated as in the case of wood or leather, or can be dried as in the case of glass or ceramics.

To catch up with previous posts on the London, here is a post commemorating the anniversary of her loss in March 1665 and another on recent archaeological work.

Wreck of the Week

A Historic England Blog

archaeological conservation

On the cleaning of metal finds from the London and the Rooswijk

Festival of Archaeology 2017

The Stirling Castle

No.83: The London, No.3: A Conservator’s Tool Kit