concretion

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

~ 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:

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

~ 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.

leather sole
Left to right: Leather sole 3141 before cleaning; during cleaning with top half cleaned; fully cleaned.

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.
Detail of the markings on the callipers
Detail of the markings on the callipers

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.

cannonball
From left to right: Cannonball before removal of concretion; during removal of concretion; after removal.

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.