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