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.

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.