How to do… archaeological conservation

An archaeological excavation can result in a huge amount of artefacts being excavated in a short period of time. The analysis of these artefacts can take many years or decades.

 

But why does conservation take so long? And what does it involve?

Maritime Archaeologist Pip and Archaeological Conservator Angela explain how they’re conserving finds from the London wreck:

Diver about to dive the London wreck June 2016 copyright Cotswold Archaeology
Diver about to dive the London wreck June 2016 copyright Cotswold Archaeology

The London was built in 1656 and in 1665 accidentally blew up in the Thames. The wealth of finds of all types preserved on the wreck provided an exceptional insight into the Navy at a time of Britain’s burgeoning naval power in Europe.

Situated adjacent to the navigation channel of the Thames, excavation works between 2014 and 2016 recovered many artefacts at risk. From glass bottles, wooden apostles (gun powder cartridges), to leather shoes and book covers.

We have to think about  the long-term preservation of artefacts recovered from the sea at the moment they come out of the water.

ornate 4cm wide Bronze sundial compass copyright Steven Ellis high res
Ornate 4cm wide Bronze sundial compass © Steven Ellis

Wet and Dirty

The first thing we do is to mimic the environment that the object has been in, whilst further conservation work can be planned. In essence, this means we keep them wet!

This is referred to as ‘passive’ conservation. This can be simply submerging the artefact in fresh, clean water. At the end of the excavation the objects are sent to a lab facility for further conservation work – at Fort Cumberland (our facility in Portsmouth), this is affectionately known as the ‘wet and dirty’ lab.

Personal items such as shoes really help illustrate the human side of the London wreck story. (c) Historic England
Personal items such as shoes really help illustrate the human side of the London wreck story. © Historic England

Desalination

The next important step is ‘desalination’ – removing the salt from the finds.  Degradation and corrosion that can be caused by the presence of salt can cause damage during drying, so it is important to remove as much salt as possible before this process begins.

This is done by placing the artefacts into distilled water, rather than either seawater or tap water. Distilled water does not contain any salts.

IMG_6684
Conductivity meter showing the conductivity of the water of a leather shoe from the London.

In order to track the progress of desalination, and to determine when the artefacts are ready to move onto the next step in the conservation process, we monitor the reduction of salt levels using a conductivity meter.

A small object with a more open cell structure, such as leather belts and straps, will take less time to desalinate than say a large metal cannon, which has a much larger mass and a much denser microstructure for the salts to come out of.

Conserving Wood

A wooden pulley from the London about to undergo remedial conservation. (c) Luke Mair
A wooden pulley from the London about to undergo remedial conservation. © Luke Mair

After desalination some materials, such as wood need to go through further conservation steps, namely impregnation with a wax-like material and vacuum-freeze drying.

Wood is a food source for many organisms. However, under some circumstances, this natural recycling process is slowed down to such a degree that wood as old as Neolithic structures can survive.

One of those circumstances is the marine environment. The limited amount of oxygen means that only few organisms can digest the wood. Decay will always start from the outside and move slowly towards the centre. Soft rot fungi and some bacteria will colonise the wood and start digesting it. This results in microscopic material loss such as cavities or tunnels in the wooden cell walls. All these cavities will be filled up with water. This means that the wood is physically weakened but in most cases retains its shape.

close up of worms
Detailed image of the sea-worms which are boring into the wood of the gun carriage, causing damage. Climate change is bringing more of these destructive wood-boring sea worms into warmer English waters. © MSDS Marine/Cotswold Archaeology

 

If such wood is simply air dried it will shrink, split and warp. We would lose valuable information that can be obtained from this kind of object. So what do we do? In order to preserve waterlogged archaeological wood, the water has to be replaced with something else, giving the weakened cell structure strength and stability.

We place the wood in a solution of PEG (polyethylene glycol)- a wax-like material. Slowly water moves out of the wood and PEG moves into the wooden cell structure. This is a slow process that can take several months or even years

To remove the remaining water we use a process called vacuum-freeze-drying. For this the wood is frozen to -30°C. It is then placed inside a vacuum-freeze-drying chamber. Here water is removed from the solid (frozen) state to the gaseous (vapour) state – this is called sublimation. Once again, this is a very slow process taking weeks to months.

Vaccum-freeze-dryer showing the build-up of ice in the condenser. L: at the beginning of the drying process R: at the end.

We can track the drying by monitoring the amount of ice we are getting inside the condenser and by weighing the wood. As the ice sublimes, the wood will become lighter.

The example given below is a wooden linstock. This is effectively an oversized matchstick, which is used to light the gun powder inside a cannon. This particular artefact spent 2 years in desalination, 25 weeks in impregnation and 22 days in the vacuum freeze dryer.

IMG_6131
A wooden linstock during vacuum-freeze drying and weight recording. © Historic England

So the excavation and lifting of this one artefact took maybe a couple of minutes. But the subsequent conservation took well over 2.5 years. This is by no means unusual. The post-excavation phase of a lot of archaeological excavations can go on for many years; but it means that these objects are saved for the future

Written by Pip Naylor (Marine Planning Archaeological Officer) and Angela Middleton (Archaeological Conservator) at Historic England

Further Reading

 

4 responses to How to do… archaeological conservation

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