Looking inside: Investigation of marine archaeological artefacts

Angela Middleton holds a degree in archaeological conservation from the University of Applied Sciences, Berlin/ Germany and an MSc in Maritime Conservation Science from the University of Portsmouth/ UK. She has worked for the Newport Medieval Ship Project and the Michael Faraday Museum of the Royal Institution before joining English Heritage as an Archaeological Conservator in 2007. She advises on and undertakes research and investigative conservation on material retrieved from land and marine sites, with a special interest in the conservation of waterlogged organic materials.

My role as an archaeological conservator gives me the opportunity to work on some fantastic finds from protected shipwreck sites. Preservation under water can be remarkable and artefacts are often discovered in a very good condition. However, certain steps have to be taken to ensure the survival of artefacts after they have been raised from the seabed. I want to give you an insight into my work and tell you how we unlock information from artefacts.

Diving on shipwrecks protected under the Protection of Wrecks Act 1973 is regulated by a licence system, which is managed by English Heritage on behalf of the Secretary of State. Over the years English Heritage has built many fruitful relationships with divers, who survey and monitor wrecks. Occasionally the lifting of artefacts is appropriate where they are shown to be at risk, a specific licence is granted for the recovery and eventual deposition in a museum is agreed in advance. And this is where I come in.

Artefacts from the marine environment have to be stored in a special way. So we always advise to keep artefacts, wet, cool and dark. If artefacts accidentally dry out or dry out too fast valuable information is lost forever.

I recently received some artefacts from the London, a Protected Wreck Site in the Thames Estuary, including a pair of dividers, personal adornment, glass and ceramic vessels, leather artefacts and a wooden pulley block. The ship sank following a gunpowder explosion in March 1665. On arrival in the conservation lab, I inspected the artefacts, made sure they were packed and stored correctly and then started to document them. This is creating a record of the artefact as it is at that moment. This normally involves photography, drawings and X-radiography.

X-radiography  is commonly used in archaeological conservation. It allows us to look beneath surface layers and record unstable objects, such as iron. An X-radiograph (x-ray) shows the shape of an artefact, which can sometimes be heavily disguised by overlying corrosion layers. It shows the condition of an artefact, such as extent of corrosion, cracks or damage from marine boring organisms and it can show construction and decoration details, such as joints, precious metal inlays or coatings.

In some cases an X-ray is all that is needed. An archaeologist can use this information to identify and date an object and help to understand the shipwreck. In other cases some cleaning work is required to clarify the shape of an object.

To begin with artefacts are stored in tap water as they contain salts (which have accumulated from the seawater). These have to be removed to ensure the long term survival of archaeological finds. The water is monitored for salt levels and changed at regular intervals. Once the salt levels stay consistently low, most artefacts can be dried slowly. Some materials, such as leather, textile and wood require more treatment before they can be dried. Their weakened cell or fibre structure means that they need to be bulked out, with a chemical called Polyethylene glycol, before they can be dried, normally by freeze drying. This is a drying technique widely used in the food industry. It is carried out in a vacuum chamber and relies on sublimation, whereby frozen moisture (in this case water) is transferred from the solid ice-phase to the gas vapour-phase with no intermediate liquid water-phase. This eliminates surface tension of evaporating water which normally damages the fibre and cell structure of wood, leather or textile.

Work on the London artefacts provided English Heritage with the opportunity for collaboration between the license holder and the archive repository, Southend Museums Service. Once all conservation and analysis work is complete, the finds will be transferred to Southend, where the can be accessed and maybe even be displayed.