Historic England – Conservation of artefacts from the ‘London’ shipwreck

As part of British Science Week 2017, the National Heritage Science Forum is once again featuring blog posts from heritage scientists from across its member organisations. This year’s theme is ‘sharing heritage science’ and the blogs over the rest of the week will give an insight into the many different forms that heritage science can takes, as well as some of the different ways of getting involved.

Next up is a post by Dr. Eric Nordgren of Historic England, talking about the conservation of artefacts on the wreck of the 1665 London ship…

The London was a Royal Navy warship that sank in the Thames Estuary following an explosion in 1665. A program of work to better understand this protected shipwreck has been underway since 2014, resulting in surface recovery of exposed objects and in two seasons of underwater excavation and recovery of hundreds of artefacts made of wood, leather, rope, ceramic, glass, iron, copper and lead. The LondonWreck1665 project is a collaboration between the protected wreck licensee, maritime archaeologists from Cotswold Archaeology, scientists and conservators at Historic England’s Fort Cumberland site in Portsmouth and the Southend Museums Service, where the artefacts and scientific data will be deposited.

The goals of conservators working on the London material are to assess the condition of the artefacts and apply investigative and remedial conservation techniques to make them as stable as possible for study and display at Southend Museums while learning as much as possible about the materials they were made from, how they were produced, and their working life on-board ship. The conservation process offers lots of opportunities to collaborate with specialist scientists to gain information such as the species of wood used to make artefacts such as musket powder cartridge bottles, the type of fibres used in rigging and cordage or the elemental composition of glass and metal finds. All of this adds to our knowledge of 17th Century seafaring and life on-board the London in particular.

All materials change and as conservators we try to understand these changes and slow them down. Iron artefacts for example often develop a thick coating of rust and marine growth called ‘concretion’ that can hide their true shape. Conservators use X-radiography to get a closer look inside the objects. These X-rays help conservators plan micro-excavation of concreted artefacts in the laboratory or can sometimes reveal the shape of objects which are no longer preserved.

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Photo (left) and digital X-radiograph (right) of an iron artefact from the wreck of the London. The object is covered by rust and concretion but radiography allows conservators to look inside and see the original size of the iron ring and how much metal still remains. (Images: Historic England 2017)

 

Conservators working on London artefacts use a variety of scientific principles and analytical techniques in their daily work. An example is the monitoring of salt removal (or desalination). Both organic (wood, leather) and inorganic (ceramics, glass, metal) materials can be damaged if allowed to dry out while they still contain soluble salts such as sodium chloride. Artefacts are soaked in baths of distilled water which allows salts to diffuse out, allowing them to be safely dried.

Some objects such as a cast iron cannon ball can’t be desalinated effectively using distilled water alone. For marine iron, 0.5 Molar sodium hydroxide solution in distilled water (pH 13.5) is used to extract chlorides more effectively. The concentration of chloride salts measured in parts per million (ppm) in the desalination bath is monitored weekly, allowing us to track chloride release, determine when the bath needs changing and when the treatment is finished (normally when very low levels of chloride are detected in solution).

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Graph showing concentration of chloride extracted (in parts per million) from a cast iron cannonball from the London into a sodium hydroxide (NaOH) desalination solution over 50 days. The concentration reaches 550 ppm after about 35 days, indicating it is time for a fresh desalination bath. (Images: Historic England 2017)

The Historic England conservation team working on the London at Fort Cumberland, Portsmouth includes Angela Middleton and Eric Nordgren.

Find out more about the London by following #LondonWreck1665!

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