Professor Dave Watkinson, Professor of Conservation
Brunel’s 1843 engineering masterpiece SS Great Britain, was the first iron hulled propeller driven ocean going liner in the world (figure 1). After a successful life at sea, she ended up an abandoned hulk in the Falkland Islands (figure 2). In 1970, the ship was towed across the Atlantic and up the river Avon (figure 3) to be reunited with the dry dock in which she was originally constructed. By the late 1990’s she was in poor condition and a bold plan to control corrosion of the hull by desiccation was being considered by Matthew Tanner, Director of the ss Great Britain Trust, and Robert Turner of EURA Conservation. They approached David Watkinson at Cardiff University for his advice, as since the 1980’s he had focused Cardiff University conservation research on designing and evaluating treatments that aim to control the corrosion of archaeological iron.
The innovative approach to control corrosion would involve sealing off the top of the dry dock with a glass roof extending from the dockside to the waterline of the hull, controlling ingress of the air into the hull interior from its deck and, aided by a huge desiccation plant input air that would surround the hull in a ‘dry’ envelope (figure 4). Above the waterline the hull was in much better condition and its exterior could be protected by the application of a coating.
Damp air drives the electrochemical corrosion process that converts metallic iron to rust, aided by residual chloride from salt water (figure 5). Removing water from the air can prevent corrosion but the unanswered question that underpinned the desiccation plan was; how dry did the air have to be to prevent iron corrosion? This would influence both the design of the plant and the cost of operating it, as the amount of gas used to dry the air would rise as the target operating humidity for delivering it to the dock fell.
Brunel built SS Great Britain using wrought iron, similar to archaeological wrought iron that had been studied in Cardiff. Consequently, the SS Great Britain Trust funded a Cardiff research programme that would identify the operating humidity for the desiccation plant. A Cardiff graduate, Mark Lewis, was the Research Assistant working on this project (Figure 6). After 2 years of experimental study, a no corrosion relative humidity of 12% was identified for preventing corrosion (Watkinson and Lewis 2004; Watkinson et. al. 2005a). Also, the incremental impact of rising relative humidity on corrosion rate was identified by studying how chloride bearing corrosion products interacted with iron at differing humidity. This delivered a ‘risk scale’ for corrosion, according to prevailing dampness of the air (Watkinson and Lewis 2005b) and improved understanding of iron corrosion throughout the heritage sector, informing management decisions for display and storage.
Achieving 12% relative humidity would be expensive but since Cardiff’s research identified there was a minimal increase in corrosion rate until 35% relative humidity was exceeded, the decision was made to deliver air at 20% relative humidity into the dock (figure 7). The desiccated air is vented over the hull surface via adjustable jets (figure 8) and the glass roof was flooded with water (figure 9). Visitors on the dock-side will see the ship ‘floating’, while those inside the dry dock will view it from ‘underwater’ (figure 10).
Undergraduate and taught postgraduate dissertation collaborations with SS Great Britain Trust have determined the distribution of humidity over the hull and within the dock, revealing that the plant is working effectively to minimise the risk of further corrosion. Since this research was completed, we have produced further research detailing how chloride and humidity influence corrosion, supported by a £447k AHRC grant (Watkinson et al. 2019).
The conservation of the ship has proved to be a great success, attracting visitors from across the globe who support the local economy and fund the continued preservation of this world renowned historical technological milestone. In 2006, SS Great Britain Trust was awarded the prestigious Gulbenkian Museum prize and in 2007 Dave Watkinson received the Anna Plowden medal for his innovative research within conservation. Mark Lewis later produced his PhD reporting these experimental studies and is now Senior Curator at the Roman Legionary Museum in Caerleon, where he continues his research collaborations with Cardiff University. While this project successfully underpinned the preservation of the SS Great Britain, it also established a Cardiff University research platform focused on corrosion and corrosion control, which continues to deliver outputs that can evidence decision making for managing heritage metals.
Watkinson D. E., Rimmer M. B. and Emmerson N. J. (2019) The Influence of Relative Humidity and Intrinsic Chloride on Post-excavation Corrosion Rates of Archaeological Wrought Iron. Studies in Conservation. Published on line open access February 8th 2019 https://www.tandfonline.com/doi/full/10.1080/00393630.2018.1565006
Watkinson, D and Tanner, M. (2008) ss Great Britain: conservation and access – synergy and cost. In Conservation and Access; contributions to the London Congress 15-19 September 2008, Saunders, D., Townsend, J. and Woodcock, S. (eds), The International Institute for the Conservation of Historic and Artistic Works, London 109-114.
Watkinson, D., Tanner, M., Turner R. and Lewis M. (2005a) ss Great Britain: teamwork as a platform for innovative conservation, The Conservator, 29 73-86.
Watkinson D. and Lewis M. (2005b) Desiccated storage of chloride contaminated archaeological iron objects. Studies in Conservation, 50 241-252.
Watkinson D. and Lewis M. (2004) ss Great Britain iron hull: modelling corrosion to define storage relative humidity, Metal 04 Proceedings of the international conference on Metals Conservation, Ashton J. and Hallam D. (eds.), Canberra Australia 4-8 October 2004, 88-103 National Museum of Australia.