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Evidencing risk to inform best practice: the limitations of detecting tinning layers by visual analysis during the removal of corrosion from archaeological iron

Watkinson, David, Crepeau, Michelle and Emmerson, Nicola 2019. Evidencing risk to inform best practice: the limitations of detecting tinning layers by visual analysis during the removal of corrosion from archaeological iron. Presented at: Metal 2019: Interim Meeting of the ICOM-CC Metals Working Group, Neuchatel, Switzerland, 2-6 September 2019.

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Abstract

Evidence for tinning on corroded archaeological iron is visible in X-radiographs as distinct thin high density white lines. These are used to guide the removal of overlying iron corrosion to reveal the tin layer. Airbrasion with 53µm aluminium oxide, at low pressure and powder flow, was used to remove iron corrosion incrementally from a tinned medieval key. Appearance of the tin layer visible on the radiograph was aided by optical microscopy. At selected points during the cleaning, SEM-BEI imaging with EDA spot analysis was used to determine if the tinning layer had been reached. Comparing optical assessment with SEM-BEI images revealed that the occurrence of tinning could not be detected with the naked eye aided by optical microscopy. No metallic tin was observed and it is presumed to be present as tin dioxide (SnO2). The presence of tinning was confirmed by SEM-EDA spot analysis. It is likely that parts of the tinning layer were lost during airbrasion. This identifies the risk of removing overlying iron corrosion products to expose tinning layers. Conservation strategies for preserving tinning layers on iron need to be formulated according to evidence that tinning cannot be readily detected by optical microscopy and elemental surface analysis is required to confirm its presence.

Item Type: Conference or Workshop Item (Paper)
Date Type: Completion
Status: Unpublished
Schools: History, Archaeology and Religion
Subjects: C Auxiliary Sciences of History > CC Archaeology
Q Science > Q Science (General)
T Technology > T Technology (General)
Date of First Compliant Deposit: 11 March 2019
Last Modified: 06 Sep 2019 02:45
URI: http://orca.cf.ac.uk/id/eprint/119710

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