AIC's 46th Annual Meeting

X-radiographs are important guides for the air abrasive cleaning of archaeological iron. What happens then when an important feature, such as a finishing surface, recorded by an x-radiograph fails to materialize? Is this merely human error on the part of the conservator? Can the x-radiographic signatures of these surfaces be caused by other factors? Or have residual finishing surfaces simply degraded past the point of un-assisted visual detection? This presentation will discuss how the combination of spectral imaging and elemental analysis can contribute to x-radiographic interpretations of non-ferrous finishing surfaces on archaeological iron and inform decisions as to the practical recovery of such surfaces. Tinned surfaces are fairly ubiquitous in the archaeological record and are frequently documented in x-radiographs. Actual recovery of these surfaces, however, is under-reported in academic literature. Due to the nature of tin corrosion and its products, tin-plate is often assumed to be a visually discrete, recoverable surface. This is an assumption seemingly supported by the presence of distinct areas of differential density known as ‘tinning lines’ on x-radiographs. However, the extent to which these lines reflect the actual condition of the underlying tinned surface and can predict the success of practical recovery is not well documented. This is especially true in the context of highly mineralized artifacts in which metallic tin may no longer exist. The aim of this project is to positively identify and characterize presumptive tinning surfaces on a highly-mineralized iron artifact using SEM-BEI imaging and SEM-EDX elemental analysis to corroborate x-radiographic and optical microscopy evidence of tinning. This project uses an archaeological wrought iron key dating from the late medieval period of the deserted English village of West Whelpington as its subject. Previous conservation indicates that the artifact was likely tinned. The validity of this identification is tested through a) producing an array of x-radiographs that explore variables, such as penetrative power, exposure time and geometry to confirm the presence of tinning lines, b) performing investigative cleaning via air abrasion to test recoverability of the layer based on x-radiographs, and c) sectioning the key and using spectral analysis techniques to better chemically and physically describe and corroborate the presumptive finishing surface. The presentation will also use SEM micrographs and SEM-EDX mapping to illustrate the distribution of highly mineralized tin layers in the corrosion matrix and discuss the extent to which these morphological changes can be detected in x-radiographs and used as signifiers of surface condition. Ultimately, this will prompt commentary as to what constitutes a recoverable surface and what factors a conservator will need to take into account, such as, stakeholders, work constraints, and artifact ‘value’, etc. when making decisions about whether or not to attempt recovery of a finishing surface that is analytically distinct but not necessarily visually or physically identifiable. Much like “all that is gold does not glitter” this paper will demonstrate that not all things of value are strictly material.