Michelle Crepeau, David Watkinson, and Nicola Emmerson
Abstract
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 during recovery? 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 unassisted visual detection? And if so, are x-radiographs sufficient predictors of the recoverability of these surfaces? Dissertation research conducted at Cardiff University in 2015 was designed to explore these questions and discuss how the combination of spectral imaging and elemental analysis contribute to x-radiographic interpretations of non-ferrous finishing surfaces on archaeological iron and inform decisions as to the practical recovery of such surfaces. A comprehensive paper regarding project findings is currently being written.
Tinned surfaces are 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 finishing surface on archaeological iron. This is an assumption seemingly supported by the presence of distinct areas of differential density known as ‘tinning lines’ in 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. The increased adoption and accessibility of x-radiography, air abrasion, and x-ray fluorescence (XRF) from the late 1970s onward provided solutions for the preferentially destructive conservation methods formerly used on tinned archaeological iron. The success of these methods, however, seems to have had the unintended consequence of retarding further exploration into alternative methods of recovery, documentation, or degradation mechanisms of tinned iron. This is especially true in the context of highly mineralized artifacts in which metallic tin may no longer exist and with respect to acid-base attack, electrochemical diffusion, stress responses, and other novel mechanisms of tin redistribution, such as those seen in tin whiskering and dendrite growth.
The aim of this project was therefore to positively identify and characterize presumptive tinning surfaces on a highly-mineralized iron artifact using alternative documentation methods, namely scanning electron microscope-backscattered electron imaging (SEM-BEI) and scanning electron microscope with energy dispersive x-ray spectroscopy (SEM-EDX) elemental analysis, to either corroborate or identify weaknesses in x-radiographic and optical microscopy evidence of tinning. The overarching goal is the development of practical guidelines to better inform decision making in the recovery of tinned surfaces.
This project used an archaeological wrought iron key dating from the late medieval period from the deserted English village of West Whelpington as its subject. Previous conservation indicates that the artifact was likely tinned. The validity of this identification was tested through a) the production of an array of x-radiographs that explored variables such as penetrative power, angle, and exposure time to confirm the presence of tinning lines and the likelihood of a non-ferrous material, b) the investigative cleaning of the key via air abrasion to test recoverability of the surface based on x-radiographs and optical microscopy, and c) the sectioning of the key for metallographic analysis to better chemically and physically describe the presumptive finishing surface.
The project used SEM-BEI 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, these comparisons prompted commentary as to what constitutes a recoverable surface and what factors a conservator needs to consider, such as stakeholders, work constraints, and artifact ‘value’, etc. when making decisions about whether 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 project served to demonstrate that not all things of value are immediately apparent.
For more information regarding upcoming publications regarding this research please contact the authors.
FURTHER READING
Corfield, Michael. 1985. “Tinning of Iron.” In Lead and Tin: Studies in Conservation and Technology, UKIC Occasional Papers 3, edited by Gwyn Miles and Sarah Pollard. 40-43.
De Ryck, I., E. Van Biezen, K. Leyssens, A. Adriaens, P. Storme, and F. Adams. 2004. “Study of Tin Corrosion: The Influence of Alloying Elements.” Journal of Cultural Heritage 5 (2): 189-195.
IFA. 2014. Tin Whiskers on Printed Circuit Boards: Consequences for Safety Components in Machine Construction. http://www.dguv.de/medien/ifa/en/pra/zinnwhisker/ifa_info_zinnwhisker_en.pdf
Meeks, Nigel. D. 1993. “Surface Characterization of Tinned Bronze, High-tin Bronze, Tinned Iron and Arsenical Bronze.” In Metal Plating & Patination: Cultural, Technical & Historical Developments, edited by Susan La Niece and P. T. Craddoc. 247-275.
Pollard, A. Mark. 1985. “Investigations of ‘Lead’ Objects using XRF.” In Lead and Tin: Studies in Conservation and Technology, UKIC Occasional Papers 3, edited by Gwyn Miles and Sarah Pollard. 27-32.
Turgoose, Steven. 1985. “The Corrosion of Lead and Tin: Before and After Excavation.” In Lead and Tin: Studies in Conservation and Technology, UKIC Occasional Papers 3, edited by Gwyn Miles and Sarah Pollard. 15-26.
Watson, Jacqui, K. Graham, A. Karsten, L. Skinner, U. Schaeder, S. Penton, J. Gao, V. Fell, and J. Jones. 2011. Research Department Report Series 72-2011: Townfoot Farm, Cumwhitton, Cumbria: Investigative Conservation of Material from the Viking Cemetery. London: English Heritage.
Yoo, Y. R., and Y. S. Kim. 2010. “Influence of Electrochemical Properties on Electrochemical Migration of SnPb and SnBi Solders.” Metals and Materials International 16 (5): 739-745. doi: 10.1007/s12540-010-1007-6
MICHELLE CREPEAU, MSc, is a conservator currently employed by the Queen Anne’s Revenge Project and can be found at the QAR Satellite Lab at the Maritime Museum in Beaufort, NC, where she demonstrates practical conservation work for the public. She is an alumna of the Cardiff University Conservation Practice MSc Program and has previously worked for the Ḉatalhöyük Archaeological Research Project and the Colonial Williamsburg Foundation. E-mail: michelle.crepeau@ncdcr.gov
DAVID WATKINSON, PhD, is a professor of conservation and Deputy Head of School of the School of History, Archaeology and Religion (SHARE) at Cardiff University, UK. E-mail: watkinson@cardiff.ac.uk
NICOLA EMMERSON, PhD, is a lecturer in conservation at the Cardiff University School of History, Archaeology and Religion (SHARE). E-mail: emmersonnj@cardiff.ac.uk