Examination of Gilded Bronze Artifacts Using Nondestructive Eddy Current Techniques
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Examination of Gilded Bronze Artifacts Using Nondestructive Eddy Current Techniques Johanna R. Bernstein and Blythe McCarthy1 Department of Materials Science and Engineering The Johns Hopkins University, Baltimore, Maryland 21218 1 Freer Gallery of Art and Arthur M. Sackler Gallery Smithsonian Institution, Washington, DC 20560 ABSTRACT In the examination of gilded bronze objects, the analysis of the thickness of the gilding layer, the condition of the substrate and the gilding method is often difficult to manage without taking samples. Yet, the resulting information is vital to research on ancient metalworking and can answer questions of authenticity. The use of nondestructive eddy current techniques can overcome these sampling limitations. Eddy current methods aretypically used in the automotive, power and aerospace industries for flaw inspection and alloy selection. In this method, the interaction of an electromagnetic field induced by a coil probe with the metal surface shifts the probe impedance which is measured and correlated to properties of the metal. Results are presented which show that eddy current testing can be used to characterize a gilding layer on a bronze substrate by variations in thickness or composition. INTRODUCTION An artisan’s choice of gilding technique results in a gold surface layer of specific chemical composition and thickness. The properties of the layer vary depending on the gilding method used. Previous technological studies of gilded objects have identified gilding methods and materials using chemical and microscopic techniques. However, these analytical techniques may be inadequate or inappropriate because the procedures require a sample of the object. Often, only limited sampling of museum objects and archaeological artifacts is acceptable, if sampling is permitted at all. The use of eddy current techniques, nondestructive methods involving a very low power electromagnetic field that does not affect the material as it is being examined, can overcome these sampling limitations. Eddy current methods have been used extensively in industry for the examination of metals. The use of eddy currents, wherein a probe is used to induce an electrical current in a metal, can detect flaws and areas of corrosion, measure the thickness of surface films (conducting and nonconducting) on metal substrates of differing electrical conductivity and distinguish between different alloys, heat treatments or processing methods [1]. This study will extend the use of eddy current techniques into the field of art conservation by using this method to examine gilding on bronze samples made from historic bronze compositions. Previous studies have identified several types of gilding processes on objects, including fire gilding, gilding with foil or leaf, electrochemical replacement and depletion gilding methods [2-4]. Differences in gilding layer thickness for each process are summarized in Table I. Eddy current methods have been applied to art conservation to identify areas of gilding under later paint l
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