Energy Dispersive X-Ray Spectrometry of Gold-Silver-Copper Standards for Comparison with Historic Gold Objects
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ENERGY DISPERSIVE X-RAY SPECTROMETRY OF GOLD-SILVER-COPPER STANDARDS FOR COMPARISON WITH HISTORIC GOLD OBJECTS S.M. White, R.J. Koestler, C. Blair, and N.Indictor* Metropolitan Museum of Art, Objects Conservation Department, New York City 10028 *Brooklyn College, CUNY and the Grad. Prog., Chemistry Dept., Brooklyn, NY 11210 ABSTRACT Seven ternary mixtures of gold(80-95%)-silver(4-15%)-copper(1-15%) were prepared as standards for the determination of elemental composition by energy dispersive X-ray spectrometry (EDS). Two geometric forms (flat and oblate) of the standards were prepared for comparison to the analyses of historic gold objects. Surface analyses were performed. Polished sections of the standards were also analysed Comparison between two methods for collecting EDS data is reported. The analyses are discussed in terms of developing a generalized methodology for estimating elemental composition of museum artifacts. INTRODUCTION A common and important problem facing museum analytical laboratories is the need for elemental analyses of metal objects which may not be sampled. Analyses may be used to characterize manufacturing techniques, give insight into the ores from which the object was fashioned, provide information on corrosion pathways the object has experienced, reveal cleaning or restorations to which the object has been subjected, and discover subtle, fingerprint-like details such as the presence of trace elements and surface enrichment [1]. The various probe techniques such as XRF, EDS, PIXE, etc. analyze x-ray emission from the surface of the sample (object). The portion of the surface addressed is, in general, very small (ca25gim 2-1x10 4jim2 ) and the subsurface portion of the object (sample) remains invisible to the procedure. Depending on the metallic elements on the surface and the probe technique employed, the depth of analysis is generally taken to be less than 4 lim, and, depending on the accellerating voltage of the electron beam, is generally in the range of ma 0.5 gim for gold objects[2-4]. The representativeness of the analysis becomes problematic in two distinct ways: 1) The surface composition may not represent the overall composition of the object. 2) The spot (area) analyzed may not represent either the surface composition or the overall composition of the object. ['Overall composition' is used to denote the percent composition of metals in the entire object, i.e., the average values of a large number of analyses performed at the surface and the interior.] The first difficulty has no remedy in surface analysis for real objects unless they happen to be thin sheets (Qa 1 gim) or homogeneous. The second difficulty may be obviated by multiple analyses at a variety of places on the object's surface. Only the problem of surface composition can be solved within reasonable confidence limits. If a cross section of the object is available, some insights may be gained concerning the comparison between surface analyses and overall composition, especially informative if the object has undergone s
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