M. Járó - L. Költő szerk.: Archaeometrical research in Hungary (Budapest, 1988)

Analysis - GEGUS Ernő, BORSZÉKI János: Investigation of archaeological metal findings by a laser-microspectral analysis method and characterization of results using pattern recognition methods

GEGUS Ernő*, BORSZÉKI János** INVESTIGATION OF ARCHAEOLOGICAL METAL FINDINGS BY A LASER-MICROSPECTRAL ANALYSIS METHOD AND CHARACTERIZATION OF RESULTS USING PATTERN RECOGNITION METHODS Abstract - It is known that a knowledge of the chemical composition of ancient metal findings contributes significantly to their characterization. On the basis of our investigations carried out on Roman bronze, silver and gold objects, it is stated that semiquantitative laser-microspectral analysis is especially suitable for identifying valuable coins, jewels or other articles for personal use, to reveal fakes, and to detect layers or coatings. Data processing of logarithmic intensities of analysis lines in laser spectra by using mathematical statistical pattern recognition methods created the possibility of classifying findings without determining their chemical composition. Details of the working conditions of a Q-switched laser with cross-spark excitation and of a plane-grating spectrograph with enhanced luminous intensity using a cylindrical quartz lense are described here for a practically nondestructive laser-microspectral analysis method elaborated on the basis of our experience. Furthermore, pattern recognition methods are dealt with, suggested for the mathematical statistical data processing of analysis results, which can contribute to the archaeological characterization of findings, and the classification of objects can be supported. 1. Introduction Knowledge of the chemical composition of precious archaeological bronze, silver or gold fmdings often contributes significantly to their characterization and classification [1]. Instead of chemical analysis methods requiring a sampling process, it is more advantageous or often inevitable to use nondestructive analysis methods. Particularly for serial investigations the X-ray spectral analysis method is extensively used, mainly for determining constituents of higher concentrations [2]. In consequence of diffusion and corrosion processes taking place in the surface layer, however, one must take care in utilizing the results [1], especially when determining concentration changes [3,4]. The determination of concentration ratios of minor elements in alloys by means of a suitable laser-microspectral analysis method has been found more generally applicable and more reliable for characterizing ancient coins. The analytical procedure was elaborated for determining the composition of Roman bronze coins [5], and was used for the practically nondestructive semiquantitative analysis of various metal findings. Useful information was obtained with the method in a homogeneity test carried out on the cross-section of several bronze coins [6]. It has been stated that laser shots directed to the edge of a coin gave results suitable for characterizing the* samples, on the basis of the intensity ratios of trace elements. Similarly, semiquantitative chemical composition data of Roman silver coins have been used to identify and classify findings [7,8],, whereas the element distributions are always agnificantly inhomogeneous, and the surface changes are especially large. •Research Group of Analytical Chemistry of the Hungarian Academy of Sciences H-8201 Veszprém, P.O.B. 158. ••Institute of Analytical Chemistry, Veszprém University H-8201 Veszprém, P.O.B. 158.