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Corrosion potential study of the Western Australian 1911 Coronation medal

Paper Number: 28
Author/s: Walter R Bloom2 and Ian D MacLeod1
Organisation: 1 Western Australian Maritime Museum, Fremantle, Australia 2 School of Engineering and Information Technology, Murdoch University, Murdoch, Australia

Abstract

The analysis of coins and medallions should only involve methods that do not physically interfere with the actual objects. Much of the collecting and commercial value of numismatic items resides in the quality of their surfaces. Previous studies (MacLeod & Ritchie 1981) had determined that valuable information regarding the nature of the surfaces and underlying alloys of suspected forged coins can be obtained by monitoring the corrosion potential over periods of several days. Since there was uncertainty in numismatic circles regarding the true nature of the Western Australian 1911 Coronation medals, which are variously listed as having been struck in silver or silvered bronze, it was decided to monitor their corrosion potentials (Ecorr) to see if this approach could distinguish between the two materials. The density of the medallions could not be determined as they were intimately bound with silk ribbons and pin bars.

Eleven examples of the King George V medallions were examined and compared with a standard silver-copper alloy from a Western Australian Museum medal. Analysis of the kinetics of the Ecorr plots showed typical sigmoidal curves for silver plated objects. Solid alloys showed Ecorr plots which were linear with the square root of immersion time. The nature of the plated and solid surfaces was compared with results with those obtained using a hand-held (bench-mounted) X-Ray Fluorescence (Bruker Tracer III) instrument for elemental analysis. Data from automatic profilometer studies determined the thickness of the electroplated layers.