K. Mayerhofer

Brown spot corrosion on historic gold coins and medals

Abstract In 1998, in the Coin Cabinet of the Kunsthistorisches Museum, Vienna, Austria, brown spot corrosion was detected on about 400 gold coins and medals minted in Central Europe between the late 18th and early 20th century. The cause of the brown spots, their temporal and spatial frequency, the development of the corrosion and the most suitable restoration treatments and storage conditions for these objects has been and is still being studied. For the analysis of the historic coins and medals—as well as a set of test ducats produced in cooperation with the Austrian Mint—different analytical methods, i.e. light microscopy, scanning electron microscopy (SEM), measurement of polarisation curves, and secondary ion mass spectrometry (SIMS) were combined. From these investigations and the study of historical sources it could be shown that the brown spot corrosion is due to the inclusion of silver and/or silver/copper containing particles into the surface of the coins and medals. These inclusions are incorporated during the minting process. Afterwards they corrode to silver and copper sulphides owing to the presence of air pollution and objects releasing sulphur containing compounds (i.e. gypsum casts) nearby the historical objects.