The microstructures of silver-copper alloys from archaeological and historical contexts have been of particular interest since age-related changes at grain boundaries were first mooted as an indicator of antiquity and authenticity. Subsequent discussion has focused on how such structures might be reproduced by appropriate heat treatments but there was only limited experimental investigation of these precipitation phenomena. A second strand of interest has been the embrittlement of archaeological silver by segregation of impurities to grain boundaries. More recently industrial interest has developed in silver-copper alloys because of their use as solders and in electrical contact, and a growing number of papers on sterling and other silver alloy microstructures is being published.
To interpret the microstructures of ancient and historic silver the key question is how to distinguish between the respective contributions of manufacture, age and the environment. This paper will describe and discuss a series of heat treatment experiments on wrought Britannia and Sterling silver and also on cast Sterling, the microstructure of cast silver being hitherto a rather neglected topic. The simple eutectic silver-copper system can exhibit a variety of precipitate morphologies and these have been characterised using optical microscopy, scanning and transmission electron microscopy, electron backscatter diffraction, and microhardness and nanoindenter testing. As a test case for discriminating between the effects of manufacture and age a series of medieval Islamic silver coins with a range of mint technologies has been examined in detail and the results presented here. The data will also be used to highlight the limits within which age-related modifications of the microstructure can be expected to be observed
