Silver plating technology of the late 3rd century Roman coinage
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Abstract
The discovery in 1998 of a late third century AD coin hoard at Rogiet (Monmouthshire) provided the opportunity for a technical study of the plating of the later Roman silver coinage which carried a very thin silver wash over a copper core. Optical metallography and scanning electron microscopy were carried out on taper sections on eight coins. Their plating was rarely thicker than 1-2μm and often too thin to be visible in cross section. There was no eutectic layer or diffusion zone at the interface between plating and core. The plating penetrated into cracks and crevices of the copper. It was not connected to the small amounts of silver phase within the copper core. From a comparison of these results with replication specimens it was concluded that the plating was applied in the form of a silvering paste, possibly based on silver chloride. Hot-dipping into molten silver chloride proved to be impractical in the replication experiments and was ruled out. No mercury was detected on any of the coins with EDX analysis in the electron microscope.
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