The manufacturing and plating technology used in the production of mid-3rd/ 4th century AD Roman coins - An Analytical Study
The integrated approach utilising documentary evidence, numismatics, analyses of the coins and the experimental data have combined to present a powerful model for the monumental changes that occurred in coin and metallurgical technology in the 3-4th Centuries AD. The primary objective of this research was to investigate the manufacturing technology of the nummus, the new coin introduced by Diocletian in his monetary reform (293/4AD). For the production of these coins, copper based quaternary copper-tin-lead-silver alloys were used and their surface was covered by thin silver plating. Later, the coin sample was extended to include dates before and after this period aiming to study if there were similarities in the production of these coins with the technique used for the nummi production. Could the date of the introduction of this technology be established and did it continue after the nummi? In this study 503 coins issued from 240 to 395 AD were examined. Optical microscopy SEM examination of the nummi revealed that the silver plating survived mainly in protected areas, between the letters in their inscriptions or in other features. Results from the metallographic examination showed that the plating was very difficult to trace. Its thickness was approximately a few microns. The examination of the antoniniani (coins issued before the nummi from 240-393/4 AD) showed that during Gallienu’s reign the silver content dropped drastically (less than 5%) and the applied silver layer was introduced as a plating method. During this period quaternary alloys (Cu-Sn-Pb-Ag) replaced the binary Cu-Ag alloys in coin production. The examination of the coins issued during the 4th century showed that this technology was in use for 100 years after its first introduction. A review of ancient sources and historic literature indicated that the most probable method that might have been used for the production of the plating was amalgam silvering. Combined application of LA-ICP-MS and EPMA analyses of the coins proved for the first time, that mercury was correlated with silver in the plating area. Much higher concentrations of mercury occurred on the surface than in the silver rich pools in the coin core. This evidence strongly implies that the possible method for the production of the plating was amalgam silvering. However, little is known about early amalgam silvering. Prior to this research no evidence existed for its earliest widespread use, nor was there any evidence for its early technology. A series of experiments were undertaken to investigate the technology. The results from these experiments demonstrate that a layer much lower in mercury (less than 0.5%) than previously achieved, was prepared, with a thinness (2 microns approximately) comparable to those observed in the coin plating layers.
Silver amalgam plating, Late Roman coins, Copper-tin-lead-silver (quaternary) alloys, Metallography, SEM-EDS, LA-ICP-MS, EPMA, Analysis
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