Further metallurgical analyses on silver coins of Trajan (AD 98–117)

Abstract

Within an interdisciplinary project, preliminary results of which were published in 2007, a group of 65 silver coins (denarii) of Trajan, evenly distributed over the entire span of his reign (2nd to 6th consulate), as well as 3 coins from the reign of his predecessor, the emperor Nerva (AD 96-98), had been acquired on the coin market. These pieces could be cross-sectioned in order to carry out analyses. Measurements were performed with m-XRF and m-SRXRF to check the fineness of the denarius alloy as well as to investigate if the coins of the different consulates showed distinct traits concerning the main impurities which could suggest a change of ore sources. Furthermore, it was examined whether the influx of precious metals into the Roman treasury after Trajan's Dacian campaigns (AD 101/102 and 105/106) could have had any repercussions on the composition of the denarius alloy; comparative data was provided by the analysis of nine imitations of Roman Republican denarii presumably struck in Dacia. Documentation of the cross-sections was performed with SEM/EDX in order to clarify details concerning the specific microstructure of the coins. An enrichment of Ag of approx. 100 to 200 mm in the near surface regions of most of the coins could be observed, which is due to corrosion effects. Previous metallographic analyses done by several scholars had been inadequate to clarify the central numismatic and economic questions. With the presented analyses, it was possible to correct the results of the earlier research and to provide, for the first time, a secure basis of data for the investigation of the development of the silver alloy used for denarii in Trajan's reign.

A Multi-Analytical Approach on Silver-Copper Coins of the Roman Empire to Elucidate the Economy of the 3rd Century A.D.

“…Looking at the overall data ( Table 3 ), 5 of the 12 coins have a lead content greater than 0.5% indicating low grade silver. Bismuth is an impurity in argentiferous galena, which was often removed by the purification process: this agrees with the low concentrations found in the coins [ 17 ].…”

“…Sn and Cu are positively correlated (0.56), as well as Ni and Cu (0.68), supporting the hypothesis of re-melted brass or bronze for coining, instead of pure copper [ 4 ]. On the other hand, Ag shows a positive correlation with Bi (0.82), as well as Pb and Bi (0.50), as foreseeable with the use of argentiferous galena as a mineral for silver extraction [ 17 ].…”

Análisis arqueométrico de denarios romanos del tesoro de Llíria (Valencia, España) mediante espectrometría XRF portátil

“…La Figura 4 muestra la relación entre las concentraciones de Ag y Cu en los denarios analizados. La correlación negativa entre Cu y Ag es un indicio de que el cobre podría haber sido añadido intencionadamente a la plata durante el proceso de copelación (Rodrigues et al, 2011). Por el contrario, las concentraciones de Ag y Pb no están correlacionadas, lo que indica que la presencia de plomo no ha sido añadida intencionalmente (Del Hoyo-Meléndez et al, 2015).…”

Atomic spectrometry update—X-ray fluorescence spectrometry

“…A range of Sn/Cu mass ratios from 0.0101 to 0.0111 (9%) was detected and one coin from Luxemburg showed differences of up to a factor of 1.5 over its surface showing that even modern metal artifacts display differences in composition that can be correlated with their production and place of use, an observation that is relevant to variations found in archaeometallurgical studies. Rarely is it possible to make cross sectional measurements on coins of archaeological interest, but Rodrigues et al 285 did so using m-XRF and m-SRXRF to check the fineness of the alloy used to manufacture silver denarius coins of the Trajan period (98)(99)(100)(101)(102)(103)(104)(105)(106)(107)(108)(109)(110)(111)(112)(113)(114)(115)(116)(117) and to see if the presence of impurities would allow identification of ore sources. Additional measurements were undertaken by ED-SEM and the authors found that the Ag content was enriched in the 100 to 200 mm surface region due to the effects of corrosion, an observation that was made in a further contribution from this group 286 and attributed to the depletion of copper in surface layers to form a typical green surface patina.…”