Metallurgical investigations of Indo-Sasanian Copper-Silver alloy coins of Gurjara-Pratiharas dynasty

Five Indo-Sasanian copper-silver alloy coins were examined to determine the chemical composition and fabrication route. Based on iconography, the investigated coins were dated in the range 8th to 9th century CE. The chemical composition of the coins confirmed that the coins were made of copper-silver alloy. The percentage of silver was found to be in the range from 14 wt.% to 16 wt.%, and other elements iron and lead were present as impurities. There was no significant difference in the chemical composition from the surface to the center. Chlorine was detected in the localized green corrosion, which confirms the presence of active corrosion, and coins must be conserved by giving proper conservation treatment. The formation of unusual active corrosion compounds i.e. paratacamite was identified by the XRD, and this may be formed due to the exposure of coins to river water and soil. Optical microscopy revealed the dendritic and dual-phase structure, and the presence of dendrite showed that the coins were manufactured by the casting. It was confirmed from the optical microscopy that coins were not subjected to heat treatment and deformation. The microstructure consists of dual phases, in which the white phase is silver-rich, and the black phase is rich in copper. The morphology and chemical composition of the corrosion compound formed on the surface of the coins mainly indicated the formation of copper oxide with different morphology.