Technical studies on the mechanisms of corrosion and state of conservation of two hoards of Persian copper coins (folus) from the Safavid period in Iran

Abstract

A series of corroded Safavid copper coins (folus) dating to the seventeenth century CE and minted in Isfahan were discovered in Hamedan, Western Iran. A selection of these coins was studied to identify the minting process, corrosion morphology and mechanisms, and their state of conservation. Eight coins from two hoards were analysed using metallography, ICP–OES, SEM–EDS and XRD to identify their metal/alloy compositions and microstructures, as well as the chemical compositions of any corrosion layers and products. The results showed that the coins were minted from blanks using thermo-mechanical processing to produce their relief-like surfaces. The corrosion morphology was found to include a two-layered corrosion crust in which the original surface was well-retained, and the corrosion mechanisms identified were internal dissolution and oxidation of copper and the formation of copper (II) compounds over the original surface. The presence of chloride compounds in the corrosion products showed that the coins have been suffering from ‘bronze disease’ so that any preventive conservation must include keeping the relative humidity of their storage and exhibition environments below 45%.