Hydrochloric Acid Concentration, Exposure Time and Material Loss in Cleaning Early Uninscribed Copper Cast Coins

Abstract Severe corrosion obscures the surface symbols of Early Uninscribed Copper Cast Coins (EUCCC), and mechanical cleaning could not remove compact encrustations from coins from Malpar, India. Five coins were characterised by X-ray fluorescence and X-ray diffraction, which indicated copper with surface cuprous oxide and siliceous deposits. Each coin was then cleaned in a different hydrochloric acid (HCl) concentration (10–50%) in 10-minute stages, with weight loss recorded and dissolved copper measured by atomic absorption spectroscopy. Total weight loss ranged from 5.75% to 12.61% and copper loss from 2.43% to 6.89%, both highest at 30% HCl; at 50% HCl the visual end point was reached in one stage. Because each concentration was tested on a single coin, concentration effects cannot be separated from coin-to-coin variation. After rinsing and Paraloid B-72 coating, symbols were legible and no macroscopic damage was seen. Findings are exploratory and need replicated, microscopic validation.