Experimental Study of Chalconatronite: From Its Identification to the Treatment of Copper Alloy Objects

Abstract

On the occasion of the reopening of the Dobrée Museum (Nantes, France), two statuettes of Egyptian origin, representing Harpocrate and Isis, were studied to shed light on the presence of the blue-green efflorescence on their surface. The efflorescence on the Harpocrate statuette was identified as being chalconatronite, while that which was present on the Isis statuette corresponded to sodium copper formate/acetate, probably due to the evolution of chalconatronite in an environment containing VOCs. The efflorescence appeared to be sensitive to the cyclic variation in relative humidity whereas it seemed stable. An experimental curative treatment to halt the reappearance was carried out. A series of pure water baths extracted a significant quantity of sodium. The treatment appeared effective and reduced the risk of a recrudescence of the efflorescence for both statuettes. However, when the efflorescence was dissolved on the Isis statuette, other compounds appeared to react with water, leading to acidification and a potential reaction with the lead in the alloy. A layer of lead carbonate/acetate on the surface appeared. The objects were then dried and protected with a highly concentrated acrylic varnish. They are currently being monitored to identify any new efflorescence that may appear during display.