The hidden secrets of copper alloy artifacts in the Athenian Agora

Three copper corrosion products on copper alloy objects in the Athenian Agora collection at the British Museum were analysed by X-ray Diffraction (XRD), Fourier transform Infrared (FTIR) spectroscopy, scanning electron microscopy coupled with energy-dispersive analysis X-ray (SEM-EDAX), and ion chromatography (IC). The blue/green corrosion matrix was identified as sodium copper carbonate acetate, white crystals as sodium acetate trihydrate, a turquoise blue corrosion as spertinite (copper (II) hydroxide) and dark brown corrosion as a mixture of cassiterite and cuprite. The factors which may have led to the development of these compounds include cleaning and stabilisation agents such as sodium hydroxide and sodium sesquicarbonate, and acetic acid from wooden storage materials. The influence that the equilibrium Relative Humidity (eqRH) of these compounds may have on continued deterioration leads to a consideration of methods for their removal.