Identification and interpretation of pale blue copper corrosion products formed on metal objects in museum collections

Different from corrosion products on freshly excavated bronzes, pale blue copper corrosion products have often been observed on metal objects in museum collections. These were identified by a combination of scanning electron microscopy - energy dispersive X - ray spectrometry (SEM - EDX), X-ray diffractometry (XRD) and Raman spectrometry as copper organic salts including simple (or basic) copper acetate and copper formate, as well as some complex compounds such as sodium copper carbonate acetate NaCu(CO₃) (CH₃CO₂) and sodium copper formate hydroxide oxide hydrate Cu₄Na₄O(HCOO)₈ (OH)₂, 4H₂O. The formation of this type of copper corrosion product was caused by poor environment of display or storage and/or residual chemicals used in previous conservation treatments. To prevent formation of these compounds, the objects need to be displayed or stored in an organic acid - free environment with controlled low relative humidity. Chemicals used for conservation treatment should completely be removed from the objects to prevent further deterioration. One needs to be especially cautious to treat and preserve objects with composite materials, as different environmental conditions are required for the safe preservation of different materials.