Green degradation products of azurite in wall paintings: identification and conservation treatment

After the restoration of the fourteenth- to fifteenth-century wall painting of San Antonio Abate in the church of San Pietro at Quaracchi near Florence, using the well-known treatment involving application of ammonium carbonate followed by barium hydroxide, a small region of the green degradation products of azurite changed colour to a dark blue. After two years, this blue had again changed colour, showing that the conservation procedure could cause chemical alterations, and that it had produced an unstable material. This study investigated the chemistry of these phenomena, in order to determine whether a conservation procedure based on the application of ammonium carbonate and barium hydroxide is suitable and appropriate in the presence of certain green degradation products of azurite. Fourier transform infrared spectroscopy (FTIR) and X-ray diffractometry (XRD) showed that the main green degradation product was not malachite but paratacamite, a basic copper chloride which forms when there is a source of chloride ions in proximity to the painting. The blue colour that appeared after the treatment was identified as copper hydroxide. Experiments on plaster samples painted with paratacamite showed that this compound is easily soluble in a solution of ammonium carbonate, while treatment with barium hydroxide produced a blue copper hydroxide which is unstable and can form either black copper oxide or, in the presence of chloride ions, a mixture of atacamite and paratacamite. This paper also showed that, when basic copper chlorides are present as green degradation products of azurite, the conservation procedure based on ammonium carbonate and barium hydroxide caused cyclic reactions resulting in the dissolution of paratacamite from the surface, formation of copper hydroxide and degradation of the blue copper hydroxide into either green paratacamite if the chloride ion source is not removed, or to black copper oxide. Great care must, therefore, be taken by conservators and restorers when these green degradation products of azurite are present in a wall painting.