Analyses of a Green Copper Pigment Used in a Thirteenth-Century Wall Painting

A thirteenth-century sacred and profane wall painting has been rediscovered during renovation work in a city castle in Krems, Lower Austria. A frieze with alternating blue circles and green diamonds attracted attention to the unusual green colour. Several analyses by means of light microscopy, scanning electron microscopy-energy-dispersive X-ray analysis and laboratory and synchrotron X-ray diffraction show that the green colour stems mainly from clinoatacamite, a copper trihydroxychloride polymorph of Cu-2(OH)(3) Cl which has probably been synthesized for this masterpiece. The pigment has been mixed in a lime matrix.