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Copper-zinc hydroxychlorides

Dpto. de Construcciones Arquitectónicas II, Universidad de Sevilla, EUAT, Avda./Reina Mercedes n° 4 A, 41012 Sevilla, Spain

^* Corresponding author, e-mail: falejan{at}us.es

This paper deals with the characterization of a green paint layer covering ornamental stone elements in the Presbytery of Arcos de la Frontera's Chapel of Mercy (Cádiz, Spain), a 16^th-century building. Optical microscopy, X-ray diffraction, Fourier-transformed infrared spectroscopy and scanning electron microscopy were used to determine that the layer derived from a treatment of the stone (calcarenite) consisting of applying mixtures of gypsum and zinc-copper basic chlorides as paint pigments. Among the different salts, zinc-stabilized paratacamite, clinoatacamite and atacamite could be present. Very scattered dark-maroon speckles appear on the green layer (mainly composed of cuprite). Both these salts and other greenish, synthetic copper-containing compounds may be products deriving from artificial corrosion of brass or other copper alloys.

Key-words: copper-zinc hydroxychlorides, polymorphous compounds, verdigris paint pigments, brass corrosion products, Chapel of Mercy.