Micro-Raman spectroscopy for the characterization of artistic patinas produced on copper-based alloys

In order to contribute to the improvement of restoration and conservation sciences of metallic artefacts belonging to Cultural Heritage, a wide research has been started on the study of artificial (artistic) and natural (corrosion or alteration) patinas of copper-based alloys. Micro-Raman spectroscopy (mRS) coupled with scanning electron microscopy (SEM) equipped with energy Dispersive X-Ray Spectroscopy (EDXS) were applied for the characterization of a number of patinas made on metallic substrate reproducing the typical copper-based alloys used for statuary. A non alloyed copper sheet has been also used to simulate the architectural cover typically used after a specific cold spray coating. To reach these goals, a number of patinas were experimentally produced in our laboratory using the torch technique and reactive solutions based on water as a solvent containing, respectively, copper nitrate, iron nitrate and potassium sulfide (‘liver of sulfur’). The only exception is a sample reproducing the architectural cover of a recent building nearly completely covered with copper sheets coated with copper salts. All the produced patinas were aged in a salt spray chamber and then studied with mRS, SEM and EDXS in order to understand their evolution between the two known terms and therefore the reactions involved in all patinas. mRS allowedus todetect asmain constituents of the ‘artistic’ patinas, before and after ageing: copper oxides (Cuprite and Tenorite), copper nitrates (Rouaite and Gerhardtite), copper chlorides (Botallackite, Atacamite and Paratacamite), a copper sulfate (Posnjakite), iron oxides (Magnetite and Hematite) and an iron hydroxide (Limonite).