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

Abstract

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)

Raman spectroscopy characterization of 10‐cash productions from the late Chinese emperors to the Republic

“…Figure compares the Raman spectra of cuprite and tenorite with that of coin Peiyang #01. The spectrum of cuprite shows two main bands at 114 and 220 cm −1 , accompanied by shoulders at 145 and 186 cm −1 , which are in agreement with literature data . Weak, broad bands also appear at 420, 525, and 625 cm −1 .…”

“…Weak, broad bands also appear at 420, 525, and 625 cm −1 . These last bands can be assigned to Cu 2 O amorphous or finely divided, and characterize defective cuprite forming native passive layer on copper/bronze objects . The spectrum of tenorite was dominated by the band at 297 cm −1 , accompanied by weak signals at 346 and 631 cm −1 , also in agreement with literature …”

“…In a first series of experiments, the spectra of Chinese 10‐cash coins were recorded on at least six different regions on each coin in order to obtain information on the composition of the external surface of the coin. Although obscured by background fluorescence, such spectra presented well‐defined cuprite bands at 90 and 215 cm −1 , accompanied by signals at 145, 525, and 620 cm −1 . The principal tenorite band at 297 cm −1 was weak or almost absent whereas the signal at 631 cm −1 possibly appeared as broadening the cuprite band at 625 cm −1 .…”

“…Due to its sensitivity and noninvasive character, Raman spectroscopy is being increasingly used in the study of artistic and archaeological objects . In particular, Raman spectroscopy has been used for identifying corrosion products in copper/bronze and other metals and characterizing artistic patinas produced in copper‐base alloys …”

A multi-analytical approach to monitor three outdoor contemporary artworks at the Gori Collection (Fattoria di Celle, Santomato, Pistoia, Italy)

“…However, the other compounds such as gerhardtite, phosphates and copper sulfide were probably applied by the foundry, according to the common practice for patination in contemporary art production [36][37][38][39]. This hypothesis was also suggested by the morphology of the patinas, which were superficial and had not penetrated the alloy [35].…”