Characterisation of artificial patinas on bronze sculptures of the Carlo Bilotti Museum (Rome)

Abstract

Two bronze sculptures, Ettore e Andromaca, a reproduction of a plaster model by Giorgio de Chirico, and Cardinale, a cast made from an original by Giacomo Manzù, stand outside the Carlo Bilotti contemporary art museum in Villa Borghese park (Rome). The composition of the artificial brown patina present on the statues’ surface, which was applied for aesthetic purposes, is unknown. This paper reports analysis carried out to identify the composition of the artificial patina and describe the corrosion products formed in outdoor conditions. Raman spectroscopy and scanning electron microscopy equipped with X-ray microanalysis were performed on sample fragments and powder scrapings taken from the bronze statues. X-ray powder diffraction was used whenever possible and subject to conservation priorities. Our data revealed, in the artificial brown patina, the formation of copper oxides (cuprite and tenorite) on the surface of both sculptures as possible result of oxidisation treatments performed with a blowtorch before the artificial patination process began. Furthermore, a copper nitrate (gerhardtite) was identified as an ingredient in the preparation applied to the bronze surfaces. The green areas revealed the presence of corrosion products as copper sulphate hydroxide (brochantite) and copper sulphate-chloride (connellite), which form under acid rains conditions

Artistic and Laboratory Patinas on Copper and Bronze Surfaces

“…Copper nitrates and FeCl3 are in fact often used in the artificial patination process [51]. In the work of both Bracci et al [59] and Casanova Municchia et al [56], relevant amounts of natural corrosion products (e.g., brochantite and atacamite) were detected, confirming that the artificial corrosion layers tend to transform upon exposure to the atmosphere.…”

“…One of the reasons could be that the distinction between artistic artificial patinas and natural corrosion layers on ancient works of art is quite difficult [54], as the composition of natural and artificial corrosion products is often the same [44]. Moreover, artificial patinas evolve with time, taking part in the natural corrosion process and changing their chemical compositions [55,56]. Therefore, few works in recent decades have reported on the compositional characterisation of corrosion layers on copper-based artefacts of supposed artificial production.…”

“…Bracci et al [59], as an example, identified gehrardtite (a basic copper nitrate), copper phosphates and copper sulphide (probably chalcocite) on a contemporary bronze sculpture realised in 1988 and exposed to outdoor environment. Casanova Municchia [56] characterised instead the composition of the artificial patinas (mainly dark-brown) on two contemporary sculptures of the Carlo Bilotti contemporary art museum in Rome. On the first sculpture, the dark patina resulted constituted by cuprite and tenorite, while on the other cuprite was associated with copper sulphur, employed probably to obtain a darker colour shade.…”

“…In this work, in particular, an artificial ageing in spray chamber of the artistic patinas was performed, and the changing in their composition upon such treatment was assessed. In addition, in several works, potassium sulphide associated with the heating of the metallic substrate, have been employed to realise black patinas, according to traditional recipes used also in modern artistic foundries [49,52,56,[60][61][62][63].…”