Laser cleaning of a nineteenth-century bronze sculpture: In situ multi-analytical evaluation

Laser cleaning of bronze artefacts is a very promising conservation approach and its wider distribution would benefit from further studies of its advantages and drawbacks. Some important references are available, yet laser cleaning cannot be considered a traditional method for bronze surfaces. In this paper, we report on the evaluation of cleaning tests performed on an important statue of Napoleon, depicted as Mars the Peacemaker, cast by Righetti in 1811 after a model by Antonio Canova. The sculpture is exposed to the outdoor environment in the main courtyard of the Brera Gallery, Milan, Italy. An analytical survey of corrosion products was carried out using X-ray diffraction, micro-Fourier transform infrared spectroscopy (?FTIR), and ?Raman spectroscopy; and scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis was applied to cross-sections. The results showed the ubiquitous presence of brochantite Cu<inf>4</inf>(SO<inf>4</inf>)(OH)<inf>6</inf>. Antlerite Cu<inf>3</inf>SO<inf>4</inf>(OH)<inf>4</inf> is also occasionally present. Residues of an past conservation treatment (Incralac^®) and of proteins were detected by ?FTIR. Laser cleaning tests were carried out with an El.En. Combo laser, both in long Q-switched and short free-running regimes. Evaluation of the effects of cleaning was carried out with a portable microscope and a reflectance colorimeter. Moreover, a portable FTIR spectrometer was used in order to evaluate the presence of coating residues and corrosion products after cleaning. Electrochemical impedance spectroscopy was used in situ on a selection of the tested areas in order to characterize the effect of laser cleaning on the rate of corrosion. A specially designed contact probe was used for measurements in the frequency range 100 kHz-10 mHz both before and after cleaning.