Brownish alterations on the marble statues in the church of Orsanmichele in Florence: what is their origin?

Monica Galeotti,Adriana Rizzo,Daniela Pinna

doi:10.1186/s40494-015-0038-1

Monica Galeotti, Adriana Rizzo

Open Access

https://doi.org/10.1186/s40494-015-0038-1

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Abstract

IntroductionThe paper describes the complex investigations carried out to understand the sources of the brown chromatic changes that have occurred on ten marble statues dated back to the first half of the 15th century and now located in the Church of Orsanmichele in Florence, Italy. When the statues were removed from the outdoor niches for restoration in the 1980s, they appeared covered with a dark brown patina (called ‘bronzatura’) that dated to interventions occurred just after 1789. Archival documents confirmed that they had been carried out to make the marble statues look like bronzes. Because of the removal of the dark patina carried out by the Opificio delle Pietre Dure in the 1990s, the marble almost regained its distinguishing clear tonality even if darker areas remained. After the statues were placed indoors at the end of restoration, brownish discolorations started to appear on their surfaces.ResultsResearch began by using various non-invasive analyses (photographs under ultraviolet illumination, fluorescence lifetime imaging, x-ray fluorescence). The results of UV fluorescence tests and FLIM showed that the fluorescence emission’s distribution map does not have distinctive and homogeneous characteristics in relation to the areas with the discoloration. Therefore, it is not a superficial film, but rather a phenomenon affecting the marble structure. Then we performed invasive analyses on samples from some statues. The results of optical microscopy, ATR-FTIR and Raman spectroscopy on cross sections, pyrolysis-gas chromatography/mass spectrometry on powdered samples from the surfaces and/or the bulk of the marble allowed the identification of the various substances involved in the chromatic alteration. Most important was the detection of weddellite and gypsum on the surface and within the marble. This finding, combined with the presence of lipids inside the stone, suggests that mineralization of treatments have occurred, causing discolorations.ConclusionsEven though the study focuses on the statues from Orsanmichele, their issue nevertheless should not be considered specific to them. Marble discolorations are a widespread phenomenon whose complete understanding needs a complex series of analyses and only the combination of non-invasive and invasive analyses can fulfill this goal.

Highlights

The paper describes the complex investigations carried out to understand the sources of the brown chromatic changes that have occurred on ten marble statues dated back to the first half of the 15th century and located in the Church of Orsanmichele in Florence, Italy
Research began by using various non-invasive analyses
The present paper describes the complex investigations carried out to understand the sources of the brown chromatic changes that have occurred on ten marble statues dated back to the first half of the 15th century and located in the Church of Orsanmichele in Florence, Italy

Summary

Introduction

The paper describes the complex investigations carried out to understand the sources of the brown chromatic changes that have occurred on ten marble statues dated back to the first half of the 15th century and located in the Church of Orsanmichele in Florence, Italy. When the statues were removed from the outdoor niches for restoration in the 1980s, they appeared covered with a dark brown patina (called ‘bronzatura’) that dated to interventions occurred just after 1789. Because of the removal of the dark patina carried out by the Opificio delle Pietre Dure in the 1990s, the marble almost regained its distinguishing clear tonality even if darker areas remained. The study of marble discolorations, typically ranging from black to grey and from brown to orange, is a relevant subject in the field of conservation of cultural heritage. When stone artefacts have metal components, either as part of the object or as elements inserted into the stone (bars, rods or pins), stains can be produced as a result of metal ions released in the stone [1]

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