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

Abstract

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 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. Results: Research 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.

Research of a complex fire-induced pollution on the marble relief from the Pushkin state museum of fine arts collection

“…Fundamental restoration of sculptures is impossible without conducting integrated studies of contamination composition and exhibit surface condition and accurate identification of the materials of sculptures, bas-reliefs and coatings. Studying the contamination before cleaning marble objects has been common practice in recent years [1][2][3][4][5][6][7][8][9][10][11][12]. Most often, these are pollutions associated with the environment [1][2][3][4][5][6][7][8][9] or appeared due to contact with other materials [10][11][12].…”

“…Studying the contamination before cleaning marble objects has been common practice in recent years [1][2][3][4][5][6][7][8][9][10][11][12]. Most often, these are pollutions associated with the environment [1][2][3][4][5][6][7][8][9] or appeared due to contact with other materials [10][11][12]. A wide range of methods is used to study pollutants: IR-spectroscopy [1][2][3][4][5][6][7][8], optical microscopy [1,2,5,7], scanning electron microscopy [1][2][3]5], gas chromatography [7][8][9], energy-dispersive X-ray microanalysis [1,5], X-ray fluorescence [7] and others.…”

Effective cleaning of copper stained calcareous stone

“…Discolouration due to metal corrosion is observed when composite artefacts of uncoated metal and porous stone are located in an outdoor environment [2][3][4]. The corrosion mechanism observed in connection with uncoated bronze statues and monuments is generally well understood [1,2,5].…”

Effective cleaning of rust stained marble

“…Although marble is a relatively stable material, the desired white surface is unfortunately prone to tarnishing when used in outdoor environments [1]. One of the major sources of tarnishing is iron.…”

“…One of the major sources of tarnishing is iron. In addition to the oxidation of internal iron compounds present in stone like pyrite (FeS 2 ) and siderite (FeCO 3 ) [1,2], contact with iron-rich ground water when marble is used in, for example, garden fountains, results in severe and unsightly discolouration [3]. Another cause is the proximity to iron metal, which is oxidized by air in the presence of rain.…”