This paper presents the data obtained from a study of the alterations and chemical composition of some of the vitreous tesserae in the mosaic depicting the "Annunciation" in the lunette above the Porta della Mandorla in the cathedral of Santa Maria del Fiore in Florence (Italy). Made by the brothers, Domenico and David Ghirlandaio, it is one of the finest examples of wall mosaics ever made; the central portion of the mosaic bears the date of 1490. The mosaic's tesserae, primarily of glassy nature, present a good state of conservation, except for the development of powdery, whitish materials of different consistencies on some of the black and green tesserae. These tesserae are irregularly distributed throughout the mosaic. These superficial phenomena are the result of the formation of different sulfates: potassium calcium sulfate (syngenite) was found on black tesserae, while anglesite (PbSO4) was found on tiles ranging from green to black. The black tesserae with syngenite are characterized by significant amounts of potassium, and all the tesserae with surface deposits of anglesite have high lead contents. The different nature of the patinas, as related to the chemical composition of the underlying tesserae made it possible to select the appropriate restoration procedures. Manual removal of the powder with a brush proved sufficient for the tesserae with surface syngenite, whereas tests with micro-sandblasting technique were needed to remove all the anglesite. With a manual removal of the syngenite based patina the chromatic differences between altered and unaltered tesserae diminished but it was not sufficient to restore the original color and shine; in fact the cleaned tesserae have a grayish cast. Micro-sandblasting, on the other hand, totally removed the lead sulfate, and restored the original colors and did not cause any microcracking in the tessera. A three-year permanent monitoring station will make it possible to assess any further degradation. Different colored glassy tesserae (black, yellow, green, violet) have been analysed and these analyses provided useful information concerning the techniques the artists used to make the mosaic. Gilded and burnished gilded tesserae were found near the classic glass and glass paste tiles.
The alteration phenomena and chemical composition of the vitreous tesserae of the lunette mosaic above the Porta della Mandorla in Santa Maria del Fiore (Florence
Susanna Bracci;Emma Cantisani;Fabio Fratini;
2004
Abstract
This paper presents the data obtained from a study of the alterations and chemical composition of some of the vitreous tesserae in the mosaic depicting the "Annunciation" in the lunette above the Porta della Mandorla in the cathedral of Santa Maria del Fiore in Florence (Italy). Made by the brothers, Domenico and David Ghirlandaio, it is one of the finest examples of wall mosaics ever made; the central portion of the mosaic bears the date of 1490. The mosaic's tesserae, primarily of glassy nature, present a good state of conservation, except for the development of powdery, whitish materials of different consistencies on some of the black and green tesserae. These tesserae are irregularly distributed throughout the mosaic. These superficial phenomena are the result of the formation of different sulfates: potassium calcium sulfate (syngenite) was found on black tesserae, while anglesite (PbSO4) was found on tiles ranging from green to black. The black tesserae with syngenite are characterized by significant amounts of potassium, and all the tesserae with surface deposits of anglesite have high lead contents. The different nature of the patinas, as related to the chemical composition of the underlying tesserae made it possible to select the appropriate restoration procedures. Manual removal of the powder with a brush proved sufficient for the tesserae with surface syngenite, whereas tests with micro-sandblasting technique were needed to remove all the anglesite. With a manual removal of the syngenite based patina the chromatic differences between altered and unaltered tesserae diminished but it was not sufficient to restore the original color and shine; in fact the cleaned tesserae have a grayish cast. Micro-sandblasting, on the other hand, totally removed the lead sulfate, and restored the original colors and did not cause any microcracking in the tessera. A three-year permanent monitoring station will make it possible to assess any further degradation. Different colored glassy tesserae (black, yellow, green, violet) have been analysed and these analyses provided useful information concerning the techniques the artists used to make the mosaic. Gilded and burnished gilded tesserae were found near the classic glass and glass paste tiles.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
