The deterioration of metamorphic serpentinites used in historical architecture under atmospheric conditions.

Dark metamorphic stone has traditionally been employed for decorative purposes and for architectural cladding, but after prolonged outdoor exposure the characteristic surface colour tends to fade. Oira stone, a durable lithotype belonging to the ultramafic rock group, was employed during the 19th century restoration of the Renaissance facade of the cathedral in Monza, Italy, and the features and conservation state of the stone were studied following more than a century of exposure as part of a pilot conservation project. A marked discolouration characterised by diffuse fading of the typical intense dark colour of Oira stone was observed on all exposed surfaces of the facade, irrespective of its location. The formation of a fragile superficial layer with scaling and detachment was also observed. Analysis of samples of the stone showed that the colour variation is associated with chemical alteration, resulting in selective leaching of Mg ions from the phyllosilicate microstructure of the mineral lizardite to a depth of about 500 micrometres. Simultaneous surface recrystallisation of mainly low-order SiO2 also occurred. Accelerated laboratory ageing tests were also conducted on representative serpentinite samples which provided comparable results in relation to colour alteration, but with less intense increase in lightness.
Dark metamorphic stone has traditionally been employed for decorative purposes and for architectural cladding, but after prolonged outdoor exposure the characteristic surface colour tends to fade. Oira stone, a durable lithotype belonging to the ultramafic rock group, was employed during the 19th century restoration of the Renaissance facade of the cathedral in Monza, Italy, and the features and conservation state of the stone were studied following more than a century of exposure as part of a pilot conservation project. A marked discolouration characterised by diffuse fading of the typical intense dark colour of Oira stone was observed on all exposed surfaces of the facade, irrespective of its location. The formation of a fragile superficial layer with scaling and detachment was also observed. Analysis of samples of the stone showed that the colour variation is associated with chemical alteration, resulting in selective leaching of Mg ions from the phyllosilicate microstructure of the mineral lizardite to a depth of about 500 micrometres. Simultaneous surface recrystallisation of mainly low-order SiO2 also occurred. Accelerated laboratory ageing tests were also conducted on representative serpentinite samples which provided comparable results in relation to colour alteration, but with less intense increase in lightness.