The color of outdoor stones is never stable in time, since they are easily attacked by atmospheric agents, thus undergoing different kinds of reactions. The degradation of stone evolves over time, and can be accelerated by the presence of different agents (chemical, physical and/or biological). In particular water, acidic substances and atmospheric particulate favor chemical degradation; the action of wind and thermal shocks facilitates mechanical disruption, while the activity of micro-organisms, algae and fungi are the basis of biological degradation [1-2]. Since the color of a stone object largely affects its aesthetic aspect, it is important to define protocols for measuring the color of the surface in a given time, in order to monitor possible color changes, and to choose the best conservation treatments, if needed. Colorimetric analysis, widely used in several applicative areas including the art conservation field, may offer an adequate solution to this problem. This method allows determination of the color parameters of an object by measuring the visible spectral reflectance of the object itself. The measurements are carried out non-invasively using portable colorimeters or spectrophotometers, supported by sophisticated software which allow a rapid and reproducible determination of the color parameters of interest .[3-10]. Studies dealing with colorimetric measurements aimed at documenting and supporting interventions on works of art have to be correctly programmed, since they need to be performed before, during, and after any interventions. However, one of the main drawbacks of surveys based on colorimetric analysis is connected with the repeatability and reproducibility of the measurements. This fact is of extreme importance and it is not an easy task to achieve especially when chromatic variations have to be monitored on 3D artworks, such as statues or ornamental stones. In this paper we will report a study on color measurements performed during a biennial diagnostic survey on the Ratto della Sabina (1583), a marble sculpture by Giambologna, that is located outdoors, under the Loggia dei Lanzi in Piazza della Signoria, Florence (Fig. 1). The sculpture underwent restoration in 2001. Since then, it has been constantly monitored with diagnostic surveys for the purposes of assessing its real condition and identifying the best cleaning/protective treatments. The last monitoring program took place in the years 2011-2013 and was coordinated by Magnolia Scudieri, SSPSAE of Florence, and Mauro Matteini with the technical support of the restorer Alberto Casciani. In the framework of this survey, which included several analyses (such as photographic documentation, Fourier Transform Infrared Spectroscopy, Micro-photogrammetry and water absorption measurements), the IFAC-CNR team carried out a campaign of colorimetric measurements aimed at defining the most appropriate cleaning treatment of the marble, and supporting the conservation intervention by measuring the color variations associated to the cleaning tests. This work reports the main results of the colorimetric analyses, with focus on methodological issues that have been tackled, such as the problem of repeatability of color measurements due to the modelling of the figures, the difficult access to the areas to be investigated, and the influence of weather conditions.
Color measurement procedures on 3D artworks: a case study
Veronica Marchiafava;Costanza Cucci;Marcello Picollo
2015
Abstract
The color of outdoor stones is never stable in time, since they are easily attacked by atmospheric agents, thus undergoing different kinds of reactions. The degradation of stone evolves over time, and can be accelerated by the presence of different agents (chemical, physical and/or biological). In particular water, acidic substances and atmospheric particulate favor chemical degradation; the action of wind and thermal shocks facilitates mechanical disruption, while the activity of micro-organisms, algae and fungi are the basis of biological degradation [1-2]. Since the color of a stone object largely affects its aesthetic aspect, it is important to define protocols for measuring the color of the surface in a given time, in order to monitor possible color changes, and to choose the best conservation treatments, if needed. Colorimetric analysis, widely used in several applicative areas including the art conservation field, may offer an adequate solution to this problem. This method allows determination of the color parameters of an object by measuring the visible spectral reflectance of the object itself. The measurements are carried out non-invasively using portable colorimeters or spectrophotometers, supported by sophisticated software which allow a rapid and reproducible determination of the color parameters of interest .[3-10]. Studies dealing with colorimetric measurements aimed at documenting and supporting interventions on works of art have to be correctly programmed, since they need to be performed before, during, and after any interventions. However, one of the main drawbacks of surveys based on colorimetric analysis is connected with the repeatability and reproducibility of the measurements. This fact is of extreme importance and it is not an easy task to achieve especially when chromatic variations have to be monitored on 3D artworks, such as statues or ornamental stones. In this paper we will report a study on color measurements performed during a biennial diagnostic survey on the Ratto della Sabina (1583), a marble sculpture by Giambologna, that is located outdoors, under the Loggia dei Lanzi in Piazza della Signoria, Florence (Fig. 1). The sculpture underwent restoration in 2001. Since then, it has been constantly monitored with diagnostic surveys for the purposes of assessing its real condition and identifying the best cleaning/protective treatments. The last monitoring program took place in the years 2011-2013 and was coordinated by Magnolia Scudieri, SSPSAE of Florence, and Mauro Matteini with the technical support of the restorer Alberto Casciani. In the framework of this survey, which included several analyses (such as photographic documentation, Fourier Transform Infrared Spectroscopy, Micro-photogrammetry and water absorption measurements), the IFAC-CNR team carried out a campaign of colorimetric measurements aimed at defining the most appropriate cleaning treatment of the marble, and supporting the conservation intervention by measuring the color variations associated to the cleaning tests. This work reports the main results of the colorimetric analyses, with focus on methodological issues that have been tackled, such as the problem of repeatability of color measurements due to the modelling of the figures, the difficult access to the areas to be investigated, and the influence of weather conditions.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
