Colour measurement procedures on outdoor 3D artworks: a case study

Colour in the Cultural Heritage field is one of the parameters currently used to document artworks appearance, to support and to monitor conservation / restoration procedures on them. Studies dealing with colorimetric measurements aimed to document and support interventions on works of art have to be correctly programmed, since they have to be performed before, during, and after any interventions. However, one of the main drawbacks of surveys based on colorimetric analysis is linked to the repeatability and reproducibility of the measurements. Indeed, the definition of reliable methods for identifying the location of investigated areas is essential, particularly when the analysis has to be repeated on the same spots at a later date. This fact is of extreme importance and it is not an easy task to achieve mainly when chromatic variations have to be monitored on 3D artworks, such as statues and/or ornamental stones. In this paper we will report a study on colour measurements performed during a biennial diagnostic survey on the Ratto della Sabina (1583), a marble sculpture by Giambologna, placed outdoor under the Loggia dei Lanzi in Piazza della Signoria, Florence. The diagnostic survey, performed in the 2011-13, was aimed at evaluating the most appropriate protective treatment of the marble and was coordinated by Magnolia Scudieri, SSPSAE of Florence, and Mauro Matteini with the technical support of Alberto Casciani. This study illustrates the main results of the colorimetric analysis, with a focus on methodological issues, such as the repeatability of colour measurements due to the modelling of the figures, the access to the areas to be investigated, and the weather conditions. Finally, limits and advantages of the application of this technique on the study of 3D artworks will be described.