Portable instrumentation for the study of wall paintings: the case of S. Maria ad Undas in Idro (BS-Italy)

The parish church of Santa Maria ad Undas, on the Idro Lake shores, externally looks quite simple and austere while the inner single nave possesses a rich decorative apparatus, currently mostly covered by a white lime layer applied during the seventeenth century plague period. Its unique characteristic is the painted masonry altar, discovered in 1955 after the removal of an eighteenth-century altarpiece. It is a very rare artifact comparable only with two other altars, both located in the Brescia area. The style and the oriflamme of San Bernardino of Siena suggest that such decoration was realized shortly after 1443, when the symbol was formally canonized. Being the altar immovable a preliminary in-situ study was mandatory to plan possible consequent laboratory analysis to finally assembly a valorization project also including a restoration and conservation plan. A set of three non invasive techniques was used to characterize both the author palette and neoformed secondary phases that may be present: Multispectral Imaging, Raman spectroscopy and Reflectance Spectroscopy (FORS). Images acquired under UV light, in both fluorescence and reflectance mode, gave useful information about the presence of organic residual materials due to the presence of a binding medium used for the application of currently lost gilding details [1]; at the same time the readability of some details was improved with respect to what observable in visible light. Images collected in Reflected Near Infra Red (RNIR) light led to formulate initial hypothesis about some of the pigments used by the artist, such as hematite, goethite and green earths [2]. The combined use of the two spectroscopic techniques allowed to clearly identifying the pigments and the alteration phases on the altar: the red and yellow areas showed signals typically attributed to hematite and goethite, confirming what supposed from RNIR imaging. Furthermore the presence of white lead was stated by Raman spectroscopy, while the use of FORS resulted fundamental torecognize the blue and green pigments as azurite and green earths [2]. As concerning the secondary phases, quite common species such as gypsum, niter and hydromagnesite were revealed due to the detrimental effect of the average church environmental conditions, such as very high humidity and relatively low temperature. The same combination is a fundamental condition for the formation of Plattnerite as alteration phase of white lead when applied in frescoes [3]; other black areas resulted covered by a thin carbon-based layer attributable to the burning of candle