LEDμSF: A new portable device for fragile artworks analyses. Application on medieval pigments

Abstract

Spectrofluorimetry is a method, among others, with the utmost sensitivity suitable for the analysis of organic materials. Commonly available devices allow us to work on small objects, or powders or on samples deposited on quartz plates. However, the study of fragile artworks (canvas paintings, pastels, medieval manuscripts) on which sampling is prohibited and which cannot be moved to laboratories requires light portable equipment for the in situ analysis of the artwork without contact or preparation. The system has to be sensitive, non-invasive, time-effective, and with a small footprint in order to perform the data acquisition within the conservation or restoration room for the sake of minimal disturbance. A new spectrofluorimeter device using LEDs sources (LEDμSF) has been designed and its performance evaluated for the in situ characterization of artworks of our Cultural Heritage components. This equipment, invented by the authors of this paper, has recently been patented.The system has two excitation sources (LEDs at 285 and 375nm). The excitation light is focused onto a small spot on the sample and the excited fluorescence is optically collected and coupled into the fiber of the spectrometer. This confocal arrangement of the measurement head eliminates most of the stray light. Filters are associated with the LED to cut their reflection and their second-order effects (570 and 750nm). The signal emitted by the sample is transmitted by an optical fiber connected to a Thorlabs mini-spectrometer (200–1000nm). The choice of LEDs is based on various studies of the analysis of binders and pigments. 285nm is used to study the fluorescence of blue or yellow pigments (lapis-lazuli, azurite, Egyptian blue; orpiment, lead tin yellow, stil-de-grain...) and some organic binders (protein glue, gum Arabic). 375nm induces fluorescence of lipid binders (egg yolk, linseed oil) and red pigments (red lead, cinnabar, cochineal...). The measurements obtained have been compared with conventional laboratory equipment currently used on pigments in mediaeval paintings.The analyses have defined the characteristic bands of the fluorescence emission of some red, blue, and yellow pigments (e.g. the red lead gives maximum fluorescence at 580nm; cinnabar at 610nm, bucktooth around 530nm...). The characteristic maxima emission wavelengths allow us to discriminate most of the pigments mixed with different binders, deposited on quartz plates, on parchment or on illuminated facsimile.