Modern White Pigments: Their Identification by means of Noninvasive Ultraviolet, Visibile, and Infrared Fiber Optic Reflectance Spectroscopy

In the first half of the twentieth century, the chemical industry provided artists with a new white pigment, titanium white, in its two crystalline forms, anatase and rutile. These were utilized along with other common whites such as lead white, zinc white, and lithopone. The identification of these white pigments is of particular interest in regard to the evolution of their use in art. Ultraviolet (UV), visible and infrared (IR) fiber optic reflectance spectroscopy (FORS) is a noninvasive analytical technique that is appropriate for in situ pigment identification. It can also be a complementary method to other noninvasive techniques. In the present work, FORS results on modern white pigments (including kaolin from the canvas preparation) will be presented. These pigments can be positively identified by their reflectance spectra in the 270-1700 nm range. Indeed, zinc sulfide (specifically one of the lithopone compounds), zinc white, and titanium white (in both the anatase and rutile crystalline forms) are clearly identified in the 330-420 nm region, where their spectral slopes make it possible to unambiguously distinguish them. In addition to the UV and visible regions, the 1300-1600 nm range is considered. There, kaolin, together with lead white and gypsum, reveals characteristic absorption features due to the hydroxyl groups (from the first overtone of stretching-mode vibrations). Finally, several examples of FORS application on twentieth-century artworks are reported. The cases selected for investigation belong to three different collections, all located in Florence: the Self-Portrait Collection of the Galleria degli Uffizi; the Fondo Pasolini, Archivio Contemporaneo "Alessandro Bonsanti" of the Gabinetto Vieusseux; and the Iconographic Collection of the publishing house Giunti Editori S.p.A.