Novel optical photothermal infrared (O-PTIR) spectroscopy for the noninvasive characterization of heritage glass-metal objects.

Optical photothermal infrared (O-PTIR) is a recently developed molecular spectroscopy technique that allows to noninvasively obtain chemical information on organic and inorganic samples at a submicrometric scale. The high spatial resolution (≈450 nm), lack of sample preparation, and comparability of the spectral results to traditional Fourier transform infrared spectroscopy make it a promising candidate for the analysis of cultural heritage. In this work, the potential of O-PTIR for the noninvasive characterization of small heritage objects (few cubic centimeters) is demonstrated on a series of degraded 16th century brass and glass decorative elements. These small and challenging samples, typically encountering limitations with existing noninvasive methods such as macroscopic x-ray powder diffraction and µRaman, were successfully characterized by O-PTIR, ultimately identifying the markers of glass-induced metal corrosion processes. The results clearly demonstrate how O-PTIR can be easily implemented in a noninvasive multianalytical strategy for the study of heritage materials, making it a fundamental tool for cultural heritage analyses.

In situ investigations of vault paintings in the Antwerp cathedral.

X-ray fluorescence spectroscopy (XRF) and Raman spectroscopy have been used to examine 15th century mediaeval and 16th century renaissance vault paintings in the Our Lady's Cathedral (Antwerp, Belgium) in view of their restoration. The use of mobile instruments made it possible to work totally non-destructively. This complementary approach yields information on the elemental (XRF) and on the molecular composition (Raman) of the pigments. For the 15th century vault painting the pigments lead-tin yellow (Pb(2)SnO(4)), lead white (2PbCO(3)xPb(OH)(2)), vermilion (HgS), massicot (PbO) and azurite (2CuCO(3).Cu(OH)(2)) could be identified. The pigments used for the 16th century vault painting could be identified as red lead (Pb(3)O(4)), hematite (Fe(2)O(3)), lead white (2PbCO(3)xPb(OH)(2)) and azurite (2CuCO(3)xCu(OH)(2)). For both paintings the presence of the strong Raman scatterer calcite (CaCO(3)) resulted in a difficult identification of the pigments by Raman spectroscopy. The presence of gypsum (CaSO(4)x2H(2)O) on the mediaeval vault painting probably indicates that degradation took place.