Dual wavelength excitation for the time-resolved photoluminescence imaging of painted ancient Egyptian objects

Background: The scientific imaging of works of art is crucial for the assessment of the presence and distribution of pigments and other materials on surfaces. It is known that some ancient pigments are luminescent: these include pink red-lakes and the blue and purple pigments Egyptian Blue (CaCuSi4O10), Han blue (BaCuSi4O10) and Han purple (BaCuSi2O6). Indeed, the unique near-infrared luminescence emission of Egyptian blue allows the imaging of its distribution on surfaces. Results: We focus on the imaging of the time-resolved photoluminescence of ancient Egyptian objects in the Burri Collection from the Civic Museum of Crema and of the Cremasco (Italy). Time-resolved photoluminescence images have been acquired using excitation at 355 nm for detecting the ns-emission of red lakes and binding media; by employing 532 nm excitation Egyptian blue is probed, and the spatial distribution of its long-lived microsecond emission is imaged. For the first time we provide data on the photoluminescence lifetime of Egyptian blue directly from objects. Moreover, we demonstrate that the use of a pulsed laser emitting at two different wavelengths increases the effectiveness of the lifetime imaging technique for mapping the presence of emissions from pigments on painted surfaces. Laser-induced luminescence spectra from different areas of the objects and traditional digital imaging, using led-based lamps, long pass filters and a commercial photographic camera, complement the results from photoluminescence lifetime imaging. We demonstrate the versatility of a new instrumental setup, capable of recording decay emission kinetics with lifetimes from nanosecond to microseconds. Conclusions: While the combined wavelength approach for the imaging of emissions from different materials has been demonstrated for the study of ancient Egyptian pigments (both organic and inorganic), the method could be extended to the analysis of modern pigments and paintings.