Abstract
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. 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. 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.
Highlights
The scientific imaging of works of art is crucial for the assessment of the presence and distribution of pigments and other materials on surfaces
Preliminary PL spectroscopy analysis of a model sample painted with the commercially available Egyptian blue pigment reveals a characteristic emission spectrum with a maximum emission at 920 ± 6 nm and a monoexponential emission lifetime of 138 ± 4 μs (95 % confidence) (Fig. 1), in good agreement with the spectral data reported by others from pigments from the same supplier [7, 19]
The emission lifetime measured in our commercial pigment is in agreement with Borozov et al [19] but differs with respect to that reported for the synthetic mineral calculated using single photon counting and 637 nm excitation (τ = 107 μs [6])
Summary
The scientific imaging of works of art is crucial for the assessment of the presence and distribution of pigments and other materials on surfaces. The unique near-infrared luminescence emission of Egyptian blue allows the imaging of its distribution on surfaces. The imaging of paintings and painted objects relies on the interaction between radiation and matter and the detection of reflected or emitted photons. Among ancient Egyptian pigments, Egyptian blue and madder-based red lakes are luminescent and are routinely identified with the aid of microscopic and spectroscopic analysis [2]. Whereas ultraviolet (UV) light sources based on Mercury lamps require filtering of spurious radiation emitted in the visible to render them useful for imaging and photography, innovative uses of Xenon-flashes and digital photography have demonstrated the peculiar and noteworthy infrared emissions from Egyptian blue on paintings and painted objects from the British Museum and, more recently, on Fayoum Portraits [7,8,9]. Many other examples of digital imaging of Egyptian blue from objects have been documented
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