Photoluminescence of Egyptian Blue

Abstract

Visible induced luminescence (VIL) is a powerful imaging technique applicable mostly to Egyptian Blue (EB hereafter). It is based on the ability of EB to absorb visible light and luminesce in the near‐infrared region. Han Blue Han Purple and some cadmium‐based pigments also luminesce, although with much lower intensity. The basic tools needed for the imaging are a digital camera with the IR filter removed, and a source of visible light without an IR component. The environmental IR noise needs to be reduced to a minimum. Many different variations of this basic setting have been proposed, from LEDs to flash with IR cutting filters. The presence of EB traces, even small, on works of art is a strong indication of their authenticity, since it is believed that the EB manufacturing technique was lost with the fall of the Roman Empire, although exceptions have been found. Using VIL, some information can be gathered on the EB history.

Preliminary multi-spectral imaging investigation on items from the Aga Khan III necropolis, Aswan (Egypt)

“…In addition, the luminescence of Egyptian blue can be observed very clearly from the VIL image (fig 4, bottom right). The fluorescence of Egyptian blue proved to be so peculiar and intense that it could be used as a specific tool for the detection of that pigment on the majority of the analysed objects [5,6]. In fact, the small statue that was aforementioned in the experimental section was examined at 360°, but no evidence of brilliance was obtained in none of the photos taken through the VIL setup.…”

“…The same experiment was repeatedly done on the majority of the excavated objects, especially because the founding of Egyptian blue is per se a kind of both dating and authentication mean, since its use is quite specific for objects made in Egypt in between the 3rd millennium B.C. and the Roman period, after which the pigment was almost abandoned [5,6,7]. In fact, it is reported in the literature that the secret recipe to make Egyptian blue was lost at some time in the late Roman period and the research of Orna et al on this topic, did not find any written evidence that Egyptian blue was produced after the Roman period [8].…”

Real-time identification and visualization of Egyptian blue using modified night vision goggles

“…Overall, EB exhibits exceptionally strong (and relatively longlasting) NIR luminescence when irradiated with visible light (Accorsi et The modi cation consists of removing the standard lter that prevents the NIR radiation from reaching the sensor. A NIR long-pass lter is then added in front of the camera (typically 800-850 nm) to eliminate visible light and allow only NIR light to pass to the sensor (Chiari 2018). The modi ed camera can then be used to acquire VIL images.…”

“…Interference can come from NIRcontaining light, such as natural light, incandescent light bulbs, and security NIR illuminators. Vivid illumination must then be provided using NIR-free sources such as white, green, or red LED light (Verri 2009a), or camera ashes properly ltered to remove the NIR component (Verri and Saunders 2014;Chiari 2018). With such a setup, the only NIR light that can reach the sensor should be the one emitted by photoluminescent materials.…”

The Young Lady in Pink. New Light on the Life and Afterlife of an Ancient Portrait

“…Unfortunately, the size of the modern secondary support hindered visible induced luminescence (VIL) analysis of the necklace area due to the object size limits of the setup that was used. VIL is an effective indicator for the presence of Egyptian blue in an object on account of the pigment's luminescence in the near-infrared region (Chiari 2017). VIL analysis of the Young Lady in Pink suggests that Egyptian blue is present throughout the incarnate (van Daal 2019, 17 fig.…”