A technical step forward in the integration of visible-induced luminescence imaging methods for the study of ancient polychromy

Abstract

Photo-induced luminescence imaging techniques, such as UV-induced visible luminescence (UVL) and the more recently developed technique of visible-induced infrared luminescence (VIL), have been invaluable for the study of ancient polychromy, allowing the detection and mapping of luminescent materials, such as varnishes, consolidants, organic binders, and crucially, traces of pigments, organic and inorganic, that are often not visible to the naked eye. In the context of works from the Hellenistic period onwards, the detection of two pigments, Egyptian blue and rose madder lake, has been particularly pivotal in advancing the field. Current conventional methodologies for the digital mapping of these two luminescent pigments rely on the separate application of two techniques (VIL and UVL), each requiring a different illumination source and acquisition set-up. In this study, a novel approach is proposed, combining the use of visible-induced infrared luminescence and visible-induced visible luminescence to locate these two pigments. As the source of illumination in both cases is the same system of LEDs, the set-up has the advantage of requiring only minor filter changes between luminescence modes. The increased portability and safety compared to the use of methodologies that employ UV radiation represent notable advantages of this integrated system. The interchangeability between highly selective excitation sources, also significantly simplifies the experimental set-up and the need to adjust the object or equipment between acquisitions, ensuring better reproducibility of the data acquired and facilitating any post-processing procedures. This results in a user-friendly methodology for both experts and non-specialists alike. Three Hellenistic period terracottas; two from Canosa di Puglia, Italy (270-200 BC) and one from Myrina, Turkey (c. 100 BC), all characterised by large well-preserved areas of decoration in Egyptian blue and red lake, were studied in order to trial the approach. Comparisons were made with the more standard techniques of VIL and UVL, and it was shown that the combined method proposed efficiently detects and maps both of these pigments with analogous results to those obtained by more established methodologies. The observations made from the multispectral images acquired were verified by analysis of small samples of the pigments, using FTIR and Raman spectroscopy and HPLC-DAD analysis.

A multispectral imaging approach integrated into the study of Late Antique textiles from Egypt

“…Post-processing procedures for the VIVL and MBR images are not currently supported by the BM_workspace. However a post-processing protocol for VIVL images was described in Dyer et al [ 2 ]. Image subtraction of the MBR images was undertaken in Adobe Photoshop to provide the final image [ 12 ].…”

“…Although the use of MSI techniques is well-established in the study of polychrome surfaces [ 2 – 5 ], these have only been used sparingly in the investigation of historical or archaeological textiles. Coremans’ use of near infrared (NIR) photography in 1938, to document tapestry restorations [ 6 ], was an early adoption of employing wavelengths of light outside of the capabilities of the human eye in the investigation of textiles.…”

“…The first of these is a luminescence technique analogous to UVL imaging, but which uses visible light, specifically blue light in this instance, as the excitation source. Visible-induced visible luminescence imaging (VIVL) has recently been applied to the study of polychrome sculpture with, among others, the notable advantage that it dispenses with the use for UV sources, thus making it safer for both user and object [ 2 ]. In its application to textiles the VIVL technique has a further advantage over UVL imaging, in that the longer excitation wavelength used avoids the absorption maximum of most fibres [ 11 ], thus suppressing background luminescence from these and minimising the distracting and colour-modifying effects which this can cause.…”

Shedding light on the colours of medieval alabaster sculptures: Scientific analysis and digital reconstruction of their original polychromy

“…18 Other complementary techniques were used, such as technical photography (TP) and fiber optic reflectance spectroscopy (FORS), which can be used noninvasively to highlight the presence of certain pigments and colourants. [19][20][21][22] An initial survey with FORS was useful to plan the sampling and analyses, and TP allows the more extensive investigation and comparison of areas on the objects, which bore similarity to areas where the presence of specific materials had been identified by analyses, thus minimizing the number of samples needed. The experimental details of all these techniques are presented in the following.…”

New insights into the composition of Indian yellow and its use in a Rajasthani wall painting

“…Most analytical techniques are increasingly being adapted to offer portability and noninvasiveness (XRF, FTIR, Raman) [14][15][16] and, when combined with imaging techniques [12,[17][18][19][20], can provide very useful information, leading to the identification of materials and aiding in their condition assessment. However, when micro-samples can be taken, the level of detailed chemical information that can be obtained increases significantly, especially for organic materials: the observation and analysis of cross-sections (SEM-EDS) [21,22] coupled with chromatographic analysis (GC-MS, HPLC-MS, Py-GC-MS) [23] is a powerful combination.…”