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.
Highlights
In recent years, the study of polychromy on ancient sculpture, either free standing or from architectural contexts, has evolved into an interdisciplinary field, which by in-depth examination of these works of art, seeks to reconsider the production and decoration of antique sculpture, throughout a better understanding of the materials and techniques employed [1, 2]
In each figure, (a) and (b) show the visible-reflected (VIS) and UV-induced visible luminescence (UVL) images, which are used as a comparison to the visibleinduced visible luminescence (VIVL) images shown in (c) and (d)
The UVL/ visible-induced visible luminescence (VIVL) and visible-induced infrared luminescence (VIL) images shown in Figs. 5, 6 and 7 are given ‘as shot’, for the purpose of comparison, while selected images after post-processing are presented in Figs. 11, 12 and 13, in the “Discussion” section
Summary
The study of polychromy on ancient sculpture, either free standing or from architectural contexts, has evolved into an interdisciplinary field, which by in-depth examination of these works of art, seeks to reconsider the production and decoration of antique sculpture, throughout a better understanding of the materials and techniques employed [1, 2]. In Hellenistic times, polychromy gains its literal meaning, as in terms of aesthetic value, the colourful aspect of sculptural works from this period often rivals, if not prevails, over the form itself. A rich colourful corpus representing the polychrome appearance of Hellenistic sculptures, though not equivalent in several aspects with the marble large scale sculptures [6], is that of the terracotta figurines known as «Ταναγραίες» from the Boeotian Site of Tanagra, where they were discovered [7]. The privileged use of a pair of colours in the adornment of Tanagra figurines; Egyptian blue and rose pink, in the form of a madder lake, could be considered a signature of a purely Hellenistic style [6, 9, 10]
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