Archaeological materials science or archaeometry is the scientific study of material culture, which plays an important role in the development and interpretation of archaeological theory by establishing links between an object's materiality and its societal context. Driven by the complex nature of archaeological materials and the necessity to improve analysis, interpretation, and access of material culture and scientific research in the field, in the last decade there has been a significant advancement in instrumentation development for rapid, non-invasive and high-specificity materials characterization. To this end, imaging spectroscopy and portable chemical sensing modalities have played a revolutionary role in the identification and chemical mapping of constituent materials in ancient and historical wall paintings and other immovable artifacts.
In this research, different imaging and spectroscopic modalities are explored: direct-detection terahertz (THz) imaging, hyperspectral imaging spectroscopy (HSI), and high-resolution scanning reflectance spectroscopy. First, a novel direct-detection THz imaging system, adopted from biomedical imaging applications, was used to probe hierarchically-complex painting targets for sub-surface imaging of hidden decorative features and structural defects that are difficult to discern by X-ray and infrared imaging techniques. The imaging system’s deep signal penetration depth and high contrast sensitivity can successfully penetrate overlaying layers of strong signal scatterers such as lead white and chalk and to identify internal voids, hidden text, and topographic details of concealed iconography, which has important implications for future applications of this system on the study of wall paintings covered by whitewash or plaster layer(s).
Investigations of two different collections of painted artifacts were performed using novel imaging spectroscopy applications, supplemented by forensic photography and portable chemical sensing modalities, fiber-optic reflectance spectroscopy (FORS) and X-ray fluorescence (XRF) spectroscopy: (1) Cypriot wall paintings, and (2) Greco-Roman funerary portraits from Egypt. A comprehensive in situ characterization of Hellenistic, Roman and Byzantine Cypriot wall paintings in the region of Paphos, Cyprus was achieved for the first time, applying a non-invasive multi-scale approach, employing a commercial hyperspectral imaging (HSI) camera operated in reflectance and luminescence modes and a custom-made high-resolution scanning reflectance spectroscopy system developed and adapted for field research. The HSI and the scanning reflectance spectroscopy system offered complementary, powerful high-spectral and spatial resolution 3D data cubes to reconstruct the palette of Cypriot painters. From an archaeological materials science perspective, the characterization and mapping of pigments through derivative spectral analysis provided important information on pigment layering and mixtures used to produce complex hues and special optical effects, such as shading and translucency. Combined with FORS and XRF, these techniques offered fast, in-depth studies of large painted surfaces, inferring material and artistic choices and the chaine operatoire of production technology. Similarly, the analysis of funerary portraits of ancient Egypt (first to fourth century AD) identified technological choices, materials selection and application methodology, revealing the vogue of Greco-Roman society.
From a materials engineering perspective, the study of these paintings lead to adaptations and development of novel methods of analysis: luminescence imaging spectroscopy was for the first time employed to collect and map luminescence signatures of Egyptian blue and madder lake, two of the most important synthetic pigments of antiquity, over painting surfaces; and the success of forensic and spectral imaging in producing luminescence/chemical maps across 2D surfaces led to the development of a streamlined, accessible methodology to construct luminescence-textured 3D models for new visualizations and analyses of high-relief/3D polychrome artifacts that feature photoluminescent pigments and conservation materials.