Abstract
Imaging spectroscopy (IS) is an important tool in the comprehensive technical analysis required of archaeological paintings. The complexity of pigment mixtures, diverse artistic practices and painting technologies, and the often-fragile and weathered nature of these objects render macroscale, non-invasive chemical mapping an essential component of the analytical protocol. Furthermore, the use of pigments such as Egyptian blue and madder lake, featuring diagnostic photoluminescence emission, provides motivation to perform photoluminescence mapping on the macroscale. This work demonstrates and advances new applications of dual-mode imaging spectroscopy and data analysis approaches for ancient painting. Both reflectance (RIS) and luminescence (LIS) modes were utilized for the study of a Roman Egyptian funerary portrait from second century CE Egypt. The first derivative of the RIS image cube was analyzed and found to significantly improve materials separation, identification, and the extent of mapping. Egyptian blue and madder lake were mapped across a decorated surface using their luminescence spectral signatures in the region of 540–1000 nm as endmembers in LIS analyses. Linear unmixing of the LIS endmembers and subsequent derivative analyses resulted in an improved separation and mapping of the luminescence pigments. RIS and LIS studies, combined with complementary, single-spot collection elemental and molecular spectroscopy, were able to successfully characterize the portrait’s painting materials and binding media used by the ancient artist, providing key insight into their material use, stylistic practices, and technological choices.
Highlights
Archaeological panel paintings and polychrome monuments and objects, such as decorated ceramics, painted facades, and sculptures, are artifacts of artistic and anthropological value
Using spectroscopic tools such as Raman spectroscopy and reflectance, luminescence, or X-ray fluorescence (XRF) spectroscopy, in 2D spatial scanning configurations, can mitigate challenges posed by degradation products that conceal original decoration and help rebuild a pictorial scheme when much of the original material seems to have been lost over time
The woman’s flesh tones, clearly the most complex portion of the painting with respect to materials mixing and layering, are represented by five different endmembers, constituting five unique mixtures predominantly comprised of ochre pigments of varying amounts mixed with a white pigment to adjust their hues
Summary
Archaeological panel paintings and polychrome monuments and objects, such as decorated ceramics, painted facades, and sculptures, are artifacts of artistic and anthropological value. The study of these artifacts, in particular the materials used to construct them, help provide insight into sources of these materials, the trade routes. Difficulties encountered in studying the materials in ancient painting originate from natural aging and deterioration due to adverse environmental and anthropogenic effects Using spectroscopic tools such as Raman spectroscopy and reflectance, luminescence, or X-ray fluorescence (XRF) spectroscopy, in 2D spatial scanning configurations, can mitigate challenges posed by degradation products that conceal original decoration and help rebuild a pictorial scheme when much of the original material seems to have been lost over time. Such information can help understand artist working methods and the state of conservation of the artwork, as well as a sense of the original extent and appearance of the painting [11,12,13,14,15,16]
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