Inspection of panel paintings beneath gilded finishes using terahertz time-domain imaging

Abstract

Scientific analysis of panel paintings could provide key information to art historians and conservators about the composition and condition of the constituent layers. Knowledge of the structure, stratigraphy, and condition of the subsurface layers of these objects is highly relevant to conservation measures, when stability problems such as delamination or internal cracking are considered. Terahertz time-domain imaging (THz-TDI) in reflection geometry is capable of highlighting interfaces between layers in a stratigraphic buildup, and thus makes structural information available in the form of subsurface 3D images and stratigraphic images (B-scans). The technique is contactless and non-invasive, and has been successfully applied to investigation of panel paintings. Regarding gilded panel paintings, the extremely high reflectivity of metals at submillimeter wavelengths generally precludes the transmission of electromagnetic waves through metallic films. Therefore, imaging of subsurface features below gilded finishes may be regarded as Very challenging. A small but non-negligible direct transmission through metal films occurs if the film thickness is of the order of the skin depth of the metal in the terahertz range, due to the skin effect. The thickness of gold leaf, a fraction of a micrometer, matches the skin depth of gold in the terahertz frequency range covered by THz-TDI devices. We therefore investigated and imaged subsurface features of panel paintings through gilded finishes with THz-TDI. Subsurface layers of three gilded panel paintings (two contemporary tempera panel replicas and one fourteenth-century icon) have been successfully imaged behind gold finishes with THz-TDI.

Fra Angelico’s painting technique revealed by terahertz time-domain imaging (THz-TDI)

“…9c, d) demonstrates how the signal is almost entirely reflected back by the gilded areas, suggesting a gold thickness that is greater than the skin depth of the metal at THz frequencies (*250 nm at 0.1 THz, *80 nm at 1 THz in case of pure gold [8,25]). …”

“…The possibility of obtaining stratigraphic information noninvasively [1][2][3] within a large portion of a panel painting led to considering THz-TDI as a suitable technique to give insight into the construction technique of panel paintings, which is closely connected to the provenance, age, and attribution of the paintings [4][5][6][7][8][9][10][11].…”

Terahertz and Cultural Heritage Science: Examination of Art and Archaeology

“…THz is used mostly to locate the gilding leaves under the paint layer. Figure 3 shows a detail from a 14th century icon which was analyzed with both infrared imaging and THz [75]. In the THz-TDI image the square gilding leaves are clearly visible at the contours of the figures' faces while infrared reflectography (IRR) imaging cannot reveal them [78].…”

“…Ultraviolet radiation is used for the examination of the varnish, infrared to reveal underdrawing and X-rays to study underpaint and the wooden support. THz has been successful to investigate all of these layers, even reaching the wood support, providing cross-section (B-scan) and top-view (C-scan) images of the artworks [75].…”

Non-destructive Investigation of Paintings on Canvas by Continuous Wave Terahertz Imaging and Flash Thermography

“…Simultaneously, Koch Dandolo et al also explored and imaged subsurface features of panel paintings through gilded finishes with THz-TDI. Subsurface layers of three gilded panel paintings (two contemporary tempera panel replicas and one fourteenth-century icon) have been successfully imaged behind gold finishes with THz-TDI [36]. Instead, Abraham et al revealed buried layer information such as a graphite handmade sketch covered by several layers of paint.…”