Combination of noninvasive imaging techniques to characterize pigments in Buddhist thangka paintings

Abstract

Scanning macro-XRF, XRF point measurements and hyperspectral reflectance imaging in the visible and near-infrared range were combined to characterize the pigments of a Tibetan painting. The complementary use of the different investigation techniques allowed us to (i) clearly identify the pigments used by the artist, (ii) strengthen the few data present in historical and contemporary sources and (iii) provide data necessary for any accurate conservation treatment of the painting. The proposed methodology allows an in-depth characterization of the materials used in thangkas, with the identification of all the pigments used by Tibetan artists, as shown by the analyses of reference samples. In the present study traditional pigments, commonly cited in historical sources, were identified such as vermilion, minium, orpiment and azurite, but also more unusual compounds such as brochantite and antlerite (in blue/green areas). Elemental distribution images confirmed that some parts of the thangka were restored, without taking into account the original pigments or the original design. Underlying annotations and a carbon black under-drawing were highlighted by the application of near-infrared hyperspectral imaging technique, which proved to be particularly useful for the study of such paintings.

Artificial Intelligence for Pigment Classification Task in the Short-Wave Infrared Range

“…The blue and green areas of the painting present visible signs of degradation at the surface of the object. As already discussed in [ 26 ], a strong darkening of azurite is very commonly observed on thangkas, although not chemically characterized yet. However, the observation of lacunas in degraded areas reveals the presence of undegraded paint material underneath the top layer (a few microns thick) with an intricate mixture of blue and green particles ( Figure 2 b).…”

“…Whereas the add-in of kaolinite pigment results in absorptions at ca. 1390, 1410, 1910, and 2200 nm [ 26 ], most pigment mixtures with kaolinite are mislabeled, and are assigned to the carmine class. Similarly, for two- and three-pigment mixtures, azurite is the only pigment properly labeled; moreover, in the specific case of azurite and malachite mixtures, pixels are systematically labeled as azurite, malachite being rarely identified in the mixture.…”

“…1490 nm ( Figure 3 c, spectrum W1). Their positions are characteristic of cellulose fibers (such as cotton, most probable in the case of Tibetan artworks) [ 26 ]. Vibrational features from the (C = O), N–H, and CH 2 groups of the animal skin glue, which include features near 2041 nm and 2176 nm, can be differentiated [ 11 ].…”

“…Paintings from one mockup and one historical thangka are used to test the robustness of the model. Thangkas are sacred paintings depicting a Buddhist deity; their main constituent materials are quite well known and described in the literature (thanks to literary sources [ 23 , 24 ], oral testimonies of modern-day painters but also previous analytical results [ 25 , 26 ]) as cotton cloth or silk, natural glue sizing and a coating traditionally composed of chalk or kaolinite, and several colored layer(s) made of mineral pigments and organic dyes.…”

Nondestructive Evaluation of Heritage Object Coatings with Four Hyperspectral Imaging Systems

“…According to Gabrieli et al [19], the wavelength position of this lipid band can be used to discriminate between a drying oil (2304 nm), egg yolk (2309 nm), or wax (2312 nm). Close bands at 1910 nm and 2330 nm were also identified as kaolin and calcite by Brocchieri et al [62], who performed SWIR mapping in parallel with XRF measurement. Analogous bands are also evident in Loadings 2 and 3.…”