Pigment Identification by Fiber-Optics Diffuse Reflectance Spectroscopy

Abstract

Fiber-optics reflectance spectroscopy is used to identify pigments in pictorial layers of works of art thanks to a spectra database of dry powdered mineral pigments. Measurements are noninvasive, without any contact, and can be implemented in situ, without moving the work of art under investigation from its conservation place. The experimental device, using the special back-scattering configuration, is briefly presented. The protocol leading to the constitution of the spectra database of dry mineral pigments is described. Unlike other studies, this protocol has been developed to emphasize multiple scattering of light by elementary pigments in comparison with specular reflection on the surface of the sample. In these conditions, the diffuse reflectance spectrum is the label of the mineral pigment. The numerical processing of pigment identification is detailed. Both the influences of the roughness of the studied surface and of a possible varnish layer are taken into account when numerical identification is implemented. Several applications on patrimonial works of art are reported.

Color change due to surface state modification

“…4 For art restorers, another study has shown that it is possible to recognize an unknown pigment, in a pictorial layer, by comparison of its spectrum with a database made of elementary pigment spectra. 7 Because the spectrum shape is not affected by the translation due to a surface state change, this recognition remains possible, even if the surface of the painting is not planar.…”

Pigments, dyes, and black enamel—the colorants of reverse paintings on glass

“…A convenient technique for the nondestructive investigation of colored materials is the use of visible reflectance spectroscopy (Leona and Winter 2001;Dupuis et al 2002;Bacci et al 2009). In our study, we investigated dyes using the color spectrometer SPM 100 (Gretag-Imaging AG, Regensdorf, Switzerland), which measures the reflection of visible light (from 380 to 730 nm, spectral resolution 10 nm).…”

Color change due to a varnish layer

“…Finally, pigment identification under a varnish layer using diffuse reflectance spectrometry has already been performed on works of art thanks to the following homothetic approximation. 12 …”