International audienceThe first purpose of this paper is to underline a relevant colorimetric co-ordinate characterizing the colour of ochres within their extremely wide range, from pale yellow to dark red. The second purpose is to link together quantitatively the variations of this colorimetric co-ordinate and the various chemical compositions of the samples, mainly hematite, goethite and white pigments. A group of 30 modern ochres and a group of 20 ancient ochres have been investigated. All these natural pigments have been commercialized. Diffuse reflectance spectrometry allows to calculate the colorimetric co-ordinates in the CIE-L*a*b* space and the position of the absorption band of each sample. Physico-chemical analysis has been obtained by quantitative X-ray diffraction, scanning and transmitting electronic microscopy and particle-size analysis by laser diffraction. The positive a* co-ordinate (redness) has been underlined, for the first time, to be the only relevant colorimetric parameter to characterize the colour of the ochres. Its variations are quantitatively connected to the shift of the absorption band due to the charge transfer between the ligand (OH− or O2−) and the Fe3+ ion contained in goethite and/or hematite. For ochres containing both hematite and goethite, the a* co-ordinate linearly increases with the relative amount of hematite while the absorption band progressively shifts towards the high wavelengths. Such a linear shift of the absorption band has never been underlined before. For ochres containing only one iron oxide, a* linearly decreases with the amount of white pigments, whatever the nature of the white charges. Moreover, this study gives the opportunity to show that only the nature, the amount and the size distribution of the white charges allow to discriminate the ochres according to their geographic origin

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.

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The technique used by Leonardo da Vinci to paint flesh tints--the sfumato--has never been scientifically depicted until now. From 100,000,000 reflectance spectra recorded on Mona Lisa, a virtual removal of the varnish is first obtained. A unique umber pigment is then identified in the upper layer and an exceptional maximum of the color saturation is underlined, both characteristics of a glaze technique. The modeling calling upon the radiative transfer equation confirms this maximum of saturation, the identification of an umber in the upper layer, and moreover underlines a mixture of 1% vermilion and 99% lead white in the base layer. Finally, the modeling, using the auxiliary function method, explains the spectacular maximum of saturation by the multiple scattering.

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The identification of the chemical nature of varnish is essential for art restorers in order to choose a suitable solvent during its removal. Until today, such identification has been performed using chemical analysis after sampling. An innovative technique is presented here, using ultraviolet (UV) fluorescence spectroscopy. The method is nondestructive, workable in situ, and leads to results in real time. It is based on the comparison between the emission spectrum of an unknown varnish with those of fresh, artificially aged, or old reference resins and varnishes, for different monochromatic excitation wavelengths. The resin and the nature of the varnish as spirit, oil, or mixed can thus be identified. Various examples are presented on home-made samples applied on fluorescent backgrounds and on real works of art.

Optical coherence tomography (OCT) is especially attractive for the study of cultural heritage artifacts because it is noninvasive and nondestructive. We have developed an original full-field time-domain OCT system dedicated to the investigation of varnished and painted artifacts: an interferometric Mirau objective allows one to perform the scan without moving the works of art. The axial and transverse high resolution (respectively, 1.5 and 1 microm) are well adapted to the detection of the investigated structures (pigment grains, wood fibers, etc.). The illumination spectrum is in the visible range (centered at 630 nm, 150 nm wide) to potentially allow us to perform spectroscopic OCT on pigment particles. The examination of wood samples coated with a traditional finish, demonstrates the ability of the system to detect particles, characterize layers thickness, and image the three-dimensional wood structures below the varnishes. OCT has finally been applied to study in situ the coated wood surface of an 18th century Italian violin and provides important information for its conservation treatment.

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