According to the epoch and place, the artists use the same pigments to paint the faces but change their techniques
and the binders, which modify the visual appearances (color, gloss) of the works of art. Various techniques
(encaustic, tempera, oil, synthetic binder) are investigated on model samples containing the same pigments but
different binders and on a old painting. The influence of the techniques on the visual rendering is investigated
experimentally. Goniophotometry and optical coherent tomography are used to determine the surface state, the
gloss and the surface scattering. The spectrometry in the visible range allows to characterize the bulk scattering
by determining the absorption and scattering coefficients of the pigments according to the binders. A particular
oil technique, the art glaze is also studied on a real painting.
The identification of varnishes may be essential for the choice of the appropriate solvent during their removal by
restorers. This recognition is obtained by UV-fluorescence emission spectroscopy with a quasi-monochromatic UVexcitation.
A new portable instrument has been developed in order to implement non-destructive, contactless and in situ
measurements, providing results in real time.
The method is applied to the analysis of a real ancient painting. The resin-based varnish, the recipe and the state of
degradation of the varnish are deduced in different locations of the painting by comparison with a database of reference
varnishes. Moreover, spectral data are compared with the Fluorescence Lifetime Imaging (FLIM) analysis, performed on
the same painting. Different areas containing the same varnish can then be localized on the whole painting. These results
show that both UV-fluorescence methods are complementary for rapid and in situ analysis of varnishes of an entire work
Nevertheless, the paint layer beneath the varnish modifies the varnish fluorescence spectrum thus complicating its
recognition. Indeed, the possible fluorescence of the binder of the paint layer or its reflectance spectrum must be taken
into account. A systematic experimental study on fresh and aged model samples made of different varnishes, pigments
and binders is presented in this work. It shows that UV-fluorescence emission spectra and diffuse reflectance spectra
must be coupled to extract the fluorescence of the single varnish. Both spectra can be recorded by the presented
instrument. A new theoretical approach is summarized in order to explain this phenomenon.
A non-destructive technique giving an early diagnosis of glass degradation is presented here. It allows to
implement a new sol-gel method that stops further deterioration of the glass and avoid the classical removing of
crusts. Reflectance spectroscopy in the visible range is used with a portable fiber-optics spectrophotometer in a
back-scattered configuration and underlines very fine levels of glass alteration.
Quantitative validations on reference samples are first presented. The reference glasses have varying amounts of
lead and have been damaged using a defined and controlled degradation process. The deterioration begins by an
increase in the roughness of the upper surface, quantified by an upward vertical translation of the reflectance
spectra. For glasses containing an important amount of lead, it is followed by a decrease in this roughness and
the apparition of periodic oscillations of spectrum, due to interferences inside a transparent layer created by the
loss of lead. The analysis of these oscillations leads to the quantification of the thickness of the altered layer and
to its evolution with the degradation. These results have been validated by RBS analysis.
The same measurements have been implemented on archeological samples that have been damaged by naturally
occurring reactions while being buried in soil. Comparisons of features and oscillations of the reflectance spectra discriminate non-, weakly and strongly altered areas of the glasses, no matter the amount of lead in the samples.
Optical Coherence Tomography (OCT) is a non-invasive and non-destructive technique which is very interesting
for the study of works of art. Based on a Michelson interferometer, our device has been adapted for the recording
of information in the visible range. This system allows three-dimensional imaging of painting with a micrometer
resolution. 3D imaging of pictorial layers is presented where the pigments embedded in the binder are clearly
visible. Furthermore, an adapted signal processing gives access to the spectral information issued from pigments.
Tests on pigments in powder are presented and show that their spectral responses are different.
Qualitative UV-fluorescence of varnishes is commonly used to locate repaints on paintings or to specify the homogeneousness of a varnish layer. Photographers can now use flash UV-lamps coupled with a CCD camera to obtain colour images of the fluorescence of paintings, unveiling thus both interest and difficulty to interpret these colours.
Starting from this point of view, UV-fluorescence spectra appear to be a potential technique to characterize the nature of varnishes and, if possible, their state of degradation. This identification will be non-invasive, without contact, obtained in real time and workable in situ, as the identification of pigments or dyes by reflectance spectrometry which is already done in our group. The last goal will be to realize both identifications with the same device.
Emission fluorescence spectra are implemented with the Jobin-Yvon Fluorolog-3, providing an incident wavelength laying between 200 and 850 nm. The emission spectra are implemented with an optical fiber linked to a Jobin-Yvon spectrometer HR460 and a multi-channel CCD detector.
In a first step, popular, fresh, raw resins used between the XVI th and the XIX th century, as mastic, dammar and sandarac, have been used to prepare varnishes films with different solvents. The fluorescence spectra of these films have been carried out at different excitation wavelengths to build databases. After having tested the coherence, the limits and the accuracy of the method, we suggest different applications of our method. A synthesis of the results will be presented to characterize each varnish by their fluorescence spectra.
Conference Committee Involvement (1)
O3A: Optics for Arts, Architecture, and Archaeology