Gilles Wallez

Revealing the Origin and History of Lead-White Pigments by Their Photoluminescence Properties

The lead white pigment, composed of two main mineral phases cerussite PbCO3 and hydrocerussite 2PbCO3·Pb(OH)2, has been used in paintings since the Antiquity. The study of historical sources revealed that a large variety of lead white qualities were proposed, depending on the degree of sophistication of the pigment synthesis. Investigation of photoluminescence of the two constitutive mineral phases gave insight into the origin of the visible emission of these materials and emphasized the influence of structural defects on their photoluminescence properties. These effects were observed by combining emission and excitation spectra in two-dimensional representations. For each excitation wavelength, between 250 and 400 nm (4.9-3.1 eV), luminescence spectra were collected between 400 and 800 nm (3.1-1.5 eV). Two types of emission-excitation bands were identified: an emission excited in the optical bandgap of the compounds (about 5 eV), which depends on the constitutive phase (2.8 eV in cerussite and 2.1 eV in hydrocerussite), and broad emission bands in the same energy range excited below the optical gap, which are sensitive to the synthesis method and the nature of postsynthesis treatments. It is proposed that this sensitivity of photoluminescence properties of lead-white pigments could be used as fingerprints of their origin and history.

Synchrotron-based high angle resolution and high lateral resolution X-ray diffraction reveals lead white pigment qualities in Old Masters paintings

Microsamples collected on 27 major paintings by Old European Masters dating from the 14th to the late 19th centuries were analyzed using synchrotron-based X-ray diffraction. Two complementary analytical configurations were used at beamlines ID22 (high angle resolution) and ID21 (high lateral resolution), in order to highlight markers of the different grades of the lead white pigments (mixture of cerussite PbCO3 and hydrocerussite Pb3(CO3)2(OH)2). Rietveld analysis and crystalline phases mapping at the microscale revealed the composition and microstructure of the pigments, shedding light on the preparation recipes and pigment choices of the artists through History.

Effect of the sintering method on microstructure and thermal and mechanical properties of zirconium oxophosphate ceramics Zr 2 O(PO 4 ) 2

Due to an ultra-low thermal expansion, Zr2O(PO4)2 could find many applications as a thermal shock resistant material. To this end, ceramic processing is a key step in order to reach best properties. In this work, Zr2O(PO4)2 was sintered by conventional sintering and by the spark plasma sintering technique (SPS) with and without additive. Samples made by conventional sintering with ZnO as sintering aid have a maximum relative density of around 92 %. Microstructure is composed of large grains and microcracks can be observed. When doped 2 with 5 wt. % of MgO, samples can be densified by SPS up to 99.6 % of the relative density and the grain size maintained between 0.5 and 1.5 μm. Thermal conductivity and Vickers microhardness were investigated as a function of the microstructure. Best values were obtained for the ceramic doped with 5 wt.% MgO and sintered by SPS, thanks to a fine microstructure and a small amount of residual microcracks.

Ferroic phase transitions in K2ZnGeO4 and related compounds

K2ZnGeO4 belongs to the ‘stuffed’ cristobalite structural type. The α-form (SG Pca21) changes to β-form (SG F 3m) above 1049 K by a displacive/rotational transformation with ferroelectric/ferroelastic character, in which the strain is only coupled with the order parameter. The distortion leading from β-HT to α-LT form results from important strains and geared rotations of the ZnO4 and GeO4 tetrahedra. The broad permittivity peak around the transition temperature is typical of a first-order transition. A comparison with isotypic compounds shows that the covalency of the structural skeleton plays a major role in the transition phenomenon.