Giacomo Chiari

The Water Molecule in Crystalline Hydrates Studied by Neutron Diffraction

A survey of the geometry and environment of water molecules in crystalline hydrates is presented. Histograms and correlation analyses are based on 97 crystal structures accurately determined by neutron diffraction, and include data from 183 water molecules, each donating two hydrogen bonds. The sample is analyzed according to the nature of the hydrogen-bond acceptors and of the coordinated cations. Cases of bifurcated hydrogen bonds are also discussed. The hydrogen bonds tend to be linear and the acceptors tend to be located close to the water plane. The tendency of coordination bonds to be collinear with the direction of one of the lone pairs of the O or their bisector increases with the strength of the bonds. The length of the W-H bond, which, on average, is shorter than in the gas phase, is correlated to the strength of the donated hydrogen bond and to the nature of the acceptor as well as to the type of coordination. On average, W--H is shorter for weaker hydrogen bonds, lower electronegativity of the acceptors and a triangular (as opposed to tetrahedral) type of coordination. The H-W-H angle is, on average, about 2.5 o wider than in the gas phase; this widening is correlated to the type of coordination (larger for triangular), to the strength of the hydrogen bond, and to the A... W...A angle between acceptors. Differences between the dimensions of water molecules in gas and solid phases are shown to be effective and not due to uncorrected thermal effects.

Crystal structure refinement of Maya Blue pigment prepared with deuterated indigo, using neutron powder diffraction

Maya Blue, a synthetic pigment produced by the ancient Mayas in pre-Columbian America, is the combination of a fibrous clay (palygorskite) and an organic blue dye (indigo). The main features of the structure of Maya Blue are already known, although the specific interactions occurring between clay and dye have yet to be completely explained. The details of the structure were studied using the Rietveld method on neutron powder diffraction patterns collected at POLARIS (Spallation Neutron Source ISIS) on a Maya Blue sample freshly synthesized using deuterated indigo. The position of the dye molecule in the structure of Maya Blue and the nature of its interactions with the clay framework were described. In the pigment, indigo lies within the palygorskite channels partially substituting the zeolitic water previously expelled during the synthesis. The dye and the water mutually compete to occupy the clay microchannels, although some portions may be empty, containing neither indigo nor water. Furthermore, the encapsulation of indigo increases the disorder in the disposition of the residual zeolitic water. The occupancy of indigo results to be higher in the orthorhombic than in the monoclinic polymorph, presumably for the faster loss of zeolitic water in the former polymorph during heating. The quantity of indigo in the pigment is rather low ( b axis. The stability of Maya Blue is guaranteed by strong H-bonds formed between the clay structural water and the indigo C=O group. All the present results are coherent with data obtained in previous studies.

An easy non-invasive X-ray diffraction method to determine the composition of Na-pyroxenes from high-density `greenstone' implements

A large number of polished stone implements from Palaeolithic to Bronze Age sites of Northern Italy and Southern France are made of high-pressure (HP) metamorphic rocks (eclogite and related rocks), mainly consisting of Na-pyroxene (jadeite to omphacite) from the metamorphic belt of the Western Alps. The standard archaeometric study of prehistoric stone implements follows a procedure that is invasive, expensive and time-consuming. Since Na-pyroxenes may show a large compositional range, a thorough study of the variations affecting the dhkl values, obtained by X-ray diffraction, of three selected reflections as a function of different chemical composition was carried out, in order to determine the chemistry of Na-pyroxene isomorphic mixtures and roughly evaluate their relative amounts. These reflections (bar221, 310, 002) are sharp, intense and sensitive to the variation of pyroxene chemical composition. Using such dhkl values measured on pyroxenes of known chemistry, a Ca-pyroxene(Di)-jadeite(Jd)-aegirine(Ae) compositional diagram was constructed, from which the composition of an unknown pyroxene can be estimated within an error of about 5%. When the size of the object is relatively small and a flat polished surface is present, the proposed analytical procedure becomes totally non-invasive. The data obtained shed light on the provenance sources of such implements and the prehistoric trade routes.

Materials and techniques of gilding on a suite of French eighteenth-century chairs

Abstract This paper describes the technical study of a suite of French Rococo chairs at the J. Paul Getty Museum with original eighteenth-century gilding preserved under layers of restoration. A variety of analytical methods was employed to identify and characterize the materials of the preparatory layers and gold alloys including optical microscopy, digital image analysis, polarized light microscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), Raman spectroscopy, and X-ray diffraction (XRD). The materials and techniques were compared with descriptions of French eighteenth-century gilding practices in contemporary artists’ treatises. Both burnished and unburnished original gilding, employing distinctive techniques, were found. The study was particularly focused on the gold alloys and the support layer for burnished gilding, a mixture known as assiette in French and as bole in English. In addition to the original gilded surface, several subsequent campaigns of gilding executed with the same eighteenth-century techniques were present on the chairs and visually indistinguishable from the original gilding. The alloy of the gold leaf used in each campaign was characterized through quantitative SEM-EDX via a calibration generated from the SEM-EDX data from gold standards. Characterization of the gold alloys proved to be a vital tool for the interpretation of the layer structure and identification of original gilding.

Lichens on Wyoming Sandstone

Seven samples of sandstone covered with lichens from a Wyoming canyon that support petroglyphs were studied using a number of different techniques including COLORMOD, a porosimetry measurement based on color mode analysis of images, obtained from thin sections with impregnating resin containing a blue dye. This technique also allows one to measure the porosity gradient. Porosity was less toward the outside, since in the outer layer lichens occluded the pores. When the lichen body was counted as well, the porosity was proved to be the same as in the core of the rock. The sandstone composed largely of quartz, is homogeneous in grain size within a single rock, but differs greatly from one rock to another. ESEM imaging of lichen interaction with the sandstone showed a superficial layer (live lichens and small mineral particles) clearly distinguishable from the bulk (larger grains and pores). Consolidation tests were carried out using ethyl silicate [Wacker OH (with) and Monsanto Silbond (without catalyst)], on samples with the greatest and least porosity. Porosity decreased less for Silbond, since, without a catalyst and an active surface, it hardly polymerized. Therefore, if an ethyl silicate treatment is planned, it is advisable to use catalyzed products. The grain size distribution of the sandstone controls the physical properties: the larger the grains, the greater the porosity, water absorption, fragility and de-cohesion of the rock. Based on observations of thin sections, grain dislodgment due to lichens is difficult to demonstrate because lichen hyphae fill the gaps between grains, which under dry conditions seem to be large enough to accommodate the hyphae without exercising undue pressure on the structure of the rock. This result may help to determine the advisability of removing the lichens from the rock surface.

Contribution of crystallography to the preservation of art and cultural heritage

This lecture will begin by providing a definition of Cultural Heritage from the perspective of Conservation Science, taking account of the radical challenge that the advent of new materials in contemporary art has posed to the field of conservation. Over the past 45 years I have applied my experience as a crystallographer to answer questions and find solutions to many problems raised by the material dimension of cultural heritage. This lecture will showcase a series of crystallographic interventions in the field of conservation science, from Maya Blue structure, elucidated using synchrotron and neutron powder diffraction to Tutankhamen tomb paintings, from Michelangelo censure panels in the Sistine chapel to a hidden face under a Rembrandt painting. My talk will address some of the ways crystallography has and can contribute to conservation science in the areas of archaeometry and conservation, such as: