Chiara Anselmi

Noninvasive Testing of Art and Cultural Heritage by Mobile NMR

Nuclear magnetic resonance (NMR) has many applications in science, medicine, and technology. Conventional instrumentation is large and expensive, however, because superconducting magnets offer maximum sensitivity. Yet NMR devices can also be small and inexpensive if permanent magnets are used, and samples need not be placed within the magnet but can be examined externally in the stray magnetic field. Mobile stray-field NMR is a method of growing interest for nondestructive testing of a diverse range of materials and processes. A well-known stray-field sensor is the commercially available NMR-MOUSE, which is small and can readily be carried to an object to be studied. In this Account, we describe mobile stray-field NMR, with particular attention to its use in analyzing objects of cultural heritage. The most common data recorded are relaxation measurements of (1)H because the proton is the most sensitive NMR nucleus, and relaxation can be measured despite the inhomogeneous magnetic field that typically accompanies a simple magnet design. Through NMR relaxation, the state of matter can be analyzed locally, and the signal amplitude gives the proton density. A variety of stray-field sensors have been designed. Small devices weighing less than a kilogram have a shallow penetration depth of just a few millimeters and a resolution of a few micrometers. Access to greater depths requires larger sensors that may weigh 30 kg or more. The use of these sensors is illustrated by selected examples, including examinations of (i) the stratigraphy of master paintings, (ii) binder aging, (iii) the deterioration of paper, (iv) wood density in master violins, (v) the moisture content and moisture profiles in walls covered with paintings and mosaics, and (vi) the evolution of stone conservation treatments. The NMR data provide unique information to the conservator on the state of the object--including past conservation measures. The use of mobile NMR remains relatively new, expanding from field testing of materials such as roads, bridge decks, soil, and the contents of drilled wells to these more recent studies of objects of cultural heritage. As a young field, noninvasive testing of artworks with stray-field NMR thus offers many opportunities for research innovation and further development.

An Integrated Experimental and Theoretical Approach to the Spectroscopy of Organic‐Dye‐Sensitized TiO2 Heterointerfaces: Disentangling the Effects of Aggregation, Solvation, and Surface Protonation

We report a joint experimental and computational study into the spectroscopic properties of a prototypical D5 organic dye, both in solution and adsorbed on a TiO2 surface, with the aim of modeling and quantifying the UV/Vis spectral shifts that occur in the different explored environments. Going from the dye in solution to dye-sensitized TiO2, various factors may shift the position of the UV/Vis absorption maximum, both towards longer and shorter wavelengths. Here we have focused on the effect of dye aggregation on TiO2, surface protonation, and solvent effects. The D5 dye forms stable aggregates on the TiO2 surface that cause spectral blueshifts. We used different sensitization conditions to vary the dye loading and thus the extent of dye aggregation. For each sensitization condition, we explored protonated and native TiO2 films. Computational modeling of different dimeric aggregates with increasing intermolecular interactions and simulation of the associated optical responses also confirm the observed spectral blueshifts. Our results show that both the presence of surface protons and solvent stabilize the excited state of the adsorbed dye molecules, which causes a marked redshift in the absorption maximum and thus moves in the opposite direction to the shift due to the increase in the surface coverage.

Scientific Investigation of an Important Corpus of Picasso Paintings in Antibes: New Insights into Technique, Condition, and Chronological Sequence

Abstract The Musée Picasso in Antibes (France) houses a unique collection of 23 paintings and 44 works on paper by Pablo Picasso, completed during the fall of 1946 by the artist, working on the same premises occupied today by the Museum. Picasso painted with readily available materials including oleoresinous enamel paints, fibrocement, wood panels, paper sheets, and re-used canvases. In this paper the results of an extensive campaign of scientific analysis of 17 of these works with both non-invasive and micro-invasive techniques are described. The project elucidated the full palette of the paintings, dispelling myths about their execution solely with the renowned brand of enamel paint Ripolin. The effective combination of elemental and spectroscopic methods of analysis enabled fine discriminations among various types of white enamel paint used by Picasso in Antibes. Because the artist appeared to have used such paints in chronological sequence, the precise identification of the type of white paint present on each of the works allowed the assignment of revised dates to some of the undated paintings. Important new information on surface coatings of wax and modern polymeric varnishes, as well as the widespread presence of metal soaps including zinc oxalates, was also uncovered.

MOLAB® meets Persia: Non-invasive study of a sixteenth-century illuminated manuscript

Abstract Using the MOLAB® non-invasive analytical mobile laboratory, we studied a finely illuminated sixteenth-century Persian manuscript at the Fitzwilliam Museum, Cambridge, UK, in collaboration with its Department of Manuscripts and Printed Books. Three miniatures belonging to the manuscript, but ascribable to different periods, were analyzed in order to identify similarities and differences in the painting materials and techniques used by Safavid artists over a period of 150 years. The use of multiple analytical techniques indicated a common palette characterizing the three decorative schemes, along with some differences mainly regarding the pigment mixture used to obtain brown hues in the first scheme, as well as the presence of different mixtures in green and dark purple areas in the third scheme.

New portable instrument for combined reflectance, time-resolved and steady-state luminescence measurements on works of art.

In this paper a new compact and portable instrument for combined reflectance, time-resolved and steady-state fluorescence is presented. All the optical parts of the apparatus, carefully described in the text, were chosen after an extensive market survey in order to obtain the best performances coupled with the smallest dimensions. This instrument through the use of a dedicated multiple fiber optic probe, allows the complete photophysical behaviors of investigated materials to be collected from the same point of the analyzed surface. In this way, the resultant instrumental setup is a portable device, usable in situ for non destructive and non invasive diagnostic purposes in the field of cultural heritage. Preliminary results concerning organic dyes characterization, which is the main application of luminescence-based diagnostic techniques in artworks, are presented and compared with those previously obtained using separate devices. Concerning reflectance data, improvements in the deep detectable UV spectral range have been achieved switching from the integrating sphere of the old instrument to bifurcated optical fibers used as probe in the new one. Special attention was devoted to test the instrument capability in order to obtain the true emission spectrum, corrected for the selfabsorption effect, for which good results were found. This particular experimental procedure is strongly recommended, by a diagnostic point of view, to avoid distortions in the instrumental responses, namely the spectral shape and emission maximum wavelength of a fluorophore as function of the color saturation.

A non-invasive NMR relaxometric characterization of the cyclododecane-solvent system inside porous substrates.

With the aim of deepening the knowledge on the behavior of cyclododecane (CDD) as a temporary consolidant agent for weathered stones, NMR longitudinal and transverse relaxation decays have been exploited to follow the distribution of cyclododecane solutions into porous matrices. By measuring as function of time the relaxation decay constants of CDD solutions dropped onto porous supports, it has been possible to differentiate the step encompassing the solvent evaporation, which determines the consolidant migration within the matrix, from that governing the consolidant sublimation, which is related to the consolidation effectiveness over time. Copyright

Molecular and structural characterization of some violet phosphate pigments for their non-invasive identification in modern paintings

Abstract A complete non-invasive characterization by XRF, XRD, near-FTIR and UV–Vis reflectance spectroscopy has been performed on some commercially available violet pigments as well as on pure violet Co-salts also known to be used as pigments. The obtained results show that, after a preliminary elemental characterization, the studied pigments can be easily identified by near-FTIR and UV–Vis spectroscopies since they exhibit peculiar spectral bands in these regions. Among the analyzed samples emerged that the pigment 45350 - “Manganviolett” from Kremer consists of two α- and β-NH4MnP2O7 polymorphs, being α-NH4MnP2O7 the most abundant one; furthermore we found that the pigment R1215D -“Cobalt violet” by Winsor & Newton (no longer available since 2006) displays spectral features that match exactly those of 45820-“Kobaltviolett hell” from Kremer and both are composed by cobalt ammonium phosphate hydrate. Such non-invasive study allowed for the identification of “Manganese Violet” (α-NH4MnP2O7) and anhydrous cobalt phosphate (Co3(PO4)2) on some Boccionis paintings during MOLAB in situ measurements at the Museo del Novecento (Milano).

Raphael’s workshop at Villa Farnesina in Rome: the frescoed vault of Cupid and Psyche investigated by macro-X-ray fluorescence scanning

An acquisition campaign of macro-X-ray fluorescence scanning measurements on the vault of the Loggia of Cupid and Psyche, frescoed by Raphael and his workshop in the Villa Farnesina (Rome), supplied the occasion of testing the potentialities for such diagnostic tool/technique in studying wall paintings in situ. In this paper, the methodological approach is considered in depth, to highlight significant problematics deriving from nature of materials, painting technique and spatial peculiarities of this kind of artwork, as well as from the treatment and decoding of raw data. Data obtained through a single-spot XRF campaign, carried out during the past restorations of the Loggia, have been considered for comparisons.Graphical abstract