Synchrotron DUV luminescence micro-imaging to identify and map historical organic coatings on wood.

Deep ultraviolet (DUV) photoluminescence (PL) microimaging is an emerging approach to characterise materials from historical artefacts (see M. Thoury, J.-P. Echard, M. Réfrégiers, B. H. Berrie, A. Nevin, F. Jamme and L. Bertrand, Anal. Chem., 2011, 83, 1737-1745). Here we further assess the potential of the method to access a deeper understanding of multi-layered varnishes coating wooden violins and lutes. Cross-section micro samples from important 16(th)- to 18(th)-century instruments were investigated using synchrotron PL microimaging and microspectroscopy. Excitation was performed in the DUV and the near ultraviolet (NUV) regions, and emission recorded from the DUV to the visible region, at a submicrometric spatial resolution. Intercomparison of microspectroscopy and microimaging was made possible by radiometrically correcting PL spectra both in excitation and emission. Based on an optimised selection of emission and excitation bands, the specific PL features of the organic binding materials allowed a vastly enhanced discrimination between collagen-based sizing layers and oil/resin-based layers compared to epiluminescence microscopy. PL therefore appears to be a very promising analytical tool to provide new insights into the diversity of surface coating techniques used by instrument-makers. More generally, our results demonstrate the potential of synchrotron PL for studying complex heterogeneous materials beyond the core application of the technique to life sciences.

A multiscalar photoluminescence approach to discriminate among semiconducting historical zinc white pigments.

In order to fully characterize the zinc white artists' pigment (ZnO), much used since the mid-nineteenth century, three samples collected in the early 20th century were studied using a combination of synchrotron and macroscopic photoluminescence spectroscopy and imaging. An improved microscope setup based on synchrotron microspectroscopy and microimaging was used to study the powders dispersed onto indium foil. The synchrotron setup offered a diffraction-limited resolution of 153 nm. The PL spectra of individual grains were measured and the distribution of particles' emission spectra was mapped at the nanoscale. The results revealed that while the samples have apparent homogeneous photoluminescence behavior at the macroscale (bulk), their PL signatures are inhomogeneous below 20 µm. At the nanoscale the three powder samples have quite different PL signatures. Different sources, perhaps even different batches, of zinc white might be readily differentiated using this method.

In situ 3D characterization of historical coatings and wood using multimodal nonlinear optical microscopy.

We demonstrate multimodal nonlinear optical imaging of historical artifacts by combining Second Harmonic Generation (SHG) and Two-Photon Excited Fluorescence (2PEF) microscopies. We first identify the nonlinear optical response of materials commonly encountered in coatings of cultural heritage artifacts by analyzing one- and multi-layered model samples. We observe 2PEF signals from cochineal lake and sandarac and show that pigments and varnish films can be discriminated by exploiting their different emission spectral ranges as in luminescence linear spectroscopy. We then demonstrate SHG imaging of a filler, plaster, composed of bassanite particles which exhibit a non centrosymmetric crystal structure. We also show that SHG/2PEF imaging enables the visualization of wood microstructure through typically 60 µm-thick coatings by revealing crystalline cellulose (SHG signal) and lignin (2PEF signal) in the wood cell walls. Finally, in situ multimodal nonlinear imaging is demonstrated in a historical violin. SHG/2PEF imaging thus appears as a promising non-destructive and contactless tool for in situ 3D investigation of historical coatings and more generally for wood characterization and coating analysis at micrometer scale.

Synchrotron UV-visible multispectral luminescence microimaging of historical samples.

UV-visible luminescence techniques are fre-quently used for the study of cultural heritage materials, despite their limitations for identification and discrimination in the case of complex heterogeneous materials. In contrast to tabletop setups, two methods based on the vacuum ultraviolet (VUV)-UV-visible emission generated at a bending magnet of a synchrotron source are described. The main advantages of the source are the extended wavelength range attained, the continuous tunability of the source, and its brightness, leading to a submicrometer lateral resolution. Raster-scanning microspectroscopy and full-field microimaging were implemented and tested at the DISCO beamline (synchrotron SOLEIL, France). Investigative measurements were performed on a sample from a varnished musical instrument and a paint sample containing the pigment zinc white (ZnO) in order to illustrate some of the challenges analyzing heterogeneous cultural heritage cross-section samples with the novel imaging approach. The data sets obtained proved useful for mapping organic materials at the submicrometer scale and visualizing heterogeneities of the semiconductor pigment material. We propose and discuss the combined use of raster-scanning microspectroscopy and full-field microimaging in an integrated analytical methodology. Synchrotron UV luminescence appears as a novel tool for identification of craftsmen's and artists' materials and techniques and to assess the condition of artifacts, from the precise identification and localization of luminescent materials.

Identification of the finishing technique of an early eighteenth century musical instrument using FTIR spectromicroscopy.

The study of varnishes from musical instruments presents the difficulty of analysing very thin layers of heterogeneous materials on samples most of which are generally brittle and difficult to prepare. Such study is crucial to the understanding of historical musical instrument varnishing practices since written sources before 1800 are very rare and not precise. Fourier-transform infrared (FTIR) spectroscopy and imaging methods were applied to identify the major chemical components within the build-up of the varnish layers on a cello made by one of the most prominent French violin-makers of the eighteenth century (Jacques Boquay, ca. 1680-1730). Two types of FTIR imaging methods were used: scanning with a synchrotron-based microscope and full-field imaging using a 2D imager with a conventional source. An interpretation of the results obtained from these studies on the Boquay cello is that the maker first applied a proteinaceous layer, probably gelatine-based animal glue. He later applied a second layer based on a mixture of a drying oil and diterpenic resin from Pinaceae sp. From an historical perspective, the results complement previous studies by describing a second technique used for musical instrument finishes at the beginning of the eighteenth century in Europe.

Structural and optical properties of wood and wood finishes studied using optical coherence tomography: application to an 18th century Italian violin.

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.

Implementation of a neural network for multispectral luminescence imaging of lake pigment paints.

Luminescence multispectral imaging is a developing and promising technique in the fields of conservation science and cultural heritage studies. In this article, we present a new methodology for recording the spatially resolved luminescence properties of objects. This methodology relies on the development of a lab-made multispectral camera setup optimized to collect low-yield luminescence images. In addition to a classic data preprocessing procedure to reduce noise on the data, we present an innovative method, based on a neural network algorithm, that allows us to obtain radiometrically calibrated luminescence spectra with increased spectral resolution from the low-spectral resolution acquisitions. After preliminary corrections, a neural network is trained using the 15-band multispectral luminescence acquisitions and corresponding spot spectroscopy luminescence data. This neural network is then used to retrieve a megapixel multispectral cube between 460 and 710 nm with a 5 nm resolution from a low-spectral-resolution multispectral acquisition. The resulting data are independent from the detection chain of the imaging system (filter transmittance, spectral sensitivity of the lens and optics, etc.). As a result, the image cube provides radiometrically calibrated emission spectra with increased spectral resolution. For each pixel, we can thus retrieve a spectrum comparable to those obtained with conventional luminescence spectroscopy. We apply this method to a panel of lake pigment paints and discuss the pertinence and perspectives of this new approach.

Excitation emission and time-resolved fluorescence spectroscopy of selected varnishes used in historical musical instruments.

The analysis of various varnishes from different origins, which are commonly found on historical musical instruments was carried out for the first time with both fluorescence excitation emission spectroscopy and laser-induced time-resolved fluorescence spectroscopy. Samples studied include varnishes prepared using shellac, and selected diterpenoid and triterpenoid resins from plants, and mixtures of these materials. Fluorescence excitation emission spectra have been collected from films of naturally aged varnishes. In parallel, time-resolved fluorescence spectroscopy of varnishes provides means for discriminating between short- (less than 2.0 ns) and long-lived (greater than 7.5 ns) fluorescence emissions in each of these complex materials. Results suggest that complementary use of the two non destructive techniques allows a better understanding of the main fluorophores responsible for the emission in shellac, and further provides means for distinguishing the main classes of other varnishes based on differences in fluorescence lifetime behaviour. Spectrofluorimetric data and time resolved spectra presented here may form the basis for the interpretation of results from future in situ fluorescence examination and time resolved fluorescence imaging of varnished musical instruments.