Noninvasive quantitative assessment of collagen degradation in parchments by polarization-resolved SHG microscopy.

Nondestructive and noninvasive investigation techniques are highly sought-after to establish the degradation state of historical parchments, which is up to now assessed by thermal techniques that are invasive and destructive. We show that advanced nonlinear optical (NLO) microscopy enables quantitative in situ mapping of parchment degradation at the micrometer scale. We introduce two parameters that are sensitive to different degradation stages: the ratio of two-photon excited fluorescence to second harmonic generation (SHG) signals probes severe degradation, while the anisotropy parameter extracted from polarization-resolved SHG measurements is sensitive to early degradation. This approach is first validated by comparing NLO quantitative parameters to thermal measurements on artificially altered contemporary parchments. We then analyze invaluable parchments from the Middle Ages and show that we can map their conservation state and assess the impact of a restoration process. NLO quantitative microscopy should therefore help to identify parchments most at risk and optimize restoration methods.

Ion beam analysis of silver leaves in gilt leather wall coverings.

An analytical methodology involving Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectroscopy (RBS) was implemented to respectively characterize the composition and the thickness of silver leaves on gilt leather decors. These objects, ancestors of our wallpapers, are nowadays still difficult to date and their provenance is generally determined from stylistic studies. The initial aim of this study was to identify markers that could be correlated with the object provenance to help distinguishing the different gilt leathers workshops in Europe. The analytical methodology was validated on modern samples and applied to a corpus of 58 ancient gilt leathers from four countries. This study provided an assessment of the sensitivity of the ion beam techniques used, and highlighted the complexity of such analyses on thin silver leaves due to the different factors affecting them, and the composite nature of the object. Thus, the thicknesses calculated from the RBS analyses presented a great variability that seems to be related to the leaf characteristics, the manufacturing process and/or the life of the decor. Nevertheless, observations suggest that silver leaves coming from the Netherlands are thicker than the ones from Spain, Italy or France. Concerning the elemental composition, the results discarded previous hypotheses and the focus was made on gold and mercury trace elements, thus it was shown that leaves in Italian decors seem to have generally a low content of these two elements. Despite the large number of decor analyzed, the corpus should be expanded over to confirm the hypotheses raised by this research. Nevertheless the results gained from this work bring new light on the factors affecting thin metal leaves in general, which will be beneficial to all fields dealing with their analysis.

Correlative nonlinear optical microscopy and infrared nanoscopy reveals collagen degradation in altered parchments.

This paper presents the correlative imaging of collagen denaturation by nonlinear optical microscopy (NLO) and nanoscale infrared (IR) spectroscopy to obtain morphological and chemical information at different length scales. Such multiscale correlated measurements are applied to the investigation of ancient parchments, which are mainly composed of dermal fibrillar collagen. The main issue is to characterize gelatinization, the ultimate and irreversible alteration corresponding to collagen denaturation to gelatin, which may also occur in biological tissues. Key information about collagen and gelatin signatures is obtained in parchments and assessed by characterizing the denaturation of pure collagen reference samples. A new absorbing band is observed near the amide I band in the IR spectra, correlated to the onset of fluorescence signals in NLO images. Meanwhile, a strong decrease is observed in Second Harmonic signals, which are a structural probe of the fibrillar organization of the collagen at the micrometer scale. NLO microscopy therefore appears as a powerful tool to reveal collagen degradation in a non-invasive way. It should provide a relevant method to assess or monitor the condition of collagen-based materials in museum and archival collections and opens avenues for a broad range of applications regarding this widespread biological material.

Investigation of the discoloration of smalt pigment in historic paintings by micro-X-ray absorption spectroscopy at the Co K-edge.

Smalt was commonly used as a pigment by artists between the 16th and 18th centuries. It is a powdered blue potash glass colored by cobalt ions and often degrades causing dramatic changes in the appearance of paintings. The aim of the work presented in this paper was to investigate the changes in the structure and environment around the cobalt ion on deterioration, to further our understanding of the basis of the loss of color. Particles of well-preserved and altered smalt in microsamples from paintings in the National Gallery, London, and the Louvre, Paris, were analyzed using synchrotron micro-X-ray absorption spectroscopy at the Co K-edge. X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) measurements showed that in intense blue particles the cobalt is predominantly present as Co(2+) in tetrahedral coordination, whereas in colorless altered smalt the Co(2+) coordination number in the glass structure is increased and there is a shift from tetrahedral toward octahedral coordination. The extent of this shift correlates clearly with the alkali content, indicating that it is caused by leaching of potassium cations, which act as charge compensators and stabilize the tetrahedral coordination of the cobalt ions that is responsible for the blue color.

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.