In situ Raman mapping of art objects

Developing Macro-Raman Mapping as a Tool for Studying the Pigment Distribution of Art Objects

Raman spectroscopy is a well-appreciated technique in cultural heritage research for its ability to obtain molecular information nondestructively. During Raman mapping experiments, advantage is taken of the excellent spatial resolution of the approach, allowing to visualize the spatial distribution of the molecules. In the current research, macro-Raman mapping is proposed, allowing us to map large areas of an artwork (typically tens or hundreds of square centimeters). Therefore, a new setup is made, using a commercially available mobile Raman spectrometer and fast translation stages. Moreover, the probe is equipped with a triangulator to measure the distance to the surface of the artwork and thus achieving accurate focusing of the Raman probe. Finally, the correct setup is guaranteed by using a calibration module that is designed to allow for spectral calibration and aligning all components of the probe. The use of the method is demonstrated by three cases, where different data processing techniques are illustrated.

Standardization of Raman spectra using variable penalty dynamic time warping

Raman spectroscopy can provide structural fingerprints to identify molecules by means of spectral library searching. However, it is difficult to share the spectral library between different Raman spectrometers because of the nonlinear displacement in Raman shift. In this study, we propose a Raman spectra Standardization method using Variable Penalty dynamic time warping (RS-VPdtw), which can synchronize the nonlinear displacement between spectra acquired with different spectrometers. We have compared the standardization performance of RS-VPdtw and MWFFT on the spectra of 13 real samples acquired with 6 different spectrometers. The mean spectral similarity of RS-VPdtw and MWFFT increased from 0.79 to 0.97 and 0.91 respectively. Results show that RS-VPdtw is significantly better than MWFFT in Raman spectra standardization. The Raman spectra acquired with different spectrometers can be standardized by RS-VPdtw to search the same spectral library, which can avoid the time-consuming and labor-intensive reestablishment of spectral libraries for different spectrometers. This means that RS-VPdtw is a promising and valuable method to solve the spectra standardization problem in large-scale applications of Raman spectroscopy.

Springtail coloration at a finer scale: mechanisms behind vibrant collembolan metallic colours

The mechanisms and evolution of metallic structural colours are of both fundamental and applied interest, yet most work in arthropods has focused on derived butterflies and beetles with distinct hues. In particular, basal hexapods-groups with many scaled, metallic representatives-are currently poorly studied and controversial, with some recent studies suggesting either that thin-film (lamina thickness) or diffraction grating (longitudinal ridges, cross-ribs) elements produce these colours in early Lepidoptera and one springtail (Collembola) species. Especially the collembolan basal scale design, consisting of a single lamina and longitudinal ridges with smooth valleys lacking cross-ribs, makes them an interesting group to explore the mechanisms of metallic coloration. Using microspectroscopy, Raman spectroscopy, electron microscopy and finite-difference time-domain optical modelling, we investigated scale colour in seven springtail species that show clear metallic coloration. Reflectance spectra are largely uniform and exhibit a broadband metallic/golden coloration with peaks in the violet/blue region. Our simulations confirm the role of the longitudinal ridges, working in conjunction with thin-film effects to produce a broadband metallic coloration. Broadband coloration occurs through spatial colour mixing, which probably results from nanoscale variation in scale thickness and ridge height and distance. These results provide crucial insights into the colour production mechanisms in a basal scale design and highlight the need for further investigation of scaled, basal arthropods.

Raman imaging of microbial colonization in rock-some analytical aspects

Raman imaging allows one to obtain spatially resolved chemical information in a nondestructive manner. Herein, we present analytical aspects of effective in situ and in vivo Raman imaging of algae and cyanobacteria from within their native rock habitats. Specifically, gypsum and halite inhabited by endolithic communities from the hyperarid Atacama Desert were analyzed. Raman imaging of these phototrophic colonization reveals a pigment composition within the aggregates that helps in understanding some of their adaptation strategies to survive in this harsh polyextreme environment. The study is focused on methodical aspects of Raman imaging acquisition and subsequent data processing. Point imaging is compared with line imaging in terms of their image quality, spatial resolution, spectral signal-to-noise ratio, time requirements, and risk of laser-induced sample alteration. The roles of excitation wavelength, exposure time, and step size of the imaging grid on successful Raman imaging results are also discussed. Graphical abstract.

Raman imaging to quantify the thermal transformation degree of Pompeian yellow ochre caused by the 79 AD Mount Vesuvius eruption

Most of the wall paintings from Pompeii are decorated with red and yellow colors but the thermal impact of 79 AD Mount Vesuvius eruption promoted the partial transformation of some yellow-painted areas into red. The aim of this research is to develop a quantitative Raman imaging methodology to relate the transformation percentage of yellow ochre (goethite, α-FeOOH) into red color (hematite, α-Fe₂O₃) depending on the temperature, in order to apply it and estimate the temperature at which the pyroclastic flow impacted the walls of Pompeii. To model the thermal impact that took place in the year 79 AD, nine wall painting fragments recovered in the archeological site of Pompeii and which include yellow ochre pigment were subjected to thermal ageing experiments (exposition to temperatures from 200 to 400 °C every 25 °C). Before the experiments, elemental information of the fragments was obtained by micro-energy dispersive X-ray fluorescence (μ-ED-XRF). The fragments were characterized before and after the exposition using Raman microscopy to monitor the transformation degree from yellow to red. The quantitative Raman imaging methodology was developed and validated using synthetic pellets of goethite and hematite standards. The results showed almost no transformation (0.5% ± 0.4) at 200 °C. However, at 225 °C, some color transformation (26.9% ± 2.8) was observed. The most remarkable color change was detected at temperatures between 250 °C (transformation of 46.7% ± 1.7) and 275 °C (transformation of 101.1% ± 1.2). At this last temperature, the transformation is totally completed since from 275 to 400 °C the transformation percentage remained constant.

Raman spectrometer for the automated scan of large painted surfaces

In this work, a novel Raman scanner capable of performing point-to-point mapping of relatively large surface of paintings is presented. This device employs an excitation wavelength of 1064 nm, and it is equipped with a high efficiency probe in order to collect the back-scattered light from each point of analysis. The use of long depth-of-field optics as well as an autofocus system allowed maintaining the best conditions for the Raman signal acquisition during the scanning, regardless of the surface irregularities. The small dimension of the optical components and the reasonable size of mechanical parts made this instrumentation particularly suitable for on-site measurements. Finally, the Raman scanner was also equipped with an online temperature control using a thermal sensor, which allows modulating automatically the output power of the laser source in order to prevent overheating and alteration effects during the scanning process. The capabilities of the Raman scanner were evaluated scanning two valuable paintings one attributed to Ambrogio Lorenzetti and the other to Duccio da Buoninsegna of the fourteenth century.

Archaeological investigations (archaeometry)

Archaeometry is the research area on the edge between humanities and natural sciences: it uses and optimises methods from chemistry, spectroscopy, physics, biology, etc. to help answering research questions from humanities. In general, these objects are investigated for several reasons. Besides the fundamental interest to know about the materials that were used in the past, the study of artefacts can support their preservation, either by helping to select optimal storage or display conditions, either by investigating decay pathways and suggesting solutions. Other reasons for art analysis include provenance studies, dating the artefact or identifying forgeries. Since several years, Raman spectroscopy is increasingly applied for the investigation of objects of art or archaeology. The technique is well-appreciated for the limited (or even absent) sample preparation, the relative straightforward interpretation of the spectra (by fingerprinting - comparing them against a database of reference pigments) and its speed of analysis. Moreover, the small spectral footprint – allowing to record a molecular spectrum of particles down to 1 µm, the typical size of pigment grains – is certainly a positive property of the technique. Raman spectroscopy can be considered as rather versatile, as inorganic as well as organic materials can be studied, and as the technique can gather information on crystalline as well as on non-crystalline phases. As a consequence, Raman spectroscopy can be used to study antique objects and twentieth-century synthetic (organic) materials – illustrating the wide range of applications. Finally, the technique is as non-destructive, provided the laser power is kept sufficiently low not to damage the artwork. In literature, the terms “non-invasive” and “non-destructive” are used, where the first term means that no sampling is involved, and the latter term indicates that no sample is taken or that during analysis the sample is not consumed (destroyed) and remains available for further analysis.