Marco Realini

Subsurface Raman Analysis of Thin Painted Layers

Here we present, for the first time, an extension of spatially offset Raman spectroscopy to thin (tens of micrometers thick), highly turbid stratified media such as those encountered in paintings. The method permits the non-destructive interrogation of painted layers in situations where conventional Raman microscopy is not applicable due to high turbidity of the top layer(s). The concept is demonstrated by recovering the pure Raman spectra of paint sub-layers that are completely obscured by paint over-layers. Potential application areas include the analysis of paintings in art preservation and restoration avoiding the cross-sectional analysis used currently with this type of samples. The technique also holds promise for the development as a non-destructive subsurface tool for in situ analysis using portable instruments.

Stability and transformation mechanism of weddellite nanocrystals studied by X-ray diffraction and infrared spectroscopy.

This study is focused on the stability of weddellite, the dihydrate phase of calcium oxalate [CaC(2)O(4)·(2 + x)H(2)O], mainly detected in kidney stones and in oxalate films found on the surfaces of several ancient monuments. Its occurrence is a critical issue since, at environmental conditions, weddellite is unstable and quickly changes into whewellite, the monohydrate phase of calcium oxalate (CaC(2)O(4)·H(2)O). New single crystal X-ray diffraction experiments have been carried out, which confirm the structural model of weddellite previously published. Synthesised nanocrystals of weddellite have been kept under different hygrometric conditions in order to study, by X-ray powder diffraction, the influence of humidity on their stability. Moreover, the mechanism of transformation of weddellite nanocrystals has been investigated by infrared spectroscopy using D(2)O as a structural probe.

Influence of atmospheric pollutants on the biodeterioration of stone

Abstract Studies have been carried out on specimens of Saltrio stone, Carrara and Candoglia marbles, exposed for some years to the polluted atmosphere of Milan. The test specimens, which were directly exposed to the washing away, showed alterations in the form of grey-black stains which were different in dimensions and quantity according to the stone material, while those sheltered showed only a uniform grey deposit of particulate matter. The decay on the exposed specimens has been tested by gravimetric analyses and scanning electron microscopy (SEM) observations. Microbiological investigations evidenced that microorganisms, either chemoautotrophic or photosynthetic, were not present in any sample. On the washed out test specimens, the presence of heterotrophic bacteria was considerable, while the sheltered specimens showed negligible values. Chemical analysis, carried out through gas chromatography–mass spectrometry (GC/MS), of the particulate deposited on the test specimens, evidenced the presence of a wide range of aromatic and aliphatic hydrocarbons and fatty acids. Cultural tests of mixed cultures and isolated strains were carried out by using a mineral medium added with 14 organic compounds, typical representatives of atmospheric pollution, as a sole source of carbon and energy. The results evidenced that the mixed cultures in particular, as well as some of the isolated strains, are capable of using many of the tested molecules (fatty acid, aliphatic compounds, p-xylene).

Rate of formation of black crusts on marble. A case study

Abstract The formation of black crusts on stone monuments is an important process in stone deterioration. The aim of this work is to study the rate of formation of crusts in an urban area for which pollution levels are well known. Samples of crust were collected from measured areas of two sculptural groups (made from white Carrara marble) inaugurated in 1937 on the front of Milan General Hospital and never restored. Analyses were carried out on ground samples by XRD, ionic chromatography and SEM. Gypsum is the main component followed by carbonaceous particles and iron oxides. The rate of formation of the crust, calculated considering the average crust thickness, the sample weight, the area of sampling and the length of exposure to the atmospheric pollution (54 years), is 2–5 μm per year. The amount of gypsum formed per unit surface (5–13 mg/cm2) has been calculated from the sulphate content and the sample weight per unit surface; the rate of gypsum formation in the black crust is about 0.2 mg/cm2 per year.

Noninvasive analysis of thin turbid layers using microscale spatially offset Raman spectroscopy

Here, we demonstrate, for the first time, the extension of applicability of recently developed microscale spatially offset Raman spectroscopy (SORS), micro-SORS, from the area of cultural heritage to a wider range of analytical problems involving thin, tens of micrometers thick diffusely scattering turbid layers. The method can be applied in situations where a high turbidity of layers prevents the deployment of conventional confocal Raman microscopy with its depth resolving capability. The method was applied successfully to detect noninvasively the presence of thin, highly turbid layers within polymers, wheat seeds, and paper. An invasive, cross sectional analysis confirmed the micro-SORS findings. Micro-SORS represents a new Raman imaging modality expanding the portfolio of noninvasive, chemically specific analytical tools.

Nd:YAG laser effects on inorganic pigments

Abstract The effects of a Q-switched Nd:YAG laser irradiation on a group of supports, painted with six different inorganic pigments, was investigated. The pigments were chosen from among the most utilized on painted historical surfaces: red ochre, yellow ochre, chromium green, ultramarine azure, white chalk and carbon black, and they were distempered with two binders – linseed oil and gypsum – on primed wood panels. The pigments were characterized by means of X-ray diffraction and Fourier transform infrared spectroscopy. Chromatic characteristics, chemical composition and surface morphology of the painted layers were investigated before and after the laser irradiation. Three different fluences were used to detect the correlation between laser parameters and changes in painting layer characteristics.

Synthesis of calcium oxalate trihydrate: New data by vibrational spectroscopy and synchrotron X-ray diffraction.

Calcium oxalate is found in nature in three different crystalline states determined by the number of H2O in the unit formula (whewellite CaC2O4·H2O, COM; weddellite CaC2O4·(2+x)H2O, COD and caoxite CaC2O4·3H2O, COT). The properties of these materials are relevant in the field of biomedicine, cultural heritage and mineralogy. In two previous papers, we have used X-ray diffraction and vibrational spectroscopy (infrared and Raman) to derive information on crystal and molecular structures of COM and COD. In this paper, we complete the synthesis and analysis on the third form, COT, and present a comparative study of the data collected from the three crystalline states. The experiments clearly highlight the role played by the H2O molecules linked within the structure by different kinds of hydrogen bonds. The vibrational assignment of the infrared and Raman bands are critically proposed. The fact relevant for the work in biomedicine, cultural heritage and crystallography is that a simple examination of the spectra allows quickly to determine the chemical nature of the material in an unknown sample even in a minute quantity or in awkward experimental conditions.

Monte Carlo simulations of subsurface analysis of painted layers in micro-scale spatially offset Raman spectroscopy.

A recently developed micrometer-scale spatially offset Raman spectroscopy (micro-SORS) method provides a new analytical capability for investigating nondestructively the chemical composition of subsurface, micrometer-scale-thick, diffusely scattering layers at depths beyond the reach of conventional confocal Raman microscopy. Here we provide, for the first time, the theoretical foundations for the micro-SORS defocusing concept based on Monte Carlo simulations. Specifically, we investigate a defocusing variant of micro-SORS that we used in our recent proof-of-concept study in conditions involving thin, diffusely scattering layers on top of an extended, diffusely scattering substrate. This configuration is pertinent, for example, for the subsurface analysis of painted layers in cultural heritage studies. The depth of the origin of Raman signal and the relative micro-SORS enhancement of the sublayer signals reached are studied as a function of layer thickness, sample photon transport length, and absorption. The model predicts that sublayer enhancement initially rapidly increases with increasing defocusing, ultimately reaching a plateau. The magnitude of the enhancement was found to be larger for thicker layers. The simulations also indicate that the penetration depths of micro-SORS can be between one and two orders of magnitude larger than those reached using conventional confocal Raman microscopy. The model provides a deeper insight into the underlying Raman photon migration mechanisms permitting the more effective optimization of experimental conditions for specific sample parameters.

Molecular Tools Applied to the Study of Deteriorated Artworks

Traditional methods normally used to study the microbial populations present on artwork surfaces are time-consuming and often do not reveal the specific characteristics of the microbial ecotypes. The development of new powerful tools from molecular biology (such as polymerase chain reaction [PCR], simple and effective cloning and electrophoretic systems [SSCP, DGGE, TGGE] for nucleic acids separation, automated DNA sequencing facilities and wide databases which rapidly accumulate sequences of signature bacterial genes such as ribosomal genes) have greatly amplified the resolution power of microbial communities analysis (Rolleke et al., 1996; Wollenzien et al., 1997; Moreira and Amils, 1996). In this report, we analyse the contribution of different molecular methods and techniques in investigating the microbial diversity on stoneworks, in relation to the microbe itself and to genes which may play some role in the metabolism of organic air pollutants, and in detecting, by means of molecular probes, particularly dangerous microorganisms whose presence is suspected. A study of the microbial biodiversity of Carrara marble and other stone materials, particularly the presence of Geodermatophilus-like organisms, is reported. Among the isolated strains, a strain phenotypically similar to Geodermatophilus but genotypically quite divergent from this species has been identified. The partial sequence of the 16S rDNA showed that the strain may represent a new species and even a new genus of the actinomycetes. Moreover, a method to detect directly on the stone samples the presence of genes involved in aromatic hydrocarbon biodegradation is proposed in order to demonstrate the capability of microorganisms to grow by degrading aromatic atmospheric pollutants. Finally, a method is proposed for the detection of Bacillus strains. In particular, a PCR-based system has been developed to monitor directly on stone samples and without isolation the members of the B. cereus group, which are widespread in the environment and are frequently found on altered stoneworks (Zanardini et al., 1997). In several cases the bio-molecular techniques proved to be a valid tool for the investigation of artwork biodeterioration.

Discovering Hidden Painted Images: Subsurface Imaging Using Microscale Spatially Offset Raman Spectroscopy

We demonstrate for the first time the mapping capability of micro-spatially offset Raman spectroscopy (micro-SORS). The technique enables to form noninvasive images of thin sublayers through highly turbid overlayers. The approach is conceptually demonstrated on recovering overpainted images in situations where conventional Raman microscopy was unable to visualize the sublayer. The specimens mimic real situations encountered in Cultural Heritage that deal, for example, with hidden paintings vandalized with graffiti or covered by superimposed painted layers or whitewash. Additionally, using a letter as a hidden image, we demonstrated the micro-SORS potential to reconstruct also a hidden writing covered, for example, with paper sheets that cannot be easily removed. Potential applications could also include other disciplines such as polymers, biological, catalytic, and forensic sciences where thin, highly turbid layers mask chemically distinct subsurface structures.