Rooted in the long tradition of Raman spectroscopy of cultural heritage materials, in this work we provide a personal perspective on recent applications and new frontiers in sampling modalities, data processing, and instrumentation.

https://hal.archives-ouvertes.fr/hal-01455522/document

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

Raman spectroscopy is widely applied in the cultural heritage field to perform non-destructive measurements in situ, in order to identify materials, specifically pigments. The spectra collected can be challenging to interpret because certain source laser wavelengths may be absorbed by specific pigments, leading to large fluorescence backgrounds which obscure the weak Raman signals, or worse cause photodegradation of the sample. Furthermore, the reference spectra for a specific pigment obtained from a particular laser wavelength is not always available and is a crucial step in the detective work of pigment identification, especially when the resonance Raman effect can enhance some signals. As the range of lasers available increases, spectral libraries do not always record spectra acquired with the same wavelength used to carry out the measurements in field. In this work, reference spectra of 32 different compounds, mostly used in mediaeval manuscripts as pigments and inks, are recorded. Five different wavelengths were used as excitation sources. The aim is to provide a useful and more complete reference source to enable better planning of which laser wavelength is the most appropriate to study a specific set of pigments, and to allow comparisons between spectra acquired with the same wavelength, leading to the unequivocal pigment identification in a step by step manner.

/pdf/raman-spectroscopic-library-of-medieval-pigments-collected-13ryh7n0tt.pdf

Raman spectroscopic library of medieval pigments collected with five different wavelengths for investigation of illuminated manuscripts

Weathering steel was designed to resist against the atmospheric impact due to the development of a characteristic rust layer that protects the metal by reducing the corrosion rate. This rust layer is formed by different iron (oxy-hydr)oxides that acts as a barrier and provides the protective ability. In order to check the conservation state trough the protective capability, it is necessary to calculate the so-called protective ability index (PAI). It mainly takes into account the ratio (α/γ) between the mass of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), present in the rust layer of weathering steel. Raman spectroscopy, apart from a qualitative technique, is recently becoming a semi quantitative approach. Therefore, it is a valuable tool for the calculation of the mentioned index. This paper is a widening of a previous work in which it was uncovered a problem in different weathering steel sculptures which are exposed to a Cl− and SO2-rich urban atmosphere because they present different esthetical problems on their surfaces, such as detachments of steel chips, discolorations and irregularities. In order to relate these problems on the surface and the rust layer composition, PAI calculations were performed from the phase identification obtained from Raman imaging and using spectra decomposition to quantify each compounds. Although goethite and lepidocrocite were the most commonly detected compounds, hematite, akaganeite and magnetite were also identified. Hematite is a sub product formed in rich SO2 atmospheres; therefore, a PAI with its content is proposed. Protective indexes obtained for the different artworks exposed in different places provide quantitative clues for the conservation diagnosis of weathering steel structures. Copyright © 2014 John Wiley & Sons, Ltd.

Protective ability index measurement through Raman quantification imaging to diagnose the conservation state of weathering steel structures

Raman spectroscopy is considered as one of the most appreciated analytical techniques in the cultural heritage research field, thanks to some of its favourable characteristics such as the relatively quick time of analysis, identification of both inorganic and organic molecules. The necessity of conducting the research non-destructively on-site and directly on the artefact brought the technique out of the laboratory. Since 2004, when the first non-commercial mobile Raman instrument for in situ applications on cultural heritage objects was realized, years of continuous technical and instrumental developments have passed. Today, on-site Raman spectroscopy is considered as a cornerstone technique for the documentation of materials of works of art and the assessment of their conservation state without jeopardizing the integrity of the artefact. This paper reviews some of the breakthroughs of Raman analysis of cultural heritage objects, conducted on field (museum environment, outdoors and archaeological complexes).

In situ Raman spectroscopy for cultural heritage studies

The built Heritage constantly suffers different deterioration processes caused by the action of external agents, being one of the main consequences of the formation of soluble salts. These salts appear as efflorescences or subefflorescences that by hydration and dehydration cycles and/or dissolution processes producing crystallization of salts within the pores may promote internal fractures and material loss. The assessment of building material conservation on a 15th century Palace house, located close to the Urola river (Azpeitia, Basque Country, North of Spain), has been performed by in-situ characterization of pathologies on sandstone, mortar and limestone affected by infiltration waters. Portable Raman and X-ray fluorescence spectrometers have been used to characterize the bulk materials as well as the salts present along the different walls of the Palace. The in-situ analysis by those portable equipments has been complemented with laboratory measurements on selected samples, taken close to the spots analysed by the portable instruments. Different carbonates (natrite, termonatrite, natron, calcite and gaylussite), oxides (hematite, limonite and rutile), feldspar (orthoclase) and silicates (quartz) have been identified as original compounds. Furthermore, nitrates (niter, nitrocalcite and nitratine) and sulphates (gypsum and thenardite) have been found as decaying compounds permitting to establish the degradation processes of the attack produced by the infiltration waters on the different building materials. Moreover, the study revealed that certain materials used in a previous restoration produced new degradation processes, evidencing the importance of a proper selection of the materials to be used in the interventions. Copyright © 2013 John Wiley & Sons, Ltd.

In-situ spectroscopic assessment of the conservation state of building materials from a Palace house affected by infiltration water

Weathering steel, and particularly CorTen steel, is a very used material for modern artworks exhibited outdoors One of the characteristic that makes this material so attractive to artists is the property to develop a protective rust layer composed by iron oxides which preserves its metallic core from atmospheric corrosion This study was conducted to evaluate the conservation state of four CorTen sculptures by Eduardo Chillida, located in different places of Bilbao city (north of Spain) and affected by different factors (environmental among others) by using Raman spectroscopy Measurements were performed in situ with a handheld Raman spectrometer mounted on a tripod with x–y–z axes motorization at the micron level The most common oxyhydroxides detected were lepidocrocite (γ-FeO(OH)), goethite (α-FeO(OH)), hematite ( -Fe2O3) and magnetite (Fe3O4), being goethite the most stable phase All the iron oxyhydroxides were identified in all of the studied sculptures but their relative amounts were different for each sculpture The consequences of the marine aerosols exposure in the steel surface were also studied, detecting limonite (FeO(OH)·nH2O) and akaganeite (β-FeO(OH)) The results confirmed that the evolution of the rust layer present on the analyzed weathering steels is different, depending both on the exposure and the particular type of the steel Copyright © 2012 John Wiley & Sons, Ltd

Portable Raman study on the conservation state of four CorTen steel‐based sculptures by Eduardo Chillida impacted by urban atmospheres

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Portable Raman study on the conservation state of four CorTen steel‐based sculptures by Eduardo Chillida impacted by urban atmospheres