Raman Spectroscopy

Raman analyses with portable instrumentation (pRS) on the objects and with microscope (μRS) on the samples

Raman spectroscopy is a powerful analytical technique with excellent characteristics for the analysis of cultural heritage: materials are identified from their fingerprint, a set of signals in the spectrum that correspond to the vibrations of the molecular bonds [Edwards and Chalmers 2005, Vandenabeele and Edwards 2018], and especially in pigments characterization [Bell et al 1997, Bersani and Lottici 2016]. Portable instruments are nowadays available, which allow the study of pigments directly in the museum or conservation studio, minimizing sampling and handling of objects. The analysis of ancient inscriptions with Raman spectroscopy benefits from the fact that ancient red pigments are well known and limited to a small range (haematite, realgar, red lead, cinnabar, realgar), with known economic and symbolic value [Payne and Booms 2014, Barone et al 2018, Coccato et al 2024]. On the other hand, both practical and analytical challenges have to be faced. First of all, most of the inscriptions are engraved, with the paint applied in the grooves, which limits the application of the hand-held probe to letters painted on the flat surface of the stone. Secondly, inscriptions have been subjected to cleaning and "squeezing" (the practice of beating a piece of wet filter paper in the grooves to create a 3D reproduction of the text [Gordon 1983]), which means that traces of paint have been removed and/or transferred to the paper, which makes the analysis of the coloured areas difficult due to the poor preservation of the paint layer. Nevertheless, it has been demonstrated that a minimally-invasive sampling can be carried out with a dry cotton-swab rolled on the preserved paint [Coccato et al 2016]: this is sufficient to remove a few particles of colouring matter, which are then tapped on a glass slide for micro-Raman analyses on individual pigment grains. This does not affect the appearance of the object and allows characterization of the materials.

Spectra acquisition

The in situ campaign was conducted using an i-Raman spectrometer (B&WTek Inc.), equipped with a 785 nm diode laser (maximum power of 300 mW), which can be connected to a 1.5 m fibre-optic handheld probe-head (<100μm spot-size), or attached to a portable microscope head with digital camera (ca. 1mm spot-size for the 4× objective). In the first case, to ensure light blocking and focussing, the head is equipped with a plastic spacer and the measurements are performed in contact; in the latter, the microscope and camera allow focussing on the desired area, minimizing the collection of stray environmental light. The background signal ("dark") is nevertheless measured and subtracted by the operating software, to minimize interferences. The backscattered Raman radiation is dispersed by a holographic grating on a Peltier-cooled CCD detector (−4◦C). The nominal spectral resolution of the instrument is 4.5 1/cm. The parameters of acquisition were adapted to each sample, taking into account the positioning system (for hand-held analyses a maximum of 1 minute total acquisition time was the standard). Power was kept below 30mW, time between 1 and 10s with accumulations between 3 and 60.

In the laboratory, a Jasco NRS3100 Micro-Raman spectrometer was used. The instrument is equipped with 50× LWD (long working distance) and 100× objectives, two lasers (532nm and 785nm), a Notch filter and a Peltier-cooled (−50 ◦C) 1024 × 128 CCD detector. A spectral resolution of 1 1/cm is reached (1800 gr/mm grating), while the minimum lateral and depth resolutions can be as low as 1 µm by means of a confocal slit. Calibration was checked with the 520.7 1/cm Raman band of silicon before each experimental session. The parameters of acquisition were adjusted for each sample taking into account its colour and the arising fluorescence. Power was kept below 10mW, time between 10 and 60s with accumulations between 3 and 100.

Spectra processing

All the spectra were elaborated in LabSpec (baseline subtraction, band deconvolution, smoothing, etc) and compared to reference databases, such as RRUFF [Lafuente et al 2015] and published collections [Bell et al 1997, Caggiani et al 2016, Marucci et al 2018].

References

• Barone, G., et al. "In situ Raman and pXRF spectroscopic study on the wall paintings of Etruscan Tarquinia tombs." Dyes and Pigments 150 (2018): 390-403. https://doi-org.ezproxy-prd.bodleian.ox.ac.uk/10.1016/j.dyepig.2017.12.008
• Bell, Ian M., Robin JH Clark, and Peter J. Gibbs. "Raman spectroscopic library of natural and synthetic pigments (pre-≈ 1850 AD)." Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 53.12 (1997): 2159-2179. http://dx.doi.org/10.1016/S1386-1425(97)00140-6
• Bersani, Danilo, and Pier Paolo Lottici. "Raman spectroscopy of minerals and mineral pigments in archaeometry." Journal of Raman Spectroscopy 47.5 (2016): 499-530. https://doi-org.ezproxy-prd.bodleian.ox.ac.uk/10.1002/jrs.4914
• Caggiani, Maria C., Antonino Cosentino, and Annarosa Mangone. "Pigments Checker version 3.0, a handy set for conservation scientists: A free online Raman spectra database." Microchemical Journal 129 (2016): 123-132. http://dx.doi.org/10.1016/j.microc.2016.06.020
• Coccato, Alessia, et al. "Pigment particles analysis with a total reflection X-ray fluorescence spectrometer: study of influence of instrumental parameters." Applied Physics A 122.12 (2016): 1051. https://doi.org/10.1007/s00339-016-0581-y
• Coccato, Alessia, et al. "Initial investigations of rubricated inscriptions from Roman Sicily: Comparing the material evidence with ancient writers’ ideals." Technè. La science au service de l’histoire de l’art et de la préservation des biens culturels 57 (2024): 38-47. https://doi.org/10.4000/12cw2
• Edwards, Howell GM, and John M. Chalmers, eds. Raman spectroscopy in archaeology and art history. Vol. 9. Royal Society of Chemistry, 2005.
• Gordon, Arthur E. Illustrated introduction to Latin epigraphy. Univ of California Press, 1983, p. 30-32.
• Lafuente, Barbara, et al. "1. The power of databases: The RRUFF project." Highlights in mineralogical crystallography. De Gruyter (O), 2015. 1-30. http://dx.doi.org/10.1515/9783110417104-003
• Marucci, Giorgia, et al. "Raman spectroscopic library of medieval pigments collected with five different wavelengths for investigation of illuminated manuscripts." Analytical methods 10.10 (2018): 1219-1236. http://dx.doi.org/10.1039/c8ay00016f
• Payne, Emma, and Dirk Booms, "Analysis of pigment palettes as evidence for room status in Nero’s Golden House." The British Museum Technical Research Bulletin 8 (2014): 117-126. https://web.archive.org/web/20180410093851id_/http://www.britishmuseum.org/pdf/BMTRB_8_Payne_and_Booms.pdf
• Vandenabeele, Peter, and Howell Edwards, eds. Raman Spectroscopy in Archaeology and Art History: Volume 2. Royal Society of Chemistry, 2018.