Raman Focal Point On Roman Egyptian Blue Elucidates Disordered Cuprorivaite Green Glass Phase And Trace Compounds | Scientific Reports Nature.com
Raman Focal Point On Roman Egyptian Blue Elucidates Disordered Cuprorivaite, Green Glass Phase And Trace Compounds | Scientific Reports – Nature.com https://collincountynewsonline.com/raman-focal-point-on-roman-egyptian-blue-elucidates-disordered-cuprorivaite-green-glass-phase-and-trace-compounds-scientific-reports-nature-com/
Scientific Reports volume 12, Article number: 15596 (2022) Cite this article
Abstract
The discussed comparative analyses of Roman Imperial pigment balls and fragmentary murals unearthed in the ancient cities of Aventicum and Augusta Raurica (Switzerland) by means of Raman microspectroscopy pertain to a predecessor study on trace compounds in Early Medieval Egyptian blue (St. Peter, Gratsch, South Tyrol, Northern Italy). The plethora of newly detected associated minerals of the raw materials surviving the synthesis procedure validate the use of quartz sand matching the composition of sediments transported by the Volturno river into the Gulf of Gaeta (Campania, Southern Italy) with a roasted sulphidic copper ore and a mixed-alkaline plant ash as fluxing agent. Thus, the results corroborate a monopolised pigment production site located in the northern Phlegrean Fields persisting over the first centuries A.D., this in line with statements of the antique Roman writers Vitruvius and Pliny the Elder and recent archaeological evidences. Beyond that, Raman spectra reveal through gradual peak shifts and changes of band width locally divergent process conditions and compositional inhomogeneities provoking crystal lattice disorder in the chromophoric cuprorivaite as well as the formation of a copper-bearing green glass phase, the latter probably in dependency of the concentration of alkali flux, notwithstanding that otherwise solid-state reactions predominate the synthesis.
Introduction
During the Roman period Egyptian blue was circulated throughout the Empire in the quasi standardised form of small balls of around 15 to 20 mm in diameter, thus the painter defined the respective grain size and by that the shade of blue and the covering capacity of the ground up artificial pigment himself1,2. In the first century B.C. Vitruvius provided the following guidance for its preparation in his architectural textbook De architectura libri decem (Liber VII, Caput XI), leaving out any details on quantities and processing temperature: “The recipes for [sky] blue were first discovered in Alexandria, and subsequently Vestorius began to manufacture it in Puteoli as well. […] Sand is ground with flower of natron […] so finely that it almost becomes like flour. Copper [ore], broken by coarse files until it is like sawdust, is sprinkled with this sand until it clings together. Then it is formed into balls by rolling it between the hands and bound together to dry. Once dry, the balls are put into a ceramic pitcher, and the pitchers are put into a kiln”3. In view of archaeological evidence and the concordant information given by Vitruvius as well as Pliny the Elder (first century A.D.)4, current research assumes a monopolised production site in the area of the ancient cities of Cumae and Liternum (Gulf of Pozzuoli, Campania, Southern Italy), whereas manufacture in Central Europe is excluded due to a very probable lack of technological abilities5,6,7,8,9,10. According to modern laboratory experiments, Egyptian blue is synthesised from a raw material blend of quartz sand, limestone, sulphidic copper or copper carbonate ore and alkali flux in the form of either natron or ash from halophytes (salt plants) at temperatures between 850 and 1000 °C under oxidising conditions9,11,12,13,14,15,16.
Only recently, a study on a monochrome blue mural fragment belonging to the Early Medieval church of St. Peter above Gratsch (South Tyrol, Northern Italy, fifth/sixth century A.D.) by means of area-covering Raman microspectroscopic imaging resulted in the identification of 26 minerals down to the sub-permille level in addition to the chromophoric cuprorivaite CuCaSi4O10—an assemblage suggestive of type and provenance of the raw materials and of chemical reactions occurring during pigment manufacture and application as well as ageing of the pictorial layer17. Especially some accessory minerals attributable to the quartz sand, which survived processing without thermal alterations, were indicative of an import of the Egyptian blue in question from the northern Phlegrean Fields in Campania. As detailed below, analogous analyses of pigment balls and of a fragment of a wall painting unearthed in the archaeological remains of the ancient Roman cities of Aventicum and Augusta Raurica (Switzerland) (Fig. 1) further extended the plethora of hitherto uncovered trace compounds and revealed, beyond that, particularities concerning the formation of crystalline and amorphous phases or the thermal history of the artificial blue, respectively.
Figure 1
Egyptian blue pigment balls and mural fragment unearthed in the archaeological remains of the ancient Roman cities of Aventicum (top) and Augusta Raurica (bottom).
Samples
Table S1 in the Supplementary Information provides an overview of the Egyptian blue samples analysed in the present and predecessor paper17. The pigment balls under study (Fig. 1) are dated via adjoining archaeological finds, especially ceramics, to the second or third quarter of the first century A.D. (Roman Colonia Augusta Raurica) and the second half of the first century A.D. or the beginning of the second century A.D. (Aventicum, principal place of the Roman Civitas Helvetiorum). The monochrome blue mural fragment (excavated in the remains of the city of Augusta Raurica) in turn is assigned to the first half of the third century A.D.
The modern Egyptian blue used as reference material for Raman experiments is part of the range of re-enacted historical pigments of the colour mill Kremer Pigmente (Aichstetten, Germany).
Methods
Raman microspectroscopic imaging
Raman spectra were acquired using a Horiba JobinYvon Labram HR800 Raman microscope with 532 nm continuous-wave laser excitation (diode-pumped solid-state laser, 40 mW maximum power at the sample surface, reduced to 20 mW by a neutral density filter). The laser light was focused onto the sample surface and the reflected and/or scattered light was collected in upright configuration by using a 50×/N.A. = 0.55 long-working-distance microscope objective (with N.A. denoting the numerical aperture) leading to a focus diameter of approximately 1.2 µm. Dispersion of the Stokes-Raman-scattered light in a 800 mm spectrometer was carried out with a 300 mm−1 grating, and spectra were detected by a Peltier cooled (− 60 °C) charge coupled device (CCD) Syncerity camera (Horiba JobinYvon) having 1024 pixels along the wavenumber axis, resulting in spectra ranging from approx. 70 cm−1 to 3250 cm−1 with a spectral resolution of 3.7 cm−1 to 2.6 cm−1 per CCD pixel. Raman maps were gathered by software-controlled (Horiba JobinYvon LabSpec 6) stepwise movement of the sample stage through the laser focus with a step size of 1 µm. Typical acquisition time per pixel or spectrum, respectively, was 0.5 s with 10 to 40 accumulations, chosen depending on the signal to noise ratio and available measurement time. Single spectra, acquired independently of mappings, were typically measured within 1 min split into several accumulations (e.g., 6 × 10 s). See Ref.18 for further specifics of the employed instrument and an introduction to Raman microspectroscopic imaging, and Ref.17 for details on the optimisation of the measurement parameters adopted from the predecessor study on trace compounds in Early Medieval Egyptian blue. As the conditions cannot be adjusted to every mineral individually, the chosen irradiance reflects a compromise between sensitivity and non-destructiveness. Therefore, thermal conversion of coloured sulphides and oxysalts cannot be ruled out completely (see the section ‘Contaminations from adherent soil minerals’ below as well as the Supplementary Information of the predecessor study17 and references therein), which was considered in the interpretation of the results.
Measurement areas were randomly selected on the surfaces of the four samples. Mapping sizes were chosen depending on local sample roughness and ranged from 28 × 40 to 161 × 141 pixels. For each sample 12 to 16 Raman maps were acquired within a typical measurement time of 50 h each (when assuming 100 × 100 pixels and 36 × 0.5 s per pixel as typical mapping conditions). Altogether, 100,016 (pigment ball Aventicum forum; see top-left image in Fig. 1), 100,318 (pigment ball Aventicum insulae 15; Fig. 1, top right), 101,091 (pigment ball Augusta Raurica) and 99,885 spectra (mural fragment Augusta Raurica), respectively, were collected. These 401,310 spectra were evaluated by using own (T.S.) LabView-based (National Instruments, Austin, TX, USA) software developed for analysing Raman maps and enabling the calculation of two-dimensional distributions of baseline-corrected peak intensities of each Raman band found in a dataset and extracting their individual spectra. The latter were assigned to mineral phases by comparison with reference data from the RRUFF spectral library (https://rruff.info)19 or from the literature (see Figs. S3–S29, S35, S36, S42, S46, S49, S50, S53 and S54 in the Supplementary Information).
Raman spectroscopy for analysing pyrometamorphic conversions
For simulating the effect of heat in an ancient furnace onto selected mineral phases, some preliminary in situ Raman measurements were carried out by employing a TS-1500 heating stage from Linkam Scientific Instruments Ltd. (Redhill, Surrey, UK) with a T96-LinkPad controller, placed under the same microscope objective with approx. 1 cm working distance mentioned above. Overall, the same typical measurement parameters were used. Such temperature-dependent experiments began with the measurement of the room-temperature spectrum of the sample in the 7 m...
