Raman spectroscopy as a tool for provenancing black limestones (bigi morati) used in antiquity

Abstract

Among precious stones used in antiquity, bigi morati stand out for their importance. They consist in black limestones mainly employed in sculptures and architectural heritages and quarried from several sites through the Mediterranean area. Their provenance is difficult to assess due to their similar appearance to the naked‐eye observation and the possible confusion with black marbles (i.e., bigi antichi). Classical methods used for studying ancient marbles, such as minero‐petrographic and stable isotopes analyses (δ13C and δ 16O), resulted to be powerful in provenancing these black lithotypes. However, these methods are microdestructive and cannot be always applied in the case of precious artefacts for which sampling is not allowed. Bearing carbonaceous matter conferring them the typical black‐dark grey colour, Raman spectroscopy might support studies on these limestones, bringing important information about the structural state of organic/carbonaceous matter and their geological evolution. In this study, the potential of Raman spectroscopy in determining geological and geographic provenance of bigi morati was tested on quarry samples from six among the most famous ancient exploitation sources. The obtained results demonstrated that the use of Raman parameters typical of low‐ordered carbon matter is an effective tool in discriminating provenance. Moreover, the laboratory‐based approach lays the groundwork for future development and validation of the method for in situ measurements on artefacts by using portable equipment.

Provenance of the white and polychrome marbles used for the architecture and sculpture of roman Sentinum (Sassoferrato, Marche, Italy)

“…Its source quarries were situated near the ancient city of Teos, in the vicinity of the present-day village of Sigacik, in the province of Izmir. The quarries also yielded a variety of bigio antico (Brilli et al, 2010;Raneri et al, 2021), with production probably forming the focus once supplies of the finer breccia were depleted.…”

Applications of Raman spectroscopy in art and archaeology

“…These applications are restricted or impossible in cases of precious artefacts where sampling is not allowed. Raneri et al [ 30 ] tested laboratory‐based μ‐Raman spectroscopy to establish the provenance of grey and black limestones used in antiquity. The applied non‐destructive method is based on spectroscopic parameters extracted from Raman spectra of carbonaceous components in the rocks.…”

Evaluation of carbonification of coals using a portable Raman spectrometer

“…[ 13–16 ] Raman microspectroscopy can be applied as a supporting method in determination of provenance of carbonates, respectively, quartz tool stones containing traces of carbonaceous matter (marbles and limestones). [ 17–19 ] Graphite, carbonaceous matter from rocks and other carbons (e.g., from meteorites), is commonly studied mostly using green Ar lasers with the typical wavelength of 514.5 nm. [ 10,20,21 ]…”

“…This method was recently used for a provenience study of black limestones (bigi morati) used in antiquity by Raneri et al (2021). [ 18 ]…”

“…[13][14][15][16] Raman microspectroscopy can be applied as a supporting method in determination of provenance of carbonates, respectively, quartz tool stones containing traces of carbonaceous matter (marbles and limestones). [17][18][19] Graphite, carbonaceous matter from rocks and other carbons (e.g., from meteorites), is commonly studied mostly using green Ar lasers with the typical wavelength of 514.5 nm. [10,20,21] The study of carbonaceous matter is complex and it requires to consider also heterogeneity in the case of coals and anisotropy in the case of graphitic matter during the acquisition of Raman spectra.…”