*2.1. The Archaeological Samples under Investigation*

2.1.1. Samples Taken from Three Finds of Purple Pigment Excavated in Lump Form


2.1.2. Microsample Taken from the Wall Painting Excavated at Raos, Thera

• RAOS-KAL1. A microsample taken from a rosette purple detail on a wall-painting fragment from the excavated site of Raos, Thera. The site of Raos is located on the south side of the caldera, with an excellent view of the Kamenes islets, Therasia, and north Thera. A tiny portion of the sample was embedded in epoxy resin (Struers CaldoFix-2) and prepared in a polished cross-section to be studied with microRaman spectroscopy. The rest of the sample was used for HPLC analysis, first reported in [30] and interpreted comparatively in this work, along with the microRaman spectroscopic results, presented here for the first time.

#### *2.2. MicroRaman Spectroscopy Experimental Details*

Analyses focusing, under microscope, on the purple particles were made with a Horiba Jobin-Yvon HR800 dispersive Raman spectrometer equipped with Edge filters and using the 458 nm excitation wavelength of an air-cooled (Melles Griot) Ar<sup>+</sup> laser. Laser power at the sample was settled on 100 μW in order to preserve the sample integrity. Sample visualization and Raman scattering collection were performed through a reverse microscope notably equipped with a 100× objective, which allows a spatial resolution close to 1 μm (Note: In the ideal case the spot is ~0.5 μm—the diffraction limit). The 600 groves/mm grating used gives a spectral resolution of about 3 cm−<sup>1</sup> for the signal recorded with an air-cooled CCD detector. The spectra presented were baseline corrected to remove the intense fluorescence contribution exhibited by the measured materials.

#### *2.3. FTIR Spectroscopy Experimental Details*

A Biorad FTS 175 system was employed for the acquisition of the FTIR spectra covering the wavenumber range of 4000–400 cm−1. A few particles (~1 mg) from each of the raw pigment samples were set aside and mixed with anhydrous KBr and then pressed into pellets. The spectra were collected in transmission mode and converted to absorbance mode with a spectral resolution of 4 cm−<sup>1</sup> representing an average of 64 scans.
