**2. Materials and Methods**

Previously studied assemblages of archaeological glass available at the Conservation Science Laboratory of the Department of Cultural Heritage (University of Bologna-Ravenna Campus) have been selected for this study: 13th–14th century ampoules, nuppenbecher and kropfflasche from the monastic complex of San Severo (Classe, Ravenna, Italy) [4]; 14th–16th century gambassini drinking vessels from the monastic complex of San Severo (Classe, Ravenna, Italy) and from the Rontana Castel (Brisighella, Ravenna, Italy) [5]; 8th century mosaic glass tesserae from the qasr of Khirbet al-Mafjar (Jericho, Palestine) [6]; and 8th century mosaic glass tesserae from the Great Mosque of Damascus (Syria) [7,8]. When other assemblages of materials from previous studies which have not been carried out by the authors are taken as examples, references are provided in the text.

An Olympus S761 stereomicroscope (magnification up to 45×, Olympus Corporation, Shinjuku, Tokyo, Japan) associated with an Olympus Soft Imaging Solutions GMBH model

SC100 camera (Olympus Corporation, Shinjuku, Tokyo, Japan) was used for preliminary morphological observations and documentation.

For Visible Reflectance Spectrometry (VIS–RS), a MINOLTA CM-2600d portable spectrometer was used. The system is equipped with an internal integrating sphere of 56 mm diameter, in reflectance geometry d:8◦, with three Xenon pulsed lamps, and a D65 illuminant was used; calibration was performed against a BaSO4 standard plate; the spectral range is 400–700 nm, with a spectral resolution of 10 nm and the area of sight of 3 mm diameter. Spectra Magic software was employed to elaborate data; specular component excluded (SCE/0) was selected, according to the literature [9].

Microsamples were taken from either objects or diagnostic fragments for performing micromorphological, microtextural, and compositional analyses. Sections were prepared by embedding microsamples in a polyester resin then polished by using abrasive papers of decreasing grain in order to expose the embedded samples avoiding any losses.

A Scanning Electron Microscope (SEM) back-scattered electron signal (BSE) was used for the inspection of the morphological features of the inclusions coupled with Energy Dispersive Spectroscopy (EDS) spot measurements to achieve a preliminary qualitative and semiquantitative elemental analysis of the inclusions themselves. Analyses were carried out on polished and carbon-coated sections. Images and EDS spectra were collected on carboncoated crosssections, using an ESEM FEI Quanta 200, equipped with an EDAX energy dispersive spectrometer. Analyses were performed in high-vacuum using an acceleration voltage of 25 kV and an energy resolution of ~200 eV; working distance was set at 10 mm, spot size was between four and five μm.

Micro-Raman spectra were collected by using a Bruker Senterra dispersive Raman spectrometer equipped with an integrated Olympus BX40 microscope. A 785 nm He– Ne laser was employed, in the 300–3500 cm−<sup>1</sup> region. Analytical measurements were performed with a 50× long working distance objective, operating at a power of 10 mW (red and blue samples) or 25 mW (yellow and green samples) with a spectral resolution of 3.5 cm−1. Raman measurements were performed on polished sections after carboncoating removal.

X-ray Powdered Diffractometry (XRPD) analyses were performed on finely powdered samples manually pressed on a Ag sample holder in a Rigaku Miniflex diffractometer employing CuKα<sup>1</sup> radiation in the range 2θ: 4◦–64◦, θ scan speed: 1◦min<sup>−</sup>1.

To determine the bulk chemistry of all samples under study, Electron Probe Microanalysis (EPMA) was performed on polished and carbon-coated sections. Chemical analyses of major and minor elements (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Co, Cu, Sn, Sb, and Pb) were performed using a CAMECA–CAMEBAX equipped with four scanning Wavelength Dispersive Spectrometers (WDS). A beam current of 2 nA, an acceleration voltage of 20 kV, and a spot size of 5 μm were used for Na, K, Si, and Al; for all other elements, a beam current of 20 nA, an acceleration voltage of 20 kV, and a spot size of 1 μm were used. Synthetic pure oxides were used as standards for Al, Cr, Fe, and Sn, synthetic MnTiO3 for Mn and Ti, wollastonite for Si and Ca, albite for Na, periclase for Mg, PbS for Pb, orthoclase for K, apatite for P, sphalerite for S, Sb2S for Sb, and pure elements for Co, Cu, and Ni. SMITHSONIAN GLASS A standard [10] was also employed as a reference sample. Ten points were analysed on each sample, and the mean values were calculated. The measured accuracy for the analysed elements was better than 3%. The standard deviations among the analysed points resulted in between 1% and 3% and 3% and 5% for major and minor constituents, respectively. The detection limit for the minor elements was between 0.01 wt% and 0.04 wt%. The correction program was based on the PAP method [11] and was used to process the results for matrix effects.

Laser Ablation–Inductively Plasma Coupled–Mass Spectrometry (LA–ICP–MS) was carried out to determine the concentration of 37 trace elements. Analyses were performed by a Thermo Fisher X-Series II quadrupole based ICP–MS coupled with a New Wave ablation system with a frequency quintupled (λ = 213 nm) Nd:YAG laser. Laser repetition rate and laser energy density on the sample surface were fixed at 20 Hz and ∼18 J/cm2, respectively. Analyses were carried out using a laser spot diameter of 100 μm on the same polished samples used for EPMA after carbon-coating removal. Due to the highly heterogeneous microstructure of the tesserae, six points were analysed on each sample and the mean values were then calculated. External calibration was performed using NIST 610 and 614 glass as external standards; NIST 612 was also used as a secondary reference sample to check precision and accuracy (Pearce et al. 1997). 29Si was employed as an internal standard, whose concentration was determined by EPMA following the method proposed by Longerich and colleagues [12]. The distribution of REE and of the other trace elements was analysed by normalising the data to the upper continental crust [13].
