Archaeological contest and glass sample characterization
The glass sample analyzed, a fragment of intense blue color (Fig. 1), was found in an amphora containing fragmented and intact glass manufactures in Pompeii’s excavation, Reg. I, Insula 14, Casa 14 (Originally numbered as Reg. II, Insula 14, and successively, during 1950s excavations, changed to Reg. I, Insula 14). Insula 14 is located in the eastern area of the Regio I of Pompeii, in the median sector of the insulae gravitating on the eastern side on via di Nocera and on the northern side on via di Castricio that determines the prevailing orientation of the housing units. The first information about insula 14 dates back to 1954, the period in which Amedeo Maiuri began an important excavation season to bring to light the entire southeastern sector of the city. In the first phase, the investigations were limited to freeing the southern front between insulae 13 and 14, and only in 1957, the southeast corner was reached, identifying a thermopolium pertaining to the current number 15. After the excavation and consolidation of the wall hills that emerged, the research activity was interrupted and resumed in 1984 . The archaeological period of the glass finds was attributable to the earthquake described by Tacitus and Seneca that seriously damaged Pompeii in A.D. 62. However, according to some views, the A.D. 62 earthquake (defined as terminus post quem) was not a single event but other seismic activity, occurred over a certain number of years (Neronian and early Flavian periods) , generated stratigraphic sequence due to the subsequent demolition and rebuilding thus, suggesting that not all the glass finds have been precisely contemporaneous. All excavated specimens should have been preserved from subsequent potential deterioration caused by the eruption of Vesuvius in 79 A.D. (terminus ante quem) . Dr. Piccioli, C., an official of the former Archaeological Superintendence of Naples and Caserta (SANC) selected the distinctive sample that was considered, according to archaeological caution, well preserved and of great significance, because its intense color that appeared identical to that of other intact glass manufactures. The artifact (about 2x3 cm) was carefully handled to avoid additional contamination and softly cleaned with a brush and wet bibula paper to remove dust deposits. The fragment was then stored in a preserved area to avoid further environmental deterioration. The sample did not show any opacity. The glass color, according to the Munsell notation, was organoleptically corresponding to a saturated and intense color . Reflected light microscopy observation revealed some technical properties such as the absence of bubbles and the refinement in cooking which explained the durability of the material. The glass surface appeared non-homogeneous highlighting forms of yellow, white, and dark blue pitting alterations most likely attributable to the chromophoric elements responsible for the blue color (Fig. 2) whereas, no crystalline phases were observed by XRD (not shown), thus suggesting the absence of devitrification and excellent quality of the glass also in consideration of the elapsed time.
Raman and FT-IR spectroscopy
Raman spectrum of the sample, reported in Fig. 3a, highlighted the presence of two major peaks at 1090 and 584 cm-1 with and two well-defined components at 945 and 995 cm-1. This signature corresponded to common lime-based glass (typically having a composition with about 10 to 15% Na2O, and about 8 to 15% CaO). In some cases, only one shoulder was observed at 950 or 995 cm-1 . The two major signatures are associated with the Si-O bending (~550 cm-1) components of SiO4 entities of the more or less polymerized (Si-O)n framework, and Si-O stretching (~1090 cm-1) . The feature at 773 cm-1 is usually assigned to the νQ0 mode of isolated not-bridged SiO4 entities . The position of the maximum of the SiO4 bending and stretching bands are reported in Table 1. The maximum of the SiO4 bending and stretching bands in the general database was determined from the Raman characterization of hundreds of different types of glassy silicate whose elemental compositions were determined by classical methods, thus allowing the identification of different types of glass. . Studies made by Colomban et al., 2006, highlighted families of glasses based on the relationship between the Raman peak area ratio (A500/A1000), defined as polymerization index (Ip), of envelopes and wavenumbers of the different Si-O stretching components. The empirical relationship between Ip, glass composition, and the processing temperature was rather well documented . According to this classification, the Ip value calculated from Raman spectra collected in a different area of the sample (Ip = 0.6 ± 0.05) would correspond to a family of silicate-based glasses characterized by an intermediate ratio between flux components (Na2O + K2O + CaO) with a very low content of PbO and most likely processed at medium temperature. Regarding the blue color and opacifiers, Raman features did not suggest either the detectable amount of lazurite (Na,Ca)8(SO4,S,Cl)2(AlSiO4)6  or Ca2Sb2O7 (no 672 cm-1 bands) .
The FT-IR spectra of the sample (Fig. 3b) showed a profile with the presence of bands consistent with the Raman result. Typical spectra showed broadband in the 3590 cm−1 regions arising from stretching of the –OH most likely assigned to the silanol group or in adsorbed water in the sample. The spectrum was characterized by bands at 2926–2844 cm−1 (functional groups region) and 1725–1586 cm−1 (double bond stretching) regions because of C-H bending [36, 37], which could be due to some original organic contamination of organic structures on the silica's surface. Peaks at 1264, 893, and 798 cm-1 could be assigned to Si-O symmetrical stretching vibration, Si-OH bending, and SiO2 stretching, respectively [38-40]. All the major FT-IR bands observed and the peak assignments are shown in Table 2.
Scanning Electron Microscope
BSE in SEM imaging showed non-homogeneity of the sample (Fig 4a). The semi-quantitative analysis was characterized by a great variability (high S.D. and CV%) because observations were made on different areas of the glass surface. Areas of the sample showed the presence of CoO up to a concentration of about 6.0 wt% (Fig. 4b and 4c, whitish zones) whereas, in other areas, the CoO content ranged from zero to about 1.0 wt%. PbO ranged from zero to about 6.0 wt%. Ti and Fe were also detected as well as minerals attributable to the group of zeolites likely formed by the alteration of glass . Table 3 shows the average composition of the glass. The sample appeared as a soda-lime-silica glass with the average concentration of SiO2, Na2O, and CaO of 61.71 wt%, 1.44 wt%, and 5.16 wt% respectively, although the average Na2O concentration resulted lower compared to that reported in the literature data on glasses of the period  whereas, a higher average concentration of MnO, FeO, CoO, and PbO was observed. The latter data suggested that cobalt was most likely the key chromophoric element responsible for the sample blue color. [43-48].
Inductively coupled plasma mass spectrometry
Because ICP-MS measurements of silicon at m/z 28 suffer from numerous spectral interferences that could include C, O, and N (the latter most likely coming from nitric acid), we first analyzed the presence of trace elements (TE) in the Pompeian glass powder after its digestion in HF and HCl in a 1:3 ratio (ICP-MSa). We expected a Si concentration above detection limits thus, it was not necessary to pre-concentrating the sample. Silicon does not require significant amounts of strong acid and low levels (< ~10 ppm) are soluble and stable in water. Moreover, the ICP-MS used allowed Si measurements in the range of a few ppm. The ICP mass analytical results are summarized in Table 4. In particular, the sample showed a relatively high content of Si, Na, and Ca and a lower content of Fe, Al, Co, Mn, Cu, Sb, Pb, and K. The amount of these elements, converted in the corresponding oxides, highlighted a composition similar to that observed in several Roman glasses . In fact, the percentage (w%) of SiO2, Na2O, CaO, Al2O3, K2O, MgO, FeO, MnO, PbO, and CuO were 43.8, 5.6, 5.4, 1.0, 0.66, 0.28, 0.7, 0.24, 0.09, and 0.08, respectively. The MgO and K2O compositions were less than 1.5%. This data suggested that natron was the primary alkali flux for this glass [49-51]. It is also worth noting that the amount of Sb observed (0.99 w%) was not sufficient as an antimony-based opacifier as instead observed for other Roman and Pompeian glasses [42, 52].
ICP-MS was used was also performed after-treatment of the sample with HCl, HNO3, and HBF4 at a 2:1:1 ratio (v/v) in a temperature ramp (ICP-MSb). The results are reported in Table 4. Although with this procedure it was not possible to detect Si, K, and Na, also here was observed a moderate-high content of Fe, Al, Ca, Co, Mn, Cu, and Mg. The weight percentage (w%) of the corresponding oxides FeO, Al2O3, CaO, CoO, MnO, CuO, and MgO were 0.79, 2.74, 8.06, 0.20, 0.52, 0.68, and 0.65, respectively . The CaO content evaluated by ICP-MSa and ICP-MSb (5.4 w% and 8.05 w%, respectively) was slightly higher compared to that reported for Pompeian glasses by  (average 7.215%) and according to the literature data on a glass of the period [43-48]. This value could be related to the percentage of sodium present in the natron to flux the silica [6, 54] and the higher quantity of lime was used to stabilize the glass thus, suggesting that the sample represented a specialized production, perhaps using a plant-ash component, during the 1st century A.D.
The relationship between the composition of Al2O3 (1.0 w% by ICP-MSa and 2.74 w% by ICP-MSb) and that of CaO showed to be very close to that reported for Pompeii glasses within the area of Roman Western European sites and in the Mediterranean area in the 1st - 3rd century A.D. . As suggested, these values could be due to the employment for the glass productions of similar raw materials along with the Empire and most likely from the Middle-East region [39, 44, 56-58].
The relatively high content of FeO (0.7 w% by ICP-MSa and 0.79 wt% by ICP-MSb) as already reported by  can be found in blue-colored Roman glasses. However, its amount might be depending upon the MnO concentration. In this case, the manganese oxide concentration (0.24 w% by ICP-MSa and 0.52 wt% by ICP-MSb) was within natural limits, thus suggesting the use of iron-containing raw material that was most likely not subjected to the decoloring procedure. Decolorized glasses generally show MnO concentration > of 0.5 wt% probably due to the addition of manganese as pyrolusite (MnO2). The latter was particularly widespread in the Roman period to neutralize the color due to the iron oxides naturally present in the primary raw materials [44, 59, 60, 61]. These findings support the hypothesis that the Pompeian glass could have been produced from the sands from the Middle-East region .
Also, copper and cobalt, contained in the sample at a concentration of 5467 ppm and 1615 ppm, respectively (0.20 w% and 0.68 w% by ICP-MS) were important coloring agents in the ancient glass-making workshop . For instance, copper might produce blue color depending on its interaction with iron and on some level with manganese and lead. However, deep blue glass showed significant amounts of copper and cobalt in the order of 1930 ppm and 1453 ppm . Therefore, the deep blue color of the sample, besides the iron present in the raw material, might be due to the presence of copper and cobalt probably added as 2Co2O • CuO • 6H2O (trianite). This compound was often used for the production of Roman blue glass . Therefore, as also clearly shown by BSE imaging (Fig. 4b and 4c), the blue color of the glass sample was essentially due to cobalt probably used in Pompeian secondary furnaces for glasses manufacture production .
ICP-MSb was also used to determine the content of rare earth elements (REE). The results, reported in Table 4, showed that neodymium (Nd) was the most preponderant rare earth element present in the sample with a concentration of 13.629 ppm. This element belongs to the light rare earth elements (LREE) of the lanthanide series and its concentration is in the range of the concentration of Nd in silica-based, non-carbonaceous sediments and sedimentary rocks that generally is in the order of 5–50 ppm . Neodymium content in glass components such as shell and limestone as well as natron is much lower (around 0.5-10 ppm, and 20-40 ppb, respectively [64-66]. These findings suggest that Roman glasses were originated from heavy minerals or a fraction of non-quartz minerals of the silica-based raw material . Under this aspect, sands from the Campanian beaches by the Garigliano and Volturno Rivers were likely not used in this case  since this area all contained more Nd, even up to 296 ppm . Moreover, these sands contained high percentages of heavy minerals, resulting in high Fe2O3 and Al2O3 levels, making them unsuitable for glass production [46, 69].