Chemical and Petrographic Characterization of Stone and Glass Tesserae in the Nereid and Geometric Mosaics from the S. Aloe Quarter in Vibo Valentia–Calabria, Italy

: Vibo Valentia’s S. Aloe quarter is an archaeological area which has three beautiful mosaic ﬂoors, dated between the centuries I BC and V AD. This work reports the results obtained on 22 glass and stone tesserae collected from the Nereid and Geometric mosaics during a recent restoration of the site. The analyses were carried out through a multi-analytical approach. The petrographic study of the stone tesserae was carried out using polarizing optical microscopy while the geochemical one was conducted using two micro-analytical techniques: the electron probe micro-analyzer with energy-dispersive X-ray spectrometry and a combination of laser ablation with inductively coupled plasma mass spectrometry for determining the major, minor, and trace element concentrations. The research highlights the use of di ﬀ erent kinds of stones such as marble, volcanic, and sedimentary rocks. The glasses show the typical soda–lime–silica composition indicating the use of natron as a ﬂux. The trace element concentrations prove the use of Pb-antimonates to create yellow glass. The bronze scrap was used to obtain the green color, while cobalt and copper were used to obtain di ﬀ erent gradations of blue. These results conﬁrm the high technological level reached by glassmakers in the Imperial Age, thus highlighting the importance of the S. Aloe archeological site.


Introduction
In the imperial period, private and public buildings were often decorated by mosaics, made of different kinds of materials, including pottery, stone, and glass.
Mosaics were considered luxury goods and, therefore, they were not available for everyone to have at home, especially if the tesserae were made of rare and expensive materials such as marble and/or colored glass [1]. Because of the difficulties in finding this material during the Roman age, glass was considered as precious as a gem or a semi-precious stone used to make jewels. For this reason, its production was exclusively for aristocrats. between the elements emerged and immersed in water are not highlighted. The emblem of the Nereid is inserted in a thin frame that separates it from a band decoration with wild ducks and a wader. A black and white checkered pattern delimits the frame with ducks and other animals. In the corners of the mosaic, four roundels are set with the same amount of fish and still lifes, all enclosed in an additional checkerboard frame.
A T-shaped floor was discovered to the north of the caldarium. It is composed of a mosaic with a black background littered with black and white stone tesserae and rare colored glasses (Table 1). It was dated between the II and III centuries AD [21].
The mosaic carpet, almost entirely preserved and restored to its original chromatic and stylistic splendor in 2008 [22], is organized in two spaces. The first one, next to the entrance (highlighted by a red arrow in the Figure 1b), includes a rectangular horizontal space composed of white tesserae and it covers an area of 4.50 m × 5.95 m. The second one, with a vertical development, includes a carpet of rectangular shape in which, on parallel vertical rows, a motif with white crosses develops and it covers an area of 6.70 m × 3.00 m. A band of white tesserae, adorned with swastikas and black squares, indicates the passage from one field to another of the mosaic. These characteristics allow us to define it as "geometric" (Figure 1b).
It is difficult to contextualize these mosaics, as no detailed information about the construction phases of the buildings is available [19,23].
Minerals 2019, 9, x FOR PEER REVIEW 3 of 20 Long horizontal grey-blue colored lines draw the water where the hippocampus swims and from which three dolphins spring. There are no bright tones among the mosaic's chromatic range; shades of grey and blue are used for depicting dolphins, waves, the Nereid's mantle, and the hippocampus' body. Different color gradations render the movement between light and shadow. The chromatic differences between the elements emerged and immersed in water are not highlighted. The emblem of the Nereid is inserted in a thin frame that separates it from a band decoration with wild ducks and a wader. A black and white checkered pattern delimits the frame with ducks and other animals. In the corners of the mosaic, four roundels are set with the same amount of fish and still lifes, all enclosed in an additional checkerboard frame.
A T-shaped floor was discovered to the north of the caldarium. It is composed of a mosaic with a black background littered with black and white stone tesserae and rare colored glasses (Table 1). It was dated between the II and III centuries AD [21].
The mosaic carpet, almost entirely preserved and restored to its original chromatic and stylistic splendor in 2008 [22], is organized in two spaces. The first one, next to the entrance (highlighted by a red arrow in the Figure 1b), includes a rectangular horizontal space composed of white tesserae and it covers an area of 4.50 × 5.95 m. The second one, with a vertical development, includes a carpet of rectangular shape in which, on parallel vertical rows, a motif with white crosses develops and it covers an area of 6.70 × 3.0 m. A band of white tesserae, adorned with swastikas and black squares, indicates the passage from one field to another of the mosaic. These characteristics allow us to define it as "geometric" (Figure 1b).
It is difficult to contextualize these mosaics, as no detailed information about the construction phases of the buildings is available [19,23].

Material and Methods
The sampling of the tesserae from the Nereid and the Geometric mosaics was conducted in summer 2018. The collection was conditioned by the sample availability. However, it was executed with the aim to take all the various colors and textures of stones and glass tesserae.
Eleven stone tesserae and ten glass tesserae were sampled from the Nereid mosaic. They were collected during the excavation phase, from parts of the flooring that were detached but certainly pertaining to the mosaic (Figure 1d,e).
As for the sampling of the Geometric one, only two glass tesserae were collected. Because the mosaic had been subjected to previous restoration, it was in a good state of preservation and the tesserae were firmly fixed. Therefore, in order to avoid any kind of damage, the stone tesserae were not collected.
All tesserae were labelled using an "N" as the first letter for the Nereid mosaic and a "G" for the Geometric one. Table 1 lists all samples studied, separating the stone from the glass tesserae, and giving information about provenance (Nereid or Geometric), petrographic nature of the stone tesserae, and color of both stone and glass tesserae.
Different and complementary techniques were used with the aim to conduct a petrographic and chemical characterization of stone and glass tesserae. The petrographic study of stone tesserae was conducted by polarizing optical microscopy (POM) on polished cross-thin sections, using a Zeiss Axiolab Optical microscope equipped with a camera Zeiss AxioCam MRc, (Zeiss, Oberkochen, Germany), useful for acquiring photomicrographs.
After a preliminary observation using a stereomicroscope, the glass tesserae were cross-cut and then polished with diamond cloths. Samples were then fixed on slides, with the fresh side facing upward. Due to their small size, three or four samples were positioned on each slide.
To determine major and minor element concentrations, electron probe micro-analyzer with energy-dispersive X-ray spectrometry (EPMA/EDS) analyses were carried out using a JEOL-JXA 8230 (JEOL Ltd, Tokyo, Japan), equipped with an EDS-JEOL EX-94310FaL1Q spectrometer (JEOL Ltd, Tokyo, Japan). Before the EPMA/EDS analyses, the polished surfaces were covered by a carbon sputter coating. The EPMA/EDS was used under the following operating conditions for the image acquisition: acceleration voltage 15 kV, probe current at 10-20 nA, using a solid state detector (SSD), (JEOL Ltd, Akishima, Tokyo, Japan) and Everhart Thornley detector (SE, JEOL Ltd, Akishima, Tokyo, Japan); for chemical analyses: acceleration voltage 15 kV, probe current 10 nA, probe diameter 1 µm, standardless quantification was performed using a Quant Correction ZAF (Z = atomic number correction, A = absorption correction, F = fluorescence correction). Furthermore, to avoid the loss of Na and ensure the correct acquisition of other element concentrations, an acquisition time of 15 s was selected. The detection limits for most elements were about 0.4 wt %. The reliability of the results was verified by measuring the SRM610 500 ppm by NIST (National Institute of Standards and Technology) and the BCR 2G (Columbia River Basalt) by USGS (United States Geological Survey) glass reference materials.
The EMPA/EDS glass analyses were executed using a defocused beam (square areas of about 5 × 5 µm 2 ) to avoid the loss of alkali. The analyses of micrometric crystals (ranging in size from 5 to 40 µm and sometimes dispersed within the glasses or, as accessory minerals, in the marble tessera NB4) were executed in spot manner to avoid the interference of glass and calcite compositions respectively.
Trace element concentrations in the glass tesserae were determined by LA-ICP-MS. Analyses were carried out using an Elan DRCe instrument (Perkin Elmer Inc., Waltham, MA, USA), connected to a New Wave UP213 Nd-YAG laser probe 213 nm (New Wave Research, Inc., Fremont, CA, USA). Samples were ablated by a laser beam in a cell, under a moderate flow of pure helium. Then, behind the cell, the ablated material was flushed in a continuous flow of a helium and argon mixture to the ICP system, where it was atomized and ionized for quantification in the mass spectrometer [24].
In this work, ablation was performed with spots of 50-80 microns, with a constant laser repetition rate of 10 Hz and fluence of about 20 J cm −2 . The procedures for data acquisition were those normally used in the Mass Spectroscopy Laboratory of the Department of Biology, Ecology and Earth Sciences, University of Calabria (Italy) [3,25,26]. In particular, for all analyses a transient signal of intensity versus time was obtained for each element using a 60 s background level (acquisition of gas blanks) followed by 40 s of ablation and then 60 s of post-ablation at background levels. Data were transmitted to a PC and processed by the Glitter program. Three point analyses were carried out on each glass tessera selecting the portions without alterations. The detection limits for most elements were about 0.01 ppm. Calibration was performed on glass reference material SRM 612-50 ppm by the NIST (National Institute of Standards and Technology) in conjunction with internal standardization, applying SiO 2 concentrations [27] from EPMA/EDS analyses. In order to evaluate possible errors within each analytical sequence, determinations were also made on the SRM 610-500 ppm by NIST and on BCR 2G by USGS glass reference materials as unknown samples, and element concentrations were compared with reference values from literature [28,29]. Table 2 lists the accuracies of all analyzed elements calculated as the relative difference from reference values [28].

Black Tesserae
Two of the four black tesserae NN1 and NN2 are fine-grained volcanic rocks. They show an intergranular fabric with microliths and rare phenocrysts of plagioclase and pyroxene. Resorbed olivine and oxide are recognizable only in the partially vitreous groundmass [30][31][32] (Figure 2a).
The microscopic study suggests that these tesserae may be classified as basalt or andesitic-basalt. The other two tesserae NN3 and NN4 are sedimentary rocks. NN3 is a biomicritic limestone [33]. This limeclast is highly oxidized, and with many unoriented microveins filled with microsparite.
NN4 is a microsparitic limestone [33]. This sample is highly altered with neoformation of ferruginous products totally pervading the entire limeclast. Pinkish/Reddish Tesserae NRR1 is a microsparitic limestone (Figure 2f). This sample is mainly unaltered even if some accessory phases or very rare opaque crystals occur, probably as a result of pyrite growth during the eodiagenetic processes of the source rock [35].
The fourth white stone tessera (NB4) is made of marble (Figure 2d,e). The petrographic analysis indicates that the marble sample is characterized by a fine grain size calcite (eight measurements among the larger grains allow to obtain a maximum grain size (MGS) ranging from 0.6 to 0.8 mm), with curved and straight boundaries, and its crystals show evident traces of cleavage, while the texture is generally heteroblastic (Figure 2d,e).
Pinkish/Reddish Tesserae NRR1 is a microsparitic limestone (Figure 2f). This sample is mainly unaltered even if some accessory phases or very rare opaque crystals occur, probably as a result of pyrite growth during the eodiagenetic processes of the source rock [35].
NRR2 is a micritic limestone lacking grains and bioclasts [33]. This finely-grained limeclast is unaltered and structureless and it has a fine pattern of veins, which are filled by a mosaic of coarser calcite crystals cutting the carbonate mud.
Greenish Tessera NVR1 is a micritic limestone lacking grains and bioclasts [33]. This finely-grained limeclast is unaltered. It has retained some microlayering filled with iron oxides and/or hydroxides consisting of opaques which are difficult to identify under thin section only through polarizing optical microscope [36,37]. As a matter of fact, opaque minerals such as magnetite, ilmenite, pyrite, and hematite, are usually problematic grains [38], identified through microprobe analysis technique, aimed at their geochemical discrimination [39,40].

Discussion
The petrographic study highlights the extensive use of stone tesserae of sedimentary origin for the manufacturing of Nereid mosaic. It is quite difficult to precisely determine the places where all the stone tesserae were quarried, considering that near Vibo Valentia some sedimentary successions constituted by late Miocene silicoclastic and carbonate sediments outcrop, unconformably overlying the Hercynian crystalline basement rocks ( Figure 3) [41,42]. However, it is possible to hypothesize that stones of local geological formations were used for the production of white, pinkish/reddish, greenish and black tesserae. On the contrary, the individuation of two basaltic/andesitic basalt and one marble tesserae-both rocks absent in the geology of the area-also indicate the use of extrabasinal raw material in the manufacture of the mosaic.
As concerns the marble provenance, considering the fact that white marbles were widely employed in antiquity and particularly during Roman times, numerous researches were focused on archaeometric surveys of ancient marbles. In general, over the past few decades numerous authors have suggested a multi-analytical approach to ascertain the provenance of such stone materials [43][44][45][46][47][48][49]. Among the distinctive properties of marbles, particularly discriminant is the variety of textural properties especially regarding the maximum grain size (MGS) of grains within a section.
A comparison among the values of MGS measured on the NB4 white marble tesserae and the MGS literature data for ancient white marble in the Mediterranean region is reported in Figure 4. The diagram variation of the MGS of classical marbles [44] is expressed as bars and boxes, with boxes comprising 70% of the total values, and each line within the boxes indicating the median. The black line in the diagram indicates the value of MGS (0.8 mm) measured in NB4 marble, showing that the sample falls within the Aphrodisias (Aydin), Carrara, Docimium (Afyon), Paros-1, Paros-2(3), and Pentelicon, areas [46][47][48][49][50]. However, considering that the average Carrara marble shows a homeoblastic texture and a fabric of "mosaic" type, with small to very small crystals, in some parts forming triple points at 120 • , it is possible to exclude the Carrara provenance for NB4 tesserae [46][47][48][49][50].
pinkish/reddish, greenish and black tesserae. On the contrary, the individuation of two basaltic/andesitic basalt and one marble tesserae-both rocks absent in the geology of the area-also indicate the use of extrabasinal raw material in the manufacture of the mosaic.
As concerns the marble provenance, considering the fact that white marbles were widely employed in antiquity and particularly during Roman times, numerous researches were focused on archaeometric surveys of ancient marbles. In general, over the past few decades numerous authors have suggested a multi-analytical approach to ascertain the provenance of such stone materials [43][44][45][46][47][48][49]. Among the distinctive properties of marbles, particularly discriminant is the variety of textural properties especially regarding the maximum grain size (MGS) of grains within a section.  [46][47][48][49][50]. However, considering that the average Carrara marble shows a homeoblastic texture and a fabric of "mosaic" type, with small to very small crystals, in some parts forming triple points at 120°, it is possible to exclude the Carrara provenance for NB4 tesserae [46][47][48][49][50].

Stone Tesserae
White Tesserae The EPMA/EDS results allowed the identification of accessory minerals, which are often small, sporadic, and difficult to be recognized by using only POM. The above-mentioned analyses indicate that calcite is the principal mineralogical phase, whereas quartz, muscovite, Ti-oxide, apatite, biotite, and Fe-sulphide are accessory minerals ( Figure 5, Table 3).
These crystals are rounded and very tiny, ranging in size from 10 to 40 µm. Considering that the texture of the NB4 marble excludes a Carrara provenance, the accessory minerals allow the

White Tesserae
The EPMA/EDS results allowed the identification of accessory minerals, which are often small, sporadic, and difficult to be recognized by using only POM. The above-mentioned analyses indicate that calcite is the principal mineralogical phase, whereas quartz, muscovite, Ti-oxide, apatite, biotite, and Fe-sulphide are accessory minerals ( Figure 5, Table 3).

Major and Minor Elements
The results of EPMA/EDS analyses on the twelve glass tesserae are reported in wt % of the oxides in Table 4.
As suggested in previous studies [4], to detect the base glass compositions, the content of colorants and opacifiers, i.e., copper and lead (see the next paragraph for discussion) was subtracted from the composition of the tesserae and the remaining oxides were normalized to 100% ( Table 5).
All the samples analyzed are soda-lime-silica glasses with SiO2 content ranging between 68.5 and 73.6 wt %, Na2O from 12.6 and 19.4 wt % and CaO from 4.1 to 8.3 wt %. The Al2O3 content varies between 1.9 to 3.2 wt %. The levels of CaO and Al2O3 roughly match the ratio found in typical Roman glasses, suggesting the use of sands containing feldspar as impurities, and, as a consequence, the use of a common silica source site [51]. MgO, K2O, and FeO are lower than 1.5 wt These crystals are rounded and very tiny, ranging in size from 10 to 40 µm. Considering that the texture of the NB4 marble excludes a Carrara provenance, the accessory minerals allow the discrimination among Aphrodisias (Aydin), Docimium (Afyon), Paros, and Pentelicon possible provenance. As highlighted by Capedri et al., [46], the presence of albite, quartz, apatite, Fe-oxide, phengitic muscovite, and paragonite is typical of Aphrodisias (Aydin) marble; apatite, phengitic muscovite, and phlogopite are accessory phases of Paros sources; apatite, Fe-oxide, chlorite, kaolinite, pyrophyllite, muscovite, and dolomite are accessory minerals in the marble of Docimium (Afyon) [46]. On the contrary, quartz, muscovite, Ti, Fe, Mn, Cr-oxides, apatite, chlorite, Fe-sulphide, and subordinate dolomite can be found in Pentelicon source [46]. Therefore, considering the accessory minerals detected, it is possible to exclude Aphrodisias (Aydin), Docimium (Afyon), and Paros sources and it is possible to hypothesize that the marble tessera NB4 comes from the Greek area of Pentelicon.

Major and Minor Elements
The results of EPMA/EDS analyses on the twelve glass tesserae are reported in wt % of the oxides in Table 4. As suggested in previous studies [4], to detect the base glass compositions, the content of colorants and opacifiers, i.e., copper and lead (see the next paragraph for discussion) was subtracted from the composition of the tesserae and the remaining oxides were normalized to 100% (Table 5). All the samples analyzed are soda-lime-silica glasses with SiO 2 content ranging between 68.5 and 73.6 wt %, Na 2 O from 12.6 and 19.4 wt % and CaO from 4.1 to 8.3 wt %. The Al 2 O 3 content varies between 1.9 to 3.2 wt %. The levels of CaO and Al 2 O 3 roughly match the ratio found in typical Roman glasses, suggesting the use of sands containing feldspar as impurities, and, as a consequence, the use of a common silica source site [51]. MgO, K 2 O, and FeO are lower than 1.5 wt % with the exception of the NV4 sample with K 2 O of 1.88 wt % and GBl with K 2 O of 1.57 wt% and FeO with 1.85 wt %. The low contents of MgO and K 2 O (Figure 6a) suggest that natron (a sodium carbonate mineral) was used as a flux for all samples according to the typical Roman production technology [5][6][7]. The rectangle drawn in Figure 6a indicates the area where Natron glasses plot. The presence of Cl (ClO varying from 1 to 2.8 wt % and SO 3 (0.08 to 0.3 wt %) is due to natron which contains NaCl and Na 2 SO 4 as contaminants [13].
The data were plotted in the ternary diagram for the normalized concentration of Na 2 O, CaO, and K 2 O + MgO (Figure 6b) which allow the identification of the alkali source used as a flux in the glass production [52,53].  (Figure 6a) suggest that natron (a sodium carbonate mineral) was used as a flux for all samples according to the typical Roman production technology [5][6][7]. The rectangle drawn in Figure 6a indicates the area where Natron glasses plot. The presence of Cl (ClO varying from 1 to 2.8 wt % and SO3 (0.08 to 0.3 wt %) is due to natron which contains NaCl and Na2SO4 as contaminants [13]. The data were plotted in the ternary diagram for the normalized concentration of Na2O, CaO, and K2O + MgO (Figure 6b) which allow the identification of the alkali source used as a flux in the glass production [52,53].   In this study, the glass tesserae plot in the silica-soda-lime glass area, regardless of their provenance, thus highlighting a typical Roman production ( Figure 6).
The concentrations of major elements in glass reflect variations in the proportions of the two major raw materials mixed by the glassmakers. Therefore, the overall homogeneity of our samples is comparable to the compositional glass groups within later Roman glass [8,14], suggesting the use of similar sand sources and highly standardized glass manufacture procedures.
Regarding the PbO contents (Table 4), samples GBl, NV1, NV2, NV4, and NGl show concentrations higher than 1 wt %, the NBl tessera contains 0.42 wt % PbO, and for the remaining samples, the values of PbO are below the detection limit of EPMA/EDS. The Pb contents were also measured by LA-ICP-MS (see the next paragraph for discussion).

Trace Elements
The results of LA-ICP-MS analyses are listed in Table 6. The values of manganese, copper, and lead are reported in both Tables 4 and 6. Before giving information about the trace elements analysis results, it is fundamental to re-state that the EMPA/EDS detection limits do not allow to determine an element's concentration when it is below 0.4 wt %. As a consequence, for this study, it has been necessary to carry out analyses also using LA-ICP-MS. In detail, as for Mn, when comparing the concentrations of GBl tesserae obtained by using both EMPA/EDS and LA-ICP-MS, it is possible to observe a good agreement between the two data (MnO = 0.76 wt % by EMPA/EDS, recalculated MnO = 0.73 wt % by LA-ICP-MS). The comparison of Cu and Pb concentrations show some differences and, in general, the PbO, concentrations obtained by LA-ICP-MS are underestimated. This is because the two methods differ in many ways: in their information depth, in lateral resolution, in sensitivity (detection limit) and in the level of non-destructiveness. In general, the results obtained by using EMPA/EDS must be preferred for major and minor elements, while the results obtained by LA-ICP-MS must be preferred for trace elements analyses i.e., when the concentrations are below 1000 ppm (see also Angelini et al. [54] and reference therein).
In the present study, the values of PbO obtained by EMPA/EDS (when detected) are higher than 3 wt %-only the PbO of the tessera NBl is 0.42 wt %-therefore they must be preferred to the corresponding concentrations obtained by LA-ICP-MS.
The contents of trace elements and, in particular, Sr, Zr, and Ba values are related to the characteristics of the sand used for glass production. Generally, low zirconium (about 60 ppm) and high strontium (about 400 ppm) indicate the use of Mediterranean coastal sand [5]. In addition, the content of barium, which enters in the structure of alkali feldspars as a vicariant of potassium, is related to the content of alkali feldspars in the sand [7,55]. All glass tesserae have low zirconium (ranging from 26 to 60 ppm), high strontium (from 282 to 486 ppm) and barium, ranging from 138 to 270 ppm, suggesting that beach sand with different amounts of alkali feldspars was used in their production.
The glass samples show different shades of colors (blue, light blue, dark and light green, green/turquoise, and yellow; Table 1) in relation to the introduction of transition metals into the glassy matrix, sometimes combined in various concentrations and oxidation states [9]. Common colorants were cobalt oxide for dark blue, iron oxide for green, blue or amber and copper oxide (CuO) for turquoise, blue or green [10]. In the twelve tesserae studied, copper and cobalt are the two main coloring ions identified for blue/light blue and shades of green. In fact, the colors of glass are strictly correlated to their concentrations.
Copper is the main coloring agent in the green samples with increasing content from the Nereid mosaic green tesserae to the Geometric mosaic one (from 5789 to 17,189 ppm).
The blue colors are produced using copper and cobalt. Indeed, the blue and light blue tesserae show high values of both elements. In the two blue-colored tesserae (NBl and GBl), Co and Cu contents are higher than in the other samples.
As for the two blue tesserae, the higher concentrations of copper and cobalt were measured in the sample GBl of the Geometric mosaic (Co = 2024 ppm and Cu = 2293). The light blue tesserae of the Nereid mosaic (NA1, NA2, NA3, NA4) contain smaller amounts of copper and cobalt compared to the blue ones ( Table 6). The NA2 sample has the highest Co and Cu content overall (Co = 189 ppm and Cu = 2548 ppm). Considering the relation between Cu and Sn (see the diagram Cu vs. Sn in Figure 7a), the shades of green and blue/light-blue colors are probably related to the presence of Cu 2+ ions in the glass network [9][10][11]-introduced in the batch by the addition of bronze or bronze scraps during glass production-as suggested by the relative percentages of copper and tin of 99%-90.2% and 1%-9.8%. Only two tesserae collected from the Nereid mosaic (NBl blue and NV2 green) show different Cu-Sn relative percentages (86%-87.5% and 14%-12.5%).
show high values of both elements. In the two blue-colored tesserae (NBl and GBl), Co and Cu contents are higher than in the other samples.
As for the two blue tesserae, the higher concentrations of copper and cobalt were measured in the sample GBl of the Geometric mosaic (Co = 2024 ppm and Cu = 2293). The light blue tesserae of the Nereid mosaic (NA1, NA2, NA3, NA4) contain smaller amounts of copper and cobalt compared to the blue ones ( Table 6). The NA2 sample has the highest Co and Cu content overall (Co = 189 ppm and Cu = 2548 ppm).
Considering the relation between Cu and Sn (see the diagram Cu vs. Sn in Figure 7a), the shades of green and blue/light-blue colors are probably related to the presence of Cu 2+ ions in the glass network [9][10][11]-introduced in the batch by the addition of bronze or bronze scraps during glass production-as suggested by the relative percentages of copper and tin of 99%-90.2% and 1%-9.8%. Only two tesserae collected from the Nereid mosaic (NBl blue and NV2 green) show different Cu-Sn relative percentages (86%-87.5% and 14%-12.5%). As regards the cobalt source, the correlation with the Ni content in the blue and light-blue tesserae (Figure 7b) suggests the probable use of a cobalt ore containing nickel, such as skutterudite as a coloring agent [3,10,11].
As for the glass opacity, all the tesserae show high contents of antimony, which acts as an opacifier. The specific nature of antimonates varies in chemical composition from calcium antimonates to lead antimonates. Considering the lead content, the tesserae can be divided into high lead (HPb) including the green, blue and yellow tesserae, and low lead (LPb) for the light-blue tesserae. The PbO content reaches a peak of 7.85 wt % (Table 4) in the yellow tessera NG1, which As regards the cobalt source, the correlation with the Ni content in the blue and light-blue tesserae (Figure 7b) suggests the probable use of a cobalt ore containing nickel, such as skutterudite as a coloring agent [3,10,11].
As for the glass opacity, all the tesserae show high contents of antimony, which acts as an opacifier. The specific nature of antimonates varies in chemical composition from calcium antimonates to lead antimonates. Considering the lead content, the tesserae can be divided into high lead (HPb) including the green, blue and yellow tesserae, and low lead (LPb) for the light-blue tesserae. The PbO content reaches a peak of 7.85 wt % (Table 4) in the yellow tessera NG1, which also shows high Ti (807 ppm), Sn (574 ppm), and Sb (11215) values ( Table 6). The lead and antimony high concentrations in the NG1 sample are related to the presence of lead antimonate Pb 2 Sb 2 O 7 microcrystals that vary in size and shape, and are homogenously distributed in the glass, as it is clearly distinguishable in the Si-Pb-Sb maps (Figure 8).
Lead and antimony act not only as opacifiers but also as colorants [12]. Indeed, lead antimonates Pb 2 Sb 2 O 7 are responsible for the yellow color of the tesserae.
Additionally, the green (NV1, NV2, NV3, and NV4) tesserae show high concentrations of Pb and Sb, confirming, as demonstrated in previous studies [10], that lead antimonate is present in Roman opaque green glass.
In addition, the presence of lead antimonate suggests that the mosaic's tesserae were produced by Roman glassmakers with a deep technological knowledge of the material's properties. Previous studies [9][10][11][12]56] showed that if antimony and lead compounds such as galena (PbS) and stibnite (Sb 2 S) were added directly to a natron base glass, the lead oxide dissolved and calcium antimonate crystallized instead of lead antimonate. To avoid the dissolution of lead oxide, the glassmakers at first produced a glass mass (corpo) containing the yellow opacifier, which was successively added to the base glass to obtain a homogeneous mixture.
High concentrations of lead and antimony were also measured in the two blue tesserae (GBl and NBl). However, the chemical study by EPMA/EDS highlights the presence of microcrystals of calcium antimonate (ranging in size from 5 to 20 µm) (Figure 9), suggesting that in these tesserae the base glass reacted with antimony and lead compounds and calcium antimonates precipitated.
The light-blue tesserae (NA1, NA2, NA3, and NA4) are different from the others, as they contain high concentrations of antimony and low content of lead. Calcium-antimonates (Ca 2 Sb 2 O 7 or CaSb 2 O 6 ) were used to opacify light-blue tesserae as testified by the little crystals detected by EPMA (Figure 9). The presence of these crystals suggests that the introduction of antimony as stibnite (Sb 2 S 3 ) or antimony oxide, produced them by reacting with calcium from the soda-lime-silica glass itself [8].
also shows high Ti (807 ppm), Sn (574 ppm), and Sb (11215) values ( Table 6). The lead and antimony high concentrations in the NG1 sample are related to the presence of lead antimonate Pb2Sb2O7 microcrystals that vary in size and shape, and are homogenously distributed in the glass, as it is clearly distinguishable in the Si-Pb-Sb maps (Figure 8).
Lead and antimony act not only as opacifiers but also as colorants [12]. Indeed, lead antimonates Pb2Sb2O7 are responsible for the yellow color of the tesserae.
Additionally, the green (NV1, NV2, NV3, and NV4) tesserae show high concentrations of Pb and Sb, confirming, as demonstrated in previous studies [10], that lead antimonate is present in Roman opaque green glass.
In addition, the presence of lead antimonate suggests that the mosaic's tesserae were produced by Roman glassmakers with a deep technological knowledge of the material's properties. Previous studies [9][10][11][12]56] showed that if antimony and lead compounds such as galena (PbS) and stibnite (Sb2S) were added directly to a natron base glass, the lead oxide dissolved and calcium antimonate crystallized instead of lead antimonate. To avoid the dissolution of lead oxide, the glassmakers at first produced a glass mass (corpo) containing the yellow opacifier, which was successively added to the base glass to obtain a homogeneous mixture.
High concentrations of lead and antimony were also measured in the two blue tesserae (GBl and NBl). However, the chemical study by EPMA/EDS highlights the presence of microcrystals of calcium antimonate (ranging in size from 5 to 20 µm) (Figure 9), suggesting that in these tesserae the base glass reacted with antimony and lead compounds and calcium antimonates precipitated.
The light-blue tesserae (NA1, NA2, NA3, and NA4) are different from the others, as they contain high concentrations of antimony and low content of lead. Calcium-antimonates (Ca2Sb2O7 or CaSb2O6) were used to opacify light-blue tesserae as testified by the little crystals detected by EPMA ( Figure 9). The presence of these crystals suggests that the introduction of antimony as stibnite (Sb2S3) or antimony oxide, produced them by reacting with calcium from the soda-limesilica glass itself [8].
. Figure 8. Sb, Si, Pb distribution map in the NG1 yellow tessera from the Nereid mosaic. The distribution of these elements highlights the presence of lead antimonate Pb2Sb2O7 microcrystals.

Conclusions
The petrographic and geochemical approach for the study of the two mosaic's tesserae is a powerful tool to obtain information about raw materials and glass-working techniques.
The results obtained on the stone and glass tesserae collected in the Nereid and Geometric mosaics in the archaeological area of S. Aloe quarter in Vibo Valentia have shown that: (1) The white, pinkish/reddish and black tesserae are all made of stones. The majority of them are limestones, except for one white tessera constituted by marble, probably of Greek provenance (Pentelicon) and two black tesserae made of volcanic rocks; (2) The blue/ light-blue, yellow tesserae are made of glass; (3) The green tesserae are made of two types of materials: stone and glass.
Although it is quite difficult to determine the quarrying places of the stone tesserae, it is possible to hypothesize that stones of local sedimentary formations were employed. The use of different varieties of stones for the flooring of this ancient Roman domus, testifies the greatness of this archaeological site. In addition, the results on the provenance of the marble support the hypothesis that this material comes from the Greek area, remarking the importance of the S. Aloe quarter.
The glass tesserae were used for those colors that were not readily available among stones, such as the analyzed blue/light-blue, shades of green and yellow tesserae. The analyses of the glass suggest the use of natron as a flux according to the Roman tradition. In particular, the coloring agents are bronze and cobalt for the blue /light-blue and green tesserae, and the lead antimonate for yellow.
All the tesserae were also opacified using lead and calcium antimonates, testifying that the glasses were produced by experienced Roman glassmakers with a deep technological knowledge of the material's properties.
The geochemical study of glass tesserae provides important information about their production techniques and the provenance of the raw materials used in their preparation. This

Conclusions
The petrographic and geochemical approach for the study of the two mosaic's tesserae is a powerful tool to obtain information about raw materials and glass-working techniques.
The results obtained on the stone and glass tesserae collected in the Nereid and Geometric mosaics in the archaeological area of S. Aloe quarter in Vibo Valentia have shown that: (1) The white, pinkish/reddish and black tesserae are all made of stones. The majority of them are limestones, except for one white tessera constituted by marble, probably of Greek provenance (Pentelicon) and two black tesserae made of volcanic rocks; (2) The blue/ light-blue, yellow tesserae are made of glass; (3) The green tesserae are made of two types of materials: stone and glass.
Although it is quite difficult to determine the quarrying places of the stone tesserae, it is possible to hypothesize that stones of local sedimentary formations were employed. The use of different varieties of stones for the flooring of this ancient Roman domus, testifies the greatness of this archaeological site. In addition, the results on the provenance of the marble support the hypothesis that this material comes from the Greek area, remarking the importance of the S. Aloe quarter.
The glass tesserae were used for those colors that were not readily available among stones, such as the analyzed blue/light-blue, shades of green and yellow tesserae. The analyses of the glass suggest the use of natron as a flux according to the Roman tradition. In particular, the coloring agents are bronze and cobalt for the blue /light-blue and green tesserae, and the lead antimonate for yellow.
All the tesserae were also opacified using lead and calcium antimonates, testifying that the glasses were produced by experienced Roman glassmakers with a deep technological knowledge of the material's properties.
The geochemical study of glass tesserae provides important information about their production techniques and the provenance of the raw materials used in their preparation. This means that the S. Aloe quarter has a relevant significance while studying Calabrian archaeological sites.