A non-invasive study of Roman Age mosaic glass tesserae by means of Raman spectroscopy

Journal of Archaeological Science 36 (2009) 2551–2559

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A non-invasive study of Roman Age mosaic glass tesserae by means of Raman spectroscopy Paola Ricciardi a, *, Philippe Colomban a, Aure´lie Tournie´ a, Michele Macchiarola b, Naceur Ayed c a

LADIR, CNRS and Universite´ Paris 06 UPMC, 2 rue Henri Dunant, 94320 Thiais, France CNR, Dipartimento Patrimonio Culturale, Istituto di Scienza e Tecnologia dei Materiali Ceramici, Via Granarolo 64, 48018 Faenza (RA), Italy c Unite´ de Recherche 1201, Institut des Sciences Applique´es et de Technologie, BP 676, 1080 Tunis, Tunisia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 September 2008 Received in revised form 6 July 2009 Accepted 9 July 2009

Ancient mosaic tesserae are a range of materials of very varied and complex nature, including pottery, stone and glass. Raman spectroscopy is a powerful tool for the analysis of all these kinds of materials. In the particular case of glasses, this technique can be used both for a study of surface weathering and for the characterization of bulk structure, but it has not yet been extensively used for the characterization of mosaic glass tesserae. We carried out Raman analyses on a set of Roman and Late Antiquity period mosaic glass samples, which allowed a good characterization of both the glass matrix and the crystalline inclusions. All the samples show the typical Raman signatures of soda-lime-silicate glasses. Several crystalline phases were also identified, being relics of raw materials used during the glass manufacturing process, such as quartz and feldspars, or linked to the glass color/opacification, such as bindheimite and cuprite. The analyses also led to the identification in some blue, turquoise and green tesserae of calcium antimonate, whose Raman signature has only recently been recognized in the scientific literature on mosaic glasses. Some emphasis is given to the analysis of red lead-containing tesserae, colored with Cuþ ions or even Cu0 (or Au0) metal nanoparticles. Samples with peculiar compositions, as well as ‘‘modern’’ (and restoration) samples, could quite easily be distinguished from the ancient ones by their Raman spectra. Published by Elsevier Ltd.

Keywords: Glass Mosaics Raman spectroscopy Roman age Opacifiers Calcium antimonate Copper nanoparticles

1. Introduction Mosaic tesserae are a range of materials of very varied and complex nature, including pottery, stone and glass. Ancient glasses were generally obtained by a mixture of naturally occurring materials containing silica, alkali and lime. Beach sand and a crude source of alkali were typical ingredients, with both the sand and the alkali containing enough lime or magnesia to give adequate chemical stability. Since about 1000 B.C., eastern Mediterranean glasses used natron (hydrated Na2CO3) available from Northern Egypt as favored source of alkali; this practice continued all across the Mediterranean region through late antiquity (Freestone, 2005). Glass was usually colored by adding small amounts of certain salts (mostly of copper, iron, and manganese); this addition of colorants was probably the first example of the use of minor ingredients to

* Corresponding author. Present address: Scientific Research Department, National Gallery of Art, 4th Street and Constitution Avenue, 20001 Washington DC, United States. E-mail addresses: [email protected] (P. Ricciardi), philippe.colomban@ glvt-cnrs.fr (P. Colomban), [email protected] (A. Tournie´), michele. [email protected] (M. Macchiarola), [email protected] (N. Ayed). 0305-4403/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.jas.2009.07.008

change glass properties to produce a desired effect. Opaque glasses were obtained by suspension of crystals, quite insoluble within the glass matrix, such as calcium- and lead-antimonate, and tin oxide (Verita`, 2000). Raman spectroscopy is a powerful tool for the analysis of glasses in the field of cultural heritage, both for a study of surface weathering (Colomban et al., 2006a; Robinet et al., 2004, 2006a) and for the characterization of bulk structure (Colomban and Tournie´, 2007; Colomban et al., 2006b; Robinet et al., 2006b; Welter et al., 2007). In spite of this, very few Raman data have been published regarding the analysis of ancient mosaic glasses (Colomban et al., 2003; Galli et al., 2004).

1.1. Research aims The cited work by Colomban et al. (2003) was focused on the study of glass beads, rings and mosaic tesserae from Tunisia. In that case, most of the analyzed samples showed the typical signatures of soda-lime-silicate glasses, with the exception of a few lead-based glasses for red tesserae. Several mineral phases were identified as pigments and opacifiers, but a reliable set of reference spectra was

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still missing. Some blue glasses were found to be the result of a synthesis in which (cobalt-containing) blue glass ingots were ground and then sintered in the presence of a liquid phase enriched in tin oxide. Finally, a red sample showed different Raman signatures in different areas, and an unusually high Au content. This would imply that the red coloration was obtained by a colloidal dispersion of metal copper and/or gold, as in the case of the Lycurgus Cup (Barber and Freestone, 1990). This work aims at providing further Raman spectroscopic data on Roman Age mosaic glasses by augmenting the number of analyzed Tunisian samples and especially by enlarging the research to mosaic tesserae found in several archaeological sites in Italy. We want to confirm the usefulness of this technique for a completely non-invasive study of this type of samples, and we attempt to help clarify the correspondence existing between the Raman spectra and the results of traditional analyses. Particular attention will also be given to the relationship between the spectra and the glass coloration.

2. Materials and methods For this work a total of 26 soda-lime-silicate ancient glasses were analyzed, coming from excavations in different Italian regions and from ancient Carthage in Tunisia (Fig. 1), and dating to the first five centuries AD. Different typologies and colors are represented, together with some basic colorless glass fragments; all colored glasses are opaque. Additionally, a blue restoration tessera from Pompeii, and a modern turquoise transparent tessera with golden foil

and cartellina were also analyzed. A synthetic description of the provenance, typology, age, and color of the samples is given in Table 1, along with the codes used to identify them; images of all the samples are shown in Fig. 2. Almost all of the Italian samples have already been the object of archaeometric studies dealing with specific historical and/or technological issues, most of which have been published (Abu Aysheh, 2006; Boschetti et al., 2007; Corradi et al., 2005; Macchiarola and Fontanelli, 2008; Macchiarola et al., 2006, 2007; Santagostino Barbone et al., 2008; Santoro et al., 2006). Table 2 contains selected chemical compositions determined by ICPOES analyses (some of which during these previous analytical works, as referenced in the table). As for the Tunisian tesserae, they belong to one of the mosaics of what is considered to be the most remarkable of the Roman villas in Carthage, the ‘‘Maison de la Volie`re’’, i.e. the ‘‘House of the Aviary’’ (Ennabli, 2000; Ennabli and Ben Osman, 1983), and have not been previously analyzed. Photographic images of one of the sampled mosaics are presented in Fig. 3. Raman spectroscopic analyses were performed using two different instruments. A Dilor XY spectrometer was used in macroscopic configuration, equipped with a Krþ laser at 406.7 nm. The analyzed volume corresponds to about 0.3  0.3  0.3 mm3. A Labram Infinity spectrometer was also used, equipped with a green Nd:Yag laser (532 nm), in microscopic configuration (50 objective, for a total magnification of 500 times). In this case, the analysis is made on a volume of 5  5  10 mm3. The attribution of the Raman signatures of crystalline phases was made by comparison with data present in the literature as well as in on-line databases (RRUFF Project, 2008; University of Parma, 2007). In order to undertake a curve fit of the spectra, a linear baseline was first subtracted using the LabSpecÒ software, as previously discussed (Colomban, 2003). In the case of glassy spectra, the analysis was limited to the range between 200 and 1300 cm1 in order to proceed to their deconvolution and to the calculation of a series of Raman parameters linked to the composition and structure of the glassy phase (Colomban et al., 2006b). 3. Results 3.1. Identification of crystalline phases

Fig. 1. Location of the archaeological sites of provenance of the analyzed mosaic tesserae.

Besides the typical features of a glassy phase, which are discussed in the next section, most spectra show also the characteristic peaks of one or more crystalline phases, which serve as pigments and/or opacifiers within the glass matrix. A synthesis of the identified phases can be found in Table 3, and representative spectra are shown in Fig. 4. Four samples (MAC3, MAC11, MAC12 and MAC17) show a couple of peaks of equal intensity at about 480–485 and 633 cm1, which have sometimes been attributed to cassiterite SnO2 (Galli et al., 2004). Sample MAC17 shows also additional peaks at 317, 387, 663 and 703 cm1. Recent works have identified this series of peaks as being the Raman signature of calcium antimonate Ca2Sb2O7 (Gedzeviciuˆte¨ et al., 2009). This attribution seems more reliable than that to tin oxide, especially as the chemical analysis of these samples indicates very small or null Sn contents. Two of these samples (MAC3 and MAC11), as well as samples MAC13 and MAC19, show a strong peak at 671 cm1, sometimes together with several less intense bands (at about 238, 325, 338 and 521 cm1). The same series of peaks had been identified in two blue mosaic tesserae (n. 5 and n. 6, only the main peak in the latter) in the above-mentioned work by Colomban et al. (2003), and there assigned to cassiterite. Galli et al. (2004) observed a narrow peak at 670 cm1 in a blue mosaic tessera from a Roman villa and proposed an attribution to an iron and/or manganese oxide. It seems more likely that these peaks should be assigned to calcium antimonate in its CaSb2O6

P. Ricciardi et al. / Journal of Archaeological Science 36 (2009) 2551–2559 Table 1 Synthetic description of the provenance, typology, age, and color of the analyzed samples. Sample name

Provenance

Description

Period

Color

MAC1 MAC2 MAC3 MAC4 MAC5 MAC6 MAC7 MAC8

Pompeii (NA) Collesalvetti (LI) Collesalvetti (LI) Suasa (AN) Suasa (AN) Suasa (AN) Suasa (AN) Pietratonda (GR)

I AD III AD III AD II AD II AD II AD II AD II–III AD

Colorless Blue Turquoise Red Red/striped Green Green Colorless

MAC9

Pietratonda (GR)

II–III AD

Colorless

MAC10

Pietratonda (GR)

II–III AD

Colorless

MAC11

V–VI AD

Green

MAC12 MAC13

S. Maria di Olivola (FG) Faragola (FG) Faragola (FG)

Basic glass tessera tessera tessera tessera tessera tessera Transparent fragment Transparent fragment Transparent fragment tessera

V AD IV AD

Blue Turquoise

MAC14

Faragola (FG)

IV AD

Green

MAC15

Faragola (FG)

IV AD

Red

MAC16

Faragola (FG)

IV AD

Dark/black

MAC17

V–VI AD

Light blue

MAC18 MAC19 MAC20

S. Maria di Olivola (FG) Pompeii (NA) Pompeii (NA) Pompeii (NA)

tessera tessera, opus sectile tessera, opus sectile tessera, opus sectile tessera, opus sectile tessera tessera tessera tessera

I AD I AD I AD

MAC21 MAC22

Pompeii (NA) –

tessera tessera

Restoration Modern

MLVa MLVb MLVc MLVd MLVe MLVf

Carthage Carthage Carthage Carthage Carthage Carthage

tessera tessera tessera tessera tessera tessera

III–IV III–IV III–IV III–IV III–IV III–IV

Red/striped Blue Red altered to green Blue Turquoise (transparent) with golden leaf and cartellina Orange Blue Green Red Orange Blue

AD AD AD AD AD AD

form. Reference spectra for this compound are rare, but can be found both in a publication on a completely different subject (Husson et al., 1984) and in the recent work of Gedzeviciuˆte¨ et al. (2009). A further support for this attribution comes from an SEM– EDS inspection of samples MAC3 and MAC13, aimed precisely at trying to identify the nature of the whitish crystals of irregular shape that could be observed throughout the glass matrix (Fig. 5). The two forms of calcium antimonate are thus shown to sometimes coexist within the same sample; the use of both of them as opacifiers for Roman glasses is well documented (Mass et al., 1998; Verita`, 2000). The spectra of three green samples (MAC6, MAC7 and MAC14) consistently show three wide peaks (at about 339, 455 and 510 cm1), plus a very strong peak at 141 cm1 (not shown in the figure). Numerous pale crystals can be observed through the microscope to be dispersed within the glassy matrix. By acquiring the spectra of such crystals, it is evident that they are responsible for the three bands which are superimposed to the first massif in the macroscopic spectra. Such peaks are associated with the presence of bindheimite Pb2Sb2O7 (Bouchard and Smith, 2003; Colomban et al., 2001). Antimony-based opacifying agents were largely used in glassmaking throughout the Roman world up to 4th–5th century AD, when they were largely substituted by tin oxide while continuing to be employed in Italy until the 13th century (Tite et al., 2008).

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Sample MAC20 contains white acicular crystals which yield peaks at 218 and 615 cm1, attributed to cuprite Cu2O (Welter et al., 2007), which is also detected by the EDS analysis. Additional peaks at 278 and 326 cm1 can tentatively be attributed to a copper salt which might be residual of the raw material used to produce this red glass. The two orange samples from Tunisia (MLVa and MLVe) also show a strong peak at 218 cm1, together with much weaker bands at 415 and 630 cm1. They can all probably be attributed to cuprite Cu2O (Welter et al., 2007). All the spectra of sample MAC21 show the main peak of fluorite CaF2 (at 320 cm1), which must have been used as a flux, and whose presence is confirmed by the SEM–EDS analysis. This is an evident difference between this restoration sample and all the ancient ones; the use of fluorite is here a proof of a more recent production of this glass with respect to the others. Additional crystalline phases have been detected, which can be attributed to residues of raw materials used during the glass manufacturing process. The macroscopic analysis of sample MAC2 shows a peak at 518 cm1, which is also found, together with weaker bands at 466, 570, and 832 cm1, in the analysis of white crystals within the glass. The main peak can also be identified in the spectrum of sample MLVb. This signature can be attributed to undissolved feldspars, as done by Colomban et al. (2003), which found the very same series of peaks in a blue glass bead (n. 62) and in a green mosaic tessera (n. 13), both from Tunisia. Additional phases which have been identified include quartz (main peak at 457–465 cm1) in five samples, calcite (main peak at 1088 cm1) in two samples, and b-wollastonite (peaks at 638 and 973 cm1) in one sample. The presence of calcite could be due to the secondary precipitation phenomena in micro fractures of the glass. The attribution of a sharp peak at about 995–998 cm1, found in four samples (MAC2, MAC11, MAC13 and MAC19), is still undetermined. It sometimes appears in combination with much weaker peaks at about 467, 617, 638 and 1078 cm1. We recently detected a similar intense peak at w995 cm1 in the blue glazes and pigments of 18th century porcelain samples, and assigned it to a Ca and/or Al-silicate, incorporating cobalt ions which were certainly responsible for the deep blue color of those materials (Ricciardi P., unpublished results). This Raman signature does correspond to a mixed compound built with isolated tetrahedral. The model compound is glaserite (K2)2þ(KNa)2þ(SO4)4 2 , and alkali sulphates have been identified in ancient glasses (Rehren, 2008). A partial substitution with Co may be possible. 3.2. Characterization of the glass structure Before proceeding to the analytical deconvolution of the Raman spectra, a few comments can already be made on a simple visual basis, looking at the representative spectra shown in Fig. 6a. All comparisons in this section are made with respect to the seven ‘‘glass families’’ (GF) which are described by Colomban et al. (2006b), and all the numerical values cited also refer to the same work. The shape of most of the spectra is that typical of alkali-silicate glasses (i.e. soda-lime-silicate glasses, corresponding to GF n. 3), which matches the available chemical compositions. Besides, all these spectra show a small band centered at 990 cm1, which was not initially included in the general deconvolution model describing the vibration modes of the silicate tetrahedral network (Mysen et al., 1982), but which seems to be one of the salient features of the Raman spectra of alkali-silicate glasses (Robinet et al., 2006b). The only exceptions are the spectra relative to samples MAC20–22 and MLVa, d, and e. The three Italian samples are indeed ‘‘outliers’’ with respect to the homogeneous corpus of the other pieces. Sample MAC20 is in fact a Pb–Na-silicate glass (GF n. 5), while the restoration tessera shows a very peculiar spectrum, mixing features of

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Fig. 2. The analyzed glass samples, grouped by color.

P. Ricciardi et al. / Journal of Archaeological Science 36 (2009) 2551–2559

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Table 2 Selected chemical composition of the analyzed samples, as results from ICP-OES analyses (wt.% oxides normalized to 100; n.d. ¼ not determined). Samples MAC19 and MAC21 contain 0.12 and 0.7 wt.% of CoO, respectively. SiO2

Al2O3

Fe2O3

MgO

MnO

K2O

Na2O

SnO2

CuO

Sb2O3

P2O5

SO3

Ref.

MAC1 MAC2

66.98 67.17

2.63 1.99

0.38 0.64

0.67 0.56

CaO 8.81 6.24

0.50 0.06

0.91 0.48

18.33 19.66

0.11 0.04

0.00 0.01

0.04 1.82

0.04 1.28

0.17 0.04

0.43 n.d.

MAC3

68.15

2.08

0.58

0.65

5.46

0.13

0.58

20.02

0.25

0.05

1.32

0.70

0.05

n.d.

MAC4 MAC5 MAC6 MAC7 MAC8 MAC9 MAC10 MAC11 MAC13 MAC14 MAC15 MAC16 MAC17 MAC18 MAC19 MAC20 MAC21

59.65 63.98 66.48 68.17 69.48 69.50 71.71 62.21 72.66 67.08 62.85 67.51 68.99 63.27 69.72 41.74 68.81

2.07 2.18 2.50 2.21 2.15 2.21 2.14 3.04 2.98 2.74 2.52 2.63 2.87 2.52 2.17 1.49 4.43

1.90 2.09 0.76 0.84 0.61 0.65 0.33 0.54 0.52 0.56 1.01 1.69 0.41 1.77 1.13 0.64 0.11

1.86 0.67 0.59 0.61 0.64 0.61 0.39 0.47 0.39 0.54 2.27 0.54 0.45 1.63 0.62 0.39 0.09

6.78 5.64 5.83 5.58 7.14 7.21 5.68 5.85 5.78 6.08 10.14 5.83 7.65 6.83 6.50 3.58 12.60

0.29 0.12 0.33 0.44 0.04 0.04 0.40 0.28 0.16 0.41 0.26 0.24 0.39 0.55 0.72 0.10 0.00

1.52 0.81 0.78 0.77 0.68 0.72 0.57 0.88 0.57 0.68 2.23 0.68 0.57 1.14 0.52 0.45 0.42

15.01 16.77 18.19 16.85 18.28 18.17 18.65 16.83 14.48 18.32 15.86 19.05 16.47 16.54 17.76 10.69 12.08

7.96 5.72 2.85 2.35 0.09 0.01 0.00 0.13 0.15 1.56 0.19 0.48 0.00 3.28 0.00 29.97 0.00

0.21 0.03 0.00 0.18 0.00 0.00 0.00 0.32 0.00 0.07 0.21 0.04 0.00 0.02 0.00 0.15 0.00

1.72 1.11 1.16 1.49 0.01 0.01 0.05 3.29 1.28 1.02 1.41 0.49 0.04 1.95 0.25 8.78 1.09

0.42 0.79 0.50 0.41 0.75 0.75 0.00 5.35 0.82 0.73 0.35 0.66 1.87 0.00 0.40 1.78 0.00

0.60 0.07 0.03 0.08 0.14 0.12 0.07 0.13 0.07 0.08 0.60 0.07 0.10 0.49 0.21 0.23 0.37

n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.66 0.13 0.14 0.09 0.10 0.18 n.d. n.d. n.d. n.d.

Boschetti et al. (2007) Macchiarola and Fontanelli (2008) Macchiarola and Fontanelli (2008) Macchiarola et al. (2007) Macchiarola et al. (2007) Macchiarola et al. (2007) Macchiarola et al. (2007)

calcium-silicate and alkali-silicate glasses (GF n. 2 and 3). Finally, the modern sample is still a soda-lime-silicate glass, but its spectrum is clearly different and easily recognizable from those of the ancient ones, probably because of the obvious technological differences. The spectra of the three Tunisian samples are heavily influenced by the presence of crystalline phases, namely quartz (MLVc) and cuprite (MLVa and MLVe), which hinder an easy identification of glass type. For sample MLVe it was actually impossible to obtain a representative spectrum of the glassy phase alone, due to a very strong absorption of the light, as observed for Cu0 containing glass (see Sections 4.1 and 4.2 for details). Comments hereafter will refer only to the homogeneous corpus of samples MAC1–19 and MLVb, d, and f. Table 4 contains the values of Raman parameters calculated from one representative spectrum per sample: the polymerization index Ip and the position of the maxima of the two ‘‘massifs’’ nMAX and dMAX Si–O. When compared with a ‘‘typical’’ spectrum of a soda-lime-silicate glass, our spectra show a compatible position of nMAX Si–O (1096 vs. 1090 cm1), while dMAX Si–O is shifted towards lower wavenumbers (560 vs. 580 cm1), and halfway towards a typical value for Na-silicate glasses (540 cm1, GF n. 4). It should be observed that the average CaO content of our samples (6.6  1.2% CaO) is lower than the

PbO

Macchiarola Macchiarola Macchiarola Macchiarola

et et et et

al. al. al. al.

(2006) (2006) (2006) (2006)

calcium content of two representative samples of GF n. 3 (which contain 8.3 and 9.2% CaO, respectively), so that the sodic component of the glass gives evidently a greater contribution to the Raman spectrum. Values of the polymerization index Ip vary in the range 0.73–1.92, with an average of 1.24  0.30, which is totally comparable to the average value of GF n. 3 (Ip ¼ 1.02). Fig. 7 shows a plot of Ip vs. dMAX Si–O values for all the analyzed samples (closed symbols), pointing out the differences existing between samples MAC20 (red, degraded), MAC21 (restoration), MLVc (green, opacified with quartz), and the group of the remaining samples (ancient and modern). Open symbols refer to samples belonging to all seven glass ‘‘families’’, analyzed during previous works. It is clear from the very small variation range of the Raman parameters how technologically homogeneous all the analyzed Roman glasses are. 4. Discussion: relationship between the Raman spectra and glass color Color differences among the glasses are not always reflected in marked differences between their spectra. For example, the turquoise, blue, and transparent and colorless samples do not seem to show spectral features which distinguish them on the base of

Fig. 3. Details of sampled mosaics in Suasa (Ancona), showing a variety of glass and stone tesserae.

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Table 3 Raman signatures of the crystalline phases identified within the glass samples. Sample

Position of peaks (cm1)

MAC2

457 305, 466, 518, 570, 832 995 465 671 480, 633 141, 340, 456, 508 141, 337, 453, 510 1088 1088 462 238, 325, 338, 521, 671 485, 633 467, 617, 638, 997, 1078 482, 633 238, 324, 338, 671 996 141, 339, 455, 510 460 317, 387, 483, 623–637, 663, 703 671 996 218, 615 278, 326 320 220, 305, 407, 620 518 205, 265, 358, 390, 400, 465, 695, 797, 805, 1068, 1082, 1163 220, 305, 407, 620 638, 973

MAC3

MAC6 MAC7 MAC8 MAC9 MAC10 MAC11

MAC12 MAC13 MAC14 MAC15 MAC17 MAC19 MAC20 MAC21 MLVa MLVb MLVc

MLVe

Phase assignment SiO2 alkali sulphate? SiO2 CaSb2O6 Ca2Sb2O7 Pb2Sb2O7 Pb2Sb2O7 CaCO3 CaCO3 SiO2 CaSb2O6 Ca2Sb2O7 alkali sulphate? Ca2Sb2O7 CaSb2O6 alkali sulphate? Pb2Sb2O7 SiO2 Ca2Sb2O7 CaSb2O6 alkali sulphate? Cu2O CuFeS2 CaF2 Cu2O

Quartz Feldspars (See text) Quartz Ca-antimonate Ca-antimonate Bindheimite Bindheimite Calcite Calcite Quartz Ca-antimonate Ca-antimonate (See text) Ca-antimonate Ca-antimonate (See text) Bindheimite Quartz Ca-antimonate

SiO2

Ca-antimonate (See text) Cuprite Chalcopyrite Fluorite Cuprite Feldspars Quartz

Cu2O CaSiO3

b-wollastonite

Cuprite

their color. Some interesting comments can nonetheless be made, because the color depends on the chemical composition of the samples, and the latter is often related to specific Raman spectral features, as in the case of red, orange and green samples. Representative spectra of differently colored glasses are shown in Fig. 6b. 4.1. Red (and dark/black) glasses Most of these samples (MAC4, MAC5, MAC15, MAC16, MAC18 and MLVd) do not show any peaks which can be related to crystalline phases giving origin to their color, but in their spectra the

band at about 950 cm1 seems to be particularly intense, possibly due to the abundance of precipitated Ca-rich nanosilicates. Historically, red glasses obtained without copper, with hematite a-Fe2O3 are only common starting from Islamic and Venetian times (Freestone and Stapleton, 1998), while most known ancient opaque red glasses and enamels were obtained by the combined use of copper and iron, usually in a lead-rich matrix. The color was due to precipitated copper compounds, in the form of metal and/or oxide (Santagostino Barbone et al., 2008; Barber et al., 2009). Iron served the purpose of obtaining increasingly dark shades of red, and also as a flux to promote the formation of coloring particles (Verita`, 2000). The red coloration in our samples was obtained in this way; the red and dark samples of which the chemical composition is known do in most cases contain relevant amounts of Cu and Fe (0.5–2% CuO, 1–2% Fe2O3). They are also all relatively rich in lead (3–8% PbO vs. an average of about 0.5% PbO for the remaining samples), with the exception of samples MAC15 and MAC16 from Faragola (<0.5% PbO). These samples belong to the category of the so-called ‘‘low-lead and low-copper’’ red glasses whose color is usually due to the presence of metallic copper, possibly in addition to cuprite, if the Pb concentration is sufficiently high (Barber et al., 2009). Finally, these samples also have a medium to high potassium and magnesium content. This agrees with the fact that in Roman times, natron was the only flux used to produce all glasses with the exception of red ones, for which ashes (and lead) were seemingly employed (Verita`, 2000). The Raman analysis of glasses colored by metallic particles can be quite a challenge, depending on the laser wavelength and on the experimental setup (macro- vs. microscopic). The absorption of light is very strong below the Surface Plasmon Resonance (SPR), i.e. w400 nm for Cu0 (Colomban and Schreiber, 2005), making the recording of a ‘‘good’’ spectrum difficult (Colomban, forthcoming). Fig. 8 shows the representative spectra of red sample MAC15, which illustrate well this situation. The microscopic analysis (Fig. 8a) yields a weak Raman signal; the subtraction of a linear baseline (Fig. 8c) allows distinguishing the general signature of a silicate glass, whose shape is however strongly modified in the low wavenumber range because of the proximity of a metal copper particle. When a laser energy far from the SPR peak is used, a macroscopic spectrum (Fig. 8b and d, after baseline subtraction) shows, on the other hand, only the expected Raman signature of a soda-lime-silicate glass. Fragment MAC20 stands as an exception among the red samples; it belongs to a rare typology of red mosaic glasses found at Pompeii, which are strongly altered and have developed degradation

Fig. 4. Representative spectra of the crystalline phases identified within the studied glass samples; (a) Ca2Sb2O7, (b) CaSb2O6, (c) Pb2Sb2O7, (d) Cu2O, (e) CaF2, (f) feldspars, (g) unidentified phase (alkali sulphate?).

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sample have been made on the red (unaltered) part; the analysis of the green (altered) parts yielded only a very intense background signal. As already stated, its spectra are typical of a Pb–Na-silicate glass (Fig. 6a), and the cuprite signature can be identified in some of the microscopic spectra. Therefore, despite the different compositions of the glass matrix, its red color is still due to the presence of copper, although in the form of cuprite rather than as metallic particles. 4.2. Orange glasses Orange glasses are usually investigated in relationship to opaque red glasses, and often considered to be a variant of the latter. The orange color is most often attributed to the presence of cuprite crystals (Santagostino Barbone et al., 2008), as it is the case of these two Tunisian samples. Most of their spectra show exclusively the main peaks of copper oxide, while the identification of the signature of the lead or soda–lead-based glass matrix is extremely rare. Their coloration could therefore be due to the peculiar orange pigment called ‘‘becco di merlo’’, obtained by crystallization of cuprite in a lead-based glass containing antimonium, whose production is attested until about the 7th century AD (Verita`, 2000). The color shift from red to orange in similarly manufactured glasses is linked to differences in cuprite crystals size and/or in different distances between Cu0 nanoparticles. 4.3. Green glasses

Fig. 5. Representative SEM image and EDS point spectrum of a whitish crystal in sample MAC13.

products in the form of Pb-carbonates and Cu-oxides/hydroxides (Corradi et al., 2005), and a consequent greenish appearance (‘‘turquoise’’ color due to Cu2þ). Its chemical composition, extremely rich in copper (w9% CuO) and lead (w30% PbO), makes this sample more similar to European Iron Age opaque red (enamel) glasses than to Roman ones (Stapleton et al., 1999). All Raman analyses on this

The Raman spectra of three of the green samples (MAC6, MAC7 and MAC14) contain the signature of bindheimite Pb2Sb2O7, in agreement with the fact that the Pb content of these samples is comprised between 1.54 and 2.84% PbO, which are the secondhighest values after those of the above-mentioned red samples. All other samples have a lead content smaller than 0.5% PbO. Bindheimite acts as both a coloring and an opaquening agent in these glasses. They contain as much Cu as the blue and turquoise ones, but more lead; copper would therefore yield a blue color, but these samples appear green because of the contemporary presence of the yellowish crystals of bindheimite, which also accounts for the opacification of the glass. Sample MAC11 is visually different and shows different spectral features from the other green samples; it

Fig. 6. Representative Raman spectra of samples; (a) MAC9 and MAC20 (ancient), MAC21 (restoration), MAC22 (modern); (b) red (MAC4), orange (MLVa), green (MAC6 and MAC11).

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Table 4 Raman parameters extracted from the spectra of all the samples, with the exception of MLVe of which no ‘‘good’’ spectrum of the glassy phase was obtained (wavenumber positions in cm1). Sample

Ip

dMAX Si–O

nMAX Si–O

MAC1 MAC2 MAC3 MAC4 MAC5 MAC6 MAC7 MAC8 MAC9 MAC10 MAC11 MAC12 MAC13 MAC14 MAC15 MAC16 MAC17 MAC18 MAC19 MAC20 MAC21 MAC22 MLVa MLVb MLVc MLVd MLVf

1.76 1.31 1.07 0.84 0.87 1.23 1.38 1.31 1.39 1.92 1.11 1.49 1.29 1.20 1.53 1.18 1.40 0.76 1.23 0.43 1.46 0.54 0.55 1.22 2.58 0.73 1.16

570 556 556 559 565 559 557 555 556 543 548 554 550 559 582 563 559 571 556 484 491 570 543 556 463 592 561

1096 1098 1101 1092 1098 1095 1100 1100 1097 1096 1092 1098 1093 1097 1101 1101 1098 1095 1096 1060 1089 1083 988 1094 1087 1081 1099

contains calcium antimonate, which accounts for the opacification of this material. It has probably been colored with copper oxides, of which no trace is evident in the spectra. This possibility is supported by the fact that this sample contains the highest amounts of Sb, Cu, and S among all the samples of which the chemical composition is known. Sample MLVc probably also owes its green color to copper salts, but is has most likely been opacified by means of quartz inclusions, whose intense Raman signature characterizes all the spectra of this sample. 5. Conclusions For the analysis of a set of Roman Age mosaic glasses, Raman spectroscopy showed its utility allowing the non-invasive identification of coloring and opacifying agents. The information obtained

Fig. 8. Representative Raman spectra of red sample MAC15; (a) microscopic analysis, laser 532 nm, raw spectrum; (b) macroscopic analysis, laser 406.7 nm, raw spectrum; (c) microscopic analysis, laser 532 nm, baseline subtracted; (d) macroscopic analysis, laser 406.7 nm, baseline subtracted.

on the glass matrix allowed its characterization but did not help in making a distinction among different provenances, because of the extreme homogeneity of the spectra. This study also led to the identification in some blue, turquoise and green glass tesserae of calcium antimonate, whose Raman signature has only recently been recognized in the scientific literature on mosaic glasses. Ca- and Pbantimonates are responsible for the opacification of most samples, while cassiterite SnO2 was never detected. Interesting results were obtained on red glasses, confirming the use of copper (metallic or as an oxide) to obtain this coloration, and which show once again the high-level technology involved in their production. The extreme homogeneity of the Raman spectra of most of the ancient tesserae well reflected the limited variability of their chemical composition, giving yet another proof of the high level of standardization of glass production in the Roman Age, through a completely non-destructive analytical approach. This homogeneity is also well reflected in the very small variances of all parameters calculated from the spectra, also justifying the difficulty in finding ‘‘trends’’ based on glass provenance. Finally, samples with peculiar compositions, as well as ‘‘modern’’ (and restoration) samples could quite easily be distinguished from the ancient ones by their Raman spectra. Acknowledgements The authors wish to thank Mr. Abdelmajid Ennabli, former conservator of the Carthage museum, for allowing sampling the ‘‘Maison de la Volie`re’’ mosaic. References

Fig. 7. Plot of Ip vs. dMAX Si–O values calculated from representative Raman spectra of samples analyzed in this work (closed symbols), and of a set of previously analyzed samples, representing seven different ‘‘glass families’’ (see Colomban et al., 2003).

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