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The role of copper on colour of palaeo-Christian glass mosaic tesserae: An XAS study
Journal of Cultural Heritage 13 (2012) 137–144
Original article
The role of copper on colour of palaeo-Christian glass mosaic tesserae: An XAS study Alberta Silvestri a,∗ , Serena Tonietto a,1 , Francesco D’Acapito b,2 , Gianmario Molin a,3 a b
Dipartimento di Geoscienze, Universita di Padova, Via Gradenigo 6, 35131 Padova, Italy CNR-IOM-OGG c/o ESRF BP220, 6, rue Jules-Horowitz, 38043 Grenoble, France
a r t i c l e
i n f o
Article history: Received 30 March 2011 Accepted 1st August 2011 Available online 15 September 2011 Keywords: Glass Mosaic tesserae Colour Copper XAS EMPA
a b s t r a c t This work reports mainly the results of an X-ray Absorption Spectroscopy (XAS) study carried out on coloured glass tesserae from the palaeo-Christian mosaic which decorated the votive chapel of St. Prosdocimus (Padova) until its replacement with the current frescoes of Renaissance age, and which is one of the only two known mosaics in the Veneto region (Italy). The study aims at clarifying how the different local structure, oxidation state and quantity of copper influenced colour. Analysis of high-resolution Cu-K edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectra showed that copper is present as cuprite (Cu2 O) in orange samples and as metallic copper in red and brown ones. These phases are responsible for both the colour and opacity of the samples. In addition, Cu1+ ions linked to the oxygen atoms of the glass framework were identified in ratios of about 60% and 30% of total copper in orange and red/brown samples, respectively. In blue and green samples, copper is dispersed in the glass matrix as a mixture of Cu1+ and Cu2+ ions, and no crystalline phases are visible. In this context, the Cu1+ and Cu2+ contents in glass were also quantified thanks to suitable standards, demonstrating that, when Cu2+ is the main chromophorous ion, colour intensity is directly correlated to its content in the glass. In particular, in green and blue samples, coloured by copper, Cu2+ content varies from 26% to 56% of total copper, and the higher contents of Cu2+ are shown by more intensely coloured samples. It should be stressed here that the green colour of the analysed tesserae is given by the physical interaction of blue colour, due to Cu2+ ions, and yellow colour, due to Pb antimonates used as opacifiers. © 2011 Elsevier Masson SAS. All rights reserved.
1. Research aims
2. Introduction
The present study, carried out on coloured glass tesserae from the palaeo-Christian mosaic which decorated the votive chapel of St. Prosdocimus (Padova, Italy) until its replacement by the current frescoes of Renaissance age, reports results from X-ray Absorption Spectroscopy (XAS) and Electron Microprobe (EMPA) analyses, coupled in selected samples with X-ray Powder Diffraction (XRPD), Scanning Electron Microscopy With Energy Dispersive X-ray Spectrometer (SEM-EDS) and colorimetric analyses, aimed at clarifying how the different local structure, oxidation state and quantity of copper could influence the colour of glass tesserae.
It is well known that colour in glass is determined by a complex combination of chemical and textural effects. It may be related to the oxidation state and the electronic configuration of the metal ions in the glass (usually transition metal ions, e.g., Fe, Co, Mn, Cu), which leads to selective absorption of electromagnetic radiation in the visible band and causes the metal to act as a colouring agent. Other colouring effects can be produced when a metal is dispersed as minute particles in the glass, the colouring depending upon the extent of colloidal dispersion, or when a pigment, micrometric in size, is dispersed in the glassy matrix [1]. In this context, copper was one of the most frequently used colouring elements in the ancient glass industry because, according to its different oxidation states, it can impart different colours to glass when it is dispersed in the glassy matrix and/or is arranged differently in crystalline structures (essentially metallic copper or cuprite) [2]. The results of an XAS study carried out on coloured glass mosaic tesserae from the disrupted palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy) are reported here. It has been extensively demonstrated that XAS is a powerful means yielding structural information on a specific absorbing element. The
∗ Corresponding author. Tel.: +39 04 98 27 91 42; fax: +39 04 98 27 91 34. E-mail addresses: [email protected] (A. Silvestri), [email protected] (S. Tonietto), [email protected] (F. D’Acapito), [email protected] (G. Molin). 1 Tel.: +39 04 98 27 91 10; fax: +39 04 98 27 91 34. 2 Tel:+33 04 76 88 24 26; fax +33 04 76 88 27 43. 3 Tel.: +39 04 98 27 91 41; fax: +39 04 98 27 91 34. 1296-2074/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2011.08.002
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technique has the advantage of being non-destructive, elementselective, and sensitive to low concentrations down to a few tens of parts per million, and is thus suitable for cultural heritage studies. In recent years, the techniques of X-ray Absorption Near-Edge Structure (XANES) and X-ray Absorption Fine Structure (EXAFS) have been successfully applied to the study of colour of several ancient glass and lustre samples, providing information on the chemical state of the colouring agents [3–14]. The present work is part of a much wider project focusing on the chemico-physical and mineralogical study of more than 200 tesserae coming from the only two palaeo-Christian mosaics known in the Veneto region (i.e., St. Prosdocimus in Padova, and St. Maria Mater Domini in Vicenza). The final aims of the study are characterisation of materials, and identification of production technologies and of possible raw materials [15].
3. Materials and experimental The tesserae examined here are all dated to the 6th century AD and were preliminarily characterised from the chemicomineralogical viewpoint by conventional analytical techniques such as SEM-EDS for high-resolution morphologic inspection of glass and qualitative chemical analyses, EMPA to determine bulk chemistry, XRPD to define the crystalline phases of opacifiers, and colorimetric analyses to identify colourants and chromatic coordinates; detailed results are reported in Tonietto [15]. Thirteen tesserae were selected for XAS analysis. They are characterised by the presence of copper, presumed to be the main colouring agent, but have different colours (blue, green, red, brown and orange) and copper and lead contents, and generally contain very little cobalt (Table 1). In addition, according to the classification of Fiori et al. [16], based on the ratio PbO/(SiO2 + Na2 O + CaO), the tesserae RO4, TU4, B4, BS4, AZ6 and CE4 are soda-lime in type, because the above ratio is less than 0.01, those labelled M4, VG1, VS4, AQ5, and BO4 are soda-lime-lead, because the ratio is between 0.01 and 0.1, and only two, labelled AV4 and VP1, are leaded because it is greater than 0.1. They were all made with natron as flux, as suggested by the low MgO and K2 O (both lower than 1.5 wt%) and in accordance with data already reported in the literature for coeval tesserae (e.g., [17–21] and references therein). XAS measurements were carried out at the Cu–K edge on the GILDA-CRG beamline [22], with the ESRF storage ring running at 6 GeV. Measurements were carried out in both the XANES and EXAFS regions of spectra. A dynamically sagittally-focussing monochromator with Si (311) crystals was used [23]. A pair of Pd-coated mirrors working in grazing incidence ( = 3.3 mrad, Ecutoff = 18 keV) was used to reject harmonics. The energy scale was calibrated by attributing the value Eedge = 8979 eV [24] to the first inflection point of the absorption spectrum of a Cu metallic foil. The procedure was repeated during data collection to check the stability of the energy calibration. Data collection was carried out at room temperature in fluorescence mode by a 13-element High Purity Ge detector. Due to the good energy resolution of this instrument (E = 200 eV in this energy region), the Cu–K␣ emission line (8.054 keV) was selected for collecting spectra. The maximum count rate per element was limited to 50 kcps in order to avoid non-linear responses by the detector. The incident beam was monitored through a N2 -filled ion chamber. For each sample, two or four spectra were collected and averaged in order to minimise noise, according to how much copper had previously been measured by the electron microprobe (higher or lower than 0.5 wt% as CuO, respectively). The reference spectra of some model compounds (metallic Cu and Cu2 O) were collected in transmission mode, whereas the spectrum of a silica-lead glass
containing only Cu2+ “SLG”1 was collected in fluorescence mode. As the absorber species (Cu in this case) were diluted in the samples, the signal from fluorescence detection was strictly proportional to the absorption coefficient [25] and direct comparisons with the reference compounds collected in transmission mode were valid. XAS spectra were extracted following the standard procedure [26], that is, by subtracting a linear background from the pre-edge region and a spline approximation from the post-edge region by means of the ATHENA code [27]. Quantitative analysis was based on ab initio calculations of the back-scattering phase and amplitude functions with the FEFF8.10 code [28]. Quantitative analyses were carried out with the known crystallographic structures of metallic Cu (Space group Fm3 m, lattice parameter a = 3.6150 A˚ [29]) and cuprite Cu2 O (Space group Pn3n, lattice parameter a = 4.2696 A˚ [29]) as references. Potentials were calculated through the Muffin Tin approximation, with the complex Hedin–Lunqvist approximation for the correlation-exchange part [28]. In particular, the Cu–Cu first shell path from the metal and the Cu–O and Cu–Cu (first and second shells, respectively) paths from the oxide were used to model the corresponding phases. The data were Fourier-transformed in the interval k = [3.0–10.0] A˚ −1 with a k2 weight and a Hanning window function. The fits of the experimental data with theoretical models were carried out in R space with the ARTEMIS code [27]. The EMPA used for quantitative analysis of Cu, Co and Pb in the glassy matrix of tesserae was a CAMECA SX50, equipped with four wavelength-dispersive spectrometers (WDS). Operating conditions were 20 kV and 30 nA sample current, with a counting time for peak and background of 40 s and 20 s, respectively. Pure elements Cu and Co and lead sulphide (PbS) were used as standards for quantitative analyses of copper, cobalt and lead, respectively. With EMPA, the Cu, Co and Pb contents of the analysed samples were identified by random point microanalyses (generally 10 per sample) and mean and standard deviations were calculated. The complete EMPA analytical conditions used for the determination of major, minor and trace elements in the glassy matrix of tesserae are reported in Silvestri and Marcante [30] as the present samples are subjected to the same analytical protocol. SEM analysis was performed on a FEI Quanta 200 FEG-ESEM instrument. Semi-quantitative elemental analyses were performed on an EDAX Genesys energy-dispersive X-ray spectrometer, with accelerating voltage of 25 keV. XRPD data were obtained on a computer-controlled Philips X’Pert PRO, with Bragg-Brentano - geometry. The normal-focus Cu X-ray tube (Cu K␣1 = 0.154056 nm) operated at 40 KV and 20 mA. Data were recorded in the 2◦ –70◦ 2 range, in step-scan mode with step width increments of 0.02◦ and a step counting time of 10 s. Data collected were processed by the X’Pert HighScore (PANalytical copyright); 2 and d values were calculated with the second-derivative algorithm of Savitzky and Golay [31]. All XRPD diffraction profiles were carried out on whole samples for conservation purposes. Colorimetric analyses are carried out by means of imaging spectroscopy. The acquisition of multispectral images revealed the most reliable asset for faithful colour reproduction, where faithfulness is measured in terms of independence from illumination and acquisition device. In the present study, the multispectral images of the selected tesserae were acquired by means of the imaging spectrograph Imspectror V10 by SPECIM, mounted on a rotary device. The illumination set-up chosen consisted of a metallic iodide lamp to measure the reflectance between 420–500 nm, in combination with an incandescent lamp for that between 500 and 800 nm. After
1 Sample SLG was kindly provided by Dr. Laura Cartechini (CNR-ISTM, Perugia, Italy).
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acquisition of the full 2D image, reflectance spectra and colorimetric coordinates (L*a*b*) were obtained by means of suitable software. Full details on this technique are reported in Paviotti et al. [32]. 4. Results and discussion 4.1. Blue and green tesserae The XANES spectra of samples TU4 (turquoise), AQ5 (aquamarine), CE4 (pale blue), B4 (blue), AZ6 (azure), BO4 (opaque blue), BS4 (dark blue), VG1 (yellow-green), VP1 (green) and VS4 (dark green) are shown in Fig. 1. All samples show resonance on the rising part of the spectrum and an edge position of 8982 eV. This means that Cu is predominantly in the +1 state, as widely reported in the literature [14], with a minor contribution of Cu2+ , the true glass chromophorous agent. XANES data can be used to quantify the Cu1+ /Cu2+ ratio, as the spectra of the pure phases (Fig. 1, BS4 and SLG curves in bold) are markedly different in this region. Cu1+ (BS4) presents a resonance at 8993.0(5) eV which is completely absent in the Cu2+ sample (SLG). Therefore, using as models the spectra of the sample, in which copper is present exclusively as Cu1+ (sample BS4 was chosen from the results of the EXAFS analysis) and of a silica-lead glass containing only Cu2+ ions (SLG), the XANES data of other samples can be reproduced as linear combinations of the models. The fits are of very good quality, as shown in the inset of Fig. 1, and the quantitative data (Table 2) reveal a mixture of Cu1+ and Cu2+ . In the case of sample
Fig. 1. Normalised XANES spectra of blue (TU4 -turquoise; AQ5 -aquamarine; CE4 pale blue; AZ6 -azure; BS4 -dark blue; B4 -blue; and BO4 -opaque blue) and green samples (VG1 -yellow-green; VP1 -green; and VS4 -dark green) together with model for Cu2+ in glass SLG. Spectra normalised by setting at one the atomic absorption far from the edge, in order to allow comparisons between them, irrespective of Cu concentration in various samples. Inset: typical fit of edge of samples as linear combination of model compounds for Cu1+ (BS4) and Cu2+ (SLG), shown in bold line in main plot.
Table 1 List of analysed samples, indicating colour type, chromatic group, and label. Also shown: colorimetric coordinates (L*a*b*), CuO, CoO and PbO contents for each sample. Colour type
Chromatic group
Sample
Colour
CuO (wt%)
CoO (wt%)
Blue
Turquoise
TU4
1.13 ± 0.03
< 0.03
0.45 ± 0.08
Pale blue
CE4
0.72 ± 0.02
< 0.03
0.22 ± 0.08
Aquamarine
AQ5
0.99 ± 0.04
< 0.03
0.53 ± 0.04
Blue
B4
0.17 ± 0.02
0.05 ± 0.02
0.34 ± 0.10
Opaque blue
BO4
0.12 ± 0.01
0.07 ± 0.01
0.52 ± 0.16
Dark blue
BS4
0.09 ± 0.02
0.12 ± 0.02
0.39 ± 0.08
Azure
AZ6
0.05 ± 0.01
< 0.03
Dark green
VS4
1.62 ± 0.38
< 0.03
1.37 ± 0.22
Yellow-green
VG1
0.80 ± 0.07
< 0.03
1.67 ± 0.29
Green
VP1
0.68 ± 0.03
< 0.03
9.39 ± 0.79
Red
Red
RO4
1.04 ± 0.31
< 0.03
0.41 ± 0.11
Brown
Brown
M4
1.62 ± 0.61
0.06 ± 0.02
6.92 ± 0.16
Orange
Orange
AV4
L* = 60.8 a* = –32.5 b* = –23.3 L* = 71.1 a* = –15 b* = –1 L* = 41.7 a* = –17.7 b* = –9.2 L* = 28.4 a* = 0.6 b* = –14.8 L* = 31.8 a* = 6.4 b* = –16 L* = 8.6 a* = 3.1 b* = –16 L* = 79.9 a* = –7.1 b* = –7 L* =37.4 a* = –15.4 b* = 2 L* =67.2 a* = –21.6 b* = 33.7 L* = 55.4 a* = –21.8 b* = 18.9 L* = 41.4 a* = 24.8 b* = 18.4 L* = 34.1 a* = 16.4 b* = 10.7 L* = 81.1 a* = 51.3 b* = 66.5
7.69 ± 0.28
< 0.03
Green
PbO (wt%)
< 0.08
27.66 ± 0.23
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Fig. 2. EXAFS spectra (a) and Fourier Transforms (b) of EXAFS data for samples AZ6 (azure), BS4 (dark blue), B4 (blue), and BO4 (opaque blue), characterised by CuO contents lower than 0.5 wt%. Transformations performed in interval k = [3.00–10.00] A˚ −1 with k2 weight and Hanning window.
Fig. 3. EXAFS spectra (a) and Fourier Transforms (b) of EXAFS data for samples TU4 (turquoise), AQ5 (aquamarine), CE4 (pale blue), VG1 (yellow-green), VP1 (green) and VS4 (dark green), characterised by CuO contents higher than 0.5 wt%. Transformations performed in interval k = [3.00–10.00] A˚ −1 with k2 weight and Hanning window.
AZ6, which has very little total copper (CuO = 0.05 ± 0.01 wt% – Table 1) and as this element is mainly present as Cu1+ ion (74 ± 5%), not as a chromophorous ion, we can confirm that the true chromophorous element is cobalt, the presence of which was confirmed by colorimetric analysis, where the typical Co optical absorptions at 530, 590 and 645 nm [33] were identified. EXAFS data were fitted by building a model which exploits the co-presence of the two sites (corresponding to the two oxidation states) for Cu evidenced by XANES. One is relative to Cu1+ which ˚ and the other is relis 2-coordinated with oxygen ions at ∼1.85 A, ˚ Free ative to Cu2+ , 4-coordinated with O neighbours at ∼1.95 A. parameters were the fraction of Cu1+ and the bond distances, as
Table 2 Quantitative analysis of XAS data for blue and green samples. Column 1: percentages of copper oxidation states, estimated from XANES analysis. Columns 2, 3, 4 and 5: analysis of EXAFS spectra, reproduced as a linear combination with contributions from Cu1+ and Cu2+ . Debye-Waller factor is fitted on sample BS4 and fixed in the others. Errors on last figure are shown in brackets. Sample
Cu1+ XANES
Cu1+ EXAFS
TU4 AQ5 CE4 B4 BO4 BS4 AZ6 VS4 VG1 VP1
46(5) 44(5) 74(5) 95(5) 95(5) 95(5) 74(5) 60(5) 63(5) 66(5)
56(5) 59(4) 90(10) 90(10) 90(10) 80(10) 98(1) 55(6) 70(10) 65(6)
R1 (Å) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2)
2 (Å2 ) 0.005 0.005 0.005 0.005 0.005 0.005(2) 0.005 0.005 0.005 0.005
R2 (Å) 1.94(3) 1.94(3) – – – – – 1.93(3) 1.97(3) 1.94(3)
well as a global edge position E0 . In order to reduce the number of free parameters, a single Debye-Waller factor 2 was used for these two shells. Actually, the site of Cu2+ should be octahedral rather than square planar, i.e., it should contain two more oxygen atoms (axial). However, it is well known that Cu2+ –O complexes undergo a Jahn-Teller tetrahedral distortion which takes the axial O considerably farther than the planar ones (for an example in water solution, see [34]). In the present case, attempts to include them in the fit were unsuccessful and the final model retained only 4 O. The spectra are shown in Figs. 2(a)–3(a) and the related Fourier Transforms (FT) of EXAFS data in Figs. 2(b)–3(b). The two contributions overlap in the main single peak visible in the FT. The results of quantitative fits are listed in Table 2. In all samples, Cu is coordinated with O indicating that Cu (in both valence states) is present as ions dispersed in the glassy matrix, as no further peaks for crystalline phases are visible. The model described above allows direct quantification of the Cu1+ /Cu2+ ratio and this value can be compared with that derived from XANES. It is worth noting that samples with higher Cu content (TU4, AQ5, VS4, VP1, VG1) exhibit a detectable fraction of Cu2+ , whereas in those with lower Cu, almost only the Cu1+ state is generally present. As regards hues, in the case of blue samples with low copper (B4, BO4, BS4, AZ6), in which non-chromophorous Cu1+ is mostly present, colour is presumably due to the presence of cobalt (Table 1), an element which has an extremely high colouring capacity, even at very low concentrations, and confers a deep blue colour on glass when it adopts tetrahedral coordination [2]. In the case of other samples with higher copper contents and no cobalt at detectable levels (TU4, CE4, AQ5, VS4, VG1, VP1 – Table 1),
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Fig. 4. XRPD profile of sample VG1. Note presence of peaks at 2 = 29.65, 2 = 34.31◦ , 2 = 49.21 and 2=58.36◦ , due to most intense reflections of structural planes (222), (400), (440) and (622) of lead antimonate, respectively. Same phase also identified in other green tesserae.
the colour is due to the presence of Cu2+ chromophorous ions. In addition, the quantification of Cu1+ and Cu2+ in these samples demonstrates that, taking into account the total content of copper, the colour intensity of the tesserae is directly correlated to the Cu2+ content of the glass, because the more intensely coloured samples show the highest amount of Cu2+ ions. In particular, in blue samples, coloured by copper (TU4, AQ5, CE4), Cu2+ varies from 26% to 56% of total copper, and its lowest contents appear in the less intensely coloured samples, e.g., CE4, pale blue (Table 2). The same happens for green tesserae (VS4, VG1, VP1), where the tesserae with darker shades of green have higher contents of Cu2+ ions dispersed in the glass matrix, e.g., sample VS4, dark green in colour. In any case, it should be stressed here that the different green hues of the Paduan tesserae are due to the physical interaction of the blue due to Cu2+ ions, identified by XAS, and the yellow due to Pb antimonates used as opacifiers, as identified by SEM-EDS and XRPD analyses in variable ratios (Fig. 4). 4.2. Orange, red and brown tesserae The XANES spectra of samples AV4 (orange), RO4 (red) and M4 (brown) are shown in Fig. 5, in which the edge positions of the spectra are marked by dots. Sample AV4 has an edge at around 8982.0(5) eV, whereas samples M4 and RO4 are at 8979.0(5) eV. Fig. 5 also shows the reference spectra of metallic copper (Cu Met) and cuprite (Cu2 O), with edges at 8979.0(5) eV and 8981.0(5) eV, respectively.
As the edge position reflects the oxidation state of the ion, we conclude that the Cu ions are predominantly in the metallic state in samples RO4 and M4, whereas the signal in sample AV4 exhibits features very similar to those of cuprite. The EXAFS data and related FT of samples RO4, M4 and AV4 are shown in Figs. 6(a) and (b) respectively, together with the data for reference compounds, metallic Cu and Cu2 O. In both cases, samples RO4 and M4 strongly resemble metallic Cu (oscillations with the same frequency and phase, giving rise to the same pattern in the spectrum), with the exception of the slightly lower intensity of the oscillations and a small shoulder at about 1.5 A˚ in the FT. This shoulder is due to the presence of an amorphous oxidised phase, as confirmed by quantitative analysis. Sample AV4 has an EXAFS spectrum similar (again in terms of frequency and phase of oscillations) to that of Cu2 O and the same peaks in the FT, although the peak at about 2.8 A˚ (due to Cu-Cu coordination) is smaller than that of the crystal. This suggests an excess of Cu-O bonds with respect to Cu-Cu, indicating a mixture of Cu1+ ions dispersed in the glass together with Cu2 O particles. Note that Cu is in the +1 valence state in both phases. EXAFS data of samples RO4 and M4 were quantitatively analysed by a two-phase model. One is a single shell Cu1+ -O relative to metal ions dispersed in glass, as already described in the previous section. The other is a metallic phase for Cu and this is described by single- and multiple-scattering paths up to the 5th coordination shell. Copper in the oxide phase is assumed to be 2coordinated, whereas the coordination numbers in the metal phase are given by the crystallographic data: the relative amount between these two phases is left as a free fit parameter and provides the fraction between oxidised and metallic Cu. The metallic phase is described in such a way as to reduce the number of free parameters as much as possible. All bond length values were derived from a single variable lattice parameter a; for the Debye-Waller Factors, a free 2 parameter was considered for the first shell, and all the others were derived from a correlated Debye model [35] as implemented in the ARTEMIS code [27] and described by a single effective Debye Temperature TD . The results are listed in
Table 3 Results of EXAFS analysis for brown (M4) and red (RO4) samples. Structural parameters of reference compound (Cu ref.) from Crystallographic Information File also shown.
Fig. 5. XANES spectra of samples RO4 (red), M4 (brown) and AV4 (orange), compared with those of metallic copper (Cu Met) and cuprite (Cu2 O). Dots: edge positions.
Sample
Metal fraction (%)
Latt. Par (Å)
First shell 2 (Å2 )
T Debye (K)
Cu ref. M4 RO4
100 70(5) 70(5)
3.615 3.61(1) 3.60(1)
– 0.0090(10) 0.0085(7)
315 310(20) 320(15)
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Fig. 6. EXAFS spectra (a) and Fourier Transforms (b) of EXAFS data for samples RO4 (red), M4 (brown) and AV4 (orange), compared with those of metallic copper (Cu Met) and cuprite (Cu2 O). Transformations performed in interval k = [3.00–10.00] A˚ −1 with k2 weight and Hanning window.
Table 3, which clearly shows that 70% of total Cu is in the crystalline metallic phase. The presence of metallic copper in samples RO4 and M4 was also confirmed by XRPD analyses, in which peaks at 2 = 43.3, 2 = 50.44◦ and 2 = 74.14◦ , related to the most intense reflections of structural planes (111), (200) and (220) of metallic copper, respectively, were identified. Sample AV4 has two well-defined peaks in the FT, at about 1.4 A˚ and 2.7 A˚ (Fig. 6(b)). The first peak coincides with the first peak of cuprite and is due to the Cu–O bond of Cu2 O; the second falls in the range of the first Cu–Cu shell of the same cuprite. As already noted, the Cu-O peak is higher than the Cu-Cu peak, as instead found in the pure Cu2 O crystal, because part of the CuO bonds comes from an amorphous oxide that does not possess the second coordination shell. Quantitative analysis, carried out as a linear combination of the amorphous oxide plus the crystalline oxide described by two shells Cu-O and Cu-Cu, provided a crystalline oxide fraction of 39(5)%, a first Cu-O shell (common to the ˚ and a second Cu-Cu amorphous and crystalline phases) at 1.85 A, ˚ as expected for this crystalline phase. The presence shell at 3.02 A, of cuprite in sample AV4 was also confirmed by XRPD analysis, in which peaks at 2 = 29.58, 2 = 36.43◦ , 2 = 42.32◦ , 2 = 61.40◦ , 2 = 73.56◦ and 2 = 77.41◦ , related to the most intense reflections of structural planes (110), (111), (200), (220), (311) and (222) of cuprite, respectively, were identified. These results were also confirmed independently by the combined linear fitting of XANES data, with the spectra of samples BS4 and metallic copper as references. The presence of Cu1+ ions confirms the highly reducing conditions of the kiln during the glass
production process, which stimulated the precipitation of metallic copper in the case of samples RO4 and M4. Taking into account the equal content of metallic copper in both tesserae (Table 3), the different colour may be related to the different crystal sizes, metallic copper particles being smaller than 150 nm in size in sample RO4 (red) and larger than 150 nm in sample M4 (brown) (Fig. 7). The literature has clearly established that the grain-size of inclusions, particularly in red glasses, influences the final colour hue, which darkens with increasing crystal size ([36], and references therein). In addition to those already reported in the literature on red and orange tesserae and obtained by means of conventional analytical techniques (e.g., [36–39] and references therein), these results are also similar to those already reported for lustreware [5,6,8,11] and for Roman [9], late Roman [13], old Japanese [3] and Portuguese [10] glasses. They also confirm the hypothesis that the red glass with low copper and low lead (e.g., samples RO4 and M4 – Table 1), is generally coloured by metallic copper nano-particles, which are exsolved due to melt reduction. This may have been induced by heat treatment at around 1000 ◦ C and/or the addition of reducing agents such as carbon or metals like tin, antimony and iron in proper oxidation states [40,41]. In sample RO4, the role of reducing agents was almost certainly played by iron and antimony (FeO and Sb2 O3 , with values of 2.4 ± 0.5 and 0.4 ± 0.4 wt%, respectively; EMPA data) and in sample M4 by both iron and tin (FeO and SnO2 , at 2.4 ± 0.2 and 1.8 ± 0.1 wt%, respectively; EMPA data). Instead, the formation of cuprite requires high copper and lead contents in the glass, as in the case of sample AV4 (Table 1): with too little copper, cuprite
Fig. 7. SEM-BSE images of tesserae RO4 (a) and M4 (b). Note different sizes of inclusions, composed of metallic copper, greater in sample M4.
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would be unlikely to form; with too little lead, it would not form the dendritic structures which produce an orange/red hue [41]. It should be stressed here, as already observed by Fiori [21] for coeval red tesserae from Ravenna, that the Paduan red/brown tesserae, coloured by means of metallic copper, are also characterised by different lead contents (Table 1) confirming that the glass batch does not necessarily need lead to stimulate the precipitation of metallic copper, as reported in Barber et al. [41]. According to experimental works (e.g., [42]), the role of lead is that of improving the rates of nucleation and growth of inclusions. This is also confirmed in the Paduan tesserae, where sample M4, which shows higher lead content than sample RO4 (Table 1), is characterised by the greater size of metallic copper crystals (Fig. 7). 5. Conclusions The present study yielded better knowledge of colour generation in palaeo-Christian glass mosaic tesserae and confirmed the complexity of this topic, which requires a combined approach, involving integration of data, obtained by various analytical techniques, also non-conventional, to be solved. The integration of XAS and EMPA data coupled with XRPD, SEM and colorimetric analyses, as carried out in this study, showed that variations in colour may be the result of deliberate technical operations — in particular, redox conditions in kilns and the composition of glass batches — which were applied with the aim of obtaining various colours. In detail, copper imparts different colours and hues depending on its state of oxidation and quantification. In the case of the blue tesserae, the colour is due both to Cu2+ ions dispersed in the glassy matrix (samples AQ5, TU4, CE4) and to other chromophorous ions, such as cobalt (samples B4, BS4, BO4, AZ6), because copper is mainly present as Cu1+ ions, not chromophorous. In green tesserae, copper is mainly present as Cu2+ ions and their colour is due to physical interactions between the blue, due to Cu2+ ions, and the yellow, due to Pb antimonates used as opacifiers. In addition, quantification of Cu1+ and Cu2+ ions in the analysed samples demonstrated that, when Cu2+ is the main chromophorous ion, colour intensity is directly correlated to its content in the glass. In the case of the red and brown tesserae, colour is mainly due to the presence of metallic copper, and to cuprite in the orange tesserae. In this context, the hypothesis of Barber et al. [41] was also confirmed: in addition to the redox conditions in kilns, these authors relate the precipitation of the above phases to differing amounts of copper and lead in the samples. It should be stressed here that, considering the whole range of colours of the Paduan tesserae, the copper and lead contents are not related, as already observed by Fiori [21] for coeval tesserae from Ravenna. In fact, in the blue tesserae, mainly coloured by copper, lead is not present at detectable levels; in the green ones, lead measured in the glassy matrix is probably due to partial dissolution of lead antimonates used as opacifiers; and in orange, red and brown ones, the addition of varying amounts of lead, in addition to the redox conditions of kilns, is functional to the precipitation and growth of cuprite and/or metallic copper, which act as opacifiers and pigments. Lastly, the similarity of the present results with those already reported in the literature indicates routine glass production processes for the same set of colours, notwithstanding their different age, provenance and type. Acknowledgements The authors thank the “Soprintendenza per il patrimonio storicoartistico ed etnoantropologico per le province di Belluno, Padova, Rovigo e Treviso”, and in particular Dr. A. Spiazzi, for authorising the
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present study and the Abbot of the Basilica of St. Justine for providing glass samples. They are also grateful to F. Gallo (Department of Geosciences, University of Padova, Italy), R. Carampin (CNRIGG, Padova, Italy), F. Zorzi (Department of Geosciences, University of Padova, Italy), P. Guerriero (CNR–ICIS, Padova, Italy) and G. M. Cortelazzo, F. Ratti and A. Paviotti (Department of Information Engineering, University of Padova, Italy) for their kind collaboration in execution of XAS, EMPA, XRPD, SEM and colorimetric analysis, respectively, and Fila Industria Chimica S.p.a. of Padova, for use of the FEI Quanta 200 FEG ESEM instrument. This work was carried out with the financial support of Progetto di Ricerca di Ateneo, University of Padova (Project No. CPDA088419).
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Original article
The role of copper on colour of palaeo-Christian glass mosaic tesserae: An XAS study Alberta Silvestri a,∗ , Serena Tonietto a,1 , Francesco D’Acapito b,2 , Gianmario Molin a,3 a b
Dipartimento di Geoscienze, Universita di Padova, Via Gradenigo 6, 35131 Padova, Italy CNR-IOM-OGG c/o ESRF BP220, 6, rue Jules-Horowitz, 38043 Grenoble, France
a r t i c l e
i n f o
Article history: Received 30 March 2011 Accepted 1st August 2011 Available online 15 September 2011 Keywords: Glass Mosaic tesserae Colour Copper XAS EMPA
a b s t r a c t This work reports mainly the results of an X-ray Absorption Spectroscopy (XAS) study carried out on coloured glass tesserae from the palaeo-Christian mosaic which decorated the votive chapel of St. Prosdocimus (Padova) until its replacement with the current frescoes of Renaissance age, and which is one of the only two known mosaics in the Veneto region (Italy). The study aims at clarifying how the different local structure, oxidation state and quantity of copper influenced colour. Analysis of high-resolution Cu-K edge X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectra showed that copper is present as cuprite (Cu2 O) in orange samples and as metallic copper in red and brown ones. These phases are responsible for both the colour and opacity of the samples. In addition, Cu1+ ions linked to the oxygen atoms of the glass framework were identified in ratios of about 60% and 30% of total copper in orange and red/brown samples, respectively. In blue and green samples, copper is dispersed in the glass matrix as a mixture of Cu1+ and Cu2+ ions, and no crystalline phases are visible. In this context, the Cu1+ and Cu2+ contents in glass were also quantified thanks to suitable standards, demonstrating that, when Cu2+ is the main chromophorous ion, colour intensity is directly correlated to its content in the glass. In particular, in green and blue samples, coloured by copper, Cu2+ content varies from 26% to 56% of total copper, and the higher contents of Cu2+ are shown by more intensely coloured samples. It should be stressed here that the green colour of the analysed tesserae is given by the physical interaction of blue colour, due to Cu2+ ions, and yellow colour, due to Pb antimonates used as opacifiers. © 2011 Elsevier Masson SAS. All rights reserved.
1. Research aims
2. Introduction
The present study, carried out on coloured glass tesserae from the palaeo-Christian mosaic which decorated the votive chapel of St. Prosdocimus (Padova, Italy) until its replacement by the current frescoes of Renaissance age, reports results from X-ray Absorption Spectroscopy (XAS) and Electron Microprobe (EMPA) analyses, coupled in selected samples with X-ray Powder Diffraction (XRPD), Scanning Electron Microscopy With Energy Dispersive X-ray Spectrometer (SEM-EDS) and colorimetric analyses, aimed at clarifying how the different local structure, oxidation state and quantity of copper could influence the colour of glass tesserae.
It is well known that colour in glass is determined by a complex combination of chemical and textural effects. It may be related to the oxidation state and the electronic configuration of the metal ions in the glass (usually transition metal ions, e.g., Fe, Co, Mn, Cu), which leads to selective absorption of electromagnetic radiation in the visible band and causes the metal to act as a colouring agent. Other colouring effects can be produced when a metal is dispersed as minute particles in the glass, the colouring depending upon the extent of colloidal dispersion, or when a pigment, micrometric in size, is dispersed in the glassy matrix [1]. In this context, copper was one of the most frequently used colouring elements in the ancient glass industry because, according to its different oxidation states, it can impart different colours to glass when it is dispersed in the glassy matrix and/or is arranged differently in crystalline structures (essentially metallic copper or cuprite) [2]. The results of an XAS study carried out on coloured glass mosaic tesserae from the disrupted palaeo-Christian glass mosaic of St. Prosdocimus (Padova, Italy) are reported here. It has been extensively demonstrated that XAS is a powerful means yielding structural information on a specific absorbing element. The
∗ Corresponding author. Tel.: +39 04 98 27 91 42; fax: +39 04 98 27 91 34. E-mail addresses: [email protected] (A. Silvestri), [email protected] (S. Tonietto), [email protected] (F. D’Acapito), [email protected] (G. Molin). 1 Tel.: +39 04 98 27 91 10; fax: +39 04 98 27 91 34. 2 Tel:+33 04 76 88 24 26; fax +33 04 76 88 27 43. 3 Tel.: +39 04 98 27 91 41; fax: +39 04 98 27 91 34. 1296-2074/$ – see front matter © 2011 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2011.08.002
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technique has the advantage of being non-destructive, elementselective, and sensitive to low concentrations down to a few tens of parts per million, and is thus suitable for cultural heritage studies. In recent years, the techniques of X-ray Absorption Near-Edge Structure (XANES) and X-ray Absorption Fine Structure (EXAFS) have been successfully applied to the study of colour of several ancient glass and lustre samples, providing information on the chemical state of the colouring agents [3–14]. The present work is part of a much wider project focusing on the chemico-physical and mineralogical study of more than 200 tesserae coming from the only two palaeo-Christian mosaics known in the Veneto region (i.e., St. Prosdocimus in Padova, and St. Maria Mater Domini in Vicenza). The final aims of the study are characterisation of materials, and identification of production technologies and of possible raw materials [15].
3. Materials and experimental The tesserae examined here are all dated to the 6th century AD and were preliminarily characterised from the chemicomineralogical viewpoint by conventional analytical techniques such as SEM-EDS for high-resolution morphologic inspection of glass and qualitative chemical analyses, EMPA to determine bulk chemistry, XRPD to define the crystalline phases of opacifiers, and colorimetric analyses to identify colourants and chromatic coordinates; detailed results are reported in Tonietto [15]. Thirteen tesserae were selected for XAS analysis. They are characterised by the presence of copper, presumed to be the main colouring agent, but have different colours (blue, green, red, brown and orange) and copper and lead contents, and generally contain very little cobalt (Table 1). In addition, according to the classification of Fiori et al. [16], based on the ratio PbO/(SiO2 + Na2 O + CaO), the tesserae RO4, TU4, B4, BS4, AZ6 and CE4 are soda-lime in type, because the above ratio is less than 0.01, those labelled M4, VG1, VS4, AQ5, and BO4 are soda-lime-lead, because the ratio is between 0.01 and 0.1, and only two, labelled AV4 and VP1, are leaded because it is greater than 0.1. They were all made with natron as flux, as suggested by the low MgO and K2 O (both lower than 1.5 wt%) and in accordance with data already reported in the literature for coeval tesserae (e.g., [17–21] and references therein). XAS measurements were carried out at the Cu–K edge on the GILDA-CRG beamline [22], with the ESRF storage ring running at 6 GeV. Measurements were carried out in both the XANES and EXAFS regions of spectra. A dynamically sagittally-focussing monochromator with Si (311) crystals was used [23]. A pair of Pd-coated mirrors working in grazing incidence ( = 3.3 mrad, Ecutoff = 18 keV) was used to reject harmonics. The energy scale was calibrated by attributing the value Eedge = 8979 eV [24] to the first inflection point of the absorption spectrum of a Cu metallic foil. The procedure was repeated during data collection to check the stability of the energy calibration. Data collection was carried out at room temperature in fluorescence mode by a 13-element High Purity Ge detector. Due to the good energy resolution of this instrument (E = 200 eV in this energy region), the Cu–K␣ emission line (8.054 keV) was selected for collecting spectra. The maximum count rate per element was limited to 50 kcps in order to avoid non-linear responses by the detector. The incident beam was monitored through a N2 -filled ion chamber. For each sample, two or four spectra were collected and averaged in order to minimise noise, according to how much copper had previously been measured by the electron microprobe (higher or lower than 0.5 wt% as CuO, respectively). The reference spectra of some model compounds (metallic Cu and Cu2 O) were collected in transmission mode, whereas the spectrum of a silica-lead glass
containing only Cu2+ “SLG”1 was collected in fluorescence mode. As the absorber species (Cu in this case) were diluted in the samples, the signal from fluorescence detection was strictly proportional to the absorption coefficient [25] and direct comparisons with the reference compounds collected in transmission mode were valid. XAS spectra were extracted following the standard procedure [26], that is, by subtracting a linear background from the pre-edge region and a spline approximation from the post-edge region by means of the ATHENA code [27]. Quantitative analysis was based on ab initio calculations of the back-scattering phase and amplitude functions with the FEFF8.10 code [28]. Quantitative analyses were carried out with the known crystallographic structures of metallic Cu (Space group Fm3 m, lattice parameter a = 3.6150 A˚ [29]) and cuprite Cu2 O (Space group Pn3n, lattice parameter a = 4.2696 A˚ [29]) as references. Potentials were calculated through the Muffin Tin approximation, with the complex Hedin–Lunqvist approximation for the correlation-exchange part [28]. In particular, the Cu–Cu first shell path from the metal and the Cu–O and Cu–Cu (first and second shells, respectively) paths from the oxide were used to model the corresponding phases. The data were Fourier-transformed in the interval k = [3.0–10.0] A˚ −1 with a k2 weight and a Hanning window function. The fits of the experimental data with theoretical models were carried out in R space with the ARTEMIS code [27]. The EMPA used for quantitative analysis of Cu, Co and Pb in the glassy matrix of tesserae was a CAMECA SX50, equipped with four wavelength-dispersive spectrometers (WDS). Operating conditions were 20 kV and 30 nA sample current, with a counting time for peak and background of 40 s and 20 s, respectively. Pure elements Cu and Co and lead sulphide (PbS) were used as standards for quantitative analyses of copper, cobalt and lead, respectively. With EMPA, the Cu, Co and Pb contents of the analysed samples were identified by random point microanalyses (generally 10 per sample) and mean and standard deviations were calculated. The complete EMPA analytical conditions used for the determination of major, minor and trace elements in the glassy matrix of tesserae are reported in Silvestri and Marcante [30] as the present samples are subjected to the same analytical protocol. SEM analysis was performed on a FEI Quanta 200 FEG-ESEM instrument. Semi-quantitative elemental analyses were performed on an EDAX Genesys energy-dispersive X-ray spectrometer, with accelerating voltage of 25 keV. XRPD data were obtained on a computer-controlled Philips X’Pert PRO, with Bragg-Brentano - geometry. The normal-focus Cu X-ray tube (Cu K␣1 = 0.154056 nm) operated at 40 KV and 20 mA. Data were recorded in the 2◦ –70◦ 2 range, in step-scan mode with step width increments of 0.02◦ and a step counting time of 10 s. Data collected were processed by the X’Pert HighScore (PANalytical copyright); 2 and d values were calculated with the second-derivative algorithm of Savitzky and Golay [31]. All XRPD diffraction profiles were carried out on whole samples for conservation purposes. Colorimetric analyses are carried out by means of imaging spectroscopy. The acquisition of multispectral images revealed the most reliable asset for faithful colour reproduction, where faithfulness is measured in terms of independence from illumination and acquisition device. In the present study, the multispectral images of the selected tesserae were acquired by means of the imaging spectrograph Imspectror V10 by SPECIM, mounted on a rotary device. The illumination set-up chosen consisted of a metallic iodide lamp to measure the reflectance between 420–500 nm, in combination with an incandescent lamp for that between 500 and 800 nm. After
1 Sample SLG was kindly provided by Dr. Laura Cartechini (CNR-ISTM, Perugia, Italy).
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acquisition of the full 2D image, reflectance spectra and colorimetric coordinates (L*a*b*) were obtained by means of suitable software. Full details on this technique are reported in Paviotti et al. [32]. 4. Results and discussion 4.1. Blue and green tesserae The XANES spectra of samples TU4 (turquoise), AQ5 (aquamarine), CE4 (pale blue), B4 (blue), AZ6 (azure), BO4 (opaque blue), BS4 (dark blue), VG1 (yellow-green), VP1 (green) and VS4 (dark green) are shown in Fig. 1. All samples show resonance on the rising part of the spectrum and an edge position of 8982 eV. This means that Cu is predominantly in the +1 state, as widely reported in the literature [14], with a minor contribution of Cu2+ , the true glass chromophorous agent. XANES data can be used to quantify the Cu1+ /Cu2+ ratio, as the spectra of the pure phases (Fig. 1, BS4 and SLG curves in bold) are markedly different in this region. Cu1+ (BS4) presents a resonance at 8993.0(5) eV which is completely absent in the Cu2+ sample (SLG). Therefore, using as models the spectra of the sample, in which copper is present exclusively as Cu1+ (sample BS4 was chosen from the results of the EXAFS analysis) and of a silica-lead glass containing only Cu2+ ions (SLG), the XANES data of other samples can be reproduced as linear combinations of the models. The fits are of very good quality, as shown in the inset of Fig. 1, and the quantitative data (Table 2) reveal a mixture of Cu1+ and Cu2+ . In the case of sample
Fig. 1. Normalised XANES spectra of blue (TU4 -turquoise; AQ5 -aquamarine; CE4 pale blue; AZ6 -azure; BS4 -dark blue; B4 -blue; and BO4 -opaque blue) and green samples (VG1 -yellow-green; VP1 -green; and VS4 -dark green) together with model for Cu2+ in glass SLG. Spectra normalised by setting at one the atomic absorption far from the edge, in order to allow comparisons between them, irrespective of Cu concentration in various samples. Inset: typical fit of edge of samples as linear combination of model compounds for Cu1+ (BS4) and Cu2+ (SLG), shown in bold line in main plot.
Table 1 List of analysed samples, indicating colour type, chromatic group, and label. Also shown: colorimetric coordinates (L*a*b*), CuO, CoO and PbO contents for each sample. Colour type
Chromatic group
Sample
Colour
CuO (wt%)
CoO (wt%)
Blue
Turquoise
TU4
1.13 ± 0.03
< 0.03
0.45 ± 0.08
Pale blue
CE4
0.72 ± 0.02
< 0.03
0.22 ± 0.08
Aquamarine
AQ5
0.99 ± 0.04
< 0.03
0.53 ± 0.04
Blue
B4
0.17 ± 0.02
0.05 ± 0.02
0.34 ± 0.10
Opaque blue
BO4
0.12 ± 0.01
0.07 ± 0.01
0.52 ± 0.16
Dark blue
BS4
0.09 ± 0.02
0.12 ± 0.02
0.39 ± 0.08
Azure
AZ6
0.05 ± 0.01
< 0.03
Dark green
VS4
1.62 ± 0.38
< 0.03
1.37 ± 0.22
Yellow-green
VG1
0.80 ± 0.07
< 0.03
1.67 ± 0.29
Green
VP1
0.68 ± 0.03
< 0.03
9.39 ± 0.79
Red
Red
RO4
1.04 ± 0.31
< 0.03
0.41 ± 0.11
Brown
Brown
M4
1.62 ± 0.61
0.06 ± 0.02
6.92 ± 0.16
Orange
Orange
AV4
L* = 60.8 a* = –32.5 b* = –23.3 L* = 71.1 a* = –15 b* = –1 L* = 41.7 a* = –17.7 b* = –9.2 L* = 28.4 a* = 0.6 b* = –14.8 L* = 31.8 a* = 6.4 b* = –16 L* = 8.6 a* = 3.1 b* = –16 L* = 79.9 a* = –7.1 b* = –7 L* =37.4 a* = –15.4 b* = 2 L* =67.2 a* = –21.6 b* = 33.7 L* = 55.4 a* = –21.8 b* = 18.9 L* = 41.4 a* = 24.8 b* = 18.4 L* = 34.1 a* = 16.4 b* = 10.7 L* = 81.1 a* = 51.3 b* = 66.5
7.69 ± 0.28
< 0.03
Green
PbO (wt%)
< 0.08
27.66 ± 0.23
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Fig. 2. EXAFS spectra (a) and Fourier Transforms (b) of EXAFS data for samples AZ6 (azure), BS4 (dark blue), B4 (blue), and BO4 (opaque blue), characterised by CuO contents lower than 0.5 wt%. Transformations performed in interval k = [3.00–10.00] A˚ −1 with k2 weight and Hanning window.
Fig. 3. EXAFS spectra (a) and Fourier Transforms (b) of EXAFS data for samples TU4 (turquoise), AQ5 (aquamarine), CE4 (pale blue), VG1 (yellow-green), VP1 (green) and VS4 (dark green), characterised by CuO contents higher than 0.5 wt%. Transformations performed in interval k = [3.00–10.00] A˚ −1 with k2 weight and Hanning window.
AZ6, which has very little total copper (CuO = 0.05 ± 0.01 wt% – Table 1) and as this element is mainly present as Cu1+ ion (74 ± 5%), not as a chromophorous ion, we can confirm that the true chromophorous element is cobalt, the presence of which was confirmed by colorimetric analysis, where the typical Co optical absorptions at 530, 590 and 645 nm [33] were identified. EXAFS data were fitted by building a model which exploits the co-presence of the two sites (corresponding to the two oxidation states) for Cu evidenced by XANES. One is relative to Cu1+ which ˚ and the other is relis 2-coordinated with oxygen ions at ∼1.85 A, ˚ Free ative to Cu2+ , 4-coordinated with O neighbours at ∼1.95 A. parameters were the fraction of Cu1+ and the bond distances, as
Table 2 Quantitative analysis of XAS data for blue and green samples. Column 1: percentages of copper oxidation states, estimated from XANES analysis. Columns 2, 3, 4 and 5: analysis of EXAFS spectra, reproduced as a linear combination with contributions from Cu1+ and Cu2+ . Debye-Waller factor is fitted on sample BS4 and fixed in the others. Errors on last figure are shown in brackets. Sample
Cu1+ XANES
Cu1+ EXAFS
TU4 AQ5 CE4 B4 BO4 BS4 AZ6 VS4 VG1 VP1
46(5) 44(5) 74(5) 95(5) 95(5) 95(5) 74(5) 60(5) 63(5) 66(5)
56(5) 59(4) 90(10) 90(10) 90(10) 80(10) 98(1) 55(6) 70(10) 65(6)
R1 (Å) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2) 1.85(2)
2 (Å2 ) 0.005 0.005 0.005 0.005 0.005 0.005(2) 0.005 0.005 0.005 0.005
R2 (Å) 1.94(3) 1.94(3) – – – – – 1.93(3) 1.97(3) 1.94(3)
well as a global edge position E0 . In order to reduce the number of free parameters, a single Debye-Waller factor 2 was used for these two shells. Actually, the site of Cu2+ should be octahedral rather than square planar, i.e., it should contain two more oxygen atoms (axial). However, it is well known that Cu2+ –O complexes undergo a Jahn-Teller tetrahedral distortion which takes the axial O considerably farther than the planar ones (for an example in water solution, see [34]). In the present case, attempts to include them in the fit were unsuccessful and the final model retained only 4 O. The spectra are shown in Figs. 2(a)–3(a) and the related Fourier Transforms (FT) of EXAFS data in Figs. 2(b)–3(b). The two contributions overlap in the main single peak visible in the FT. The results of quantitative fits are listed in Table 2. In all samples, Cu is coordinated with O indicating that Cu (in both valence states) is present as ions dispersed in the glassy matrix, as no further peaks for crystalline phases are visible. The model described above allows direct quantification of the Cu1+ /Cu2+ ratio and this value can be compared with that derived from XANES. It is worth noting that samples with higher Cu content (TU4, AQ5, VS4, VP1, VG1) exhibit a detectable fraction of Cu2+ , whereas in those with lower Cu, almost only the Cu1+ state is generally present. As regards hues, in the case of blue samples with low copper (B4, BO4, BS4, AZ6), in which non-chromophorous Cu1+ is mostly present, colour is presumably due to the presence of cobalt (Table 1), an element which has an extremely high colouring capacity, even at very low concentrations, and confers a deep blue colour on glass when it adopts tetrahedral coordination [2]. In the case of other samples with higher copper contents and no cobalt at detectable levels (TU4, CE4, AQ5, VS4, VG1, VP1 – Table 1),
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Fig. 4. XRPD profile of sample VG1. Note presence of peaks at 2 = 29.65, 2 = 34.31◦ , 2 = 49.21 and 2=58.36◦ , due to most intense reflections of structural planes (222), (400), (440) and (622) of lead antimonate, respectively. Same phase also identified in other green tesserae.
the colour is due to the presence of Cu2+ chromophorous ions. In addition, the quantification of Cu1+ and Cu2+ in these samples demonstrates that, taking into account the total content of copper, the colour intensity of the tesserae is directly correlated to the Cu2+ content of the glass, because the more intensely coloured samples show the highest amount of Cu2+ ions. In particular, in blue samples, coloured by copper (TU4, AQ5, CE4), Cu2+ varies from 26% to 56% of total copper, and its lowest contents appear in the less intensely coloured samples, e.g., CE4, pale blue (Table 2). The same happens for green tesserae (VS4, VG1, VP1), where the tesserae with darker shades of green have higher contents of Cu2+ ions dispersed in the glass matrix, e.g., sample VS4, dark green in colour. In any case, it should be stressed here that the different green hues of the Paduan tesserae are due to the physical interaction of the blue due to Cu2+ ions, identified by XAS, and the yellow due to Pb antimonates used as opacifiers, as identified by SEM-EDS and XRPD analyses in variable ratios (Fig. 4). 4.2. Orange, red and brown tesserae The XANES spectra of samples AV4 (orange), RO4 (red) and M4 (brown) are shown in Fig. 5, in which the edge positions of the spectra are marked by dots. Sample AV4 has an edge at around 8982.0(5) eV, whereas samples M4 and RO4 are at 8979.0(5) eV. Fig. 5 also shows the reference spectra of metallic copper (Cu Met) and cuprite (Cu2 O), with edges at 8979.0(5) eV and 8981.0(5) eV, respectively.
As the edge position reflects the oxidation state of the ion, we conclude that the Cu ions are predominantly in the metallic state in samples RO4 and M4, whereas the signal in sample AV4 exhibits features very similar to those of cuprite. The EXAFS data and related FT of samples RO4, M4 and AV4 are shown in Figs. 6(a) and (b) respectively, together with the data for reference compounds, metallic Cu and Cu2 O. In both cases, samples RO4 and M4 strongly resemble metallic Cu (oscillations with the same frequency and phase, giving rise to the same pattern in the spectrum), with the exception of the slightly lower intensity of the oscillations and a small shoulder at about 1.5 A˚ in the FT. This shoulder is due to the presence of an amorphous oxidised phase, as confirmed by quantitative analysis. Sample AV4 has an EXAFS spectrum similar (again in terms of frequency and phase of oscillations) to that of Cu2 O and the same peaks in the FT, although the peak at about 2.8 A˚ (due to Cu-Cu coordination) is smaller than that of the crystal. This suggests an excess of Cu-O bonds with respect to Cu-Cu, indicating a mixture of Cu1+ ions dispersed in the glass together with Cu2 O particles. Note that Cu is in the +1 valence state in both phases. EXAFS data of samples RO4 and M4 were quantitatively analysed by a two-phase model. One is a single shell Cu1+ -O relative to metal ions dispersed in glass, as already described in the previous section. The other is a metallic phase for Cu and this is described by single- and multiple-scattering paths up to the 5th coordination shell. Copper in the oxide phase is assumed to be 2coordinated, whereas the coordination numbers in the metal phase are given by the crystallographic data: the relative amount between these two phases is left as a free fit parameter and provides the fraction between oxidised and metallic Cu. The metallic phase is described in such a way as to reduce the number of free parameters as much as possible. All bond length values were derived from a single variable lattice parameter a; for the Debye-Waller Factors, a free 2 parameter was considered for the first shell, and all the others were derived from a correlated Debye model [35] as implemented in the ARTEMIS code [27] and described by a single effective Debye Temperature TD . The results are listed in
Table 3 Results of EXAFS analysis for brown (M4) and red (RO4) samples. Structural parameters of reference compound (Cu ref.) from Crystallographic Information File also shown.
Fig. 5. XANES spectra of samples RO4 (red), M4 (brown) and AV4 (orange), compared with those of metallic copper (Cu Met) and cuprite (Cu2 O). Dots: edge positions.
Sample
Metal fraction (%)
Latt. Par (Å)
First shell 2 (Å2 )
T Debye (K)
Cu ref. M4 RO4
100 70(5) 70(5)
3.615 3.61(1) 3.60(1)
– 0.0090(10) 0.0085(7)
315 310(20) 320(15)
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Fig. 6. EXAFS spectra (a) and Fourier Transforms (b) of EXAFS data for samples RO4 (red), M4 (brown) and AV4 (orange), compared with those of metallic copper (Cu Met) and cuprite (Cu2 O). Transformations performed in interval k = [3.00–10.00] A˚ −1 with k2 weight and Hanning window.
Table 3, which clearly shows that 70% of total Cu is in the crystalline metallic phase. The presence of metallic copper in samples RO4 and M4 was also confirmed by XRPD analyses, in which peaks at 2 = 43.3, 2 = 50.44◦ and 2 = 74.14◦ , related to the most intense reflections of structural planes (111), (200) and (220) of metallic copper, respectively, were identified. Sample AV4 has two well-defined peaks in the FT, at about 1.4 A˚ and 2.7 A˚ (Fig. 6(b)). The first peak coincides with the first peak of cuprite and is due to the Cu–O bond of Cu2 O; the second falls in the range of the first Cu–Cu shell of the same cuprite. As already noted, the Cu-O peak is higher than the Cu-Cu peak, as instead found in the pure Cu2 O crystal, because part of the CuO bonds comes from an amorphous oxide that does not possess the second coordination shell. Quantitative analysis, carried out as a linear combination of the amorphous oxide plus the crystalline oxide described by two shells Cu-O and Cu-Cu, provided a crystalline oxide fraction of 39(5)%, a first Cu-O shell (common to the ˚ and a second Cu-Cu amorphous and crystalline phases) at 1.85 A, ˚ as expected for this crystalline phase. The presence shell at 3.02 A, of cuprite in sample AV4 was also confirmed by XRPD analysis, in which peaks at 2 = 29.58, 2 = 36.43◦ , 2 = 42.32◦ , 2 = 61.40◦ , 2 = 73.56◦ and 2 = 77.41◦ , related to the most intense reflections of structural planes (110), (111), (200), (220), (311) and (222) of cuprite, respectively, were identified. These results were also confirmed independently by the combined linear fitting of XANES data, with the spectra of samples BS4 and metallic copper as references. The presence of Cu1+ ions confirms the highly reducing conditions of the kiln during the glass
production process, which stimulated the precipitation of metallic copper in the case of samples RO4 and M4. Taking into account the equal content of metallic copper in both tesserae (Table 3), the different colour may be related to the different crystal sizes, metallic copper particles being smaller than 150 nm in size in sample RO4 (red) and larger than 150 nm in sample M4 (brown) (Fig. 7). The literature has clearly established that the grain-size of inclusions, particularly in red glasses, influences the final colour hue, which darkens with increasing crystal size ([36], and references therein). In addition to those already reported in the literature on red and orange tesserae and obtained by means of conventional analytical techniques (e.g., [36–39] and references therein), these results are also similar to those already reported for lustreware [5,6,8,11] and for Roman [9], late Roman [13], old Japanese [3] and Portuguese [10] glasses. They also confirm the hypothesis that the red glass with low copper and low lead (e.g., samples RO4 and M4 – Table 1), is generally coloured by metallic copper nano-particles, which are exsolved due to melt reduction. This may have been induced by heat treatment at around 1000 ◦ C and/or the addition of reducing agents such as carbon or metals like tin, antimony and iron in proper oxidation states [40,41]. In sample RO4, the role of reducing agents was almost certainly played by iron and antimony (FeO and Sb2 O3 , with values of 2.4 ± 0.5 and 0.4 ± 0.4 wt%, respectively; EMPA data) and in sample M4 by both iron and tin (FeO and SnO2 , at 2.4 ± 0.2 and 1.8 ± 0.1 wt%, respectively; EMPA data). Instead, the formation of cuprite requires high copper and lead contents in the glass, as in the case of sample AV4 (Table 1): with too little copper, cuprite
Fig. 7. SEM-BSE images of tesserae RO4 (a) and M4 (b). Note different sizes of inclusions, composed of metallic copper, greater in sample M4.
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would be unlikely to form; with too little lead, it would not form the dendritic structures which produce an orange/red hue [41]. It should be stressed here, as already observed by Fiori [21] for coeval red tesserae from Ravenna, that the Paduan red/brown tesserae, coloured by means of metallic copper, are also characterised by different lead contents (Table 1) confirming that the glass batch does not necessarily need lead to stimulate the precipitation of metallic copper, as reported in Barber et al. [41]. According to experimental works (e.g., [42]), the role of lead is that of improving the rates of nucleation and growth of inclusions. This is also confirmed in the Paduan tesserae, where sample M4, which shows higher lead content than sample RO4 (Table 1), is characterised by the greater size of metallic copper crystals (Fig. 7). 5. Conclusions The present study yielded better knowledge of colour generation in palaeo-Christian glass mosaic tesserae and confirmed the complexity of this topic, which requires a combined approach, involving integration of data, obtained by various analytical techniques, also non-conventional, to be solved. The integration of XAS and EMPA data coupled with XRPD, SEM and colorimetric analyses, as carried out in this study, showed that variations in colour may be the result of deliberate technical operations — in particular, redox conditions in kilns and the composition of glass batches — which were applied with the aim of obtaining various colours. In detail, copper imparts different colours and hues depending on its state of oxidation and quantification. In the case of the blue tesserae, the colour is due both to Cu2+ ions dispersed in the glassy matrix (samples AQ5, TU4, CE4) and to other chromophorous ions, such as cobalt (samples B4, BS4, BO4, AZ6), because copper is mainly present as Cu1+ ions, not chromophorous. In green tesserae, copper is mainly present as Cu2+ ions and their colour is due to physical interactions between the blue, due to Cu2+ ions, and the yellow, due to Pb antimonates used as opacifiers. In addition, quantification of Cu1+ and Cu2+ ions in the analysed samples demonstrated that, when Cu2+ is the main chromophorous ion, colour intensity is directly correlated to its content in the glass. In the case of the red and brown tesserae, colour is mainly due to the presence of metallic copper, and to cuprite in the orange tesserae. In this context, the hypothesis of Barber et al. [41] was also confirmed: in addition to the redox conditions in kilns, these authors relate the precipitation of the above phases to differing amounts of copper and lead in the samples. It should be stressed here that, considering the whole range of colours of the Paduan tesserae, the copper and lead contents are not related, as already observed by Fiori [21] for coeval tesserae from Ravenna. In fact, in the blue tesserae, mainly coloured by copper, lead is not present at detectable levels; in the green ones, lead measured in the glassy matrix is probably due to partial dissolution of lead antimonates used as opacifiers; and in orange, red and brown ones, the addition of varying amounts of lead, in addition to the redox conditions of kilns, is functional to the precipitation and growth of cuprite and/or metallic copper, which act as opacifiers and pigments. Lastly, the similarity of the present results with those already reported in the literature indicates routine glass production processes for the same set of colours, notwithstanding their different age, provenance and type. Acknowledgements The authors thank the “Soprintendenza per il patrimonio storicoartistico ed etnoantropologico per le province di Belluno, Padova, Rovigo e Treviso”, and in particular Dr. A. Spiazzi, for authorising the
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present study and the Abbot of the Basilica of St. Justine for providing glass samples. They are also grateful to F. Gallo (Department of Geosciences, University of Padova, Italy), R. Carampin (CNRIGG, Padova, Italy), F. Zorzi (Department of Geosciences, University of Padova, Italy), P. Guerriero (CNR–ICIS, Padova, Italy) and G. M. Cortelazzo, F. Ratti and A. Paviotti (Department of Information Engineering, University of Padova, Italy) for their kind collaboration in execution of XAS, EMPA, XRPD, SEM and colorimetric analysis, respectively, and Fila Industria Chimica S.p.a. of Padova, for use of the FEI Quanta 200 FEG ESEM instrument. This work was carried out with the financial support of Progetto di Ricerca di Ateneo, University of Padova (Project No. CPDA088419).
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