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Spectrochemical and structural studies on a roman sample of Egyptian blue
Pergamon
Spectrochimica Acta, Vol. 51A, No. 3, pp. 437-446, 1995 Copyright(~ 1995 ElscvierScicncc Ltd Printed in Great Britain. All rights reserved 0584-8539(94)E0108-M 0584-8539/95 $9.50+ 0.00
Spectrochemical and structural studies on a Roman sample of Egyptian blue P. MIRTIt Dipartimento di Chimica analitica, Universit~ di Torino, via Giuria 5, 1-10125 Torino, Italy
L. APPOLONIA Soprintendenza ai Beni culturali e ambientali, piazza Narbonne 3, I-11100 Aosta, Italy
A. CASOLI Dipartimento di Chimica generale e inorganica, chimica analitica, chimica fisica, Universith di Parma, viale delle Scienze 78, 1-43100 Parma, Italy
R. P. FERRARI and E. LAURENTI Dipartimento di Chimica inorganica, chimica fisica e chimica dei materiali, Universith di Torino, via Giuria 7, 1-10125 Torino, Italy and
A. AMISANO CANESI and G. CHIARI Dipartimento di Scienze mineralogiche e petrologiche, Universith di Torino, via Valperga Caluso 37, 1-10125 Torino, Italy (Received 1 February 1994; in final form 25 March 1994; accepted 28 March 1994)
Abstraet--A ball of Egyptian blue excavated on the archaeological site of Augusta Praetoria (Aosta, Italy) has been investigated by several techniques. Optical microscopy and X-ray diffraction gave proof of the identity of the pigment, while indicating that silica phases had also been formed because of sand excess in the reaction mixture. No evidence was found of the presence of tin compounds, thus excluding the use of bronze scraps in the preparation of the pigment. The crystal structure of the pigment was refined using single-crystal X-ray diffraction data; Fourier transform infrared spectroscopy, UV-vis-NIR reflectance spectroscopy and electron paramagnetic resonance spectroscopy were used to characterize further the pigment. Scanning electron microscopy, coupled with energy dispersive detection of emitted X-rays, was finally used to investigate morphology and determine the composition of representative areas of both pigment and impurities.
INTRODUCTION EGYPTIAN b l u e was t h e first s y n t h e t i c p i g m e n t e v e r p r o d u c e d by m a n . It a p p e a r e d in E g y p t d u r i n g t h e f o u r t h d y n a s t y in t h e 3rd m i l l e n n i u m a c a n d a d e v e l o p m e n t at a b o u t the s a m e t i m e , o r e v e n e a r l i e r , m i g h t h a v e o c c u r r e d in M e s o p o t a m i a [1]. It was u s e d as a highly p r i z e d b l u e p i g m e n t a n d to p r o d u c e small o b j e c t s ( b e a d s , s t a t u e t t e s , s c a r a b s , inlays) in E g y p t a n d t h e N e a r E a s t a l m o s t c o n t i n u o u s l y t h r o u g h o u t the w h o l e p e r i o d f r o m its first a p p e a r a n c e s d o w n to R o m a n times. F r o m E g y p t a n d the N e a r E a s t the use o f the p i g m e n t s p r e a d to M i n o a n C r e t e a n d t h e G r e e k w o r l d , a n d t h e n to the R o m a n w o r l d . D u r i n g t h e R o m a n E m p i r e it still was a highly p r i z e d p i g m e n t , in use f r o m B r i t a i n to N o r t h e r n A f r i c a a n d A s i a M i n o r . F o l l o w i n g the fall o f the R o m a n E m p i r e its use r a p i d l y d i s a p p e a r e d , a p a r t f r o m a few a r e a s , m a i n l y in the B y z a n t i n e w o r l d , w h e r e it is still d o c u m e n t e d for a while. I n d e e d , it is p o s s i b l e t h a t use o f E g y p t i a n blue in p o s t R o m a n t i m e s m a y c o n c e r n a r e u t i l i z a t i o n o f l u m p s p r o d u c e d in R o m a n times. It is a fact t h a t a n y t e c h n o l o g i c a l k n o w l e d g e , on which the p r e p a r a t i o n o f this high q u a l i t y b l u e p i g m e n t r e l i e d , s e e m s to h a v e b e e n lost by t h e e n d o f t h e first m i l l e n n i u m AD, a n d o n l y d i s c o v e r e d again in the d e c a d e s at t h e t u r n o f the p r e s e n t c e n t u r y [2, 3]. t Author to whom correspondence should be addressed. 437
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Egyptian blue (EB hereafter) is a crystalline compound whose composition corresponds to a copper calcium tetrasilicate, CaCuSi4Oi0, which is often referred to as cuprorivaite. Indeed, cuprorivaite is the name given to a very rare mineral of the same composition, which was discovered in 1938 in the lava of Mt Vesuvius, near Naples [4]. The structure of the mineral is still unknown, while the structure of the synthetic compound EB was solved by Pabst [5]. EB was prepared by heating a mixture formed by sand, limestone (or calcareous sand), copper compounds (copper ores or bronze scraps) and flux (natural s o d a - - n a t r o n from lower Egypt, or plant ash) [1, 6]. The preparation was described in the first century ac by Vitruvius (Marcus Vitruvius Polio, De architectura libri decem, VII, 11), who observed it during a visit to a glass factory situated in Pozzuoli, near Naples (EB and glass could be obtained by the same basic raw materials). However, as the factory did probably use calcareous sand of the Volturno river, Vitruvius did not mention calcium carbonate as a separate ingredient for obtaining EB (which he names coeruleum); medieval transcribers, which could not be aware of this particular situation, went on omitting calcium carbonate in copying Vitruvius' text, thus transmitting a recipe, which, when a non-calcareous sand was used, could not lead to the formation of the blue pigment. In its place, a green pigment (green frit) was obtained [1]. It is possible that this contributed to the rapid decline in the use of EB in post-Roman times. The synthesis of EB has been carried out in recent times by many scholars with the purpose of shedding light on the ancient procedure [1, 6-8]. Opinions do not completely agree, but a few points stand firm. It appears that the blue pigment was obtained by mixing together the raw materials with the proper amount of flux and heating to a temperature between 850 and 1000°C. The most suitable temperature, and whether sintering or melting was actually achieved (by controlling the flux quantity) is still controversial. However, the pigment has been shown to decompose when heated beyond 1050°C and if a reducing atmosphere does prevail in the furnace [1, 7]. It has also been proposed that a two-stage process could be a most suitable one to obtain a high quality pigment [1, 6]; the second stage should have consisted in reheating the crystals obtained by the first heating, after having ground them to a fine powder. It appears that an accurate control of the reaction conditions is critical for obtaining a high quality blue pigment. In fact, if the raw ingredients are not mixed in the required stoichiometric ratios, other compounds may form in addition to EB. These may include silica phases (e.g. quartz or tridymite) if silica sand is in excess of the other reagents, wollastonite (calcium silicate, CaSiO3) with an excess of both sand and limestone (or calcareous sand), copper oxides (tenorite, CuO, or cuprite, Cu20) when an excess of copper ingredients is present, and glass phase (coloured blue or b l u e - g r e e n by copper) if silica sand and flux are in excess [8]. One has to note here that the introduction of copper into the reaction mixture was not necessarily performed by using copper ores (malachite, basic copper carbonate, CuCO3. Cu(OH)2, for example), but scraps or filings of metallic copper or copper alloys could be used in its place. This could lead to the formation of cassiterite (SnO2) or malayaite (CaSnSiOs) if, for example, tin-copper alloys (bronze) were used. Indeed, arsenic, tin and lead containing compounds have been recognized in samples of EB collected from Egyptian monuments dating from the 3rd millennium BC down to Roman times; this was related to the progressive development of copper alloying from arsenical copper to bronze and leaded bronze [9]. The present study deals with a ball of EB found in archaeological excavations carried out on the site of Augusta Praetoria (Aosta, Italy). The archaeological context enabled dating of the ball to Roman imperial times. The ball was a friable mass made up of blue, white and b l u e - g r e e n grains. Vitruvius reports that sand, copper compounds and soda were ground to a fine powder, mixed together and wet with water to form little balls, which were dried and heated to produce EB. The find from Aosta matches this report, and it seemed worthwhile to perform a comprehensive spectrochemical and structural characterization of the raw pigment. Various techniques were used for this purpose, including optical microscopy, X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, U V - v i s - N I R reflectance spectroscopy, electron paramagnetic resonance
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( E P R ) s p e c t r o s c o p y , a n d s c a n n i n g e l e c t r o n m i c r o s c o p y ( S E M ) c o u p l e d with e n e r g y d i s p e r s i v e X - r a y d e t e c t i o n for m i c r o p r o b e analysis.
EXPERIMENTAL The ball of pigment was set free from the excavation sand by sonication in water. Repeated treatments allowed complete separation and cleaning of the pigment grains from sand because of density difference. At this stage the raw pigment was no longer in the form of a ball because the various grains (blue, white and blue-green) were set loose by sonication. Most studies were performed on sample aliquots collected after the ultrasonic treatment, without separation of the differently coloured grains; in a few instances, however, a separation of the blue, white and b l u e green grains from each other proved to be useful. This was achieved by careful hand selection under a Wild Leitz M8 zoom stereomicroscope at 50 x magnification. Optical microscopy examination was carried out by a Polivar POL microscope using either reflected or transmitted light; working magnification ranged from 100 x to 200 x with both normal and polarized light. X-ray powder diffraction was performed by a Rigaku Rotaflex rD/max instrument equipped with a copper rotating anode ( 2 = 1.5418 ]~), operated at the following working conditions: accelerator voltage 40 kV, current 30 mA, response time 2 s, scanning time 1 deg min -~, reading step 0.02 deg. X-ray diffraction intensities from a single crystal were collected on a Siemens R3 four-circle diffractometer, graphite monocromatized M o K a radiation (2=0.71073,~), using the to scan technique, 4 ° < 20 < 50 °, - 9 < h < 9, - 9 < k < 9, 0 < l < 18, 2 ° scanning range, background/scan r a t i o = 0 . 5 , scan speed variable from 2 to 20 ° min -l, 2 standard reflections monitored every 50 measurements. Fourier transform infrared spectra were recorded by a Bruker FS25 spectrometer, equipped with a CsI window. Spectra were recorded in the wavelength range 4000-250 c m - l (2500-40,000 nm) with a resolution of 4 c m - i. Samples were prepared as KBr pellets, and 3 mm micropellets were used in some cases. U V - v i s - N I R diffuse reflectance spectra were recorded in the 250-2500 nm wavelength region (40,000-4000cm -l) by a Varian Cary 5 U V - v i s - N I R spectrophotometer, equipped with a polytetrafluoroethylene (PTFE) coated integrating sphere and performing reflectance measurements against a PTFE coated reference disk. Electron paramagnetic resonance spectra were recorded by a Varian El09 X-band spectrometer equipped with a TE,02 double cavity, an Oxford Instruments ESR900A continuous flow cooling system (range 3.8-300 K) and a Stelar A Q M Auriga/XT data system. A MgO matrix, containing Mn(II) ion in the form of a very low common impurity and addition of a known quantity of DPPH (a,a-diphenyl-fl-picrylhydrazyl) powder, was used as a double standard situated in the reference cavity. The well-known DPPH powder g factor (2.0037+0.0002) and manganese field-scan calibration (g = 2.0012 _+0.0002 and A = 178.21[ x 10 -4 c m - ~) [10] made possible the correct calculation of the g values to the fourth decimal place. Spectra were recorded in the temperature range 77-298 K using the following experimental parameters: magnetic field scan range = 4000 G, microwave power = 2-20 m W and modulation amplitude = 1-4 G. SEM examination was performed by a Jeol JSM 6400 scanning electron microscope; samples were glued on stubs and coated with carbon using a Jeoi JEE-4X vacuum evaporator. Images were obtained by either secondary electrons (acceleration voltage 15 kV, working distance 15 mm, spot 1 2 - - C L coarse, objective aperture 3) or backscattered electrons (acceleration voltage 15 kV, working distance 15 mm, spot 9 - - C L course, objective aperture 2). Microprobe analyses were performed by a Tracor energy dispersive system microprobe (acceleration voltage 15 kV, current 0.62 mA, working distance 39 mm, spot 1 0 - - C L coarse, objective aperture 2), using the SQ program for quantitative determination.
RESULTS AND DISCUSSION
Optical microscopy A p a r t f r o m t h e a r c h a e o l o g i c a l e v i d e n c e , o p t i c a l m i c r o s c o p y was u s e d for the identific a t i o n o f the b l u e g r a i n s as crystals o f E B . T h e s e a r e b l u e c o l o u r e d crystals s h o w i n g p l e o c h r o i s m a n d b i r e f r i n g e n c e [11]; c h e m i c a l m i c r o t e s t s easily a v o i d c o n f u s i o n with
P. MIRTI et
440 5k - -
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! i
i
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Fig. 1. Powder diffractograms recorded on ground aliquots of blue (above) and white grains (below), respectively, separated from the archaeological sample of Egyptian blue.
azzurrite (basic copper carbonate, 2 C u C O 3" C u ( O H ) 2 ) , which dissolves by acid attack, while EB does not [12].
X-ray powder diffraction Further and definite identification of the blue grains as EB was obtained by X-ray powder diffraction. Figure 1 shows the diffractograms recorded on aliquots of blue and white grains, respectively, collected by hand separation under the stereomicroscope and ground to fine powder in an agate mortar. It easily allows one to identify the blue grains as crystals of EB, as all the cuprorivaite peaks are recognized, apart from that corresonding to d = 3.66 (crystal plane 200); however, relative peak intensities do not match the cuprorivaite JCPDS card (card 12-512) [13] because of preferred orientations arising from sample preparation. On the other hand, the diffractogram recorded on the white grains, apart from the occurrence of peaks due to residual EB, allows one to identify these grains as quartz crystals; no evidence is gained here for the presence of woilastonite, thus suggesting that limestone was not in excess of the other reagents. Moreover, evidence for the presence of cassiterite or malayaite is not found either. This would exclude the use of bronze scraps for introducing copper into the reaction mixture. Thus either a copper ore or copper filings should have been used for this purpose.
Refinement of crystal structure A finely shaped transparent blue crystal (dimension 0.12×0.1 ×0.05 mm 3) was selected and mounted on a Siemens R3 four-circle diffractometer. The cell parameters obtained by a least squares refinement on 52 reflections measured in the range 1 1 ° < 2 0 < 3 4 ° are a0=7.309+0.001 and c 0 = 1 5 . 1 0 7 + 0 . 0 0 5 , ~ , in agreement with the values reported by Pabst [5] which are a, = 7.30 +_0.01 and co = 15.12 + 0.02 A. The analysis of systematic absences confirmed the space group to be P4/ncc. The 2924 reflections measured in half Ewald sphere, in order to obtain better statistics, were corrected for Lorentz polarization effects and the absorption correction was based on the ~p scan method [14]. The number of unique reflections was 362, of which 315 with I > 4 o ( 1 ) were considered observed and used in the following calculations, carried out using the S H E L X T L system [15], The structure was solved anew by direct methods which allowed the positioning of all atoms. The origin was taken at the symmetry centre, with a shift of (1/4x, - 1/4y) with respect to the choice of Pabst [5], The refinement was carried out by full-matrix least-squares using atomic scattering factors for neutral atoms
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Table 1. Fractional atomic coordinates and equivalent U values (/~2) for a crystal of Egyptian blue (e.s.d. in parentheses) Atom Ca Cu Si O(1) 0(2) 0(3)
X/A 1/4 1/4 0.0068(3) 0.9618(4) 0.7082(3) 0.3535(3)
Y/B 3/4 1/4 -.0732(1) 0.0382(4) 0.0016(4) 0.0073(3)
Z/C 0 0.08173(5) 0.14732(6) 1/4 0.1263(2) 0.0819(2)
U~q 0.0068(3) 0.0054(2) 0.0050(3) 0.0187(9) 0.0110(8) 0.0094(7)
provided by S H E L X T L , unit weights and anisotropic thermal parameters for all atoms. The final R value was 1.97% for 38 parameters, goodness of fit = 1.5. The maximum peak in the final difference Fourier was 0.3 e ,~-3. The present refinement confirms the crystal structure proposed by Pabst [5], but shows e.s.d, smaller by one order of magnitude. Table 1 gives the fractional atomic coordinates and the equivalent U, and Table 2 the relevant distances and angles. In Fig. 2, a drawing of two unit cells projected on (010) shows the sheets orthogonal to z and composed of units (Cu25i8020) -4. The Cu atoms are in square planar co-ordination. These sheets are connected by interlayer Ca atoms in distorted antiprismatic coordination. One can see from Table 2 that the two independent C a - O distances are very different from one another. This can be explained in terms of charge balance. In fact 0 ( 2 ) is bonded to two Si and one Ca atom and its bond valence sum, according to Pauling [16] is 2.25 v.u., while 0 ( 3 ) is bonded to one Si, one Cu and one Ca and its Pauling bond valence sum is 1.75 v.u. The distances of 0 ( 3 ) from all the cations tend therefore to be shorter than usual in order to compensate for the lack of charge. The opposite is true for the oversaturated 0 ( 2 ) . The sums of the bond strengths, calculated on the basis of bond length [17] on 0 ( 2 ) is 2.13v.u. and on 0 ( 3 ) is 1.95v.u. The fact that the structure refinement proceeds in an excellent way can be ascribed to the good quality of the crystal synthesized in Roman times.
FTIR spectroscopy Fourier transform infrared spectra were recorded on mixed grains, as well as on blue, white and b l u e - g r e e n grains, separately. The obtained spectra, reported in Fig. 3, were compared with those of standards of quartz and fragments of archaeological glass, containing and not containing copper ions. Figure 3(a) reports the spectrum recorded on the blue grains separated from the archaeological sample of EB; this spectrum shows band shifts with respect to both quartz and archaeological glass in the regions of symmetrical and antisymmetrical S i - O - S i stretching, as well as in the O - S i - O bending region. In more detail, antisymmetrical S i - O - S i stretching bands are observed for EB at 1230, 1160, 1056 and 1008 cm-1, while symmetrical S i - O - S i stretching bands are centred at 800, 755,664 and 595 cm-I; the bands centred at 1230, 1008 and 595 cm-I appear to be peculiar to EB, as they are not Table 2. Atomic distances (,~,) and bond angles (deg) for a crystal of Egyptian blue (e.s.d. in parentheses) Ca-O(2) Ca-O(3) O(2)-Ca-O(3) O(3)-Ca-O(3) O(2)-Ca-O(3) O(2)-Ca-O(3) Cu-O(3) O(3)-Cu-O(3) SA(A) 51:3-J
2.652(3) 2.375(3) 78.2(1) 105.8(1) 59.9(1) 75.6(1) 1.928(3) 90
Si-O(1) Si-O(2) Si-O(3) O(l)-Si-O(2) O(l)-Si-O(2) O(2)-Si-O(2) O(1)-Si-O(3) O(2)-Si-O(3) O(2)-Si-O(3)
1.601(1) 1.629(3) 1.585(3) 110.0(1) 107.9(1) 108.3(1) 114.6(1) 103.3(1) 112.9(1)
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Fig. 2. Projection of the structure of Egyptian blue (cuprorivaite) on the (010) plane, showing the arrangement of the SiO4 tetrahedra. Smaller spheres represent copper atoms, larger ones calcium atoms.
matched in the quartz and glass spectra. As for the O - S i - O bending region, bands are observed for EB at 521 and 484 cm-1. F T I R spectra recorded on white and b l u e - g r e e n grains are shown in Figs 3(b) and (c); in these cases 3 mm micropellets were used, due to the reduced amount of sample available. The spectrum of the white transparent grains proved to match completely the quartz spectrum, as expected on the basis of the previous information from X-ray powder diffraction; b l u e - g r e e n grains, on the contrary, which were much less abundant in the sample and were silent to X-ray diffraction, gave a spectrum matching those of the archaeological glass fragments.
=~ 75
.3 E
50
(a) ~
25 i 2000 1900
I
1
I
r
1800 1700 1600 1500
T
I
I
1400 1300 1200 1100 1000
900
800
700
600
500
400
Wave number (cm-1) Fig. 3. FTIR spectra recorded on aliquots of blue (spectrum a), white (b) and blue-green (c) grains separated from the archaeological sample of Egyptian blue. The spectra are not on the same transmittance scale: the given scale refers only to spectrum a.
Spectrochemical and structural studies on a R o m a n sample of Egyptian blue
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100
r.-. 50
1600
Nanometres
20'00
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Fig. 4. Reflectance spectrum of an unground aliquot of the archaeological sample of Egyptian blue, recorded in the UV-vis-NIR spectral region.
Reflectance spectroscopy U V - v i s - N I R reflectance spectra and colour measurements were performed on both unground and ground aliquots of the raw pigment, without separation of the blue grains from the archaeological specimen, due to the non-negligible amount of sample required. Thus allowance must be made for the presence of the white grains within the sample, which enhance colour luminosity and diminish saturation. Colour coordinates were obtained in the CIEL*a*b* colour space as follows: L* = 41.98, a* = 14.82, b* = - 38.16 for the unground sample, and L * = 5 9 . 6 9 , a*--0.15, b * = - 2 8 . 3 3 for the ground specimen, respectively; these account for the less saturated colour of the ground pigment. The reflectance spectrum, recorded in the 250-2500 nm wavelength range on the unground sample, is reported in Fig. 4 (a similar spectrum, shifted towards higher reflectance values, was recorded on the ground sample). This shows two major absorption bands at about 560 and 628 nm (some 17,850 and 15,900 cm- t, respectively), and a minor one centred at about 790 (some 12,650 cm-I; automatic change of detector may somehow bias an accurate determination of this absorption maximum). These d-d bands are due to optical absorption by copper(II) ions in the Big ground state, either in EB or in the glass phase. It is evident from Fig. 4 that the intense colour of the blue pigment is principally due to the absorption bands at 560 and 628 nm, which can be assigned to the transitions dx2_y2~...-dxv (Blg~---B2g) and dx2_.v,-~-dxz,yz (Blg'~-Eg), respectively. This is consistent with an effective D4h square planar geometry of coordination of the copper(I1) ion in the EB crystals. In contrast, absorption bands centred at about 800 nm are commonly assigned to the transition dx,-_y,_*--d~2(B ig*--A~) and may be associated with the presence of copper(II) ions, both in solution and in the solid state, in a distorted (tetragonal elongated) octahedral coordinaton [18]. This might be related, in the present case, to the occurrence of glass phase within the raw pigment.
EPR spectroscopy E P R spectra were recorded on both unground and ground aliquots of the archaeological sample, as well as on a ground aliquot homogeneously diluted in a diamagnetic matrix: these spectra were strictly similar to each other in the temperature range considered. Figure 5 shows the E P R powder spectral pattern of the undiluted ground pigment recorded at 77 K. This consists of two absorption lines in the perpendicular and parallel regions of the spectrum, respectively, in agreement with the occurrence of a unique copper(II) paramagnetic species (S = 1/2), with the unpaired electron in the Big ground state in an apparent ligand field axial geometry D4h. The obtained values &-- 2.3220 and g , = 2.0744 (g~oc,~c= 2.1570), with the trend glt > g . > 2, can be in accord with an oxygen
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(a)
J 30b0
35'00
H (gauss) Fig. 5. First derivative EPR powder spectrum recorded at 77 K on a ground aliquot of the archaeological sample of Egyptian blue. Microwave power 10 mW, modulation amplitude 1 G, time constant 0.128 s. (a) DPPH and *Mn(II) in MgO matrix patterns.
atom ligand field in either a tetragonal elongated octahedral or in a square planar geometry. Theoretical g values (gtl = 2.37; g i = 2.10; g~so= 2.19) have been calculated introducing the transition energies, as obtained by reflectance spectroscopy (AE~dxv-dx2 v2)= 17,850 cm -~ and AE~dx~.~.~-dx2v2)= 15,900 cm-l), and the Cu(II) free ion spin-0rbit Coupling 2 = - 829 cm- 1 in the well-known empirical formulas: gll = 2.0023 g i = 2.0023 -
8)~/AE~d~v-ax2v2) 22/AE~a~:.~_<2 ~2)
holding for a system with S = 1/2 and 2 ~ AE. These theoretical values are higher than the experimental ones, due to the introduction of the free ion spin-orbit coupling in the g empirical equation, while in general some metal-ligand covalent bonding degree occurs in complexes, and 2 becomes smaller. Indeed, using a literature value (2 = - 710 cm -1) obtained for some copper(II) complexes [19], theoretical g values became g11=2.32, g~ =2.09 and g~so=2.17, in better agreement with our experimental values. Most common copper(II) complexes are characterized by an octahedral tetragonal distorted geometry and show, in general, higher experimental g values (g11~2.4, g ~ 2 . 1 ) in agreement with lower transition energies, due to the presence of strong axial ligands [20]. Thus E P R spectroscopy (in agreement with the complementary electronic reflectance spectroscopy) confirms the presence of a copper(II) site surrounded by four oxygen ligand atoms in a square planar geometry, as put into evidence by the X-ray structure. Such a structure has been observed for only a limited number of Cu(II) complexes, in diluted solution, as powders [20] or as oriented monocrystals [21]. The occurrence of an octahedral distorted geometry of Cu(II) complexes in the glass phase (blue-green grains) is not revealed in the E P R spectrum, due to the likely superimposition of the strongly predominant EB spectral pattern. No evidence of copper hyperfine electron-nuclear interaction ( l c , = 3 / 2 ) has been gained in the working temperature range, due to the line broadening resulting from electric crystalline field effects, to the electrostatic interactions present in the solid state phase, and to the spin rapid relaxation through the spin-orbit interactions typical of the paramagnetic transition metal complexes [22].
Scanning electron microscopy EB crystals and unreacted silica, together with minor amounts of glass phase, were observed in SEM images. The length of the EB crystals, which aggregate to form definite
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Fig. 6. SEM image (secondary electrons) showing an aggregate of cuprorivaite crystals in the archaeological sample of Egyptian blue. clusters, roughly ranges from 10 to 100/~m. Figure 6 shows a typical aggregate of EB crystals within the archaeological sample. Point analyses performed on single crystals gave the following average percent composition: SiO2 65.6 + 0.9, CaO 19.7 + 0.6, CuO 14.5 + 0.4; this has to be compared with the theoretical stoichiometric composition of EB (SIO2 63.9, CaO 21.2, CuO 14.9). Figure 7 shows a back-scattered electron image, which clearly allows for discrimination among the phases present on the basis of mean atomic number contrast: EB crystals,
Fig. 7. SEM image (backscattered electrons) showing silica lumps and glass phase amidst cuprorivaite crystals.
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glass phase and silica are viewed as increasingly dark areas (silica lumps are also clearly visible in Fig. 6 amidst the EB crystals). Point analyses performed on silica lumps gave a SiO2 content ranging between 92 and 99%, while areas of glass phase gave SiO2 contents in the range 71-80%, Na20 6 - 1 1 % , CaO 10-11%, CuO 1-2%, AI203 1%. The latter approaches the composition of a silica-soda-lime glass, apart from a rather low Na20 content. The formation of areas of glass phase depends on local excesses of flux which cause local melts to occur, and a glass to form; this is coloured blue-green by copper. The presence of sodium and the negligible amount of potassium recognized in the glass composition suggest that natron and not plant ash was used as a flux in the preparation of the pigment.
CONCLUSIONS
The present paper has given an account on the application of several spectrochemical techniques to the study of a sample of EB excavated on an archaeological site. It has been shown that the blue crystals present in the sample have the composition of cuprorivaite, and that silica sand was most probably the only reagent present in excess of the others in the reaction mixture. Areas of glass phase are not widespread, indicating a good control of temperature and flux quantity; natural soda rather than plant ash seems to have been used as the fluxing agent. No evidence has been gained for the presence of tin compounds in such a quantity to suggest the use of bronze scraps for introducing copper into the system; instead, copper ores or filings should have been used. Evidence for the presence of calcium silicates was neither found, which indicates that there was no excess of both lime and silica with respect to copper.
REFERENCES [1] D. Ullrich, Datation--caract~rization des peintures pari~tales et murales (Edited by F. Delamare, T. Hackens and B. Helly), p. 323. Council of Europe, Ravello (1987). [2] F. Fouqu6, Bull. Soc. Fr. Miner. 12, 36 (1889). [3] A. P. Laurie, W. F. P. McLintock and F. D. Miles, Proc. R. Soc. A89,418 (1914). [4] C. Minguzzi, Periodico di Mineralogia 9, 333 (1938). [5] A. Pabst, Acta Cryst. 12, 733 (1959). [6] M. S. Tite, M. Bimson and M. R. Cowell, Archaeological Chemistry (Edited by J. B. Lambert), Vol. 3, p. 215. American Chemical Society, Washington (1984). [7] H. G. Wiedemann and G. Bayer, Anal. Chem. 54, 619A (1982). [8] H. Jaksch W. Seipel, K. L. Weiner and A. El Goresy, Naturwissenschaften 70, 525 (1983). [9] S. Schiegl, K. L. Weiner and A. El Goresy, Erzmetall 43, 265 (1990). [10] A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions. Dover, New York (1986). [11] R. J. Genens and G. L. Stout, Painting Materials, a Short Encyclopedia. Dover, New York (1966). [12] Leganti, fissatioi, pigmenti. Metodi di riconoscimento. Istituto Centrale del restauro, Roma (1978). [13] Mineral Powder Diffracton File Data Bank. JCPDS, Swarthmore (1980). [14] A. C. T. North, D. C. Phillips and F. S. Mathews, Acta Cryst. A24, 351 (1968). [15] G. M. Sheldrick, S H E L X T L , Revision 3. An Integrated System for Soloing, Refining and Displaying Crystal Structures from Diffraction Data. University of G6ttingen, G6ttingen (1981). [16] L. Pauling, The Nature o f Chemical Bond. Cornell University Press, Itacha (1960). [17] I. D. Brown and D. Altermatt, Acta Cryst. B41,244 (1985). [18] M. A. Hitchman and T. D. Waite, lnorg. Chem. 15;, 2150 (1976). [19] B. Bleaney, K. D. Bowers and D. J. E. Ingram, Proc. R. Soc. A228, 147 (1955). [20] J. Ammeter, G. Rist and Hs. H. Gunthard, J. Chem. Phys. 57, 3852 (1972). [21] R. E. Coffman, J. Chem. Phys. 48,609 (1968). [22] D. Kivelson and R. Neiman, J. Chem. Phys. 35, 149 (1961).
Spectrochimica Acta, Vol. 51A, No. 3, pp. 437-446, 1995 Copyright(~ 1995 ElscvierScicncc Ltd Printed in Great Britain. All rights reserved 0584-8539(94)E0108-M 0584-8539/95 $9.50+ 0.00
Spectrochemical and structural studies on a Roman sample of Egyptian blue P. MIRTIt Dipartimento di Chimica analitica, Universit~ di Torino, via Giuria 5, 1-10125 Torino, Italy
L. APPOLONIA Soprintendenza ai Beni culturali e ambientali, piazza Narbonne 3, I-11100 Aosta, Italy
A. CASOLI Dipartimento di Chimica generale e inorganica, chimica analitica, chimica fisica, Universith di Parma, viale delle Scienze 78, 1-43100 Parma, Italy
R. P. FERRARI and E. LAURENTI Dipartimento di Chimica inorganica, chimica fisica e chimica dei materiali, Universith di Torino, via Giuria 7, 1-10125 Torino, Italy and
A. AMISANO CANESI and G. CHIARI Dipartimento di Scienze mineralogiche e petrologiche, Universith di Torino, via Valperga Caluso 37, 1-10125 Torino, Italy (Received 1 February 1994; in final form 25 March 1994; accepted 28 March 1994)
Abstraet--A ball of Egyptian blue excavated on the archaeological site of Augusta Praetoria (Aosta, Italy) has been investigated by several techniques. Optical microscopy and X-ray diffraction gave proof of the identity of the pigment, while indicating that silica phases had also been formed because of sand excess in the reaction mixture. No evidence was found of the presence of tin compounds, thus excluding the use of bronze scraps in the preparation of the pigment. The crystal structure of the pigment was refined using single-crystal X-ray diffraction data; Fourier transform infrared spectroscopy, UV-vis-NIR reflectance spectroscopy and electron paramagnetic resonance spectroscopy were used to characterize further the pigment. Scanning electron microscopy, coupled with energy dispersive detection of emitted X-rays, was finally used to investigate morphology and determine the composition of representative areas of both pigment and impurities.
INTRODUCTION EGYPTIAN b l u e was t h e first s y n t h e t i c p i g m e n t e v e r p r o d u c e d by m a n . It a p p e a r e d in E g y p t d u r i n g t h e f o u r t h d y n a s t y in t h e 3rd m i l l e n n i u m a c a n d a d e v e l o p m e n t at a b o u t the s a m e t i m e , o r e v e n e a r l i e r , m i g h t h a v e o c c u r r e d in M e s o p o t a m i a [1]. It was u s e d as a highly p r i z e d b l u e p i g m e n t a n d to p r o d u c e small o b j e c t s ( b e a d s , s t a t u e t t e s , s c a r a b s , inlays) in E g y p t a n d t h e N e a r E a s t a l m o s t c o n t i n u o u s l y t h r o u g h o u t the w h o l e p e r i o d f r o m its first a p p e a r a n c e s d o w n to R o m a n times. F r o m E g y p t a n d the N e a r E a s t the use o f the p i g m e n t s p r e a d to M i n o a n C r e t e a n d t h e G r e e k w o r l d , a n d t h e n to the R o m a n w o r l d . D u r i n g t h e R o m a n E m p i r e it still was a highly p r i z e d p i g m e n t , in use f r o m B r i t a i n to N o r t h e r n A f r i c a a n d A s i a M i n o r . F o l l o w i n g the fall o f the R o m a n E m p i r e its use r a p i d l y d i s a p p e a r e d , a p a r t f r o m a few a r e a s , m a i n l y in the B y z a n t i n e w o r l d , w h e r e it is still d o c u m e n t e d for a while. I n d e e d , it is p o s s i b l e t h a t use o f E g y p t i a n blue in p o s t R o m a n t i m e s m a y c o n c e r n a r e u t i l i z a t i o n o f l u m p s p r o d u c e d in R o m a n times. It is a fact t h a t a n y t e c h n o l o g i c a l k n o w l e d g e , on which the p r e p a r a t i o n o f this high q u a l i t y b l u e p i g m e n t r e l i e d , s e e m s to h a v e b e e n lost by t h e e n d o f t h e first m i l l e n n i u m AD, a n d o n l y d i s c o v e r e d again in the d e c a d e s at t h e t u r n o f the p r e s e n t c e n t u r y [2, 3]. t Author to whom correspondence should be addressed. 437
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Egyptian blue (EB hereafter) is a crystalline compound whose composition corresponds to a copper calcium tetrasilicate, CaCuSi4Oi0, which is often referred to as cuprorivaite. Indeed, cuprorivaite is the name given to a very rare mineral of the same composition, which was discovered in 1938 in the lava of Mt Vesuvius, near Naples [4]. The structure of the mineral is still unknown, while the structure of the synthetic compound EB was solved by Pabst [5]. EB was prepared by heating a mixture formed by sand, limestone (or calcareous sand), copper compounds (copper ores or bronze scraps) and flux (natural s o d a - - n a t r o n from lower Egypt, or plant ash) [1, 6]. The preparation was described in the first century ac by Vitruvius (Marcus Vitruvius Polio, De architectura libri decem, VII, 11), who observed it during a visit to a glass factory situated in Pozzuoli, near Naples (EB and glass could be obtained by the same basic raw materials). However, as the factory did probably use calcareous sand of the Volturno river, Vitruvius did not mention calcium carbonate as a separate ingredient for obtaining EB (which he names coeruleum); medieval transcribers, which could not be aware of this particular situation, went on omitting calcium carbonate in copying Vitruvius' text, thus transmitting a recipe, which, when a non-calcareous sand was used, could not lead to the formation of the blue pigment. In its place, a green pigment (green frit) was obtained [1]. It is possible that this contributed to the rapid decline in the use of EB in post-Roman times. The synthesis of EB has been carried out in recent times by many scholars with the purpose of shedding light on the ancient procedure [1, 6-8]. Opinions do not completely agree, but a few points stand firm. It appears that the blue pigment was obtained by mixing together the raw materials with the proper amount of flux and heating to a temperature between 850 and 1000°C. The most suitable temperature, and whether sintering or melting was actually achieved (by controlling the flux quantity) is still controversial. However, the pigment has been shown to decompose when heated beyond 1050°C and if a reducing atmosphere does prevail in the furnace [1, 7]. It has also been proposed that a two-stage process could be a most suitable one to obtain a high quality pigment [1, 6]; the second stage should have consisted in reheating the crystals obtained by the first heating, after having ground them to a fine powder. It appears that an accurate control of the reaction conditions is critical for obtaining a high quality blue pigment. In fact, if the raw ingredients are not mixed in the required stoichiometric ratios, other compounds may form in addition to EB. These may include silica phases (e.g. quartz or tridymite) if silica sand is in excess of the other reagents, wollastonite (calcium silicate, CaSiO3) with an excess of both sand and limestone (or calcareous sand), copper oxides (tenorite, CuO, or cuprite, Cu20) when an excess of copper ingredients is present, and glass phase (coloured blue or b l u e - g r e e n by copper) if silica sand and flux are in excess [8]. One has to note here that the introduction of copper into the reaction mixture was not necessarily performed by using copper ores (malachite, basic copper carbonate, CuCO3. Cu(OH)2, for example), but scraps or filings of metallic copper or copper alloys could be used in its place. This could lead to the formation of cassiterite (SnO2) or malayaite (CaSnSiOs) if, for example, tin-copper alloys (bronze) were used. Indeed, arsenic, tin and lead containing compounds have been recognized in samples of EB collected from Egyptian monuments dating from the 3rd millennium BC down to Roman times; this was related to the progressive development of copper alloying from arsenical copper to bronze and leaded bronze [9]. The present study deals with a ball of EB found in archaeological excavations carried out on the site of Augusta Praetoria (Aosta, Italy). The archaeological context enabled dating of the ball to Roman imperial times. The ball was a friable mass made up of blue, white and b l u e - g r e e n grains. Vitruvius reports that sand, copper compounds and soda were ground to a fine powder, mixed together and wet with water to form little balls, which were dried and heated to produce EB. The find from Aosta matches this report, and it seemed worthwhile to perform a comprehensive spectrochemical and structural characterization of the raw pigment. Various techniques were used for this purpose, including optical microscopy, X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, U V - v i s - N I R reflectance spectroscopy, electron paramagnetic resonance
Spectrochcmical and structural studies on a Roman sample of Egyptian blue
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( E P R ) s p e c t r o s c o p y , a n d s c a n n i n g e l e c t r o n m i c r o s c o p y ( S E M ) c o u p l e d with e n e r g y d i s p e r s i v e X - r a y d e t e c t i o n for m i c r o p r o b e analysis.
EXPERIMENTAL The ball of pigment was set free from the excavation sand by sonication in water. Repeated treatments allowed complete separation and cleaning of the pigment grains from sand because of density difference. At this stage the raw pigment was no longer in the form of a ball because the various grains (blue, white and blue-green) were set loose by sonication. Most studies were performed on sample aliquots collected after the ultrasonic treatment, without separation of the differently coloured grains; in a few instances, however, a separation of the blue, white and b l u e green grains from each other proved to be useful. This was achieved by careful hand selection under a Wild Leitz M8 zoom stereomicroscope at 50 x magnification. Optical microscopy examination was carried out by a Polivar POL microscope using either reflected or transmitted light; working magnification ranged from 100 x to 200 x with both normal and polarized light. X-ray powder diffraction was performed by a Rigaku Rotaflex rD/max instrument equipped with a copper rotating anode ( 2 = 1.5418 ]~), operated at the following working conditions: accelerator voltage 40 kV, current 30 mA, response time 2 s, scanning time 1 deg min -~, reading step 0.02 deg. X-ray diffraction intensities from a single crystal were collected on a Siemens R3 four-circle diffractometer, graphite monocromatized M o K a radiation (2=0.71073,~), using the to scan technique, 4 ° < 20 < 50 °, - 9 < h < 9, - 9 < k < 9, 0 < l < 18, 2 ° scanning range, background/scan r a t i o = 0 . 5 , scan speed variable from 2 to 20 ° min -l, 2 standard reflections monitored every 50 measurements. Fourier transform infrared spectra were recorded by a Bruker FS25 spectrometer, equipped with a CsI window. Spectra were recorded in the wavelength range 4000-250 c m - l (2500-40,000 nm) with a resolution of 4 c m - i. Samples were prepared as KBr pellets, and 3 mm micropellets were used in some cases. U V - v i s - N I R diffuse reflectance spectra were recorded in the 250-2500 nm wavelength region (40,000-4000cm -l) by a Varian Cary 5 U V - v i s - N I R spectrophotometer, equipped with a polytetrafluoroethylene (PTFE) coated integrating sphere and performing reflectance measurements against a PTFE coated reference disk. Electron paramagnetic resonance spectra were recorded by a Varian El09 X-band spectrometer equipped with a TE,02 double cavity, an Oxford Instruments ESR900A continuous flow cooling system (range 3.8-300 K) and a Stelar A Q M Auriga/XT data system. A MgO matrix, containing Mn(II) ion in the form of a very low common impurity and addition of a known quantity of DPPH (a,a-diphenyl-fl-picrylhydrazyl) powder, was used as a double standard situated in the reference cavity. The well-known DPPH powder g factor (2.0037+0.0002) and manganese field-scan calibration (g = 2.0012 _+0.0002 and A = 178.21[ x 10 -4 c m - ~) [10] made possible the correct calculation of the g values to the fourth decimal place. Spectra were recorded in the temperature range 77-298 K using the following experimental parameters: magnetic field scan range = 4000 G, microwave power = 2-20 m W and modulation amplitude = 1-4 G. SEM examination was performed by a Jeol JSM 6400 scanning electron microscope; samples were glued on stubs and coated with carbon using a Jeoi JEE-4X vacuum evaporator. Images were obtained by either secondary electrons (acceleration voltage 15 kV, working distance 15 mm, spot 1 2 - - C L coarse, objective aperture 3) or backscattered electrons (acceleration voltage 15 kV, working distance 15 mm, spot 9 - - C L course, objective aperture 2). Microprobe analyses were performed by a Tracor energy dispersive system microprobe (acceleration voltage 15 kV, current 0.62 mA, working distance 39 mm, spot 1 0 - - C L coarse, objective aperture 2), using the SQ program for quantitative determination.
RESULTS AND DISCUSSION
Optical microscopy A p a r t f r o m t h e a r c h a e o l o g i c a l e v i d e n c e , o p t i c a l m i c r o s c o p y was u s e d for the identific a t i o n o f the b l u e g r a i n s as crystals o f E B . T h e s e a r e b l u e c o l o u r e d crystals s h o w i n g p l e o c h r o i s m a n d b i r e f r i n g e n c e [11]; c h e m i c a l m i c r o t e s t s easily a v o i d c o n f u s i o n with
P. MIRTI et
440 5k - -
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! i
i
,
50
90
Fig. 1. Powder diffractograms recorded on ground aliquots of blue (above) and white grains (below), respectively, separated from the archaeological sample of Egyptian blue.
azzurrite (basic copper carbonate, 2 C u C O 3" C u ( O H ) 2 ) , which dissolves by acid attack, while EB does not [12].
X-ray powder diffraction Further and definite identification of the blue grains as EB was obtained by X-ray powder diffraction. Figure 1 shows the diffractograms recorded on aliquots of blue and white grains, respectively, collected by hand separation under the stereomicroscope and ground to fine powder in an agate mortar. It easily allows one to identify the blue grains as crystals of EB, as all the cuprorivaite peaks are recognized, apart from that corresonding to d = 3.66 (crystal plane 200); however, relative peak intensities do not match the cuprorivaite JCPDS card (card 12-512) [13] because of preferred orientations arising from sample preparation. On the other hand, the diffractogram recorded on the white grains, apart from the occurrence of peaks due to residual EB, allows one to identify these grains as quartz crystals; no evidence is gained here for the presence of woilastonite, thus suggesting that limestone was not in excess of the other reagents. Moreover, evidence for the presence of cassiterite or malayaite is not found either. This would exclude the use of bronze scraps for introducing copper into the reaction mixture. Thus either a copper ore or copper filings should have been used for this purpose.
Refinement of crystal structure A finely shaped transparent blue crystal (dimension 0.12×0.1 ×0.05 mm 3) was selected and mounted on a Siemens R3 four-circle diffractometer. The cell parameters obtained by a least squares refinement on 52 reflections measured in the range 1 1 ° < 2 0 < 3 4 ° are a0=7.309+0.001 and c 0 = 1 5 . 1 0 7 + 0 . 0 0 5 , ~ , in agreement with the values reported by Pabst [5] which are a, = 7.30 +_0.01 and co = 15.12 + 0.02 A. The analysis of systematic absences confirmed the space group to be P4/ncc. The 2924 reflections measured in half Ewald sphere, in order to obtain better statistics, were corrected for Lorentz polarization effects and the absorption correction was based on the ~p scan method [14]. The number of unique reflections was 362, of which 315 with I > 4 o ( 1 ) were considered observed and used in the following calculations, carried out using the S H E L X T L system [15], The structure was solved anew by direct methods which allowed the positioning of all atoms. The origin was taken at the symmetry centre, with a shift of (1/4x, - 1/4y) with respect to the choice of Pabst [5], The refinement was carried out by full-matrix least-squares using atomic scattering factors for neutral atoms
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Table 1. Fractional atomic coordinates and equivalent U values (/~2) for a crystal of Egyptian blue (e.s.d. in parentheses) Atom Ca Cu Si O(1) 0(2) 0(3)
X/A 1/4 1/4 0.0068(3) 0.9618(4) 0.7082(3) 0.3535(3)
Y/B 3/4 1/4 -.0732(1) 0.0382(4) 0.0016(4) 0.0073(3)
Z/C 0 0.08173(5) 0.14732(6) 1/4 0.1263(2) 0.0819(2)
U~q 0.0068(3) 0.0054(2) 0.0050(3) 0.0187(9) 0.0110(8) 0.0094(7)
provided by S H E L X T L , unit weights and anisotropic thermal parameters for all atoms. The final R value was 1.97% for 38 parameters, goodness of fit = 1.5. The maximum peak in the final difference Fourier was 0.3 e ,~-3. The present refinement confirms the crystal structure proposed by Pabst [5], but shows e.s.d, smaller by one order of magnitude. Table 1 gives the fractional atomic coordinates and the equivalent U, and Table 2 the relevant distances and angles. In Fig. 2, a drawing of two unit cells projected on (010) shows the sheets orthogonal to z and composed of units (Cu25i8020) -4. The Cu atoms are in square planar co-ordination. These sheets are connected by interlayer Ca atoms in distorted antiprismatic coordination. One can see from Table 2 that the two independent C a - O distances are very different from one another. This can be explained in terms of charge balance. In fact 0 ( 2 ) is bonded to two Si and one Ca atom and its bond valence sum, according to Pauling [16] is 2.25 v.u., while 0 ( 3 ) is bonded to one Si, one Cu and one Ca and its Pauling bond valence sum is 1.75 v.u. The distances of 0 ( 3 ) from all the cations tend therefore to be shorter than usual in order to compensate for the lack of charge. The opposite is true for the oversaturated 0 ( 2 ) . The sums of the bond strengths, calculated on the basis of bond length [17] on 0 ( 2 ) is 2.13v.u. and on 0 ( 3 ) is 1.95v.u. The fact that the structure refinement proceeds in an excellent way can be ascribed to the good quality of the crystal synthesized in Roman times.
FTIR spectroscopy Fourier transform infrared spectra were recorded on mixed grains, as well as on blue, white and b l u e - g r e e n grains, separately. The obtained spectra, reported in Fig. 3, were compared with those of standards of quartz and fragments of archaeological glass, containing and not containing copper ions. Figure 3(a) reports the spectrum recorded on the blue grains separated from the archaeological sample of EB; this spectrum shows band shifts with respect to both quartz and archaeological glass in the regions of symmetrical and antisymmetrical S i - O - S i stretching, as well as in the O - S i - O bending region. In more detail, antisymmetrical S i - O - S i stretching bands are observed for EB at 1230, 1160, 1056 and 1008 cm-1, while symmetrical S i - O - S i stretching bands are centred at 800, 755,664 and 595 cm-I; the bands centred at 1230, 1008 and 595 cm-I appear to be peculiar to EB, as they are not Table 2. Atomic distances (,~,) and bond angles (deg) for a crystal of Egyptian blue (e.s.d. in parentheses) Ca-O(2) Ca-O(3) O(2)-Ca-O(3) O(3)-Ca-O(3) O(2)-Ca-O(3) O(2)-Ca-O(3) Cu-O(3) O(3)-Cu-O(3) SA(A) 51:3-J
2.652(3) 2.375(3) 78.2(1) 105.8(1) 59.9(1) 75.6(1) 1.928(3) 90
Si-O(1) Si-O(2) Si-O(3) O(l)-Si-O(2) O(l)-Si-O(2) O(2)-Si-O(2) O(1)-Si-O(3) O(2)-Si-O(3) O(2)-Si-O(3)
1.601(1) 1.629(3) 1.585(3) 110.0(1) 107.9(1) 108.3(1) 114.6(1) 103.3(1) 112.9(1)
442
P. MIRT!et al.
Fig. 2. Projection of the structure of Egyptian blue (cuprorivaite) on the (010) plane, showing the arrangement of the SiO4 tetrahedra. Smaller spheres represent copper atoms, larger ones calcium atoms.
matched in the quartz and glass spectra. As for the O - S i - O bending region, bands are observed for EB at 521 and 484 cm-1. F T I R spectra recorded on white and b l u e - g r e e n grains are shown in Figs 3(b) and (c); in these cases 3 mm micropellets were used, due to the reduced amount of sample available. The spectrum of the white transparent grains proved to match completely the quartz spectrum, as expected on the basis of the previous information from X-ray powder diffraction; b l u e - g r e e n grains, on the contrary, which were much less abundant in the sample and were silent to X-ray diffraction, gave a spectrum matching those of the archaeological glass fragments.
=~ 75
.3 E
50
(a) ~
25 i 2000 1900
I
1
I
r
1800 1700 1600 1500
T
I
I
1400 1300 1200 1100 1000
900
800
700
600
500
400
Wave number (cm-1) Fig. 3. FTIR spectra recorded on aliquots of blue (spectrum a), white (b) and blue-green (c) grains separated from the archaeological sample of Egyptian blue. The spectra are not on the same transmittance scale: the given scale refers only to spectrum a.
Spectrochemical and structural studies on a R o m a n sample of Egyptian blue
443
100
r.-. 50
1600
Nanometres
20'00
2400
Fig. 4. Reflectance spectrum of an unground aliquot of the archaeological sample of Egyptian blue, recorded in the UV-vis-NIR spectral region.
Reflectance spectroscopy U V - v i s - N I R reflectance spectra and colour measurements were performed on both unground and ground aliquots of the raw pigment, without separation of the blue grains from the archaeological specimen, due to the non-negligible amount of sample required. Thus allowance must be made for the presence of the white grains within the sample, which enhance colour luminosity and diminish saturation. Colour coordinates were obtained in the CIEL*a*b* colour space as follows: L* = 41.98, a* = 14.82, b* = - 38.16 for the unground sample, and L * = 5 9 . 6 9 , a*--0.15, b * = - 2 8 . 3 3 for the ground specimen, respectively; these account for the less saturated colour of the ground pigment. The reflectance spectrum, recorded in the 250-2500 nm wavelength range on the unground sample, is reported in Fig. 4 (a similar spectrum, shifted towards higher reflectance values, was recorded on the ground sample). This shows two major absorption bands at about 560 and 628 nm (some 17,850 and 15,900 cm- t, respectively), and a minor one centred at about 790 (some 12,650 cm-I; automatic change of detector may somehow bias an accurate determination of this absorption maximum). These d-d bands are due to optical absorption by copper(II) ions in the Big ground state, either in EB or in the glass phase. It is evident from Fig. 4 that the intense colour of the blue pigment is principally due to the absorption bands at 560 and 628 nm, which can be assigned to the transitions dx2_y2~...-dxv (Blg~---B2g) and dx2_.v,-~-dxz,yz (Blg'~-Eg), respectively. This is consistent with an effective D4h square planar geometry of coordination of the copper(I1) ion in the EB crystals. In contrast, absorption bands centred at about 800 nm are commonly assigned to the transition dx,-_y,_*--d~2(B ig*--A~) and may be associated with the presence of copper(II) ions, both in solution and in the solid state, in a distorted (tetragonal elongated) octahedral coordinaton [18]. This might be related, in the present case, to the occurrence of glass phase within the raw pigment.
EPR spectroscopy E P R spectra were recorded on both unground and ground aliquots of the archaeological sample, as well as on a ground aliquot homogeneously diluted in a diamagnetic matrix: these spectra were strictly similar to each other in the temperature range considered. Figure 5 shows the E P R powder spectral pattern of the undiluted ground pigment recorded at 77 K. This consists of two absorption lines in the perpendicular and parallel regions of the spectrum, respectively, in agreement with the occurrence of a unique copper(II) paramagnetic species (S = 1/2), with the unpaired electron in the Big ground state in an apparent ligand field axial geometry D4h. The obtained values &-- 2.3220 and g , = 2.0744 (g~oc,~c= 2.1570), with the trend glt > g . > 2, can be in accord with an oxygen
444
P. MIRTI et al. DPPH
(a)
J 30b0
35'00
H (gauss) Fig. 5. First derivative EPR powder spectrum recorded at 77 K on a ground aliquot of the archaeological sample of Egyptian blue. Microwave power 10 mW, modulation amplitude 1 G, time constant 0.128 s. (a) DPPH and *Mn(II) in MgO matrix patterns.
atom ligand field in either a tetragonal elongated octahedral or in a square planar geometry. Theoretical g values (gtl = 2.37; g i = 2.10; g~so= 2.19) have been calculated introducing the transition energies, as obtained by reflectance spectroscopy (AE~dxv-dx2 v2)= 17,850 cm -~ and AE~dx~.~.~-dx2v2)= 15,900 cm-l), and the Cu(II) free ion spin-0rbit Coupling 2 = - 829 cm- 1 in the well-known empirical formulas: gll = 2.0023 g i = 2.0023 -
8)~/AE~d~v-ax2v2) 22/AE~a~:.~_<2 ~2)
holding for a system with S = 1/2 and 2 ~ AE. These theoretical values are higher than the experimental ones, due to the introduction of the free ion spin-orbit coupling in the g empirical equation, while in general some metal-ligand covalent bonding degree occurs in complexes, and 2 becomes smaller. Indeed, using a literature value (2 = - 710 cm -1) obtained for some copper(II) complexes [19], theoretical g values became g11=2.32, g~ =2.09 and g~so=2.17, in better agreement with our experimental values. Most common copper(II) complexes are characterized by an octahedral tetragonal distorted geometry and show, in general, higher experimental g values (g11~2.4, g ~ 2 . 1 ) in agreement with lower transition energies, due to the presence of strong axial ligands [20]. Thus E P R spectroscopy (in agreement with the complementary electronic reflectance spectroscopy) confirms the presence of a copper(II) site surrounded by four oxygen ligand atoms in a square planar geometry, as put into evidence by the X-ray structure. Such a structure has been observed for only a limited number of Cu(II) complexes, in diluted solution, as powders [20] or as oriented monocrystals [21]. The occurrence of an octahedral distorted geometry of Cu(II) complexes in the glass phase (blue-green grains) is not revealed in the E P R spectrum, due to the likely superimposition of the strongly predominant EB spectral pattern. No evidence of copper hyperfine electron-nuclear interaction ( l c , = 3 / 2 ) has been gained in the working temperature range, due to the line broadening resulting from electric crystalline field effects, to the electrostatic interactions present in the solid state phase, and to the spin rapid relaxation through the spin-orbit interactions typical of the paramagnetic transition metal complexes [22].
Scanning electron microscopy EB crystals and unreacted silica, together with minor amounts of glass phase, were observed in SEM images. The length of the EB crystals, which aggregate to form definite
Spectrochemical and structural studies on a Roman sample of Egyptian blue
445
Fig. 6. SEM image (secondary electrons) showing an aggregate of cuprorivaite crystals in the archaeological sample of Egyptian blue. clusters, roughly ranges from 10 to 100/~m. Figure 6 shows a typical aggregate of EB crystals within the archaeological sample. Point analyses performed on single crystals gave the following average percent composition: SiO2 65.6 + 0.9, CaO 19.7 + 0.6, CuO 14.5 + 0.4; this has to be compared with the theoretical stoichiometric composition of EB (SIO2 63.9, CaO 21.2, CuO 14.9). Figure 7 shows a back-scattered electron image, which clearly allows for discrimination among the phases present on the basis of mean atomic number contrast: EB crystals,
Fig. 7. SEM image (backscattered electrons) showing silica lumps and glass phase amidst cuprorivaite crystals.
446
P. MIRTI et al.
glass phase and silica are viewed as increasingly dark areas (silica lumps are also clearly visible in Fig. 6 amidst the EB crystals). Point analyses performed on silica lumps gave a SiO2 content ranging between 92 and 99%, while areas of glass phase gave SiO2 contents in the range 71-80%, Na20 6 - 1 1 % , CaO 10-11%, CuO 1-2%, AI203 1%. The latter approaches the composition of a silica-soda-lime glass, apart from a rather low Na20 content. The formation of areas of glass phase depends on local excesses of flux which cause local melts to occur, and a glass to form; this is coloured blue-green by copper. The presence of sodium and the negligible amount of potassium recognized in the glass composition suggest that natron and not plant ash was used as a flux in the preparation of the pigment.
CONCLUSIONS
The present paper has given an account on the application of several spectrochemical techniques to the study of a sample of EB excavated on an archaeological site. It has been shown that the blue crystals present in the sample have the composition of cuprorivaite, and that silica sand was most probably the only reagent present in excess of the others in the reaction mixture. Areas of glass phase are not widespread, indicating a good control of temperature and flux quantity; natural soda rather than plant ash seems to have been used as the fluxing agent. No evidence has been gained for the presence of tin compounds in such a quantity to suggest the use of bronze scraps for introducing copper into the system; instead, copper ores or filings should have been used. Evidence for the presence of calcium silicates was neither found, which indicates that there was no excess of both lime and silica with respect to copper.
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