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First evidence for 1st century AD production of Egyptian blue frit in Roman Italy
Journal of Archaeological Science 53 (2015) 578e585
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First evidence for 1st century AD production of Egyptian blue frit in Roman Italy * Lorenzo Lazzarini, Marco Verita IUAV di Venezia, San Polo 2468/B, 30125 Venezia, Italy Laboratorio di Analisi dei Materiali Antichi, Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 June 2014 Received in revised form 30 October 2014 Accepted 3 November 2014 Available online 12 November 2014
The considerable amount of research carried out up to present time in an attempt to understand the production technique of Egyptian blue frit is based on analyses of the ancient pigment and its laboratory synthesis. The fortunate finding in Roman Liternum of 1st century AD fragments of ceramic crucibles with adhering remains of a blue pigment allowed laboratory analyses (OM, XRD, SEMeEDS) to be performed on original production debris. The results confirm that the crucibles had been used for the synthesis of Egyptian blue frit and throw new light on the controversial issue of the Roman manufacturing technique of Egyptian blue frit. The results of the analyses indicate that the Egyptian blue frit was produced with a mixture of a local source of silicaelime sand, a fluxer (not identified) and copper scale; the crucibles were also made with local raw materials. The firing of the batch was performed at a relatively low temperature, probably around 850 C and firing at higher temperatures would have hindered the formation of the cuprorivaite crystals. The analyses suggest that the evaporation of copper during firing of the batch attested in this study is one of the most critical parameters influencing the final blue pigment hue. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Liternum Egyptian blue Crucibles Pigment SEMeEDS XRD PM
1. Introduction Liternum, presently Literno (Naples, Italy), was a small Roman town 8 km north of the more famous Greek and Roman town of Cuma, one of the oldest Greek colonies in Southern Italy. Liternum was founded in 194 B.C. by 500 Roman veterans as a colonia maritima, with a defensive role. Placed by the sea on a marshy area near the Lake Patria, it always remained a small settlement playing a negligible role in Roman history, and is remembered only for the presence of the villa and the tomb of Scipio the African. It reached a certain importance at the end of the first century A.D. when it was crossed by the Via Domitiana, the main road of ancient Campania. Ruins of a Forum, Basilica, Capitolium, Theatre and Amphitheatre are still visible on the site (Maiuri, 1958; De Caro and Greco, 1981). Archaeological excavations on the theatre started in 1932, lasted for five years and were then abandoned, being resumed in the nineties. In 1996, near the town walls on the left shore of the Lake Patria a large amount of fragments of ceramic pots (clearly to be identified
* Corresponding author. Tel.: þ39 041 2571459; fax: þ39 041 1434. E-mail addresses: [email protected] (L. Lazzarini), [email protected] (M. Verit a). http://dx.doi.org/10.1016/j.jas.2014.11.004 0305-4403/© 2014 Elsevier Ltd. All rights reserved.
as kiln wastes) and of crucibles, some with attached blue incrustations (Fig. 1) dated not later than 1st c. AD was found together with a number of small spheres of blue colour (Gargiulo, 1997, 1998), very similar to the ones found at Pompeii (Augusti, 1967) and in other Roman towns, for example at Lugdunum (modern Lyon, France). The hypothesis made by the archaeologists was that the area was used for depositing waste pieces of one or more kilns (the kilns have not been found so far) producing pottery and Egyptian blue. For the importance and rarity of this latter find, it was decided to submit samples of the crucibles and of its residues to laboratory analysis with the purpose of collecting information on the pigment and its production technology. Egyptian blue (indicated hereafter as EB) is the most important blue pigment of antiquity. Manufactured for the first time around 2000 BC in Mesopotamia, it was first identified by Laurie et al. (1914) in Egypt, hence its name. This pigment was known in Minoan and Mycenaean times when it was extensively used in the wall paintings of Knossos, Pylos and Thera, and continued to be largely employed in Greek times when it was called kyanos, in the frescoes of the famous Macedonian tombs, in polychrome funerary stelae, for the decoration of temple roofs, precious pottery, etc. The Romans increased its use quantitatively, sometimes extending it to
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Fig. 1. The examined fragments of crucibles with incrustations of blue pigment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mosaic tesserae; it was then named caeruleum, and was considered one of the “colores floridi”. It cannot be excluded that the production of this pigment ended at the collapse of the Roman Empire, although its use continued in Rome until at least the 11th c. AD (Lazzarini, 1982; Santopadre et al., 2011). Other analyses seem to point to an even later use at Genoa and Treviso (Italy) during the 13th c. (Gaetani et al., 2004). The great success of Egyptian blue throughout the centuries is certainly due to its beautiful blue hue, to the rather cheap manufacture and high stability in all techniques, although its poor hiding power and transparency may favour the blackening or browning of paints produced with the addition of organic media such as the gum arabic used in Pharaonic times (Daniels et al., 2004).
1.1. The manufacture of Egyptian blue in antiquity Egyptian blue is a multicomponent material whose colour is due to the presence of calciumecopper tetrasilicate crystals (cuprorivaite: CaCuSi4O10) produced by firing a batch (mixture) of quartz, lime, copper and a small amount of a soda flux. Vitruvius (7, Ch. 11, 1)1 describes the way of producing Egyptian blue in Pozzuoli, near Pompei, during the first century B.C., by adding copper to a ground mixture of sand (harena) and natron2 (flos nitri). This mixture was shaped into small balls, dried in air and fired in earthenware pots. Vitruvius's text does not mention any addition of limestone or lime. This is not a surprise: the same situation can be observed for the production of glass in Roman time. The source for calcium was natural sand, like the one
1 Vitruvius 7, cap. 11, 1 (Morgan, 1960): “Blue was first manufactured at Alexandria, and afterwards by Vestorius at Pozzuoli. The method of making it, and the nature of the ingredients, merit our attention. Harena (sand) is ground with flos nitri (natron), till the mixture is as fine as flour, to which coarse filings of Cyprian copper are added, so as to make a paste when moistened with water; this is rolled into balls with the hand, and dried. The balls are then put into an earthen vessel, and that is placed in a furnace. Thus the copper and sand heating together by the intensity of the fire, impart to each other their different qualities, and thereby acquire their blue colour”. 2 A mixture of natural evaporate minerals (mainly sodium carbonatebicarbonate, secondarily chlorides and sulphates).
extracted from the river Volturnus (north of Naples), not far from Pompei and Liternum, indicated by Pliny for the manufacture of glass (Pliny the Elder, Naturalis Historia book 36, 66); in this sand silica and lime are naturally mixed in the right ratio to make glass , 2002).3 (Vallotto and Verita EB has always been of interest to archaeologists and archaeometrists since the beginning of the last century. The latter were also interested in understanding the production technique reproducing it experimentally (for instance: Schippa and Torraca, 1957; Riederer, 1997; Hatton et al., 2008). At this time a production site for EB dating to the Roman period has been found at Memphis in Egypt (Petrie, 1909; Nicholson, 2003). Fragments of ceramic vessels, belonging to crucibles either cylindrical or globular in shape, were lined with a white slip (up to 2e3 mm in thickness, lime based) to which a layer of Egyptian blue frit (up to about 9 mm in thickness) adhered. This frit contains less glassy phase and more abundant quartz particles than the EB balls from the same site and, consequently, it has a much more open microstructure. The cuprorivaite crystals in the frit layers are also smaller (up to 40 mm in length), whereas the remaining quartz particles (also cristobalite and tridymite were identified) tend to be larger (Tite and Hatton, 2007). Chemical analysis by SEMeEDS of the ceramic vessels allows concluding that they were made of a non-calcareous, iron rich Nile silt. The interaction occurred between the ceramic vessel and the EB layer during preparation of the EB was not investigated. Chemical analyses of the Memphis EB balls and EB layers are reported and compared with analyses of EB samples from Delos, Pompeii, Rome, England and Malta. The relatively high alumina and iron contents and the good correlation between the potash and alumina contents suggest the use for the preparation of EB pigment of a mixture of silicaelime sand, copper oxide scale or leaded copper and a sodium source. For EB from Pompei, Rome, England and Malta the high amount of potash introduced along with the feldspar in the sand does not allow distinguishing between natron and soda plant as the sodium
3 The importance of lime for glassmaking will be fully understood only since the end of the 17th century and after this date lime will be added to the glassmaking batch as a separate component.
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source. Pompei and Rome samples are recognised by their high potash and alumina (Pompeii also high magnesia) contents as compared to the EB samples found in Memphis and in the other sites. The authors conclude that the production debris excavated in Memphis is consistent with the description of the EB production given by Vitruvius. An EB production is also attested at Cuma and Puteoli (Pozzuoli), two towns very close to Liternum, where ceramic fragments with incrustations of EB dating to the 1st c. AD have been found. This discovery made it possible to reconstruct the cylindrical shape of the crucibles, whose diameter varies between 30 and 50 cm. An intact specimen about 50 cm in height used as a burial urn has been excavated at Cuma (Caputo and Cavassa, 2009; Gargiulo, 1998). A relationship between the findings at Cuma and Liternum and the production of EB blue tesserae in Roman wall mosaics of the 1st century AD is suggested in (Boschetti et al., 2012). The blue incrustation of one of the ceramic fragments found in Liternum was investigated with several analytical techniques by Platania et al. (1998). The authors concluded that the blue incrustation is the result of the process of preparation of Egyptian blue fired on the surface of a ceramic crucible. In this paper five fragments of crucibles from Liternum with abundant incrustations of a blue porous frit on their internal surface are investigated by several analytical techniques in order to get information about the production technology of EB and to verify similarities with published compositions of this pigment. 2. Materials and methods 2.1. Samples Five fragments of crucibles with consistent residues of a blue porous frit on their inner surface (Fig. 1) were kindly given to the authors by the local archaeological suprintendency (Soprintendenza Archeologica delle Province di Napoli e Caserta, Dr. Stefano De Caro) for archaeometric analysis. The five fragments before laboratory examination were described as follows for their macroscopic aspect: Sample 1: yellowish ceramic body 12 mm thick containing few brown inclusions, with a 3e5 mm thick deposit of a porous blue pigment; Sample 2: as for sample 1, but the body and the deposit are slightly thinner, 10 mm and 3 mm thick, respectively, and the pigment is lighter in colour; Sample 3: the fragment is probably broken near its bottom-part having a thickness variable from 10 to 22 mm; the colour of the pottery is brownish and shows a vitrified body with several shrinkage cracks due to a higher firing temperature. It is covered inside by a rather thin (1e2 mm) greenish-blue glassy porous deposit, with lumps of the same material on the outside surface; Sample 4A: brownish slightly vitrified ceramic body 10 mm thick, with a 1e3 mm thick dark blue deposit. Presence of diffused bluish and black deposit on the outside of the potsherd; this sample was investigated only by optical microscopy and XRD; Sample 4B: the ceramic body is yellowish, with brown inclusions as in sample 1; some areas are covered by thin blue pigment deposits alternating with others with black shining crystals. The latter are also diffusely present on the outside of the potsherd. The small size of the ceramic fragments does not allow the shape and size of the crucibles to be recognized. A reconstruction of the crucibles with the Cuma fragments is described in (Caputo and Cavassa, 2009).
2.2. Analytical methods The ceramic bodies and the adhering residues of pigments were sampled for the preparation of powders for X-Ray diffraction analysis (Cu, Ka/Ni at 40 KV, 20 mA, Panalitical Empyrean). Small chips including the ceramic bodies with their pigment residue were cut with a thin diamond wheel to be prepared in thin sections and then studied under a polarizing optical microscope in transmitted/ reflected light (PM). Other chips were embedded in an acrylic resin and prepared in polished cross sections down to 1 mm grain size diamond pastes. After optical microscopy observation (OM) in reflected light, samples were carbon coated and examined under scanning electron microscopy (Philips XL 30) coupled with an energy-dispersive X-ray spectrometer (Edax) for chemical analyses (analytical conditions: 25 kV, 1 nA). SEM observation was made in backscattered electrons mode (SEMeBSE) and allowed distinguishing phases with different chemical composition by different grey hues. Average quantitative chemical composition of the ceramic body and of the EB layer were obtained by X-ray microanalysis (SEMeEDS) by scanning the electron beam on large areas (for the ceramic body, 1 mm 0.3 mm areas, as far as possible from the interface with the EB layer). For the analysis of the glassy phase the electron beam was scanned on areas as large as possible, following et al., 1994) to avoid alkali the general conditions reported in (Verita depletion during the analysis. The averaged values of three to five analyses in different areas (the routine number of three analyses was increased when large deviations in composition from point to point were observed) were considered.
3. Results 3.1. Ceramic body of the crucibles 3.1.1. Minero-petrographic analyses The XRD analyses of all the ceramic bodies are reported in Table 1. They reveal the ubiquitous presence of considerable, although variable amounts of quartz and plagioclase, and the presence of pyroxene. The microscopic study of the thin sections showed for all the ceramic bodies a very porous (porosity measured by visual evaluation is around 30%), semi-isotropic/isotropic matrix containing some rounded, small vitrified ARF (Argillaceous Rock Fragments) with abundant sandy skeleton. The latter exhibits a bi-modal grain size formed by a fine detritic fraction belonging to the original clay, and a relatively abundant added temper. The detritic fraction is composed of very small (0.02e0.05 mm) angular quartz crystals, with a small amount of twinned plagioclase and K-feldspar; very
Table 1 Results of the X-Ray powder diffraction analysis: Qz: quartz; Pl: plagioclase; P: pyroxene; Cu: cuprorivaite; Cr: cristobalite; Te: tenorite; (þþþ: very abundant; þþ: abundant; þ: present; ±: trace).
Ceramic Body
EB layer
Black Cryst.
Sample N.
Qz
Pl
P
1 2 3 4A 4B 1 2 3 green 4A 4B 4B
þþ þþ þ þþþ þþþ þ
þþþ þþ þþþ þþ þþ ± ±
þþ þ þ
þ þ þ þ
Cu
Cr
þþþ þþþ þþþ þþþ þþ
±?
Te
þ
±? þþ þþþ
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Table 2 Average chemical compositions (wt% of the oxides) of the ceramic body and of the layer at the interface with the EB frit (sample 4). Ceramic body
Na2O
MgO
Al2O3
SiO2
1 2 3 4B 4B interface
6.5 6.5 8.5 6.0 8.0
2.3 2.2 2.0 2.0 1.9
13.5 13.3 15.5 14.0 13.0
61.0 55.0 53.0 61.5 51.0
P2O5 1.3 0.4
abundant are opaque and semi-opaque minerals to be identified as iron-ores (haematite, ilmenite, magnetite). The added temper is made of larger single clasts, mostly angular, sometimes subrounded, with dimensions ranging from 0.2 to 1.1 mm. They are formed by (in increasing abundancy): - K-feldspar sometimes identified as microcline, in rare prismatic crystals with their typical twinning, sometimes slightly opacified by caolinisation; - clinopyroxene, mostly in subrounded individuals, rarely twinned, with the optical characteristics of diopside: because of its size (several micrometres in length) a neoformation has to be excluded for this mineral; - rock fragments, mostly of acidic lavas showing a groundmass with plagioclase microliths and abundant iron ore (haematite and some magnetite), and plagioclase and clinopyroxene phenocrysts; - quartz, in angular mostly single (rarely polycrystalline and subrounded) individuals; - plagioclase, in prismatic, often twinned crystals of sodic composition, sometimes slightly opacified by caolinisation. Some remarkable differences can be observed in sample 2, i.e. the presence in the skeleton of isoriented illite needles (indicating the use of a turning wheel for the manufacture of this crucible) in the matrix, a small amount of biotite flakes pervasively altered into haematite, and of clasts of a quartzearenite cemented by limoniteehaematite. In sample 3 the composition is quite similar to that of sample 2, with a definitely isotropic matrix indicating a somewhat higher firing temperature. Sample 4B differs from the others for the presence of several calcite “ghosts” and of a few clasts of chert. 3.1.2. SEM microscopy and X-ray microanalysis The chemical compositions of the ceramic bodies are reported in Table 2. The results are similar in four samples (sample 4A excluded) examined: silica and alumina are the main components and relatively high flux content (Na2O 6.0e6.5%, higher in sample 3: 8.5%; K2O 1.4e1.6%, higher in sample 2: 2.6%) alkaline earths (CaO 6.8e8%, higher in sample 2: 9.7%; MgO 2e2.3%), iron (Fe2O3 4.5e5.4%) and titanium (TiO2 0.7e1.0%) are present. In samples 2 and 3 a certain amount of phosphorous and sulphur was also detected. The copper, sulphur and chlorine contents (and probably also part of the sodium and calcium contents) are related to migration of volatile batch components deeply into the pores of the ceramic body during firing of the EB batch. 3.2. EB frit layer 3.2.1. Minero-petrographic analyses The results obtained from the XRD analysis of the coloured incrustations are reported in Table 1. They indicate the rather obvious presence in all samples of cuprorivaite (EB), of quartz in most samples (1, 3, 4A and 4B), of traces of plagioclase in samples 1 and 2, and of dubious traces of cristobalite in samples 1 and 3, a mineral
SO3 2.0 3.0 1.5
Cl
K2O
CaO
TiO2
Fe2O3
CuO
0.20 0.40 0.20 0.20 0.30
1.60 2.60 1.60 1.40 1.75
6.8 9.7 8.0 7.0 10.0
0.75 1.00 0.77 0.70 0.70
4.5 5.0 4.8 5.4 6.5
2.8 1.0 2.3 1.8 5.3
definitely present in sample 4A. The black crystalline deposit on the sample 4B (inside and outside) is made of tenorite (CuO). The quite thick (max. 5 mm) deep-blue incrustation on samples 1 and 2 examined under the microscope in plane-polarised light shows a sort of honeycomb fabric consisting of a large amount of pores of irregular and globular shape (Fig. 2), and a net of blue needles and small prisms of EB with a maximum grain size (MGS) of 0.30 mm. A brownish glassy phase (Fig. 2) glues the crystals together and the EB layer to the ceramic body. Under crossed polars the EB shows its typical bluish strong pleochroism and high polychrome interference colours,4 and is often encircling quartz clasts. In fact, sand clasts with a MGS of 0.08e0.4 mm are scattered in the glass and EB, mostly subangular/subrounded in shape and consisting of quartz, plagioclase, acidic lava (Fig. 3) with abundant plagioclase microliths, rare pervasively caolinised K-feldspars, clinopyroxenes and opaque minerals. Similar observations were made on the blue incrustation of sample 4A, while that on sample 3 is quite different, for it contains a very small amount of EB crystals with MGS of 0.10 mm embedded in an abundant glassy phase: this feature explains the overall greenish colour of this incrustation. Abundant unmelted sand clasts of rounded quartz and rare Kfeldspars are also present.
3.2.2. SEM microscopy and X-ray microanalysis The thick EB layer of samples 1 and 2 consists of clusters of elongate EB crystals (white in SEM micrographs) surrounding partially reacted angular quartz grains bonded together by a scant glassy phase leaving abundant voids and forming a porous, brittle layer (Fig. 4a, b). Few grains of unreacted feldspars can also be observed in both samples. In sample 3 round quartz grains prevail, and only rare EB crystals were detected; the glassy phase is relatively abundant forming strong bonds with the underlying compact ceramic body (Fig. 5). The average chemical compositions of the EB blue layers, determined by X-ray microanalysis scanning the electron beam on three different large areas, are given in Table 3. No lead and tin were detected (estimated lower limits of detection: PbO 0.2%; SnO2 0.3%), thus excluding the use of a bronze scale as a source of copper. Compositions are quite similar in samples 1, 2 and 4B (the mean and std deviation of 9 measures for sample 4B are given in Table 3), while in sample 3 (greenish layer) the copper (CuO 3.7% instead of 8.9%) and lime contents (CaO 3% instead of 9%) are lower and silica is higher (SiO2 85% instead of 72.7%), indicating a greater quartz grains content. The EB crystals surround the quartz grains; they are up to 100 mm in length (sample 4B) but generally up to 50 mm in length. Their mean chemical composition is similar in samples 1, 2 and 4B (no EB crystals were found in the polished section of sample 3): SiO2 65.5%; CaO 14%, CuO 20%; Fe2O3 0.5%. These values match well
4 The optical properties of EB are so peculiar under the polarizing microscope as to allow this pigment to be easily identified. Its exceptional near-infrared luminescence also allows it to be identified in visible light very quickly and definitely even if present in traces (Accorsi et al., 2009).
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Fig. 2. Sample 1, PM micrograph of a thin section of the blue incrustation showing its high porosity, the brown glass glueing the incrustation to the ceramic body and the EB particles together. N//(long side: 1.03 mm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the theoretical stoichiometric composition of cuprorivaite, except for the presence of iron impurities. Angular quartz crystals (SiO2 100%) up to 0.8 mm in length (in sample 4B the grain length is less than 0.3 mm) and less abundant rounded grains are generally observed (in sample 3 rounded grains prevail). Rare grains of feldspars and of a mineral of average composition: SiO2 24%; Al2O3 21%; Fe2O3 28%; CuO 5%; Na2O 7.5%; MgO 7.5%; CaO 5%, were detected in samples 1, 2 and 4B. Small amounts of glassy phase were observed in samples 1, 2 and 4B, while larger amounts were observed in sample 3. The chemical compositions of the glassy phases (mean of three to five analyses in different areas) are given in Table 3. They are of a sodae(lime)esilica type with a fairly high alumina (Al2O3 6.5%) and potassium (K2O 2.7e4.6%) and low calcium (CaO 1.7e2.7%) contents. Traces of phosphorous (P2O5 0.5e1.0%) were also detected in some of the measurements in all the samples. By analysing different glassy areas of a same EB layer, an important heterogeneity of this phase was detected (see std deviation calculated for five measurements in sample 3, Table 3). These data suggest a nonequilibrium condition in which some batch components did not
Fig. 3. As for Fig. 2, detail of the incrustation in cross-polarised light showing two clasts of acidic lavas (one top left rich in plagioclase microliths/phenocrysts).
Fig. 4. a,b e Sample 1, SEM micrographs (low and high magnifications) of the polished cross section of the EB layer: quartz grains (grey) are surrounded by EB crystals (white) and bonded together by a glassy phase (light grey); porosity appears dark grey.
have sufficient reaction time (or a sufficiently high temperature) to melt completely and form a homogeneous glassy phase.
3.3. EB frit layer/ceramic body interface The EB layer/ceramic body interface is fairly clear, without any secondary phases (neocrystallizations) that could indicate a
Fig. 5. SEM micrograph of the polished cross section of sample 3 showing the compact ceramic body (light grey, lower part of the figure) and the blue layer made of angular and rounded quartz grains (grey) and EB crystals (white) embedded in the glassy phase; the porosity appears black.
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Table 3 Average bulk chemical compositions (wt% of the oxides) of EB layers (for sample 1, 2 and 4B also the standard deviation is reported of the 9 measures) and of the glassy phase of the EB layers (the std. dev. of 5 measures on sample 3 are also reported). P2O5 in the range 0.5e1% was occasionally found in all the EB layers. Lead and tin were analysed but not detected (LLD SnO2 0.3%; PbO 0.2%). Sample
BE layer
3 1,2,4B
Green layer, mean Blue layer, mean [9] std dev
1 2 4B 3
Glassy phase Glassy phase Glassy phase Glassy phase [5] std dev
Na2O
MgO
Al2O3
SiO2
1.6 1.7 0.6
0.70 0.80 0.21
3.0 3.4 0.8
85.0 72.7 4.0
9.2 8.0 10.0 8.8 0.5
0.90 0.90 0.60 1.1 0.6
6.2 6.7 6.4 6.5 2.6
68.0 67.7 60.3 66.5 1.9
reaction between the two layers. In samples 1 and (more evident) 2, a layer 1e3 mm thick of compact and less porous earthenware material is observed at the interface (Fig. 6). SEM investigation indicates that the compact layer is generally formed by several layers 0.1e0.3 mm thick of more or less porous ceramic body material (Fig. 7a,b). No slip layers were observed at the EB-ceramic body interface of the analysed samples. The SEM observation of the cross section of samples 1 and 4B (not of samples 2 and 3) reveals at the interface the presence of a thin (0.1e0.4 mm), compact layer, which is less porous as compared to the ceramic body (Fig. 7 a). In sample 4B deposits of copper (white areas in Fig. 7b) are located between the compact layer and the ceramic body. There are probably tenorite crystals formed inside the porosity of the ceramic body by redeposition of evaporated copper during firing of the EB. The composition of the ceramic body at the EB interface was determined by scanning the electron beam on three areas (0.5 mm in length, 0.05 mm thickness) of each cross section and considering the average values. Significant differences in composition as compared to the ceramic body were observed only in sample 4B (see Table 2), with a lower silica content and larger contents of Na, K, Ca, Fe and Cu. No significant differences were found in the other samples. 3.4. Black crystals Black deposits of a well crystallized material can be observed on an area free of EB frit of the inner surface of sample 4B, as well as on the outer surface of this crucible fragment, and of that of sample 4A (Fig. 8). The X-ray microanalysis detected only copper and oxygen
SO3
0.20 0.40 0.30 0.53 0.06
Cl
K2O
CaO
TiO2
Fe2O3
CuO
0.20 0.33 0.14
1.10 1.23 0.35
3.0 9.0 2.4
0.25 0.37 0.20
1.5 1.6 0.8
3.7 8.9 3.2
0.93 0.80 1.70 0.55 0.04
3.3 3.6 4.6 2.7 0.6
1.8 1.7 2.7 2.5 0.3
0.29 0.15 0.08 0.31 0.03
1.9 3.3 3.3 2.1 0.4
7.2 6.7 10.0 8.5 2.5
in these crystals, thus confirming the presence of tenorite revealed by the XRD analysis (see above). 4. Discussion 4.1. Ceramic body of the crucibles The minero-petrographic analyses allow us to conclude that the clays used for the manufacture of the five crucible fragments were quite similar, very likely weakly illitic and slightly calcareous. This clay, probably quarried in the neighbourhood of Liternum, had a quartz-rich detritic fraction, to which slightly different tempers were added, a coarser one for samples 1, 3, 4A and 4B, and a finer one for sample 2. Such tempers were probably taken from local river sands containing a mixture of acidic lavas (andesites) and small amounts of sedimentary rocks (quartzarenites, chert, limestone) and their relative single mineral components (unpublished information from professor Alessio Langella, Sannio University, Benevento, Italy). This clay was likely fired at a temperature of about 850 C as indicated by the total decomposition of calcite (Maggetti, 1982) and the formation of a semi-vitrified matrix. The absence of mullite and cristobalite in the XRD spectra confirms a maximum firing temperature lower than 900 C. The formation of a thin low-porosity layer at the EB layerceramic body interface in samples 1 and 4B cannot be explained by the chemical composition of the ceramic body, which results to be similar in all the analysed samples (see Table 2). Nevertheless, it must be remembered that the ceramic body composition is affected by volatile batch components (mainly sodium sulphate and chloride and copper oxide), penetrated in the pores of the crucibles during firing. The higher soda content (Na2O 8.4%) and the lower silica content (SiO2 53%) of the ceramic body of sample 3 may be responsible for its higher compactness and lower porosity. However, in this sample these aspects could be also attributed to prolonged firing and/or dwelling at higher temperatures; this hypothesis is supported by the fact that an abundant glassy phase, the rounded quartz grains and only few EB crystals are the components of the greenish frit layer of this sample. 4.2. EB frit layer
Fig. 6. OM micrograph of the polished cross section of the EB frit-ceramic body interface in sample 1 (long side: 12 mm).
As mentioned above, Egyptian blue frit was produced from a mixture of quartz and lime (both probably present in a natural sand), copper and natron; the batch was similar to the one used for glass melting, but the proportions of the batch components were different (Pradell et al., 2006). The results of the analyses do not allow us to confirm the use of natron as a flux, because significant amounts of K, Mg and P (fingerprint elements to recognise the natron-type glass) entered the composition of the glassy phase
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Fig. 7. a,b e SEM micrograph of the polished cross sections of samples 1 (left) and 4 (right); the arrows indicate the interface layer spearing the ceramic body (lower part of the figures) and the EB layers made of quartz grains (grey), surrounded by EB crystals (white) and bonded together by a glassy phase (light grey); porosity appears dark grey (a) or black (b).
through feldspars and other components of the silica source. The high levels of Al, Fe and Ti suggest that natural sand was used in which quartz was mixed with other minerals such as observed in fluvial sands. As indicated by Pliny, a local source of silicaelime sand used for glassmaking could be the mouth of the Volturno river, near Cuma (Vallotto and Verit a, 2002), a few kilometres from Liternum. The mostly predominant angular shape of the quartz grains indicates the addition of sand crushed to fine powder. This observation is not in contrast with the use of fluvial sands; in fact, the use of crushed grains increases the quartz surface. This feature, and the control of the grain size were critical parameters for the synthesis of the blue pigment, for nucleation and growth of the cuprorivaite crystals occurs on the surface of the quartz grains. The composition of the EB frit layers of Liternum differs from the composition of EB frit layers from Memphis (Tite and Hatton, 2007). These differences are mainly related to the use of different silica sources. An interesting difference consists in the high temperature silica polymorphs detected by XRD: in the Memphis samples both cristobalite and tridymite were detected, while trydimite was not
Fig. 8. Deposits of tenorite black crystals (dark) on the inner surface of ceramic body 4, at the interface with the EB layer (porous material on the low-right side of the figure); (long side: 4.3 mm).
identified in any EB frit layer from Liternum, and an evident presence of cristobalite was found only in sample 4B (dubious traces in samples 1 and 3, absent in samples 2 and 4A). These results allow us to conclude that Liternum EB was fired at lower temperatures and/ or for a shorter time as compared to the Memphis EB. The chemical composition of the glassy phase of EB frit layers from Liternum differs significantly from the composition of EB frits from other centres of the Roman Empire. The glassy phase has an unusual composition, with low CaO and relatively high MgO levels. The high amounts of K2O and Al2O3 confirm the presence of K-feldspars in the used silica source. A very good similarity is found only for the composition of the glassy phase of EB pigments (wall painting) of the 1st c. AD from Pompei (Tite and Hatton, 2007; Tite and Shortland, 2008, Tab. 8.9, pp.182e183) and two mosaic EB tesserae from Rome. The concordance with the Pompeii samples is almost perfect; the fingerprinting characteristic of the EB frits from Liternum and Pompeii is the unusually high alumina content (Al2O3 6.2e8.9%). Another similarity is that neither contain measurable tin and lead oxides, in contrast to many other Roman EB samples (included the two EB mosaic tesserae from Rome). An important heterogeneity in composition was ascertained in Liternum by analysing EB glassy phase in different areas in the same sample. This heterogeneity could be attributed to a too short reaction time, or most probably to a low firing temperature. Unfortunately, the homogeneity of the glassy phase was not investigated by other authors studying EB Roman samples and, therefore, it is impossible to ascertain whether this is a specific phenomenon of Liternum EB frit production or was a common characteristic of the EB glassy phase. The presence on the surface of the ceramic body deposits of well crystallized tenorite (CuO) crystals, sometimes covered by the EB layer, testifies to an oxidizing firing and the addition of copper in the form of a metallic (or oxide) powder. The presence of tenorite also inside the pores of the ceramic body and on the external surface of two crucibles suggests that copper did sublimate at the beginning of the firing of the raw materials, condensing and depositing as tenorite before the formation of the EB layer. It is very likely that one of the possible causes of the pale colour often observed in EB is the shortage of copper in the batch because of its sublimation, Cu being a key component of the blue crystals. Finally, the absence of lead and tin in the EB layers and in the glassy phase demonstrates that copper was added in form of pure oxide and not
/ Journal of Archaeological Science 53 (2015) 578e585 L. Lazzarini, M. Verita
as a metal alloy (bronze), as verified in the analysis of other historical EB samples (Ingo et al., 2013). 5. Conclusions The ancient sources of Vitruvius and Plinii, as well recent archaeological excavations clearly indicate that an important production of EB took place in the 1st century AD in the Phlegrean Fields (Cuma, Liternum and Puteoli), taking advantage of the presence of local silica-lime sand and the vicinity of the harbour of Pozzuoli from which all kinds of imported goods arrived from all the provinces of the empire. The analyses of 1st c. AD fragments of crucibles used for firing EB frit excavated at Liternum (Naples, Italy) allow some conclusions to be drawn about the firing technology of this important blue pigment in Roman time. 5.1. EB-firing technology The crucibles were made with local raw materials in direct connection with the igneous-sedimentary formations of the hills and plane around Liternum. They were fired at temperatures above 850 C, as testified by the total decomposition of the calcite originally present in the clay. The poor EB frit-ceramic body interaction indicates that the firing of the batch was performed at a relatively low temperature, probably around 850 C. The heterogeneity of the glassy phase resulting from the chemical analysis, confirms that the firing temperature of the batch should have been not too high, so as to assure the formation of a viscous liquid phase in which the blue crystals of cuprorivaite could easily nucleate and grow. Also the absence of mullite in the ceramic body of the crucibles and the presence of cristobalite in the EB layers but not in the ceramic body confirm a low firing temperature. In fact, it is known that cristobalite forms in a glassy phase surrounding the quartz crystals, also at temperatures lower than those expected. On the other hand, the abundant glassy phase and the rare EB crystals in sample 3 confirm that firing at higher temperatures with formation of a more fluid glassy phase do not favour the formation of the EB crystals. These conclusions are consistent with the laboratory investigation of (Bianchetti et al., 2000) revealing that the maximum growth of cuprorivaite occurs around 850 C. 5.2. Colour of the EB frit The analyses allow some conclusions to be drawn about the colour of the EB frit. As observed by several authors, the intensity of the colour of this pigment is not uniform and frequently it appears quite whitened. The amount and size of cuprorivaite crystals and of residual quartz grains, as well as the amount of green glassy phase should be the most important parameters influencing the final blue pigment hue. These parameters were influenced by the composition of the fired batch; the evaporation of copper (added in the form of copper scale: no tin and lead were detected) attested in this study (that could lead to a shortage of this element in the glassy phase and therefore to a lower amount of cuprorivaite crystals) could be the most critical parameter in this sense. The total surface of the residual white quartz crystals had probably a similar effect and probably for this reason crushed quartz of relatively small grain size was used in the EB batch. Finally, a too high firing temperature would lead to a too much fluid glassy phase that would hinder the nucleation and growth of the blue cuprorivaite crystals.
585
Acknowledgements The Soprintendenza Archeologica delle Province di Napoli e Caserta is kindly acknowledged for supplying the samples for analyses. References Accorsi, G., Verri, G., Bolognesi, M., Armaeroli, N., Clementi, C., Milliani, C., Romani, A., 2009. The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue). Chem. Comm. 3392e3394. Augusti, S., 1967. I Colouri Pompeiani, Roma. Bianchetti, P., Talarico, F., Vigliano, M.G., Fuad Ali, M., 2000. Production and characterization of Egyptian blue and Egyptian green frit. J. Cult. Herit. 1 (2), 179e188. Boschetti, C., Leonelli, C., Corradi, A., 2012. The earliest wall mosaics and the origin of Roman glass in Italy: archaeological considerations for an archaeometric study. In: Ignatiadou, Despina, Antonaras, Anastassios (Eds.), Proceedings of the AIHV 18th Congress, pp. 139e144. Thessaloniki. gyptien a Cumes. In: Brun, J.-P. Caputo, P., Cavassa, L., 2009. La fabrication du bleu e langes offerts a Maria Fran(Ed.), Artisanats antiques d'Italie et de Gaule: Me rard, vol. 32. arche ologie de cesca Buonaiuto, Collection du Centre Jean Be l'artisanat antique, 2, Naples, pp. 169e179. Daniels, V., Stacey, R., Middleton, A., 2004. The blackening of paint containing Egyptian Blue. Stud. Conserv. 49 (4), 217e230. De Caro, S., Greco, A., 1981. Campania, Guide Archeologiche Laterza. Bari, pp. 90e91. Gaetani, M.C., Santamaria, U., Seccaroni, C., 2004. The use of Egyptian blue and Lapis Lazuli in the Middle Ages. Stud. Conserv. 49 (1), 13e22. Gargiulo, P., 1998. Contenitori con depositi di colore blu egiziano e officine vetrarie nell'area dell'antica Liternum e nel territorio flegreo. Aspetti tecnologici e prospettive di studio. In: Il vetro dall'antichit a all'et a contemporanea: aspetti tecnologici, funzionali e commerciali, 2e Giornate Nazionali di Studio AIHV, December 1996, Milan, pp. 61e65. Gargiulo, P., 1997. Su un'officina per la pasta vitrea a Liternum. Noi Chim. Anno XXIV 2, 11e14. Hatton, G.D., Shortland, A.J., Tite, M.S., 2008. The production technology of Egyptian blue and green frits from second millennium BC Egypt and Mesopotamia. J. Archaeol. Sci. 35, 1591e1604. lou, Di Carlo, G., Batmat, A., De Caro, T., Riccucci, C., Parisi, E.I., Ingo, G.M., Çilingirog Faraldi, F., 2013. Egyptian Blue cakes from the Ayanis fortress (Eastern Anatolia, Turkey): micro-chemical and -structural investigation for the identification of manufacturing process and provenance. J. Archaeol. Sci. 40, 4283e4290. Laurie, A.P., Bimson, M., Cowell, M.R., 1914. Technological examination of Egyptian Blue. Proc. Rot. Soc. A 89, 19e29. Lazzarini, L., 1982. The discovery of Egyptian Blue in a Roman fresco of the Medieval period (ninth century A.D.). Stud. Conserv. 27 (2), 84e86. Maggetti, M., 1982. Phase analysis and its significance for technology and origin. In: Archaeological Ceramics. Smithsonian Institution Press, Washington, D.C, pp. 121e133. Maiuri, A., 1958. I Campi Flegrei, Roma. Morgan, R.H., 1960. Vitruvius: the Ten Books of Architecture (7, 11, 1). Dover Publications, Mineola, N.Y. Nicholson, P.T., 2003. New excavations at a Ptolemaic-Roman faience factory at s de l'Association Internationale Memphis, Egypt. In: Annales du 15e Congre pour l'Histoire du Verre, AIHV, Nottingham, pp. 49e52. Petrie, W.M.F., 1909. Memphis I. British School of Archaeology in Egypt, London. Platania, R., Armiento, A., Attanasio, D., Cinquegrana de Divitiis, L., 1998. Caratterizzazione chimico fisica di un ‘Blu Egizio’ proveniente da Liternum. In: D'Amico, C., Albore Livadie, C. (Eds.), Le Scienze della Terra e l'Archeometria, atti della Quarta giornata di studio Il futuro della memoria (Napoli, febbraio 1997). Napoli, pp. 323e328. Pradell, T., Salvado, N., Hatton, G.D., Tite, M.S., 2006. Physical processes involved in production of the ancient pigment, Egyptian blue. J. Am. Ceram. Soc. 89, 1426e1431. Riederer, J., 1997. Egyptian blue. In: Fitzhugh, E. (Ed.), Artist's Pigments. A Handbook of Their History and Characteristics, vol. 3, pp. 23e45. Oxford. Santopadre, P., Tamanti, G., Bianchetti, P., Sidoti, G., 2011. Studio delle tracce di pigmenti azzurri in due affreschi della chiesa inferiore di San Clemente a Roma. Boll. ICR 22e23, 85e95. Schippa, G., Torraca, G., 1957. Contributo alla conoscenza del “Blu Egiziano”. Boll. ICR 31e32, 97e107. Tite, M.S., Hatton, G.D., 2007. The production technology of, and trade in, Egyptian blue pigment in the Roman world. In: Gosden, C., Hamerow, H., de Jersey, P., Lock, G. (Eds.), Communities and Connections: Essays in Honour of Barry Cunliffe. Oxford University Press, Oxford, pp. 75e92. Tite, M.S., Shortland, A.J., 2008. Production Technology of Faience and Related Early Vitreous Materials. In: Monograph 72. Oxford University School of Archaeology, Oxford, pp. 147e184. Vallotto, M., Verit a, M., 2002. Glasses from Pompeii and Herculaneum and the sands of the rivers Belus and Volturno. In: Renn, J., Castagnetti, G. (Eds.), Homo Faber: Studies on Nature, Technology and Science of the Time of Pompei. L'Erma di Bretschneider, Rome, pp. 63e73. Verit a, M., Basso, R., Wypyski, M.T., Koestler, R., 1994. X-ray microanalysis of ancient glassy materials: a comparative study of wavelength dispersive and energy dispersive techniques. Archaeometry 36, 241e251.
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First evidence for 1st century AD production of Egyptian blue frit in Roman Italy * Lorenzo Lazzarini, Marco Verita IUAV di Venezia, San Polo 2468/B, 30125 Venezia, Italy Laboratorio di Analisi dei Materiali Antichi, Universita
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 June 2014 Received in revised form 30 October 2014 Accepted 3 November 2014 Available online 12 November 2014
The considerable amount of research carried out up to present time in an attempt to understand the production technique of Egyptian blue frit is based on analyses of the ancient pigment and its laboratory synthesis. The fortunate finding in Roman Liternum of 1st century AD fragments of ceramic crucibles with adhering remains of a blue pigment allowed laboratory analyses (OM, XRD, SEMeEDS) to be performed on original production debris. The results confirm that the crucibles had been used for the synthesis of Egyptian blue frit and throw new light on the controversial issue of the Roman manufacturing technique of Egyptian blue frit. The results of the analyses indicate that the Egyptian blue frit was produced with a mixture of a local source of silicaelime sand, a fluxer (not identified) and copper scale; the crucibles were also made with local raw materials. The firing of the batch was performed at a relatively low temperature, probably around 850 C and firing at higher temperatures would have hindered the formation of the cuprorivaite crystals. The analyses suggest that the evaporation of copper during firing of the batch attested in this study is one of the most critical parameters influencing the final blue pigment hue. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Liternum Egyptian blue Crucibles Pigment SEMeEDS XRD PM
1. Introduction Liternum, presently Literno (Naples, Italy), was a small Roman town 8 km north of the more famous Greek and Roman town of Cuma, one of the oldest Greek colonies in Southern Italy. Liternum was founded in 194 B.C. by 500 Roman veterans as a colonia maritima, with a defensive role. Placed by the sea on a marshy area near the Lake Patria, it always remained a small settlement playing a negligible role in Roman history, and is remembered only for the presence of the villa and the tomb of Scipio the African. It reached a certain importance at the end of the first century A.D. when it was crossed by the Via Domitiana, the main road of ancient Campania. Ruins of a Forum, Basilica, Capitolium, Theatre and Amphitheatre are still visible on the site (Maiuri, 1958; De Caro and Greco, 1981). Archaeological excavations on the theatre started in 1932, lasted for five years and were then abandoned, being resumed in the nineties. In 1996, near the town walls on the left shore of the Lake Patria a large amount of fragments of ceramic pots (clearly to be identified
* Corresponding author. Tel.: þ39 041 2571459; fax: þ39 041 1434. E-mail addresses: [email protected] (L. Lazzarini), [email protected] (M. Verit a). http://dx.doi.org/10.1016/j.jas.2014.11.004 0305-4403/© 2014 Elsevier Ltd. All rights reserved.
as kiln wastes) and of crucibles, some with attached blue incrustations (Fig. 1) dated not later than 1st c. AD was found together with a number of small spheres of blue colour (Gargiulo, 1997, 1998), very similar to the ones found at Pompeii (Augusti, 1967) and in other Roman towns, for example at Lugdunum (modern Lyon, France). The hypothesis made by the archaeologists was that the area was used for depositing waste pieces of one or more kilns (the kilns have not been found so far) producing pottery and Egyptian blue. For the importance and rarity of this latter find, it was decided to submit samples of the crucibles and of its residues to laboratory analysis with the purpose of collecting information on the pigment and its production technology. Egyptian blue (indicated hereafter as EB) is the most important blue pigment of antiquity. Manufactured for the first time around 2000 BC in Mesopotamia, it was first identified by Laurie et al. (1914) in Egypt, hence its name. This pigment was known in Minoan and Mycenaean times when it was extensively used in the wall paintings of Knossos, Pylos and Thera, and continued to be largely employed in Greek times when it was called kyanos, in the frescoes of the famous Macedonian tombs, in polychrome funerary stelae, for the decoration of temple roofs, precious pottery, etc. The Romans increased its use quantitatively, sometimes extending it to
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Fig. 1. The examined fragments of crucibles with incrustations of blue pigment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mosaic tesserae; it was then named caeruleum, and was considered one of the “colores floridi”. It cannot be excluded that the production of this pigment ended at the collapse of the Roman Empire, although its use continued in Rome until at least the 11th c. AD (Lazzarini, 1982; Santopadre et al., 2011). Other analyses seem to point to an even later use at Genoa and Treviso (Italy) during the 13th c. (Gaetani et al., 2004). The great success of Egyptian blue throughout the centuries is certainly due to its beautiful blue hue, to the rather cheap manufacture and high stability in all techniques, although its poor hiding power and transparency may favour the blackening or browning of paints produced with the addition of organic media such as the gum arabic used in Pharaonic times (Daniels et al., 2004).
1.1. The manufacture of Egyptian blue in antiquity Egyptian blue is a multicomponent material whose colour is due to the presence of calciumecopper tetrasilicate crystals (cuprorivaite: CaCuSi4O10) produced by firing a batch (mixture) of quartz, lime, copper and a small amount of a soda flux. Vitruvius (7, Ch. 11, 1)1 describes the way of producing Egyptian blue in Pozzuoli, near Pompei, during the first century B.C., by adding copper to a ground mixture of sand (harena) and natron2 (flos nitri). This mixture was shaped into small balls, dried in air and fired in earthenware pots. Vitruvius's text does not mention any addition of limestone or lime. This is not a surprise: the same situation can be observed for the production of glass in Roman time. The source for calcium was natural sand, like the one
1 Vitruvius 7, cap. 11, 1 (Morgan, 1960): “Blue was first manufactured at Alexandria, and afterwards by Vestorius at Pozzuoli. The method of making it, and the nature of the ingredients, merit our attention. Harena (sand) is ground with flos nitri (natron), till the mixture is as fine as flour, to which coarse filings of Cyprian copper are added, so as to make a paste when moistened with water; this is rolled into balls with the hand, and dried. The balls are then put into an earthen vessel, and that is placed in a furnace. Thus the copper and sand heating together by the intensity of the fire, impart to each other their different qualities, and thereby acquire their blue colour”. 2 A mixture of natural evaporate minerals (mainly sodium carbonatebicarbonate, secondarily chlorides and sulphates).
extracted from the river Volturnus (north of Naples), not far from Pompei and Liternum, indicated by Pliny for the manufacture of glass (Pliny the Elder, Naturalis Historia book 36, 66); in this sand silica and lime are naturally mixed in the right ratio to make glass , 2002).3 (Vallotto and Verita EB has always been of interest to archaeologists and archaeometrists since the beginning of the last century. The latter were also interested in understanding the production technique reproducing it experimentally (for instance: Schippa and Torraca, 1957; Riederer, 1997; Hatton et al., 2008). At this time a production site for EB dating to the Roman period has been found at Memphis in Egypt (Petrie, 1909; Nicholson, 2003). Fragments of ceramic vessels, belonging to crucibles either cylindrical or globular in shape, were lined with a white slip (up to 2e3 mm in thickness, lime based) to which a layer of Egyptian blue frit (up to about 9 mm in thickness) adhered. This frit contains less glassy phase and more abundant quartz particles than the EB balls from the same site and, consequently, it has a much more open microstructure. The cuprorivaite crystals in the frit layers are also smaller (up to 40 mm in length), whereas the remaining quartz particles (also cristobalite and tridymite were identified) tend to be larger (Tite and Hatton, 2007). Chemical analysis by SEMeEDS of the ceramic vessels allows concluding that they were made of a non-calcareous, iron rich Nile silt. The interaction occurred between the ceramic vessel and the EB layer during preparation of the EB was not investigated. Chemical analyses of the Memphis EB balls and EB layers are reported and compared with analyses of EB samples from Delos, Pompeii, Rome, England and Malta. The relatively high alumina and iron contents and the good correlation between the potash and alumina contents suggest the use for the preparation of EB pigment of a mixture of silicaelime sand, copper oxide scale or leaded copper and a sodium source. For EB from Pompei, Rome, England and Malta the high amount of potash introduced along with the feldspar in the sand does not allow distinguishing between natron and soda plant as the sodium
3 The importance of lime for glassmaking will be fully understood only since the end of the 17th century and after this date lime will be added to the glassmaking batch as a separate component.
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source. Pompei and Rome samples are recognised by their high potash and alumina (Pompeii also high magnesia) contents as compared to the EB samples found in Memphis and in the other sites. The authors conclude that the production debris excavated in Memphis is consistent with the description of the EB production given by Vitruvius. An EB production is also attested at Cuma and Puteoli (Pozzuoli), two towns very close to Liternum, where ceramic fragments with incrustations of EB dating to the 1st c. AD have been found. This discovery made it possible to reconstruct the cylindrical shape of the crucibles, whose diameter varies between 30 and 50 cm. An intact specimen about 50 cm in height used as a burial urn has been excavated at Cuma (Caputo and Cavassa, 2009; Gargiulo, 1998). A relationship between the findings at Cuma and Liternum and the production of EB blue tesserae in Roman wall mosaics of the 1st century AD is suggested in (Boschetti et al., 2012). The blue incrustation of one of the ceramic fragments found in Liternum was investigated with several analytical techniques by Platania et al. (1998). The authors concluded that the blue incrustation is the result of the process of preparation of Egyptian blue fired on the surface of a ceramic crucible. In this paper five fragments of crucibles from Liternum with abundant incrustations of a blue porous frit on their internal surface are investigated by several analytical techniques in order to get information about the production technology of EB and to verify similarities with published compositions of this pigment. 2. Materials and methods 2.1. Samples Five fragments of crucibles with consistent residues of a blue porous frit on their inner surface (Fig. 1) were kindly given to the authors by the local archaeological suprintendency (Soprintendenza Archeologica delle Province di Napoli e Caserta, Dr. Stefano De Caro) for archaeometric analysis. The five fragments before laboratory examination were described as follows for their macroscopic aspect: Sample 1: yellowish ceramic body 12 mm thick containing few brown inclusions, with a 3e5 mm thick deposit of a porous blue pigment; Sample 2: as for sample 1, but the body and the deposit are slightly thinner, 10 mm and 3 mm thick, respectively, and the pigment is lighter in colour; Sample 3: the fragment is probably broken near its bottom-part having a thickness variable from 10 to 22 mm; the colour of the pottery is brownish and shows a vitrified body with several shrinkage cracks due to a higher firing temperature. It is covered inside by a rather thin (1e2 mm) greenish-blue glassy porous deposit, with lumps of the same material on the outside surface; Sample 4A: brownish slightly vitrified ceramic body 10 mm thick, with a 1e3 mm thick dark blue deposit. Presence of diffused bluish and black deposit on the outside of the potsherd; this sample was investigated only by optical microscopy and XRD; Sample 4B: the ceramic body is yellowish, with brown inclusions as in sample 1; some areas are covered by thin blue pigment deposits alternating with others with black shining crystals. The latter are also diffusely present on the outside of the potsherd. The small size of the ceramic fragments does not allow the shape and size of the crucibles to be recognized. A reconstruction of the crucibles with the Cuma fragments is described in (Caputo and Cavassa, 2009).
2.2. Analytical methods The ceramic bodies and the adhering residues of pigments were sampled for the preparation of powders for X-Ray diffraction analysis (Cu, Ka/Ni at 40 KV, 20 mA, Panalitical Empyrean). Small chips including the ceramic bodies with their pigment residue were cut with a thin diamond wheel to be prepared in thin sections and then studied under a polarizing optical microscope in transmitted/ reflected light (PM). Other chips were embedded in an acrylic resin and prepared in polished cross sections down to 1 mm grain size diamond pastes. After optical microscopy observation (OM) in reflected light, samples were carbon coated and examined under scanning electron microscopy (Philips XL 30) coupled with an energy-dispersive X-ray spectrometer (Edax) for chemical analyses (analytical conditions: 25 kV, 1 nA). SEM observation was made in backscattered electrons mode (SEMeBSE) and allowed distinguishing phases with different chemical composition by different grey hues. Average quantitative chemical composition of the ceramic body and of the EB layer were obtained by X-ray microanalysis (SEMeEDS) by scanning the electron beam on large areas (for the ceramic body, 1 mm 0.3 mm areas, as far as possible from the interface with the EB layer). For the analysis of the glassy phase the electron beam was scanned on areas as large as possible, following et al., 1994) to avoid alkali the general conditions reported in (Verita depletion during the analysis. The averaged values of three to five analyses in different areas (the routine number of three analyses was increased when large deviations in composition from point to point were observed) were considered.
3. Results 3.1. Ceramic body of the crucibles 3.1.1. Minero-petrographic analyses The XRD analyses of all the ceramic bodies are reported in Table 1. They reveal the ubiquitous presence of considerable, although variable amounts of quartz and plagioclase, and the presence of pyroxene. The microscopic study of the thin sections showed for all the ceramic bodies a very porous (porosity measured by visual evaluation is around 30%), semi-isotropic/isotropic matrix containing some rounded, small vitrified ARF (Argillaceous Rock Fragments) with abundant sandy skeleton. The latter exhibits a bi-modal grain size formed by a fine detritic fraction belonging to the original clay, and a relatively abundant added temper. The detritic fraction is composed of very small (0.02e0.05 mm) angular quartz crystals, with a small amount of twinned plagioclase and K-feldspar; very
Table 1 Results of the X-Ray powder diffraction analysis: Qz: quartz; Pl: plagioclase; P: pyroxene; Cu: cuprorivaite; Cr: cristobalite; Te: tenorite; (þþþ: very abundant; þþ: abundant; þ: present; ±: trace).
Ceramic Body
EB layer
Black Cryst.
Sample N.
Qz
Pl
P
1 2 3 4A 4B 1 2 3 green 4A 4B 4B
þþ þþ þ þþþ þþþ þ
þþþ þþ þþþ þþ þþ ± ±
þþ þ þ
þ þ þ þ
Cu
Cr
þþþ þþþ þþþ þþþ þþ
±?
Te
þ
±? þþ þþþ
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Table 2 Average chemical compositions (wt% of the oxides) of the ceramic body and of the layer at the interface with the EB frit (sample 4). Ceramic body
Na2O
MgO
Al2O3
SiO2
1 2 3 4B 4B interface
6.5 6.5 8.5 6.0 8.0
2.3 2.2 2.0 2.0 1.9
13.5 13.3 15.5 14.0 13.0
61.0 55.0 53.0 61.5 51.0
P2O5 1.3 0.4
abundant are opaque and semi-opaque minerals to be identified as iron-ores (haematite, ilmenite, magnetite). The added temper is made of larger single clasts, mostly angular, sometimes subrounded, with dimensions ranging from 0.2 to 1.1 mm. They are formed by (in increasing abundancy): - K-feldspar sometimes identified as microcline, in rare prismatic crystals with their typical twinning, sometimes slightly opacified by caolinisation; - clinopyroxene, mostly in subrounded individuals, rarely twinned, with the optical characteristics of diopside: because of its size (several micrometres in length) a neoformation has to be excluded for this mineral; - rock fragments, mostly of acidic lavas showing a groundmass with plagioclase microliths and abundant iron ore (haematite and some magnetite), and plagioclase and clinopyroxene phenocrysts; - quartz, in angular mostly single (rarely polycrystalline and subrounded) individuals; - plagioclase, in prismatic, often twinned crystals of sodic composition, sometimes slightly opacified by caolinisation. Some remarkable differences can be observed in sample 2, i.e. the presence in the skeleton of isoriented illite needles (indicating the use of a turning wheel for the manufacture of this crucible) in the matrix, a small amount of biotite flakes pervasively altered into haematite, and of clasts of a quartzearenite cemented by limoniteehaematite. In sample 3 the composition is quite similar to that of sample 2, with a definitely isotropic matrix indicating a somewhat higher firing temperature. Sample 4B differs from the others for the presence of several calcite “ghosts” and of a few clasts of chert. 3.1.2. SEM microscopy and X-ray microanalysis The chemical compositions of the ceramic bodies are reported in Table 2. The results are similar in four samples (sample 4A excluded) examined: silica and alumina are the main components and relatively high flux content (Na2O 6.0e6.5%, higher in sample 3: 8.5%; K2O 1.4e1.6%, higher in sample 2: 2.6%) alkaline earths (CaO 6.8e8%, higher in sample 2: 9.7%; MgO 2e2.3%), iron (Fe2O3 4.5e5.4%) and titanium (TiO2 0.7e1.0%) are present. In samples 2 and 3 a certain amount of phosphorous and sulphur was also detected. The copper, sulphur and chlorine contents (and probably also part of the sodium and calcium contents) are related to migration of volatile batch components deeply into the pores of the ceramic body during firing of the EB batch. 3.2. EB frit layer 3.2.1. Minero-petrographic analyses The results obtained from the XRD analysis of the coloured incrustations are reported in Table 1. They indicate the rather obvious presence in all samples of cuprorivaite (EB), of quartz in most samples (1, 3, 4A and 4B), of traces of plagioclase in samples 1 and 2, and of dubious traces of cristobalite in samples 1 and 3, a mineral
SO3 2.0 3.0 1.5
Cl
K2O
CaO
TiO2
Fe2O3
CuO
0.20 0.40 0.20 0.20 0.30
1.60 2.60 1.60 1.40 1.75
6.8 9.7 8.0 7.0 10.0
0.75 1.00 0.77 0.70 0.70
4.5 5.0 4.8 5.4 6.5
2.8 1.0 2.3 1.8 5.3
definitely present in sample 4A. The black crystalline deposit on the sample 4B (inside and outside) is made of tenorite (CuO). The quite thick (max. 5 mm) deep-blue incrustation on samples 1 and 2 examined under the microscope in plane-polarised light shows a sort of honeycomb fabric consisting of a large amount of pores of irregular and globular shape (Fig. 2), and a net of blue needles and small prisms of EB with a maximum grain size (MGS) of 0.30 mm. A brownish glassy phase (Fig. 2) glues the crystals together and the EB layer to the ceramic body. Under crossed polars the EB shows its typical bluish strong pleochroism and high polychrome interference colours,4 and is often encircling quartz clasts. In fact, sand clasts with a MGS of 0.08e0.4 mm are scattered in the glass and EB, mostly subangular/subrounded in shape and consisting of quartz, plagioclase, acidic lava (Fig. 3) with abundant plagioclase microliths, rare pervasively caolinised K-feldspars, clinopyroxenes and opaque minerals. Similar observations were made on the blue incrustation of sample 4A, while that on sample 3 is quite different, for it contains a very small amount of EB crystals with MGS of 0.10 mm embedded in an abundant glassy phase: this feature explains the overall greenish colour of this incrustation. Abundant unmelted sand clasts of rounded quartz and rare Kfeldspars are also present.
3.2.2. SEM microscopy and X-ray microanalysis The thick EB layer of samples 1 and 2 consists of clusters of elongate EB crystals (white in SEM micrographs) surrounding partially reacted angular quartz grains bonded together by a scant glassy phase leaving abundant voids and forming a porous, brittle layer (Fig. 4a, b). Few grains of unreacted feldspars can also be observed in both samples. In sample 3 round quartz grains prevail, and only rare EB crystals were detected; the glassy phase is relatively abundant forming strong bonds with the underlying compact ceramic body (Fig. 5). The average chemical compositions of the EB blue layers, determined by X-ray microanalysis scanning the electron beam on three different large areas, are given in Table 3. No lead and tin were detected (estimated lower limits of detection: PbO 0.2%; SnO2 0.3%), thus excluding the use of a bronze scale as a source of copper. Compositions are quite similar in samples 1, 2 and 4B (the mean and std deviation of 9 measures for sample 4B are given in Table 3), while in sample 3 (greenish layer) the copper (CuO 3.7% instead of 8.9%) and lime contents (CaO 3% instead of 9%) are lower and silica is higher (SiO2 85% instead of 72.7%), indicating a greater quartz grains content. The EB crystals surround the quartz grains; they are up to 100 mm in length (sample 4B) but generally up to 50 mm in length. Their mean chemical composition is similar in samples 1, 2 and 4B (no EB crystals were found in the polished section of sample 3): SiO2 65.5%; CaO 14%, CuO 20%; Fe2O3 0.5%. These values match well
4 The optical properties of EB are so peculiar under the polarizing microscope as to allow this pigment to be easily identified. Its exceptional near-infrared luminescence also allows it to be identified in visible light very quickly and definitely even if present in traces (Accorsi et al., 2009).
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Fig. 2. Sample 1, PM micrograph of a thin section of the blue incrustation showing its high porosity, the brown glass glueing the incrustation to the ceramic body and the EB particles together. N//(long side: 1.03 mm). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
the theoretical stoichiometric composition of cuprorivaite, except for the presence of iron impurities. Angular quartz crystals (SiO2 100%) up to 0.8 mm in length (in sample 4B the grain length is less than 0.3 mm) and less abundant rounded grains are generally observed (in sample 3 rounded grains prevail). Rare grains of feldspars and of a mineral of average composition: SiO2 24%; Al2O3 21%; Fe2O3 28%; CuO 5%; Na2O 7.5%; MgO 7.5%; CaO 5%, were detected in samples 1, 2 and 4B. Small amounts of glassy phase were observed in samples 1, 2 and 4B, while larger amounts were observed in sample 3. The chemical compositions of the glassy phases (mean of three to five analyses in different areas) are given in Table 3. They are of a sodae(lime)esilica type with a fairly high alumina (Al2O3 6.5%) and potassium (K2O 2.7e4.6%) and low calcium (CaO 1.7e2.7%) contents. Traces of phosphorous (P2O5 0.5e1.0%) were also detected in some of the measurements in all the samples. By analysing different glassy areas of a same EB layer, an important heterogeneity of this phase was detected (see std deviation calculated for five measurements in sample 3, Table 3). These data suggest a nonequilibrium condition in which some batch components did not
Fig. 3. As for Fig. 2, detail of the incrustation in cross-polarised light showing two clasts of acidic lavas (one top left rich in plagioclase microliths/phenocrysts).
Fig. 4. a,b e Sample 1, SEM micrographs (low and high magnifications) of the polished cross section of the EB layer: quartz grains (grey) are surrounded by EB crystals (white) and bonded together by a glassy phase (light grey); porosity appears dark grey.
have sufficient reaction time (or a sufficiently high temperature) to melt completely and form a homogeneous glassy phase.
3.3. EB frit layer/ceramic body interface The EB layer/ceramic body interface is fairly clear, without any secondary phases (neocrystallizations) that could indicate a
Fig. 5. SEM micrograph of the polished cross section of sample 3 showing the compact ceramic body (light grey, lower part of the figure) and the blue layer made of angular and rounded quartz grains (grey) and EB crystals (white) embedded in the glassy phase; the porosity appears black.
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Table 3 Average bulk chemical compositions (wt% of the oxides) of EB layers (for sample 1, 2 and 4B also the standard deviation is reported of the 9 measures) and of the glassy phase of the EB layers (the std. dev. of 5 measures on sample 3 are also reported). P2O5 in the range 0.5e1% was occasionally found in all the EB layers. Lead and tin were analysed but not detected (LLD SnO2 0.3%; PbO 0.2%). Sample
BE layer
3 1,2,4B
Green layer, mean Blue layer, mean [9] std dev
1 2 4B 3
Glassy phase Glassy phase Glassy phase Glassy phase [5] std dev
Na2O
MgO
Al2O3
SiO2
1.6 1.7 0.6
0.70 0.80 0.21
3.0 3.4 0.8
85.0 72.7 4.0
9.2 8.0 10.0 8.8 0.5
0.90 0.90 0.60 1.1 0.6
6.2 6.7 6.4 6.5 2.6
68.0 67.7 60.3 66.5 1.9
reaction between the two layers. In samples 1 and (more evident) 2, a layer 1e3 mm thick of compact and less porous earthenware material is observed at the interface (Fig. 6). SEM investigation indicates that the compact layer is generally formed by several layers 0.1e0.3 mm thick of more or less porous ceramic body material (Fig. 7a,b). No slip layers were observed at the EB-ceramic body interface of the analysed samples. The SEM observation of the cross section of samples 1 and 4B (not of samples 2 and 3) reveals at the interface the presence of a thin (0.1e0.4 mm), compact layer, which is less porous as compared to the ceramic body (Fig. 7 a). In sample 4B deposits of copper (white areas in Fig. 7b) are located between the compact layer and the ceramic body. There are probably tenorite crystals formed inside the porosity of the ceramic body by redeposition of evaporated copper during firing of the EB. The composition of the ceramic body at the EB interface was determined by scanning the electron beam on three areas (0.5 mm in length, 0.05 mm thickness) of each cross section and considering the average values. Significant differences in composition as compared to the ceramic body were observed only in sample 4B (see Table 2), with a lower silica content and larger contents of Na, K, Ca, Fe and Cu. No significant differences were found in the other samples. 3.4. Black crystals Black deposits of a well crystallized material can be observed on an area free of EB frit of the inner surface of sample 4B, as well as on the outer surface of this crucible fragment, and of that of sample 4A (Fig. 8). The X-ray microanalysis detected only copper and oxygen
SO3
0.20 0.40 0.30 0.53 0.06
Cl
K2O
CaO
TiO2
Fe2O3
CuO
0.20 0.33 0.14
1.10 1.23 0.35
3.0 9.0 2.4
0.25 0.37 0.20
1.5 1.6 0.8
3.7 8.9 3.2
0.93 0.80 1.70 0.55 0.04
3.3 3.6 4.6 2.7 0.6
1.8 1.7 2.7 2.5 0.3
0.29 0.15 0.08 0.31 0.03
1.9 3.3 3.3 2.1 0.4
7.2 6.7 10.0 8.5 2.5
in these crystals, thus confirming the presence of tenorite revealed by the XRD analysis (see above). 4. Discussion 4.1. Ceramic body of the crucibles The minero-petrographic analyses allow us to conclude that the clays used for the manufacture of the five crucible fragments were quite similar, very likely weakly illitic and slightly calcareous. This clay, probably quarried in the neighbourhood of Liternum, had a quartz-rich detritic fraction, to which slightly different tempers were added, a coarser one for samples 1, 3, 4A and 4B, and a finer one for sample 2. Such tempers were probably taken from local river sands containing a mixture of acidic lavas (andesites) and small amounts of sedimentary rocks (quartzarenites, chert, limestone) and their relative single mineral components (unpublished information from professor Alessio Langella, Sannio University, Benevento, Italy). This clay was likely fired at a temperature of about 850 C as indicated by the total decomposition of calcite (Maggetti, 1982) and the formation of a semi-vitrified matrix. The absence of mullite and cristobalite in the XRD spectra confirms a maximum firing temperature lower than 900 C. The formation of a thin low-porosity layer at the EB layerceramic body interface in samples 1 and 4B cannot be explained by the chemical composition of the ceramic body, which results to be similar in all the analysed samples (see Table 2). Nevertheless, it must be remembered that the ceramic body composition is affected by volatile batch components (mainly sodium sulphate and chloride and copper oxide), penetrated in the pores of the crucibles during firing. The higher soda content (Na2O 8.4%) and the lower silica content (SiO2 53%) of the ceramic body of sample 3 may be responsible for its higher compactness and lower porosity. However, in this sample these aspects could be also attributed to prolonged firing and/or dwelling at higher temperatures; this hypothesis is supported by the fact that an abundant glassy phase, the rounded quartz grains and only few EB crystals are the components of the greenish frit layer of this sample. 4.2. EB frit layer
Fig. 6. OM micrograph of the polished cross section of the EB frit-ceramic body interface in sample 1 (long side: 12 mm).
As mentioned above, Egyptian blue frit was produced from a mixture of quartz and lime (both probably present in a natural sand), copper and natron; the batch was similar to the one used for glass melting, but the proportions of the batch components were different (Pradell et al., 2006). The results of the analyses do not allow us to confirm the use of natron as a flux, because significant amounts of K, Mg and P (fingerprint elements to recognise the natron-type glass) entered the composition of the glassy phase
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Fig. 7. a,b e SEM micrograph of the polished cross sections of samples 1 (left) and 4 (right); the arrows indicate the interface layer spearing the ceramic body (lower part of the figures) and the EB layers made of quartz grains (grey), surrounded by EB crystals (white) and bonded together by a glassy phase (light grey); porosity appears dark grey (a) or black (b).
through feldspars and other components of the silica source. The high levels of Al, Fe and Ti suggest that natural sand was used in which quartz was mixed with other minerals such as observed in fluvial sands. As indicated by Pliny, a local source of silicaelime sand used for glassmaking could be the mouth of the Volturno river, near Cuma (Vallotto and Verit a, 2002), a few kilometres from Liternum. The mostly predominant angular shape of the quartz grains indicates the addition of sand crushed to fine powder. This observation is not in contrast with the use of fluvial sands; in fact, the use of crushed grains increases the quartz surface. This feature, and the control of the grain size were critical parameters for the synthesis of the blue pigment, for nucleation and growth of the cuprorivaite crystals occurs on the surface of the quartz grains. The composition of the EB frit layers of Liternum differs from the composition of EB frit layers from Memphis (Tite and Hatton, 2007). These differences are mainly related to the use of different silica sources. An interesting difference consists in the high temperature silica polymorphs detected by XRD: in the Memphis samples both cristobalite and tridymite were detected, while trydimite was not
Fig. 8. Deposits of tenorite black crystals (dark) on the inner surface of ceramic body 4, at the interface with the EB layer (porous material on the low-right side of the figure); (long side: 4.3 mm).
identified in any EB frit layer from Liternum, and an evident presence of cristobalite was found only in sample 4B (dubious traces in samples 1 and 3, absent in samples 2 and 4A). These results allow us to conclude that Liternum EB was fired at lower temperatures and/ or for a shorter time as compared to the Memphis EB. The chemical composition of the glassy phase of EB frit layers from Liternum differs significantly from the composition of EB frits from other centres of the Roman Empire. The glassy phase has an unusual composition, with low CaO and relatively high MgO levels. The high amounts of K2O and Al2O3 confirm the presence of K-feldspars in the used silica source. A very good similarity is found only for the composition of the glassy phase of EB pigments (wall painting) of the 1st c. AD from Pompei (Tite and Hatton, 2007; Tite and Shortland, 2008, Tab. 8.9, pp.182e183) and two mosaic EB tesserae from Rome. The concordance with the Pompeii samples is almost perfect; the fingerprinting characteristic of the EB frits from Liternum and Pompeii is the unusually high alumina content (Al2O3 6.2e8.9%). Another similarity is that neither contain measurable tin and lead oxides, in contrast to many other Roman EB samples (included the two EB mosaic tesserae from Rome). An important heterogeneity in composition was ascertained in Liternum by analysing EB glassy phase in different areas in the same sample. This heterogeneity could be attributed to a too short reaction time, or most probably to a low firing temperature. Unfortunately, the homogeneity of the glassy phase was not investigated by other authors studying EB Roman samples and, therefore, it is impossible to ascertain whether this is a specific phenomenon of Liternum EB frit production or was a common characteristic of the EB glassy phase. The presence on the surface of the ceramic body deposits of well crystallized tenorite (CuO) crystals, sometimes covered by the EB layer, testifies to an oxidizing firing and the addition of copper in the form of a metallic (or oxide) powder. The presence of tenorite also inside the pores of the ceramic body and on the external surface of two crucibles suggests that copper did sublimate at the beginning of the firing of the raw materials, condensing and depositing as tenorite before the formation of the EB layer. It is very likely that one of the possible causes of the pale colour often observed in EB is the shortage of copper in the batch because of its sublimation, Cu being a key component of the blue crystals. Finally, the absence of lead and tin in the EB layers and in the glassy phase demonstrates that copper was added in form of pure oxide and not
/ Journal of Archaeological Science 53 (2015) 578e585 L. Lazzarini, M. Verita
as a metal alloy (bronze), as verified in the analysis of other historical EB samples (Ingo et al., 2013). 5. Conclusions The ancient sources of Vitruvius and Plinii, as well recent archaeological excavations clearly indicate that an important production of EB took place in the 1st century AD in the Phlegrean Fields (Cuma, Liternum and Puteoli), taking advantage of the presence of local silica-lime sand and the vicinity of the harbour of Pozzuoli from which all kinds of imported goods arrived from all the provinces of the empire. The analyses of 1st c. AD fragments of crucibles used for firing EB frit excavated at Liternum (Naples, Italy) allow some conclusions to be drawn about the firing technology of this important blue pigment in Roman time. 5.1. EB-firing technology The crucibles were made with local raw materials in direct connection with the igneous-sedimentary formations of the hills and plane around Liternum. They were fired at temperatures above 850 C, as testified by the total decomposition of the calcite originally present in the clay. The poor EB frit-ceramic body interaction indicates that the firing of the batch was performed at a relatively low temperature, probably around 850 C. The heterogeneity of the glassy phase resulting from the chemical analysis, confirms that the firing temperature of the batch should have been not too high, so as to assure the formation of a viscous liquid phase in which the blue crystals of cuprorivaite could easily nucleate and grow. Also the absence of mullite in the ceramic body of the crucibles and the presence of cristobalite in the EB layers but not in the ceramic body confirm a low firing temperature. In fact, it is known that cristobalite forms in a glassy phase surrounding the quartz crystals, also at temperatures lower than those expected. On the other hand, the abundant glassy phase and the rare EB crystals in sample 3 confirm that firing at higher temperatures with formation of a more fluid glassy phase do not favour the formation of the EB crystals. These conclusions are consistent with the laboratory investigation of (Bianchetti et al., 2000) revealing that the maximum growth of cuprorivaite occurs around 850 C. 5.2. Colour of the EB frit The analyses allow some conclusions to be drawn about the colour of the EB frit. As observed by several authors, the intensity of the colour of this pigment is not uniform and frequently it appears quite whitened. The amount and size of cuprorivaite crystals and of residual quartz grains, as well as the amount of green glassy phase should be the most important parameters influencing the final blue pigment hue. These parameters were influenced by the composition of the fired batch; the evaporation of copper (added in the form of copper scale: no tin and lead were detected) attested in this study (that could lead to a shortage of this element in the glassy phase and therefore to a lower amount of cuprorivaite crystals) could be the most critical parameter in this sense. The total surface of the residual white quartz crystals had probably a similar effect and probably for this reason crushed quartz of relatively small grain size was used in the EB batch. Finally, a too high firing temperature would lead to a too much fluid glassy phase that would hinder the nucleation and growth of the blue cuprorivaite crystals.
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Acknowledgements The Soprintendenza Archeologica delle Province di Napoli e Caserta is kindly acknowledged for supplying the samples for analyses. References Accorsi, G., Verri, G., Bolognesi, M., Armaeroli, N., Clementi, C., Milliani, C., Romani, A., 2009. The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue). Chem. Comm. 3392e3394. Augusti, S., 1967. I Colouri Pompeiani, Roma. Bianchetti, P., Talarico, F., Vigliano, M.G., Fuad Ali, M., 2000. Production and characterization of Egyptian blue and Egyptian green frit. J. Cult. Herit. 1 (2), 179e188. Boschetti, C., Leonelli, C., Corradi, A., 2012. The earliest wall mosaics and the origin of Roman glass in Italy: archaeological considerations for an archaeometric study. In: Ignatiadou, Despina, Antonaras, Anastassios (Eds.), Proceedings of the AIHV 18th Congress, pp. 139e144. Thessaloniki. gyptien a Cumes. In: Brun, J.-P. Caputo, P., Cavassa, L., 2009. 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