Egyptian blue in the Castelseprio mural painting cycle: imaging and evidence of a non-traditional manufacture Marco Nicolaa,e, Maurizio Acetob*, Vincenzo Gheroldic, Roberto Gobettoa, Giacomo Chiarid
a Dipartimento di Chimica, Università degli Studi di Torino, Via P. Giuria, 7 - 10125 Torino, Italy. Email:
nicola@adamantionet.com
b Dipartimento di Scienze e Innovazione Tecnologica (DiSIT) & Centro Interdisciplinare per lo Studio e la
Conservazione dei Beni Culturali (CenISCo), Università degli Studi del Piemonte Orientale, Viale T. Michel, 11 -15121 Alessandria, Italy. Email: maurizio.aceto@uniupo.it
c Scuola di Specializzazione in Beni Storici e Artistici, Università degli Studi di Bologna, Piazzetta Morandi, 1
-Bologna, Italy. Email: vincenzo.gheroldi@unibo.it
d Getty Conservation Institute – Los Angeles (retired). Email: gc.giacomochiari@gmail.com e Adamantio S.r.l., Via G. F. Napione, 29/a - 10124 Torino, Italy. Email: nicola@adamantionet.com
* Corresponding author. Email: maurizio.aceto@uniupo.it
ABSTRACT
The mural paintings in Santa Maria foris portas church at Castelseprio (Lombardy) are a precious early medieval cycle datable between 9th and 10th century CE and are therefore among the oldest Carolingian in Italy. The painting cycle has
been analysed in situ, to characterise the palette used in its decoration, with non-invasive molecular and elemental techniques. In addition, Visible Induced Luminescence (VIL) has been applied to provide an assessment of the extended use of Egyptian blue within the painting cycle. The palette, beside the use of the expected pigments, is characterised by the diffuse presence of Egyptian blue, more than 500 years after the fall of the Western Roman Empire. A remarkable feature in the composition of this pigment is the presence of non-negligible amounts of zinc, indicating a non-traditional production of Egyptian Blue. In fact, the use of a copper/zinc alloy rather than the more common copper/tin or copper/arsenic, proper of bronze residues, has never been previously reported.
KEYWORDS
Egyptian blue, mural paintings, Egyptian blue rich in zinc, FORS, XRF, VIL, Castelseprio, early medieval period, Carolingian
1 INTRODUCTION
The church of Santa Maria foris portas at Castelseprio (Varese, Lombardy – Italy) has the peculiarity of having three apses instead of the common design of only one apse typical of the medieval period (Figure 1).
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Figure 1. a) The church of Santa Maria Foris Portas in Castelseprio, Varese. Two of the three apses are shown. The larger central one (to the right) is entirely painted inside. b) Adorazione dei Magi scene as an example of the frescoes.
The mural paintings located in the eastern apse portrait scenes of Infanzia di Cristo taken from the Protoevangelium of James and from the Gospel of Pseudo-Matthew. Some scholars consider these painting as a product of western art, others as an artwork belonging to a Byzantine Middle Eastern tradition. The different chronological attributions vary between 6th to 10th century CE (Bertoni, 2003). These dating rely on an ante-quem only, consisting of the graffito image
of a presbiter recalling, apparently some years later, his own consecration under the episcopacy of Arderico (archbishop of Milan between 936 and 948). Technical and stratigraphical investigations showed that the paintings did not constitute the first finishing of the apse (Franzini and Gratziu, 1988). The church was in use for several years before the execution of the paintings (Gheroldi, 2013) which was carried out on a surface already involved in a previous collapse (Gheroldi, 2017). Moreover, it can be seen that a fragment of painted plaster is set without breaking against the clamped beam in the ring cant apse over the Andata a Betlemme scene (Brogiolo, 2013; Gheroldi, 2013). Recently the ante-quem term for the paintings has more accurately established by the 14C wiggle-match dating (WMD) of the main timber beam
(Brogiolo, 2013; Gheroldi, 2013; Gheroldi, 2017). This provides a date between the end of 9th and the half of 10th
century CE. (Martinelli and Pignatelli, 2013).
One of the most important technical features concerning these paintings is the abundance of use of the blue colour. In situ microscopy investigation by means of 100x and 200x magnification evidenced large blue grains dispersed in a white matrix. 33
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54Figure 2. Magnified image of a blue coloured area on the halo of the priest in Acque Amare scene
These grains were particularly abundant in the incarnate shadows and in some architectural details. For example, in the area between the cheek and the neck of the Virgin in the Annunciazione, in the Visitazione and in the Acque amare scene, as well as in the incarnate of the Cristo Pantocrator and in details such as the column in the Annunciazione (see images below). Layers with the same pigment, but characterised by a higher concentration were found on the red ochre surfaces of the mantle of the Virgin in all the scenes of the upper zone. The background of the Cristo Pantocrator or the pants of the Adorazione dei Magi are examples of the violaceous tone of the blue produced by the mixing of translucent blue colour with a base of red ochre.
In 2009 a campaign of measurements carried out by X-ray fluorescence spectrometry (XRF) showed the presence of significant amounts of copper in the blue areas together with iron, zinc and lead. These data were interpreted as an indication of the presence of azurite (Cagnana et al., 2009). This attribution, however, can be disproved by some clues. For instance, the painted surfaces, in spite of being subjected to strong whitewash and to the long exposure to humidity, did not show evidence of the colour change from blue to green typical of azurite particles, even under a microscopic investigation at 200x magnification. It may be worth to note that the alteration has not occurred even if the blue grains are dispersed in a calcium carbonate matrix present as ligand from the fresco painting, or as a whitener/ligand used in mixture with the blue pigment.
These evidences strongly suggest that azurite is not the pigment in question in the blue areas of the Castelseprio painting cycle. It was therefore suggested that the blue colour was instead Egyptian blue (EB hereafter), largely used by all ancient civilizations, from the Egyptians to the Greeks and to the Romans. It is common belief that after the fall of the Roman Empire the technology of production of EB was lost in Western Europe and the pigment almost forgotten.
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The hypothesis that EB was the pigment used at Castelseprio was already proposed in a technical report in 1948 (Bognetti et al., 1948). Later, analyses carried out by the Istituto Centrale per il Restauro identified this pigment, always together with silica and carbonated lime (ICR, 1967), on the mantle of the Virgin in the Natività scene, on the blue pants of a Wise Man and on the mantle of the leaning figure in the Adorazione dei Magi scene. Such interpretation of analytical data is supported by specific studies carried out by the ICR in previous years on the identification of Egyptian blue on artworks; examples are reported by Schippa and Torraca (1957). See also Nicolini and Santini (1958) and Tabasso Laurenzi (1967). Therefore, it seems convenient to further investigate the Castelseprio cycle of paintings using, in particular, imaging techniques that were not available to the researchers who previously studied this remarkable church.
2 MATERIALS AND METHODS 2.1 Identification of Egyptian Blue
To confirm the identification of Egyptian blue in the Santa Maria foris portas mural painting cycle, to acquire more information on the nature of the blue pigment and to identify the palette used by the painters, a non-invasive diagnostic campaign was carried out using in situ portable instrumentations. UV-visible diffuse reflectance spectrophotometry with optic fibres (FORS) was used for the preliminary identification of most pigments. XRF spectrometry was employed to gain elemental information on the chemical nature of EB, with concern to the source of copper inside the cuprorivaite.
Finally, Visible Induced Luminescence (VIL) was used for the identification of EB and to accurately identify its distribution in the paintings, including some places where it was not visible by naked eye observation. VIL is the best technique to identify and image EB in paintings and polychrome statues (Verri, 2009a). It is based on the simple observation (Ajo’ et al.,1996; Pozza et al., 2000) that Egyptian blue absorbs energy in the visible light, specifically in the red region since the light that it re-emits is blue, (Accorsi et al., 2009) and luminesces in the near-infrared region. Absorption and excitement spectra of synthetic cuprorivaite show 3 different electronic transitions: 2B
1g →2B2g , 2Eg e 2A
1g. They are due to Cu2+ ions, which should be the only photoluminescent centers in cuprorivaite. The main excitation
peak is centered in the red range of the electromagnetic spectrum (about 630 nm), while the emission spectrum is centered in the infrared range (about 910 nm) and is linked to the lowest energy electronic transition (2B
2g→2B1g). This
transition is strictly parity forbidden, as suggested by the long luminescence decay (107 ms). (Pozza et al. 2000). Cuprorivaite, and its artificial counterpart Egyptian blue, crystallizes in the tetrahedral space group P4/ncc. The luminescent properties of cuprorivaite (CaCuSi4O10) are based on the Cu2+ ion located on the fourfold axis, where the
square planar coordinated Cu2+ ion links together rings of four SiO
4 tetrahedra (Kendrick et al., 2007). In this
environment of tetrahedral symmetry, the d9 configuration of Cu2+ makes it subject to Jahn–Teller distortion (Cotton et
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al., 1980). In total, this leads to a descent in symmetry from Oh (octahedral) to D4h (tetragonal) and to a splitting of the
t2g and eg orbitals, so that the two eg orbitals give rise to a non-degenerate orbital of a1g and b1g whilst the three t2g
orbitals give rise to a non-degenerate orbital of b2g symmetry and a doubly degenerate pair of orbitals of eg symmetry
(Putnis 1992; Jones 2002). The splitting of the orbitals makes room for the light used to induce the luminescence, so absorption at an energy of 12,500 cm−1 (800 nm) can take place followed by an emission transition at about 10,500 cm−1
(952 nm) (Pozza et al. 2000). Han Blue, Han Purple and some cadmium based pigments also luminesce approximately in this region, although with much lower intensity (Kriss et al., 2016; Chiari, 2017). Verri and his co-workers (Verri, 2009a; Verri, 2009b; Dyer et al., 2013) took advantage of this characteristic to perfect the VIL technique, now broadly used by many researchers. He used a modified digital camera in which the filter blocking the IR radiation had been removed and, for excitation he used a light source lacking IR component (LED). The CCD detector of the camera is sensitive to visible light, in particular to the one used for the excitation and reflected by the objects. This signal is order of magnitude stronger than the VIL signal and it would dwarf the IR luminescence. Therefore, a long pass filter (850 nm is adequate) needs to be inserted in front of the lens to block the visible light. This assembly works well in environments in which there is no parasitic IR light (the Tutankhamen tomb was a typical example, see Chiari, 2017), but a studio environment does fulfil these requirements quite well. If the condition of absence of IR radiation cannot be obtained (i.e. museum exhibits, archaeological sites with presence of daylight) one must use a different equipment. Instead of a LED source that does not contain IR but is relatively weak, one can use a much more powerful photography flash, provided that the IR component emitted by it is filtered away by a short pass filter (e.g. 750 nm). By doing so the excitation power is strongly increased, and the VIL signal can be picked up even in daylight. If the surface to be imaged is large, more excitation power may be needed, and two or more synchronized flashes should be used. This is the situation of the frescoes in the church of Castelseprio.
VIL imaging carried out in Castelseprio could not be performed at night. Although it was possible to black out the two large windows in the back of the apse, environmental IR was strongly present. Furthermore, the frescoes are several meters in size, and to include the whole scene in the picture frame one would have to stand at a prohibitive distance from the paintings, since the intensity of the excitation and the emitted signal diminish with the square of the distance. We therefore used two synchronized flashes and took a series of pictures to be collaged afterwards into a mosaic of frames to image the whole wall. The problem remained that many sections were about 3 m high. To reach this height without using scaffolding (which was not allowed), we located the small adapted camera (Figure 3) on top of a tripod that was lifted at the proper height in front of the painting.
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Figure 3. a) The equipment used for VIL imaging. The filter blocking IR and UV light has been removed from the camera; b) the same camera coupled with two synchronized flashes, each having a screen to cut the IR emission
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A delayed shot allowed for the time needed to lift the camera to the proper position. This procedure had the advantage that the pictures were taken perpendicularly to the painting, and therefore with reduced perspective deformation. The disadvantage is that only a small section of the painting is captured at each time, and therefore a lot of mosaicking using Adobe Photoshop was needed. As an alternative, we used two synchronized flashes remotely controlled by the camera (Figure 3b). This method partially solved the problem of obtaining a larger and more intense illumination, but the perspective deformation of the images in part remained. This was corrected using the program NIP2 (Cupitt et al., 2015), which allowed obtaining a proper mosaicking. VIL images showing EB luminescence only are rare. On the contrary, most images present a faint residue of IR reflected by the objects, especially the whither parts of them. Therefore, the VIL pictures taken at Castelseprio need a post treatment. These false signals need to be subtracted using one of the image treating programs commonly available, by converting the image to black and white and then, using the histogram function or the combination of brightness and contrast, by separating the VIL signal from the rest. A visible image of the frescoes should be taken from the same spot as the VIL one, when possible. A similar camera and the same focal length to avoid distortions must be used. For the very high locations, this was not possible and the two images, visible and VIL, were rectified using the (NIP2) program and superposed and cross-faded using the program IMAGEj (Ferreira and Rasband, 2009). This operation is particularly useful for those cases in which there are only small, difficult to be located, details containing EB.
2.2 X-ray Fluorescence spectrometry
For in situ XRF analysis the instrumentation used was a portable XRF spectrophotometer Assing (Monterotondo -Roma, Italy) LITHOS 3000 model, working with monochromatic excitation energy of molybdenum Kα, voltage 25 kV,
0.3 mA, measurement area of ca. 50 mm2, Si-PIN detector Peltier cooled. The distance between probe and surface was
kept at 10 mm (standard distance); measuring time: 120 s. The technique allows to detect elements of atomic number 15 (phosphorus) or greater.
2.3 UV-visible diffuse reflectance spectrophotometry
FORS analysis was performed with an Avantes (Apeldoorn, The Netherlands) AvaSpec-ULS2048XLSB2-UA model spectrophotometer. The instrument is equipped with a 2048x14 pixels back illuminated CCD detector and a 300 lines/mm grating; useable range is 200-1100nm. With this configuration, the spectral resolution was 2.5 nm. An Energetiq (Woburn, Massachusetts, USA) EQ-99 model Laser-Driven Light Source was used for illumination. An Ocean Optics (Dunedin, Florida) R-400 model probe was used to drive light on the sample and to collect reflected light from the sample (spot diameter = 3 mm). Light source and detector were connected with fibre optic cables. Both the incident and detecting angles were 45° from the surface normal, to exclude specular reflectance. Diffuse reflectance 142
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spectra of the samples were referenced against an Avantes WS-2 reference tile made of PTFE, guaranteed reflective at 98% or more in the spectral range investigated. Instrumental conditions were the following: integration 20 ms, 250 scans giving a total of 5 seconds for each spectrum. The whole system was managed through AvaSoft v.7 dedicated software running under Microsoft Windows 7 operating system.
2.4 Scanning Electron Microscopy – Energy Dispersive X-ray (SEM-EDX) analysis
SEM-EDX measurements were carried out on a Quanta 200 FEI (Hillsboro, Oregon) Scanning Electron Microscope equipped with EDAX (Mahwah, New Jersey) EDS attachment, using a tungsten filament as electron source at 20 KeV. The instrument was used in E-SEM mode (90 mbar of water pressure in chamber) in order to avoid samples metallisation.
3 RESULTS AND DISCUSSION
Non-invasive measurements were carried out using VIL, XRF and FORS on all pigments, in order to describe the complete palette. The combination of surface techniques (FORS and VIL) and an in-depth one (XRF) allowed obtaining abundant molecular and elemental information, together with mapping the distribution of, at least, EB. For the XRF spectroscopy the extensive analysis was also made to identify any possible source of environmental interference (i.e. if elements other than Cu could eventually originate by a source different from Egyptian Blue such as plaster or other pigments). A total of 47 points were analysed, 36 of which from the apse half-dome and 11 from the triumphal arc. Results of the analyses are shown and discussed in the following paragraph. Particular effort was dedicated to the identification and chemical characterisation of EB, due the historical relevance of this pigment. Imaging of Egyptian Blue was carried out using VIL technique. A SEM-EDX analysis was carried out on a loose blue microscopic fragment.
3.1 Blue
Particular emphasis is dedicated to the blue, since to our knowledge, no other medieval fresco cycles show such an extensive use of Egyptian Blue, in an epoch in which, according to a diffuse consensus among scholars, Egyptian Blue should be almost absent in Western Europe.
Egyptian Blue is present not only in the visibly blue areas, but was also used to produce other hues, where it was mixed to other pigments for colour special effects.
Figure 4 shows a comparison between the FORS spectrum of a standard sample of Egyptian Blue, of a blue painted area in the apse half-dome and of a turquoise section.
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Figure 4 - FORS spectra of standard Egyptian blue (solid line) and a blue painted area (dashed line) and a turquoise painted area (dotted line) in Castelseprio paintings.
The diagnostic study carried out with FORS and XRF allowed identifying unequivocally the diffuse use of EB in the painting cycle (Aceto, 2013; Nicola, 2013). Measurements were performed both on the best preserved blue areas, and on worn-out zones.
The reflectance minima, located at 629 and 778 nm, can be considered as maxima in apparent absorbance coordinates (Aceto et al., 2014) and identify the pigment, as already suggested by Vezin and Roger (2007) in their analytical study on some 8-10th century CE illuminated manuscripts; the spectral features in the region 700-1100 nm confirm the typical
luminescence of the pigment. EB was found also in several turquoise and green painted areas. In these cases, FORS spectra (Figure 4, dotted line) showed the characteristic reflectance minimum at 629 nm but the maximum was located inside the green spectral region, so that we can hypothesise that Egyptian blue had been mixed with a yellow or white pigment, which caused a bathochromic shift of the maximum. In fact, XRF analysis highlighted the presence of Fe in these hues, so most probably yellow ochre was the yellow pigment added to EB.
It would be of the highest interest to evaluate the provenance of copper used as raw material in the production of the pigment. Information is particularly rich for what concerns Egyptian epoch for which the source of copper could be a mineral (i.e. azurite or malachite), the pure metal or bronze (Fenoglio et al. 2012); in the latter case, according to the chronology suggested by the literature (El Goresy et al. 1996), it is possible to distinguish between arsenical bronze, used from Ancient Kingdom to middle 15th century BCE, bronze based on tin (from 1479 to 1425 BCE) and bronze with tin and lead that was used later. As assessed by XRF analysis, the Egyptian Blue found in Castelseprio wall paintings showed the presence of Zn together with Cu, possibly indicating that the raw materials were neither copper
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ores nor bronze, but rather a copper/zinc alloy i.e. a brass. XRF analysis performed in situ (Figure 5 and Tables 1 and 2) highlighted the fact that the average semi quantitative Cu/Zn ratio varied in the range 3.5 – 4.3 in all blue areas analysed.
Figure 5 - XRF spectrum of a light blue painted area in Castelseprio paintings (reference point X2). The Kα peak of zinc
is evident at 8.637 Kev.
Poin
t Description Aspect Ca Fe Cu Zn Pb
X1 grey band grey on red 48,4 10,4 0,6 0,6
-X2 blue ribbon light blue 19,3 4,8 31,6 7,4 2,8
X3 yellow over blue ribbon yellow on lightblue 36,6 9,9 19,2 4,7 2
X4 deep red under Wise Men red 42,2 27,8 0,4 0,6 0,7
X5 blue ribbon under Wise Men light blue 23,3 5,1 27,3 6,3 2,5 X6 blue on Saint Joseph’s cloak light blue 29,4 4,8 13,7 3,9 1,7
X7 deep blue on Saint Joseph’s cloak blue 28,3 7 15,7 4 1,9
X8 black on Saint Mary’s shoe, near WiseMen black 19,4 4,9 0,7 0,6 23,6
X9 blue on Wise Men’s cloak light blue 36,8 3,9 13,8 3,4 1,4
X10 blue on Wise Men’s footwear light blue 20,2 9,5 19,4 4,2 1,2 X11 yellow background near Wise Men’s cloak yellow 41,9 8 - 0,3 0,5
X12 background white (ground) 53,3 6,9 0,4 0,5 0,7
X13 green on Saint Joseph’s pillow green 23 10 0,9 0,6 0,4
X14 blue on Angel’s dress light blue 30,4 4,5 9,5 2,7 1,1
X15 black on Saint Mary’s foot black on red 17,5 5 0,6 0,7 28,9 227
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The presence of copper and thus of Egyptian Blue, also confirmed by FORS analysis, seems evident in the points X2, X3, X5, X6, X7, X9, X10 e X14 indicated on Figure 6.
Figure 6. Points of analysis.
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Aspect
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Zn
Pb
Fe
Cu/Zn Pb/Zn Cu/Fe Cu/PbX2
lightblue 31,6 7,4 2,8 4,8 4,3 0,4 6,5 11,2X3
on lightyellow blue 19,2 4,7 2,0 9,9 4,1 0,4 1,9 9,4X5
lightblue 27,3 6,3 2,5 5,1 4,4 0,4 5,4 11,0X6
lightblue 13,7 3,9 1,7 4,8 3,5 0,4 2,9 7,9X7
blue 15,7 4,0 1,9 7,0 3,9 0,5 2,3 8,1X9
lightblue 13,8 3,4 1,4 3,9 4,1 0,4 3,6 9,7X10
lightblue 19,4 4,2 1,2 9,5 4,7 0,3 2,0 16,1X14
lightblue 9,5 2,7 1,1 4,5 3,5 0,4 2,1 8,5Table 2 - Semi-quantitative elemental data as obtained by XRF spectroscopy in blue points. Cu/Zn is quite regular ranging normally in a 3,9±0,5 value.
Sampl e O Na M g Al Si Ca Fe Cu Zn Pb Cu/Z n a 46, 6 2, 3 1,0 1, 4 20, 0 5, 0 2, 0 7, 3 3, 0 5, 6 2,5 b 49, 8 1, 2 1,5 1, 2 19, 9 7, 2 1, 1 5, 1 0, 9 2, 1 5,6 c 50, 9 2, 2 0,4 2, 8 24, 9 5, 0 0, 5 6, 5 2, 6 1, 8 2,5
Table 3 - Quantitative elemental data as obtained by SEM-EDX analysis on a loose micro sample from a light blue area. In the case of Castelseprio paintings, elemental analyses by means of in situ XRF spectrometry were followed by more accurate and precise SEM-EDX analyses carried out on single crystals isolated using a bland acidic dissolution of the carbonate substrate. Both techniques identified in all instances significant amounts of zinc (See table 2 and table 3) with lower amounts of lead and excluded the presence of tin. The data acquired suggest the possibility of use of scrapes of brass as a copper source for the production of Egyptian Blue. As seen in tables 2 and 3, the concentration of zinc with 245
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respect to copper would be consistent with the values normally found on brass alloys with ca 20% zinc. That kind of alloy is compatible with the ones present in that period and commonly obtained via cementation process, the standard method of producing brass in Europe since at least the 1st century BCE (Bourgarit and Thomas, 2011).
The intentional use of brass is coherent in that the brass was increasing in popularity just during the early Middle Ages. In Middle Eastern Asia and Eastern Mediterranean area, early copper/zinc alloys are known in small numbers from some third millennium BC sites (Weirong 2001). It has been suggested that the disruption in the trade of tin for bronze from Western Europe may have indeed contributed to the increasing popularity of brass in the East and by the 6th–7th centuries CE it is estimated that over 90% of copper alloy artefacts from Egypt were made of brass (Craddock et al. 1990). It may be worth to report that the use of brass for the production of a synthetic blue pigment is attested in at least one ancient recipe of the end of 15th century. Limature octonis, indeed, is documented in recipe n. 37 Ad azurrum
faciendum contained in ms. 2861 also known as Manoscritto Bolognese, kept at Biblioteca Universitaria in Bologna (Guerrini and Ricci, 1887; Muzio, 2012). However the recipe is not related to the production of Egyptian Blue but to another copper bearing blue pigment.
The use of a copper/zinc alloy as source of copper for Egyptian blue has never been reported in ancient or scientific literature. Zinc was shown to be absent in the Egyptian Blue cakes in Egypt and in Mesopotamia (Jaksch et al. 1983, Tite and Hatton, 2007, Hatton et al. 2008). Recently Ingo et al. (2013) found zinc as minor element in Egyptian Blue cakes excavated at Ayanis fortress (Eastern Anatolia, Turkey), datable to the 7th century BCE. The content of zinc determined, though, ranged between 0.54 to 1.15 wt %, so that it could not be due to a copper/zinc alloy; in fact the authors put it in relation with the local metal ores sources exploited in ancient times. We can therefore assume that the production of the pigment used at Castelseprio was the result of an alternative technology with respect the ancient world.
The high chemical resistance of EB to alkaline environment renders this pigment suitable for fresco painting or other techniques involving the use of chalk or lime media. The microscopic image acquired (Figure 2) show the blue pigment in a white matrix. Together with the evidence of substantial coats retaining the imprint of the brush stroke and tending to detach from the support, well observable, for example, in the blue ribbon painted above the curtain under the Presentazione al tempio scene, allows hypothesising the use of Egyptian blue in lime painting rather than in true fresco painting technique. A clue is brought by the reflection of the column represented behind the Virgin in the Annunciazione scene, where the transparent coat is made of EB grains dispersed lime wash.
3.2 VIL imaging of Egyptian blue areas
Visible Induced Luminescence was applied to the Castelseprio painting cycle to obtain a complete map of the distribution of EB. Figures 7, 8, 9 and 10 show details of the frescoes in visible light (a) and VIL (b).
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Figure 7. a) The Nativity scene in visual mode. b) The same in a mosaic of VIL pictures (EB only is visible).
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Figure 8. The central Christ Pantocrator. The halo behind the figure appears blue, and therefore there is no surprise in seeing that it is made of EB. The incarnates of the face instead contain EB not for its colour but for the characteristic of making the white whiter . This was observed already in Egyptian and Roman paintings.
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Figure 10. Apparizione dell'angelo a Giuseppe scene. It is interesting to note that all the halos but the one of the Christ and the Virgin, which are made in yellow ochre to simulate gold, are done using EB . Also, the shadows on the face of the angel is realized with EB, like other shadows.
3.3 The use of Egyptian Blue after Roman epoch
The identification of the use of EB in the paintings of Castelseprio is not a definitive clue for dating the pictorial cycle. Some scholars considered this evidence as an indication for a very early dating. In fact, it is common believe that with the fall of the Roman Empire the manufacturing technique of EB was lost. However, there is a significant number of identification of this pigment beyond the classical Egyptian-Greek-Roman age, proving that EB was used in the whole Mediterranean area. Its use is in fact reported in Italian mural paintings at least up to 10th century CE (Lazzarini, 1982;
Bensi, 1990; Gaetani et al., 2004; Delamare, 2013) but other instances, though sporadic, are cited up to Renaissance (Bredal-Jørgensen et al., 2011); outside Italy it was reported, among others, in the already cited 8-10th century A.D.
illuminated manuscripts (Roger et al., 2004; Vezin and Roger, 2007, Roger, 2007), in 9th century Carolingian wall
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paintings in Müstair (Switzerland) (Mairinger and Schreiner, 1986; Emmenegger, 1986; Riederer, 1997) and in 11th
century Romanesque mural paintings in Catalunia (Lluveras et al., 2010). The cycle of paintings at Castelseprio, therefore, is well inserted in the period in which the pigment was historically known to be used, even if no medieval literary sources are known witnessing that the production was still alive.
More interesting is the constant use of the pigment for the glazes on the shadows, which represents a relevant cultural attestation. The particular depiction code of the shadows is attested neither in Lungubardian age (Gheroldi, 2014; Fiorin and Salvadori, 2014), nor in the subsequent Carolingian figurative tradition (Mairinger and Schreiner, 1986; Emmenegger, 1986; Goll et al., 2007). It is instead a figurative code found in the Byzantine culture, in particular in the Constantinople tradition (Gheroldi, 2017).
Figure 11. Blue shadows on Saint Mary’s face in Acque Amare scene. On the eyelid it’s possible to see a large EB particle.
In studies more closely related to the paintings of Castelseprio from the historical point of view, there is scarce information concerning elemental analysis of late Antique and early Middle Ages paintings. In the work of Gaetani et al. (2004) EB was identified in the paintings of the church of San Saba in Rome, datable to 8th century CE. SEM-EDX,
though, did not identify any relevant impurities, suggesting the use of pure copper as raw material. In the study of Howard (2001) on painted stuccoes at San Vincenzo al Volturno (Molise) datable to 9th century CE, impurities of lead
were identified. Unfortunately, in most of these cases EB was just identified and not fully characterised from the elemental point of view.
We can therefore assume that the use of the pigment at Castelseprio has been the result of a non-traditional technology, compared to the Western European norm. The scarcity of analytical data from the scientific literature suggests the need of elemental measurements on other painting cycles of Late Antique and Early Middle Age epochs in Italy, to clarify the technological aspects of the blue pigment found at Castelseprio.
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3423.4 Other Pigments Green and blue-green
Two different pigments were used for the green. In several green and turquoise areas a mixture of Egyptian Blue and, possibly, yellow ochre was used. In other sections, though, iron instead of copper was detected by XRF, suggesting the presence of green earth. FORS analysis (see Figure 4) supported this identification according to the reflectance minimum located at 745 nm and to the general shape of the spectrum, while the minimum at 629 nm characterizing Egyptian Blue was less pronounced but still perceivable. This choice of one or the other green could be motivated by the need to attribute symbolic meanings to certain pictorial features, possibly more important if painted in Egyptian Blue, less if painted in green earth. An alternative explanation based on artistic reasons could be to have a larger variety of greens at disposal by skilfully mixing a blue and yellow pigment. Further hypothesis, although not documented, is the late intervention of another artist.
Black
In most black painted areas a carbon based pigment was used. In few instances, though, the presence of black material on a reddish background was noted. In these points, XRF analysis showed the presence of abundant lead, while on most black areas only calcium and iron, likely from the underlying player, were found. FORS analysis confirmed that for these particular black sections carbon was not used and suggested the presence of minium (Pb3O4), according to the
inflexion point located at 568 nm; the blackish tone is explained by the probable chromatic alteration of minium due to transformation in plattnerite, a black lead oxide with formula PbO2. Raman spectroscopy carried out on a micro sample
confirmed this hypothesis. The conversion of minium to plattnerite can occur under favourable conditions of pH and humidity, which are easy to occur on paintings exposed to air (Burgio et al., 2001; Aze et al., 2008; Aceto et al., 2012).
Red and yellow.
Both red and yellow colours are based on iron-oxides pigments (Nicola et al. 2016), sometime mixed with Egyptian Blue to produce purple and green shades (Aceto 2013). Red ochre is also used as a background for the blue. This is the norm for frescoes of this epoch.
White
Based on the finding of abundant Ca by XRF and the absence of lead (i.e. of lead white) one can expect a white pigment like “bianco di San Giovanni”, i.e. calcium carbonate. It is however probable that some white area were obtained letting the surface unpainted with the white ground of lime plaster on sight or using other techniques involving lime rather than using a truly white pigment.
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4 CONCLUSIONS
The enquiry on the mural paintings of the church of Santa Maria foris portas of Castelseprio is part of a vaster study on the Medieval religious art, with particular emphasis on Longobard and Carolingians periods. In elucidating the details of the paintings, non-invasive portable instrumentation only was used, i.e. FORS, XRF and VIL. This allowed clarifying the palette used by the artists, which is coherent with the period of the paintings (around the year 1000 CE) for all the pigment but for the ubiquitous presence of Egyptian Blue. This pigment, extensively used by the Egyptian, Greek and Roman artists, is commonly believed to have disappeared from Western Europe following the upheaval of the disastrous fall of the Roman Empire. In the church of Castelseprio large sectors of the paintings, which cover several square meters of wall, are painted using EB, either pure, to obtain an intense blue colour, or mixed with yellow ochre to get various hues of green. This is not a novelty, since the presence of EB in Castelseprio was already suggested in 1948. But the use of XRF (and in one instance of SEM-EDX on a few single crystals of EB isolated from the lime matrix to confirm the portable instrument results) showed the anomalous presence of conspicuous amount of zinc in the EB composition. This was never mentioned in the literature. The presence of zinc, and the corresponding absence of both arsenic and lead, demonstrates that the material used for the making of EB was not bronze scrapings (as typical of Egyptian and Roman EB) but a copper/zinc alloy. Brass was well known and used in the Eastern Roman Empire, and may suggest that the EB used in Lombardy was coming from Byzantium, where the destruction of Rome may not have affected a continuous production of EB. The paintings, emphasized by the further elucidation of the images due to the maps of EB obtained with the VIL imaging, show features oriental in style. This is particularly evident for the Christ Pantocrator. Other characteristics of the pictures are not similar to the typical Italian Medieval art. The relaxed Madonna lying comfortably on what seems to be a chaise longue, without Baby Jesus which is attended by two other women is definitely an unusual representation. So are the skinny young Wise Men with bright blue pants and fez-like hats. The choice of the scenes that are not taken from the orthodox Gospel but from apocryphal writings is mostly forgotten by the religious people of the epoch. So is the story of the Acque amare, in which Mary is submitted to the test of a poisonous drink to prove that she was telling the truth about her divine pregnancy. Or, next to it, the angel who reassures Joseph about Mary’s virginity. Many of these features may be due to an Oriental influence or to the work of an Oriental workshop. The extensive mapping of EB done by VIL and the possible use of inedited brass in the production of the EB add a substantial confirmation to this hypothesis, previously based on art-historical ground only.
5 ACKNOWLEDGEMENTS
We are thankful to Provincia di Varese, owner of the monument, for the authorization to access the church; we are also grateful to the agencies of Soprintendenza in charge for Castelseprio for authorizing technical analyses in 2012 and VIL 375 376 377
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imaging in 2017. We have to express out appreciation to Paola Marina De Marchi, Gian Pietro Brogiolo and Sara Marazzani for their kind cooperation.
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