Studio di vetri musivi della Basilica Alfa (VI secolo) di Nicopoli d’Epiro, Grecia
10.6092/issn.1973-9494/4186
SummaryAfter collecting the essential historical, archaeological, and artistic information relative to the ancient town of Nikopolis in Epirus and its Alpha (or Doumetios) Basilica, built in the sixth century A.D., glass tesserae belonging to degraded and lost parts of the mosaic floors of the basilica were analysed, determining their chemical composition and the possible presence of crystalline phases. As in all glasses produced in the Mediterranean area in that epoch, they are soda-lime silicate glasses, in part with lead, obtained using natron as a flux. The typology of the glass corresponds to that of Roman glasses, of the first centuries A.D., and is the same as that employed for the basilicas of Ravenna in the fifth and sixth centuries; in particular, a comparison was made with the glass tesserae of the St Vitale Basilica. The technological base for producing coloured glasses is also practically the same as that of the mosaic glasses of Ravenna. In particular, antimony was used as an opacifying element and a fusion was carried out under reducing conditions to obtain red glass with microparticles of metallic copper. Thus, it can be supposed there was a common source of production of the mosaic glasses used in Nikopolis and Ravenna in the sixth century and that this source was in the Near Orient. RiassuntoDopo la raccolta delle informazioni essenziali di carattere storico, archeologico e artistico relative all’antica città di Nicopoli d’Epiro e alla sua Basilica Alfa (o Doumetios), costruita nel VI secolo d.C., sono state analizzate tessere vitree che appartenevano a parti dei pavimenti musivi della basilica degradate e andate perdute, determinando la loro composizione chimica e la eventuale presenza di fasi cristalline. Come tutti i vetri dell’epoca prodotti in area mediterranea si tratta di vetri silicatico-sodico-calcici, in parte piombici, ottenuti usando natron quale fondente. La tipologia del vetro corrisponde a quella dei vetri Romani, dei primi secoli d.C., ed è la stessa dei vetri musivi impiegati nelle basiliche ravennati del V e VI secolo; in particolare il confronto è stato effettuato con la composizione delle tessere vetrose della basilica di San Vitale. Anche la tecnologia alla base dell’ottenimento dei vetri colorati è praticamente la stessa dei vetri ravennati. In particolare, è stato usato antimonio come elemento opacizzante e condotta una fusione in ambiente riducente per ottenere vetro rosso con microparticelle di rame metallico. Si può, dunque, fare l’ipotesi di una fonte produttiva comune dei vetri musivi impiegati a Nicopoli e a Ravenna nel VI secolo e che tale fonte fosse nel Vicino Oriente.RésuméAprès la collecte des informations essentielles à caractère historique, archéologique et artistique concernant la ville antique de Nicopolis d’Épire et sa basilique Alpha (ou Doumetios), construite au VIe siècle apr. J.-C., il a été procédé à l’analyse des tesselles de verre qui appartenaient à des parties des pavements de mosaïque de la basilique qui ont été dégradés et perdus, à la définition de leur composition chimique et au relevé d’une éventuelle présence de phases cristallines. Comme tous les verres de l’époque fabriqués dans la région méditerranéenne, il s’agit de verres silico-sodiques-calciques, en partie plombiques, qui ont été obtenus en utilisant le natron comme fondant. La typologie du verre correspond à celle des verres romains des premiers siècles apr. J.-C., et c’est la même que celle des verres de mosaïque employés dans les basiliques de Ravenne des Ve et VIe siècles. Notamment, une comparaison a été effectuée sur la composition des tesselles de verre de la basilique de Saint-Vital. La technique permettant d’obtenir des verres colorés est pratiquement la même que celle des verres de Ravenne. En particulier, l’antimoine a été utilisé comme élément opacifiant, et une fusion en milieu réducteur a permis d’obtenir un verre rouge avec des microparticules de cuivre métallique. Nous pouvons donc émettre l’hypothèse d’une source commune de fabrication des verres de mosaïque utilisés à Nicopolis et à Ravenne au VIe siècle, et que cette source se trouvait au Proche Orient. ZusammenfassungNachdem die wesentlichen historischen, archäologischen und künstlerischen Informationen über die antike Stadt Nikopolis in Epirus und ihre im 6. Jahrhundert n. Chr. errichtete Basilika Alpha (auch Doumetios) eingeholt worden waren, wurden Glassteine analysiert, die zu zerstörten und verloren gegangenen Teilen des Mosaikbodens der Basilika gehörten, um ihre chemische Zusammensetzung und den eventuellen Gehalt an Kristallphasen zu bestimmen. Wie alle anderen Glassorten, die damals im Mittelmeergebiet hergestellt wurden, handelt es sich auch hier um silikat-, natrium- und kalkhaltiges, teils auch bleihaltiges Glas, das unter Verwendung von Natron als Schmelzmittel hergestellt wurde. Diese Glassorte entspricht derjenigen der Römischen Gläser aus den ersten Jahrhunderten n. Chr. und es ist die gleiche Glassorte, wie die der Glassteine, die in den Basiliken von Ravenna aus dem 5. und 6. Jahrhundert verwendet wurden.  Im Einzelnen wurde der Vergleich mit den Mosaiksteinen aus Glas in der Basilika San Vitale vorgenommen. Auch die Technik der Herstellung farbigen Glases ist praktisch die gleiche, die in Ravenna zum Einsatz kam. Insbesondere wurde Antimon zur Opakisierung und ein Reduktions-Schmelzverfahren zur Herstellung roten Glases mit Mikropartikeln metallischen Kupfers verwendet. Es kann also angenommen werden, dass die Mosaik-Glassteine in Nikopolis und Ravenna im 6. Jahrhundert aus der gleichen Produktionsquelle stammten, und dass sich diese Quelle im Vorderen Orient befand.ResumenTras haber recogido la información esencial desde el punto de vista histórico, arqueológico y artístico sobre la antigua ciudad de Nicópolis de Epiro y de su Basílica Alfa (o de Dumetios), construida en el siglo VI d.C., se han analizado las teselas de vidrio pertenecientes a partes del pavimento musivario de la basílica, las cuales han sufrido degrado o han desaparecido, para determinar su composición química y la eventual presencia de fases cristalinas. Al igual que los demás vidrios producidos en el área del Mediterráneo en aquella época, se trata de vidrios de silicato sódico cálcico, en parte plúmbicos, obtenidos utilizando natrón como fundente. El tipo de vidrio corresponde al de los vidrios romanos de los primeros siglos d.C. y es el mismo de los vidrios musivarios empleados en las basílicas de Rávena en los siglos V y VI; en particular, se ha realizado una comparación con la composición de las teselas de vidrio de la basílica de San Vital. Incluso la tecnología empleada para obtener vidrios de colores es prácticamente la misma que la de los vidrios de Rávena. En particular, se ha empleado antimonio como elemento opacador y se ha conducido una fusión en ambiente reductor para obtener vidrio rojo con micropartículas de cobre metálico. Por tanto, es posible plantear la hipótesis de que habría una fuente productiva común de los vidrios musivarios de Nicópolis y de Rávena en el siglo VI y que dicha fuente se habría encontrado en el Cercano Oriente. РезюмеПосле сбора существенных исторических, археологических и художественных сведений, связанных с древним городом Никополь Эпира и его Базиликой Альфа (или Doumetios), построенной в VI в. н. э., были проанализированы стеклянные тессеры, принадлежавшие частям разрушенных полов базилики, и установлен их химический состав и возможное присутствие кристаллических фаз. Как и все стекла той эпохи, сделанные в средиземноморской зоне, это кальциево-натриевые-силикатные стекла, частично свинцовые, полученные при использовании натрона в качестве расплавителя. Типология стекла соответствует типологии римского стекла первых веков нашей эры и такая же характеризует стеклянные мозаики, украшающие базилики Равенны V и VI веков; в частности было произведено сравнение с составом стеклянных тессер базилики Сан Витале. Технология для получения цветного стекла практически такая же как и у равеннских стекол. В частности  была применена сурьма как матирующий элемент и произведено плавление в восстановительной среде для получения красного стекла с микрочастицами металлической меди. Следовательно, возможна гипотеза об общем источнике производства стеклянных мозаик, использованных в Никополе и Равенне в VI веке, и что этот источник был на Ближнем Востоке. 摘要在古老城市伊庇鲁斯尼科波尔和它建于六世纪的第一神殿(或 Doumetios神殿)的相关历史、考古和艺术等特性的重要信息进行了收集之后，学者分析了属于大殿马赛克地板的玻璃镶嵌件并确定了它们的化学成分和结晶相的存在可能，该部分已退化和丢失了。正如地中海地区当时的所有玻璃产品都是钙 - 钠硅酸盐玻璃，一部分也含铅，都用泡碱作为熔剂。该玻璃类型与公元初几个世纪罗马的玻璃相对应，也和五、六世纪的拉文纳大教堂采用的马赛克玻璃一样；尤其是与圣维塔大教堂的玻化砖成分进行了对照。其实彩色玻璃的制造技术也跟拉文纳的相同。尤其当时先用了锑为不透明元素，然后在还原环境下进行了融合，就获得了金属铜微粒的红色玻璃。因此，可以猜想到六世纪拉文纳马赛克玻璃采用的生产来源和尼科波尔相同，并且这来源地是在近东。 
application/pdf
1973-9494
4186
en
Corrado Palmiero
Conservation Science in Cultural Heritage
Ravenna
Study of mosaic glasses from the Alpha Basilica (sixth century) of Nikopolis in Epirus, Greece
Text.Serial.Journal
https://conservation-science.unibo.it/
13
Articles
2013-12-30
2014-02-09
2013-12-30
2015-11-02
231-244
CSCH
Corrado Palmiero
2013/12/30
10.6092/issn.1973-9494/4186
231
1973-9494
Ravenna
en
244
FIORI C., 2011, “Vetro musivo del VI secolo dagli scavi della basilica di S.Severo a Classe (Ravenna)”, Riv. Staz. Sperim. Vetro, XVI (1) pp. 22-34
Study of mosaic glasses from the Alpha Basilica (sixth century) of Nikopolis in Epirus, Greece
https://conservation-science.unibo.it/article/download/4186/3666
13
S    TUDY OF MOSAIC GLASSES FROM THE ALPHA BA-
     SILICA (SIXTH CENTURY) OF NIKOPOLIS IN EPIRUS,
     GREECE



   Cesare Fiori•
   Department of Cultural Heritage, University of Bologna, Italy

   Sabrina Gualtieri
   CNR-ISTEC, Faenza, Italy

   Dimitrios Chrysopoulos
   Directorate of Conservation of Ancient and Modern Monuments, Ministry of Cul-
   ture, Athens, Greece

   Corrado Palmiero
   Degree in Preservation of Cultural Heritage, University of Bologna, Italy



   Keywords: Nikopolis in Epirus, ancient glass, mosaic glass, Byzantine basilica,
   Ravenna




                                                                                                CONSERVATION SCIENCE IN CULTURAL HERITAGE
   1. Introduction

   The main objective of the present work was the characterization of the composition,
and the reconstruction of the production technology, of glass tesserae from the sam-
ples kept in the Archaeological Museum of Nikopolis in Epirus, Greece. The analysed
tesserae belonged to lost parts of mosaic floors in the Alpha Basilica (or Doumetios
Basilica), dated to the 6th century. In addition, the composition of these tesserae was
compared with that of mosaic glasses of the same century recovered in Ravenna, in
particular with the thoroughly studied mosaic glasses from the St Vitale basilica, in
order to verify a possible common technological basis and a possible common produc-
tion source for the two localities.

   1.1 Historical notes

   Nikopolis was founded by Caesar Augustus Octavian to celebrate the victory
against Mark Antony at Actium (31 B.C.), a locality near the site where the new town
stood. At that time, the area was in decline and almost abandoned, so the founding
of a new town contributed to its repopulation [1-2]. Nikopolis subsequently drew the
attention of successive emperors and obtained the statute of civitas foederata, that

• Corresponding author: cesare.fiori@unibo.it

                                                                                          231
                                                                                                          established particular tax concessions. The town was embellished under the Emperor
                                                                                                          Hadrian and later included in the province of Epirus by the Emperor Antoninus Pius.
                                                                                                          Subsequently, due to reforms by Diocletian and Constantine, the town experienced a
                                                                                                          relatively long period of peace and prosperity that was suddenly interrupted by the first
                                                                                                          great barbarian migration. In 363 A.D., Claudius Mamertinus, the prefect of Illyricum,
                                                                                                          described the dramatic condition of Nikopolis after the first barbarian incursions that
                                                                                                          damaged the population (inhabitants either killed or captured, reduced to slavery, and
                                                                                                          ransomed for exorbitant sums of money), buildings and structures (roads, bridges,
                                                                                                          dams, aqueducts), and the general economy and management of public offices (since
                                                                                                          the high levels of taxation could no longer be sustained) [3]. Thus, between  the 4th
                                                                                                          and 5th centuries the town experienced a sharp decline that put its existence at risk,
                                                                                                          due to the downfall of the Roman Empire [4].
                                                                                                             The first Visigoth invasion in Epirus can be dated to 380 A.D. In 396 the Goths of
C. Fiori, S. Gualtieri, D. Chrysopoulos, C. Palmiero - Study of mosaic glasses from the alpha basilica




                                                                                                          Alaric invaded Epirus, then returned in 406, and stayed in Nikopolis for a year. In 429
                                                                                                          the Vandals took over in North Africa, where they founded a powerful nation, and short-
                                                                                                          ly after succeeded in dominating the central Mediterranean Sea through acts of piracy
                                                                                                          and incursions in coastal provinces. In 474 the Vandals, led by Genseric, subjected
                                                                                                          Nikopolis to a particularly heavy sack. Following this event, new walls were built that,
                                                                                                          however, surrounded only a sixth of the area occupied by the town in its full splendour;
                                                                                                          an indication of the strong decline at that time.
                                                                                                             The earliest information about the birth of the Christian Church in Epirus refers to
                                                                                                          the first century A.D., following the passing of the apostle Paul; but the new religion
                                                                                                          experienced difficulties. It was only after the death of the Emperor Julian, the apostate,
                                                                                                          that there was a mass conversion. There was an attempt to limit the excessive number
                                                                                                          of ordinations of the Christian Clergy for fiscal reasons, but religious ferment led to the
                                                                                                          construction of many churches. In Nikopolis five basilicas were built in a period of one
                                                                                                          hundred years. The cultural level of the bishops was high, as they attended ecumenical
                                                                                                          councils, and as is evidenced by surviving inscriptions in the high quality of the floor
                                                                                                          mosaics.
                                                                                                             The construction of new basilicas, during the fifth and sixth centuries, also indicates
                                                                                                          a certain economic recovery of the town. The Emperor Justinian reinforced the walls of
                                                                                                          the fifth century and this saved the internal part of the town from being sacked by the
                                                                                                          Ostrogoths in 551, as reported by Procopius of Caesarea [5]. Among the four basilicas
                                                                                                          inside the new walls, was the Alpha Basilica dedicated to St Demetrius and known as
                                                                                                          Basilica Doumetios, dated to the sixth century.
                                                                                                             However, the barbarian incursions, together with the insufficient economic resourc-
                                                                                                          es and political instability, were the cause of a new, definitive decline of the area and
                                                                                                          the town of Nikopolis [2].

                                                                                                             1.2 Archaeological studies

                                                                                                             The first archaeologist to mention Nikopolis was Ciriaco Pizzecolli, whose scripts
                                                                                                          were published in the eighteenth century; there are also notes by three diplomats from
                                                                                                          the beginning of the nineteenth century: two Frenchmen, Louis Duprè and Françoise
                                                                                                          Pouqueville, and the Englishman, Martin Leake [2, 6]. Archaeological exploration was
                                                                                                          started in 1913 by Alexander Philadelphus [7] who unearthed the Alpha Basilica during
                                                                                                          the excavations of 1915-1918 and began to explore the Basilica B (or Alkison Basilica)
                                                                                                          in 1921. The first local museum was built in Preveza. Other archaeologists took part


                                                                                                    232
in the excavations: George A. Soteriou and Anastassios Orlandos who completed the
unearthing of the two basilicas in 1930. They began the excavation of the Basilica C in
1937-1938 [8-11]. The Second World War interrupted the excavations and damaged
the archaeological site, including the Preveza Museum, which was also subjected to
the sacking of its collections.
    Archaeological exploration was resumed in 1956 by Orlandos [9-10] who worked at
Basilica D and from 1956 began collaborating with D.Pallas [12]. In 1969 Sotiriou Da-
karis, archaeologist and director of Byzantine studies on Epirus, headed the construc-
tion of a new museum built in the archaeological area. Afterwards, the main activity
was dedicated to the restoration and conservation of the archaeological remains. Be-
tween 1991 and 1995, Dakaris and the Boston University archaeologist James Wise-
man collaborated to create the Nikopolis Project. It was a multidisciplinary project,
experimented in the southern part of Epirus, involving a number of prospecting tech-
niques which dealt with the question of ancient settlement patterns in the region and
management of the cultural heritage. A present day view of the Basilica Alpha remains
is shown in Figure 1; a plan of the Basilica [8] is reported in Figure 2.




                                                                                                            CONSERVATION SCIENCE IN CULTURAL HERITAGE

Figure 1. Remains of the Alpha Basilica of Nikopolis, with the built shelters of the mosaics of the
transept south and north wings) and the nartex (south wing). (Photo by C.Palmiero)

    Philadelphus [7] described the Byzantine structure of the Alpha Basilica oriented
west-east (entrance-apse). The plan with the correct measurements was mapped out
by Soteriou [8]. The Basilica was constructed in two distinct phases: (1) at the begin-
ning of the sixth century and (2) around the middle of the same century, according to
the dates proposed by Kitzinger [13]. The total length is 56 m; the main entrance has
a propylaeum, which opens onto an atrium that precedes a narthex; the main body
of the church has a nave and two aisles, with the nave separated from the aisles by
colonnades; the apse is raised 1.5 m above the nave level. R. Krautheimer [14], in his
study on paleochristian and Byzantine architecture, identified Greek churches as hav-
ing a distinct architectural style, with influences from the Eastern Church, as regards
the liturgy, and the Western Church, related to the masonry. In this period the basilicas,
of remarkable dimensions, on the outside appeared simple and bare, while all the rich-

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                                                                                                          ness was concentrated in the interior, with gilded ceilings, furnishings with gold and
                                                                                                          silver, columns made of precious marbles, and fine mosaic decorations.
C. Fiori, S. Gualtieri, D. Chrysopoulos, C. Palmiero - Study of mosaic glasses from the alpha basilica




                                                                                                          Figure 2. Plan of the Alpha Basilica of Nikopolis [8]


                                                                                                            1.3. The mosaics

                                                                                                             Kitzinger [13] based his dating of the two construction phases of the basilica on
                                                                                                          the stylistic, iconographic and epigraphic characteristics of the mosaics. Following the
                                                                                                          chronological order proposed by Kitzinger and reports of the excavation campaign
                                                                                                          by Philadelphus [7], who saw that most of the floor was still intact, M. Spiro [15] has
                                                                                                          described the mosaics in detail, analysing drawings, colours and materials. The floors
                                                                                                          of the second decorative phase are of a remarkably inferior quality in comparison to
                                                                                                          those of the first phase.
                                                                                                             The central part of the floor of the atrium, referable to the second construction
                                                                                                          phase, was decorated with a mosaic epigraph which cited the dedication of the church
                                                                                                          to St Demetrius and the name of the episcopal sponsors of the first and second phas-
                                                                                                          es: Doumetios I and his successor Doumetios II. At the western entrance was another
                                                                                                          epigraph reporting the information that Doumetios I was the promoter of the building,
                                                                                                          from its foundations to the decorative scheme [16]. The preserved epigraphs confirm
                                                                                                          the dedication of the church to St Demetrius. Part of the floor mosaic was lost after
                                                                                                          the excavations; only those of the transept and the southern wing of the narthex are
                                                                                                          preserved as they were laid on new supports and protected by the construction of
                                                                                                          shelters (Figure 1). In the mosaic carpets a clipeus decoration with figures of hunters,
                                                                                                          animals, plants, and fruits predominates. Details of the mosaics are shown in Figures
                                                                                                          3 and 4.



                                                                                                    234
   The mosaic carpets of the north (prothesis) and south (diakonikon) wings of the
transept are referable to the first phase of construction of the basilica. In the northern
wing, there is a mosaic panel (Figure 3) that, besides the epigraph, shows fruit trees
alternated with cypresses, two guinea-fowls and birds in the sky; the tones of the
colours are very strong, particularly the very dark green of the trees that contrasts
with the yellow and red of the fruits. Outside the panel, there is a frame with waves
in turn surrounded by a band in which water fowl, fish and aquatic plants are rep-
resented. In this pattern a strong Hellenistic style persists and the central panel is
probably a schematic representation of the whole world, corresponding to a popular
cosmological tradition. In the southern wing of the transept there is a mosaic panel
composed of a badly damaged central emblema surrounded by 16 clipeus, deco-
rated with acanthus and racemes, containing representations of hunters and animals
(Figure 4); on the outer band fish and aquatic plants are represented. A large section
of the hunting scene in the centre is lost, thus its interpretation is difficult; assuming
a correspondence with the northern wing, it can be supposed that it deals with the
Garden of Eden. Thus, the two panels of the transept should represent a complex
cosmological and geographical scheme of the Earth, through a precise and sche-
matic symbolism [13, 17].
   In the southern wing of the narthex, the mosaic floor shows a more common style
and subjects, such as in the outer band with polychromatic interlaced patterns and
the inner part with small squares containing pictures of animals and geometrical and
botanical motifs. The most important part is the epigraph.




                                                                                                       CONSERVATION SCIENCE IN CULTURAL HERITAGE




Figure 3. Particular of the mosaic in the southern wing of the transept. (Photo by C.Palmiero)




                                                                                                 235
C. Fiori, S. Gualtieri, D. Chrysopoulos, C. Palmiero - Study of mosaic glasses from the alpha basilica




                                                                                                          Figure 4 – Particular of the mosaic in the southern wing of the transept. (Photo by C.Palmiero)


                                                                                                             2. Archaeometric study of the glass mosaic tesserae

                                                                                                              Glass tesserae were amply used for the floor mosaics of the Alpha Basilica. This is
                                                                                                          rather unusual for that epoch, when the use of more resistant stone tesserae prevailed.
                                                                                                          However, the mosaics decorating the floors of the Basilica were reserved exclusively for the
                                                                                                          clergy and authorities and therefore less subject to deterioration from foot traffic. The glass
                                                                                                          tesserae are mainly in shades of blue and green, but tones of yellow, ochre, red and black
                                                                                                          are also present.
                                                                                                              The twelve analysed tesserae were selected from among those lost and damaged parts
                                                                                                          of the floor mosaics of the basilica. Knowing the period of construction of the church was
                                                                                                          the sixth century A.D., and that the twentieth century excavations and studies did not evi-
                                                                                                          dence either past restoration such as integrations or that any kind of intervention was pro-
                                                                                                          moted, except for the consolidation and protection of original parts, the examined glass
                                                                                                          tesserae are certainly original. Thus, their characterization is useful to trace the production
                                                                                                          technology of glass referable to a well-defined period, which may at most be extended to
                                                                                                          the second part of the previous century. Furthermore, the perfect correspondence with the
                                                                                                          construction period of the Byzantine basilicas in Ravenna (fifth/sixth centuries) makes an
                                                                                                          interesting comparison with the characteristics of the well studied mosaic glass of Ravenna
                                                                                                          and, in particular, those of St Vitale, which represent one of the most important productions
                                                                                                          and applications of mosaic glass of that period [18].

                                                                                                             2.1 Analytical techniques

                                                                                                             Determination of the composition of the glass samples was performed through chemical
                                                                                                          and mineralogical analyses. The quantitative chemical compositions were obtained using a
                                                                                                          scanning electron microscope equipped with energy dispersion microanalysis (SEM-EDS).
                                                                                                          The Cambridge-INCAX, Sight Oxford Instrument, mod.7060, with STEREOSCAN 360
                                                                                                          electronic gun and a thermo-ionic emission tungsten wire, was used. Operating conditions
                                                                                                          were as follows: a fragment of cobalt as a benchmark for energy intensity was used; the
                                                                                                          distance of the sample surface from the objective lens (WD) was of 25 mm, and the beam
                                                                                                          acceleration (ETH) was of 15 KV. Before the analysis procedure, small fragments of the

                                                                                                    236
samples were fixed on aluminium supports with adhesive cement Leitc-C and then coated
with a very fine graphite film using the instrument Assing Automatic HR Sputter Coater. With
the aim of obtaining representative results, a minimum of ten analyses was carried out for
each sample, on areas of 1 mm2. The X-ray diffractometric (XRD) analysis was carried out
using the Rigaku instrument, mod. Miniflex, with CuKα1 radiation, nickel filter, angular veloc-
ity ϑ/2ϑ of 0.25/0.5°/min, 2ϑ range from 4° to 64°. A small fragment (about 0.3 g in weight) of
each tessera was pulverized for the analysis.

   2.2 Results and Discussion

   In Table 1 the results of the chemical investigation are reported. The high percent-
age of sodium (15% Na2O approx.), as well as silica, and the relatively high quantity
of calcium (6% CaO approx.) are clearly evident. Seven samples (1,2,3,4,5,10 and 11)
are also characterised by amounts of lead oxide comprised in the range 5-12% PbO.
The remaining five samples have very low or even no lead contents. Aluminium oxide
(around 2% Al2O3), and magnesium and potassium oxides (less than 1%), as well as
chlorine, can be considered as impurities in the raw materials used to produce the
glass. In particular, the alumina content is to be ascribed to the presence of feldspars
and micas in the sand used as silicate raw material. The fairly variable amounts of ele-
ments such as iron, copper, manganese, cobalt, antimony, and tin are connected with
obtaining the various colours, as discussed below.

Table 1. Chemical composition




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                                                                                                  237
                                                                                                             On the basis of the chemical composition, the glass of the tesserae is classifiable as
                                                                                                          soda-lime silicate glass, obtained mixing natron as the sodic flux (a natural deposit of
                                                                                                          salts in Egypt consisting mainly of a sodium carbonate) and a quartziferous sand. This
                                                                                                          glass typology corresponds to that of Roman glasses of the first centuries A.D. [19-
                                                                                                          20]. The low percentages of potassium and magnesium oxides, as also illustrated in
                                                                                                          the correlation diagram of Figure 5, are indicative of the use of natron instead of plant
                                                                                                          ashes, that are a possible alternative flux for ancient glasses.
C. Fiori, S. Gualtieri, D. Chrysopoulos, C. Palmiero - Study of mosaic glasses from the alpha basilica




                                                                                                          Figure 5. MgO vs. K2O diagram, also showing the compositional field of the tesserae from
                                                                                                          S.Vitale (Ravenna)




                                                                                                          Figure 6. CaO vs. Al2O3 diagram, also showing the compositional field of the tessearae from
                                                                                                          S.Vitale (Ravenna)


                                                                                                             The presence of lead in the above mentioned samples is linked to technological re-
                                                                                                          quirements because certain colours (bright shades of green, yellow, orange, and red)
                                                                                                          could only be obtained with a lead-based glass. The lead component of the fusible mix

                                                                                                    238
could be derived from the recovery of metallic alloy scraps or from metallurgy slag [19,
21]. These seven samples are also characterised by slightly higher concentrations of
potassium and magnesium oxides than unleaded samples (Figure 5); this may be due,
for the distinct production of leaded glasses, to the use of raw materials (sand and flux)
with higher amounts of impurities containing potassium and magnesium. The “red” sam-
ple also has a potassium content over the average of natron glasses, but not so high
as to denote the use of plant ash as flux. This content is not an anomaly for red glasses
produced in that epoch; in fact, these glasses were produced under reducing conditions,
probably obtained with the introduction of fuel (wood) ash, relatively rich in potassium,
into the fusible mix [22-24]. In the MgO vs. K2O diagram, the compositional field of the
mosaic glasses of St Vitale is also reported [19], into which the samples from Nikopolis
fall (with the exception of the above mentioned red glass).
     As regards the type of quartziferous sand used, the relationship between alumina and
calcium oxide contents in Nikopolis glasses (Figure 6), in accordance with that of Byz-
antine mosaic glasses from St. Vitale, seems to confirm that a type of sand such as that
found at the mouth of the River Belus in ancient Palestine (today Haifa Bay, Israel) was
used. This sand also contained fragments of shells, and thus calcium carbonate, in an
optimal dose for providing calcium as a stabilizing element of the glass; therefore, it was
used for the primary production of glasses of the Roman epoch and glasses of the early
Byzantine period (Levantine I type), in the Egyptian-Palestinian area [20]. In the CaO vs.
Al2O3 diagram, it can be noted that the leaded glasses are slightly shifted towards the
left and downwards, maybe due to the relative decrease in the sand content as a conse-
quence of the introduction of the lead component into the fusible mix.
     In some tesserae, with the highest lead oxide content, small amounts of tin oxide were
measured. As ascertained in previous studies on mosaic glasses of the sixth century [19,
25], the presence of tin is linked to that of lead, and not due to the intentional addition
of tin as an opacifier, as antimony was used for this function. The use of tin instead of




                                                                                                       CONSERVATION SCIENCE IN CULTURAL HERITAGE
antimony as an opacifier for mosaic glasses became widespread after the sixth century.
     As regards the colouring elements, copper was used for obtaining the leaded green
glasses. The variability in the concentration of copper oxide is due to the intensity of
the final hue. These glasses also present a moderate quantity of iron contents and, in
some, traces of antimony. In the light blue and blue glasses, with PbO<0.5%, the colour
is due to very low percentages (less than 0.09% CoO) of cobalt oxide, that however has
extremely high colouring capacity; in this case too, the intensity of the colour increases
with the concentration of the colouring element. The iron content influences the colours
of the ochre, red and black glasses, naturally together with other parameters, such as the
concomitant presence of other elements and melting atmosphere conditions. More in de-
tail, the ochre glass owes its  colour to the presence of iron, lead (9% PbO approx.), and
antimony traces, while in the black glass it is the high amount of iron oxide (5% Fe2O3
approx.) and the absence of lead that are responsible. As mentioned before, the red
colour was obtained by melting in reducing conditions a glass with a moderate content
of lead oxide (around 5%) in which very fine metallic copper particles were segregated;
it is these that are actually responsible for the colour. The relatively high content of iron,
together with the probable addition of fuel (wood) ash during the fusion, contributed to
the attainment of the reducing conditions [25].
     Apart from a clearly opaque light blue glass, the tesserae are either translucent or the
colour is so intense that the glass is not transparent. The X-ray diffraction study allowed
us to determine the presence of opacifying crystalline phases in a part of the samples,


                                                                                                 239
                                                                                                          which developed in a remarkable way or only as traces, depending on the variable con-
                                                                                                          tent of antimony oxide. In some unleaded glasses (6,7, and 8) the crystalline phase of
                                                                                                          calcium antimonate, CaSb2O6, is present; this is particularly evident in the opaque light
                                                                                                          blue sample (6), as ascertained through SEM observations (Figure 7a) and XRD analy-
                                                                                                          sis (Figure 8). In green (1, 3, and 5) and ochre (10) samples containing small amounts
                                                                                                          of antimony oxide, traces of the crystalline phase of lead antimonate, Pb2Sb2O7, known
                                                                                                          to be yellow in colour, were detected (Figure 9). However, while in the green samples
                                                                                                          these traces did not significantly influence the colour due to the colouring element cop-
                                                                                                          per, in the ochre tessera, lead antimonate most likely contributed to that particular shade
                                                                                                          of colour. For the red tessera, the XRD spectrum showed some peaks attributable to Cu
                                                                                                          (Figure 10) confirming the interpretation at SEM observations (Figure 7b).
                                                                                                             The tesserae without crystalline phases (or with their presence below the detectable
                                                                                                          level) are some of the green (2 and 5), the dark blue (9) and black (12) ones.
C. Fiori, S. Gualtieri, D. Chrysopoulos, C. Palmiero - Study of mosaic glasses from the alpha basilica




                                                                                                             The results of all investigative analyses seem to indicate that the technology for the
                                                                                                          production of the coloured mosaic glasses fully corresponds to that of the mosaic glass-
                                                                                                          es of Ravenna [19].




                                                                                                               a                                                  b

                                                                                                          Figure 7. a) SEM photo of the red tessera showing the presence of the crystalline phase metallic
                                                                                                          Cu; b) SEM photo of a light blue tessera showing the presence of the crystalline phase CaSb2O6




                                                                                                          Figure 8 – X-ray diffractogram of a light-blue tessera showing the presence of the crystalline
                                                                                                          phase CaSb2O6



                                                                                                    240
   3. Conclusions

    An examination of the literature and an inspection of the site, with a visit to the lo-
cal archaeological museum, have allowed us to obtain the essential historical-artistic
information relating to the mosaic floors of the Alpha Basilica of Nikopolis in Epirus built
in the sixth century A.D.




Figure 9 – X-ray diffractogram of a green tessera showing the presence of the crystalline phase
Pb2Sb2O7




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Figure 10. X-ray diffractogram of the red tessera showing the presence of the crystalline phase
metallic Cu


   Glass tesserae belonging to lost and degraded parts of the mosaic floors of the
Basilica were analysed determining their chemical composition and the possible pres-
ence of crystalline phases.
   As in all glasses produced in the Mediterranean area in that epoch, they are soda-
lime silicate glasses, in part with lead, obtained using natron (a natural Egyptian de-
posit of salts, mainly made up of sodium carbonate) as a flux (and not plant ashes, an
alternative flux used in other areas or epochs); this is evident from the high sodium
oxide content and low concentrations of potassium and magnesium oxides. The raw


                                                                                                  241
                                                                                                          silicate material was of the kind present at the mouth of the River Belus (ancient Pal-
                                                                                                          estine). The typology corresponds to that of the primary production in the Egyptian-
                                                                                                          Palestinian area, of Roman glasses employed in the same period for the basilicas
                                                                                                          of Ravenna; in particular, the comparison was made with the widely studied glass
                                                                                                          tesserae of the St Vitale Basilica.
                                                                                                              As concerns the colours, the green tesserae were obtained with leaded glass and
                                                                                                          copper as the main colouring element; in some green glasses traces of the crystalline
                                                                                                          phase lead antimonate, with a yellow colour, are present, but they do not affect the
                                                                                                          colouring that is, instead, affected by the relatively high presence of copper. The light
                                                                                                          blue and blue colours have an unleaded base (or only traces of lead) and the colour-
                                                                                                          ing was obtained using small quantities of cobalt, the concentration of which increases
                                                                                                          accordingly with the intensity of the colour. The light blue colour is opaque due to the
                                                                                                          presence of the crystalline phase calcium antimonate. The ochre colour is due to iron
C. Fiori, S. Gualtieri, D. Chrysopoulos, C. Palmiero - Study of mosaic glasses from the alpha basilica




                                                                                                          as the main colouring element together with traces of lead antimonate which, in this
                                                                                                          case (the copper being absent), have contributed to the colour. The red leaded glass
                                                                                                          was obtained under reducing conditions that allowed the formation of metallic copper
                                                                                                          particles. The black colour of the glass, unleaded, is in relation to the particularly high
                                                                                                          concentration of iron oxide.
                                                                                                              The technology behind the production of coloured glasses is quite analogous to that
                                                                                                          of the mosaic glasses of Ravenna. It can therefore be assumed there is a common
                                                                                                          source for the production of the mosaic glasses used in Nikopolis and Ravenna in the
                                                                                                          sixth century and that this source was in the Near East, perhaps in Constantinople
                                                                                                          itself.



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                                                                                                            Biographical notes

                                                                                                             Cesare Fiori, Associate Professor of the scientific division “Chemistry of the  en-
                                                                                                          vironment and cultural heritage” at the Department of Cultural Heritage, University of
                                                                                                          Bologna, branch of Ravenna (Italy).He currently teaches “Conservation and treatment
                                                                                                          of materials” and “Restoration Chemistry” in the first level degree of Conservation of
                                                                                                          Cultural Heritage, and “Techniques for the conservation of mosaics”, in the Master of
                                                                                                          Science Degree of Conservation and Valorization of the Archaeological Heritage.

                                                                                                             Sabrina Gualtieri, Researcher at the Institute of Science and Technology of Ceram-
                                                                                                          ic Materials, National Research Council (CNR), Faenza (RA), Italy. Research Group:
                                                                                                          New technologies for the environment, construction industry and cultural heritage; sec-
                                                                                                          tor: Archaeometry, diagnostics and technology for cultural heritage conservation.
C. Fiori, S. Gualtieri, D. Chrysopoulos, C. Palmiero - Study of mosaic glasses from the alpha basilica




                                                                                                            Dimitrios Chrysopoulos, Conservator of mosaics. Head of the Department of
                                                                                                          Conservation of Byzantine Antiquities, Directorate for the Conservation of Ancient and
                                                                                                          Modern Monuments, Ministry of Culture, Athens, Greece. Currently retired.

                                                                                                             Corrado Palmiero, Degree in Conservation of Cultural Heritage, University of Bo-
                                                                                                          logna, Ravenna (Italy).




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