Spectroscopic techniques for the characterization of the potsherds from Tigranakert in Artsakh

Archaeometry and archaeological science correspond and refer to the application of scienti�c techniques to the analysis of archaeological materials, as well as the processes involved in their manufacture [Williams 2005, Virgil de la Mencia 2008, Martinon-Torres 2015]1–3]. This paper presents study of a set of ceramic samples using stratigraphic analysis coupled with SEM/EDX technique. Ceramic materials provide information on the clay materials employed in their manufacture [Hradil et al 2018, Giannosa et al 2020]4,5], hence facilitating the assignment of their autochthonous or allochthonous character [6–8]. Analysis of the cross section provides to the stratigraphic evaluation of the ceramic body (holes, cavities, imperfections, granulometry, inclusions and color), the glaze or other super�cial layers (enamels, transparency or opacity, color and saturation, defects and other): SEM/EDX technique allows obtain information on the samples (texture and microstructure, phases and minerals recognition by electron backscattered diffraction, all these analysis are carried out simply tacked a very small fragment from the surface, including glaze and ceramic body (less than a couple mm), embedded in polyester resin and grinded and polished without destruction of the specimen.


Introduction
Tigranakert in Artsakh discovered by H. Petrosyan [9][10][11][12].The late Hellenistic city of Tigranakert, now under Azerbaijani control, is located in the Askeran region of Artsakh (Nagorno-Karabagh), in the lower valley of the Khachenaget river, which is the second largest river in the highland (Fig. 1, left).It is spread over the south-eastern slopes of Mount Vankasar and is adjacent to the slopes near the "Royal Springs" (Şahbulaq).The city was founded at the end of the 90s BCE by the Armenian King Tigranes II the Great (r.95-55 BCE) and functioned until the end of the 13th century.
During fteen years of excavations directed by Hamlet Petrosyan, different sections of the city were discovered (Fig. 1, right): the Late Hellenistic forti ed district (1st c.BCE) and the citadel, the rst and second Late Hellenistic districts, the Late Hellenistic cemetery with jug and cist burials, the Early Christian rock carved complex and a rocky canal near the city the Early Christian cemetery the Early Christian square with remnants of two churches, a memorial stele, an Early Christian underground reliquary-sepulcher and a graveyard.(Fig. 2) In Middle Ages (4th-13th centuries), Tigranakert maintained its role as an important urban center.More than 4000 specimens of glazed pottery found during excavations are the most reliable evidence of urban life.The vast majority of them are open vessels: bowls, plates, saltshaker.There are also small jars, jars, oil lamps.The color of the ceramics varies with a range of brown-reddish-yellow colors on the bottom of some ceramics there are so-called main marks-different geometric compositions, which are printed with a special stamp on the wet residue of the amphora.In all these groups there are also amphorae covered with only one enamel.The main colors of the enamel and the individual paints are green, blue, yellow, brown.The compositions mainly correspond to the shape of the amphora: the arrangement of individual elements (geometric and vegetative), radius or circular, dispersion of small ornaments over the entire surface.One of the popular themes of the paint is the bird in the garden, which meets deer (images of deer).The study of technological speci cs of Early Bronze Age ceramics of Armenia was conducted by Navasardyan [13,14].

Experimental conditions
Tigranakert glazed pottery collection is presented with examples of the early faded glazing of the 9th century (sub-glazed painting with paintings, sample 20, Fig. 1), enameled pottery from the 10-11th century, engobe painting (the "engobbio" is a covering and decoration technique for terracotta and ceramics), samples 05-07, 12, 14-17 and XII-XIII century, engraved on ceramic samples 01, 02, 04.An analytical approach took place rst by observing the fragments with a stereomicroscope.Besides, also the thickness of each fragment was observed positioning the pro le of each one in a sand bed to show the whole strati cation.The surfaces and the pro les of the fragments were observed at various magni cations (0.8x − 16x) and photographed with the Leica digital camera.From each fragment a micro-sample including the entire thickness (bulk ceramic enamels and concretions) was taken; it was embedded in polyester resin (Mecaprex) and subsequently ground and polished transversely to obtain a cross-section.The samples were observed with a Zeiss optical microscope equipped with white Led and ultraviolet (Zeiss Colibri UV LED, 360 nm) light sources and objectives Epi uar with 10X to 50X magni cation; the images were recorded with a Canon EOS 1000 camera with 16 Mpx sensor and the Canon Image Browser as acquisition software.Subsequently, the same cross-sections were mounted on Al stubs (Ø=13 mm) with a conductive adhesive based on metallic Ag (Agar silver paint) and metalized by a graphite sputter (Agar scienti c sputter-coater, ca 40 mA), observing and analyzing them in high vacuum SEM (9 − 10 bar).The EDX elements were performed employing an EDS microprobe [Oxford "X-Max 80 Silicon Drift Detector" (SDD)] and the analytic results were displayed both in the form of XRF of spot or areas spectra and also as main elements distribution maps, were processed using the Oxford Instrument "AZTEC® 2.0" software.

Results and discussions
Ceramic is usually composed of different materials: clays, feldspar (sodium, potassium or both), quartz, iron oxides and alumina.Such articulated composition determines the presence of attened molecular structures called phyllosilicates.The shape of these, in the presence of water, gives to clay a certain plasticity and makes its processing easier and more pro table.Navasardyan [14] described some technical and technological speci cs of ceramics in Armenia (III-II millennia B.C).The representative images of samples are reported in Fig. 3.
They are among the most common artifacts to be found at an archaeological site, generally in the form of small fragments of broken pottery, called potsherd or sherd.Processing of collected sherds can be consistent with two main types of analysis: technical and traditional.The traditional is based on style, composition, manufacturing, and morphology.In the 13th century glazed ceramics changes its color palette.They became more monochrome.Ceramic artifacts have an important role in archaeology for understanding the culture, technology, and behavior of peoples of the past.Some fragments representatives of the type of pottery and decoration, were taken from the pieces selected for the analyses; they were photographed with the same magni cation factor resting on a bed of sand in order to be observed planarly.Each one has been oriented according to the maximum diversi cation of the surface appearance and cut with a mini diamond wheel; subsequently the sample obtained was embedded in polyester resin, ground and polished to obtain the correspondent cross sections (Fig. 4).
The technical approach to ceramic analysis involves a ner examination of the composition in order to determine the source of the material and through this possible manufacturing [15].The thicknesses of both slip and glaze covered on the ceramic body may have an active role in macroscopic identi cation.Since, single and/or multi-layers have a strong effect on optical properties (transparency level, refractive index, absorption coe cient, etc.), which are decisive factors during the naked eye examinations.Not only the thickness of glaze but also that of the interface zone are affected by some variables like ring temperature, soaking time, the composition and viscosity of the suspension to be used as glaze material depending on glazing technology (dipping, spraying, burnishing, etc.) [16].The differences in the thickness of glaze could be attributed to de ciencies caused by the used glaze application method as reported in [17].They studied on the technology issues of selected Byzantine glazed pottery and they reported that the thickness of the glazed layer was 60-100 µm, however, it could be wider and in some cases the thickness was found as 200 µm in rims or highly curved fragments [18].For this purpose, in this section, the thicknesses of both slip and glaze layers are measured in order to group the sherds before microanalytical studies.In medieval ceramics, the underlying slip has low CaO concentration and the slipped ceramic bodies were biscuit-red.The bubbles in the glaze layer may be due to low ring temperatures or not long enough soaking time [19].Finally, the temper used in potters consists of several sand-sized minerals and rock fragments, including limestone, quartz, pyroxenes, amphiboles, feldspar, and fragments of igneous and metamorphic rocks.It was observed also the use of volcanic ash temper in ceramic industries, crushed obsidian and grog, crushed obsidian and bone as arti cial temper.

Analysis of samples
Observation of fragments with the stereomicroscope demonstrates the following: Sample 03.Dark orange color, glaze blue light and dark with two layers, there are only Si and Fe, without engobe.
Sample 04.Dark orange with white regular engobe, dense, glaze is green emerald, there are three layers present.
Sample 05.Yellowish color with white regular engobe.Major elements are Si, Ba and Ca.Barium used as primary ux in glazes to produce satin matt surfaces with exciting colors.
Different types of ceramic body have been distinguished, differing in color, neness and presence of slimming or inert substances, but they can basically be reduced to three main types (Fig. 5).
The decorative layers of the ceramics are of different type: enamels are present in few cases (i.e., sample 9), the most of all are based on transparent colored glass and glaze.The thickness of the glaze and slip determines the optical features such as transparency level, refractive index, absorption coe cient of the sherds.The color tone is achieved through the superimposition of the glass on substrates of different colors, such white underlayer to obtain bright color (Fig. 5, left) or green glaze on red underlayer to reach dark or brownish color (the red color is the complement of green and in the subtractive synthesis of the colors a dark brown is obtained).Otherwise, a part of them is composed by semi opaque, colored glaze, obtained by vitri cation of mixture of glaze and quartz, silica or feldspar dust as matting material (Fig. 4, center).In several examples iridescent effects were produced by alternating colored glasses on opaque substrates (Fig. 4, right).In some samples were detected darker, intense colored lines and curves (Fig. 4, left): they were obtained by engraving the surface of the ceramic after covering it with a white base and then applying the green glaze for the last re: the thicker green trace absorbs more light giving a darker and intense color.
Many samples showed various defects and morphological characteristics attributable to aging and manufacturing techniques (Fig. 6).
The SEM/EDX analyses carried out on the cross sections observed under the optical microscope make it possible to examine the presence and distribution of the chemical elements in the various layers, thus allowing to study both the ceramic manufacturing techniques and their morphological and chemicalphysical alterations.
In Fig. 8. representative image of analyses on sample 10 have been reported.
Figure 8 shows a typical example of the analysis procedure for establishing the composition of the decorative layer of the fragment (Fig. 8, left): from obtaining the cross section to the primary electron image (BSE, Fig. 9, middle) and the subsequent pointing of the electron beam on various spots and areas of the sample (Fig. 8, right, EDX spectrum of the glass (spectrum 42).SEM/EDS analyses are very precise and effectives for the determination of the color elements and for the type of inclusion/opaci er.The Fig. 9 illustrates three samples where the BSE image makes visible the differences between the elements in order of atomic number: heavy elements (such as Pb) appear bright, light elements (C, Si, Al) appear dark, while intermediate elements take on increasingly lighter shades of gray as the number increases.Surface coating composition of the samples 03, 06 and 07 is given in Fig. 9.
The SEM/EDX management software (Oxford Instruments Aztec® 2.0) allows to graphically organize the results of the analyzes by highlighting the punctual presence of the various constituent elements in the cross section.This greatly facilitates investigations, allowing the "map", i.e., the distribution, of each single element to be evaluated at a glance.The maps thus obtained can be coded with arbitrary colors and superimposed, recreating a general map in false colors with the punctual distribution of the main elements and their associations.In Fig. 10 comparison between BSE image and the reconstructed main elements EDX map is reported.
As related in the summary table, some samples show needle-like formations at the interface between the ceramic glaze and the biscuit (samples 04, 05, 09, 10, 11).These formations are very ne and are only visible under high magni cation with SEM and by mean of BSE detector (primary electrons: high contrast among the light Al and K nuclei of the needles and the heavy Pb of the enamel, Fig. 11.
The genesis of these needle formations probably occurred during the vitri cation phase of the surface decoration through the migration into the melt of feldspars which acted as uxes and which remained con ned to the interphase during cooling [16].Figure 12 shows the different percentage composition of the elements of the enamel and needles: the latter have the typical composition of alkali feldspars.The compositional difference of the two spectrums is appreciable: the glaze has the same composition as the colored glazes based on silicon and lead, in this case a brown containing Iron and Manganese, while the needles have a composition coinciding with the potassium aluminum silicates, generically feldspars, belonging to the ceramic body.
Other samples examined showed some peculiarities due to the presence of speci c elements, such as for example Sn, in the form of oxide, index of a white enamel (this compound between lead and tin was found on the internal surface of sample 09), it is probably the glazed interior of a bowl or plate.Sample 11 also has tin dispersed in the opaque green glaze, surely it is an addition to obtain a covering color for the ceramic, not detecting any layer of white or colored slip on the terracotta (Fig. 13, left).
Finally, the presence of volcanic glass was ascertained in the terracotta of sample 9; it is composed on the basis of phosphate of some particular rare earths, such as cerium, lanthanum, neodymium, gadolinium, promethium and samarium, typical of tufa and pozzolanic soils, impurities of the clay used for the manufacturing (Fig. 13, right).
The modi ed paste region corresponds to the part of the body that has reacted with the glaze.It appears porous and more compact.Dark grains (corresponding mostly to quartz or aluminosilicates particles) lighter and SEM-EDX mapping shows that it is due to lead impregnation into the paste.It also seems are less numerous, often smaller (less than a few microns) and more altered than those.present in the less porous and more compact.Dark grains (corresponding mostly to quartz, or aluminosilicates unreacted paste (Fig. 13a).This can be explained by the fact that during the ring lead is acting as a particles) are less numerous, often smaller (less than a few microns) and more altered than those ux and induces an amorphization of the body, leading to a more homogeneous aspect with fewer resent in the unaffected paste (Fig. 14a).This can be explained by the fact that during the ring porosities and inclusions are diminish.Moreover, the modi ed paste matrix is showing a lot of submicro-compounds and lead is acting as a ux and induces an amorphization of the body, leading to a more nano-metric contrasts (visible in BSE mode), indicating that sub-micro-and nano-sized compounds have aspect with fewer porosities and inclusions.Moreover, the modi ed paste matrix is showing a lot of different chemistries present in it (not analyzable using EDX which is probing a micro-sized, sub-microand nano-metric contrasts (visible in BSE mode), indicating sub-micro-and nano-sized volume).
It is shown that the increase of iron content in clays causes a decrease of the ceramic strength.This behavior is related to the clay compositions, which contain iron in the form of goethite.During the thermal transformations, part of the iron is involved in both the structural transformations and the densi cation phenomenon of the silico-aluminate phase.But the most part of iron readily transforms into hematite crystallites, which are embedded within the silico-aluminate.
The unglazed pottery of the 9th -13th centuries determined the level of development of the ceramic production of medieval Armenia and had deserving role in the ceramic production of the Trance Caucasus and the near east.Large amounts of pithoi, jars pots were found.Already in the vessels of the 9th century, there were signi cant de ections in comparison with the earlier objects.Since the 9th century the glass-making in Armenia was developing very quickly.Free blowing which was the main technique of production, blowing in the mould connected to the mass production.In the discussed period, Artsakh was one of peripheral provinces of Armenia and plaid a role of a buffer between the central provinces of Armenia on one side and the Caucasus foothills on the other.The early group of the glazed pottery of Artsakh is represented with several shards related to the 9th century, decorated with painting under the lead glaze.The pottery of the 12-th-13th centuries was decorated by the same

Conclusions
The coexistence of ceramic groups made up of two or more typological groups demonstrates their contemporaneity.In this distinct system it is obvious that, demonstrating stratigraphically the synchronism of mixed complexes, one can speak of distinguishing a certain 'transitional period' and revealing the peculiarities of this or that 'transitional period'.The excavated material is peculiar for discovering perfect samples of polychrome pottery.
technique.The pottery of the 10th -11th c. is represented by several shards, which have very delicate engraving and were covered with polychrome glaze.The complicated geological structure in Armenia, its broken relief, abrupt changes of the climate are important factors due to which variations of clay are formed.Clay is formed by the different siliceous rocks, weathering ( uvial), by accumulating the weathered rocks in the water basin (lake, sea) or in the river beds (alluvial, proluvial), by glaciers (moraine).
The colour of glaze depends on different metal oxides, their quantity and of the type of glaze.For instance, 0,32% of copper in basic composition gives the glaze the light blue colour, while copper of the same quantity in leaden glaze gives it turquoise green colour.
Different tones of purple glaze depend on the quantity of manganese oxide e.g., in the glaze deep purple colour caused by 7.5% of MnO 2 .
Combination of Mn and Fe gives the glaze dark brown colour (Mn-1.3,Fe-2,4%) The dark blue colour is caused by 0.24% of cobalt, but in this set of samples none contain Co.
The SEM/BSE/EDX analysis technique performed on the stratigraphic sections allows us, in addition to a particularly in-depth morphological study of the decorated surface layers (phases distribution, crystals shape, defects, granulometry, mixtures), to examine not only the main components but also the presence of other micro-or oligo-elements and their quality.
The existence of Al 2 O 3 in the glass testi es the use of obsidian as additional raw material.
The glazed pottery is characteristic for the 10th -11th centuries and represented with vessels made by the method of linear ornaments and engobe removal under the transparent, green and yellowish glaze.
Glazed vessels of the 12th -13th centuries were made by the method of engraving and linear ornamentation under green and yellowish glaze.
Monochrome pottery with green glaze was the most ancient type of the glazed wares.
The white paste basis made of pure quartz sand, kaolin and feldspar (as gluing material), faience became largely used.Along with this new basis the new type of glaze was taken into circulation.
Figures   The representative images of samples.They were excavated at 2017, from Central quarter, Early Christian Square.12 th century, geometric or oral ornament Figure 4 18 sampled fragments (from top to bottom and from left to right, in order sample 3 [1] → sample 20) oriented according to the cut stratigraphic section (red line = surface of the cross section).The portion of the ground surface was oriented during the embedding phase to obtain the maximum exposition of the thickness of the complete ceramic surface.
[1] Samples 1 and 2 were lost during cutting Appearance of different type of terracotta bulk: sample 06 beige, ne-grained; sample 10 red-orange, coarse-grained; saple15 yellow, medium-grained within black charcoal and obsidian grains.
Above: embedded fragments and corresponding cross sections: the horizontal red line on the above images represents the edge of each lapped cross section.Below: layering of surface coatings: sample 04 -white underlayer beneath the green transparent glass; sample 09-semi-opaque glaze directly applied on a terracotta; sample 12 -red underlayer beneath an opaque glaze.In the left image is visible the dark green line obtained by the greater thickness of the super cial green glass layer present in the engraving.Defects of surface coatings fracture of the glaze, bubbles, inclusions are reported in g.7.
Figure 7 Defects of surface coatings: sample 03 shows fracture of the glaze white underlayer, sample 18inclusions, bubbles, and super cial deposits.
Representative image of analyses on sample 10, from left: fragment cross section (re ected light); SEM/BSE image with spot and area analyzed by EDX spectroscopy; EDX spectrum of the glass (spectrum 42, spot on green glass layer).
Comparison between two EDX spectra of glaze (brown pro le) and needle (grey bold): the needle composition is based mainly in Al, Si, K and small Fe, while glaze include Si, Pb and Mn as color element.

Figure 2 Early
Figure 2

Figure 9 Surface
Figure 9