- Research article
- Open Access
Archaeometric characterization of Byzantine pottery from Păcuiul lui Soare
Heritage Sciencevolume 7, Article number: 55 (2019)
Archaeometric investigations using OM (optical microscopy) and micro-PIXE (particle induced X-ray emission) were performed on 45 ceramic shards unearthed in archaeological excavations at Păcuiul lui Soare (southeastern Romania) and dated to the eleventh century AD. This study aimed to get clues about the raw materials and manufacturing techniques used by the potters from the Lower Danube area during the Byzantine period. The analyzed ceramic fragments were selected according to stylistic and archaeological criteria, trying to cover the entire palette of potteries discovered at this site. OM detailed the characteristics of the fabric (texture, microstructure and porosity), mineralogy, surface treatments and firing of the shards. Principal component analysis (PCA) of the PIXE data highlighted two main categories of shards with distinct compositional signatures, separated mainly by their aluminum and calcium content. Micro-PIXE maps of the interfaces between the glaze and the ceramic body showed that the green glaze is rich in lead oxide compared to the underlying ceramic body. The results of these investigations were compared to the ones previously obtained on coeval potteries from other Byzantine archaeological sites, i.e. Hârşova and Oltina, trying to get some hints about the consumption and circulation of pottery in the Lower Danube region at the beginning of the second millennium AD.
Archaeometric investigations of Byzantine ceramics are relatively scant, only the recent years witnessing an increase in the number of the publications on this topic [1,2,3,4,5,6 and references therein]. This statement also applies for the Byzantine pottery excavated on nowadays Romania’s territory [7,8,9].
Until now, the studies dedicated to the ceramic trade in Dobrudja were exclusively based on stylistic analogies between similar finds discovered at different sites [10,11,12,13]; however, these publications do not contain any analytical data.
This research was conducted on an assemblage of 45 ceramic shards excavated at the fortress from Păcuiul lui Soare (see Fig. 1) located on the eponymous island on the Danube river (see Fig. 2). The shards were found in secure archaeological contexts; all of them were dated to the eleventh century AD according to stratigraphic and typological criteria. The selected fragments can be considered as representative for all the ceramic finds discovered until now at this archaeological site.
Through this study, we try to answer the following questions: Are there any potteries different from the ones positively identified as local products on archaeological grounds? Can we identify the ceramic vessels supposedly imported to Păcuiul lui Soare based on mineralogical and chemical criteria Can we get any information about the techniques and resources employed to manufacture these utilitarian products?
To reach these goals, OM was used to describe the fabric and the mineralogy of the selected ceramic fragments, while PIXE provided the compositional characterization of the shards. The analytical information was used to get some hints about the raw materials and manufacturing techniques, trying to verify if the stylistic and typological differences between the shards are also reflected by distinctive morphological and physico-chemical features.
This study is a stage of an ongoing archaeometric project targeting the systematic multidisciplinary characterization of Byzantine ceramic dated to the eighth to twelfth centuries AD excavated in Dobrudja, the historical province located in the south-eastern part of Romania between Danube and the Black Sea—see Fig. 1. Based on this information, this project attempts to produce analytical evidences for the commercial exchanges between the centers from the Lower Danube region during the Byzantine ruling.
Historical and archaeological background
The fortress from Păcuiul lui Soare was raised by the Byzantines during the second half of the tenth century AD. Established as a naval basis , the fortress played an important role in controlling the navigation on the Danube and protecting Dorostolon, the capital of the Byzantine Theme Dristra/Paradunavon—nowadays Silistra. During the eleventh century AD, Păcuiul lui Soare lost its military importance and turned into a civilian settlement . In 1094, a Cuman attack practically stopped the habitation of the site for more than 100 years .
Păcuiul lui Soare was situated on a well-known commercial route of the epoch, namely the Danube river, that connects Central Europe with the Black Sea and the Middle East. This special location made Păcuiul lui Soare a powerful commercial center during the eleventh century AD. Commodities from Europe, Central Asia and Middle East arrived at this point, as demonstrated by the impressive archaeological discoveries and by the existence of a harbor from which only a 42 m long quay survived until today .
The ceramic material analyzed in this study comes from sealed contexts, as well as from the occupation layer. The analyzed shards were dated between 1001 and 1094 AD.
There are strong archaeological evidences indicating that ceramic vessels were produced at Păcuiul lui Soare during the first half of the eleventh century AD. Thus, during the 1973 excavation campaign, a pottery kiln was discovered inside the fortress, close to the northern gate —see Fig. 2. Inside this kiln, some ceramic vessels in fragmentary state and some refuses were found, suggesting that pots and cauldrons were produced here. In the filling of the access pit of the kiln, several amphorae fragments were also discovered. The finding of the amphorae shards nearby the kiln, as well as the discovery in 1971 of a bronze stamp for marking amphorae suggest the production of this type of potteries at Păcuiul lui Soare alongside with other types of vessels .
Materials and methods
To answer the above-mentioned questions, a set of 45 ceramic shards excavated at Păcuiul lui Soare were selected for this archaeometric study; they are considered representative for all potteries discovered until now at this site. The ceramic fragments derived from different types of vessels, being either manufactured from common clay or from “kaolinitic” clays, with fine or coarse paste, covered with olive-green glaze, golden engobe or with incised decorations, painted or not, originating from cooking or storage vessels, etc. In particular, the studied shards are fragments of cauldrons, jugs, pots with or without handles, bowls, amphorae, and rush lights. For a more detailed description of the ceramic samples under scrutiny, the reader is referred to Table 1 and Fig. 3.
Glazed shards represent 5% of the overall ceramics finds excavated until now at Păcuiul lui Soare. The glaze is olive-green, with hues varying from light to dark olive-green, sometimes with reddish reflections. The glaze is matte or shiny, being either uniformly or unevenly applied, mostly on the outer surface of the vessels. In most cases, the glaze was well preserved. Shard P2 from Fig. 3 is covered with a shining glaze without any exfoliation. However, the glaze is lacking from several small zones of this shard. This partial absence of the external decoration is not an effect of the burial, but it can be explained by the fact that the glaze was not uniformly applied on the entire surface of the vessel, suggesting a less careful manufacturing process.
An important category of ceramic finds from Păcuiul lui Soare is the one of potteries made of “whitish clays” , a term cautiously used to describe the shards presenting the visual characteristics of kaolinitic paste, i.e. light grey, yellow, pale yellow with pinkish white and pink hues and reddish rounded inclusions .
Due to the fact that they were rare occurrences among the ceramic finds from Păcuiul lui Soare, it was presumed that the glazed shards and the ones with golden engobe represent imported potteries.
In order to characterize the ceramic paste, the 45 ceramic samples were initially studied with a stereomicroscope with magnifications of × 10...× 30. This preliminary investigation showed that all the shards can be classified into four types of ceramic paste. Subsequently, 15 shards were selected to prepare thin sections that can be considered representative for the entire batch of 45 ceramic fragments. To detail the types of paste and to perform the mineralogical characterization, the thin sections were analyzed using an Olympus BX 60 polarizing petrographic microscope at magnifications varying from × 50 to × 500. The microscopic description was made following the guidelines of thin section analysis [20,21,22], based on the textural characteristics (granulometry, sorting, frequency and grain shapes), composition (nature of mineral and organic constituents), microstructure, porosity, firing and surface treatment .
PIXE measurements were performed on samples extracted from all shards at AN2000 accelerator of LNL, INFN, Italy, with a 2 MeV proton beam , using macro-beam settings (beam size ~ 3 × 3 mm2) and a preset charge of 4 μC. The IGLET-X™ HPGe detector from ORTEC® used for X-rays detection was covered with a 100 μm thick Al funny filter (0.7%) that enabled the reduction of the intense peaks of low-Z elements in the characteristic X-ray spectra. An example of a PIXE spectrum is given in Fig. 4.
Using an agate mortar, small ceramic fragments broken from each shard were transformed into powders that were subsequently pelletized. The interfaces between the glaze layers and the underlying ceramic bodies were measured with the micro-beam (beam size ~ 4 × 4 μm2), by scanning 750 × 750 μm2 areas on the shards as such. To obtain the glaze composition, small zones on the decorated surface of shards were bombarded with the proton beam without any preparation.
To obtain quantitative results, PIXE spectra were treated with GUPIXWIN software (version 2.2.4) , considering all the detected elements (Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Pb, Sb) as oxides (except for Cl), and normalizing the concentrations to 100 mass%.
Basalt BHVO-2 Certified Reference Material (CRM) was used to assess the accuracy of the quantitative results. The overall uncertainties were estimated to around 5% for the major elements, varying from 10% up to 30% for the minor elements and trace-elements.
The detection limits of the employed experimental set-up for a ceramic target are given in Table 2, while the results of the PIXE analyses—chemical composition of the ceramic body and green glaze—are presented in Tables 3 and 4, respectively.
Results and discussion
Using OM, four main types of ceramic paste were identified for the Păcuiul lui Soare shards. Their description is given in the lines below; their assignation to a certain type of paste is summarized in Table 1.
Fine paste with silty clay matrix, oriented microstructure and micro-fissures: P15, P16, P17, P18, P19, P20, P21, P22, P23, P24, P25, P26, P27, P28, P29, P30, P42.
Fine, homogeneous paste with fine silty clay matrix with rare (5–10%) silt grains, 10–50 μm, and more frequent (10–15%) fine sand grains, 100–250 μm, rarely medium sand, 300–500 μm, and accidental coarse grains, 1.0–1.4 mm; mono- and polycrystalline (for coarse sand fraction) quartz and feldspar, frequent amorphous ferruginous stains and concretions, 10–30 μm, and rarely (1–2%) much larger, i.e. 100–300 μm. Low porosity (5–15%), with fine isolated pores, 100–300 μm, and more frequent micro-fissures, 0.5–3.0 mm. The clay is birefringent, the sand grains are sub-rounded and rounded, being intentionally added to the silty clay matrix. This paste shows a good to moderate sorting; mica flakes are absent (Fig. 5a). These vessels were fired in oxidizing conditions, complete or incomplete (“banded” structure). In some cases, secondary firing is present, as some of these shards were fragments of cooking pots or because some fragments were discovered in burned dwellings.
Fine silty paste with porphyritic microstructure: P5, P8, P9, P10, P11, P12, P13, P14, P31, P32, P41, P43, P44, P45.
Fine, homogeneous paste with fine grained silty matrix, well sorted, with frequent (15–25%) grains of fine sand; mono- and polycrystalline quartz and feldspar, 50–100 μm, rarely 150–200 μm and frequent mica (predominant muscovite), 50–200 μm, frequent fine vegetal debris, 20–150 μm, ferruginous stains and concretions, 50–150 μm, rare fine carbonate grains, 100–400 μm, and accidental shell fragments. Fine porosity (5–10%), with frequent isolated and rounded voids, 50–200 μm, very rarely up to 1 mm. Rare elongated voids originating from vegetal temper are present. This paste was most probably made from alluvial sediments, well sorted silt with fine sand and clay; sand grains are sub-rounded and rounded (Fig. 5b). These shards were fired in complete or incomplete (with dark organic “core”) oxidizing conditions.
Fine carbonate paste with porphyritic microstructure: P1, P2, P3, P4, P6, P7, P36, P39.
Fine, homogeneous paste with fine carbonate matrix, with frequent (10–15%) fine silty grains, 10–50 μm, rare fine sand, 50–200 μm, and very rare (2–3%) medium to coarse sand grains, 400–750 μm; sand fraction with mono- and polycrystalline quartz and feldspars, rare (5%) fine flakes of mica, 50–150 μm, rare calcite grains, 100–200 μm and opaque grains, 50–100 μm (Fig. 5c). This paste includes areas of calcite recrystallization on voids and fine cracks. Low porosity (5–10%), with fine circular voids, 50–300 μm, and rare mm-size micro-fissures are visible. This paste was made from carbonated sediments (unconsolidated marl type), featuring a well-sorted matrix, with possible mixture of fine sand with medium to coarse sand grains, that are generally sub-rounded and rounded. The shards were fired in oxidizing conditions, featuring a banded structure and an organic “core”.
Medium granular paste with porphyritic microstructure: P33, P34, P35, P37, P38, P40.
Semi-fine, homogeneous paste, with silty clay with fine sand matrix, moderately to poorly sorted; rare silty grains (5%), 20–50 μm, and frequent very fine sand (10–20%), 50–200 μm. It includes rare coarse sand grains (1–5%), 500–1500 μm. This paste is similar with the fine silty paste (type II), with frequent fine mica flakes, 50–200 μm, but with frequent quartz and quartzite sand inclusions, with heterogeneous appearance (Fig. 5d). Fine porosity, 10–15%, isolated voids, 50–200 μm, circular and elongated, irregular, and micro-fissures, 100–300 μm, rarely 0.5–1.5 mm. The sand grains are sub-angular, sub-rounded and rounded. These shards were fired in reducing conditions, with an organic “core”, being oxidized at the exterior.
The four main types of ceramic paste identified for the ceramic fragments from Păcuiul lui Soare are similar to the ones evidenced for the coeval potteries discovered in the nearby archaeological sites Hârșova and Oltina that were previously analysed in the frame of the on-going project mentioned in “Introduction” section [7, 8].
The fine paste with silty clay matrix (type I) is made from a mixture of sediments, silty clay and fine sand, with oriented and birefringent clay, possibly kaolinitic, such as the one identified in the two aforementioned sites.
The silty paste with porphyritic structure (type II) is made of fine alluvial sediments. As in the other two archaeological sites studied until now (Hârşova and Oltina) this is indicated by the good sorting of the sedimentary matrix, the presence of the vegetal debris (partially decayed fragments of wood), and accidentally, of the shell fragments.
The fine carbonate paste (type III) is most probably made of fine alluvial sediments deposited in low energy sedimentary environments, such as marshy areas or lakes.
The medium granular paste (type IV) is made of silty fine sands with clay, very probably alluvial, mixed with coarse alluvial sands with variable granulometry.
To get a clear picture about the possible grouping of the ceramic shards according to their composition, taking into account the relatively large number of samples and variables, PCA (Principal Components Analysis) of the standardized PIXE data—Mg, Al, K, Ca, Ti, Cr, Fe, Ni, Cu, Zn, Ga, Rb, Sr and Zr oxides—was performed using STATISTICA software (version 8.0). Figure 6 shows the result of this analysis indicating the separation of the analyzed samples into two main compositional groups. A column in Table 1 also indicates to which of these two groups each sample belongs.
One of these groups (conventionally indicated with “I” in Fig. 6) is made out of 18 samples with relatively high Al2O3 (~ 24.9 mass% on average) and low CaO (~ 1.0 mass% on average) contents. This first group separates quite well from the rest of the other 27 ceramic fragments that cluster in a second group (marked with “II” in the same figure), regardless of their characteristics (decoration, granulation, firing etc.). This second group is characterized by relatively low Al2O3 (~ 17.7 mass% on average) and high CaO (~ 4.0 mass% on average) concentrations. As also visible from the loading plots shown in Fig. 6, the higher contents of Al are correlated with higher concentrations of Ti (group I), while the higher contents of Ca are correlated with higher concentrations of Mg, K and Sr (group II).
The first compositional group superposes relatively well with type I resulting from the petrographic analysis, except for sample P14 (containing 23.2 mass% Al2O3), attributed to type II from OM and sample P38 (with 24.6 mass% Al2O3) that belongs to type IV resulting from OM. Other discrepancy appears in the case of sample P42 (18.8 mass% Al2O3) that belongs to type I resulting from OM, and, at the same time, pertains to compositional cluster II, the one made out of the low-Al samples. Explanations for these discrepancies are still sought, but in any case, there seems to be a pattern that can be recognized from the analysis of the data obtained using these two different analytical techniques. Thus, paste I resulting from OM superposes quite well with the samples attributed to group I resulting from PCA; more or less, this reflects the use of a distinct type of clay that was chosen on purpose—most likely, to obtain vessels with different properties. On the other hand, the samples belonging to group II resulting from the PCA of PIXE data correspond to three types of paste (II, III and IV) revealed by OM. The most plausible explanation for this finding is that the shards belonging to these groups contain more or less the same minerals/compounds, hence the compositional similarity of the pellets (see also PIXE section). The differences between types II, III and IV of paste mainly consist in textural parameters, homogeneity and porosity, etc., and do not result from geochemistry.
A similar sub-division of potteries into two large groups, mainly separated by their Al and Ca contents, was also evidenced in the case of previously analyzed coeval ceramics from Hârșova and Oltina [7, 8].
Micro-PIXE scans of the interfaces between the decorated surfaces and the ceramic bodies were performed to identify the compound(s) responsible for the green glaze, as well as to estimate the thickness of these layers. A clear change in the chemical composition were observed for this type of decoration characterized by a large lead content (~ 64.4 mass% PbO on average, PbO content ranging from 50.4 to 76.8 mass%) compared to the ceramic body (~ 0.1 mass% PbO on average)—see Fig. 7 and Tables 3 and 4.
There are two ways of producing a Pb glazing: either by applying a Pb compound (e.g. litharge) as a suspension in water or by applying a mixture of PbO and quartz. In the former case, the glaze forms through direct reaction of PbO with the clay body; in the latter, the two components of the suspension react during firing, form a glaze that interacts with the body, and certain chemical elements (Al, Ca, Fe, Na, Mg) diffuse from the body into the glaze. These two glazing methods can be distinguished by subtracting the PbO content from the glaze composition and renormalizing the recipe to 100% and subsequently comparing this adjusted glaze composition with the one of the body [25,26,27]. Figure 8 shows that the Al2O3 content in the recast glaze composition is roughly the same as in the corresponding ceramic body, indicating that glazing was made through the application of lead oxide onto a non-calcareous ceramic body.
The micro-PIXE Pb Lα maps were used to estimate the thickness of the green glaze layer that turned out to range from 50 μm up to 300 μm. Some shards feature decorative layers with variable thickness.
Relatively small amounts of CuO (~ 1270 ppm on average) were found in the green glaze—see Table 4. The most likely explanation for the olive-green hues of the glaze is that they were induced by iron ions in reducing state [3, 4, 26, 28]. It must be mentioned here that certain amounts of Sb were determined in the composition of the green glaze covering samples P1 and P2—see Table 4. However, these particular two samples do not exhibit yellow hues, color that might have resulted from the presence of lead antimonate ; they are also olive-green, similar to the other five glazed samples analyzed in this study. The relatively small amounts of Sb from these two particular samples did not lead to any change in the appearance/coloring of their decorations. A possible explanation for the presence of Sb in the glaze is that this chemical element is a contaminant of the raw materials employed for making the glaze (in particular, of the lead compound).
The micro-PIXE scans of the interfaces did not provide any hint about what causes the shining of the golden engobe. However, the explanation was provided by OM: fine flakes of muscovite embedded in the slip produce the glittering golden shine of these shards. This decoration is the result of a different preparation manner and it did not imply the use of other raw materials.
From geological point of view, Păcuiul lui Soare area is characterized by the presence of Holocene clayey and sandy sediments, terrace deposits and Upper Pleistocene loessoid deposits .
Going south along the main valleys in the Southern Dobrudja area, beside consolidated rocks and the above-mentioned deposits, marly clays of Barremian age, sands and clays of Albian age, Ypresian clayey sands, quartz sands and clays of Tortonian age, greenish and brown clays and diatomites of Sarmatian age, gray and yellowish ferruginous sands, of the Dacian, and Levantine quartz sands with red sands intercalations can be encountered .
Summarizing, Southern Dobrudja region offers a wide range of raw materials that can be used for pottery making. The archaeometric analyses indicated that the Byzantine craftsmen from Păcuiul lui Soare chose in a natural way local sedimentary clays to produce ceramic vessels.
Kaolinitic clays were used in Dobrudja starting from the Roman period, as indicated by the discovery at Castelu of a kiln for pottery making and of some pits containing ceramic refuses . Kaolinitic clays have been also employed during the Byzantine period from the ninth century AD onwards, but mostly during the tenth to eleventh centuries AD .
In Southern Dobrudja, important deposits of kaolinitic clays are known in the area of Medgidia, Cuza Vodă, Satu Nou, Ţibrinu, and Tortoman, in the Carasu Valley, as well as on small tributaries, such as Agi Cabul and Adânca Valley. These deposits are in the form of lenticular layers separated by sand layers; this complex of Aptian age is situated above a layer of thick sands and below the Sarmatian lumachelic limestones . These clays are formed by the kaolinisation of detrital feldspathic material in a continental-lacustrine sedimentation environment. The color of the kaolinitic clay varies greatly from white to yellow, pink, and even bluish, depending on the amount and nature of the impurities (e.g. iron and manganese hydroxides) . These deposits are situated on the Danube valley, the closest to Păcuiul lui Soare (approximately 50 km away) being the one nearby Cernavodă .
Other kaolinitic clay deposits are encountered as small outcrops in the form of lenses on Gîrliţa valley, Oltina valley and southwest from Rasova, at distances between 12 and 35 km from Păcuiul lui Soare . They contain small clay lenses, along with sands, pebbles and quartzitic micro-conglomerates, with intercalations of sands and limestone. However, the use of these deposits for pottery making during the Byzantine period remains to be demonstrated.
Byzantine kaolinitic clay vessels discovered in Dobrudja are characterized by an increased mechanical resistance—i.e. it is harder to break them compared to other potteries found at the same archaeological sites.
It seems that the potters from Păcuiul lui Soare knew about the kaolinitic clay deposits situated some tens of km from their settlement and they used this type of raw materials to make pots with good mechanical resistance. Thus, they could produce vessels not only different in appearance, as demonstrated by the different light hues of the paste, but mostly with better mechanical properties, hence justifying the effort of collecting some raw materials that were not readily available nearby their settlement.
Shards P1…P7 (the green glazed shards from the studied assemblage) and P8…P11 (the ones with golden engobe) belong to types II and III of paste resulting from OM analysis, i.e. potteries made from fine alluvial sediments. Compositionally, they belong to group II emerging from PCA. At this stage of research, we do not have any analytical argument to support the idea that these shards represent imported potteries, as they are similar in chemical compositional and mineralogy to the ceramic vessels that were locally produced from raw materials available nearby the settlement.
This paper describes the OM and micro-PIXE investigations on 45 ceramic shards dated to the eleventh century AD and in discovered in well-defined archaeological contexts at Păcuiul lui Soare, Romania, in a trial to identify the raw materials and working techniques employed for the manufacturing of these potteries, as well as to check some provenance hypotheses.
OM led to the identification of four types of pastes, similar to the ones evidenced for the coeval ceramics from Hârșova and Oltina, two archaeological sites located on the bank of the Danube River, relatively nearby Păcuiul lui Soare. PCA of the PIXE data singled out two compositionally distinct groups. The first one is made of the shards produced from Al-rich clays, possibly taken from a relatively nearby location from Dobrudja. The geology of the region and the archaeological discoveries indicated a possible source of kaolinitic clays located approximately 50 km north-east from Păcuiul lui Soare, source exploited starting with the Roman period. The second compositional group is composed of shards produced from several types of sedimentary raw materials that were locally available—most likely alluvial deposits of Danube and/or nearby lakes, as suggested by the petrographic data.
The Pb-rich green glaze was manufactured through the application of lead oxide onto a non-calcareous clay body, technology often encountered during the Roman and the Byzantine periods. The mica flakes present in the yellow engobe indicate that this type of decoration was produced by covering the vessels with a slip made of silty alluvial clay containing muscovite.
The results of this study demonstrated that the eleventh century AD pottery from Păcuiul lui Soare, with various appearances and playing different roles, was most likely locally manufactured using distinct raw materials, preparation and decoration techniques.
This research showed a marked similarity between the potteries from Păcuiul lui Soare and the ones excavated in the nearby Byzantine sites Hârșova and Oltina, indicating that during the eleventh century AD ceramic manufacturing in Dobrudja involved mostly the use of common local clays, but also of some pure, birefringent (possibly kaolinitic) clays from specific deposits, that were prepared using various recipes and techniques.
The archaeometric characterization performed using OM and PIXE did not provide any argument to suggest the presence of imported potteries among the analyzed ceramics from Păcuiul lui Soare.
The data reported in this paper contribute to the understanding of the way pottery was made during the eleventh century AD in the Lower Danube region, a topic little tackled until recently, and certainly of interest for the scholars involved in the study of the Byzantine period.
Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
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Leonardo La Torre is gratefully acknowledged for his skilled technical contribution in realizing the LNL experiment. The authors would like to express their gratitude to Dr. Oana Damian from Institutul de Arheologie “Vasile Pârvan”, Bucureşti, for providing the ceramic fragments discovered at Păcuiul lui Soare investigated in this study.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme ENSAR2 under Grant agreement No. 654002.
The authors declare that they have no competing interests.
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