{"word_count": 5942, "character_count": 45484}
1. Introduction        From the 4th millennium BC        onwards in the Near East region and Egypt alkali glazes were used        first for covering steatite and ground quartz vessels (i.e. Egyptian        faience) and then clay vessels from the 15th        century BC (Tite et al., 1998).        The earliest known lead glazed ceramics in the West        were produced during the Roman era (about 1st        century BC) (Tite et al., 1998; Wolf et al., 2003; Walton and Tite, 2010) after their        appearance in China perhaps a few centuries earlier (Tite et al., 1998).        Evidence of the production of these wares comes        from several sites in Anatolia (present-day Turkey) (Walton and Tite,        2010). Subsequently, glazes of this type were extensively used for        pottery and tiles throughout the Islamic and Byzantine world and        medieval Europe (Wolf et al., 2003). At present        their use is continuing in the Near East and Europe (Tite et al., 1998; Wolf et al.,        2003). From the late 12th century AD, the high        lead glaze type was also used in China as the basis for colored        overglaze enamels (Tite et al., 1998).        An important category of glazed pottery, first        produced in Iraq in the 8th century AD is the        tin-opacified glazes, initially containing up to 1-2% PbO        (Charalambous et al., 2010). Gradually the        percentage of PbO increased and by the 10th-11th century AD the glazes typically contained 20-40%        PbO and 5-12% alkali (Wolf et al., 2003).        According to Davis and Stocker (2013) typical concentration of SnO2 in tin-opacified glazes amounts to 4-9 wt.%        Thereafter, the produced tin-opacified glazes throughout the Near East        and Europe are characterized by this composition.        The reasons for replacement of the alkali glazes        with lead glazes are lower melting temperature, higher fluidity and        thus better coating the glazed surfaces. In addition lower thermal        expansion which predetermines lower risk of glaze crazing and higher        refractive index leading to greater brilliance (Wolf et al., 2003) are factors that led to the        widespread use of high-lead and lead-alkali glazes.        There are mainly two ways to produce high lead        glazes. The first one is to use a lead compound, such as litharge        (PbO), red lead (Pb3O4), white        lead (2PbCO3.Pb(OH)2) or        galena (PbS). In the second way, a suspension of a lead compound with        silica in the form of quartz sand, ground quartz or chert pebbles is        used. In both methods, a small amount of clay, gum or starch can be        added to the glaze slurry to maintain the lead and silica particles in        suspension. The application can be to either unfired or biscuit fired        bodies (Tite et al., 1998). The lead        oxide-plus-quartz suspension can be applied either in a raw state or        after pre-fritting (Walton and Tite, 2010).        The lead glazes are easily formed, obtain        lustrousness and opacity at low temperature and are also easily        colored with oxides of other metals (Charalambous et al., 2010). Apart from the concentration, the        oxidation states of copper and iron are also important for obtaining a        particular color. The green color is traditionally related to the        presence of Cu2+(Davis and Stocker, 2013) or Fe2+ (De Benedeto et al., 2004;        Molera et al., 1997; Wakamatsu et al., 1987). If no copper has been detected in        the glazes, the change of the color must be related to the degree of        oxidation of the iron (Molera et al., 1997;        Wakamatsu et al., 1987). Fe3+        is responsible for the brown, yellow, black and red colors        (Molera et al., 1997), however, black glazes        also have Mn and yellow glazes also contain Cr (Charalambous et al., 2010). Co-based pigments were well known        for the blue color (Wolf et al., 2003) due to        CoO4 complex (Charalambous et al., 2010) or probably Co-Ni-minerals        (Kleinmann, 1991).        The production of lead glazes expanded rapidly,        attributed to the growing Roman markets and since the 7th century AD lead glazes were produced also in the        Balkans (Walton and Tite, 2010).        The production of glazed pottery in Bulgaria is        about 2% of the pottery in general. Glazed pottery has been found        during archaeological excavations in Drastar and the nearness (Koleva,        2008; Mancheva, 1986), Karaachteke near Varna (Yoleva et al., 2015), Zlatna Livada, near Chirpan (Koleva,        2015), Pliska (Djingova and Kuleff, 1992), etc. Fragments of glazed        ceramic vessels although not many, are always part of the ceramic        finds from the Early Medieval pits and houses, which lay above or are        dug into the Late Antiquity stratigraphic layer. Its fragmentation is        a result of the representative character of this valuable ceramics and        the fact that its quantity was considerable less in comparison to the        other vessels. The fragments allow identifying the major shapes of        vessels, which are very diverse – pitchers, handles, dishes, vessels,        etc [Koleva, 2009].        Glazed pottery from Bulgaria has not attracted much        interest in terms of analytical studies. The subjects of the present        investigation are fifteen medieval glazed pottery vessels. The samples        are taken from town Drastar (present day Silistra) (see Fig. 1) where        in the recent years medieval kilns for pottery production were        discovered during archaeological excavations (Angelova and Koleva,        2005; Koleva, 2008). The excavated kilns are evidence for the        production of sgraffito pottery in Drastar in the period from 13th to 14th century AD [Koleva,        2008]. A detailed description of the town Drastar can be found in        Angelova (1988).        The aims of the present paper are to determine: (i)        the chemical composition of the pottery; (ii) the type of colorants in        the glazes after chemical characterization of the glaze; (iii) the        technology of production of high lead glazes from Drastar.        Figure 1: Map of Bulgaria with the location of        Silistra / Carte de la Bulgarie avec l'emplacement        géographique de Silistra                      2. Materials ands methods                  Materials          In the the present investigation fifteen glazed          pottery fragments from Drastar were analyzed. Four of the samples          (Nrs. from 1 to 4) were found in the room leading to the uncovered          kiln. This fact is a further evidence for the existence of medieval          kilns for pottery production in the city, as already established by          Koleva, 2008.          The rest of the samples (Nrs. from 5 to 15) were          found during excavations of part of the medieval site of Drastar.          Two of the samples (Gl-P-10 and Gl-P-11) are probably imported from          Byzantium.          The investigated samples are only fragments (not          entire objects) of sgraffito and glazed ceramics which represent          parts mainly from vessels and dishes – bottom, edge and side part          and one handle. Most of the samples are glazed from one side and two          of them are glazed from both sides. The glazes have different color,          dominated by green, brown and yellow in different shades laid on a          red, grey or white clay.          A short description of the pottery samples is          given in Table 1. Pictures of the samples with numbers from Nr. 5 to          Nr. 15 are presented in Figure 2. The samples are dated          archaeologically to 10th century (Nr. 10), 11th century (Nr. 8), 10th to          12th century (Nr. 11), 13th century (Nrs. from 1 to 6, and 14), 13th-14th century (Nr. 9), 14th century (Nr. 15), 16th century (Nrs. 7 and 13), 18th century (Nr. 12).          Table 1: Description of the investigated          glazed pottery from Drastar (present day Silistra) / Tableau 1 : Description de la poterie glaçurée de          Drastar (aujourd'hui Silistra) qui a fait l’objet          d’analyses                                    Lab. code              Description                                      Gl-P-1              sgraffito pottery, 13th cent. AD,              lead glaze before firing with white-yellowish color, the find              was discovered in the room which leaded to the kiln                                      Gl-P-2              sgraffito pottery, 13th cent. AD,              lead glaze before firing with white-yellowish color, the find              was discovered in the room which leaded to the kiln                                      Gl-P-3              sgraffito pottery, 13th cent. AD,              lead glaze before firing with white-yellowish color, the find              was discovered in the room which leaded to the kiln                                      Gl-P-4              sgraffito pottery, 13th cent. AD,              lead glaze before firing with white-yellowish color, the find              was discovered in the room which leaded to the kiln                                      Gl-P-5              sgraffito pottery; 13th cent. AD,              bottom of vessel, red clay                                      Gl-P-6              sgraffito pottery, 13th cent. AD,              bottom of vessel, red clay                                       Gl-P-7              glazed pottery, 16th cent. AD, edge              of vessel, red clay                                      Gl-P-8              glazed pottery, 11th cent. AD,              handle, grey clay                                      Gl-P-9              glazed pottery, 13th to 14th cent. AD, bottom of vessel, glazed from both              sides, white clay                                      Gl-P-10              glazed pottery, 10th cent. AD,              bottom of vessel, glazed from both sides, white clay, probably              imported from Byzantium                                      Gl-P-11              sgraffito pottery, 10th to 12th cent. AD, bottom of vessel, white clay,              probably imported from Byzantium                                      Gl-P-12              glazed pottery, 18th cent. AD, side              part of vessel, red clay                                       Gl-P-13              sgraffito potery, 16th cent. AD,              side part of dish; red clay                                      Gl-P-14              sgraffito potery, 13th cent. AD,              bottom of vessel, red clay                                      Gl-P-15              sgraffito potery, 14th cent. AD;              bottom of vessel; red clay                                The surface of some glazes suffered severe          deterioration due to environmental erosive factors as humidity and          soluble salts. This effect is clearly visible on samples Gl-P-10.1,          Gl-P-10.2, Gl-P-11 (see Fig. 2). More information about the analyzed          samples is given in Koleva, 2008 and Mancheva, 1986.          Figure 2: Pictures of some of the glazed          pottery samples / Photos de certains des échantillons de poterie          glaçurée                    The analyzed samples are very common and typical          and are chosen as representative samples for the medieval glazed          pottery, excavated in the Drastar region.                          Analytical methods                      Proton induced X-ray and proton induced            gamma-ray emissions            The glaze covered pottery fragments were            analyzed using Proton Induced X-ray Emission (PIXE) and Proton            Induced Gamma Emission (PIGE). The measurements were performed at            the Tandem accelerator of the Jožef Stefan Institute in Ljubljana,            Slovenia using a proton beam of 3MeV nominal energy in air. A            combined PIXE-PIGE method was applied. The concentration of Al2O3, CaO, CuO, Fe2O3, K2O,            MgO, MnO, Na2O, PbO, SiO2,            Cr2O3 and TiO2, was determined. Elements heavier than silicon            were analysed according to their characteristic X-rays, detected            by a Si(Li) detector of 160 eV resolution at 5.89 keV. The proton            energy at the target, after passing an 8 μm aluminium window and a            1.1 cm air-gap, was 2.70 MeV. The air gap between the target and            X-ray detector was 5.7 cm, which acted as an efficient absorber of            intense silicon X-rays. The beam size at the target was a few            mm2. Typical measurements times were 300-500            seconds at a proton current of &#60;1 nA. Sensitivity for mid-Z            element was improved to about 5 mg kg-1 by an            aluminium absorber of 0.1 mm thickness, and increasing the proton            current to a few nA. Spectral deconvolution was performed by the            AXIL code.            The contents of Na, Mg and Al were determined            from the intensities of gamma rays excited by inelastic proton            scattering. As the proton exit window, a 2 μm thick tantalum foil            on a brass nozzle was used. Gamma rays were detected by a 40 %            intrinsic germanium detector. The gamma lines exploited for            analysis were 440 keV for Na, 585 keV for Mg, and 844 and 1014 keV            for Al. Line intensities were determined by the GRILS program of            the GANAAS software package. For gamma measurement, the proton            current was 2-3 nA and the collected dose was about 5 μC for the            sample, and 15 μC for the standard.            The elemental concentrations were calculated by            an iterative procedure, using simultaneously both X-ray and            gamma-ray intensities. The contents of Na, Mg and Al were            determined within the surface approximation, using NIST SRM 610            glass standard for calibration. Periodically the NIST SRM 610 and            NIST SRM 612 glass standards were measured as unknown samples. The            accuracy of the method was within 5 % for major elements, but            decreased to 10-20 % for the elements at the sub 0.1 % level.            Uncertainties are also large for Mg approaching the detection            limit; at about 0.4 % they may be as high as 30 %.                                Laser ablation mass spectrometry with            inductively plasma            The concentration of the elements in pottery            was determined by Laser Ablation Inductively Coupled Plasma Mass            Spectrometry (LA-ICP-MS). Due to the different porosity of the            samples which lead to irreproducible results by direct ablation of            the pottery, the samples were prepared in the form of pellets. A            small part of the surface ceramic body layer was removed by a            drill and a tungsten carbide cutter to eliminate possible surface            contamination effects. The samples (0.1g) were ground in agate            mortar for 20 min, then mixed with H3BO3 in ratio 1:5, well homogenised and ground again.            Afterwards the mixture was pressed into a pellet of 1.2 cm            diameter and 2 mm thickness under pressure of 2 bars for 1 min.            This approach has been developed for ED-XRF analysis of soils            (Ivanova et al., 1998). The calibration of            the LA-ICP-MS system was performed by the Certified Reference            Material Ohio Red Clay, prepared by Harbottle (1976). Pellets of            the Certified Reference Material Ohio Red Clay was prepared the            same way and used for calibration of the LA-ICP-MS system. A            summary of the analytical data for 42 elements obtained by            analysing Ohio Red Clay from different laboratories are published            in Kuleff and Djingova (1998). The published mean values of the            elements of interest were used as certified values for the            calibration of the LA-ICP-MS system.            The LA-ICP-MS conditions for measurement of the            elements of interest are given in Table 2A (ICP-MS, Perkin Elmer            SCIEX DRC-e) and 2B (LA, New Wave Research). For the simultaneous            determination of total element composition in the samples, the            signal of the matrix elements was reduced by the introduction of            the rejection parameter (RPa) to enable the high-mass cut off. The            optimized RPa values for the measured isotopes of the matrix            elements are listed in Table 2.            Table 2: LA-ICP-MS operation conditions for            the quantitative analysis of the pottery / Tableau 2 : Les conditions de fonctionnement de            LA-ICP-MS utilisées pour l'analyse quantitative de la            poterie                                          A. ICP-MS                B. LA                                            Argon plasma gas flow                15 L min-1                Wavelength                213 nm                                            Auxiliary gas flow                1.20 L min-1                Laser ablation chamber                Standard (New Wave Research)                                            Nebulizer gas flow                0.90 L min-1                Ablation mode                Continuous                                            Lens voltage                6.00 V                Line length                4000 µm                                            ICP-RF power                1100 W                Pulse duration                5 ns                                            Pulse stage voltage                950 V                Beam diameter                100 µm                                            Dwell time                50 ms                Fluence                7.5 J cm-2                                            Acquisition mode                Peak hop                Repetition rate                10 Hz                                            Peak pattern                One point per mass at maximum peak                Scanning speed                10 µm s-1                                            Number of runs                4                Carrier gas (Ar) flow rate                1.0 L min-1                                            Determined isotopes (RPa)                23Na (0.013), 27Al (0.015), 28Si                (0.016), 42Ca (0.013), 39K (0.016), 54Fe (0.015),                56Fe (0.015), 57Fe                (0.013), 24Mg (0.014), 47,                48, 49Ti, 50, 52Cr, 55Mn, 63,65Cu, 207, 208Pb                                                          SEM–EDX microanalyses            Scanning Electron Microscopy (SEM), (TESCAN,            SEM/FIB LYRA I XMU) with Energy Dispersive X-ray Spectroscopy            (EDX) (Quantax 200, Bruker) was applied used to obtain surface            images of some of the glazes and potteries. Before analysis the            samples were coated with graphite.                                3. Results        The analytical data for the composition of the clay        bodies of the fragments, determined by LA-ICP-MS are reported in Table        3.        Table 3: Chemical compositions of the pottery        (normalized to 100%) from medieval Bulgaria using LA-ICP-MS; the        number of the decimal places is related to the precision of the        measurements / Tableau 3 : Compositions chimiques de        la poterie (normalisées à 100%) de la Bulgarie médiévale obtenues par        LA-ICP-MS ; le nombre de décimales est liée à la précision des        mesures                              Sample            Na2O            MgO            Al2O3            SiO2            K2O            CaO            TiO2            Cr2O3            MnO            Fe2O3            CuO            PbO                                Gl-P-5            1.72            1.83            12.1            69.3            3.42            5.09            0.55            0.11            0.16            6.65            0.02            &#60;0.001                                Gl-P-6            1.41            2.46            10.2            73.5            2.12            5.33            0.59            0.03            0.12            5.14            0.01            0.53                                Gl-P-7            1.21            2.41            13.4            72.5            0.98            4.85            0.69            0.02            0.21            3.89            0.01            0.67                                Gl-P-8            1.12            3.02            15.9            70.0            1.40            2.48            1.10            0.04            0.28            5.61            0.04            &#60;0.001                                Gl-P-9            0.82            1.23            16.9            72.7            2.20            4.15            0.86            0.01            0.02            1.64            0.02            1.54                                Gl-P-10            0.67            0.95            13.8            68.0            2.77            11.6            0.5            0.01            0.02            1.93            0.01            1.05                                Gl-P-11            0.64            1.1            7.75            64.6            2.95            19.6            0.97            0.01            0.02            2.44            0.01            &#60;0.001                                Gl-P-12            1.12            2.74            15.9            67.7            4.41            2.3            1.06            1.02            0.14            5.77            0.01            0.04                                Gl-P-13            1.73            2.7            12.9            73.0            1.15            4.88            0.67            0.01            0.50            2.82            0.01            0.42                                Gl-P-14            1.62            1.9            8.54            74.6            2.24            4.27            0.62            0.01            0.09            6.16            0.01            &#60;0.001                                Gl-P-15            1.78            2.17            11.5            68.2            2.78            2.40            1.46            0.02            0.12            9.54            0.02            0.31                          The data show that the ceramic pastes consist        mainly of SiO2 (64-75 %), Al2O3 (9-17%), CaO (1.3-18%), Fe2O3 (1.6-10%), K2O (1-4%), MgO (1-3%) and Na2O        (0.7-1.8%).        The results from the chemical analyses of the        glazes (sample Nrs. from 5 to 15) are summarized in Table 4. Figure 3        presents the ternary diagram – SiO2, PbO, and the        sum of concentrations of Na2O+K2O+MgO+CaO+Al2O3. The average metal concentrations are about 46% PbO,        39% SiO2, 7% Al2O3, 2% Fe2O3,        1% K2O, 1% MgO and Na2O&#60;1%.        The concentration of CaO is about 2%, with the exception of Gl-P-10        and Gl-P-11, glazed over white paste, both of them dated to 10th-12th century AD and imported        from Byzantium. The investigated samples could be considered as        high-lead glazes although the concentrations of PbO are lower compared        to the glaze from other sources (see e.g. Tite et al., 1998). Literature data confirm the use of        lead glazes on the Balkans as early as the 7th        century AD (Walton and Tite, 2010).        Figure 3: Ternary diagram of the concentration        of SiO2, PbO and the sum of Na2O+K2O+MgO+CaO+Al2O3. / Diagramme ternaire de la        concentration de SiO2, PbO et la somme Na2O + K2O + MgO + CaO + Al2O3                As discussed by Tite et al.        (1998), two primary methods for production of lead glazes: either lead        oxide (or some other lead compound) is applied to the surface of the        pottery body, or a mixture of lead oxide and quartz. The two primary        glazing methods can be distinguished by subtracting the percentage of        lead oxide and any intentionally added oxide as a colorant (e.g.        copper oxide) from the glaze composition and renormalizing the        resulting composition to 100% (Hurst and Freestone, 1996; Walton and        Tite, 2010). The concentration of the metal oxides in the glaze        composition after subtraction of percentages of PbO and CuO is        presented in Table 5.              4. Discussions                  Chemical composition of the body          Table 3 presents the results from the LA-ICP-MS          analysis of the clay pellets. Based on the concentration of CaO clay          bodies are classified in two groups. Figure 4 presents the          distribution of the investigated samples (bodies and glazes) based          on the concentration of CaO and Al2O3. Most of the samples dated to 10th-18th century AD have          non-calcareous clay body (CaO≤5%). The samples from this group have          red color due to high iron oxide concentration (3-10%), low          temperature and oxidizing atmosphere of firing. Under these          conditions iron oxide, present at relatively high levels          predominantly occurs in the form of hematite, which contributes to          the red hue of the pottery (Molera et al.,          1997).          Figure 4: Distribution of the samples (bodies          and glazes) according to CaO and Al2O3 concentrations / Distribution          des échantillons (corps et glaçures) selon les concentrations de CaO          et d'Al2O3                    Comparison with other samples from Bulgaria is          rather difficult, because of the limited number of analytical data.          However, non-calcareous medieval unglazed and glazed ceramic          artifacts were also discovered in other places, e.g. during          archaeological excavations at the monastery of Karaachteke near          Varna (Bulgaria). Both the unglazed and glazed artifacts contained a          certain amount of coloring oxides (Fe2O3 + TiO2), which determined          their slightly red color (Yoleva et al.,          2015).          Two of the samples from this group show different          chemical composition and have grey (Gl-P-8, dated to 11th century AD) and white (Gl-P-9, dated to 13th-14th century AD) colored          bodies.          Sample Gl-P-8 represents a handle and is entirely          glazed. Despite the presence of high iron oxide concentration, the          body of this sample has grey color. This result may be ascribed to          firing in a reducing atmosphere at temperature over 850°C          (temperature reached while reduction is performed) and the presence          of well-developed hercynite and Fe2+ in the          silicate phase (Molera et al., 1997). The          ceramic body is brown glazed which is related to the presence of          Fe3+ in oxidizing environment. The reason for          this effect is that reducing atmosphere is created during the firing          process by the gases produced inside the ceramic body, enclosed by          glaze on all sides. During firing, reducing and inert gases such as          CO, water vapor, CO2, etc. are produced which          consume oxygen inside the kiln and the pottery cannot be completely          oxidized. During the cooling phase, there is no burning, and the          partial pressure of oxygen is 20%, and the iron of the paste is          oxidized while the temperature is relatively high (over 500°C). If          all sides are glazed the ceramic body is sealed before the oxidizing          atmosphere reaches the paste and, thus, it remains reduced due to          the protective action of the glaze (Molera et al., 1997).          Sample Gl-P-9 is characterized by lower Na2O, MgO and Fe2O3 and higher PbO concentrations. Since the sample is          glazed on both sides, the white color of the body can be also          related to a reducing atmosphere during the firing process. It is          also possible that the high CaO content (three times higher than          Fe2O3) may contribute to the          white color of the body.          The results show that two of the samples (Gl-P-10          and Gl-P-11) which are dated from 10th to 12th century AD and imported to medieval Bulgaria from          Byzantium have white colored calcareous clay bodies. They are          characterized by high CaO (&#62;10%) and low MgO (~1%), Na2O (&#62;1%) and Fe2O3 (~2%) concentrations. Besides the high CaO          concentration, sample Gl-P-10 is also glazed on both sides, which          may affect the color of the body.          Some of the ceramic bodies were investigated by          SEM-EDX. The selected samples were representative of the three          described groups: non-calcareous – red color, non-calcareous – grey          color and calcareous white color. The obtained images are presented          in Fig. 5 to Fig. 7 and demostrate the effect of glazing in reducing          and oxidizing conditions on the property of the clay bodies. Fig. 5          presents the SEM image of a red, non-calcareous sample (Gl-P-7),          showing vitrification of the ceramic body. The next image (Fig.6)          shows the fine nature of the slip layer of the non-calcareous, grey          sample (Gl-P-8). The different structure of the body of sample          Gl-P-8 compared to the “one-side” glazed non-calcareous samples is          explained with the reducing conditions during the firing process.          According to Rekecki et al., 2014, the Fe2+/Fetotal ratio played a          decisive role in the sintering process under a reducing atmosphere.          The formed glassy phase, with the presence of Fe2+, attacked the earth-alkali oxides after the          carbonate decomposition and more effectively filled smaller pores,          resulting in changed microstructural characteristics.          Figure 5: SEM image of the body and part of          the glazed layer of sample Gl-P-7 / Image SEM du corps et une partie          de la couche glaçurée d'échantillon Gl-P-7                    Figure 6 : SEM image of the body of sample          Gl-P-8. / Image SEM du corps d'échantillon          Gl-P-8                    Figure 7: SEM image of the body and part of          the glazed layer of sample Gl-P-10 / Image SEM du          corps et une partie de la couche glaçurée d'échantillon          Gl-P-10                    The SEM-image of the calcareous, white ceramic          body of the sample Gl-P-10 is shown in Fig. 7. The image shows the          presence of bridges between the clay body and the glaze. The dark          areas in the glaze are most likely CaO inclusions.                          Chemical composition of the glaze          Metal concentrations in the glazes are presented          in Table 4. In order to check the effect of the ceramic body in the          quantification of the glazed layer, calculations for a layered          structure were performed.          Table 4: Chemical composition of the glazes          (normalized to 100%) from medieval Bulgaria using PIXE-PIGE; the          number of the decimal places is related to the precision of the          measurements / Tableau 4 : Composition chimique          des glaçures (normalisée à 100%) de la Bulgarie médiévale obtenue          par PIXE-PIGE ; le nombre de décimales est liée à la précision des          mesures                    The calculations assumed that the glaze layers          were infinitely thick. For checking the influence of this assumption          on the deduced elemental concentrations we performed model          calculations for a target composed of a layer of PbO (with a density          of 9.35 g/cm3) sitting on an infinitely thick          substrate. In the first step of calculation we generated a ratio of          X-rays yields for this structure, and in the second step we          calculated the concentrations from these yields for an infinitely          thick homogeneous target. First we assumed a layer of PbO on a          substrate of SiO2. The contribution of Si K rays          was generally quite low on account of lower proton energy and strong          X-ray attenuation in the PbO layer. A significant apparent          concentration of Si in PbO (higher than 1%) was obtained for PbO          thicknesses smaller than 3 μm. We further performed calculation for          more penetrative X-rays, selecting a substrate of Fe2O3. Apparent concentration of          Fe in PbO smaller than 1% was obtained for PbO thicknesses smaller          than 10 μm. However, as the concentration of iron oxide in the          pottery body is about an order of magnitude smaller than in pure          Fe2O3 (Table 3), we also          determined the thickness of PbO which yields apparent concentration          of iron below 10%; it was 6 μm. The range of critical PbO          thicknesses is thus smaller than our actual values in the glaze, so          the effect of final glaze thickness may be neglected.          Furthermore, the obtained SEM images showed that          the actual thickness of the glaze is more than 20 μm, which ensures          that the quantitative results are not influenced by signals that          could otherwise emanate from the ceramic body.          The data show that no significant difference may          be observed among the metal oxide concentrations depending on the          sample dating and type of clay used. The analytical data indicate          that these glazes are of the high-lead type. Compared to the          eutectic composition, the glaze contains more silica and lower lead          oxide, which suggests firing at higher temperature and high          viscosity of the mixture, which would hampered the formation of thin          layer.          Higher silicon and lower lead oxide          concentrations is also visible from the plotted values in Fig. 3          compared to the diagram given by Tite et al.          (1998). Similar tendency is observed when comparing the results to          literature data for lead glazed potteries from France, England,          Serbia and Romania (Walton and Tite, 2010) and Italy (De Benedetto          et al., 2004; Walton and Tite, 2010). On the          contrary, the present results show lower silicon and higher lead          oxide compared to lead glazes composition from Paphos region, Cypros          (Charalambous et al., 2010) and similar          silicon and lower lead compared to lead-glazed pottery from Paterna,          Spain (Molera et al., 1997). This result          suggests local production of the studied samples from medieval          Bulgaria. The relatively low Na2O concentration          is indicative that the glaze was not prepared according to the          Islamic recipe which is characterized with high concentration of          sodium (see e.g. Kuleff, 2012, 350).          The presence of K, Al, Ca and Fe oxides in the          glaze may be partially attributed to the composition of the raw          materials used to make the glaze (Molera et al., 1997), but also to the diffusion of the          components from the paste to the glaze during the firing. An          evidence for the diffusion of CaO from the body to the glaze is the          high concentration of CaO in the glaze of samples Gl-P-10 and          Gl-P-11 (see Fig. 4). The rest of the metal oxides have similar          concentrations in the calcareous and non-calcareous samples which          leads to the assumption of diffusion processes rather than using          another source. The diffusion from the clay to the glaze of K, Al,          Fe and Ca has also been assumed by Molera et al. (1997). According to Walton and Tite          (2010) when the re-normalized lime content (as described in the          results section, see Table 4) in the glaze is higher than in the          body, this is probably due to efflorescence to the surface of          lime-rich salts in the clay and/or in the hard water used in forming          the body. When, the iron oxide content in the glaze is higher than          in the body, this is probably due to its addition to the glaze as a          colorant. The composition of the present samples shows that CaO          content is higher in the body clay which excludes the efflorescence          process. According to the described criteria iron oxide was          intentionally added as a colorant in samples Gl-P-10 and Gl-P-11          imported from Byzantium and Gl-P-12-1 from medieval Bulgaria.          The colors of studied samples are yellow, green,          brown, and black. As can be seen from Figure 2 most of the samples          are decorated with more than one color which proves that both iron          and copper oxides are intentionally added as colorants. The yellow          color of the studied glazes is due to Fe3+          (~1%), present most probably as hematite in oxidizing          atmosphere.          Elemental analysis of the studied glazes shows          that the green color is due to the presence of copper oxide.          Although the studied green glazes contain iron oxide at          concentrations about 1% its presence is not responsible for the          glaze color. Green color is caused by the presence of Fe2+ under reducing conditions. The green glazed          ceramics were certainly fired in oxidizing conditions, confirmed by          the red body clay and the combination of green-yellow, green-brown          and green-yellow-brown glazes in one sample, which is an indication          that copper is in the form of Cu2+, and          determines the green color of the glazes (Davis and Stocker, 2013).          The brown color is due to the presence of high concentration of          Fe3+ (~2%), or to the combination of Fe3+ and Mn3+ (samples Gl-P-8,          Gl-P-11 and Gl-P-13-2). The black color of the sample Gl-P-12-1 is          caused by the intentional addition of Fe3+ at          concentration around 10%.          Sample Gl-P-5 deserves more attention. Although          the ceramic body is red colored, which is due to the presence of          Fe3+ in oxidizing atmosphere, having in mind the          low concentration of copper oxide (0.21%) in the green glaze, its          color is more likely due to high iron oxide concentration (1.43%)          using reducing condition. The yellowish colored glaze seems to be a          second layer, covering the green glaze, applied subsequently in oxidizing environments.          The yellowish color could be explained with the presence of Fe3+ in oxidizing atmosphere. Due to environmental          erosive factors, the yellowish layer is partially removed (see Fig.          2).                          Method for glaze production          Table 5 shows the composition of the glazes          (sample Nrs 5-15) after subtraction of percentages of PbO and CuO          and re-normalized to 100%. It has been pointed out that when silica          and alumina data points fall on the unity line, the glaze and body          composition are essentially the same and, therefore, glazing was by          the application of lead oxide by itself. In contrast, when the          silica contents are richer in the glaze than in the bodies, and the          alumina and lime contents are richer in the bodies than in the          glaze, glazing was by the addition of a lead-plus-quartz mixture          (Walton and Tite, 2010). In a third variant, proposed by Tite et al., 1998 lead compound and silica are first          fired together to produce a frit which was then ground to a powder          before being used to prepare the suspension.          Table 5: Chemical composition of the glazes –          renormalized to 100% after subtraction of percentages of PbO and          CuO / Tableau 5 : Composition chimique des glaçures renormalisée à          100% après la soustraction de PbO (en pourcentage) et CuO (en          pourcentage)                    In order to distinguish between the two primary          methods of glazing, the adjusted glaze composition of SiO2, CaO, Al2O3          and Fe2O3 were plotted          versus the corresponding body composition and are presented in Fig.          8.          The plots, presented in Fig. 8 show that while          for lime (R2=0.92) and          alumina (R2=0.87) proportionality between glaze          and clay body content is established, for silica the data are poorly          correlated (R2=0.67).          Figure 8: Two-dimensional diagrams of the          renormalized glaze composition of SiO2, CaO,          Al2O3 and Fe2O3 vs. the corresponding body          composition / Diagrammes deux dimensionnels des          compositions renormalisées de SiO2, CaO, Al2O3 et Fe2O3 de la          glaçure et du corps correspondant                    The concentration of SiO2 and          Al2O3 are equal (Gl-P-8) or          higher in the body of both calcareous clay bodies (Gl-P-10 and          Gl-P-11). The higher lime contents observed in the glazes of these          samples could be due to efflorescence to the surface (Walton and          Tite, 2010). Therefore, the glazing of this group of samples was          performed by the application of lead oxide by itself.          In most of the non-calcareous samples the          concentration of silica is higher in the glaze than in the body          composition, which could be considered as evidence for glazing by          the addition of a lead-plus-quartz mixture. However, this would          suggest lower alumina and lime content in the glaze than in the body          (Walton and Tite, 2010). The close concentrations of alumina and          calcium in the body and the glaze in some of the samples (see e.g.          Gl-P-6.1, Gl-P-7-2, Gl-P-9; Gl-P-12.2, Gl-P-13.2) contradict this          assumption.          It might be also assumed that silica was added to          the lead compound to prepare the suspension (Tite et al., 1998).          Though, the presence of samples, with different          colored glazes, in which the concentration of silica is higher          (Gl-P-7.1) and comparable (Gl-P-7.2) or lower (Gl-P-12.1) and higher          (Gl-P-12.2) than the corresponding concentration in the clay bodies          further confirms that neither quartz nor silica were added to the          glaze. Thus, it can be concluded that both the calcareous and the          non-calcareous samples were glazed using lead oxide by itself.          The plot, presenting the adjusted glaze          composition of Fe2O3 versus          the corresponding body composition showed rather scattered          distribution of the respective points (R2=0.22)          which may be due to the intentional addition of iron oxide as          coloring agent.          Because of the limited number of samples analyzed          in the present work and according only to the elemental composition,          a precise geographic location of the raw material sources is not          possible to define. However, it should be pointed out that during          archeological excavations medieval kilns for pottery production were          discovered in Drastar (Angelova and Koleva, 2005; Koleva, 2008)          which is evidence for the local production of the studied glazed          potteries. Production of lead-glazed pottery using non-calcareous          clay in combination with lead oxide by itself has been established          by Walton and Tite (2010) for samples produced in the Balkans          (Serbia and Romania) from the 1st to the 4th centuries AD. Futhermore, lead-isotope analysis          of lead-glazed pottery found in Eastern Europe, proved that the lead          for the glaze was obtained using ores from Bulgaria (i.e. the          Rhodopi mountains) and it was concluded that the pottery was          produced locally within the Danubian region (Iliev, 2006; Walton and          Tite, 2010).                      5. Conclusion        The study of 15 medieval lead-glazed ceramics from        Drastar, Bulgaria provides significant information on the chemical        composition and the techniques used for their production. In        particular the present study has shown that both calcareous and        non-calcareous clay bodies were used for pottery production. The        calcareous clay bodies have white color due to the high lime content.        The majority of the non-calcareous clay bodies are red colored,        determined by the high iron oxide concentration in the form of        hematite. Despite the presence of high concentration of iron oxide,        two of the samples from this group have grey (Gl-P-8) and white        (Gl-P-9) colored clay bodies. Both of them are entirely glazed which        creates a reducing atmosphere inside the clay during the firing        process and the presence of hercynite and Fe2+.        Concerning the glazes, the yellow color is due to        the presence of Fe3+, the brown to Fe3+ in higher concentration or to the combination of        Fe3+ and Mn3+ and the green        color to Cu2+.        Further studies on more samples excavated in        Bulgaria are needed to extend the knowledge of glaze pottery        production in Bulgaria.