Material analysis and TL dating of a Renaissance glazed terracotta Madonna statue kept in the Museum of Fine Arts, Budapest

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Original article

Material analysis and TL dating of a Renaissance glazed terracotta Madonna statue kept in the Museum of Fine Arts, Budapest Bernadett Bajnóczi a,∗ , Géza Nagy a , György Sipos b , Zoltán May c , Tamás Váczi d,e , Mária Tóth a , Ildikó Boros f , Manga Pattantyús g a Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 1112 Budaörsi út 45, Budapest, Hungary b Department of Physical Geography and Geoinformatics, University of Szeged, 6722 Egyetem u. 2-6, Szeged, Hungary c Institute for Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1117 Magyar tudósok kórútja 2, Budapest, Hungary d Department of Mineralogy, Eötvös Loránd University, H-1117 Pázmány Péter sétány 1/C, Budapest, Hungary e Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, Konkoly-Thege Miklós út 29-33, 1121 Budapest, Hungary f Hungarian University of Fine Arts, Andrássy út 69-71, 1062 Budapest, Hungary g Hungarian National Gallery, Szent György tér 2, 1014 Budapest, Hungary

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Article history: Received 11 April 2017 Accepted 16 March 2018 Available online xxx Keywords: Madonna statue Glazed terracotta Tin glaze Della Robbia Buglioni TL dating

a b s t r a c t A glazed terracotta statue depicting the Virgin and the Child, dated to the turn of the 15th and 16th centuries, is a prominent object of the Collection of Sculpture before 1800 of the Museum of Fine Arts, Budapest. The provenance of the statue is unknown, it may stem from the place of its 19th-century purchase, Florence or its environs. This paper presents the material analysis and TL dating of the statue and compares the technological features to the glazed sculptural ceramics produced by the della Robbia and Buglioni workshops in the Renaissance Florence. The yellowish ceramic body was made from highly calcareous clay (25 wt% CaO content) and its mineralogical composition indicates an apparent firing temperature of ∼900–950 ◦ C. The white tin glaze is of lead-alkali type with 19.2–20.7 wt% SnO2 , 26–31 wt% PbO and 4.7–7.4 wt% Na2 O + K2 O content. Tiny green spots occur sporadically in the white glaze, where the colour is due to the presence of dissolved copper. In these spots, newly-formed potassiumaluminium silicate, calcium-tin silicate and calcium silicate crystals occur at the body-glaze interface and in the glaze. The violet-coloured glaze on the base of the statue contains a lower amount of tin oxide and a higher amount of lead oxide (11.8 wt% SnO2 , 40 wt% PbO) compared to the white glaze covering the statue. The colour was achieved by addition of manganese, and the violet-coloured glaze was applied on a white glaze covering the body of the base. Based on the TL dating, the statue is unambiguously authentic with an age of 0.58 ± 0.06 ka. © 2018 Elsevier Masson SAS. All rights reserved.

1. Introduction and research aims Glazed terracotta sculptures represent one of the major inventions of the Italian Renaissance. The production of sculptural ceramics of high artistic quality started in Florence by Luca della Robbia in the 15th century and continued by his descendants

∗ Corresponding author. E-mail addresses: [email protected] (B. Bajnóczi), [email protected] (G. Sipos), [email protected] (Z. May), [email protected], [email protected] (T. Váczi), [email protected] (M. Tóth), [email protected] (I. Boros), [email protected] (M. Pattantyús).

for over a century [1]. Other Florentine workshops also produced glazed terracotta objects, a rival one to della Robbia was the Buglioni workshop operating from the 1480s. Nowadays the material characteristics (chemical composition and microstructure) of the body and the glaze of the della Robbia works of art are wellknown thanks to the extensive scientific research using a wide variety of modern analytical techniques performed on the productions of the workshop in the last three decades [e.g. 2-16]. Similar analytical data about the Buglioni objects are also available [11,13,14]. The Collection of Sculpture before 1800 of the Museum of Fine Arts, Budapest holds a glazed terracotta statue depicting the Virgin and the Child (inv. no. 1227, Fig. 1). The 1.26 m high statue manifests the Virgin Mary sitting on a throne with lion legs and rosette

https://doi.org/10.1016/j.culher.2018.03.015 1296-2074/© 2018 Elsevier Masson SAS. All rights reserved.

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study was to assess the authenticity and age of the statue by means of thermoluminescence (TL) dating.

2. Materials and methods

Fig. 1. The ‘Virgin and Child’ glazed terracotta statue (Collection of Sculpture before 1800, Museum of Fine Arts, Budapest, inv. no. 1227).

armrests. In her lap, the infant Jesus stands on a cushion with his right arm raised to bless. The statue is made of yellowish ceramic body covered by bright white glaze, except on the base where the glaze is violet-coloured. Some parts of the statue were restored probably in the 19th century with reddish ceramic covered by ‘milky’ white glaze and on the base by purple glaze, respectively. The Madonna statue was bought in Florence in 1895 as one of the more than hundred items forming the basis of the Sculpture Collection of the Museum [17]. The provenance of the statue is unknown; it may stem from a church or a monastery in Florence or its environs. The object can be dated to the turn of the 15th and 16th centuries. Among the possible creators of the statue several names had arisen, such as Benedetto da Maiano, Giovanni della Robbia, Andrea della Robbia, Leonardo del Tasso [1] and Giovanfranceso Rustici. Recently the statue has been attributed to Baccio de Montelupo [17]. The recent restoration of the statue allows us to perform a scientific investigation, which aims to reveal the technological features, such as type of clay used, type and colorants of glazes of both the original and the formerly restored parts of the object. In this paper, we focus on the data obtained from the analysis of the original parts of the statue and compare them with the technological features of the glazed terracotta works produced by the two most important Florentine terracotta workshops, the della Robbia and the Buglioni. Results of the material analysis performed on the formerly restored parts of the object are included in the Appendix. A very similar Madonna sculpture exists in the Musée de Ceramique in Sevres (inv. no. 7160), which is now regarded as a copy after the Budapest piece [17]. Existence of two similar large-sized, hand-made Renaissance Madonna statues raises the question of originality. Consequently, in addition to the material analysis, a further aim of the present

The original ceramic body was sampled from the unglazed back of the statue at three sites in the form of several mm-cm-sized chips (Fig. 2a, b). Tiny fragments of the original white glaze were detached from the head of the Madonna, the back of the Child and the drapery on the front side of the statue. Several green spots of some mm in size occur in the white glaze, two of them were also sampled. The violet-coloured glaze was sampled from the base of the object. Polished cross-sections embedded into epoxy resin, and perpendicular to the glaze–body interface in case of glazes, were done from the samples. The microstructure of the ceramic body and glazes was examined using the BSE mode of a JEOL Superprobe-733 electron microprobe. The ‘bulk’ chemical composition of the glazes was quantitatively determined by area analyses using Oxford Instruments INCA Energy 200 EDS spectrometer attached to the electron microprobe and calibrated with cobalt standard (Co K␣: 0.20 KeV). Analytical conditions were 20 kV acceleration voltage, 5 nA beam current and a count time of 100 sec. The chemical composition of the vitreous matrix and the inclusions was measured using spot EDS analyses with count time of 40 sec. Glass standards (Corning archaeological reference glasses A, B, C and D) [18] provided by the Smithsonian Institution (USA) were used for calibration of the major elements, while SnO2 for Sn and chalcopyrite for Cu were applied. PAP correction was automatically made by the Oxford Instruments software. The method is not able to distinguish between oxidation states of polyvalent elements; therefore, all iron is expressed as FeO and all manganese as MnO. The areas of ‘bulk’ EDS analyses were set as large as possible depending on glaze thickness, therefore varied from 80 × 80 ␮m to 210 × 210 ␮m for the white glaze and from 230 × 230 ␮m to 450 × 450 ␮m for the violet-coloured glaze. The area of the ‘bulk’ EDS analyses for the green spots was 66 × 66 ␮m and 110 × 110 ␮m, respectively. Typical inclusions, such as tin oxide, quartz and feldspar particles were included in the analysed area, but the body–glaze interface was avoided. At least three area measurements were performed on each sample (except green spots), the results were averaged. The analysed elements are expressed in oxides (except chlorine), their totals are between 97 and 101 wt%, rarely as low as 92 wt%, and all data were normalised to 100 wt% total. The relative standard deviations are usually 5–10% for major elements, and up to 20% for elements present at the 1% or less level. The inclusions in the green-coloured glaze were identified by Raman microspectroscopic analysis on the polished cross-sections using a HORIBA JobinYvon LabRAM HR800 dispersive, edgefilter based confocal Raman spectrometer (focal length: 800 mm) equipped with an Olympus BXFM microscope. The spectra were collected using the 632.8 nm emission of a He-Ne red laser, a 100 × (N.A. 0.9) objective, a grating with 600 grooves/mm and a pinhole of 50 ␮m, which also acted as the entrance slit to the spectrometer. The Raman spectra of the inclusions were compared with the reference spectra of mineral phases of the RRUFF Project database (http://rruff.info). Phase composition of the ceramic body was determined on powdered samples by X-ray diffraction analysis (XRD) using a PHILIPS PW 1730 diffractometer in Bragg-Brentano geometry (instrumental and measuring parameters: CuK␣ radiation, 2–70◦ range of 2 theta scanning, 45 kV acceleration voltage, 35 mA tube current, 1 sec/0.05◦ 2 data collection speed, graphite monochromator). Chemical composition (major, minor and trace elements, including REEs) of the ceramic body was determined by inductively

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Fig. 2. a: ceramic body on the back of the statue; b: fragment of the yellowish ceramic body with thin, red-coloured stripes, and covered with white glaze (head of the Madonna); c, d: microstructure of the ceramic body with lead enrichment in and on the rim of the grains (bright patches in Fig. 2d) (BSE images); e: X-ray diffraction pattern of the ceramic body (only the highest intensity reflections of the detected phases are marked, st: standard).

coupled plasma combined with atomic emission spectroscopy (ICP-AES) and mass spectroscopy (ICP-MS), respectively. Powdered samples (20–30 mg) were digested by Multiwave 3000 (Anton-Paar) sample preparation system (microwave digestion) using 8 ml concentrated nitric acid (67%) and 0.5 ml concentrated

hydrogen fluoride (40%) solutions. After digestion 6 ml saturated boric acid solution (6%) was added to the samples and performed the complexation program with microwave system to eliminate free fluoride ions. Then the sample solutions were washed in 50 ml plastic volumetric flasks with ultrapure water and measured by a

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Spectro Genesis ICP-OES equipment with axial plasma observation and a Thermo Scientific iCAP-Q ICP-MS, respectively (after 20-fold dilution). The totals of the major elements are between 51.7 and 54.9 wt%. Concentration of major elements were calculated to concentration of oxides; and data were normalised to 100 wt% total. The firing date of the artefact was assessed by thermoluminescence dating (TL). Luminescence dating techniques are based on determining:

• the amount of radioactive dose (palaeodose) absorbed by the sample during antiquity; • the annual dose or dose rate produced in the sample matrix and its environment.

Age is provided by the ratio of these two quantities [19]. One TL sample was collected from the original part of the object, i.e. the head of the Madonna. Sampling was made using tungsten carbide drill bits at low rpm. The outer 2–3 mm of the drilling was discarded and only the inner material was used for the TL measurements to secure that only the material of the object was responsible for alpha and beta dose rates throughout antiquity. Both sampling and sample preparation was made in dark room conditions. Sample preparation followed the conventional fine grain technique: treatment with 10% HCl and 10% H2 O2 , separation of the 4–11 ␮m fraction by repeated settling in a 60 mm acetone column [20,21]. The equivalent of the absorbed palaeodose (equivalent dose: De ) was determined by using a RISØ TL-OSL DA-20 luminescence reader equipped with 90 Sr/90 Y beta and 241 Am alpha sources. Throughout the measurements, the multiple aliquot additive dose (MAAD) protocol was applied [19,22]. The natural TL and the TL resulted by three additive doses (three aliquots/dose) were used to set up a dose response curve. As a matter of possible supralinear TL growth in the beginning of natural dosing during antiquity, a correction was made by annealing further aliquots and administering three regenerative doses (three aliquots/dose) to set up a regeneration growth curve. The deviation of the x-intercept (dose axis) from zero was added to the De determined by the MAAD procedure [19]. During all TL measurements, the heating rate was set to 5 ◦ C, end temperature was 450 ◦ C and detection was made through a combination of Corning 7–59 and Schott BG 45 filters. A 30 s preheat at 230 ◦ C was applied, which was followed by a month storage at room temperature. Each TL measurement: natural TL, additive dose TL, regeneration dose TL, were made on the same day. As TL measurements were made on a polimineral sample, anomalous fading was also assessed. Laboratory fading rates were measured by administering the same beta dose to 4 groups of annealed aliquots, those used for the MAAD procedure, and inserting different length of delays between beta irradiation and TL measurements. The calculated fading rate (gvalue) was used for De correction as it is described by Aitken [19]. In order to assess annual dose, an additional 12 g of ceramic sample was collected. The radioactive element content (nat U, 232 Th, 40 K) of the material was determined using a Canberra type gamma spectrometer equipped with an extended range coaxial Ge detector. Internal dose rate, i.e. alpha and beta contribution was calculated using the conversion factors of Adamiec and Aitken [23]. In order to determine the TL efficiency of alpha radiation (a-value) further measurements were made on aliquots used for regeneration TL measurements. A known alpha dose was administered to each aliquot, then increasing additive beta doses were irradiated to groups of aliquots and a MAAD growth curve was acquired to determine the apparent beta equivalent dose of the baseline alpha dose. A moderate water content, 5 ± 5% was assumed for the calculation of wet dose rates. The environmental or external dose rate, being the sum of gamma and cosmic contribution, was estimated on the

basis of numerous indoor measurements made with a Canberra Inspector 1000 type portable NaI detector.

3. Results The BSE images show that the yellowish body is a finegrained ceramic with high porosity (Fig. 2c, d). Non-plastic components of up to 150–200 ␮m size are quartz, K-feldspar, mica (mainly muscovite) and compound particles (e.g. quartz + Kfeldspar ± muscovite, K-feldspar + amphibole) as well as accessory minerals (titanium oxide and zircon). Rarely rounded manganeseand/or iron-rich grains of ∼200 ␮m size also appear. EDS analyses indicate that calcium is enriched in the fine network of the matrix. The ceramic body near to the glaze contains thin, red-coloured, irregular stripes of up to 5 mm length, sometimes in cracks (Fig. 2b). These stripes are bright zones on the BSE images (Fig. 2d) containing enrichment in lead in and on the rim of the particles (e.g. quartz grains) with up to 14 wt% PbO content based on spot EDS analyses. No elevated concentration of iron compared to the average chemical composition of the ceramic body was detected in these zones. According to the XRD analysis (Fig. 2e) the body is composed of the following phases in decreasing amount: diopside (wollastonite?), gehlenite, quartz, plagioclase, K-feldspar, and traces of hematite and gypsum, the latter comes from the former restorations. Calcite and 10 A˚ phyllosilicate (illite-muscovite) were detected only in one of the analysed samples. Chemical analysis of the body indicates high CaO content (25 wt%) accompanied by 3 wt% MgO and 5 wt% Fe2 O3 content (Table 1). The 100 to 300 ␮m thick white glaze contains abundant, homogeneously distributed, mostly fine-grained (1–2 ␮m or less) tin oxide (cassiterite) particles and their aggregates of up to 20 ␮m size as well as less than 160 ␮m sized rounded pores after bubbles (Fig. 3a–c). Rounded and irregular quartz and feldspar particles are distributed heterogeneously in the glaze: in some parts of the statue the glaze is more enriched in quartz and feldspar inclusions (e.g. the drapery on the front, Fig. 3c) than in other parts, where these grains are rare (Fig. 3a, b). The body–glaze interface is sharp, newly-formed crystals at or near the interface are absent. Only a few calcium silicate rods (<10 ␮m, wollastonite?) occur in the glaze. ‘Bulk’ EDS analyses indicate 19.2 to 20.7 wt% SnO2 , 26 to 31 wt% PbO and 4.7 to 7.4 wt% total alkali (Na2 O + K2 O) with more than double amount of K2 O that Na2 O (K2 O/Na2 O = 2.2–2.7) (Table 2). Concentration of chlorine in the glaze is on the detection limit. Distribution of elements is not homogeneous in some parts of the glaze: the CaO concentration increases by 1–1.5 wt%, the K2 O concentration by 1–1.5 wt%, the Al2 O3 concentration by ∼1–3 wt% and the SiO2 concentration by 4–5 wt%, whereas the PbO concentration decreases by 4–7 wt% at and near the body–glaze interface compared to the outer rim of the glaze. Few tiny green spots occur sporadically in the white glaze of the Madonna head and the drapery (Fig. 3d), where the colour is due to the presence of dissolved copper (∼1–6 wt% CuO in the ‘bulk’ glaze, Table 2), no pigment particles were detected. In the greencoloured glaze several inclusions occur together with the abundant cassiterite particles, the latter ones are frequently incorporated in the inclusions (Fig. 3e–h, Table 3): • at the body–glaze interface, a few ␮m sized, hexagonal and octagonal potassium-aluminium silicate crystals with calcium, iron, magnesium and lead(?) content; • at the body–glaze interface and in the glaze up to 20 ␮m sized, acicular-tabular and rhombic calcium silicate crystals with average chemical composition of about 45 wt% CaO and 51 wt% SiO2

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B. Bajnóczi et al. / Journal of Cultural Heritage xxx (2017) xxx–xxx Table 1 Chemical composition of the ceramic body of the Madonna statue measured by ICP-AES (in wt%) and ICP-MS (in ppm) (average of the 3 analyses, major and minor elements are normalised to 100 wt%). Oxide (wt%)

Madonna

SiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O TiO2 P2 O5 PbO SO3

48.97 13.87 4.97 0.11 3.00 24.94 1.12 2.19 0.58

± ± ± ± ± ± ± ± ±

Della Robbia

0.56 0.14 0.07 0.00 0.21 0.38 0.14 0.58 0.01

52.07 13.20 4.85 0.11 2.85 22.99 1.06 2.06 0.58 0.23

± ± ± ± ± ± ± ± ± ±

2.5 0.7 0.4 0.02 1.4 2.3 0.4 0.2 0.03 0.1

Buglioni 52.89–57.27 13.28–13.71 4.54–5.32 0.10–0.12 2.42–2.68 18.34–21.63 0.97–1.20 2.05–2.13 0.58–0.60 0.17–0.27

0.04 ± 0.01 0.21 ± 0.12

Element (ppm)

Madonna

Sc Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Ce/Ce* Eu/Eu*

10.6 20.7 29.0 25.0 6.54 23.3 4.88 1.06 5.11 0.32 3.71 0.76 2.20 0.27 2.01 1.84 4.65 0.41 0.74

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.9 2.3 1.8 1.1 0.44 1.2 0.31 0.04 0.21 0.13 0.18 0.06 0.09 0,02 0.16 0.96 0.17 0.03 0.04

Della Robbia 10.3 ± 1.1 26.9 ± 2.4 47.8 ± 6.6 26.1 ± 5.9 4.34 ± 0.50 1.77 ± 1.97 1.07 ± 1.26

2.28 0.31 7.49 0.83 0.98

± ± ± ± ±

1.10 0.04 0.83 0.07 0.60

The chemical composition (major and minor elements) of the ceramic body of the glazed terracotta works made by the della Robbia and the Buglioni workshops, respectively, is shown for comparison (based on [13]; ICP-AES method; della Robbia workshop: average of 60 objects analysed from the Italian period; Buglioni workshop: 4 objects analysed, only the calcareous bodies are indicated). Trace element (including REE) concentrations of glazed terracotta medallions displayed in Portuguese museums and attributed to della Robbia Italian workshop are also indicated (data from [24]; INAA method; average of 11 objects). Cerium anomalies are calculated as Ce/Ce* = 3CeN /(2LaN + NdN ) and Eu anomalies as Eu/Eu* = 3EuN /(2SmN + TbN ) after [24], where N indicates normalised data to the average of chondrites of Evensen et al. [25].

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addition of manganese, which was dissolved in the vitreous matrix; neither relict pigment particles, nor newly-formed manganesebearing crystals were detected. In addition, no specific elements related to the manganese colorant was detected in the glaze. The iron content is in the same range as in the white glaze (0.46 wt% FeO, Table 2). The cross-section of the glaze (Fig. 3j) as well as the spot EDS measurements show that the lower part of glaze is less coloured, in fact it is whitish, in a thickness ranging from 30–40 ␮m to 150–200 ␮m and the amount of manganese dissolved in the glaze decreases to 0.3–0.6 wt% MnO. In addition, the lower part of the glaze contains unmelted quartz grains and larger pores. The boundary of the violet-coloured and the whitish glaze parts is diffuse and follows the body–glaze interface; the latter is sharp and lacks newly-formed crystals (Fig. 3j–k). Concerning the TL measurements first the existence of a glow curve plateau was checked by taking the ratio of a representative natural and an additive (+6.2 Gy) TL curve. This simple test has already shown the suitability of the sample for performing the MAAD analysis as a clear plateau was observed from 360 ◦ C (Fig. 5). Taking the 360–440 temperature range the mean ratio of the two curves was 0.35, referring to an approximate 2.14 Gy equivalent dose in the sample. The more detailed MAAD analysis allowed to check whether there is a dose plateau by calculating De at every 10 ◦ C temperature increment. The plateau could be identified at 370–420 ◦ C. Consequently, the TL signals were integrated in this range to set up the MAAD relationship (Fig. 6). The dose response curve was fitted with a linear function, and after subtracting the background a De of 2.56 ± 0.11 Gy was received. Based on the regeneration dose measurements, a 0.33 ± 0.09 Gy correction was made (Fig. 7). The fading test yielded a 2.70 ± 0.14 g-value, which meant another 11% increase in terms of De (Table 4). Radioactive element contents showed usual values. Based on the ratio of 226 Ra and 210 Pb specific activities (0.96 ± 0.11) the 238 U chain was found to be in equilibrium, no correction was made in this respect. Alpha efficiency (a-value) and consequently alpha dose rate showed considerable values. The total dose rate was therefore also relatively high, 5.50 ± 0.44 Gy/ka (Table 4). By dividing the corrected equivalent dose and dose rate, the age of the object was calculated to be 0.58 ± 0.06 ka. The achieved 10% relative error meets the expected precision of the method.

4. Discussion (CaO/SiO2 ratio of 0.9) containing magnesium as minor component (≤ 1 wt% MgO); • in the glaze up to 50 ␮m sized, tabular and rhombic calcium-tin silicate with average chemical composition of 20.0 wt% CaO, 52.5 wt% SnO2 and 22.6 wt% SiO2 (CaO/SiO2 ratio of 0.9) containing titanium and iron in average (1.7 wt% TiO2 , 1.4 wt% FeO). Raman microspectroscopic analysis revealed that potassiumaluminium silicate crystals are leucite (Table 3) as well as K-feldspar (orthoclase), whereas calcium silicate crystals are wollastonite, and calcium-tin silicate crystals are malayaite (Fig. 4). The glaze on the base of the statue (Fig. 3i) is circa 300 to 600 ␮m thick and mainly violet-coloured in cross-section (Fig. 3j). The violet-coloured glaze differs from the white glaze of the statue in that it contains smaller (5–10 ␮m) pores and rare unmelted quartz inclusions (Fig. 3k). Lots of homogeneously distributed cassiterite particles occur in the violet-coloured glaze, however, the ‘bulk’ SnO2 content is lower and the ‘bulk’ PbO content is higher (11.8 wt% SnO2 , ∼40 wt% PbO) compared to that of the white glaze (Table 2). The total alkali (Na2 O + K2 O) content is 6.2 wt% and the K2 O/Na2 O ratio is 1.6. The ‘bulk’ MnO content of the glaze is 1.9 wt% in average (Table 2) indicating that violet colour was achieved by

4.1. Technological features of the statue The high CaO content (25 wt%) and the phase composition of the ceramic body indicate that the statue was produced from calcareous illitic-chloritic clay. The disappearance of carbonate and 10 A˚ phyllosilicate (illite-muscovite), and the dominance of high-temperature calcium silicate phases (diopside–CaMgSi2 O6 , gehlenite–Ca2 Al2 SiO7 ) suggest an apparent firing temperature of ∼900–950 ◦ C [26]. Calcite detected in one of the samples can either be a primary particle, incompletely dissociated during firing most probably due to its large grain size, or of secondary origin due to contamination. According to the classification of Tite et al. [27] the white glaze is a of lead-alkali type opacified with high amount of tin oxide (∼20 wt% SnO2 ). The chemical composition of the glaze indicates that, in accordance with Piccolpasso’s treatise written about the production technology of the Italian maiolica at around 1557 [10,28], the glaze mixture was composed of lead-tin calx and alkali-rich frit. Tin was introduced into the glaze by lead-tin calx (calcine), a mixture of lead and tin oxides, which was prepared in a special furnace by melting of lead and tin metals together.

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Fig. 3. a–b: white glaze with abundant bright tin oxide inclusions, rare dark quartz and feldspar particles and rounded pores (a: head of the Madonna, b: back of the Child, BSE images); c white glaze with abundant bright tin oxide inclusions, lots of dark quartz and feldspar particles and rounded pores (drapery, BSE image); d: green spots in the white glaze on the head of the Madonna; e–f, g–h: cross-sections of green spots in the white glaze with abundant, bright tin oxide inclusions, dark potassium-aluminium silicate crystals at the body-glaze interface (spot EDS sites: a1 and a2), dark, needle-like, tabular and rhombic calcium silicate crystals (spot EDS sites: b1, b2, b3) and grey, tabular and rhombic calcium-tin silicate crystals (spot EDS sites: c1, c2, c3) (BSE images). Chemical composition of the analysed inclusions is shown in Table 3; i: violet-coloured glaze on the base of the statue; j: cross-section of the violet-coloured glaze; k: violet-coloured glaze with abundant bright tin oxide inclusions and a white glaze underneath (BSE image).

Due to its solubility in water, alkali is usually pre-fritted with silica; the latter is mainly in the form of sand, before making the glaze suspension. The K2 O/Na2 O ratio of 2.2 to 2.7 with the low amount of chlorine in the glaze indicates the use of higher amount of potassium-bearing flux, probably in the form of wine lees or tartar, than sodium-bearing flux, the latter mainly in form of common salt. Quartz and feldspar particles are relicts of the sand further added to the glaze mixture, and their rounded and irregular morphology indicates that these particles were not totally melted during glaze firing and the glaze mixture with the additional sand was not fritted before application in accordance with the

glaze preparation method described by Piccolpasso [10]. The glaze covering the different parts of the statue varies mainly in the amount of the unmelted sand particles rather than its thickness. Green spots occurring sporadically in the white glaze are most probably unintentional impurities. Green glaze drops might accidentally have appeared on the raw glaze before firing or pollutions from the furnace, and melted into the white tin glaze during firing. At the body–glaze boundary and in the glaze of the green spots, a greater diversity of inclusions was detected than in the white glaze. The idiomorphic shape of the calcium silicate (wollastonite) and calcium-tin silicate (malayaite) crystals together with the

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Table 2 Chemical composition (in wt%) of the white and violet-coloured glazes of the statue (average of ‘bulk’ area and spot EDS measurements, n: number of analyses, standard deviation in parenthesis, results are normalised to 100 wt%, na: not analysed).

White glaze 1

Area (n = 8) Spot (n = 47) Area (n = 8) Spot (n = 10) Area (n = 6) Spot (n = 20) Area (n = 1) Spot (n = 7) Area (n = 1) Spot (n = 7) Area (n = 3) Spot (n = 11)

White glaze 2

White glaze 3

Green spot 1

Green spot 2

Violet−coloured glaze

SiO2

PbO

SnO2

Na2 O

K2 O

CaO

MgO

Al2 O3

FeO

Cl

CuO

MnO

41.98 (1.17) 52.15 (2.09) 42.69 (0.68) 52.75 (1.97) 41.95 (1.4) 48.76 (2.65) 33.1

26.18 (1.43) 32.1 (2.89) 27.53 (1.69) 32.4 (1.88) 30.78 (1.32) 38.91 (2.6) 39.25

20.7 (1.45) 3.39 (1.41) 19.19 (1.08) 3.12 (0.95) 19.46 (0.49) 2.48 (1.9) 13.35

2.32 (0.26) 1.87 (0.38) 2.06 (0.18) 1.7 (0.32) 1.27 (0.21) 1.07 (0.28) 2.63

5.07 (0.73) 5.73 (0.88) 5.03 (0.21) 5.5 (0.5) 3.38 (0.14) 4.69 (0.52) 3.52

1.48 (0.39) 1.9 (0.68) 1.11 (0.23) 1.77 (0.91) 0.97 (0.24) 1.08 (0.49) 3.57

0.23 (0.08) 0.31 (0.16) 0.19 (0.05) 0.33 (0.22) 0.18 (0.04) 0.17 (0.15) 0.62

1.64 (0.25) 1.95 (0.9) 1.71 (−.29) 1.62 (1.03) 1.56 (0.23) 2.36 (0.71) 1.85

0.29 (0.13) 0.39 (0.32) 0.17 (0.16) 0.42 (0.28) 0.28 (0.16) 0.2 (0.17) 0.98

0.11 (0.15) 0.2 (0.14) 0.32 (0.04) 0.38 (0.1) 0.18 (0.11) 0.28 (0.21) −0.13

na

na

na

na

na

na

na

na

na

na

na

na

1.23

na

38.72 (1.95) 31.3

43.77 (1.89) 36.6

1.41 (0.36) 14.07

2.6 (0.26) 3.48

4.65 (0.51) 1.75

3.59 (0.93) 3.38

0.56 (0.25) 0.64

2.39 (0.7) 2.11

1.09 (0.46) 0.73

0.13 (0.14) 0

1.08 (0.46) 5.95

na

36.31 (1) 37.38 (0.58) 44.2 (2.87)

43.27 (0.78) 39.6 (1.06) 41.65 (2.37)

1.23 (1.05) 11.76 (1.25) 2.07 (0.91)

2.87 (0.45) 2.36 (0.09) 2.17 (0.28)

2.76 (0.32) 3.82 (0.05) 4.55 (0.42)

2.3 (0.55) 0.83 (0.33) 0.93 (0.79)

0.65 (0.27) 0.31 (0.21) 0.22 (0.14)

2.93 (0.57) 1.31 (0.11) 1.82 (0.77)

0.86 (0.27) 0.46 (0.2) 0.34 (0.23)

0 (0.16) 0.25 (0.09) 0.22 (0.19)

6.74 (0.57) na

na

na

na

1.92 (0.09) 1.93 (0.41)

White glaze1 is from the head of the Madonna, white glaze2 is from the back of the Child, white glaze3 is from the drapery. ‘Bulk’ EDS analyses of the green spots contain only tin oxide inclusions.

Table 3 Chemical composition (in wt%) of potassium-aluminium silicate (leucite), calcium silicate (wollastonite) and calcium-tin silicate (malayaite) crystals present at the body-glaze interface and in the green-coloured glaze measured by spot EDS analyses (for the sites of the analysis see Fig. 3f–h). Oxide wt%

SiO2 PbO SnO2 Na2 O K2 O CaO MgO Al2 O3 FeO TiO2 CuO Total

Leucite

Wollastonite

Malayaite

a1

a2

b1

b2

b3

c1

c2

c3

51.50 – – 0.35 12.33 7.87 3.36 16.93 4.7 – – 97.04

50.85 1.59 – 0.08 13.08 7.09 3.21 17.48 4.07 – – 97.46

50.11 1.09 3.45 – – 44.09 0.91 – – – – 99.65

51.11 0.92 3.25 – – 45.22 0.62 – – – – 101.12

51.36 2.22 –0.03 – – 45.19 1.03 – – – – 99.77

22.27 0.86 51.44 – – 20.26 – – 1.79 1.88 1.01 99.52

22.55 2.06 51.97 – – 19.65 – – 1.26 1.47 – 98.96

22.93 0.48 53.99 – – 20.23 – – 1.26 1.79 – 100.68

–: not analysed, lead detected in almost all measurements derives from the glaze, tin detected in wollastonite derives from the embedded tin oxide inclusions).

embedded tin oxide opacifier particles clearly indicates that they were formed in situ during firing and subsequent cooling. The higher CaO content of the green spots (> 3 wt% CaO) compared to the white glaze (< 1.5 wt% CaO) and the in situ formed calcium-rich inclusions indicate that calcium diffused from the calcareous body to the glaze during firing. The degree of diffusion was smaller from the body to the uncoloured white glaze, whereas the unintentionally copper impurity in the green-coloured glaze promoted the element diffusion and/or the in situ formation of calcium-rich phases. Calcium silicate, wollastonite (CaSiO3 ) forms from the reaction between CaO and SiO2 (in ceramics) at 800 ◦ C [26] or circa 100 ◦ C higher temperature [29]. Calcium-tin silicate, malayaite (CaSnSiO5 /CaSnOSiO4 ), belonging to the titanite group, is a mineral found in skarn deposits and typically forms under hydrothermal conditions at several hundreds of degrees temperature [e.g. 30–33]. As a synthetic ceramic pigment to produce pink-coloured glaze, malayaite is doped with transition metal ions, mainly with chromium (CPMA 12-25-5) [34]. However, malayaite is rarely observed as a phase formed in situ in glazes, for instance at the body–glaze interface of green glaze in Portuguese

Hispano-Moresque tiles [35], where its formation was facilitated by presence of transition ion like copper. Malayaite could form in the green-coloured glaze due to the reaction of tin oxide (cassiterite) and wollastonite or equimolar quantities of CaO, SiO2 and SnO2 [30–33] during firing, and subsequent cooling. In addition to calcium-rich inclusions, idiomorphic calciumiron-magnesium-lead(?)-bearing potassium-aluminium silicate (leucite, KAlSi2 O6 and K-feldspar, KAlSi3 O8 ) crystals at the body–glaze interface in the green spots are likewise newly-formed phases indicating elevated potassium content due to diffusion from the body to the glaze. Lead-bearing potassium-aluminium silicate (feldspar) typically forms at the interface between K-rich (illitic) clay body and lead-rich glaze at temperatures from 800 to 1000 ◦ C [36,37]. These crystals can form at the interface between calcareous ceramic body and lead glaze if the body contains enough potassium, i.e. illitic [37–39], as the body of the Madonna statue. Leucite crystallites of similar origin were also detected in earlier studies [39]. The violet-coloured glaze on the base of the statue is also of lead-alkali type with less unmelted sand particles and smaller

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Fig. 4. Raman spectra of the inclusions in the green-coloured glaze compared to their reference spectra from the RRUFF database: a: leucite and its reference (R060300); b: K-feldspar (orthoclase) and its reference (R050185); c: wollastonite and its reference (R120016); d: malayaite and its reference (R061106).

Fig. 5. Representative natural and natural + additive TL glow curves, and the ratio of the two curves at 20 ◦ C increments (filled circles).

Fig. 7. Normalised TL intensities as a function of delay between irradiation and measurement. Ic and tc stand for the intensity and “delay” in comparison to the prompt measurement. Table 4 Dose rate (D*), equivalent dose (De ) and age data of the investigated object.

Fig. 6. Additive (circles) and regeneration (squares) dose response functions corrected with background (triangle) TL intensities. De : equivalent dose calculated on the basis of the MAAD procedure. I: x-intercept yielding the value of supralinearity correction.

Inv. no. Lab ID. D*internal U (ppm) Th (ppm) K (%) a-value D*␣ (Gy/ka) D*␤ (Gy/ka) D* external (Gy/ka) D*total (Gy/ka) De (Gy) g-value De corr (Gy) Age (ka) Calendar Age

1227 OSZ1294 2.70 ± 0.08 8.04 ± 0.24 2.25 ± 0.07 0.22 ± 0.04 2.51 ± 0.41 2.24 ± 0.14 0.75 ± 0.10 5.50 ± 0.44 3.24 ± 0.20 2.70 ± 0.14 3.21 ± 0.22 0.58 ± 0.06 AD 1370–1490

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amount of tin oxide (11.8 wt% SnO2 ) compared to the white glaze. Comparing the chemical composition of the violet-coloured and white glazes normalised to 100 wt% with the exclusion of the tin oxide concentration, the violet-coloured glaze has higher lead content resulting in a shinier glaze with lower melting point. This indicates that the manganese-coloured glaze is chemically different from the white one that covers the statue. The lower white part of the glaze on the base of the statue with different microstructure (higher amount of sand particles and larger pores) suggests that a white glaze was applied directly on the terracotta of the base underneath the violet-coloured glaze. 4.2. Comparison with the della Robbia and the Buglioni works of art The ceramic body of the della Robbia glazed terracotta works is buff- or pink-coloured, tempered with small amount of quartz and contains dispersed clay lenses [12,13]. The terracotta bodies produced by the workshop are chemically homogeneous indicating that the quality of the clay used was consistent during generations, a calcareous clay with 20 to 25 wt% CaO content, except the works produced in France by Girolamo della Robbia [12,13] (Table 1). This chemical composition is remarkably consistent compared to that of the clay used for the Renaissance maiolica (14–25 wt% CaO) [10] due to the fact that the family has its own clay pit in the valley of Arno [1,40]. The firing of the body occurred at around 1000 ◦ C, and the temperature of the second firing after glazing was somewhat lower, at 900–950 ◦ C [12,13]. A recent study on glazed terracotta bodies attributed to the della Robbia workshop also supported that the firing temperature was around 900 ◦ C [24]. The apparent firing temperature of the Madonna statue is in accordance with the published temperature estimations. According to the chemical analysis of the ceramic body, the Buglioni workshop used at least two types of clay for the glazed terracotta: one poor in lime (circa 2 wt% CaO) and another one with high lime content (18–22 wt% CaO) [13] (Table 1). Although much less chemical data are available (altogether 6 objects [13]), major and minor element composition of the calcareous body of some Buglioni works, especially the early ones, is similar to that of the della Robbia bodies (Table 1). The Madonna statue, made of calcareous clay, chemically resembles the della Robbia as well as some Buglioni works (Table 1). A possible way to distinguish between della Robbia and Buglioni works is the comparison of trace element (including REE) pattern of their ceramic bodies. Zucchiatti et al. [5] measured the major, minor and trace element (including REE) compositions of the bodies of ten della Robbia glazed terracotta sculptures, although they did not publish the trace (REE) element concentration values. Recently Dias et al. [24] measured the chemical composition (mainly REEs) of eleven glazed medallions displayed in Portuguese museums and attributed to the della Robbia workshop (Table 1). The common feature of the sculptures studied is the negative cerium anomaly (Ce/Ce* from 0.65 to 0.96) interpreted as an indication for the use of carbonate-rich clays of marine origin [24]. Dias et al. [24] also detected both negative and positive europium anomalies (Eu/Eu* from 0.60 to 2.88) in the terracotta bodies. The body of the Madonna statue exhibits negative europium anomaly (Eu/Eu* = 0.74), but a lower cerium concentration, consequently a more negative cerium anomaly (Ce/Ce* = 0.41), compared to the Portuguese terracotta sculptures (Table 1). Therefore, a similarity to della Robbia works based on available REE concentration data cannot be proved. Fine red lenses, detected by necked eye and irregularly distributed in the body of della Robbia works, are reported [13] and attributed to the mixing of two types of clay, a light-coloured,

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calcareous one and a red-coloured, iron-rich one. No iron-bearing clay lenses were detected in the studied body samples of the Madonna statue; however, the thin, red, irregular stripes visible especially near to the glaze are enrichments of lead and interpreted as results of lead diffusion occurred to the body from the glaze. The della Robbia workshop used shiny, smooth, uniformly intense white glaze on the terracotta with the following characteristics: thickness of 150 to 400 ␮m, presence of abundant, tiny, opacifying tin oxide particles and rare unmelted quartz grains, occurrence of developed (10 to 50 ␮m) transitional zone with calcium silicate and calcium aluminium silicate phases at the body–glaze interface indicating that the physical properties of the clay and the glaze fit to each other [12,13]. The typical chemical composition of the della Robbia white glaze is 15–20 wt% SnO2 , 25–40 wt% PbO, 35–50 wt% SiO2 with K2 O/Na2 O ratio of 0.6 to 2.5 [10,14]. This composition indicates that compared to the tin glaze of the Renaissance maiolica, which typically contains 4 to 10 wt% SnO2 and has a K2 O/Na2 O ratio of 1.5 to 6, the della Robbia workshop deliberately modified the composition of the glaze: higher amount of tin oxide and lead oxide as well as higher amount of sodium-bearing flux was used [10,14]. The high amount of tin oxide was applied to increase the whiteness and opacity as well as to increase the viscosity of the glaze which can help to compensate the decrease in glaze viscosity due to its higher lead oxide content, the latter was most probably applied to decrease the melting temperature [10]. Coloured glazes preferentially used by the della Robbia workshop were directly applied on the fired clay (terracotta) without using a white glaze underneath. In addition, the colorant distributes homogeneously in the whole thickness of the glaze which resulted in more uniform and intense colour indicating that the preparation method of the coloured glazes of the sculptural ceramics was modified compared to the maiolica [2,10]. The violet or purple glaze was coloured with manganese [11,14]. There are much less published data about the chemistry of the white glaze used by the Buglioni workshop. The workshop used two types of white glaze: one with low amount of tin (in general about or less than 5 wt% SnO2 , opacified mainly by quartz and feldspar) [11] and another one with higher amount of tin, the latter in fact contains less tin than the della Robbia white glazes [14,without any exact compositional data]. Both the della Robbia and the Buglioni coloured glazes have variable tin content ranging from the typical tin content of the maiolica to the tin content of the white glaze of the terracotta produced by the same workshop, that is from ≤ 5 wt% to > 20 wt% SnO2 (Fig. 8). In some of the Buglioni works of art, there are areas in the glaze in which tin oxide grains is more concentrated and areas in which the vitreous phase is more abundant [6]. Based on the chemical composition, the white glaze of the Madonna statue fits well into the compositional range of the della Robbia white glazes, whereas the violet-coloured glaze fits into the common range of della Robbia and Buglioni coloured glazes (Fig. 8). Most of the characteristics of the glazes of the Madonna statue (thickness, inclusions, chemical composition, type of colouring) show a similarity to the della Robbia glazes. However, a welldefined transition zone (∼20 ␮m thick) with newly-formed phases occurs only in the green-coloured glaze spots, and seems not to be characteristic for the bulk white glaze. Another difference is the application of the violet-coloured glaze on a white glaze, not directly on the terracotta base of the statue. Based our observations, the base with violet-coloured glaze is as an integral and original part of the statue, and not a later addition. According to our knowledge, the ‘coloured glaze on white glaze’ technique was not described earlier, therefore it seems to be a new characteristic of Renaissance glazed terracotta sculptures.

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Fig. 8. PbO vs. SnO2 diagram for the white and violet-coloured glazes of the Madonna statue compared to the white and coloured glazes (only blue and violet-black) of della Robbia and Buglioni sculptural ceramics (data from [2,5–7,9–11,15]). Chemical compositions were measured by SEM-EDS, electron microprobe, PIXE and portable XRF analyses for della Robbia and Buglioni glazes. PbO-SnO2 concentration areas for the white glaze of the Early and Late Renaissance Italian maiolica (based on the data from [10,41–44]) are also indicated. Explanations: dark squares: della Robbia white glazes, grey squares: della Robbia coloured glazes, white squares: Buglioni coloured glazes, circles: Madonna white and violet-coloured glazes.

4.3. Age of the statue

Appendix A. Supplementary data

TL measurements verified that the original part of the object is unambiguously authentic with an age of 0.58 ± 0.06 ka (calendar age: AD 1370–1490). Although the received age places the production rather to the 15th century than to the turn of the 15th and 16th centuries as indicated by the art historical analysis, by considering the error terms of the TL date the two results overlap. A previous TL analysis of another object attributed to the della Robbia family (Madonna tondo, inv.no. 1172) in the same museum collection yielded a very similar age, i.e. 0.57 ± 0.10 [45], reinforcing the data achieved in this study. The error term in this case was quite high, but as a matter of the high uncertainties in terms of the external dose rate and the water content of the ceramic body the precision of the TL data can hardly be increased in any of the cases.

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.culher.2018.03.015.

5. Conclusions The analysis of the original ceramic body and glazes of the Madonna statue revealed the material usage and the technological features such as yellowish calcareous body fired at high temperature (∼900–950 ◦ C), thick white glaze opacified with high amount of tin (∼20 wt% SnO2 ) and violet-coloured glaze coloured with manganese. The TL dating of the object verified that it is an original Renaissance work of art, not a late copy. Material characteristics of the statue are similar to that of the contemporaneous glazed terracotta objects made by the della Robbia and the Bugloni workshops in Florence. However, yet unknown features of Renaissance glazed terracotta sculptures were also detected like green spots in the white glaze as impurities with lots of newly-formed crystals and a two-layer glazing technique (coloured glaze applied on white glaze). Present results constitute a valuable basis for future comparative analysis and a more precise determination of the production workshop.

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Please cite this article in press as: B. Bajnóczi, et al., Material analysis and TL dating of a Renaissance glazed terracotta Madonna statue kept in the Museum of Fine Arts, Budapest, Journal of Cultural Heritage (2017), https://doi.org/10.1016/j.culher.2018.03.015