Clay and alunite-rich materials in painting grounds of prominent Italian masters – Caravaggio and Mattia Preti

Applied Clay Science 185 (2020) 105412

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Clay and alunite-rich materials in painting grounds of prominent Italian masters – Caravaggio and Mattia Preti

T

⁎

David Hradila,b, , Janka Hradilováb, Giancarlo Lanternac, Monica Galeottic, Katarína Holcováb,d, Victory Jaquesb,d, Petr Bezdičkaa Institute of Inorganic Chemistry of the Czech Academy of Sciences, v.v.i., ALMA Laboratory, 1001 Husinec-Řež, 250 68 Řež, Czech Republic Academy of Fine Arts in Prague, ALMA Laboratory, U Akademie 4, 170 22 Prague 7, Czech Republic c Laboratorio Scientifico, Opificio delle Pietre Dure – MiBAC, viale F. Strozzi 1, 50129 Florence, Italy d Department of Geology and Paleontology, Faculty of Science, Charles University Prague, Albertov 6, 128 43 Prague 2, Czech Republic a

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Painting grounds Globigerina limestone Alunite Caravaggio Powder X-ray micro-diffraction

Recently, the fine arts' research is increasingly carried out by non-invasive techniques, which do not require any sampling. However, some questions cannot be answered in this way. A typical example is the provenance analysis of the natural materials used in historical paintings. Another question is how much the provenance of the material is related to the provenance of the artwork itself (because of trade with pigments, painters' migration and their artistic preferences). Often, only painter's preference and intention are mentioned, and not economic factors and regional availability of the raw material. This is also the case of clay-based preparatory layers (grounds) on Caravaggio's paintings. In order to prove a connection of the ground composition with the place of the painting's creation, the painting grounds used by two prominent Baroque Masters Caravaggio and Mattia Preti have been investigated using X-ray powder micro-diffraction and Fourier transform infrared microspectroscopy in combination with micro-palaentological analysis. It was found that grounds applied by the same painter, but in two closely related regions – Italy and Malta, differ. While pottery clays were used in Italy, weathered Globigerina limestones were applied in Malta in combination with alunite-hematite material. It is the first time ever that such material has been identified as a main component in painting grounds.

1. Introduction A development of non-invasive analytical techniques, which can be applied directly to the work of art and do not require any sampling, can be seen as one of recent trends in the analysis of the fine arts. (Alfeld and Broekaert, 2013; Legrand et al., 2018) Although the information obtained in this way is incomplete, it is sufficient to address a number of issues – visualisation of hidden structures in paints (Tonazzini et al., 2019), study of degradation processes (Vanmeert et al., 2019) etc. Therefore, the sampling of paint layers becomes more limited, highly efficient and targeted to solve only special tasks. Such a task may be, for example, a detailed description of the painting technique or the provenance analysis. The term “provenance analysis” is usually understood to be the search for such materials/technological characteristics that help to refine dating, painter's or workshop's attribution, or also the place of the painting's creation. In Middle ages and modern era, the link between a regional provenance of the material (widely studied in the

field of archaeometry) and the place of origin of the painting itself is not as straightforward as in antiquity. The reason is the developing trade with materials and products, and particularly also an increasing mobility of artists. The search of the origin of the raw material is widely represented in archaeometry - in the analysis of archaeological findings (including paints – Rodler et al., 2017), while for historical paintings is only rarely applied (Stevenson et al., 2016). Once there are written sources and the painting is well preserved (not only in fragments), the focus of provenance analysis is shifted into art-historic considerations. A great potential of modern scientific approaches is not fully exploited and misinterpretations are common. For example, it is frequently mentioned that Caravaggio (1571–1610) worked intentionally with the colour of the ground material when creating his canvas paintings. While grey grounds can be found in some of Caravaggio's early paintings in Rome, most of Caravaggio's paintings have a red-brown, somewhat translucent ground that he came to utilize in a very particular manner – he left the ground

⁎ Corresponding author at: Institute of Inorganic Chemistry of the Czech Academy of Sciences, v.v.i., ALMA Laboratory, 1001 Husinec-Řež, 250 68 Řež, Czech Republic. E-mail address: [email protected] (D. Hradil).

https://doi.org/10.1016/j.clay.2019.105412 Received 31 July 2019; Received in revised form 15 December 2019; Accepted 16 December 2019 0169-1317/ © 2019 Published by Elsevier B.V.

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

visible in the half-tones. (Weil, 2007; Falcucci, 2013) The question is, whether Caravaggio had prepared his grounds himself, or whether he had to draw on the colouring of already primed (ready-to-use) canvases. According to the investigation of Keith (1998) the reddish-brown ground on famous paintings Boy bitten by a lizard (around 1595) and Last supper at Emmaus (1601) consists of calcite (chalk), earth (clay) pigments and little lead white. These grounds are characterized by Keith (1998) as “naturally translucent as well as dark in colour”, and, as interpreted by Weil (2007) „its translucency is apparently imparted by the considerable quantity of chalk in the mixture”. Although the results of materials analyses are not presented in these articles, the above cited descriptions have to be based on the evidence of a considerable amount of Ca together with Si and Al in the ground mixture. However, the interpretation that Ca indicates an intentionally added chalk (natural calcium carbonate) to earth pigments is not the only one possible. Alternatively, the source material could be a Ca-rich marly clay – it means that both carbonate and silicate components are of the same geological origin in terms of place and time. In such a case, the idea that the painter adjusted the ratio of chalk and earth pigments in the mixture with any artistic aim, is completely odd. Against the idea that everything is subordinated to the author's intention (preferred by numerous art-historians) is the consideration accentuating the role of economic factors and local availability of certain materials. These factors are very little taken into account because materials' provenance micro-analysis is a very difficult task. For example, it seems likely that in Italy in the 17th century manufacturers (rather than painters themselves) used cheaply available calcareous pottery clay instead of earth pigments for priming canvases (Hradil et al., 2018). The dull colour of this clay was then improved by deliberately added pigments (such as, e.g., PbeSb yellow, Cu pigments, Pb white, iron oxides) (Stols-Witlox, 2012; Hradil et al., 2015) to the ground mixture. Although there is no exact prove, pottery clay, later intentionally coloured, known from the grounds of Italian 17th century Caravaggists, was very probably previously also bought and used by Caravaggio himself, when he was working in Italy. All indications, photographs (showing the brown colour of the ground (Keith, 1998) and available information on the composition (here mentioned earth + chalk mixture) are in line with this. The question that arises from this wider contextual introduction is whether it is possible, according to the composition of grounds, also to judge where the painter travelled and where the work originated. Hradil et al. (2015) already evidenced the differences between Italian and Central European clay ground types. So far, however, no one has compared in detailed the changes in mineralogical composition of the ground of one painter created in two closely related regions. The aim of this work is to compare the composition of the painting grounds created in two closely interrelated regions – Italy and Malta. Only paintings by Italian painters with clear authorship and dating to the 17th century were selected to solve this task. It must have been also obvious that selected painters spent part of their productive life in Italy and part on Malta Island in order to monitor whether changes in composition are related to a change of region or to a change in painter's intention or preference. Two Baroque Masters met this strict criterion Caravaggio and Mattia Preti. In order to prove the regional specificity of grounds micro-spectroscopic techniques were combined with X-ray powder micro-diffraction and with micro-palaeontological analysis of micro- and nannofossils. The idea of a regional specificity of grounds in 17th and 18th century is based on the assumption that in that time workshops experimented with new earthy materials (Stols-Witlox, 2012) and cheaply available coloured clays were not economically advantageous to transport over longer distances.

Fig. 1. Detail from the oil-on-canvas painting Beheading of Saint John the Baptist (1608), 370 × 520 cm, St. John's Co-Cathedral, Valletta, Malta, signed by Michalengelo Merisi (known as “Caravaggio”); adopted from www. slavneobrazy.cz

2. Materials and methods 2.1. Analysis of artworks' micro-samples 2.1.1. Selection of micro-samples Three microsamples from one of Caravaggio's latest paintings, which he painted for the Cathedral in Valetta, Malta, were examined (Fig. 1). The origin of this painting on the island of Malta is documented elsewhere (e.g. Ciatti and Lalli, 2013). Caravaggio came to Malta in 1608, was knighted by Grand Master of Vignacourt, departed, and died in 1610 (Calleja, 1881). Along with that, seven microsamples from the paintings by Mattia Preti were selected, both from those he had created with a high probability in Italy (and commissioned either by Dubrovnik noble families or wealthy commoners; now located in Dubrovnik, Croatia - Pustić, 2009, Lalli et al., 2014) and those that were created on Malta after 1660 (Pelosi et al., 2017; Napolillo, 2013). Mattia Preti came to Malta in 1661 at the invitation of the Grand Master de Eedin at resided here until his death at 1699 (Calleja, 1881). All samples came from the archive of Opificio delle Pietre Dure in Florence, Italy, and have been carefully selected in order to contain a sufficiently preserved bottom ground layer. According to information collected in the archive all the samples contained „earthy pigments “or „iron ochres “in the ground, without any further specification. High contents of chalk were mentioned particularly in samples J1863 and J1864. Also in these samples, numerous visually discernible microfossils were observed and reported – again, without any further specification. All the samples and artworks are summarized in Table 1. For each painting, the number of samples collected from different locations on the painting is specified. Since the ground layer is the same throughout the whole painted area, only one sample from each painting (the most suitable in terms of preservation and thickness of the ground layer) was selected for mineralogical and micro-destructive paleontological analysis. Elemental composition was analysed in all samples and then averaged. 2.1.2. Micro-sample preparation and light microscopy (LM) The micro-samples were first observed by stereoscope Leica S8 APO Stereozoom. Subsequently, they were embedded in Polylite 32,032–20 polyester resin and, after hardening, processed into grinded cross-sections by LaboPol-5 grinding machine made by the Struers company. A small portion of each sample was left untreated for the purpose of diffraction and palaeontological studies. Zeiss Axio Imager A.2 light microscope with the Olympus DP 74 digital camera was employed for detailed visual observation of microsamples and their cross-sections. Photographs were taken in the white reflected light as well as in the UV 2

Applied Clay Science 185 (2020) 105412 Malta (Ciatti and Lalli, 2013; Calleja, 1881) Italy (Pustić, 2009, Lalli et al., 2014) Italy (Pustić, 2009, Lalli et al., 2014) Italy (Pustić, 2009, Lalli et al., 2014) Italy (Pustić, 2009, Lalli et al., 2014) Malta (Pelosi et al., 2017; Napolillo, 2013) Malta (Pelosi et al., 2017; Napolillo, 2013) Cattedrale di S. Giovanni, La Valetta (Malta) Church of St. Blaise/Dubrovnik (Croatia) Church of Our Lady of Carmel/Dubrovnik (Croatia) Church of Our Lady of Carmel/Dubrovnik (Croatia) Church of Our Lady of Carmel/Dubrovnik (Croatia) Museum of Wignacourt (Malta) Żabbar Sanctuary Museum, Zabbar (Malta) Beheading of Saint John the Baptist (1608) Four Evangelists (cycle), St. Mathew (before 1660) Four Evangelists (cycle), St. Luke (before 1660) Four Evangelists (cycle), St. John (before 1660) Four Evangelists (cycle), St. Mathew (before 1660) St. Peter blessing (1690) Our Lady of Carmel and souls from Purgatory (after 1660) Caravaggio (1571–1610) Mattia Preti (1613–1699) Mattia Preti (1613–1699) Mattia Preti (1613–1699) Mattia Preti (1613–1699) Mattia Preti (1613–1699) Mattia Preti (1613–1699) J1866/3 J1857/1 J1858/1 J1859/2 J1860/1 J1863/1 J1864/1

Country of origin Current location Title (dating) Painter Code/no. of samples

Table 1 List of studied artworks.

D. Hradil, et al.

light (365 and 470 nm) using Colibri 2 fluorescence module. 2.1.3. Electron microscopy and microanalysis (SEM-EDS) Scanning electron microscope (SEM) JEOL JSM6510 with detectors of back-scattered electrons (BSE) and secondary electrons (SE), coupled with INCA-EDS detecting unit (energy-dispersive X-ray spectroscopy) for microanalysis, was used to describe the elemental composition of individual layers and grains. Light elements performance technology (LEAPC) allowed detection of the elements heavier than Be (Z > 4) at spectral resolution of 125 eV. Measurements were carried out in low vacuum mode, which allowed analysis of the samples without conductive coating of their surface. Coating of artworks' samples is not a recommended procedure, because it requires subsequent re-polishing of the surface, which is not a fully non-destructive process. It increases the risk of losing information in the case of very rare, small and extremely heterogeneous samples of paints. Standardless quantification using ZAF correction (Genesis Spectrum SEM Quant ZAF, version 3.60) was applied to calculate the elemental composition; the typical counting time was 60 s. Simultaneously, some of samples were measured also on scanning electron microscope ZEISS “EVO 25”, coupled with EDS probe Oxford X-MAX 80, 80 mm2, under following conditions: accelerating voltage: 20 KV, beam current: 350 pA, vacuum: 5 ∙ 10–6 KP, secondary electrons (SE) and back scattered electrons detectors (QBSD), graphite coating (thickness ≈ 200 nm). AZTEC® 2.0 software by Oxford was used for data interpretation. 2.1.4. Powder X-ray micro-diffraction (micro-pXRD) Micro-diffraction patterns of painting microsamples (either fragments or their cross-sections) were collected as described elsewhere (Švarcová et al., 2010; Hradil et al., 2016) using a PANalytical X'Pert PRO diffractometer. A CoKα tube with point focus, an X-ray mono-capillary with diameter of 0.1 mm in the primary beam path, and a multichannel detector X'Celerator with an anti-scatter shield in the diffracted beam path were used. A sample holder was adapted by adding z-(vertical) axis adjustment (Huber 1005 goniometric head). Xray patterns were measured in the range of 4 to 80° 2Θ with a step of 0.0334° and 2200 s counting per step. Anti-scatter slit (2.5 mm) and Fe beta-filter were used in the diffracted beam. The duration of the scan was ca. 12 h. Qualitative analysis was performed with the HighScorePlus software package (PANalytical, The Netherlands, version 4.7.0) and powder diffraction file (PDF) database provided by the International Centre for Diffraction Data (ICDD) of the Joint Committee on Powder Diffraction Standards (JCPDS PDF-4 database, 2018). Clay minerals were interpreted according to Moore and Reynolds (1997). Quantification of the experimental data was performed using the Rietveld method (Rietveld, 1969). As the clay minerals exhibit a wide range of disorders (stacking faults in the layer structure), the BGMN software was used for all calculations (Bergmann et al., 1998). This program includes a code which permits the use of structural models correctly describing the disorder models (Ufer et al., 2008). Structural models were described as standard Rietveld models. (ICSD, 2018). 2.1.5. Fourier transform infrared micro-spectroscopy (micro-FTIR) Infrared spectra of individual layers in microsamples were measured in attenuated total reflection (ATR) mode with Ge ATR crystal for contact measurement. The Hyperion 3000 infrared microscope coupled with VERTEX spectrometer (Bruker Optics, Germany) and equipped with MCT Wide-Band detector was used, which enabled to cover the MIR spectral region of 4000–450 cm−1. Spectra were collected in the resolution of 4 cm−1 and with a typical number of scans 64. Subsequently, they were analysed using Opus 7.8 software (Bruker Optics, Germany). 2.1.6. Micro-palaentological analysis Considering the scarcity of the material, an innovative approach of 3

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

sample preparation has been applied to mechanically separated portions of artworks' microsamples. Solid-liquid separation was performed by evaporation and not by decantation in order to avoid any loss of material. Approximately 1 cubic millimetre of the ground material was carefully mixed with 7% H2O2 and 99% isopropanol for the chemical dissolution of aged organic binders in a 5 ml eppendorf. The solution was then heated up to 70 °C for protein denaturation and liquefaction of specific binders until the solution was half evaporated. 10 s in ultrasonic bath was added to the procedure for micro-destruction of interstitial cement. The suspension was dripped and dried on two ultra-thin cover plate (24 × 50 mm) and mounted permanently with venetianer balsam. The slides were analysed through optical microscope (Zeiss Axio Imager A.2) with 960× (non-oil immersion) and 1000× magnification (oil immersion objective) in parallel and crossed nichols. Determination of nannoplankton and stratigraphical range was based on the data from Young et al. (2018). Further, scanning electron microscope (SEM) FEI Nova NanoSEM 450 equipped with CBS detector was employed for detailed visual observation of micro- and nannofossils in cross-sections and also in untreated mechanically separated material. 2.2. Analysis of rock samples 2.2.1. Selection of rock samples Two samples have been collected at the outcrop in St. Thomas Bay, Malta (GPS coordinates 35°50′59.6”N 14°33′55.8″E) - the first represented white-pinkish Globigerina Limestone (STB-lime) and the second the red earth material from the up to 1 m thick layer, which was sandwiched between two limestone layers (STB-earth, Fig. 2) and which represented an intermediate layer with prevailing sedimentation of clastic material. This selection was based on the assumption that in the past, the red earths were mined out for the painting purposes either (i) separately or (ii) they remained as waste from mining of limestones as for building stones.

Fig. 2. Natural outcrop of Middle Globigerina limestones at St. Thomas Bay, Malta, with indication of sampling locations in lower limestone (STB-lime) and upper red earth (STB-earth) layers, and with information about the average thickness of the red earth layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2.2.2. Powder X-ray diffraction Diffraction patterns of side loaded rock samples were collected with a PANalytical X'PertPRO diffractometer equipped with a conventional X-ray tube (CoKα radiation, 40 kV, 30 mA, line focus) and an X'Celerator detector with an anti-scatter shield. (Hradil et al., 2016) Xray patterns were measured in the range of 4–95°2Θ with a step of 0.0167° and 1050 s counting per step. Conventional Bragg Brentano geometry was used with the following parameters: 0.02 rad Soller slit, 0.25° divergence slit, 0.5° anti scatter slit, and 15 mm mask in the incident beam, 5.0 mm anti-scatter slit, 0.02 rad Soller slit and Fe betafilter in the diffracted beam. The duration of the scan: ca. 13 h.

ground could be in both cases an attempt to modify the background colour and change the overall tone of the painting. While Mattia Preti has changed the original brown to rusty red, Caravaggio has darkened the original red by admixing a dark pigment to the upper layer (Fig. 3). Each individual painting thus represents a different story, and must always be addressed separately. Despite the visual similarity of the ground layers (colour, morphology), the elemental composition indicated clear differences between the paintings created in Malta and in Italy. The composition of all the grounds of M. Preti's paintings, which are now located in Dubrovnik, but which were created in Italy during his Roman (before 1651) or Neapolitan (between 1653 and 1660) periods (Lalli et al., 2014), is remarkably similar to clays used by other Italian Caravaggists in the same period (Table 2, Fig. 4). As previously published (Hradil et al., 2018), they represent pottery clays, which were also used for terracotta sculpture in Italy. Several possible locations of their extraction was already suggested (as, e.g., Sassuolo district in Emilia Romagna). As can be seen from Fig. 4a, there are no discernible differences between the lower and upper layers of the ground, which means that the same material was used in both cases. Therefore, the difference in the colour (Fig. 3) corresponds solely to the additional colouring of the upper layer by clearly visible red Fe-containing grains. A detailed analysis of grains and heterogeneities in individual layers revealed also other natural and artificial admixtures (Table 2) – in both lower and upper layers, framboidal pyrite (Fe, S) was common (found in J1859 and J1860) and carbon black (C) was artificially admixed. In upper layers, locally increased contents of Fe and Ti indicated the presence of

2.2.3. Micro-palaeontological analysis Calcareous nannoplankton was studied using the light microscope technique. Smear slides from reference rock samples were prepared by a conventional decantation method described by Švábenická, 2002. 3. Results and discussion 3.1. Visual inspection and elemental composition Visual differences between the grounds can be the first indication of their different origins. At the same time, however, visual comparison may be confusing if there were any further modifications to the material - for example, the mixing or additional colouring. The samples analysed in this work can be divided into single-layer (brown ground in J1858, J1859 and red grounds in J1863 and J1864) and double-layer grounds (lower brown/grey + upper red in J1857 and J1860, and lower red + upper red/grey in J1866). Double-layer grounds were typical for Mattia Preti on his Italian paintings and for Caravaggio on his painting from Malta. The reason for applying the second layer of the 4

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

Fig. 3. Example of double-layer earthy grounds by (a) Caravaggio - J1866 and by (b) Mattia Preti – J1860; 1 – lower layer of the ground, 2 – upper layer of the ground, intentionally coloured (re-priming), 3 – oil-based paint.

and S correlate in all studied artworks; the difference is only in relative content of sulphates. Thus, it appears that the Maltese grounds contain two components - limestone (most represented in sample J1863) and a colouring component with a high content of Al sulphates (most represented in sample J1866). The difference between the painters is given only by different relative proportions of these two components.

Fe and Ti oxides (J1857, J1859). Also sulphates cannot be excluded due to locally increased content of S. As a very rare admixture barite (Ba, S) was indicated in one grain (J1860). Up to 4 at.% of Pb indicates an admixture of the lead white (basic lead carbonate), although larger grains are only rarely present in the layer (applies for both lower and upper layers). Compared to grounds created in Italy, the grounds from Malta are clearly different (Table 2, Fig. 4). In addition, there are differences between Maltese paintings by Preti and by Caravaggio. Both Preti's paintings are characterized by a high Ca content (above 30 or even 40 at.%), indicating the use of limestone/chalk as a pre-dominant component. On the contrary, the Ca content in Caravaggio's ground is even lower than in Italian pottery clays and, inevitably, in his previous paintings created in Italy. For his Maltese painting, Caravaggio used a very specific material, characterized in particular by a high proportion of Al (Si/Al ˂ 1), Fe and also S. The same material has been applied in both ground layers. A carbon-based black has been then admixed to the upper layer in a relatively high quantity in order to mute an intense red colour. Concerning admixtures, lead white is again present in all layers. Some content of P is typical for all Maltese grounds (around 1 at. % in average). Specifically, in the case of Carravaggio, dark bitumen grains (exhibiting a high sulphur content) were locally identified. His red ground is also characterized by some Mn (< 1 at.% in average), which is accompanying iron (Table 2). The question that arises from this preliminary screening is whether the Maltese Preti's and Caravaggio's grounds have any interrelation or if they represent completely different raw materials whose common feature is only the red colour. To solve this question, correlation of chemical elements has been performed. Probably the most interesting is the positive correlation of Al, S and K in all samples indicating clearly the presence of potassiumaluminium sulphates, as, e.g., alunite (KAl3(SO4)2(OH)6. If one compares the results of correlation with the Al/S ratio in the structure of alunite, a slight excess of Al can be observed in most of cases (Fig. 5). On the other hand, K/S ratio fits better the reference line. The causes of these variations can be both analytical and materials-based. One of analytical causes could be, for example, an underestimation of sulphur, especially at low concentrations and in the presence of Pb (which is admixed to the layer in the form of lead white) due to the overlap of their characteristic lines. However, this does not explain the excess of Al in all cases, especially in some heterogeneities. In addition to analytical constrains (underestimation of sulphur, or even overestimation of Al), the presence of other Al-rich phases has to be considered (particularly in points inside the ellipse in Fig. 5). On the other hand, an excess of S could indicate the presence of other sulphates, as, e.g., gypsum. K, Al

3.2. Mineralogical analysis 3.2.1. Paintings created in Italy Results of mineralogical analysis are summarized in Table 3. It is evident that mineralogical composition of all Preti's grounds prepared in Italy (J1857, J1859, J1860) is in line with the composition of reference Italian 17th century ground (S1855) and with previously published data for pottery clays (Hradil et al., 2018). Quartz, chlorite, calcite, dolomite, plagioclase and mica as the key components, and also characteristic impurities, such as, e.g., framboidal pyrite, have been documented. The micro-ATR FTIR method allowed further characterisation of the samples and confirmed the results obtained by micropXRD. In the spectra collected in the mid-IR region (4000–450 cm−1), characteristic OH and SieO bending and stretching vibrations of layered silicates were identified (Bishop et al., 2008). Interestingly, the spectra of Preti's grounds are strikingly similar with already published spectra of pottery clays in grounds of Carravaggists' paintings (Hradil et al., 2018; Fig. 6) - both the positions and intensities of the bands are highly alike. Position of Si-O-Si(Al) stretching band is in the range 1001–1005 cm−1. Al-Al-OH bending (at 914 cm−1) and stretching (at 3620 cm−1, not shown) vibrations occur in both spectra in similar intensities. Al-Fe-OH bending vibration is overlapped by the carbonate band at 875 cm−1. The presence of carbonates in samples is further characterized by a strong CO32– stretching vibration at 1413 cm−1 and by 712 cm−1 band resulting from in-plane and out-of-plane deformation vibrations of CO32– (Bruckman and Wriessnig, 2013). A very weak Al-Mg-OH bending vibration at 847 cm−1 can be observed in both spectra, which corresponds with higher content of Mg in chlorites. Bands at 798, 780 and 695 cm−1 refers to silica (e.g. Madejová, 2003 or Tinti et al., 2015). 3.2.2. Paintings created in Malta The ground of Preti's Maltese painting St. Peter blessing (J1863) consist mainly from calcite, which is accompanied by ankerite, gypsum, goethite and hematite (Table 3). Although, according to EDS analysis, the layer contains up to 33 at. % Si, no Si-containing crystalline phase has been identified. This may be caused by a presence of amorphous 5

wt% at.% wt% at.%

Mattia Preti/Malta J1863 2 analyses (avg.) J1864 4 analyses (avg.)

6 11.17 14.67 11.60 16.95 11.16 15.7 13.49 16.38 13.11 16.48 12.59 15.40 12.59 17.17 14.98

Unknown Caravaggist/Italy (Hradil et al., 2018) S1855 at.% 50.38

4.25 6.67 11.40 16.10

18.64 26.51 17.39 26.05

( ± 0.30)

Al

36.70 46.29 28.22 39.63 35.23 47.63 45.51 53.1 40.98 49.48 46.14 54.21 46.14 51.00

wt% at.% wt% at.% wt% at.% wt% at.% wt% at.% wt% at.% wt% at.%

14.55 19.89 13.26 19.09

wt% at.% wt% at.%

Caravaggio/Malta J1866 - lower 3 analyses (avg.) J1866 - upper 2 analyses (avg.)

Mattia Preti/Italy J1857- lower 3 analyses (avg.) J1857 - upper 3 analyses (avg.) J1858 3 analyses (avg.) J1859 - lower 6 analyses (avg.) J1859 - upper 2 analyses (avg.) J1860 - lower 3 analyses (avg.) J1860 - upper 4 analyses (avg.)

( ± 0.34)

Standard deviation (avg., wt%.)

21.97 33.12 20.43 27.71

Si

Element

1.59

0.00 n.d. 0.00 n.d. 1.31 2.17 1.01 1.44 1.02 1.5 0.86 1.23 0.86 1.37

0.01 0.01 0.00 n.d.

0.70 1.17 0.80 1.40

( ± 0.27)

Na

Table 2 Average composition of ground layers according to SEM-EDS data.

4.01

4.07 3.69 3.89 3.92 5.31 5.16 5.17 4.33 5.12 4.44 4.66 3.93 4.66 3.99

1.56 1.69 3.45 3.36

5.54 5.44 5.76 5.96

( ± 0.22)

K

0.41

0.55 0.41 0.73 0.6 0.71 0.56 0.70 0.48 0.44 0.31 0.68 0.47 0.68 0.58

0.01 0.01 0.00 n.d.

0.27 0.22 0.00 0.00

( ± 0.17)

Ti

4.01

4.69 6.84 4.25 6.9 2.78 4.34 3.03 4.09 3.09 4.31 3.31 4.50 3.31 3.78

1.12 1.95 0.99 1.55

0.01 0.01 0.62 1.03

( ± 0.20)

Mg

20.15

22.52 19.91 22.08 21.73 16.98 16.09 15.53 12.7 18.65 15.78 14.66 12.07 14.66 9.63

45.14 47.68 33.69 32.02

1.86 1.78 3.24 3.27

( ± 0.31)

Ca

3.72

9.93 6.3 8.27 5.84 5.96 4.05 11.04 6.48 9.35 5.68 10.73 6.34 10.73 8.25

5.49 4.16 15.91 10.85

46.00 31.62 40.49 29.31

( ± 0.38)

Fe

0.05

0.00 n.d. 0.43 0.53 0.00 n.d. 0.26 0.27 0.42 0.44 1.04 1.07 1.04 0.95

0.44 0.58 5.35 6.35

9.38 11.23 8.79 11.08

( ± 0.33)

S

n.d.

0.00 n.d. 0.00 n.d. 0.00 n.d. 0.00 n.d. 0.00 n.d. 0.00 n.d. 0.00 n.d.

0.00 n.d. 0.85 1.05

1.04 1.29 0.99 1.29

( ± 0.25)

P

n.d.

0.23 0.15 0.00 n.d. 0.00 n.d. 0.00 n.d. 0.00 n.d. 0.00 n.d. 0.00 n.d.

0.00 n.d. 0.00 n.d.

0.64 0.45 1.07 0.79

( ± 0.27)

Mn

0.61

10.12 1.73 20.54 3.91 19.92 3.65 4.17 0.66 7.46 1.22 5.40 0.86 5.40 1.39

19.97 4.08 8.49 1.56

1.19 0.22 8.51 1.66

( ± 0.70)

Pb

(Fe,S)

(Ba,S), (Fe,S)

(Fe,S)

(Ti), (Fe)

(Fe,S)

(Fe)

(Fe), (Pb)

(Fe), (Pb), (Fe,S,K)

(Fe,Mn)

(Fe,Mn), (S)

Grains (elements increased)

D. Hradil, et al.

Applied Clay Science 185 (2020) 105412

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

Fig. 5. Al/S (a) and K/S (b) ratios based on EDS measurements (at. %) of Maltese grounds by M. Preti (J1863 – grey circles, J1864 – black circles) and Caravaggio (empty circles); average analyses in the layer and analyses of individual grains are not differentiated; a high excess of Al over S and also Si (not shown) in points inside the ellipse could indicated the presence of Al-(hydro) oxides in some grains.

Fig. 4. Concentration of selected elements (at. %) in (a) M. Preti's grounds created in Italy (J1857, J1858, J1859 and J1860) and (b) M. Preti's (J1863 and J1864) and Caravaggio's (J1866) grounds created in Malta, in comparison with composition of a typical Italian ground of the 17th century, containing pottery clays, analysed on the painting by an unknown Caravaggist (black circles, adopted from Hradil et al., 2018).

John the Baptist (J1866) sample was measured only as an untreated fragment (and not as cross-section) clay minerals may have been completely overlooked. Another possibility why EDS and XRD data do not match at this point may be the presence of Al-containing amorphous phases. Calcite is almost completely missing in the Caravaggio's ground. The dominant phases are hematite and alunite only, gypsum, goethite, quartz, and possibly also jarosite are further present. Jarosite was indicated only locally due to the increased contents of Fe, K and S in some grains (Table 2); in the diffraction pattern, its determination is at the limit of detection and thus unreliable. Based on these results, it can be concluded that instead of clays, altered limestones were used in grounds of Maltese paintings by M. Preti, together with sulphates (gypsum + alunite), opal and some other silicates, hematite and also goethite. The highly variable ratio of characteristic minerals calcite (on one hand) and alunite (on the other hand) is interesting. While in J1863 calcite dominates and alunite is almost absent (only few grains were identified by EDS), both minerals are clearly represented in J1864. Alunite positively correlates with hematite. In the Maltese ground by Caravaggio, limestone is missing and only alunite-hematite/goethite component is present together with gypsum and some quartz. Due to the variability described above, there are two possible explanations - it could be either two raw materials available (limestone and alunitehematite), which were mixed together in variable ratios, or, alternatively, there was a single source where the altered limestones were in contact with sulphate-rich deposits. In any event, it is for the first time when the alunite-hematite material was identified as the main component in historical painting grounds.

SiO2 in the sample, along with materials heterogeneity. Unfortunately, measurement of another part of the layer was not possible; the sample was too small and the layer too thin. On the next Maltese painting, Our Lady of Carmel and souls from Purgatory (J1864), two measurements of the ground layer have been performed – on the untreated fragment and on the cross-section (Table 3). The results of these two measurements obtained are quite comparable, differences result from the sample heterogeneity. M. Preti used a similar ground as in the previous case, but the prevailing calcite is accompanied by higher amounts of hematite, and also alunite, which was indicated by a positive correlation of Al, S and K in EDS data (Chapter 3.1). The presence of opal was confirmed in the sample J1864, together with some quartz. Further, unspecified clay minerals together with an artificial addition of the lead white have been indicated. (Fig. 7, Table 3) The finding of clay minerals would be in accordance with the assumption based on EDS data that additional Al-containing phases must be present in the ground, not just sulphates. However, the possible presence of alumosilicates was indicated only when measuring the ground in the cross-section (which better corresponds to the EDS area), and not when measuring untreated fragment from the bottom. Differences resulting from materials heterogeneity, as well as from the impossibility to homogenize the sample prior to measurement, are typical for the analysis of painting microsamples. Particle size and crystallinity variations play more significant role if irradiated area is small – ca 0.15 mm in our case (Švarcová et al., 2010). Since the ground of Caravaggio's Maltese painting Beheading of Saint 7

8

22.5 ± 0.7 n.d. 26.1 ± 0.7 12.7 ± 0.8 n.d. n.d. n.d. 1.0 ± 0.3 n.d. 15.5 ± 1.2 17.5 ± 1.1 4.7 ± 1.0 n.d. n.d. n.d. n.d. n.d. n.d. 8.76 5.92

10.78 ± 0.8 n.d. 35.16 ± 0.6 17.49 ± 0.4 n.d. n.d. n.d. n.d. n.d. 5.12 ± 0.4 16.85 ± 0.7 12.38 ± 0.8 n.d. n.d. n.d. n.d. n.d. n.d. 12.82 6.80

Quartz SiO2 Opaline silica/cristoballite SiO2 Calcite CaCO3 Dolomite CaMg(CO3)2 Ankerite Ca(Fe,Mg,Mn)(CO3)2 Alunite KAl3[(OH)6|(SO4)2] Gypsum CaSO4·2H2O Hematite Fe2O3 Goethite FeO(OH) Clinochlore 1 M IIb (Mg,Fe,Al)6(Si,Al)4O10(OH)8 K-mica (illite) K1-xAl2.0(AlxSi4−x)O10(OH)2 Kaolinite Al2Si2O5(OH)4 Plagioclase (albite) NaAlSi3O8 K-feldspar (microcline) KAlSi3O8 Phase at d = 1.64 nmc Halite NaCl Hydrocerussite 2PbCO3 ·Pb(OH)2 Cerussite PbCO3 Weighted profile R-factor (Rwp) Expected R-factor (Rexp)

28.9 ± 0.8 n.d. 11.4 ± 0.5 2.5 ± 0.3 n.d. n.d. n.d. 2.7 ± 0.5 n.d. 24.2 ± 1.2 24.5 ± 1.0 n.d. 5.9 ± 0.9 n.d. n.d. n.d. n.d. n.d. 9.33 6.25

J1859/lower 31.3 ± 0.8 n.d. 20.2 ± 0.6 n.d. n.d. n.d. n.d. 0.5 ± 0.3 n.d. 12.8 ± 1.3 20.4 ± 0.9 14.9 ± 1.2 n.d. n.d. n.d. n.d. n.d. n.d. 9.33 5.98

J1860/lower n.d. n.d. 73.1 ± 0.8 n.d. 10.7 ± 0.5 n.d. 6.4 ± 0.5 2.3 ± 0.6 7.5 ± 0.6 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 10.10 6.99

J1863 3.0 ± 0.5 6.3 ± 1.1 57.7 ± 1.3 n.d. n.d. 12.3 ± 0.8 n.d. 12.8 ± 0.9 n.d. n.d. n.d. n.d. n.d. n.d. unclear n.d. 3.1 ± 0.2 4.9 ± 0.3 8.31 6.65

J1864 n.d. 8.6 ± 0.3 58.3 ± 0.9 n.d. n.d. 9.2 ± 0.4 n.d. 10.6 ± 0.4 n.d. n.d. 4.6 ± 1.1 n.d. n.d. n.d. 8.5 ± 1.3 n.d. 0.2 ± 0.1 n.d. 6.27 6.01

J1864/csb

5.7 ± 0.5 n.d. n.d. n.d. n.d. 24.6 ± 0.6 16.4 ± 0.8 33.7 ± 0.9 19.6 ± 0.9 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 6.25 4.77

J1866/lower

26.7 ± 0.2 n.d. 15.1 ± 0.2 n.d. n.d. 1.2 ± 0.2 0.5 ± 0.1 0.1 ± 0.1 n.d. n.d. 32.0 ± 0.5 10.2 ± 0.3 5.0 ± 0.2 6.0 ± 0.2 n.d. 3.3 ± 0.1 n.d. n.d. 4.09 4.55

STB-earth

3.1 ± 0.6 n.d. 90.3 ± 0.4 n.d. n.d. n.d. n.d. 0.2 ± 0.1 n.d. 0.8 ± 0.2 2.5 ± 0.4 1.2 ± 0.2 n.d. n.d. n.d. 1.9 ± 0.5 n.d. n.d. 5.00 6.90

STB-lime

n.d. not detected. a In the case of double-layer grounds the analytical information comes only from lower layers, because upper layers were not accessible for the method (if bottom parts of untreated fragments were only analysed) or they were not thick enough to get information non-affected by the surroudings. b Measuring of the same fragment, but from its cross-section. c Diffraction line at 1.64 nm could represent d(001) line of smectite shifted by interaction with an organic binder in the ground layer (as already experimentally proved by Hradil et al., 2016); nevertheless, the interpretation is not definitive.

J1857/lower

S1855

Code/Ground layer indicationa (applied for double-layer grounds)

Table 3 Mineralogical composition (wt%) of studied samples vs. reference samples – ground of 17th oil-on-canvas painting by an unknown Caravaggist containing pottery clays (S1855; Hradil et al., 2018), Globigerina limestone (STB-lime) and red earth (STB-earth) from St. Thomas Bay, Malta.

D. Hradil, et al.

Applied Clay Science 185 (2020) 105412

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

Fig. 6. Characteristic vibrations in the mid-IR spectra – fingerprint region; (a) a comparison of Italian M. Preti painting (J1859) with a painting by an unknown Caravaggist published in Hradil et al., 2018 (Ref.) and (b) a comparison of Maltese paintings by M. Preti (J1864) and by Caravaggio (J1866).

micro-palaeontological method has been used in last decades to specify the place of origin of the chalk used as for grounds of orthodox icons from the museum in Krakow (Kędzierski and Kruk, 2018) or to specify the geological age and possible source areas of pottery clay used as for grounds in Baroque Italy (Hradil et al., 2018). Unfortunately, the amount of disintegrated material and the number of fossils obtained from samples of Italian paintings by M. Preti were too small to determine for sure the origin of the material – both the Cretaceous and Oligocene-Eocene species were found. On the other hand, both grounds of M. Preti created in Malta contained a sufficient number of fossils – both the microfossils and nannofossils (Figs. 8, 9). Microfossils were usually poorly preserved, crashed or cracked and mainly recrystallized. Nannoplakton were scarce and corroded, dominated by Coccolithus pelagicus. The co-occurrence of Helicosphera ampliaperta (NN2-NN4; Young et al., 2018) and Sphenolithus calyculus (NP25-NN2) enable to relate this material with Middle Globigerina Limestone (Aquitanian-Burdigalian, Miocene, Fig. 10). Beside the biostratigraphical correlation also the lack of phosphorus corresponds to Middle Globigerina Member, in which phosphatic admixtures were not recorded in contrast to Lower and Upper Globigerina Member

In addition to micro-pXRD (Fig. 7), the presence of alunite can also be indicated by spectroscopic measurements. In infrared spectra, alunite is represented by the following bands: ν3(SO4)2− at approx. 1220 and 1180 cm−1 (overlap with SieO bonds), ν4(SO4)2− at 669 and 617 cm−1, δ(OH) at 1025 cm−1 and γ(OH) at 591 cm−1. Positions more likely correspond to K-alunite (Bishop and Murad, 2005) and they are identical for the grounds of both painters – Caravaggio and M. Preti. Other similarity can be seen in intensities and positions of vibrations in Si-O-Si stretching region (1075 and 1042 cm−1), SieO vibrations at 798 and 782 cm−1 (quartz) and in weak vibrations at 680 cm−1 (CO32– deformation of hydrocerussite – lead white) and 545 cm−1 (probably FeeO of hematite). Therefore, the only difference is the content of calcite in M. Preti's ground (J1864) represented by CO32– stretching vibration at 1410 cm−1, and by CO32– deformation vibrations at 872 and 712 cm−1 and also by an admixture of clay component represented by Al-Al-OH vibration at 914 cm−1. (Bishop et al., 2008) 3.3. Micropaleontological analysis Despite numerous limiting factors and micro-destructiveness, the 9

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

Fig. 7. Part of micro-diffraction patterns of Maltese grounds by M. Preti as measured on an untreated fragment (J1864) or its cross-section (J1864/cs), and Caravaggio (J1866); G – gypsum, A – alunite, H – hematite, Go – goethite, Q – quartz, O – opal containing both cristoballite and tridymite components, C – calcite, M – mica (e.g. illite), L – lead white (hydrocerussite and/or cerussite); minerals highlighted by circles are present in both paintings and one unknown phase is indicated by its diffraction line at d = 1.64 nm (for possible interpretation, see Table 3).

(Soldati et al., 2010).

3.4. Comparison with rock samples It is highly probable that Globigerina limestone (Lower Miocene, 23/ 14 Ma), which is a dominant component of M. Preti's grounds came from the local Maltese source. The weathered parts of these limestones are mostly light yellow, but also pinkish or reddish due to the presence of hematite beside goethite. In Malta, goethite and hematite are commonly found together in limonite-rich sediments or terra rossa formations developed on limestones, which are, however, more typical for Upper Coralline Limestone Formation (Upper Miocene, 10/6 Ma) than for Globigerina limestone. Red hematite-containing positions in Maltese Globigerina limestone are referred to as “Hard Grounds”, which contain not only hematite and goethite, but also phosphates, as, e.g. francolite (Soldati et al., 2010). Since there is no evidence from which parts of the Middle Globigerina limestone formation the material was obtained in the past, a randomly selected position of red earths was used for a comparison. It is represented by the sample STB-earth (St. Thomas Bay profile, Fig. 2). It is conceivable that precisely such positions, mined together with the surrounding limestone, could be suitable for making colours in the past. However, results of pXRD analysis showed important differences between reference STB-earth sample and ground layers of Preti's and Carravaggio's paintings created in Malta (Table 3). Analysis of the underlying limestone (STB-lime, Table 2) shows that in addition to dominant calcite, the same phases as in the red layer (quartz, kaolinite, illite, hematite) are present in very small amounts. Therefore, the only difference is the proportion of clastic and chemical sediments within the same strata. Both sulphate minerals - gypsum and alunite - are present, but in low amounts only. Pure limestones of the Middle Globigerina Member (here represented by the sample STB-lime) contain abundant and well preserved nannoplankton. Besides taxa which occur in painting grounds (Coccolithus pelagicus, Cyclicargolithus floridanus, Helicosphaera ampliaperta, oligomiocene reticulofenestras), there are also discoasters. In red earth layer (STB-earth) the nannoplankton is rather scarce (2–3 coccoliths/field of microscope view instead 30–50 in pure limestone), which is similar to the abundance in painting grounds and which simply corresponds with lower content of carbonates. Assemblages are very similar to those found in the painting grounds - coccoliths are corroded and low-diversified containing Coccolithus pelagicus, Cyclicargolithus floridanus and Helicosphaera ampliaperta.

Fig. 8. Globigerinids (sample J1864) observed with optical microscope under UV365 nm light; two large globular tests with 3 and 4 visible chambers slightly embracing in a spiral shape - the chambers are rapidly increasing in size.

Fig. 9. Reticulofenestra ex gr. minuta Roth, 1970 (sample J1864); observed with scanning electron microscope under CBS detector - very small (< 5 μm) and strongly recrystallized coccolith broadly elliptical, large elevated central-area closed by a plate from imbricated arms, a longitudinal canal is visible; the rim is made of anti-clockwise overlapping elements.

10

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

Fig. 10. Chronostratigraphy of the calcite-rich ground material used by M. Preti on his paintings created in Malta, based on the identification of micro- and nannofossils.

Perhaps a more acceptable interpretation may be that alunite-hematite material was imported by shipping trade routes from other location in the Mediterranean and available on the Malta's market. Then it was intentionally admixed within the ground preparation in order to improve its colour. It could come directly from the area of economically significant exploitation of alunite. Since Middle Ages, alunite was used to produce alum, which subsequently served as a mordant in a textile dyeing process and was also use in papermaking, pharmacy and other fields (Derry and Williams, 1961). In Italy, alunite has been known since 1400 and its extraction was concentrated to regions of South Tuscany and Latium (on the axis Siena – Rome), although they were also other places, as, e.g., in Sicily. Probably the best known is (still active) Tolfa/Allumiere deposit north of Rome. (Lombardi, 1977) Transporting the Italian alunite to Malta would be logical, as the textile industry on this island has a very long tradition – it existed already before the third century B.C. (Busuttil, 1966). However, there is no written evidence that alunite from these deposits was also used for painting and the other question is the content of hematite. Raw alunite from Tolfa is very light, with a low iron content. There are other numerous mineralogical occurrences of alunite along with hematite in Italy (for example in sulphuric acid caves, D'Angeli et al., 2018), but it cannot be assumed that these were economically significant in the studied period. In summary, the origin of the hematite-alunite component in Maltese grounds, whether used separately (by Caravaggio) or mixed with a local limestone (by Preti), cannot be determined. Since this material does not appear in North-Italian paintings (Hradil et al., 2015), but only in Maltese paintings, it had to be either extracted on Malta or imported by the sea trade routes in the Mediterranean (despite the

3.5. Discussion of the provenance While the regional provenance of limestone is obvious, the origin of the hematite-rich component still remains unexplained. An important guideline in this case is the association of hematite and alunite, which appears in one of the grounds of M. Preti (J1864, with limestone) and especially in the Caravaggio's ground (J1866, almost without limestone). There are other evidences of the presence of alunite in Fe-based reds in Italian paintings, but in all these cases, alunite appears as an accidental admixture and not as a main component. According to nonpublished reports archived in the Academy of Fine Arts in Prague – laboratory ALMA alunite was found e.g. in three 18th century paintings attributed to Appolonio Domenichini (Venice, 1715 - c.1770), both in paint layers and also in the ground (in one of them only). This is probably related to the fact that alunite is a very abundant admixture in weathering crust on volcanic rocks in many places in Italy. However, in none of these cases, its average concentration in the layer reach 1 at.% it is in contrast with 25 at.% of alunite in the case of Caravaggio (Table 3). Although alunite is only rarely described in the geology of Malta, there are some references. Soldati et al. (2010) states that the most common sulphate on Malta – gypsum is usually ascribed to an arid phase, during which opal, gypsum and alunite would have formed. All these three phases have been found in Maltese grounds in various proportions and further, some alunite has been also identified in the red earth sample from the St. Thomas Bay, Malta (STB-earth). Although it is not possible to prove the Maltese origin of alunite on the basis of these data, it cannot be excluded either. 11

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

References

locality of its origin). However, these links must be examined within a broader comparative research in the future.

ICSD database FIZ Karlsruhe, 2018. Germany Release 2017/2. Alfeld, M., Broekaert, J.A.C., 2013. Mobile depth profiling and sub-surface imaging techniques for historical paintings – a review. Spectrochim. Acta Part B 88, 211–230. Bergmann, J., Friedel, P., Kleeberg, R., 1998. BGMN — a new fundamental parameters based Rietveld program for laboratory X-ray sources, its use in quantitative analysis and structure investigations, CPD (Commission of Powder Diffraction, International Union of Crystallography). Newsletters 20, 5–8. Bishop, J.L., Murad, E., 2005. The visible and infrared spectral properties of jarosite and alunite. Am. Mineral. 90, 1100–1107. Bishop, J.L., Lane, M.D., Dyar, M.D., Brown, A.J., 2008. Reflectance and emission spectroscopy study of four groups of phyllosilicates: smectites, kaolinite, serpentines, chlorites and micas. Clay Miner. 43, 35–54. Bruckman, V.J., Wriessnig, K., 2013. Improved soil carbonate determination by FTIR and X-ray analysis. Environ. Chem. Lett. 11 (1), 65–70. Busuttil, J., 1966. The Maltese textile industry in Antiquity. Melita historica. J. Malta Hist. Soc. 4, 215–219. Calleja, G., 1881. The Works of Art in the Churches of Malta and the Governor's Palace. Maltese Press L'Immacolata. Internet Archive, Valletta. https://archive.org/details/ worksartinchurc00callgoog/page/n4. Ciatti, M., Lalli, C., 2013. Caravaggio's the beheading of Saint John the Baptist in Malta. In: Ciatti, M., Brunetti, B.G. (Eds.), Caravaggio's Painting Technique – Proceedings of the CHARISMA Workshop. Nardini Editore, Florence, Italy, pp. 11–23. D'Angeli, I.M., Carbone, C., Nagostinis, M., Parise, M., Vattano, M., Madonia, G., De Waele, J., 2018. New insights on secondary minerals from Italian sulfuric acid caves. Int. J. Speleol. 47 (3), 271–291. Derry, T.K., Williams, T.I., 1961. A Short History of Technology from the Earliest Times to A.D. 1900. Oxford University Press, New York. Falcucci, C., 2013. 2013. Practices and processes, from the Odescalci Conversion of Saint Paul to the Adoration of the Sheperds. In: Ciatti, M., Brunetti, B.G. (Eds.), Caravaggio's Painting Technique – Proceedings of the CHARISMA Workshop. Nardini Editore, Florence, Italy, pp. 31–40. Hradil, D., Hradilová, J., Bezdička, P., Švarcová, S., 2015. Differentiation between anonymous paintings of the 17th and the early 18th century by composition of claybased grounds. Appl. Clay Sci. 118, 8–20. Hradil, D., Bezdička, P., Hradilová, J., Vašutová, V., 2016. Microanalysis of clay-based pigments in paintings by XRD techniques. Microchem. J. 125, 10–20. Hradil, D., Hradilová, J., Holcová, K., Bezdička, P., 2018. The use of pottery clay for canvas priming in Italian Baroque – an example of technology transfer. Appl. Clay Sci. 165, 135–147. JCPDS PDF-4 database, 2018. International Centre for Diffraction Data, Newtown Square, PA, USA. Kędzierski, M., Kruk, M.P., 2018. Similarity and provenance of underpainting chalk grounds based on their nannofossil assemblages cluster analysis. J. Cult. Herit. 34, 13–22. Keith, L., 1998. Three paintings by Caravaggio. Nat. Gallery Tech. Bull. 19, 37–51. Lalli, C.G., von Breska-Ficović, N., Innocenti, F., Kolić Pustić, M., 2014. Le due serie dei quattro "Evangelisti" di Mattia Preti a Dubrovnik: un progetto multidisciplinare e di collaborazione tra il Laboratorio Scientifico dell'Opificio e l'Istituto Croato di Restauro. In: OPD Restauro n° 26. vol. 2014. CentroDi, Firenze, pp. 231–248. Legrand, S., Riccardi, P., Nodari, L., Janssens, K., 2018. Non-invasive analysis of a 15th century illuminated manuscript fragment: point-based vs imaging spectroscopy. Microchem. J. 138, 162–172. Lombardi, G., 1977. Alunite in Italy. Occurrence and Genesis. In: Galan, E. (Ed.), Proceedings of the 8th kaolin symposium and meeting on alunite, Madrid-Rome, September 7–16, 1977, A-3, 19 pp. Madejová, J., 2003. FTIR techniques in clay mineral studies. Vib. Spectrosc. 31, 1–10. Moore, D.M., Reynolds, R.C., 1997. X-Ray Diffraction and the Identification and Analysis of Clay Minerals. Oxford University Press, UK. Napolillo, V., 2013. Mattia Preti – Artefice del Seicento. Edizioni Orizzonti Meridionali, Cosenza, Italy. Pelosi, C., Agresti, G., Baraldi, P., 2017. Analysis of three paintings by Mattia Preti from Wignacourt college museum, Rabat (Malta). In: Rusu, I., Nechita, M.T., Drăgoi, E.N., Apostolescu, N. (Eds.), 9th European Symposium on Religious Art, Restoration & Conservation – Proceedings Book, Kermes, Torino, pp. 22–25. Pustić, M.K. (Ed.), 2009. The four Evangelists in the Œuvre of Mattia Preti. Hrvatski Restauratorski Zavod – Restauratorski Odjel Dubrovnik 2009 (ISBN: 978-953-738908-6). Rietveld, H.M., 1969. A profile refinement method for nuclear and magnetic structures. J. Appl. Crystallogr. 2, 65–71. Rodler, A.S., Artioli, G., Klein, S., Petschick, R., Fink-Jensen, P., Brons, C., 2017. Provenancing ancient pigments: Lead isotope analyses of the copper compound of egyptian blue pigments from ancient mediterranean artefacts. J. Archaeol. Sci. Rep. 16 (1–18). Soldati, M., Barbieri, M., Biolchi, S., Buldrini, F., Devoto, E., Forte, E., Furlani, S., Gualtieri, A., Lugli, S., Mantovani, M., Mocnik, A., Padovani, V., Pasuto, A., Piacentini, D., Prampolini, M., Remitti, F., Schembri, J., Tonelli, C., Vescogni, A., 2010. A.Multidisciplinary Geological Excursion in the Open-Air Laboratory of the Island of Malta. 11–18 November 2010. Field-Trip Guide. Dipartimento di Scienze della Terra, Università degli Studi di Modena e Reggio Emilia (pp 41). Stevenson, R.K., Moffatt, E.A., Corbeil, M.-C., Poirier, A., 2016. Pb and Sr Isotopes and the Provenance of the Painting Materials of Cornelius Krieghoff in 19th-Century Canada. Archaeometry 58, 673–687. Stols-Witlox, M., 2012. Grounds 1400–1900. In: Hill Stoner, J., Rushfield, R. (Eds.),

3.6. Notes to the painting technique The last interpretation relates to the painting and technological context. From the above documented data it can be reliably proven that both authors - M. Preti and Caravaggio - meet both the influences mentioned in the introduction - the availability of materials (economic factor) and their own intention (artistic factor) in the preparation of their grounds. When looking again at double-layered grounds presented in Fig. 3 one can try (at least speculatively) to understand the thinking of painters who intended to work with the colour of the ground, but they were limited by the availability of materials in the place where they were working. During his time in Italy, Caravaggio used grey and later brown grounds represented by calcareous pottery clays (like most other Italian painters at that time). Their colour was never deep red. In Malta, however, he had available a deep red hematite-alunite material for the ground. In order to dampen the red and gain colour similar to the one he was using before, he added dark carbonaceous material to the top layer. Mattia Preti tells a completely opposite story, because he preferred the red colour of the ground. Therefore, he added hematite to the top layer of the ground to revive the dull colour of pottery clays (available for grounds in Italy), or probably also added hematite-alunite component to intensify the red colour of Globigerina limestones (available for grounds in Malta). 4. Conclusions It has been sufficiently proved that the composition of the grounds of Italian painters of the 17th century was related to the place where the work originated and was not just a painter's intention. Although the source materials were subsequently treated and additionally coloured, various raw materials were clearly distinguished. Therefore, the composition of grounds seems to be an efficient tool to differentiate the works of one particular painter according to the place of their origin. It is evident that Matia Pretti painted on Italian pottery clays in Italy, while in Malta he painted on altered Globigerina limestone of a local origin. In one of his latest works “Beheading of St. John the Baptist”, Caravaggio used a completely different material for the ground than in his previous works - with a dominant content of alunite and hematite. It is the first time ever that such material has been identified in painting grounds. Alunite-hematite material is also associated with the place of painting's origin (Malta), because in painting grounds created in Italy it was never identified. Mattia Preti used the same material to intensify the red colour of his limestone-based grounds. Nevertheless, the source locality of alunite remains unclear. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to thank their colleagues from ALMA Laboratory, particularly Silvia Garrappa and Lenka Rydlová for their versatile assistance and performing micro-ATR FTIR measurements. The study has been supported by Czech Science Foundation (project no. 17-25687S). Access to OPD data has been made possible through the ARCHLAB Programme within the FP7 EU project CHARISMA 20092014 (grant number 228330). 12

Applied Clay Science 185 (2020) 105412

D. Hradil, et al.

manuscripts and paintings: a review. J. Adv. Res. 17, 31–42. Ufer, K., Kleeberg, R., Bergmann, J., Curtius, H., Dohrmann, R., 2008. Refining real structure parameters of disordered layer structures within the Rietveld method. Z. Krist. (Suppl. 27), 151–158. Vanmeert, F., Keyser, N., van Loon, A., Klaassen, L., Noble, P., Janssens, K., 2019. Transmission and reflection mode Macroscopic X-ray Powder Diffraction Imaging for the noninvasive visualization of paint degradation in still life paintings by Jan Davidsz. de Heem. Anal. Chem. 91 (11), 7153–7161. Weil, P.D., 2007. Technical art history and archaeometry II. An exploration of Caravaggio's painting techniques. Revista Brasileira de Arqueometria. Restauração e Conservação 1 (3), 106–110. Young, J.R., Bown, P.R., Lees, J.A. (Eds.), 2018. Nannotax3 Website. International Nannoplankton Association 17 Apr. 2018. URL: http://ina.tmsoc.org/Nannotax3.

Conservation of Easel Paintings. Routledge, Oxon, Great Britain, pp. 889. Švábenická, L., 2002. Calcareous nannofossils of the Upper Karpatian and lower Badenian deposits in the Carpathian Foredeep, Moravia (Czech Republic). Geol. Carpath. 53 (3), 197–210. Švarcová, S., Kočí, E., Bezdička, P., Hradil, D., Hradilová, J., 2010. Evaluation of laboratory powder X-ray micro-diffraction for applications in the field of cultural heritage and forensic science. Anal. Bioanal. Chem. 398, 1061–1076. Tinti, A., Tugnoli, V., Bonora, S., Francioso, O., 2015. Recent applications of vibrational mid-infrared (IR) spectroscopy for studying soil components: a review. J. Cent. Eur. Agric. 16, 1–22. Tonazzini, A., Salerno, E., Abdel-Salam, Z.A., Harith, M.A., Marras, L., Botto, A., Campanella, B., Legnaioli, S., Pagnotta, S., Poggialini, F., Palleschi, V., 2019. Analytical and mathematical methods for revealing hidden details in ancient

13