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Applied Clay Science 165 (2018) 135–147
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Applied Clay Science
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Research paper
The use of pottery clay for canvas priming in Italian Baroque – An example
of technology transfer
David Hradila,b, , Janka Hradilováb, Katarína Holcováb,c, Petr Bezdičkaa
Institute of Inorganic Chemistry of the Czech Academy of Sciences, 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
Department of Geology and Paleontology, Faculty of Science, Charles University Prague, Albertov 6, 128 43 Prague 2, Czech Republic
Italian Baroque paintings
Clay-based grounds
Powder X-ray micro-diffraction
In the Baroque European painting technology, various coloured clays had been used to prime canvases. These
clays are generally considered to be carefully selected in terms of colour and other technological properties
(adhesiveness, ductility etc.), as the painting represented the most delicate field of fine art. However, it seems
that the availability of the material at a given place as well as its price often played a much more significant role
than previously thought. It led to the usage of highly heterogeneous cheap pottery clays in painting, even though
they often had to be additionally coloured. For the first time, a clear evidence is provided that a very similar
pottery clay material was applied in three different technological ways: i) as a clay body of an unfired terracotta
statue created in Florence or Bologna at the end of 16th century, ii) as a secondary putty on the Renaissance
painting by Antonello da Saliba (1466–1535), and iii) as a preparation layer – ground – of an oil-on-canvas
paintings attributed to Italian Caravaggists (17th century) or also to Carlo Maratta or workshop (1655–1713).
The identity of the material was confirmed by mineralogical analyses as well as description of nannofossils,
which enable to date the clay to Eocene – Oligocene.
1. Introduction
In the traditional European art, two main types of priming layers
(grounds) appear on easel paintings: white gypsum- or chalk-based and
colour clay-based ones (Hradil et al., 2015; Stols-Witlox, 2012). From
Byzantium to Gothic, the white/grey grounds prevailed; while in the
Central Europe, their usage persisted up to the second half of the 16th
century and the early 17th century, concurrently, the artists in Italian
environment had already started to use the clay-based (earthy) grounds
typical for Baroque. (Duval, 1994; Bergeon and Martin, 1994; Roy,
1999) Based on their chemical and mineralogical composition, the
Italian clay-based grounds differ significantly from those used in Central Europe (Hradil et al., 2015), which is clearly related to the availability of various regionally important types of clays. This regional
specificity of Baroque grounds makes them important for provenance
analysis of anonymous works of art and may be conveniently applied in
the interdisciplinary fields of research, such as, e.g., technical art history.
Provenance analysis of the fine art is a complex discipline, combining artistic, historical and materials/technological issues. It includes
the authorship and/or workshop attributions, but also period and
geographical relationships. The composition of clay-based grounds
corresponds to the regional provenance of the painting, because coloured clays were cheaply available in many places and therefore, it did
not make sense to transport them for a longer distance. It is also necessary to take into account that in Baroque, the painters usually
bought the already primed canvases in the place where they were
working. Therefore, the grounds refer rather to the place of a painting
creation, and not to the painter. It also means that according to
grounds, we can distinguish the paintings of one particular painter
created in various locations during his life. To trace the regional provenance the description of sufficiently conclusive signs that allow a safe
identification of the material within comparative studies is particularly
important. These “fingerprints” can be found in variability of, e.g.,
crystal structures of minerals, admixtures, trace elements, isotopes, and
also microfossils. Especially the micropalaeontological analysis in claybased artistic materials remains almost completely neglected by researchers in the cultural heritage field, although it can provide accurate
information about the geological age of the material used and significantly reduce the number of potential source localities.
Clay-based (earthy) pigments are very common in nature – they are
most often products of silicate rocks weathering (red/yellow earths as
Corresponding author.
E-mail address: (D. Hradil).
Received 28 May 2018; Received in revised form 8 August 2018; Accepted 9 August 2018
0169-1317/ © 2018 Published by Elsevier B.V.
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
well as white earths – kaolins), or, eventually, products of hydrothermal/volcanic alterations (green earths – celadonites) or diagenesis
(glauconites). Ferric ochres and reds cover also materials from ore deposits oxidation zones, but these usually contain only a small proportion of alumosilicates (clays). In particular, earthy pigments from Italy
were well-known to painters and sold all over the world. Italy was not
only a major centre for trade in and processing of the pigments themselves, but became, by the fourteenth and fifteenth centuries, a centre
for the production of artists' manuals and collections of recipes for
paints, dyes and inks. (Friedman and Figg, 2000)
Probably the most famous ones are “Terra di Siena” (poorly crystallized brown-yellow goethite from lacustrine sediment of Monte
Amiata near Siena, southern Tuscany) (Manasse and Mellini, 2006;
Manasse and Viti, 2007) and “Italian green earth” (celadonite from the
Monte Baldo area near Verona, Veneto) (Grissom, 1986). There are
several other areas in Italy known for the production of earthy pigments
– for example Lessinni mountains in Veneto (exploited since Prehistory,
Cavallo et al., 2016), Piedmont (Cavallo and Gianoli, 2015) and the
southern part of Sardinia. The Sardinian earths are still available on the
European market of artistic pigments (see, e.g. product No. 40490,
Rosso Sartorius, offered by Kremer-Pigmente, Ltd.). However, the
variety of earthy pigments is vast and it seems to be difficult to match
the pigment with its source. The only way to distinguish, for example,
Italian and Cypriot type of the green earth, is to determine the geological age of celadonite using the K/Ar method – however, it cannot be
done non-destructively and with insufficient amounts of a pure material. The matter is further complicated by the fact that it was probably
not economically feasible to use high-quality pigments for grounds as
they were usually thicker than the paint layers – more material was
needed to cover the entire surface of the canvas. Moreover, as documented in 2015 by Hradil et al., 2015., a frequently used Italian pale
brown ground (designated as “D type” by the authors) has been often
intentionally coloured by various pigments – therefore, the starting clay
material was probably not a pigment at all.
The priming of canvases with clays first started in Italy; there must
have been some source of inspiration as well as available material for
such experiment, which eventually resulted in an extensive change in
the European painting technology. One reasonable alternative to earth
pigments (however never analytically confirmed) were the cheap and
widely available pottery clays adapted for an experimental application
in painting. The tradition of clay pottery and sculpture (all included
under the Italian term “terracotta”) is very old and date back to the
Etruscans – the ancient inhabitants of the Apennine peninsula (de
Thomson Grummond and Simon, 2006). The revival of terracotta and a
renewed popularity of clay sculptures took place in Italian Renaissance
(15th to 16th century), when a new artistic centre was formed in
Tuscany. The materials of clay bodies of terracotta sculptures (either
fired or unfired) represent a largely unstudied topic. For the period of
interest, i.e., from the 15th to the 18th century, mineralogical data in
the scientific literature are sporadic.
The research of terracotta polychrome sculptures from the 16th
(and the 17th) century usually focuses only on the polychrome layers
due to conservation issues (Colombo et al., 2011a; Pelosi et al., 2017).
Other authors studied also the clay body, but the information is incomplete, which is related also to their methodological approach. If the
sculptures or taken samples are studied only by portable X-ray fluorescence (e.g., Križnar et al., 2009 or Colombo et al., 2011b) or, in addition, by spectroscopic methods (e.g., Amadori et al., 2013 or Colombo
et al., 2011b), nothing can be deduced about the source of the clay
materials. Potentially very promising is the information provided by
Zucchiatti et al. (2003), who analysed clay bodies of a group of Italian
Renaissance glazed terracotta angels by ICP-MS and a total of 53 chemical elements including the trace elements were identified. Unfortunately, only ten major components are listed in the results.
Nevertheless, it can be clearly seen that the clay bodies were calcareous
with 20–25 wt% CaO content, which is in agreement with other
sources, for example the contribution by Hykin et al., 2007, at the interim meeting of the ICOM-CC Working Group in 2007. The authors
mentioned that light-coloured calcareous clay bodies were used for
glazed terracottas throughout the Italian Renaissance (see also Tite,
1991; Olson and Barbour, 2001; Bouquillon et al., 2004). This information is particularly important as the “D-type” grounds of Italian
Baroque paintings described by Hradil et al. (2015) are characterised by
a high Ca-content. It represents just an indication and the credible
methodology to prove the identity of the employed materials has yet to
be developed. The main difficulty is the limited availability of the
material from paintings and sculptures and, therefore, the necessity for
either a completely non-invasive, or micro-analytical approach applied
on heterogeneous samples that are typically smaller than 1 mm.
In this paper, X-ray micro-diffraction analysis, Fourier-transformed
infrared micro-spectroscopy and detailed clay structure description are
combined with micropalaeontology (applied on the carbonate component) in order to compare clays used in the grounds of the 17th century
Italian paintings with the clay body of unfired terracotta sculpture from
Tuscany (dated to the end of the 16th century) and to discuss their
possible source in nature. Due to the scarcity of the samples, microdestructive approaches are very frequently limited. Therefore, an additional aim of this work is to show that in some cases, a tiny amount of
consumed material is sufficient to obtain results of fundamental importance for provenance determination and that micro-destructive
analysis (often completely forbidden) can be used in a targeted and
statistically significant way. The new here-suggested approaches may
be used in the future for a wider comparative analysis of clay and
carbonate-based materials in the fine art.
2. Materials and methods
2.1. Studied artworks
Five paintings of the Italian Baroque art were selected for the purpose of this research. Their description is given in Table 1. Two of them
(M1010 and S1855) were recently attributed to “Italian Caravaggists”
(followers of Caravaggio) – “An old woman with coins” (M1010) and
“David with Goliath's head” (S1855). On the next canvas (J1709) the
Old Testament scene of Rebecca and Eliezer at the Well (Gn 24, 1–49) is
depicted (Fig. 1). The historical and artistic comparison of this newly
discovered painting (J1709) with an identical composition in the Indianapolis Museum of Art in the U.S.A. have put it in association with
the work of a 17th century Roman painter Carlo Maratta (1655–1713)
or his workshop (Hradilová et al., 2017). The Italian provenance of two
other paintings (J1601 and J1633) is not as obvious as in the abovementioned cases; it is formulated only vaguely by art-historians. The
ground layers of all these paintings were first characterised based on
their chemical and mineralogical composition. In order to provide a
detailed micropalaentological examination, two of them (M1010 and
J1709) were then selected for a micro-destructive procedure. Within a
comparative study, the same procedure was applied to examine the clay
body of an unfired terracotta statuette dated to the end of the 16th
century and attributed to a Florentine or Bolognese master (ClayS1)
(Fig. 1), and also the secondary clay filling of damaged parts of the
Italian Renaissance painting by Antonello da Saliba (J1536). (Table 1)
This putty was included to the research because, according to stratigraphy of layers, it certainly represents a material from a non-original
intervention, which is visually very similar to the material of Italian
Baroque grounds. It was therefore very likely that comparative research
would make it possible to specify this intervention not only chronologically, but also regionally.
2.2. Sample preparation and light microscopy
The micro-samples were first observed by stereoscope Leica S8 APO
Stereozoom. Subsequently, they were embedded in Polylite 32,032–20
Applied Clay Science 165 (2018) 135–147
SEM-EDS, pXRD, micro-FTIR, micropalaeontology
and/or Neukadur PE 45 polyester resin and, after hardening, processed
into polished 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. Olympus BX
60 light microscope equipped with Olympus DP 74 digital camera and/
or Zeiss Axio Imager A.2 light microscope with the Olympus DP 73
digital camera were 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 light (365 and 470 nm) using
Colibri 2 fluorescence module.
2.3. Electron microscopy and microanalysis
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.
Further, scanning electron microscope (SEM) TESCAN VEGA 3 XMU
was used for a documentation of microfossils and framboidal pyrites
under following conditions: accelerating voltage 25 kV, working distance 12–15 mm, BSE mode, carbon coating (if feasible).
2.4. Laboratory powder X-ray diffraction (pXRD) and micro-diffraction
Micro-samples from the terracotta statuette (ClayS1) and clay-based
putty (J1536) were mixed in an agate mortar to form a suspension with
cyclohexane. This suspension was then placed on top of a silicon zero
background sample holder. After evaporation of solvent, thin layer of as
prepared sample was analysed by a PANalytical X'Pert PRO diffractometer equipped with a conventional X-ray tube (CoKα radiation,
40 kV, 30 mA, line focus) and a multichannel detector X'Celerator with
an anti-scatter shield. X-ray patterns were measured in the range of
4–95° 2 Θ 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 beta-filter in the diffracted beam. The duration of the scan: ca. 13 h.
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
end of 16th century
SEM-EDS, pXRD, micro-FTIR, micropalaeontology
16th century
SEM-EDS, micro-pXRD, micro-FTIR
17th century
SEM-EDS, micro-pXRD, micro-FTIR
17th century
17th century
17th century
17th century
Oil-on-canvas painting by Carlo Maratta or workshop (1625–1713): Rebecca and Eliezer at the Well, Bratislava Castle, SK/
Italian provenance; analysis of the ground layer
Oil-on-canvas painting by an unknown painter (Caravaggist) “An old woman with coins”, private collection, CZ/Italian
provenance; analysis of the ground layer
Oil-on-canvas painting by an unknown painter (Caravaggist) “David with Goliath's head”, Slovak National Gallery Bratislava,
SK/Italian provenance; analysis of the ground layer
Oil-on-canvas painting by an unknown painter “Portrait of Anna of Tyrol”, National Museum in Prague, CZ/probably Italian
provenance; analysis of the ground layer
Oil-on-canvas painting attributed to “Jan Černoch” “Mercury playing his flute to Argus”, Archbishopric Olomouc, CZ/probably
Italian provenance (during the painter's stay in Italy); analysis of the ground layer
Madonna with Child, tempera on wood, Moravian Gallery in Brno, CZ/Antonello da Saliba (1466 (?) – 1535); analysis of
secondary filling (first conservation treatment)
Polychrome terracotta sculpture of Madonna, Moravian Gallery in Brno, CZ/Florence or Bologna, Northern Italy; analysis of
unfired clay body
SEM-EDS, micro-pXRD, micro-FTIR,
SEM-EDS, micro-pXRD, micro-FTIR,
SEM-EDS, micro-pXRD, micro-FTIR
Date of creation
Artwork, place/attribution
Table 1
List of analysed artworks, collected samples and methods applied.
Number of samples
Methods applied
D. Hradil et al.
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Fig. 1. Comparison of the composition of the clay
material used for priming of Italian 17th century
paintings (as, e.g., the painting by Carlo Maratta or
workshop (“Rebecca and Eliezer at the Well,
Bratislava Castle, Slovakia – on the left, photo M.
Deko) and for the creation of clay statues in northern
Italy at the same time (as, e.g. unfired terracotta
statuette of Madonna from Florence or Bologna,
dated to the end of the 16th century, Moravian
Gallery in Brno, Czech Republic – on the right, photo
I. Fogaš) is the main purpose of this paper.
material, the following methodology has been used: approximately 1
cubic millimetre of ground material was carefully separated and mixed
with distilled water and pure ethanol (98%) in various ratios in order to
disaggregate clay particles and dissolve the organic binder (mostly
proteinaceous); ca 0.5 ml of suspension has been obtained. The disintegrated material was then dripped onto an ultra-thin cover slide
(24 × 50 mm) and dried. Two permanent slides from each sample were
prepared from 0.5 ml of suspension using Canada Balsam as a mounting
medium. The slides were analysed by optic microscope, magnification
1000× (oil-immersion objective) in both parallel and crossed nicols.
Determination of nannoplakton and stratigraphical ranges of individual
taxa were based on the data from Young et al. (2018).
on Powder Diffraction Standards (JCPDS, 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). In
order to specify the structure of chlorite using BGMN software general
formula presented by Weiss and Kužvart (2005) – (Mg6-x-y,Fex,Aly,□z)
(Si2.62 Al1.38)O10(OH)8 and a structural model by Rule and Bailey
(1987) have been adopted.
3. Results and discussion
2.5. Fourier-transformed infrared micro-spectroscopy (micro FTIR)
3.1. Classification by major elements
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).
The first step in the clays' characterisation was their classification by
major elements using SEM-EDS. The most significant results of the
analyses are summarised in Table 2.
Firstly, we have applied the procedure described by Hradil et al.
(2015) for preliminary differentiation of Italian and Central-European
clay-based grounds based on characteristic K/Ti and Al/Mg ratios and
relative iron content. All studied paintings fell within the same group –
denoted as type D (North Italian). Fig.2a shows that not only the
paintings, but also the secondary filling used in restored parts of the
Italian Renaissance painting from the 17th century (J1536) as well as
the clay body of the sculpture of Madonna created at the end of the 16th
century in Florence or Bologna (ClayS1) belong to the same category.
This represents a first sign indicating the similarity of clay materials in
Italian Baroque grounds with sculpture clays used in the 17th century.
2.6. Micropalaeontological research
Micropaleontological analysis was applied on mechanically separated portions of microsamples. Calcareous nannoplankton was studied
using the light microscope technique. Considering the scarcity of the
Table 2
Characteristic elemental ratios and admixtures (averaged values).
Natural heterogeneities (grains)
Intentional admixtures
Fe, S (pyrite), Ti oxide of FeeTi (such as, e.g. ilmenite), Ba, S (barite), Zr (zircon)
Fe, S (pyrite)
Fe, S (pyrite), Ti oxide, FeeMn oxide
Fe, S (pyrite)
Fe, S (pyrite)
Fe, S (pyrite), FeeMn oxide
Ti oxide, FeeMn oxide
C (carbon black)
C (carbon black), Pb (lead white)
C (carbon black)
Sn (cassiterite), Pb (lead white, minium), C (carbon black)
C (carbon black), Ca,P (bone black), Pb (minium)
Cu (azurite), Pb, Sn, Sb (mixed yellow)
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Fig. 2. Characteristic elemental ratios in clay-based grounds
of Italian Baroque paintings (open circles; 0 – J1709, 1 –
S1855, 2 – M1010, 3 – J1633, 4 – J1601), in the secondary
putty and in the clay of unfired terracotta sculptures (full
circles; 0 – J1536, 1 – ClayS1, 2 – Matteo Civitali's sculpture,
about 1480, adopted from Chloros et al., 2010, 3 – Benedetto
da Maiano sculpture, about 1480, adopted from Chloros et al.,
2010, 4 – Giovanni de Fondulis sculpture, 1483–1487,
adopted from Chloros et al., 2010, 5 – sculptures attributed to
the della Robbia family, adopted from Zucchiatti et al., 2003,
6 – sculpture attributed to Andrea della Robbia, adopted from
Beillard et al., 2001), and in natural clays from reference
areas (rectangles; R1 – Sassuolo district, Ranzano Fm.-marly
parts, adopted from Dondi, 1999, R2 – Sassuolo district,
Ranzano Fm.-low Ca parts, adopted from Dondi, 1999, M –
Tuscan Nape - Lago Trasimeno, Macigno Fm., adopted from
Amendola et al., 2016).
influenced by the pigments, but they are all light-coloured (from grey to
beige/light red-brown) and their internal textures look highly alike
with frequent presence of framboidal pyrite. (Fig. 4) Mineralogical inhomogeneities of the clays are represented only by locally increased
abundance of Ca and/or Mg carbonates or Fe (+Mn) oxides (Fig. 3).
This is in line with the fact that the average measurements showed
varying amounts of Mg and Fe, while the concentrations of potassium
remained stable. (Fig. 2b) This enabled the division of the data into
three tentative groups (shown by dotted lines in the ternary plots).
(Fig. 2) The largest group (group 1, average Mg and Fe) encompasses
Unfortunately, published scientific data regarding terracotta are scarce,
nevertheless, similarities may be observed even with 15th century
terracotta artworks, when this field of art rose to prominence (e.g.,
works of Andrea della Robbia). (Fig. 2).
In the next step, the focus was placed on the micro-morphology,
inhomogeneities and presence of characteristic admixtures (Figs. 3, 4).
The SEM-EDS analyses showed that clays used in grounds were intentionally coloured with a variety of pigments such azurite, lead white
or bone black (of course this does not occur in the clay statuette,
ClayS1) (Table 2). The resulting colour of the studied materials was
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Fig. 3. Oil-on-canvas painting by an unknown Italian
Caravaggist “An old woman with coins” (left) and
micro-section of the sample M1010–2 in visible light
(VIS), UV luminiscence (UV 470 nm) and in backscattered electrons (SEM); description of layer stratigraphy 1 – clay-based ground containing carbonates, clay minerals and other inhomogeneities locally enriched by Fe and Mn oxides, framboidal
pyrite and intentionally added Pb and C-based pigments, 2 – proteinaceous isolation, 3 – paint layer
with lead white, bone black, earth pigments, cinnabar and organic lake, 4 - varnish.
Hradil et al., 2016), micro-pXRD is, so far, the most suitable method for
non-destructive phase microanalysis of clays. pXRD and micro-pXRD
data showed that a characteristic feature of all the studied samples is
the presence of quartz together with chlorite, clay mica (possibly a
combination of illite and muscovite), calcite and dolomite. (Fig. 5) In
most of samples a significant amount of feldspathic phases (typically
albite) was also identified. In some cases, kaolinite, ankerite, Mg-calcite, gypsum or smectite also appear. As can be seen from the picture,
micro-diffraction pattern of an untreated fragment exhibit broader
diffraction lines than the measurements performed in a conventional
Bragg-Brentano arrangement with using carefully separated microvolumes of clays placed on silicon support. It si due the fact that monocapillary produces a quasi-parallel beam (Švarcová et al., 2010).
However, analytical information is the same. The most interesting mineralogical characteristic of all studied samples is the presence of
chlorite. Although it is a mineral commonly occurring in clay sediments, it is not usually found in ground layers of painted artworks.
Consequently, it was decided to try to provide a more detailed description of its structure which is closely related to the clay's origin.
The presence of chlorites was detected based on diffraction lines
positioned at d = 1.42 (001), 0.72 (002), 0.47 (003) and 0.35 nm (004).
In 6 samples, any presence of expandable layers was excluded, because
no shift of the basal lines was observed. These shifts are usually caused
Carravaggists' paintings (M1010, S1855), painting by C. Marrata
(J1709) and one by an unknown painter (J1633), clay statuette from
the end of the 16th century (ClayS1) and several Renaissance terracotta
statues attributed to A. della Robbia (data adopted from the literature).
Slightly outside this main group, there is the painting by J. Černoch
(J1601) together with the secondary putty from the Renaissance
painting by Saliba (J1536) (group 2, increased Mg), and on the other
side is group 3 (increased Fe) consisting of several other Renaissance
terracotta sculptures. Such division is complicated by the fact that it can
only hardly be assumed that the sources of clays were the same for the
entire 200 years, and with respect to the heterogeneity of the starting
materials, it is not possible to expect complete consistency in the main
elements. Nevertheless, it is worthwhile to address the question whether – at least in some cases closely related by chemical composition –
the identity of the material can be ascertained by other specific microanalytical procedures.
3.2. Mineralogical analysis
Subsequently, all samples were studied by pXRD (ClayS1 and
J1536) and micro-pXRD (other samples). Their mineralogical composition is summarised in Table 3.
Despite all the pitfalls described elsewhere (Švarcová et al., 2010;
Fig. 4. Details of framboidal pyrites in selected samples exhibiting similarities and differences in their morphologies.
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Table 3
Mineralogical composition of the studied samples.
Mineral phase
Quartz SiO2
Plagioclase (albite) NaAlSi3O8
Chlorite (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
Smectite group mineral
Calcite/Mg-calcite CaCO3
Dolomite CaMg(CO3)2
Ankerite Ca(Fe,Mg,Mn)(CO3)2
Goethite FeO(OH)
Anatase (TiO2)
Gypsum CaSO4 · 2H2O
by interactions of proteinaceous organic binders (present in all studied
ground layers) with expandable clay structures (Hradil et al., 2016).
Such shift was found only in sample S1855, where smectite-protein
intercalate was formed and indicated by a broad diffraction line at
1.71 nm (while the diffraction lines of chlorite remained untouched). In
all of the studied samples, chlorite was identified as clinochlore of 1M
IIb-2 polytype with relatively large (but variable) amount of iron in its
structure. Using the simplified structural model according to Rule and
Bailey (1987), the calculations of Mg/Fe ratios (BGMN software) were
summarised in Table 4.
As the procedure of the micro-pXRD measurement was “non-standard” (shielding in low angles on uneven surfaces, heterogeneity of the
materials and thus lack of representativeness of the studied microsamples), it was not possible to perform quantitative mineralogical
analysis. However, for comparison purposes, the values may be expressed in the form of proportional numbers. In this respect, an interesting point is to evaluate the ratio of clay/clastic (quartz + chlorite +
mica + kaolinite) and carbonate (calcite + dolomite + ankerite)
component in the materials. It varies in a wide range from 8.10
(ClayS1) to 0.78 (J1709). The lowest carbonate content in the clay body
of the terracotta statuette (ClayS1) may either indicate that it was a
desirable property when selecting the clay and that it was not important
Table 4
Mg/Fe ratios in the chlorite structure calculated from pXRD and micro-pXRD
Code of the artwork
Clinochlore formula
in the case of paintings' ground layers or, eventually, that carbonates
from other sources (chalk, limestone) were artificially added to ground
layers in order to modify their properties. The same may apply to the
gypsum content. Therefore, we tried to pinpoint whether the clay and
carbonate (and/or sulphate) components are of the same or different
origin. An indication of the joint origin chlorite and dolomite was
provided by a comparison of the ratio of calcite to dolomite (representation of the Mg content in carbonates) and the Mg/Fe ratio in
chlorite structure (Fig. 6). The result is an inverse relationship, which
Fig. 5. Part of diffraction patterns showing mineralogical similarity of clays used in three different technological ways – as a clay body of the 17th century sculpture
(ClayS1), as a secondary putty on the Renaissance painting (J1536) and as a ground of 17th century oil-on-canvas painting (J1633); Ch – chlorite, K – kaolinite, G –
gypsum, I – illite, F – feldspar (plagioclase), Q – quartz, C – calcite, D – dolomite, A – ankerite, T – anatase; BB - Bragg-Brentano arrangement, MIC - micro-pXRD
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Fig. 6. Dolomite/calcite ratio vs. Mg/Fe ratio in the chlorite structure, as calculated from the pXRD and micro-pXRD data.
putty from the Renaissance painting by Saliba (J1536), which is again
in agreement with SEM-EDS analysis (group 2). (Figs. 2 and 7) The only
exception is represented by the sample spectrum J1633 (unknown
painter), which slightly differs from all the others.
Otherwise the micro-ATR FTIR analyses confirmed the results obtained by micro-pXRD. Positions of Si-O-Si(Al) stretching bands vary in
the range from 976 cm−1 (J1633) to 1008 cm−1 (S1855), which could
be related with varying contents of chlorite and micas (Bishop et al.,
2008; Vaculíková and Plevová, 2005). Al-Al-OH bending (at 914 cm−1)
and stretching (at 3620 cm−1) vibrations occur in all spectra in similar
intensities. Al-Fe-OH bending vibration is overlapped by the carbonate
band at 875 cm−1. An estimation of carbonate content was made on the
basis of a weak band at around 2510 cm−1, assigned to
νs(CO32−) + νas(CO32−) combination band (Legodi et al., 2001), which
is the weakest (practically invisible) in the ClayS1 sample (terracotta
statuette). The presence of carbonates in samples is further characterised by a strong CO32– stretching vibration between 1430 and
may reflect their formation or transformation in the same sedimentary
basin: when large amounts of dolomite crystallise, Mg content in the
chlorite structure decreases; at low dolomite content, the amount of Mg
in chlorite increases.
The micro-ATR FTIR method allowed further characterisation
of the samples. 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. The most significant ones
are summarised in Table 5. Interestingly, the spectra of Carravaggists'
paintings (M1010, S1855) and the painting by C. Marrata (J1709) are
strikingly similar. (Fig. 7) As both the positions and intensities of the
bands are highly alike, it probably indicates the same material as well
as the same way of preparation. The spectrum of terracotta statuette
from the end of the 16th century (ClayS1) looks very similar, but is not
fully identical. They all belong to group 1 delineated already by major
elemental composition. Another group of highly similar FTIR spectra
consists of the painting by Jan Cernoch (J1601) and of the secondary
Table 5
Mid-IR spectra interpretation (selected characteristic vibrations).
Code of the artwork
Al-Al-OH stretching
νs(CO32−) + νas(CO32−)
C=O stretching (amide I)
CO3 stretching
Si-O-Si(Al) stretching
Al-Al-OH bending
914(w), 935(vw)
Al-Fe-OH bending and/or CO32–
Al-Mg-OH bending
Si-O-Al in plane (micas)
828(w) 752(vw)
827(w) 754(w)
827(w) 754(w)
Al(Fe)-Mg-OH and/or
SieO (silica)
Si-O (silica)
CO32– deformation
Si-O (silica)
Mg-Fe-OH and/or FeeO out-of-plane
Si-O-Si(Al,Fe) tetr.
829 (vw) 752
728 (vw), 712(w)
474 + 465(s)
M–O–Si bending
505, 498, 650,
470 + 460(s),
506, 498, 650, 670
467(s), 454,
506, 498,
650, 670
471 + 461(s), 455,
501, 670
469(s), 454,
506, 650,
728(w), 713(vw)
472 + 465(s),
518 + 528(w)
650, 670
728(w), 713(vw)
474 + 465(s),
518 + 528(w)
507, 500, 650, 670
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
2017) The calcareous clay-based grounds have never been palaeontologically examined. Specifying a material in a non-destructive manner
(e.g., by electron microscopy or micro-CT) is not possible, as the larger
fossils are frequently broken (e.g., by friction of the material during its
initial preparation) and nannofossils are scarce. Therefore, the only
possible way is the material sampling. Due to the micro-destructiveness
of the procedure, it was not possible to analyse all samples. From
“group 1”, samples J1709, M1010, ClayS1 and from “group 2” sample
J1536 were selected.
Calcareous nannoplankton in separated micro-volumes of the material was rare to very rare, abundance varied from 2 to 3 coccoliths/
visual field of microscope (sample J1709), 0.5–1 coccolith/visual field
of microscope (samples ClayS1 and J1536) to < 0.01 coccolith/visual
field of microscope (sample M1010). Preservation of nannoplankton
was good, it was only rarely broken, however, in samples ClayS1, J1536
and M1010 dissolution-resistant taxa prevailed (Coccolithus pelagicus,
large Reticulofenstra spp., Watzenauria spp., massive Sphenolithus spp.).
Assemblages contained 7–14 species with exception of sample M1010
with only 3 species. This low diversity is probably caused by scarcity of
the material and/or dissolution of fragile species (Fig. 8). The small
Reticulofenestra minuta abundantly occurred in sample J1709; in this
case the dissolution is not expected. Also the diversity in this sample
was the highest (14 species).
The youngest biostratigraphical markers recorded in samples
ClayS1, J1536 and J1709 (Reticulofenestra bisecta, R. stavenensis, C.
abisectus, Sphenolithus dissimilis) indicate Oligocene, eventually late
Eocene – Oligocene age (sample J1536; Fig. 9). This stratigraphical
range certainly can be correlated with geological age of the clay material used in all these samples, although in the sample M1010 the
number of index fossils is not statistically significant. In addition, the
lower-middle Eocene Discoaster barbadoensis recorded in the sample
J1709 was with high probability reworked to the Oligocene rock.
Cretaceous taxa occur in all samples; while older Hauterive-Valanginian
markers were found in sample J1536, in other samples late Cretaceous
species are present (the Coniacian-Maastrichtian). The difference of the
paleontological record in the J1536 sample is another factor besides the
different IR spectra (Fig. 7) and the contents of the main elements
(Fig. 2) that justify the inclusion of this sample to the “group 2”. It may
indicate the difference in the mining area or period of mining.
Nannoplankton corresponds to several biostratigraphical levels (late
Eocene-Oligocene and Cretaceous) co-occurring in all samples. In general, it may be caused by either (i) natural reworking of older nannoplankton to younger rocks, which is a common phenomenon in geology,
or (ii) technological mixing of two materials, which is common phenomenon in painting technology. In this respect is perhaps the most
important fact that the materials of different ages are also found in the
sample ClayS1, where no technological admixtures of pure carbonates
into the clay body of the statuette can be reasonably assumed. Although
the late Cretaceous nannoplankton dominated by Arkhangelskiella spp.
(samples J1709, ClayS1, M1010) might indicate an admixture of the
Cretaceous chalk (particularly the Rügen chalk) which was commonly
used in ground layers from the Gothic period (Švábenická, 1994;
Švábenická et al., 2017), it has to be excluded also for other – historical
reasons. In the Gothic period, this chalk was used in Northern and
Central Europe only, not in the regions south of Alps. In Italy, gypsum
(gesso) was used instead of chalk, and thus it is not logical to expect the
chalk to be imported later in Baroque only for the purpose of mixing
with local clays. Therefore, the preferred hypothesis is that Cretaceous
carbonates represent a naturally reworked material in the late EoceneOligocene sediment.
Fig. 7. Characteristic vibrations in the mid-IR spectra– fingerprint region.
1500 cm−1. Relative amount of dolomite to calcite can be expressed by
the position ratio of the 727 cm−1 (dolomite) and 712 cm−1 band
(calcite) resulting from in-plane and out-of-plane deformation vibrations of CO32– (Bruckman and Wriessnig, 2013). The highest intensities
of 727 cm−1 vibration are in samples J1601 and J1536. On the other
hand, when the dolomite content is low, a very weak Al-Mg-OH
bending vibration at 847 cm−1 can be observed (in samples M1010,
S1855 and J1709), which corresponds with higher content of Mg in
chlorites. Bands at 825 + 750 cm−1 refers to Si-O-Al in plane vibrations
in micas (Farmer, 1968; Vaculíková and Plevová, 2005), bands at 798,
780 and 695 cm−1 to silica (e.g. Madejová, 2003 or Tinti et al., 2015).
With the exception of ClayS1 sample (terracotta statuette), the presence
of proteinaceous binder is indicated in all samples by a weak C]O
stretching vibration of amides (amide I) at 1645 cm−1.
3.3. Micropaleontological analysis
Considering a significant proportion of carbonates in the samples
and numerous microfossils observed during SEM analysis (Foraminifera
sp.), a detailed micropalaeontological research was carried out in order
to determine the geological age of the materials. Micropalaeontological
analyses of painting samples were performed only sporadically and they
dealt only with pure white calcium carbonates (chalks) used for
grounds in Gothic panel paintings (Švábenická, 1994; Švábenická et al.,
3.4. Discussion of possible sources
The following criteria were used to determine the origin of the
material: (i) occurrence of outcrops of fine clastics (claystones) of the
late Eocene-Oligocene age, (ii) marine origin of these rocks and
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Fig. 8. Gallery of nannoplacton species observed in normal (17, 24, 36b) and cross-polarized light (1–9, 11–16, 18–23, 25–35, 36a) or in back-scattered electrons SEM (10); 1 -Reticulofenestra stavensis Levin & Joerger 1967, Varol 1989 (sample J1709), 2 -Cyclicargolithus abisectus Muller 1970, Wise 1973 (sample J1709), 3, 4
and 11 -Reticulofenestra bisecta Hay, Mohler and Wade 1966, Roth 1970 (sample ClayS1), 5, 6 and 7 -Reticulofenestra minuta Roth 1970 (sample J1709), 8
-Reticulofenestra haqii Backman 1978 (sample J1709), 9 -Reticulofenestra minuta Roth 1970 (sample ClayS1), 10 -Reticulofenestra sp. – bisecta group (sample S1855), 12
-Reticulofenestra bisecta Hay, Mohler and Wade 1966, Roth 1970 (sample J1536), 13 -Helicosphaera obliqua Bramlette & Wilcoxon 1967 (sample J1709), 14
-Sphenolithus cf. dissimilis Bukry and Percival 1971 (sample ClayS1), a - 45° to polarizer, b - 0° to polarizer, 15 -Sphenolithus dissimilis Bukry and Percival 1971 (sample
ClayS1), a - 45° to polarizer, b - 0° to polarizer, 16 -Sphenolithus dissimilis Bukry and Percival 1971 (sample J1709), a - 45° to polarizer, b - 0° to polarizer, 17
-Discoaster deflandrei Bramlette & Riedel 1954 (sample J1709), 18 -Discoaster barbadiensis Tan 1927 (sample J1709), 19 -Coccolithus pelagicus Wallich 1877, Schiller
1930 (sample ClayS1), 20 -Reticulofenestra bisecta Hay, Mohler and Wade 1966, Roth 1970 (sample M1010), 21 -Reticulofenestra umbilicus Levin 1965, Martini &
Ritzkowski 1968 (sample J 1709, broken specimen), 22 -Reticulofenestra haqii Backman 1978 (sample M1010), 23 -Arkhangelskiella maastrichtiensis Burnett 1997
(sample M1010), 24 -Tubodiscus jurapelagicus Worsley 1971, Roth 1973 (sample J1536), 25 -Arkhangelskiella sp. (sample J1709, broken specimen), 26 -Micula
staurophora Gardet, 1955, Stradner 1963 (sample J1709), 27 -Micula staurophora Gardet, 1955, Stradner 1963 (sample ClayS1), 28 -Cegumentum stradneri Thierstein
in Roth & Thierstein 1972 (sample J1536), 29 and 34 -Watznaueria barnesiae Black in Black & Barnes 1959, Perch-Nielsen 1968 (sample J1536, 29 – broken
specimen), 30 and 36 -Quadrum octobrachium Varol 1992 (sample M1010), 31 -Assipetra sp. (sample J1536), 32 -Quadrum sp. (sample ClayS1), 35 -Quadrum sp.
(sample J 1536), 33 -Eprolithus moratus Stover 1966, Burnett 1998 (sample ClayS1).
potentially suitable region of Venetian Pre-Alps. Oligocene deposits in
this area are dominated by shallow-water carbonates and coarser
clastics (siltstones to sandstones), which are not suitable for pottery
material (Rasser et al., 2008). Tertiary clay shales (basin-plain turbidites) of Cormons (Venetia Julia) and Possagno Fm. (Veneto), which are
listed among major deposits of brick clays in Italy (Dondi et al., 1999),
occurrence of similar assemblages of calcareous nannoplankton, and
(iii) similar geochemical parameters of rocks. If we assume that materials for the grounds of Italian paintings came from local (Italian)
sources, we can exclude Balkanian, Pannonian, Eastern and Northern
European, Iberian, and French occurrences from the discussion about
possible origin of the material. In Italy, we can also exclude a
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Oligocene. It is a very clayey rock, greenish-grayish accompanied by
many volcanic products which is mainly found in the Teolo area (Monte
Berici), SE from Padova (Aurighi and Vittadello, 1999). Unfortunately,
no data about calcareous nannoplankton from this formation has been
Northern Apennines represent probably the most promising area of
origin of the Oligocene-Eocene fine clastics. There are several geological units which conform to the geological age and also mineralogical
and chemical composition. The Sassuolo clay district is suggested to be
the best candidate as a source area of the rocks, because their exploitation and use for ceramic tiles is widely documented (Dondi,
1999). It was found in the literature, that the production of bricks and
ceramic tiles in this region was known already in the 17th century
(Spreafico and Guaraldi, 2006) and the local clays were also used also
for sculptures at the Academies of Fine Arts in Modena, Reggio and
Carrarra (Di Brunnhoff and De'Brignoli, 1840). The Ranzano Fm. of
Epiligurian Unit is built by lower Oligocene politic turbidites. Their
chemical composition almost perfectly corresponds to the composition
of the samples studied here. (Fig. 2) The classic Ranzano shales are
characterised by the presence of calcite and dolomite, and by a significant quartz-feldspathic portion. The clay minerals are represented
by illite and chlorite associated with smaller quantities of kaolinite and
smectite. (Dondi, 1999). The lithology of the formation was studied by
Martelli et al. (1998), planktonic foraminifera by Mancini and Pirini
(2001). Calcareous nannoplankton assemblages from the Ranzano
Sandstones were biostratigraphically analysed by Catanzariti et al.
(1997). The common occurrence of large Reticulofenestra spp. described
in this work characterise also assemblages from our material. Moreover,
Discoaster barbadoensis and Cretaceous redeposits are mentioned from
field samples. On the other hand, sphenoliths from S. distensus-predistentus group as well as Helicosphaera recta common in Catanzariti's
material were not recorded in our samples. The differences may be
caused by scarcity of our material which did not provide whole assemblages, only their random representatives. Catanzariti et al. (1997)
described only biostratigraphical markers and not the whole assemblages, which also reduces the possibility of assemblage comparison.
Lithologically suitable rocks are exposed also in the Tuscan Nape.
Here, the Oligocene-Miocene foredeep turbidite formations are classically referred to Macigno Fm. (Chattian-Aquitanian). (Fig. 2) Phyllosilicates are the main mineralogical components, with prevailing micas
followed by chlorites. Other minerals are represented by quartz, feldspars (plagioclase and K-feldspar) and carbonates (calcite and dolomite). (Amendola et al., 2016) Nannoplankton assemblages are dominated by large reticulofenestras, Sphenolithus spp. and contain
Cretaceous and Eocene reworked nannoplankton. This composition is
very close especially to the sample which also does not content Coccolithus pelagicus (ClayS1). The nannoplankton assemblages have been
described near Lago Trasimeno (Barsella et al., 2009) but the Tuscan
Nape continues to NW. To the south (Lazzio, Abruzzo), these rocks are
disappearing (Cosentino et al., 2010). Facies Scaglio from the UmbriaMarche basin contains similar nannoplankton assemblages (Menichini,
1999; Coccioni et al., 2008), but lithologically (pelagic carbonate succession characterised by alternations of marly limestone, calcareous
marl, and marl beds of grey colour with some important intercalations
of biotite-rich layers) does not favour this area as suitable for pottery
4. Conclusions
Fig. 9. Chronostratigraphy of the clay material used as a clay body of the 17th
century sculpture (ClayS1), as a secondary putty on the Renaissance painting
(J1536) and as a priming material (ground) of the 17th century oil-on-canvas
paintings (J1709, M1010).
For the first time, it was proven that the calcareous clay, from which
the Italian terracotta statuette dated to the end of 16th century was
made, was also a material from which the Italian workshops were
preparing priming layers (grounds) on canvas paintings at the same
period of time. Here presented example clearly demonstrates the
technological transfer and new input into European painting, inspired
by the development of pottery and clay sculpture in Italy since the
do not meet the geological age, as they belong to early to middle Eocene. (Agnini et al., 2006 and Rasser et al., 2008) The only suitable rock
would be the “Marne Euganee” dated to Upper Eocene to the lower
Applied Clay Science 165 (2018) 135–147
D. Hradil et al.
Renaissance. Before this turning point, clays had never been used as
grounds for paintings.
In all studied samples, the material of the same lithotype (greybrown fine clastic rock with clinochlore IIb, micas, quartz, plagioclases,
dolomite, calcite and framboidal pyrite) and geological age (OligoceneEocene) was identified. The highest degree of chemical conformity was
achieved in grounds of the paintings by Italian Carravagists, the
painting by C. Marrata (or workshop) and the clay body of the terracotta statuette. Although the composition of the secondary putty used
to repair damaged parts in the Renaissance painting by A. da Saliba
differs slightly from the previous ones, it also represents a pottery clay.
Therefore, it is the third technological way of application of the same
The heterogeneity and variability in the composition may, of course,
be determined by the variability of natural sources. In Italy, the
Northern Apennines, in particular Ranzano Fm in Sassuolo district and
Macigno Fm at Tuscan Nape, appear to be the most promising possible
sources of Oligocene-Eocene fine clastics of the corresponding lithotype. A detailed micro-paleontological research, which accompanied
the mineralogical analysis of earthy materials in fine arts for the first
time, proved to be a very effective tool, even though it required sampling and consumption of a small amount of material from an artwork.
In the future, a combination of chemical, mineralogical and palaeontological analysis could become an integral part of wider comparative studies of Italian Baroque arts as well as reference clay materials from the source areas.
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The authors would like to thank Hana Kližanová (Slovak National
Museum Bratislava), Jana Želinská (The Monuments Board of the
Slovak Republic), Igor Fogaš and Marie Zmydlená (Moravian Gallery in
Brno) for providing an access to artworks and samples and for sharing
their expertise, and to our colleagues from ALMA Laboratory, particularly Zdeňka Čermáková, Kateřina Šídová and Silvia Garrappa, for their
versatile assistance in this research and performing particular investigations. The authors thank also Zuzana Korbelová (Institute of
Geology of the CAS) and restorer Kristýna Hájková for performing
useful macro- and micro-photographs. The study has been supported by
Czech Science Foundation (project no. 17-25687S) and by Institutional
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