GEOLOGICA CARPATHICA, 53, 2, BRATISLAVA, APRIL 2002
123 — 126
CLAY MINERALS IN PIGMENTS OF MEDIAEVAL
AND BAROQUE PAINTINGS
, JANKA HRADILOVÁ
and BARBORA HŘEBÍČKOVÁ
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic,
250 68 Řež, Czech Republic; firstname.lastname@example.org
Academy of Fine Arts in Prague, U Akademie 4, Prague 7, Czech Republic; email@example.com
(Manuscript received October 4, 2001; accepted in revised form December 13, 2001)
Abstract: Six different pieces of gothic wooden panels or sculptures and baroque canvas paintings were chosen to
identify the content, mineralogy and crystallinity of clay minerals present in pigments of original grounds and painting
layers. Electron microscopy and microanalysis, X-ray diffraction and electron diffraction have been used to investigate
minute heterogeneous material fragments. Reference commercial clay-rich pigments have also been measured. The
greyish basal layer of some chalk grounds of gothic paintings and sculptures is more siliceous than expected with only a
low content of clays. Baroque bole grounds are rich in clays, they contain predominantly kaolinite, mostly well-ordered;
illite is common, smectite was enriched especially in the ground of a Baroque painting by Ch. A. Coypel.
Key words: Gothic and Baroque paintings, bole grounds, ochres, clays, earthy pigments, chalk.
Within the art restoration process, material fragments are in-
vestigated in the laboratory with the aim to identify pigments
and binding media used in the original painting and to de-
scribe the stratigraphy of re-paints on cross-sections, which
is important for choosing suitable restoration procedures.
Materials research on heterogeneous fragments needs instru-
mental techniques to be used for their detailed description.
We have found optical and electron microscopic methods to
be the most suitable for the standard identification of inor-
ganic components in the painting layers regarding their mor-
phology, texture, colour, fluorescence in UV light and chemi-
cal composition as it is documented on the multiple of
research reports of the last years (e.g. Hradilová & Hradil
2001). Additionally, powder X-ray diffraction analysis
(XRD) is used if well-separated homogenous samples are
available in a sufficient amount. In the present study, we
have focused on the detailed classification of earthy pig-
ments (white, yellow, red, brown and green), known in the
painting technology as ferric ochres, siena, umbra, terra ver-
de etc. These pigments are found either in painting layers or
in clay-rich grounds. Red clay grounds, known as boles, rep-
resent one of the most typical features of Baroque painting
Scientific works referring explicitly and systematically to
the ground materials (and therefore also to clays) of mediae-
val and Baroque paintings have not been found in the litera-
ture. General information and classification criteria are given
only in the universal monographs concerning painting tech-
niques, for example, in Laurie (1967), Harley (1982) or, in
the Czech literature, Slánský (1953 and 1956). Natural clay
minerals as pigments and ochres are shortly dealt with in the
monograph of Konta (1995). The authors obviously derive
from historical sources, today accessible in reprints, for ex-
ample from Cennini (1946), Berger (1988) or Eastlake
(1960). Grounds, as coating materials used to prepare a sur-
face of panel or canvas for painting, can be subdivided ac-
cording to binding media such as glue, emulsion, oil grounds
or, evidently, according to the inorganic components as
chalk, gesso or bole grounds. If we neglect frescoes as paint-
ings traditionally built on ‘plaster grounds’, historically, the
application of grounds beneath tempera paints replaced the
ancient Greek encaustic technique (i.e. direct hot wax co-
lours application on the wood) in the period of subapostolic
icon art (Bentchev & Haustein-Bertch 1997). In medieval
Europe, grounds applied on wood panel surfaces were al-
ready very common.
Iron-rich pigments replace white chalk and gesso in the
grounds of painting of the 16
century and, generally, they
are predominant in Baroque works of the 17
ries in all European countries (Santos et al. 1998). In a nar-
row sense, the term ‘bole’ is related to the hygroscopic smec-
tite-rich material coloured by iron oxides (‘Armenian bole’)
and used especially as a ground for the gilding (Kužvart
1984). Generally, this term is used for any iron-rich ground,
typically red or orange-red, probably rich in hematite, but
commonly in casts ranging from yellow (goethite-rich yel-
low ochre pigments) to brownish, if containing some manga-
nese oxides (siena and umbra pigments). Earthy pigments
occur in nature (i) as residual laterites after intense weather-
ing of rock-forming silicates with silica removal or just after
oxidation of iron sulphides as mineral ochres in oxidation
zones, overburdens and tunnels in mining areas, or, (ii) dislo-
cated from the source area in sedimentary deposits of co-
loured clays. To distinguish among these materials, used in a
relatively long historical period and large territory, it is nec-
essary to describe clay minerals and iron oxides in detail;
their mineralogy reflects weathering conditions and should
be different case to case.
Within this study, powder X-ray diffraction (XRD), scan-
ning electron microscopy/microanalysis (SEM/EDX) and
124 HRADIL, HRADILOVÁ and HŘEBÍČKOVÁ
partially also transmission electron microscopy are used to
identify and compare clay minerals in real historical painting
fragments and in reference pigment and clay standards of
Materials and methods
Reference commercial pigments from different providers
are stored in collections of the School of Restoration of the
Academy of Fine Arts in Prague. Powders of earth pigments
from this source are mostly declared as natural analogues of
historical materials. As we have found earlier (Hradil et al.
1998), some of them do not meet this definition because of ar-
tificial admixtures, for example, of barite involving additional
tonality of the pigment. X-ray powder diffraction has been em-
ployed for mineralogical analyses of these samples using a SI-
EMENS D-5005 instrument under the following measurement
radiation, secondary monochromator, volt-
age 40 kV, current 30 mA, degree range 2
°, step 0.02°
per 8 seconds. The raw data were processed by the ZDS for
Windows program (Ondruš 1997) employing diffraction pat-
tern database (JCPDS 2000). Clay minerals were described
using conventional rules described, for example, in Moore &
Fragments of historical paintings have been collected and
treated in the following way: after wary sampling realized by
qualified restorers, fragments were pre-treated by casting them
into moulds of a polyester resin. Polished cross-sections were
then prepared on the Kompakt 1031 device to catch the com-
plete layer stratigraphy. Beside observations in the optical mi-
croscope Olympus BX-60, the scanning electron microscope
Philips XL30 CP was used in arrangement with Robinson de-
tector of back-scattered electrons and EDAX detector of X-
rays. If possible, a small part (up to 10 miligrams) of raw ma-
terial was separated, homogenized and measured by powder
X-ray diffraction method and, in some cases, also by transmis-
sion electron microscopy; transmission electron microscope
Philips EM 201 enables obtaining of electron diffraction pat-
terns of crystyalline solids.
Within this study, we have chosen six different pieces as ex-
amples – gothic wooden panels and sculptures by Master
Theodoricus (National Gallery in Prague) and unknown paint-
ers (Courtauld Museum of London), Baroque canvas paintings
by J. Hess (Church of Our Lady Victorious in Prague), Ch. A.
Coypel (Gallery of Fine Arts in Ostrava), M.V. Halbax (Na-
tional Gallery in Prague) and an unknown painter (Castle Sy-
Results and disscusion
On the basis of mineralogical composition, reference earth
ochres of today’s commercial production can easily be sorted
into groups referring to their origin as (i) French ochres with
dominant content of kaolinite and quartz, accompanied by clay
mica (illite) and hematite or goethite, (ii) German ochres also
with quartz, kaolinite, illite and hematite (or goethite), but usu-
ally accompanied by calcite and/or dolomite and (iii) Italian
ochres with dominant amounts of gypsum and anhydrite, usu-
ally also calcite, pigmented by goethite or hematite and ac-
companied by smectite group minerals and minute amounts of
kaolinite. Differences in the diffraction patterns of commercial
French and Italian ochres are shown in Fig. 1. Other unspeci-
fied ochres are similar in some content of clay minerals, but
differ significantly in crystallinity and content of artificial ad-
ditives. Samples marked as ‘green earth’ are usually rich in
glauconite (or celadonite), hematite has been found as an ad-
Fig. 1. Part of diffraction patterns of commercial pigments: a com-
parison of French and Italian ochres.
Clay minerals in paintings of the Middle Ages
The first indication of any content of clays in the studied
fragments is given by the analysis of layer stratigraphy and
chemical composition on polished cross-sections by means of
scanning electron microscopy equipped with a Robinson de-
tector of back-scattered electrons and EDAX detector of X-
rays. The relative contents of aluminium and/or magnesium
with respect to silicon involve evaluating the appropriate por-
tion of aluminosilicates within the mixture. In the mostly
chalk grounds of a number of Gothic wooden panels and
sculptures a greyish basal layer has been identified. This grey
chalk, usually termed “mountain chalk” is classically inter-
preted in painting technology as a clay-rich chalk (silt) pig-
mented by natural bitumens or graphite.
As we have found in the ground of a Gothic painting by
Master Theodoricus ‘St. Bishop’ (National Gallery in Prague),
as well as in the ground of a Gothic wooden panel ‘The Cruci-
fixion’ (unknown painter, Courtauld Museum in London), the
relative content of aluminium within this basal layer is rela-
tively small and the layer composition should be interpreted as
more siliceous. Independent measurement provided by powder
X-ray diffraction on fragments of similar ground layer break-
away from the cracked surface of a Gothic polychromed
wooden sculpture ‘Madonna from Pilsen’, has identified only
calcite and quartz as the main components. We have conclud-
ed that the composition of the basal greyish layer typical in
some gothic chalk grounds especially those of Czech prove-
CLAY MINERALS IN PIGMENTS OF MEDIAEVAL PAINTINGS 125
nance does not meet the definition of ‘mountain chalk’ and the
content of clays is lower than expected.
Clay minerals in baroque paintings
Most of measured Baroque bole grounds are aluminium-
rich, as in the case of the ground of altar piece by Jan Hess: ‘St.
Theresia’ (Church of our Lady Victorious in Prague), which
contains mainly Si, Al, Fe, K, Ti and Ca. It represents a typical
composition of red ochres used in grounds of Baroque paint-
ings; besides aluminium and silicon, usually potassium and
sometimes magnesium occurs as building elements of clays,
iron occurs mostly in oxides. Titanium oxides of natural origin
are also very common and could refer to similar associations
and sources of these materials – they have not been found in
any of the reference commercial pigments.
Mineralogical composition is difficult to distinguish if oper-
ating with only minute material fragments. However, we have
obtained interpretable diffractions, if the fragment was pre-
treated in a drop of toluene applied on the silicon slide, dried in
air and measured. Thus we have distinguished between smec-
tite-rich and kaolinite-rich grounds, as it is clearly visible from
the comparison of diffraction patterns of samples from the
Fig. 4. SEM image (back-scattered electrons) showing a cross-
section of painting layers of the painting ‘Armida watching the
destruction of a palace’ by CH.A. Coypel. Layers have been inter-
preted as: (1) Bole ground (smectite, kaolinite, illite, anatase,
quartz, well-crystallized hematite), (2) Underpainting (brown sie-
na containing clays and Mn and Fe oxides, white lead, bone
black), (3) White lead, grains of green earth (celadonite – be-
cause of higher Mg/Al ratio), (4) Dtto 3 + grains of Naples yellow,
(5) Lake double-layer (dammar with copal in poppy oil).
Fig. 3. Part of diffraction pattern of historical bole ground of Ba-
roque painting of M.V. Halbax with well-ordered kaolinite.
Fig. 2. Part of diffraction patterns of historical bole grounds: a
comparison of two Baroque paintings (by Ch. A. Coypel and un-
known author, respectively).
paintings by Ch. A. Coypel (‘Armida watching the destruction
of a palace’) and by an unknown painter (‘Portrait of the sit-
ting lady’), (Fig. 2). Identification of smectites in grounds is
important because traditional historical source material – Ar-
menian bole – should be a montmorillonitic clay with hygro-
scopic properties (e.g. Kužvart 1984). In our findings, kaolin-
ite is usually dominant. For example, the bole ground of the
painting ‘Kimon and Pera’ by M.V. Halbax contains predomi-
nantly well-ordered kaolinite accompanied by quartz; titanium
oxide can be undoubtedly identified as anatase (Fig. 3).
As a case study, the complete interpretation of original
painting layers of the Baroque painting by Ch. A. Coypel
‘Armida watching the destruction of a palace’ is shown in Fig.
4. Beside the bole ground, clays also occur in painting layers
as a main substance of Mn-rich grains of siena and as a miner-
al celadonite (green earth grains). A full description has been
done on a cross-section of the painting 1 mm thick, which
contains five different layers (Hradilová & Hradil 2001).
When studying the chemical composition, mineralogy and
crystallinity of clay minerals within minute fragments of orig-
inal historical paintings from the Middle Ages and Baroque,
we have found that:
– greyish basal layer of some chalk grounds of Gothic paint-
ings and polychromed sculptures known as ‘mountain chalk’
is more siliceous (quartz-rich) than expected; the content of
clay minerals is low;
– Baroque bole grounds contain predominantly kaolinite (as
measured on four different pieces), interpreted mostly as well-
ordered; illite is frequent, smectite was enriched in the ground
of a painting by Ch. A. Coypel from the Gallery of Fine Arts
in Ostrava; grains of the green earth found in the same paint-
ing contain celadonite.
bole ground (Ch.A. Coypel: "Armida is watching a destruction of palace")
bole ground (unknown painter:"Portrait of the sitting lady")
126 HRADIL, HRADILOVÁ and HŘEBÍČKOVÁ
Acknowledgments: This study was supported by the Ministry
of Education, Grant No. VS 97083. The authors are very grate-
ful to their colleagues in the Institute of Inorganic Chemistry,
Academy of Sciences of the Czech Republic (P. Bezdička, S.
Bakardjieva, J. Šubrt, J. Boháček) for performing analytical
measurements and to qualified restorers of the Academy of
Fine Arts and National Gallery in Prague and others for their
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