MINERALOGY OF BOHEMIAN GREEN EARTH PIGMENT AND ITS MICROANALYTICAL EVIDENCE IN HISTORICAL PAINTINGS

MINERALOGY OF BOHEMIAN GREEN EARTH PIGMENTAND ITS MICROANALYTICAL EVIDENCE IN

HISTORICAL PAINTINGS*

D. HRADIL,1,2† A. PÍŠKOVÁ,1,3 J. HRADILOVÁ,2 P. BEZDICKA,1,2 G. LEHRBERGER4

and S. GERZER5

1Institute of Inorganic Chemistry AS CR v.v.i., ALMA laboratory, 250 68 Husinec—Rež, Czech Republic2Academy of Fine Arts in Prague, ALMA laboratory, U Akademie 4, 170 22 Prague 7, Czech Republic

3Institute of Geochemistry, Mineralogy and Mineral Resources, Charles University in Prague, Albertov 6, 12843 Prague,Czech Republic

4Technische Universität München, Lehrstuhl für Ingenieurgeologie, Arcisstraße 21, 80333 München, Germany5Deutsches Museum München, Museumsinsel 1, 80538 München, Germany

While the most famous historical green earth deposits in Italy and Cyprus were exceptionallyrich in celadonite, the deposits near Kadan, Czech Republic, consisted of thin Fe-rich cela-donite impregnations accompanied by interstratified celadonite/smectite. In superimposedtuffs, smectites become prevalent and their colour changes from forest to olive green. PowderX-ray microdiffraction and infrared microspectroscopy permit us to distinguish between greenearth minerals in microsamples taken from works of art. The Ti contents and the Fe/Mg ratioshelp to indicate the presence of Bohemian green earth from Kadan. We found this material inlocal 18th-century works of art and also in paintings by Anton Kern (1709–47).

KEYWORDS: BOHEMIAN GREEN EARTH, CELADONITE, SMECTITE, ARTISTS’ PIGMENTS,MICROANALYSIS

INTRODUCTION

Green earth, similar to other earthy pigments containing clay minerals, is permanent, stable andnon-reactive and suitable for painting in any medium. The green colour of the clays is caused bythe presence of green Fe-containing minerals belonging to the group of clay micas (celadonite orglauconite), or less commonly to the group of chlorites or smectites. While celadonite andglauconite are K-rich minerals, smectites and chlorites, despite their variable composition,contain no potassium in their structure and/or their interlayer space. The increased sorptioncapacity predetermines the use of green earth as a siccative in oil paintings and it could beattributed to a higher content of smectite in the pigment; these properties are typical of thetraditional Bohemian green earth (Grissom 1986).

In Europe, the first important information on green earths appeared in the text De architectura,written by Vitruvius and dating to the first century bc. He mentioned two sources of the greenearth pigment in Roman times—Verona in Italy and Smyrna (now Izmir) in Turkey. According toOdin et al. (1988), the first location refers to celadonite found in the Late Eocene volcanites in theMonte Baldo area, near Verona, Italy, and the second location represents celadonite from theMassif of Troodos, in Cyprus, because Smyrna was only the port where the pigment was sold

*Received 17 February 2010; accepted 11 June 2010†Corresponding author: tel. +420-266172187; fax +420-20941502; email [email protected]

Archaeometry 53, 3 (2011) 563–586 doi: 10.1111/j.1475-4754.2010.00554.x

© University of Oxford, 2010

under the name ‘Earth of Smyrna’. These two localities were exceptionally rich in celadonite andthey were used throughout history up to at least the Second World War (Verona) and possiblyeven up to the present day (Cyprus).

While the artistic terms ‘Verona green earth’ and ‘Cyprus green earth’ are originally undoubt-edly connected with the above-mentioned deposits, the geographical indication ‘Bohemian greenearth’, which is also often used by artists, is not fully clear. The term appeared initially intechnological formulae in the 19th century, as ‘Kaadener Grün’ (Cechl and Hejtmánek 1993), andit was undoubtedly connected with the presence of ‘celadonite’ at Úhošt’any near Kadan, westernBohemia, Czech Republic. Later, its use became more widespread and it also included Creta-ceous glauconitic sandstones and claystones that were extracted extensively, particularly in thePrague area. The mining in the Kadan area started in the Middle Ages as an opencast venture andthe first mining galleries were opened in the 18th century (Becker 1891; Bláhová 2002; Gerzer2006). The green earth was used until the Second World War and appreciated by, for example, thedyeing industry because of its higher sorption capacity. This location also represented the mostimportant source in central Europe of pigments based on green clay, although its use in the finearts is not sufficiently documented by evidence. Its colour is reported to have been different fromsimilar pigments coming from southern Europe, but the mineralogy of the pigment was neverdescribed in detail. In central European painting, copper pigments were widely used for greensin the Middle Ages; green earths were more widely used from the 18th century onwards.

As we have reported earlier (Hradil et al. 2004), the locality of the altered volcanic tuffs atKadan, Czech Republic, is today relatively poor in celadonite and rich in the uncommon Fe-richsmectite; also, saponite has been identified in the alterations. This paper presents more detailedresults of an analysis of a much more representative set of Kadan green earths.

The minerals celadonite and glauconite have already been defined in older literature (Wise andEugster 1964; Buckley et al. 1978). The crystallochemical differences enabling the correct use ofthe appropriate terms were recommended in 1978 by the AIPEA Nomenclature Committee(Odom 1984). To summarize simply, celadonite should be close to the ideal mica composition,KMgFe3+Si4O10(OH)2, potentially with only a small substitution of Si4+ with Al3+ (or Fe3+) in itstetrahedral sheet, up to 0.2 atoms per formula unit at most. On the contrary, more extensivesubstitutions in its octahedral sheet can lead to recently defined minerals belonging tothe ‘celadonite group’—ferroceladonite, KFe2+Fe3+Si4O10(OH)2, and ferroaluminoceladonite,KFe2+AlSi4O10(OH)2 (Li et al. 1997). From the viewpoint of identification, it is important that thebetter-defined crystal structure of celadonite leads to sharper diffractions in the XRD pattern, andthe diagnostic position of the XRD 060 line should always be at a d060 value lower than 1.510 Å.The composition of glauconite is more uncertain: tetrahedral substitutions are more extensiveand the composition of octahedral sheets is more complex, with Fe3+ >> Al and Mg > Fe2+.Because of the lower negative layer charge of glauconite, the potassium content can be signifi-cantly lower than 1 atom per formula unit. Diffused diffraction lines are common and d060 isalways larger than 1.510 Å.

Glauconite pellets typically occur in sandy sediments of marine origin, as a main component ofthe so-called ‘glaucony facies’, alongside other less common greenish minerals such as nontroniteor Fe-montmorillonite (Odom 1984; Gaudin et al. 2005). Celadonite is described as an alterationproduct of basaltic rocks, usually found in fillings of vesicles and fissures, or as a replacement forprimary minerals (Odom 1984). Traditionally, its origin is considered to be hydrothermal, but Odinet al. (1988) suggested the long-term influence of low-temperature modified marine fluids as themost important factor of its formation. The idea of an exclusively marine origin for celadonite iswell documented by numerous findings of this mineral in altered oceanic basalts, often in close

564 D. Hradil et al.

© University of Oxford, 2010, Archaeometry 53, 3 (2011) 563–586

association with Mg-rich smectite—saponite (Andrews 1980; Talbi and Honnorez 2003). Findingsof celadonite in non-marine environments are relatively rare (e.g., Baker 1997).

Glauconite and celadonite can be confused analytically due to their similar chemical compo-sitions and structures. In sub-marine altered basalts, their joint occurrence has been reported byseveral authors (e.g., Clayton and Pearce 2000), although this possibility had been previouslydenied—by, for example, Odom (1984). Another evidence of the complex mineralogy of marinesediments and alteration products of volcanic rocks is the increasing number of findings ofmixed-layered structures, typically glauconite/nontronite and celadonite/saponite. Admixtures ofsmectites in green earths are probably more common than assumed in the past.

It is clear that even if a lot of pure material is available for crystallochemical analysis, the definiteclassification can remain problematic. When analysing colour layers of works of art, we can findgreen earth grains smaller than 0.1 mm as an inseparable part of the colour layer—that is, mixedwith other organic and inorganic components—and the possibility of their successful mineralogi-cal classification is further limited. Finally, the identification of green earths is further aggravatedby the fact that the terms ‘celadonite’ and ‘glauconite’ are used arbitrarily in the evaluation of thematerial analysis of fragments of works of art, irrespective of the actual classification criteria andirrespective of the ability of the employed analytical techniques to discern between the twomineralogical forms (Edreira et al. 2001; Mazzocchin et al. 2003, 2004). In the fine arts, only avisual evaluation of greens is still the prevalent practice among artists, as well as restorers. It is easyto show that the colour of the green alteration products of basaltic rocks can vary widely within onelocality and therefore such a simplified determination is hardly possible.

The objective of this work is to compare different historical samples of Bohemian green earthsfrom Kadan, obtained from mineralogical collections, and to relate the different mineralogicalcompositions to the individual phases of the mining of green earths. The measurement of archivesamples is the only way to describe the alteration processes in the locality, where only substan-dard material is now available. The second task is to summarize microanalytical measurementsof green earths found in 18th-century paintings in Czech collections and to try to find indicatorsof their possible origin. The aim is to clarify the unclear contribution of the Bohemian green earthto the regional central European market before the well-documented mining in the Kadan area inthe 19th century. The question is whether the regional green earths could be, positively andcorrectly, distinguished from pure celadonites imported from southern Europe (Italy, Cyprus) inmicrosamples of real paintings.

A search for the provenance of pigments is always a highly valuable component of researchinto works of art, because it can provide some additional clues about the provenance of the workof art itself. In spite of the great complexity of this task in the case of green earths, it is sufficientlyworthy of consideration, and it is necessary to critically evaluate whether the provenance canreally be reliably traced. This paper provides a comprehensive description of the geological andmineralogical aspects affecting the colour properties of the most famous central European greenearth deposit at Kadan, Czech Republic, and it explains the limits of microanalytical tools indetermining the provenance of those heterogeneous pigments found in real paintings.

EXPERIMENTAL

Analysis of rock samples and commercial pigments

All of the samples are listed in Table 1. Rock samples H146, H084, H087, H114, H140, H141,H150 and H215 were collected by the authors in accessible parts of historically documented

Mineralogy of Bohemian green earth pigment 565


localities of green earths in the Czech Republic. Other rock samples were taken from themineralogical archives of Technical University in Chania, Greece (H218), the Regional Museumin Chomutov, Czech Republic (H102 and H103), the Reichsstadtmuseum Weißenburg, Germany(H153) and the Faculty of Science, Charles University in Prague, Czech Republic (H215 andH219). The mineralogical samples were compared with green earth pigments currently available

Table 1 A list of green earth pigments and rock samples and their mineralogical compositions (already publisheddata are referenced)

Code and nature Description Mineralogical composition (XRD)

Bohemian green earth samples from the Kadan area (Group 1)H084, H087, H146: rock samples Green altered tuff from the

abandoned mine gallery at Brodcenear Kadan (self-collection)

Fe-smectite, saponite, biotite, anatase,quartz, titanomagnetite, calcite orMg-calcite, celadonite (very smallamount) (Hradil et al. 2004)

Bohemian green earth samples from the Kadan area (Group 2)H140, H141: rock samples Green earth from old mine dumps at

Úhošt’any, near Kadan(self-collection)

Calcite, celadonite, goethite,celadonite/smectite

Bohemian green earth samples from the Kadan area (Group 3)H151: rock sample Green earth from Kadan, 1930

production (Kremer-Pigmente,Germany)

Fe-smectite, calcite, quartz, anatase,biotite and/or celadonite,Mg-siderite

H153: rock sample Green earth from Kadan(Reichsstadtmuseum Weißenburg,Germany)

Fe-smectite, calcite, celadonite,celadonite/smectite, quartz,anatase, siderite

H102, H103: rock samples Green earth from the Úhošt’any deposit,near Kadan (Regional Museum atChomutov)

Calcite, celadonite,celadonite/smectite

Bohemian green earth samples from the Prague areaH215: rock sample Green claystone, Vidoule, Prague

(archive sample, Faculty of Science,Charles University in Prague)

Glauconite, kaolinite, illite, quartz(Hradil et al. 2004)

H114: rock sample Green claystone from Vysocany,Prague (self-collection)

Glauconite, glauconite/smectite,kaolinite, quartz

7071: Umton pigment, powder Green earth from the Prague area(Umton Colours Ltd, Decín, CzechRepublic)

Glauconite

Archive samples from other European localitiesH218: rock sample Green earth from Cyprus (Technical

University in Chania, Greece)Celadonite

K17400: K17410 pigment,powder

Green earth from Cyprus(Kremer-Pigmente, Germany)

Celadonite, smectite (very smallamount), plagioclase (Hradil et al.2004)

H219: rock sample Green earth from Monte Baldo, Italy(archive sample, Faculty ofScience, Charles University, Prague)

Celadonite (Hradil et al. 2004)

H154: rock sample Green earth from Monte Baldo, Italy,production of c. 1995(Kremer-Pigmente, Germany)

Celadonite, celadonite/smectite?,goethite

K11000: K11010 pigment, powder Green earth from Verona, Italy(Kremer-Pigmente, Germany)

Fe-smectite, plagioclase (Hradil et al.2004)



on the market: K17400, K17410, K11000 and K11010 (Kremer-Pigmente, Germany) and 7071(Umton Colours Ltd, Decín, Czech Republic), and with the Kadan green earth pigment datingfrom 1930—H151 (Kremer-Pigmente, Germany).

If not otherwise stated, all the analyses were carried out at the Institute of Inorganic Chemistryof the ASCR, v.v.i., Czech Republic. Bulk samples, as well as oriented clay fraction specimens,were analysed using a conventional powder X-ray diffractometer (XRD), a Siemens D-5005(Bruker) with CuKa radiation, a secondary monochromator and a common scintillation detector,and by an X’Pert PRO diffractometer (PANalytical) with a multichannel X’Celerator detectorand CoKa radiation. Additionally, slow scans were performed in the 2q angle region 60–77° todistinguish non-basal 060 diffractions of the individual clay minerals. HighScore (PANalytical),with the 2005 release of the PDF-2 database, was used for the phase identification. The clayminerals were interpreted according to Moore and Reynolds (1997).

For Fourier-transformed infrared spectroscopy (FT–IR) of bulk green earths, the samples werepressed with KBr (0.5 mg of sample per 200 mg of KBr) into 13 mm pellets. The spectra wereobtained using a Nicolet Magna 750 FTIR spectrometer. Repeated scans were collected (128) foreach sample at a resolution of 4 cm-1. The spectra were interpreted according to Besson and Drits(1997a,b).

The diffuse reflectance spectra (DRS) were obtained using the UV/Vis PerkinElmer Lambda35 spectrometer, equipped with a Labs sphere RSAPE-20 integrating sphere accessory. Thesamples were ground in an agate mortar and placed in 2 mm quartz cells before measurement. ABaSO4 pellet was used as a white reference. The reflection spectra were acquired with a 0.5 nmstep, converted to the Kubelka–Munk (KM) scale, smoothed by a seven-point Fourier filter andthen deconvoluted into a set of Gaussian components using the peak-fitting module in OriginPro7.0 (OriginLab Corporation).

The major element content in homogenized samples was quantified after their dissolution byatomic absorption spectroscopy (AAS) in the analytical laboratories of Gematest Ltd (Cernošice,Czech Republic). The semi-quantitative elemental composition was also measured by electronmicroscopy and microanalysis (SEM/EDS). Thin sections of heterogeneous altered tuffs from theKadan area were used for elemental mapping, using EDAX software on a Philips XL-30 CPelectron microscope.

Analysis of works of art

The list of samples is provided in Table 2. Fragments of colour layers of about 1 mm in size havebeen taken by qualified restorers from selected 18th-century paintings located in the CzechRepublic and containing the green earth pigment. The fragments were embedded in polyesterresin and polished in their cross-sections. The preparation of cross-sections is necessary for aroutine description of the stratigraphy of the colour layers, and the painting technique used.Hence the samples cannot be homogenized: the information about their heterogeneity, which isessential for their historical and technological evaluation, must not be lost. We examined thecross-sections by means of optical microscopy (Olympus BX-60 in reflected visible and UV lightmode), scanning electron microscopy (Philips XL-30 CP) with a Robinson detector of backscat-tered electrons and EDS analysis (SEM/EDS) to describe the layer stratigraphy and the elementalcomposition of the green earth grains. EDS EDAX with a compact Sapphire Si(Li) detecting unitand light elements performance technology (LEAPC) was used to allow detection of elementsheavier than Be (Z > 4) at a resolution of 135 eV. Standardless quantification using ZAF



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correction (Genesis Spectrum SEM Quant ZAF, version 3.60) was applied to calculate theelemental composition; the typical counting time was 60 s.

The X’PertPro (PANalytical) X-ray diffractometer with CoKa radiation, a monocapillaryfocusing the primary beam to a 0.15 mm diameter, and a X’Celerator multichannel detector asdescribed by Šímová et al. (2005) (micro XRD) were used for microanalysis of the phase(mineral) composition of the cross-sections, and also microfragments (<1 mm) whenever theamount of material allowed its splitting into several subsamples. HighScore (PANalytical), withthe 2005 release of the PDF-2 database, was used for the phase identification. The clay mineralswere interpreted according to Moore and Reynolds (1997).

Infrared spectra of cross-sections were measured in reflection mode, using a Continuuminfrared microscope with a Nexus microspectrometer (ThermoNicolet, USA) at the PolymerInstitute in Brno (micro FT–IR). The spectra were recorded in the region 4000–650 cm-1 with aresolution of 4 or 8 cm-1 and were analysed using Omnic 7.2 (Thermo Electron Co.).

RESULTS AND DISCUSSION

Bohemian green earth

The samples of Bohemian green earth from the Kadan area, Czech Republic, can be divided intothree groups, based on their provenance and composition (Table 1). In the first group there weresamples collected in a mining gallery at the village of Brodce, near Kadan (H084, H087 andH146). The gallery follows a sub-horizontal brown–green layer of altered basaltic tuffs ofOligocene–Miocene age superposed on a layer of limnic (freshwater) limestones. The claymineralogy of these tuffs has already been described by Hradil et al. (2004), the main compo-nents being ferruginous dioctahedral smectite, saponite, green mica (in a very small amount),anatase, quartz and titanomagnetite. The ‘Brodce’ mining gallery is currently the only accessiblegallery and it probably belongs to the last phases of mining in this area, which provided onlylow-grade material. The most interesting sign of the alteration is a joint occurrence of saponiteand green mica, and a relatively high content of titanomagnetite and anatase contributing to therelatively large content of total Ti (Table 3).

In the thin sections of the Brodce samples, several types of heterogeneities were found inthe fine-grained matrix of ferruginous smectite—angular grains of titanomagnetite with anaverage composition of Fe2.5Ti0.5O4 (obtained by SEM/EDS), pseudomorphs after augitephenocrysts, now consisting mainly of olive-green Fe-smectite, and vesicles and fracture infill-ings. In these infillings, two alteration zones are clearly visible on the EDS element mappingfigures (Fig. 1). The outer one contains K-enriched material with the average compositionK0.52(Si3.80,Al1.1,Fe0.61,Mg0.59)6.10O10(OH)2 resembling green mica, and the inner one is filled withsaponite with the average composition Ca0.33(Mg2.46,Fe0.54)3.00(Si3.52,Al0.48)4.00O10(OH)2 (Fig. 1).The presence of trioctahedral smectite is also indicated by the characteristic position of 060diffraction at d060 = 1.53 Å (Fig. 2). This type of alteration is very similar to infillings ofamygdales in sub-marine basalts, formed by percolating sea water, as described on the westernflank of the Mid-Atlantic Ridge (Andrews 1980), and similarly in the Caribbean Sea (Clayton andPearce 2000). The problem is that due to the close association of green earth at Brodce withlimestones of undoubtedly limnic (freshwater) origin, and due to its position within the firstneo-volcanic stage of the development of the Oligocene West Bohemian platform (Malkovský1985), hardly any influence of the sea water can be expected in connection with the alterationprocesses in this location. Physical mixtures of celadonite, saponite and nontronite have also been



Tabl

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84N

a 0.0

4(Si

4.03

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Fe+3

0.93

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.60)

5.98

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(OH

) 2(n

oim

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8C

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591.

386.

300.

390.

038.

870.

050.

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K0.

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) 2(t

hefo

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H) 2

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Al 0

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Fe+3

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).



described in Troodos ophiolite in Cyprus (Gallahan and Duncan 1994)—the source locality of theCyprus green earth. Because of the high iron content, several authors suggest a joint occurrenceof celadonite and glauconite in the Cyprus deposits (e.g., Despairies et al. 1988).

The second group of samples (H140 and H141 in Table 1) was collected in the old miningdumps near the village of Úhošt’any (about 5 km from Brodce). Because of their more intensegreen colour, they could represent relics of mining in the richest part of the deposit. All of thesamples contain a lot of calcite, as shown by XRD, pointing to their origin at the base of thevolcano-sedimentary series in close contact with limnic (freshwater) limestones (similar toBrodce). Mica-like minerals in these samples can be clearly recognized by XRD. No smectite ispresent as a single phase, but just beside the 001 diffraction line of clay mica a broad diffractionis visible, indicating the presence of an interstratified structure (Fig. 3). According to measure-ments of an oriented air-dried and ethylene glycol solvated specimen of sample H103 (Fig. 4), itis an almost regular mica/smectite structure of type R3 (i.e., one layer of smectite surrounded bythree layers of mica). The position of the diffraction line d060 = 1.511 Å corresponds to the claymica structures and indicates an Fe-rich phase of the glauconite type (not shown). We alsocalculated the approximate structural formula of that mica mineral, based on its bulk elementalanalysis (Table 3). The final structural formula resembles Fe-rich glauconite, but the calculation

Figure 1 Element mapping of vesicle infillings in green tuffs from Brodce, Kadan area, Czech Republic (sample H084).The mapping of Ca indicates secondary calcite. The mapping of K indicates the celadonite-rich rim of the vesicle and Mgindicates saponite in its central part. the mapping of Fe shows grains of titanomagnetite, and Ti indicates bothtitanomagnetite and anatase.



is influenced by the presence of smectite layers. Samples with other Fe-containing phases (e.g.,goethite, siderite and titanomagnetite) were excluded from the calculation.

The third group of samples (H151, H153, H102 and H103 in Table 1) represented archivematerials from regional museums and commercial pigments supplied by Kremer-Pigmente,Germany. When comparing them with samples collected in the location concerned, it is obviousthat the composition of the pigments varied over the course of the history (Table 1 and Fig. 3).While the samples from the Regional Museum in Chomutov, Czech Republic (H102 and H103)have the same composition as those from the old mining dumps at Úhošt’any (H140, H141 andH150)—that is, with calcite, green mica and interstratified green mica/smectite—the samplefrom the Reichsstadtmuseum Weißenburg, Germany (H153), contains smectite as a single phase,mica and a lower content of calcite; the mixed green mica/smectite layer is also visible as adiffuse diffraction at approximately d001 = 12.0 Å. The sample supplied by Kremer-Pigmente,Germany, identified as Kadan green earth from 1930 (H151), is very similar to the materialcollected in the mining gallery in Brodce (H146, H084 and H087), with predominantFe-smectite, saponite and anatase. Only the 001 line of smectite is shifted to the higher angles inH151, indicating the possible presence of a mixed layer with a mica/smectite structure with ahigh content of smectite layers. The mica diffraction here corresponds to macroscopic biotitebecause it is absent in the clay (fine-grain) fraction (Fig. 3).

Figure 2 Non-basal 060 diffractions of Bohemian green earth samples from different sources.



When comparing samples from all the three groups, it is possible to observe a gradual changefrom green mica to interstratified green mica/smectite and finally to pure smectite. The change isaccompanied by a gradual decrease in the calcite content and an increase in the Ti-containingphases—titanomagnetite and anatase.

Examination of the infrared spectra of the selected samples provides a correct classification ofthe green micas (Table 4 and Fig. 5). Samples H102 and H153, which probably represent thepigment production from the 19th century, resemble celadonite from the Monte Baldo deposit,with several differences. A strong Si–O–Si(–Al) stretching vibration at 970 cm-1 indicates thepresence of Al3+ in the tetrahedral sheet to a higher extent than in the Monte Baldo celadonite—this correlates with the structural formulae presented in Table 3, and smectite is indicated by thevibration at ~1020–1030 cm-1. The mica structure can be still classified as tetrasilicic, because thetypical position of this vibration in trisilicic glauconites is at 980–1025 cm-1 (similar to smectite).The high Fe content in the octahedral sheet is indicated in the OH-bending region by thevibrations at 874 cm-1 (Al–Fe–OH vibration) and ~820 cm-1 (Fe–Fe–OH vibration). The first ofthese appears in all samples from Kadan and therefore it can be attributed to the Fe-smectitepresent either in the interstratified structure or as a single phase. Sample H151 of green earth

Figure 3 Basal 001 diffractions of Bohemia green earth samples from different sources. The presence of biotitephenocrysts was confirmed by macroscopic observation of tuffs.



pigment, from 1930 (Kremer-Pigmente), is very similar to the sample from the Brodce gallery(H084), with predominant Fe-smectite. However, some of the vibrations typical of celadonitesare still visible; for example, Si–O bending vibrations at 455 and 440 cm-1, respectively, or sharpvibrations in the OH-stretching region, at 3528 and 3534 cm-1 (not shown). In the Brodce andH151 samples, smectites prevail and the Mg content is higher, as indicated by the well-developedvibrations at 3603 cm-1 (Al–Mg–OH) and 3570 cm-1 (Fe–Mg–OH) (Table 4; see also Hradilet al. 2004). Also, in cases where the Fe–Fe–OH vibration at ~820 cm-1 is missing (in the casesof the Brodce H084 and Monte Baldo H219 samples), the vibration at ~840 cm-1 should beattributed to Al–Mg–OH rather than the second Fe–Fe–OH (Gates 2005).

Finally, we can conclude that both the positions of the d060 diffraction and also the elementalcompositions of the green mica mineral in archive samples H102 and H153 are influenced by thepresence of smectite layers; and that the green mica mineral should be classified as celadoniterather than glauconite, accompanied in fact by a celadonite/smectite interstratified structure.Smectite as a single phase appears in H153 and H151 (1930 production), resembling thecomposition of the green earth available at Brodce today (H084). This classification reflects thesharpness of the mica diffraction lines, the IR spectra and the correlation of the 060 diffractionswith the content of octahedral Fe3+ proposed by Odom (1984). As clearly indicated by thecorrelation (Fig. 6), the archive Kadan green earth falls into the group of mixed-layeredceladonite/smectite structures rather than the group of glauconites.

The mineralogical differences among the samples are accompanied by important changes inelemental composition, as indicated by the IR spectra and also by the bulk chemical composition.Because of insufficient amounts of archive samples for chemical analyses, we adopted datapresented by Neužil (1955), which reflect the composition of Kadan green earth at the end of its

Figure 4 Interstratified celadonite/smectite of R3 type in the archive Bohemian green earth sample H103: a comparisonof air-dried and ethylene glycol (EG) solvated measurements for specimen C indicates celadonite, while otherdiffractions indicated by arrows represent the interstratified structure. Interpretation according to Moore and Reynolds(1997).



Tabl

e4

Cha

ract

eris

tic

vibr

atio

nsin

the

mid

-IR

spec

tra

ofK

adan

gree

nea

rth

sam

ples

and

refe

renc

ece

lado

nite

from

Mon

teB

aldo

Mon

teB

aldo

(H21

9)K

adan

GE

(H10

2):

arch

ive

sam

ple

Kad

anG

E(H

153)

:ar

chiv

esa

mpl

eK

adan

GE

(H15

1):

1930

prod

ucti

onK

adan

GE

(H08

4):

Bro

dce

min

e

OH

bend

ing

band

s(c

m-1

)A

l–A

l–O

HM

issi

ngM

issi

ngM

issi

ng91

5U

ncle

arA

l–Fe

–OH

Mis

sing

874

874

874

874

Al–

Mg–

OH

842

Mis

sing

Mis

sing

Mis

sing

840

Fe–F

e–O

HM

issi

ng82

0U

ncle

arM

issi

ngM

issi

ngA

l(Fe

)–M

g–O

H,S

i–O

amor

ph.

799

798

798

Bro

ad(@

795)

Unc

lear

Lat

tice

-def

orm

atio

nan

dSi

–Obe

ndin

gba

nds

(cm

-1)

MgF

e3+–O

Hor

Fe–O

out

ofpl

ane

682

683

682

Bro

ad(@

680)

682

coup

led

Si–O

&M

–Oou

tof

plan

eM

issi

ngM

issi

ngB

road

(@60

5)M

issi

ngU

ncle

arM

–O–S

i49

249

249

151

751

0Si

–O–S

i46

044

045

544

045

644

046

646

044

0

460

OH

stre

tchi

ngba

nds

(cm

-1)

Al–

Mg–

OH

Unc

lear

Unc

lear

Unc

lear

Unc

lear

3603

Al3+

–Fe2+

–OH

Mis

sing

Mis

sing

Mis

sing

Mis

sing

3680

Fe3+

–Mg2+

–OH

3555

3555

3554

Unc

lear

3570

Fe3+

–Fe2+

–OH

3528

3528

3527

Unc

lear

Mis

sing

Si–O

stre

tchi

ngba

nds

(cm

-1)

Si–O

–Si(

–Al)

956

977

971

1015

974

1020

1031

1027



production in the middle of the 20th century (Table 3). These chemical analyses demonstrateconsiderable variation in the MgO content among the three Kadan green earth samples collectedin different periods and at different places. This could be explained by the most probable theoryof formation of the deposit. The celadonite develops at the contact of limnic (freshwater)

Figure 5 Part of the mid-IR spectra of Bohemian green earth samples from different sources.

Figure 6 Correlation of the octahedral Fe3+ content and the 060 position according to Odom (1984).



limestones and altered volcanic tuffs, described in the old German literature as ‘sand’ (because ofits similar texture) of a greenish or brownish colour (Becker 1891; Albrecht et al. 1903). The tuffsare considered to be the source material for the green earth; they are rich in augite and aretransformed after its deposition into celadonite. The temperature of the alteration is not easy toreconstruct. We can assume a relatively thin (up to 50 cm) and irregular layer (or aligned lenses)of hydrothermal Fe-rich celadonite on the top of a limnic limestone sequence (Fig. 7). Inagreement with a general zonality of the hydrothermal deposits and with an increasing distancefrom the source of heat, first the interstratified celadonite/smectite appears, and then smectiteonly in the outer zones. In the superimposed volcanic tuffs, the abating hydrothermal effectswere combined with the previously started sub-aquatic alteration at lower temperatures, similarto the argillization in the neighbouring bentonite deposit, which has approximately the samestratigraphic position. Numerous Mg-containing phases appeared here—biotite, vermiculite,Mg-calcite (and/or siderite) and saponite—that significantly affected the Fe/Mg ratio. Here, theassociation of saponite and celadonite in vesicle fillings in samples from Brodce is of localimportance. The role of the secondary replacements remains unclear.

The older literature (Becker 1891; Albrecht et al. 1903) identified the upper green tuff layersas zones of leaching (dissolution), and the top of limestones with impregnated celadonite as azone of secondary crystallization of mineralized solutions. Because of the small thickness of the‘massive’ celadonite (if any), the mixed layers with smectite were always present in the exploitedproducts as a characteristic feature of this green earth. The lack of large quantities and thevariable thickness of the celadonite bodies were probably the reasons why the upper greenishvolcanic tuffs with prevalent smectites were also exploited, particularly in the last stages ofmining at Brodce. The colour naturally reflects the unusual and varying composition of the Kadangreen earth—the original forest green (instead of the blue–green in Cyprus and Verona) graduallychanged to olive green (Fig. 8).

This case demonstrates how difficult it can be to perform a correct mineralogical classificationof green earths. Instead of direct identification of glauconite and celadonite in samples of real

Figure 7 A sketch of the areal extent and stratigraphy of the Kadan mine for green earth, according to Albrecht et al.(1903).



paintings, it is probably more efficient to look for differences in the bulk elemental compositionand for the presence of potential admixtures. The Bohemian green earth from Kadan should beindicated in historical paintings (19th century or earlier) by a high Fe/Mg ratio, and in the lastphases of mining (20th century) the decrease in the Fe/Mg ratio will be accompanied by anincreasing Ti content.

The original Monte Baldo and Cyprus green earths are massive celadonites, close to the idealcomposition published by Odin et al. (1988) (Table 3). Celadonites of such purity were probablynever found at Kadan, or only in very small quantities. Although the alteration processes and thecomplex mineralogical composition were similar in the Cyprus and Kadan deposits, the smallsize of the deposit at Kadan resulted in the necessity of also extracting the low-grade material,which always contained some smectite layers. This is probably the only reason why the Bo-hemian green earth from Kadan is well known for its increased smectite content, which modifiesits colour from blue–green to forest or olive green and which increases its oil-sorption capacity.Smectites are also present in other deposits, but due to sufficient access to pure massive celado-nite in the Mediterranean region, there was no historical reason to exploit them. The situationcontinues to change even in those historical localities: the Monte Baldo green earth from 1995contains interstratified celadonite/smectite and the current ‘Verona’ green earth supplied byKremer-Pigmente (Germany) contains only smectite (Hradil et al. 2004). The currently availableCyprus green earth also contains some smectite admixture (Table 1).

Chemically untreated raw samples of natural pigments, as used in the fine arts, always containimpurities that affect the structural formula calculation. Differences in their chemical composi-tion can therefore be used to estimate their provenance, but not for their accurate mineralogicalclassification. For this purpose, it is necessary to obtain X-ray diffraction data and/or IR spectra.

The other localities in central Europe (the Prague area in the Czech Republic, and Poland andHungary) were even less important than Kadan. Most of them belong to sedimentary deposits of

Figure 8 The colour of mineralogical samples of green earths expressed by CIE x,y chromaticity coordinates.



glauconite, similar to those from other European localities; for example, in France (Grissom1986). In the Prague area, glauconitic claystones of Cretaceous age are typically accompanied bykaolinite. They are still exploited and currently supplied by Umton Colours Ltd (Decín, CzechRepublic), as ‘Bohemian green earth’. The mineralogy is different from that of the traditionalBohemian green earth from Kadan (Table 1).

Samples from real paintings

The typical size of samples and green earth grains are shown in Figure 9. Because of mixing ofgreen earths with other pigments in real colour layers in works of art, individual grains of asufficient purity are difficult to find. Admixtures are common and therefore the calculation ofcrystallochemical formulae should be approximate in principle. The only possible way is to lookfor the basic relations between major elements, regardless of whether they are caused bycrystallochemical variability or by the presence of physical admixtures. The crucial parameter isnaturally the potassium content, because clay micas are K-containing phases while smectites andchlorites are not. The K content is therefore related to the content of micas. It is not a trivial taskto distinguish between the micas—that is, celadonite and glauconite—provided that only semi-quantitative EDS analyses of green earth grains are available. For the first approximation, werecommend the use of the K/(Si/Al) ratio, because it works even when the most commonsmectites (montmorillonites, beidellites and also ferruginous smectites) or their mixed-layeredstructures are present in the admixture (Fig. 10). As can be seen in this figure, typical celadonites(Cyprus, Verona) have this ratio in the range from 1 to 1.5, while typical glauconites have a ratioof around 2 (Prague). The ratio does not change significantly even in the presence of somesmectite layers, because in that case the gradual decrease in the relative content of potassium isusually accompanied by an increasing relative content of aluminium. This is naturally only arough estimation, which cannot substitute for a standard mineralogical analysis by diffraction andspectroscopic methods.

The second important ratio that could reflect the provenance of the green earth is Fe/Mg. As theratio can be affected by both green micas and smectites, the total K content should be always

Figure 9 The typical size of a microsample of painting layers embedded in polyester resin (sample M0532).



taken into account. If any presence of smectites or chlorites is excluded by other methods, thenthe Fe/Mg ratio should be higher in glauconites than in celadonites (Fig. 10). All samplesincluded in this graph have significant contents of clay micas, characterized by an increased Kcontent (a minimum of 5 at%). The lowest K content, 5 at%, has been found in an analysis ofBohemian green earth from Kadan, as published in Neužil (1955). It has been clearly demon-strated that changes in the Fe/Mg ratio in the case of Kadan were caused by the graduallyincreasing content of smectites and other phases, and not by changes in the celadonite structureitself.

In most cases, the dispersion of results of analyses performed on one painting is not as high asthe differences between individual paintings, and therefore the averaged analyses can be used toclassify the paintings approximately according to the compositional similarities of the greenearths used (Fig. 10). It is obvious that the majority of green earth samples can be classified asceladonites and that none of them represents a typical glauconite. It is interesting that all samplesfrom works of art actually located in the town of Kadan contain green earths with an increasedFe/Mg ratio (particularly numbers 1 and 2) and also an increased Ti content (all of them). Noother sample contains Ti, except for J0607 (number 18). It is highly probable that the green earthfrom Kadan was also used in all of the paintings investigated by Anton Kern (numbers 5, 6, 7 and8) and in the oil-on-canvas paintings J0406 and M0225 (numbers 9 and 10), although in thoseanalyses no Ti was found. Based on this screening, all of the paintings containing Fe-richceladonites were selected for further research (numbers 1–10). None of the samples had an iron

Figure 10 Selected elemental ratios obtained by SEM/EDS analyses of mineralogical samples and microsamples of realpaintings.



content as high as the interstratified celadonite/smectite from Kadan (H102). One explanationcould be that the Fe/Mg ratio of the Kadan green earth was exposed to secondary weathering (nofresh samples are available today). The composition of the majority of other green earths from thecollection of 18th-century paintings is unusual because of the increased Al content, which isrelatively high compared to the reference celadonites (Verona, Cyprus). The relatively high Mgcontent found in some samples resembles Mg-celadonite (specifically, the wall paintings J0607and M0357, numbers 18 and 19) or unusual Mg-glauconite (numbers 21–25). For this unusualcomposition with high relative contents of K, Mg and Al together, we currently have no availablereference material. We cannot completely exclude a potential contamination of those green earthgrains with other earthy pigments; for example, yellow ochres that usually contain K- and Al-richphases, such as kaolinite and/or illite. In order to determine whether other non-mica minerals arepresent in the pigment, a direct phase analysis is necessary. Physical admixtures of other silicatesalso most probably reduce the relative content of K in some analysed grains.

The most suitable method for distinguishing between micas, chlorites and smectites isundoubtedly powder X-ray diffraction. For the microanalysis of fragments and their cross-sections, we used powder X-ray microdiffraction with a spatial resolution of about 0.15 mm. Inthe case of samples taken from real works of art, the quality of a diffraction pattern is stronglyinfluenced by the presence of amorphous organic binders and sealing compounds. We were ableto clearly identify chlorites used as green pigment in one 19th-century repaint, but in original18th-century painting layers only mica minerals were present as green pigments.

In the case of the fresco-secco painting from the Plasy monastery (M0357, number 19 inFig. 10), diffraction patterns of green layer samples, prepared by three different methods, werecompared (Fig. 11). When the sample was powdered and measured by conventional powderX-ray diffraction, only well-crystallized minerals from the sand used in the plaster were visible(low-Fe mica, chlorite, albite and quartz), together with some calcite. Microdiffraction measure-ments of both the untreated fragments from the top and cross-sections of the same green layerexcluded the possibility that chlorites were used as the green pigment—only celadonite wasfound as a colouring agent. The cross-sectioning and measuring of flat clay particles ‘from theside’ caused a decrease in the intensity of the basal 001 diffraction line of celadonite in thismeasurement. On the other hand, when the fragment was analysed ‘from the top’, the intensityof the basal diffractions of celadonite was increased and, additionally, the presence of gypsumwas identified, proving the presence of a secondary salt efflorescence on the surface of thepainting. This case can demonstrate how important it is in respect to the stratigraphy of paintinglayers to avoid any data misinterpretations. It also shows the advantages of powder X-raymicrodiffraction for direct phase microanalysis within individual layers. Unfortunately, thequality of the microdiffraction patterns was only sufficient in some cases to find the exact positionof the 060 diffraction of the mica minerals. In one wall painting (M0357) and in one oil-on-canvas painting (M0439), we found d060 in the range 1.509–1.510 Å, which indicated the presenceof celadonite. Smectites were not clearly visible, because the detection limit of poorly crystallinelayered silicates (clays) is much lower when measuring the cross-section of their basal planes.Consequently, it was not possible to confirm the presence of smectite admixtures in any of thepreselected Fe-celadonites, which are believed to originate from Kadan. IR microscopy was usedfor this purpose.

The main advantage of IR microscopy is that a single grain of the pigment can be analysed, butthis fact is also the weak point: an inherent consequence is that the specular reflection issubstantially deformed. The reasons are that the crystals or grains are larger than the wavelengthof IR radiation, and that measurements carried out on works of art must be non-destructive. In our



case, the Si–O–Si(–Al) vibrations were used to distinguish between celadonite and smectitelayers in a set of microsamples containing Fe-celadonites. As mentioned earlier, these micro-samples can be divided into three groups. The first group is represented by works of art actuallylocated in the town of Kadan (numbers 1–4 in Fig. 10), where the high Fe/Mg ratio is accom-panied by the increased Ti content—here, the provenance of the green earth pigment seems to bedefinite. The second group includes four paintings by Anton Kern (numbers 5–8 in Fig. 10)—regardless of their current location, the green earth pigment definitely comes from the samesource. The increased Fe/Mg ratio could indicate the green earth from Kadan, but no Ti wasfound. The third group includes two other paintings which, similarly to paintings by Anton Kern,have an increased Fe/Mg ratio and contain no Ti (numbers 9 and 10 in Fig. 10).

As already mentioned, the Fe/Mg ratio was gradually changing in the green earth material fromKadan and it was related to the content of smectite admixtures. Thus, the decrease in the Fe/Mgratio was accompanied by a decrease in the K content (Fig. 12). This feature is specific for thegreen earth from Kadan. When, for example, we compare the pure historical celadonites fromVerona, Italy, with the now available Verona green earth containing predominantly smectites(Fig. 12), we can see that although the K content has decreased, the Fe/Mg ratio remainsunchanged. There are considerable differences between the K contents of the microsamples ofreal paintings. While microsamples from the town of Kadan (group 1) have a relatively low Kcontent, clearly indicating the presence of a smectite admixture, the K content in other samples

Figure 11 Diffraction and microdiffraction patterns of the green layer in the wall painting from the Plasy monastery,Czech Republic (18th century). A, powdered green layer + plaster (conventional measurement); B, fragment,microdiffraction of the green layer from the top; C, cross-section, microdiffraction of the green layer from the side.



resembles that of pure celadonites. The IR spectra could help us to decide whether or not thosequestionable samples contain some smectite layers.

Figure 13 shows the region of the Si–O–Si(–Al) vibrations (950–1050 cm-1). While in theKadan reference sample H102 the vibration at ~1020 cm-1 indicates a smectite admixture beyondany doubt, in the Monte Baldo reference sample H219 this vibration is completely missing (seealso Fig. 5). When comparing the oil-on-canvas painting from the Franciscan monastery at Kadan(M0532, group 1), one of the paintings by Anton Kern (M0430, group 2) and an anonymouspainting from the monastery church at Milevsko, southern Bohemia (J0406, group 3), we seesome smectite vibrations in all of them, but their intensities are different. The highest intensityand thus probably the highest content of smectite layers was found in the painting actually locatedin the town of Kadan (M0532). The other two paintings (M0430 and J0406) definitely containsome smectites, but in lower quantities. The K content is relatively higher, but their IR spectradiffer from those of pure celadonites. This could indicate that in these two cases also, the greenearth pigment could have come from the regional source in Bohemia.

CONCLUSIONS

As the most famous earthy pigment in central Europe, the Bohemian green earth from the Kadanarea, Czech Republic, is characterized by a comparatively low celadonite content, accompaniedby interstratified celadonite/smectite or smectite as a single phase. This reflects the simultaneousexploitation of green earth from an upper horizon of greenish altered volcanic tuffs and from alower level with celadonite impregnations in and above limnic (freshwater) limestones. Themixture results from the limited availability of rich celadonite layers. While in the contact with

Figure 12 K, Mg and Fe contents in selected samples. Bohemian green earth reference samples: 1, 2, 19th-centuryproduction (sample H102 and Odin 1988); 3, 20th-century production (Neužil 1955); 4, samples available today on site(H146). Cyprus green earth reference samples: A, original celadonite (H218); B, commercial pigment available today(K17410). Verona green earth reference samples: C, original celadonite (H219); D, commercial pigment available today(K11000). Microsamples: black circles, works of art located at Kadan (M0433, J0507, M0532 and M0629); grey circles,paintings by Anton Kern (9828, 9845, M0430 and M0352); white circles, other paintings with a high Fe/Mg ratio (M0225and J0406).



limestones, celadonite and celadonite/smectite are both rich in iron, the Fe/Mg ratio decreasesgradually towards the upper zones of alteration, due to the presence of saponite, biotite andMg-carbonates. Simultaneously, the Ti content increases due to the presence of anatase, titano-magnetite and biotite. The presence of ferruginous smectites in Bohemian green earth fromKadan affects its colour significantly. Instead of the blue–green colour of the pure celadonitesknown from Monte Baldo, Italy, and from Cyprus, here a forest green colour is typical forceladonite/smectites, changing rapidly to olive green when the content of smectites increases inthe raw material.

In historical paintings, the Bohemian green earth from Kadan can be recognized by its higherFe/Mg ratio and some varying Ti content. In our set of 18th-century paintings from Czechcollections, only for those actually located in the town of Kadan can the use of local green earthbe regarded as certain (because of the high Fe/Mg ratio, the lower K content, the increased Ticontent and the presence of smectite admixtures). For all of the paintings by Anton Kern and fortwo other paintings, Kadan was identified as the possible provenance of the green earth, on thebasis of the increased Fe/Mg ratio and the presence of some smectite structures indicated by IRmicroscopy.

The results obtained from our work demonstrate that the direct mineralogical classification ofgreen earths in microsamples of colour layers from paintings is very difficult because of their

Figure 13 Part of the mid-IR spectra of selected microsamples compared to reference celadonite (H219) andceladonite/smectite (H102).



small size (less than 0.5 mm) and heterogeneity (admixtures of pigments). In addition to theprevailing green micas, chlorites have also been identified as pigments beyond any doubt bypowder X-ray microdiffraction in one 19th-century repaint.

ACKNOWLEDGEMENTS

The authors thank the restorers and students of the School of Restoration of the Academy ofFine Arts in Prague, Czech Republic, for providing samples; and Veronika Grünwaldová, SilvieŠvarcová and Tomáš Grygar for analytical measurements of green earths. This work was sup-ported by Czech Science Foundation (project No. 203/07/1324), by the Academy of Sciencesof the Czech Republic (M200320901 and AV0Z40320502) and by the Ministry of Educationof the Czech Republic (MSM6046144603). The investigations of Stefanie Gerzer werefinancially supported by Dr Georg Kremer, the founder and owner of the Kremer-PigmenteCompany.

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MINERALOGY OF BOHEMIAN GREEN EARTH PIGMENT AND ITS MICROANALYTICAL EVIDENCE IN HISTORICAL PAINTINGS