http://france.elsevier.com/direct/CULHER/

Journal of Cultural Heritage 7 (2006) 134–138

Case Study

* CorrespondinE-mail addre

jcoll.ceramica@m

1296-2074/$ - sedoi:10.1016/j.cul

EDXRF analysis of blue pigments used in Valencian

ceramics from the 14th century to modern times

Clodoaldo Roldán a,*, Jaume Coll b, José Ferrero a

aUnidad de Arqueometría, Instituto de Ciencia de Materiales. Universidad de Valencia, P.O. Box 22085. E46071 Valencia, SpainbMuseo Nacional de Cerámica y Artes Suntuarias "González Martí", Poeta Querol, 2. E46002 Valencia, Spain

Received 24 October 2005; accepted 20 February 2006

Abstract

EDXRF analyses of cobalt-blue pigments were performed on 73 pieces of Valencian ceramics from the beginning of the 14th century up tothe 20th century. In 67 of such samples, the pigment decoration was applied together with a tin opacified lead glaze cover on the clay body. Infive samples the pigment was applied on the clay body without a glaze cover. The comparison between EDXRF spectra from coloured and non-coloured areas contains information about the pigment composition. Elements like Mn, Fe, Co, Ni, Cu, Zn and As are identified as characteristicof blue pigments; different associations of these elements were found and correlated with the chronology of the samples. The results can be usedfor identifying the different types of cobalt ores employed in the manufacture of blue pigments and their provenance.© 2006 Elsevier SAS. All rights reserved.

Keywords: EDXRF; Non destructive analysis; Cobalt blue pigments

1. Introduction

When cobalt oxide became an important ceramic element inthe Iberian Peninsula, it had already been known and used inthe Middle East for several millenniums [1,2]. Chronologi-cally, the 13th century pottery productions from the Southeastare the first evidence of the use of cobalt ores as blue pigmentsfor ceramics in the Iberian Peninsula. Between the late 13thcentury and the early 14th century, the use of cobalt spreadacross the Mudejar potteries of the Christian kingdoms reach-ing Paterna and Manises in the Kingdom of Valencia [3,4].From all the productions of Medieval Valencian ceramics thosedecorated in blue are the least studied despite their large num-bers. The Valencian potteries of Manises and Paterna producedhuge amounts of blue-decorated pieces which became wide-spread utilities and whose production spanning from the Mid-dle to the Modern Ages.

g author.sses: clodoaldo.roldan@uv.es (C. Roldán),useo.mec.es (J. Coll), jose.ferrero@uv.es (J. Ferrero).

e front matter © 2006 Elsevier SAS. All rights reserved.her.2006.02.003

The analysis of these highly valuable materials requires theapplication of analytical techniques capable of providing infor-mation without altering or damaging the sample. The answers toquestions about the type and origin of the cobalt ores used inthese ceramics could be given by a systematic and exhaustivestudy of the blue pigment used by Valencian potteries. There-fore, although relative measurements for cobalt-blue pigmentsin European and Oriental ceramics have previously been madeusing several analytical methods [5–9], we only focus on thediscussion on the studies of the Valencian ceramics using theenergy dispersive X-ray fluorescence (EDXRF) method. Thispaper presents the preliminary results of a large study undertakenby the Archaeometry Unit of the Materials Science Institute ofthe Valencia University (UA-ICMUV) and the National Museumof Ceramics and Sumptuary Arts "González Martí" (MNC) tocatalogue the cobalt-blue pigments used in Valencian potterythroughout its history by means EDXRF spectrometry.

2. Material and methods

A portable EDXRF spectrometer was employed with a pal-ladium X-ray tube, and a 500 μm thick Peltier cooled Si-PIN

C. Roldán et al. / Journal of Cultural Heritage 7 (2006) 134–138 135

detector with an energy resolution of 170 eV (FWHM at 5.9keV) and an entrance beryllium window of 12.5 μm. For themeasurements, an X-ray beam with a potential of 35 kV and atube current of 100 μA was collimated with an aluminium cy-linder with a 2 mm internal hole. The tube and detector weredisposed with a 45° geometry and the ceramic sample surfacewas placed perpendicular to the X-ray beam with a sample-de-tector distance of 2 cm. A multi-channel buffer board AMP-TEK MCA Pocket was used to control data acquisition overan effective counting time of 300 s.

EDXRF analyses were conducted on 73 ceramic fragmentsfrom the 14th to the 20th century with blue decoration anddifferent typology, all of them from the MNC collection.Sixty-eighth ceramic samples came from the Valencian pot-teries of Manises-Paterna, two samples came from Catalonia,two samples came from Alcora (Castellón, Spain) and onesample came from Persia. All the samples were ceramic frag-ments with surfaces ranging from 20 to 100 cm2 and with co-balt-blue decoration applied together with a tin-opacified leadglaze, except for four samples that present the raw pigmentsover the clay body without glaze cover.

A stratigraphic section in an uncoloured area of a standardceramic piece consists of a glaze cover (less than 200 μm thickin our case) on a clay body, whereas in a blue coloured area thepigment melts into the glaze matrix as a result of the applica-tion mode (overglaze or underglaze) and the firing processes[9]. The identification of the main elements present in the co-

Fig. 1. EDXRF spectra from ceramic samples with cobalt-blue decoration. Continuoincluding glaze, pigment and clay body.

balt blue pigments and the calculation of their net fluorescencepeak areas were made by comparison and subtraction betweenthe EDXRF spectrum of a zone with glaze, pigment and claybody, and the EDXRF spectrum of a zone with just the glazeand the clay body. The WinQXAS code [11] was used in thecalculations.

Fig. 1 shows the overlay of the EDXRF spectra from twodifferent positions on four ceramic samples. The intensities ofelements like Mn, Fe, Co, Ni, Zn and As for the two spectradiffered significantly, which implied that these elements essen-tially came from the pigment. Since the glazes of the samplesare tin-opacified lead glazes, their contribution to the spectrumis marked by the presence of their main elements Si, Al, K, Ca,Fe, Sn, and Pb, while the contribution of the clay body is de-fined by the presence of Mg, Al, Si, K, Ca, Ti, Mn, and Fe[10]. Although there are some elements like Co, Ni, Zn, andAs which are exclusive to the pigment, others like Mn, Fe, andCu can contribute to the EDXRF spectrum when they are pre-sent on the glaze, the pigment, or the clay body. Trace ele-ments like In, Sb, Bi, and U, possibly linked to the local com-position of cobalt ores in Europe [6], have not been detected,maybe due to their low concentrations or, maybe due to theoverlapping of their fluorescence lines with those of the pig-ment’s major elements.

The measured intensity of the fluorescence lines from theceramic blue decoration cannot be correlated directly with theconcentration of the element in the cobalt ores since the pig-

us line: from an area including glaze and clay body. Dashed line: from an area

C. Roldán et al. / Journal of Cultural Heritage 7 (2006) 134–138136

ment is obtained from raw materials marketed under diverseforms (powder, glass ingots, etc.) and transformed with the ad-dition of other products for their use in ceramic decoration.Therefore, a qualitative analysis was performed to look for dif-ferences or resemblances in the distribution patterns of the ma-jor elements (Mn, Fe, Co, Ni, Cu, Zn and As) in the blue pig-ment, and to group the pigments on the basis of a similarassociation of these elements that help us to their classification.

3. Results and discussion

Fig. 2 shows the bar charts with the chronological distribu-tion of net areas of the Kα lines for Mn, Fe, Co, Ni, Cu and Znand the As Kβ line (since As Kα interferes with the Pb Lα linefrom the glaze). Cobalt and iron were found in all the analysedpigment samples. The presence of nickel became evident asfrom the 15th century and the presence of arsenic in the pig-ments was detected as from the 16th century. Copper and zincwere mainly found in samples previous to the 16th century.

Fig. 2. Bar charts showing the net areas of the elements Mn, Fe, Co, Ni, Cu, Znand As in the pigments of Valencian ceramics. The (*) marks the sampleswithout glaze cover.

Manganese was detected in the blue pigment of some ceramicsamples whose chronology covers the period from the 14th tothe 19th century.

A qualitative interpretation of the data shown in Fig. 2 al-lowed us to distinguish four groups of pigments that combinecobalt with different associations of the other elements.

Group A. Fe-Co-Zn association. Found in 16 ceramic sam-ples from the 14th and 15th centuries. They have characteristicspectra, like the one in Fig. 1 (group A), where Zn being theelement that identifies the group. This would show that thecobalt ores were extracted together with zinc blend [6].

Group B. Associations: Mn-Fe-Co-Zn (4 samples), Mn-Fe-Co-Ni-Cu (3 samples), Mn-Fe-Co-Ni (6 samples), Mn-Fe-Co(3 samples). The chronology of this 16-sample group goesfrom the 14th to the 19th century. Manganese is the elementthat identifies this group and the pigments have spectra similarto that in Fig. 1 (group B) where we can see that Mn, Fe, Co,and Ni are characteristic of the pigment. In view of such data,we could argue that asbolane (an impure mixture of hydratedoxides of manganese and cobalt) mixed with other mineralscontaining Fe, Cu, or Zn, were possibly used. But it is alsopossible that manganese has been added deliberately to obtaina darker blue or purplish shade, or to prevent the diffusion ofcobalt during the firing (manganese acts as an antiflux for thecobalt in the glaze) [13]. Another possibility is that the de-scribed associations with Mn may fit in with groups A, C,and D, whose characteristic elements are Zn, Ni and Ni-As,respectively.

Group C. Fe-Co-Ni-Cu, Fe-Co-Ni associations. The charac-teristic element in this group is Ni. Co-Ni-Cu associations werefound in 14 samples from the 14th and 15th centuries whileCo-Ni associations were found in 11 samples from the 15thto the 20th century. A representative spectrum of this groupis shown in Fig. 1 (group C), where we can see a clear evi-dence that Fe, Co and Ni are characteristic elements in the pig-ment. Such data suggest that the pigment used in the samplescould have been derived from the zaffre with minerals like er-ythrite ([Co,Ni]3[AsO4]2·8H2O) and/or smaltite ([Co,Ni]As3-2)but in a manufacturing process that roasts the mineral at hightemperatures and produces the loss of the arsenic [14]. Cobaltores associated with Ni but without As may have also beenused, as for example pyrites (FeS2), which have low cobaltand nickel percentages.

Group D. Fe-Co-Ni-As associations. Arsenic is the charac-teristic element in this 16-sample group. The samples date backto the 16th century and their representative spectrum is shownin Fig. 1 (group D). The pigments in group D could have thesame mineral origin as group C but a different manufacturingprocess. In this case, the arsenic remains in the pigment wouldbe explained if the cobalt ores were roasted at a lower tempera-ture than in group C [14]. Another possibility is to link this factwith a change of raw materials. However, these hypothesesrequire further analysis. Anyway, the presence of arsenic inthe cobalt pigments is not detected before the 16th centuryand could be used for dating ceramics.

Fig. 3. Chronology and grouping of the characteristic elements of the cobalt-blue pigments used in Valencian ceramics. The parentheses indicate the numberof samples of each group.

C. Roldán et al. / Journal of Cultural Heritage 7 (2006) 134–138 137

Fig. 3 shows our proposal for the grouping of the cobalt-blue pigments in Valencian ceramics with the chronology ofeach one of these groups. The pigments of the sixty-eighthceramic samples from the Valencian potteries of Manises-Pa-terna are included in groups A–D. This grouping is compatiblewith the different types of cobalt ores found by other authors inEuropean cobalt-blue glass and ceramic pigments from the13th to the 19th centuries [5,6,12]. With respect to the foreignsamples, the pigment used on the tile from Persia (14th cen-tury) corresponds to the Co-Zn grouping. The two samplesfrom Catalonia are included in group C (sample dated in the14th century) and in group D (sample dated in the 17th cen-tury). The Alcora, samples dated in the 18th century, presentthe Co-Ni-As association (group D). The classification of thesesamples is compatible with its origin and dating.

No reliable evidences exist about the supply of cobalt orefrom foreign or Spanish mines to Valencian potteries, but itwould not be far-fetched to think Valencian potters used it intheir productions. A hypothesis about the foreign origin are thecobalt mines of Germany and Middle-East, both well known inLate Antiquity [12,13]. Respect the hypothesis of the Spanishorigin, unfortunately, there are no references to cobalt oremining in Spain before the 18th century. Therefore, the onlyhypothesis left to relate pigment groupings with local produc-tions is to consider that the cobalt mineral sources used byValencian ceramists were in fact the cobalt sites recorded bythe prospective studies of the Spanish Geological and MiningInstitute within the framework of the Mining Research ActionPlan on "Cobalt in Spain" [15]. Should this hypothesis be plau-sible, the local cobalt ores used by Valencian potters wouldthen be [1,16]: asbolane (Mn-Co associations) and Co-Cu as-sociations from Chovar in the province of Castellón and closeto the Paterna and Manises potteries; erythrite from Guajar inthe Granada province; pyrites with a low Ni-Co content fromChodes in the Zaragoza province; arsenides and sulphoarse-nides like skutterudite (Co, Ni)As3 and cobaltite (CoAsS) fromGistain in the province of Huesca. Some of the samples ana-lysed in this work could have used cobalt-blue pigments man-ufactured with mineral associations from those sites; however,

there are not documentary or analytical evidence on their originand chronology of use.

4. Conclusion

Portable EDXRF spectrometry is a non-destructive analyti-cal technique successfully used in the characterisation of co-balt-blue pigments used in the decoration of Valencian cera-mics from the 14th to the 20th century. Applied to 73ceramic samples from the MNC, this technique allowed us toclass ceramic pigments into four groups corresponding to theassociations of cobalt with other minerals. Chronologicallyused in the 14th and 15th centuries only, Zn has been consid-ered a significant element of the pigment in Group A (Co-Zngrouping). The characteristic element in Group B is Mn; in thiscase, it could be cobalt minerals with presence of manganese inthe ore, or manganese deliberately added during the pigmentmanufacture to manipulate the blue shade or prevent cobaltfrom melting into the glaze cover. Groups C and D are definedby the presence of Co-Ni and Co-Ni-As associations, respec-tively. The presence of arsenic in blue pigments is seen in oursamples as from the 16th century, which is in line with otherworks on European ceramics with blue decoration [6,8,14].This fact may point to a change in the mineral used for pigmentmanufacturing or in the production process happened in theearly 16th century.

To establish a correlation between the origin of the analysedpigments and the foreign or Spanish cobalt mines, it would benecessary to carry out new and exhaustive studies.

Acknowledgements

Our acknowledgements go to the Ministerio de Ciencia yTecnología (Project Ref. BHA2003-05800) and the AgènciaValenciana de Ciència i Tecnologia (Project Ref. GRU-POS03/192) for their financial support. The authors wish tothank Dr. Claudio Seccaroni, ENEA (Roma) for his sugges-tions and useful discussions.

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