ABSTRACT. — The field of pigment analysis isexplored through a series of examples taken fromthe authors’ work experience. Some analyses areeasy to make, other may require the combination ofvarious techniques, still others require the use ofsophisticated equipment such a synchrotron orneutron diffraction analyses. Sometimes the resultsobtained do not agree with the accepted theoriesabout the use of specific pigments (e.g. EgyptianBlue), or a new, unexpected substance is found to beused as a pigment (e.g. magnesium oxalate or leadsulfate). Investigations into illuminated manuscriptsrevealed the presence of a variety of verdigris, onlymatched by our laboratory synthesis of thisvariant. The discovery that the binders used were ofsize, resulted in the presence of copper-proteinatepaints from reaction of the media with verdigris. Thered pigment in a manuscript by Von Ems was foundto be made from rhubarb, which is the first time thiscolorant has been detected from illuminatedmanuscripts. Examination of an Egyptian ushabti ofthe New Kingdom revealed a layer of tridymitewhite pigment overlying a calcite ground. Thisunusual discovery may signify that the Egyptianpigment palette is much more extensive thanpreviously thought.

RIASSUNTO. — Lo studio dei pigmenti è affrontatoattraverso una serie di esempi presi da esperienze diricerca. Alcune analisi sono molto semplici daeseguire, altre richiedono la combinazione di piùtecniche analitiche, altre ancora prevedono l’uso distrumenti sofisticati come il sincrotrone o analisi didiffrazione con neutroni.

A volte i risultati ottenuti non sono in accordo conle teorie già attestate sull’utilizzo di pigmenti specifici(es. il blu egizio), o si scopre l’utilizzo inaspettato dialcune sostanze per ottenere dei pigmenti (es. ossalatodi magnesio o solfato di piombo). Gli studi dimanoscritti miniati hanno rivelato la presenza di unavarietà di “verdigris” che è stata preparata, in questavariante, solo dal nostro laboratorio di sintesi. Lascoperta che i leganti erano nel giusto rapporto, èconfermata dalla presenza di pitture con proteinati dirame, prodotti dalla reazione del supporto con ilverdigris. In un manoscritto di Von Ems, si descrivela produzione del pigmento rosso a partire dalla piantadel rabarbaro, che è stato individuato per la primavolta in un manoscritto miniato.

Esaminando un ushabti egizio del Nuovo Regno èemerso un livello di pigmento bianco a tridimiteposto su di un fondo di calcite. Questa scopertainconsueta, può significare che la tavolozza deicolori Egizi è molto più ampia di quanto fin oraconosciuto. (tradotto dall’editor).

KEY WORDS: Pigment analysis, Maya Blue,Egyptian Greens, Verdigris

Per. Mineral. (2004), 73, 227-237 http://go.to/perminSPECIAL ISSUE 3: A showcase of the Italian research in applied petrology

Pigment analysis: potentialities and problems

GIACOMO CHIARI* and DAVID SCOTT

Getty Conservation Institute, Los Angeles, California and UCLA/Getty Archaeological and Ethnographic Conservation Program

* Corresponding author, E-mail: gchiari@getty.edu

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYand applied topics on Environment, Archaeometry and Cultural Heritage

INTRODUCTION

Practically all art employs pigments of onekind or another. This fascinating area of studyfrequently makes use of the latest scientifictests to establish their identity, yet the world ofpigments is a comparatively well-known one,which has been the subject of several classicstudies, such as the compilation by Gettens andStout, originally published in 1942. As longago as 1910, Laurie was able to write a usefulaccount of Egyptian pigments, yet such is theincrease in our knowledge that manydiscoveries since that time have resulted in theneed for more recent review, such as that forEgyptian pigments by Lee and Quirke in 2001.Despite the recent date of that review, we havediscovered the use in ancient Egypt of stillmore pigments, some of which will bediscussed in this short paper. There are manyhistoric accounts of how pigments were made,such as the compendium of principallyVenetian manuscripts by Merrifield (1849) and«La fabbrica dei Colori» (1986), whichcontains information about use andmanufacture.

For wall-paintings, we have a recent anduseful review by Howard (2003), whichconcentrates on the Medieval period, andreveals the complexity of some of the colourschemes employed by Gothic and Mediaevalartists. Over the last fifteen years a series ofvolumes on pigment has been produced by theNational Gallery of Art, Washington, whichpromise to remain standard works of referencefor many years to come (Feller 1986; Roy1993; Fitzhugh 1997), and which documentpreparation, identification, geographic locationand period of use for many of the importantpigments of ancient art.

The examination and characterization ofpigments is an interesting task for theexperienced conservation scientist, since someof them are relatively easy to identify, oncetheir characteristics and the history of their useis known and understood. The task, therefore ofanalyzing these pigments should not be toodifficult, since until recent times:

(a) Their number is limited to between 50-100 common types.

(b) The composition of most of them is wellknown and published.

(c) They can be categorized in a range ofsimple colours, so that the number of possiblealternatives is further restricted (e.g.: a fewblues, a few yellows or blacks etc.

Furthermore, one can make use of otherinformation such as the time and geographicprovenance of the work of art. In fact, it mayhave been retouched with pigments onlyavailable at a later date, in which case furtherinformation can be revealed.

All this is true when the pigments are pure andthe colour can be easily studied and identified.In this case, simple observation, by use of thenaked eye, or with the help of opticalmicroscopy can help to solve the problem ofidentity. Microscopy can be carried out directlyto observe the surface of the work of art, ormore efficiently by means of a cross section,which allows for the understanding of thesuccession of layers of pigment which may bepresent. In case of doubt, a relatively simple X-ray fluorescence analysis (XRF) can help tosolve the problem. For example, a red-orangepigment, for which XRF shows only mercury,allows for an almost certain identification:cinnabar. But it is not always that easy: in fact,X-rays penetrate beyond the surface, so thatinformation from many layers may be mixedtogether, with the result that all elements heavierthan sodium present in a multi-layered pigmentor paint layer, would be detected. Therefore,when the analysis point selected is part of awhite, pink, gray, yellow, orange, red or dark-purple colour, and XRF only reveals thepresence of lead, the choice still remains among:lead white PbCO3.Pb(OH)2, used both as whitepigment and as a substrate; massicot (yellow)PbO; litharge (orange) PbO; minium (red)Pb3O4 [PbO2.2PbO] or plattnerite (an alterationof lead white, which may be coloured darkpurple or almost black) PbO2. In fact, oxygen,carbon and hydrogen are not visible by XRF.

In such a complex situation one may have torefer to some other analytical techniques, such

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as X-ray diffraction or Raman spectroscopy,which can provide the extra informationneeded for an unambiguous identification ofthe compound rather than the elementalcomposition only, which is obtained using x-ray fluorescence analysis.

Another problem faced by the conservationscientist, is that the pigments which are presentmay be present in such low concentrations oramounts that they are not easily detected by theanalytical techniques employed. When usingmicro-invasive techniques one tends to remove

samples of the order of the milligram, using amicroscalpel or other small tool. This amountis usually sufficient to carry out an analysis,given the tremendous progress in analyticalinstrumentation provided by moderntechnology.

In the case of paintings, one must considerthe fact that the painted layer, which is theultimate goal of the analysis, is generally verythin (~ 50 µm) and may represent only a smallfraction of the sample removed. It is thereforenot unusual for the pigment of interest to fallbelow the sensitivity threshold of the methodemployed. In this case, the experiencedinvestigator may be able to work with smallclues, determined from the context of theinvestigation.

For example, on the facade of the CasaVelasquez, in Santiago de Cuba, very smallremnants of a turquoise blue were found. If thispigment proved to be Maya Blue, moreevidence of the trade of the pigment fromMesoamerica to Cuba could be ascertained. X-ray diffraction showed quartz, calcite andgypsum only (See Fig. 1a). These mineralsobviously did not justify the blue colour. Aftera simple acidic attack the pattern in Figure 1bwas produced, in which palygorskite (the claycomposing, together with indigo, Maya Blue,discussed later) is unambiguously present. Thepresence of this particular clay mineral in thecontext of this blue pigment is an importantclue for the investigator that the pigment is, infact, Maya blue.

If the pigments under investigation areknown and properly described from previousstudies the task of recognizing them is easier.Although very rarely, it is still possible to find«new pigments» or alterations such as bronzecorrosion products, never studied before. Sincethe majority of the techniques work bycomparison of the actual sample with standardreferences contained in databases, an unknownproduct may be very difficult to identify and tocharacterize.

As an example, let us consider the verdigrisgroup of pigments, which are based on copperacetate. These can be simply made by exposing

Pigment analysis: potentialities and problems 229

Fig. 1b – XRD of the same sample after acidic attack.Palygorskite was present.

Fig. 1a – XRD pattern of a blue paint residue as sampled.There is no evidence of Maya Blue.

copper sheets to stale wine or sour vinegar. Theverdigris pigment group, consisting of variouscopper acetates, have been important as paleblue or turquoise pigments from antiquity, butespecially in the late mediaeval and renaissanceperiod, when numerous alchemical recipeswere recorded for their manufacture.

Most of these recipes produce variations onthe copper acetates, usually basic salts, such asCu(CH3COO)2[Cu(OH2)]2, which is a lightblue colour, often used as pigments for avariety of purposes (Scott 2002). These basiccopper acetates could be recrystallized fromvinegar solutions to form the neutral copper (II)acetate dihydrate, Cu(CH3COO)2.2H2O, whichis green in colour, and was preferable in oilpainting, as being less susceptible toundesirable reactions or colour changes thanthe basic salts.

The identification of some later Medievalpigments based on the copper acetates can be acomplex task, because the alchemical recipesfor their preparation becomes increasinglysubtle. For example, a fifteenth-centurymanuscript, MS 1243 in the BibliotecaRiccardiana, Padua, (Merrifield 1849), givesthis recipe for a durable azure:

«Mix well one part of sal ammoniac andthree parts of verdigris with oil of tartar until itis soft and paste-like….then place it in aglassed vessel under hot dung for a day;afterwards you will find that the green hasturned to best blue…..»

«Oil of tartar» can be identified as potassiumcarbonate, and our synthesis in the laboratoryto replicate this preparation, produced amixture of blue and colourless phases, whichgave a complex set of X-ray diffraction datapartially matching potassium copper acetate2K(CH3COO).Cu(CH3COO)2 and ammoniumcopper acetate acetic acid C14H50CuN4O20.Thus, this recipe does indeed produce a durableblue colour, but identifying this particular blueconcoction in an illuminated manuscript wouldbe far for easy, especially since the pigmentmixture may react with the media. Thecomplete identification of some of these blueand green pigments in works of art continues to

be an interesting analytical challenge for theyears ahead.

For other pigments of unknown compositionit took years to unravel the mystery. Aninteresting example is that of Maya Blue. Thisbeautiful pigment was made by the Maya bymixing the clay palygorskite with indigo andwarming the mixture to around 100°C, whichproduced a blue pigment of exceptionalstability and permanence. To try to discoversome of the reasons for the remarkableproperties of this pigment and its structure, notonly the usual techniques of analysis have beenemployed, but also very advanced scientifictools from different disciplines, such as varioustypes of spectroscopy, thermal analyses,electron microscopy, synchrotron radiation andneutron radiation for highly advanceddiffraction studies. (See Chiari et al., 2003;Giustetto & Chiari, 2003 and referencestherein). For Maya Blue, even a trialexperiment for dating the pigment using 14CAMS was performed, unfortunately withdubious results.

WHY DO WE ANALYZE PIGMENTS AT ALL?

To understand and reconstruct the paintingtechnique (this has value on itself, just as inother branches of archaeometry).

To reconstruct the history of the use of agiven pigment: this may be of great help inindirectly dating a painting or even morefrequently, a retouch or repainting. A typicalexample is that of the «braghe» or censurepanels used to cover some nudity inMichelangelo’s Last Judgment. (Chiari, 1996).

To aid in the conservation of the work of artor to suggest possible problems or alterationsof existing pigments.

Sometimes one can discover unexpectedresults: for example, Egyptian blue is one of themost widely utilized pigments of Egyptian andWestern civilizations up to the fall of RomanEmpire. It was widely used throughout theAncient Middle East, starting from about 3600BC, when it was first synthesized. It was

G. CHIARI and D. SCOTT230

obtained by heating at 800-900°C a mixture ofcalcium carbonate, silica and copper compounds(possibly bronze scrapings). It is possible thatthe Egyptian Blue «secret» traveled along theSilk Road and reached China, where themanufacture was readapted to the local mineralsources. Han Blue, in fact, contains bariuminstead of calcium, and has more or less thesame colour, while its homologous membercontaining strontium is Han Purple. It isuniversally accepted that the manufacture ofEgyptian Blue, in spite of the recipe left byPliny, was lost with the advent of the MiddleAges. Only in the second half of the 20th centurywas the recipe recovered thanks to modernanalysis (Schippa and Torraca, 1957), whichproved the formula to be CaCuSi4O10, identicalto the rare mineral cuprorivaite (Pabst, 1959),although several 19th century scientists werebeginning to unravel the truth, such as SirHumphry Davy who wrote about theconstitution of Egyptian blue in 1815, and wasalso able to synthesize the pigment. Plinymentioned, as ingredients, sand and Cucompounds, without specifying if the sand waslime or silica based. Very likely, the sand thatPliny saw adding to the mix contained bothsilicon and calcium. The Renaissance trials toduplicate the process failed because the sandadded was either silica- or lime-based. Thereforethe unequivocal finding of Egyptian blue on apainting of the 12th century (Nicolaus JohannesLast Judgment in the Vatican Museums,unpublished result) may be puzzling. Thepigment is not occasional or attributable to a fewsmall retouches. On the contrary it constitutesthe whole dark blue sky in the background. Dowe have to, on the basis of one finding, changecompletely the theory regarding the history ofEgyptian blue? Perhaps we should, if all otherpossible explanations of a well documented factcan be excluded. In this case there is apossibility that the pigment derives from anancient small object made of sintered Egyptianblue, (of which there are many examples) andtherefore the direct synthesis of the pigment isnot the only possible interpretation.Nevertheless, there is a suggestion that in the

12th century somebody knew how to synthesizedthe prestigious pigment. For this reason it isimportant to analyze works of art and cumulateour knowledge, because only when severalfindings of this type will corroborate each other,an important chapter of art history may have tobe re-written.

At least in one specific case a pigment(haematite) when used on mural paintings cangive directly the date of a painting. Thistechnique is recent and not completelydeveloped (since it requires a non trivialcalibration procedure). (Chiari & Lanza, 1997,1999). It is based on the fact that the smallgrains of haematite are magnetized, andtherefore can preferentially orient themselvestoward the magnetic pole while suspended inthe medium used to execute the muralpaintings. When the paint dries, the grains aretrapped in that position (Pictorial RemanentMagnetization, PiRM) and, provided that nochange in orientation of the support took placebetween the moment of painting and themeasure, one can retrieve the position of themagnetic pole by statistically interpreting theresults of a number of samples (at least 7-8).The sensitivity of the method depends uponseveral factors, beside the measurement errors,such as the shape of the curve of the magneticpole movements at the time to be dated. TheSecular Variations of the magnetic poleposition are not well known due to the lack ofprecise data, but at least from 1820 on (whenprecise measurements started to be carried out)they are large enough to guarantee a sufficientprecision (10-50 years). The winding of thecurve creates intersections and thereforemultiple dating for the same magnetic values.For this reason, some ideas of the date may berequired to discriminate amongst variouspossible dates. The application of the method islimited since it is rather destructive (multiplesamples of about 1 cm2 need to be retrieved forthe measurement). In many situations though,given the extremely common use of haematitefor mural paintings, both in fresco and tempera,it is possible to find borders or solid redbackgrounds that can be reasonably sacrificed

Pigment analysis: potentialities and problems 231

in order to obtain a dating that cannot beotherwise achieved.

HOW DO WE ANALYZE PIGMENTS?

The number of techniques now available islarge. The specialist can therefore pick andchose according to the type of object to beanalyzed, its value, the importance of theinformation retrievable, the instruments whichare available, the time and the budget for thework. In the last few years the technologicaladvance in the instrumentation was such thatthe amount of sample needed for an analysis isoften under a milligram, thus allowing one toclassify most techniques as micro-invasive.Furthermore, the number of techniques whichcan be called non-invasive, since they do notrequire a sample to be taken at all, increasesevery day. Often the information retrievableusing non-invasive techniques is not complete,but the use of more than one complementarytechnique normally produces the requestedresults. The most important thing for theanalyst is to be result-oriented and notinstrument bound (although one tends to benaturally attached to the technique that oneuses more frequently and, knowing it better, toattribute to it a larger capability than it has).

Often the simple techniques of opticalexamination and micro chemical tests canprovide excellent results with pigmentidentification problems. This is because of thelimited number of them and the acquiredexperience of the optical microscopist, whichrenders polarized light microscopy such auseful tool for examination and identification.

A trained eye can recognize without faultseveral pigments. This may lead to thetemptation of just «trusting your eyes». Thatcan be dangerous because many times, togetherwith the main pigment that one may be able torecognize at sight, there are smaller quantitiesof other pigments, which can be interesting. Inother instances our eye may completely fail. Itis the case of a curious finding, still notunderstood at all: in the small Church of San

Fiorenzo, in Bastia Mondoví (Piedmont, Italy)there is a 1474 fresco cycle: on the cupola aChrist figure is holding a white scroll. The eyeimmediately suggests «Bianco di SanGiovanni», which is a form of good-qualitycalcite, but since it could have been lead whiteas well, a small sample was taken to beanalyzed by XRD. Very surprisingly it turnedout to be glushinskite, a magnesium oxalate.The most simple explanation was that themortar contained magnesium and the oxalatewas the result of a reaction similar to the oneproducing weddelite or whewellite (the twowell studied calcium oxalates). Analysis of themortar showed that it was made of pure calciteand glushinskite was limited exclusively to thewhite manuscript. Other white parts in thesame painting only contained calcite (pointingto Bianco di San Giovanni as white pigment).These types of findings are intriguing, puzzlingand many times are not published at all (it ishard to write even a short paper on a singleanalysis, quite simple per se, leading to asporadic finding). On the other hand publishingsporadic result may be the only way to groupthem with those of other researcher anddetermine if they are indeed a simple curiosityafter all, or the result of the application of apreviously unknown technique, although rare.In the case of San Fiorenzo, the onlyexplanation one can imagine for the result isthat a painter in 15th century found some puremagnesium oxalate (or some alchemistprepared some for him) and used it as apigment. But until at least some more examplescan be found, the whole matter remains asimple curiosity. Another example of this typeis the find of anglesite (lead sulfate) on alunetta attributed to Antelami and located onthe entrance of the Sant’Andrea church inVercelli, Piedmont, Italy (Fig. 2a). Thepreparation layer for all faces and hands of theprotagonists of the scene is made of lead white.The background, of a slightly warmer tonality,more similar to the stone used for the carving isinstead anglesite, i.e. lead sulfate. On the top ofthe lunetta there is a frieze portraying flowersand an angel who takes to heaven San Andrea

G. CHIARI and D. SCOTT232

soul. All faces and hands are made withanglesite. This substance is not known to be apigment, to our knowledge. Was it broughtback from England by the bishop who built thechurch? Was it commonly used but we do notknow it because we trust our eyes too much?

Another interesting example where simpleassumptions as to the nature of a white pigmentwere proved to be erroneous concerns the caseof an Egyptian New Kingdom wooden paintedUshabti in the collections of the Department ofReligion, University of Southern California. Amicro scalpel was used to obtain a minutesample of pigment which was mounted forpolarized light microscopy. This sample provedto have all of the optical characteristics ofcalcite, and since this is well-known to havebeen extensively employed by the Egyptians asa white ground and paint, this seemed perfectlyreasonable and in accord with visualestimation. We decided to check on theidentification of calcite by x-ray diffraction ofanother micro sample from the Ushabti, but

this produced a surprise: the white pigmentidentified proved to be tridymite, which is ametastable form of quartz, usually associatedwith the high-temperature decomposition ofalpha-quartz. As a result of this, we decidedthat this apparently simple white pigmentrequired further study: The pigment microsample used for the diffraction work wastransferred to a stub and examined in lowvacuum mode in the ESEM. The first scanningelectron photomicrograph showed the presenceof rhombohedral calcite crystals, which wasconfirmed by EDAX analyses.

Pigment analysis: potentialities and problems 233

Fig. 2 – Sant’Andrea in Vercelli. Lunetta attributed toAntelami. The white pigment on the faces of the mainscene is lead white, as expected. In the top frieze and in thebackground anglesite (lead sulfate) was used instead.

Fig. 3 – Egyptian Ushatbi. The white paint with anunderlayer of calcite and a finishing layer of tridimite canbe seen near the waist.

However, when the particle on the stub wasrotated into a different orientation, the scanningelectron image revealed an assemblage of tinyhexagonal prismatic crystals, obviously notcalcite. Further study showed that theEgyptians had used calcite for the ground ofthe Ushabti, and had employed tridymite as thefinal white paint, since the white of thetridymite is even purer and more intense thancalcite. The two whites can be clearlydifferentiated by crystal morphology.

Tridymite actually forms hexagonal crystals asa pseudomorph after beta-tridymite, and isitself monoclinic, but the preservation of thepseudohexagonal variety is quite common andan ESEM photomicrograph of the tridymitefrom the Ushabti is shown in Figure 3. Wheredoes this tridymite come from? The mineralforms from quartz on heating to temperaturesbetween 870-1470°C and can be found as anatural mineral or could have been preparedfrom heated and crushed quartz. Tridymite is ametastable polymorph of quartz, but the factthat it has survived here on the EgyptianUshabti shows that it can remain stable forthousands of years without change. Untilfurther examples are found and studied, we willnot know how common the use of this whitepigment was in ancient Egypt. (See Fig. 4).

Another difficult case is provided by aGerman illuminated manuscript of the 14thcentury AD in the collections of the J. PaulGetty Museum (Scott et al., 2001). Thismanuscript, the Barlaam und Josaphat, byRudolf Von Ems, with illustrations by theDiebolt Lauber Atelier dates to 1469 AD. Themanuscript is on paper, which has becomeembrittled, with the result that many smallpieces have broken away. One of thesedetached fragments with red, light green anddark green pigment was available for taking amicro sample for further study. The redpigment exhibits a pronounced craquelure,including severe cupping and cracking in someareas and a fine white precipitate, having theappearance of a salt, can be seen on the redunder the binocular microscope. We attemptedto identify the green pigment using in situexamination on a Siemens D5005 x-raydiffractometer equipped with Gobels mirrors.This was not successful in giving a gooddiffractogram, given the extremely smallamount of pigment. We next tried FTIR withsimilar problems: no identification could bemade from the spectrum obtained. Applicationof 0.2 microlitres of water also failed toproduce any extractable components. Becauseof these difficulties we prepared a microsample of the green pigment for polarized light

G. CHIARI and D. SCOTT234

Fig. 4 – View under the ESEM of the tridymite crystalsforming the outer white pigment layer overlying calcite inthe Ushabti shown in Figure 3.

Fig. 5 – Verdigris curved crystal array of acicularaggregates (cross polars x120) from a laboratory synthesisof the verdigris used on the German Von Ems manuscript.

microscopy, which showed particles of RI lessthan 1.662 which were not malachite, since therefractive index of malachite is greater nor ofchrysocolla, whose colour and conchoidalfracture are characteristic. A micro sample ofthe green pigment was then examined usingDebye-Scherrer x-ray powder diffraction, whena reasonable set of d-spacings could bemeasured, but once again there was a problem:the set of d-spacings did not match any knownmineral defined in the ICDD files or otherpigment types published in the conservationliterature. The problem was only solved sincefor the three years prior to this study we hadbeen working on issues of identification and synthesis of verdigris pigments, both from literature sources and historical recipesthat were replicated in the laboratory. The XRD data showed a close match toverdigris salt B4, of nominal compositionCu(CH3COO)2.Cu(OH)2.5H2O which is awater-soluble verdigris variety, made by theaction of sour vinegar on copper strips at highhumidity. The binder that had been used forthis pigment was found to be egg by GC-MS,and since the proteins in the egg mediainteracts with the verdigris pigment to producea copper proteinate green, this could help toexplain some of the difficulties of generalidentification, as well as the embrittlement ofthe paper support due to the leaching of thesoluble acetate and cupric ions, well-known tobe very damaging to cellulose fibers. A darkergreen copper-containing pigment layer whichhad been applied over this underlying greenwas also examined and its binder determined tobe glue by GC-MS analysis. This material,which is non-crystalline appears to be madefrom a verdigris pigment dissolved in glue tomake a thicker, more opaque dark green paintto apply over the verdigris in egg temperamedium used for larger green areas of thedesign. The red pigmented area also proved tobe very difficult to identify. ESEM studiesshowed that the pigment consists of a veryuniform layer with sharp-edged cleavage.ESEM-EDS examination revealed mostlycalcium, aluminum, sulfur, oxygen and carbon

as the major elements present. The results offurther studies suggested that an organic redpigment had been laked onto an aluminum-based mordant, such as alum, (potassiumaluminum sulphate), which was well-knownfrom early periods. Initially we used non-destructive florescence spectrometryemploying remote sensing with a fiber opticprobe to identify the red colourant. This wasnot successful and our attempts to match amicro sample employed for UV/Visspectroscopy with known standards also failed.We eventually succeeded using thin layerchromatography. The sample was evaluatedtogether with thirty-four standards, and a matchwas found with rhubarb. To further evaluatethe data from the UV/Vis spectrometry, a newspectrum of rhubarb root in concentratedsulphuric acid was run and found to be a closematch to our manuscript sample. Rhubarb root(colouring agent chrysophanic acid), is a red ineither acidic or alkaline conditions. Sincerhubarb was used as a fabric dye it is possiblethat this pigment began as a clothlet dye thatwas then extracted and laked with alum for useon the manuscript. The palette of this Germanmanuscript is quite restricted and uses a limitedrange of colours and inexpensive materials;which may explain the use of simple verdigrisand rhubarb as the principal palette for thisimpressive manuscript. Instead of expensivecinnabar and ground malachite, the restrictedpalette makes good use of what were probablylocally made materials. The description of thisanalysis may give an idea of how difficult theidentification of a pigment could be. (See Fig. 5).

Further difficulties with green pigments werefound with a late Greco/early Roman Egyptiancartonnage fragment from the ArchaeologyResearch Collection of the Department ofReligion at the University of SouthernCalifornia. A light green was analyzed by XRFin situ and showed strong peaks for iron, withminor peaks for potassium and magnesium,suggesting that this pigment is a green earth.XRD also in situ, using a Siemens D5005 andGobels mirrors showed that this green was amixture of celadonite and glauconite, typical

Pigment analysis: potentialities and problems 235

for green earth pigments , but very rarelymentioned in Egyptian contexts where theusual greens encountered are malachite orEgyptian green frit. The eye of Horus on thiscartonnage is painted in a dark olive-colouredgreen which is an unusual shade, and has inplaces decayed to a brown discolouredsurfaced. XRF analysis showed the presence ofcopper, while polarized light microscopyrevealed isotropic particles, clearly non-crystalline which could either be a copperresinate or proteinate. GC-MS analysisrevealed the presence of a glue media and thepresence of copper in the green pigmentsuggests that it was either made with averdigris pigment dissolved in glue to make acopper proteinate, or the verdigris has reactedwith the glue media over time to form thecopper proteinate green. Since the reactionbetween verdigris and glue is fairly rapid, thiswould soon have been noticed and may havebeen deliberately employed by the ancientEgyptians.

CONCLUSIONS

As the above examples show (and the list canbe much longer) the task of identifyingpigments on art objects can be very interestingand challenging. It is a common mistake tothink that the number of pigments used by manis limited and therefore their identification isstraightforward. In fact, often what remains onthe decorated surface is not the originalpigment but an alteration, which depends uponthe substances which came in contact with thework of art accidentally, from the exteriorworld, or that were used during themanufacture of the object for various tasks.When this happens in general any referencestandard from the pigments class is lost and thesearch extends to a much larger set ofchemicals. Unfortunately, all the restrictions insampling dictated by the necessity of nonintrusive analysis remain. This is perhaps why,beside the beauty of dealing with wonderfulmasterpieces, pigment analysis still remains avery fulfilling and challenging endeavor.

REFERENCES

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CHIARI G., GIUSTETTO R. and RICCHIARDI G. (2003)— Crystal structure refinements of palygorskiteand Maya Blue from molecular modelling andpowder synchrotron diffraction. Eur. J. Mineral.,(in press).

CHIARI G. and LANZA R. (1997) — Pictorialremanent magnetization as an indicator of secularvariation of the Earth’s magnetic field. Phys.Earth Planet. Int., 101, 79-83.

CHIARI G. and LANZA R. (1999) — Remanentmagnetization of mural paintings from theBibliotheca Apostolica Vatican, Rome). J. Appl.Geophys., 41, 137-143.

DAVY H. (1815) — Some experiments andobservations on the colours used in painting bythe ancients. Philosophical Transactions of theRoyal Society of London 105 97-124 W. Nichol:London.

FELLER R.L. (1986) — Artists’ Pigments: ahandbook of their history and characteristics Vol.I. National Gallery of Art: Washington & OxfordUniversity Press.

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