analyses of copper- and beeswax- containing green paint on egyptian antiquities

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Analyses of Copper- and Beeswax- Containing Green Paint on Egyptian AntiquitiesAuthor(s): Vincent DanielsSource: Studies in Conservation, Vol. 52, No. 1 (2007), pp. 13-18Published by: Maney Publishing on behalf of the International Institute for Conservation ofHistoric and Artistic WorksStable URL: http://www.jstor.org/stable/20619475 .

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13

Analyses of Copper- and Beeswax

Containing Green Paint on Egyptian

Antiquities Vincent Daniels

Four samples of waxy, green paint from ancient Egyptian objects have been examined. Analysis was performed using infrared and

X-ray fluorescence spectroscopy Each sample was found to contain beeswax and copper. An apparently identical paint could be

made by heating together copper and beeswax at 1 0 C or by mixing copper(II) acetate with molten beeswax. The copper salts of various palmitic and stearic acids were made and their infrared spectra obtained, which led to the conclusion that the green paint contained copper salts of beeswax fatty acids.

INTRODUCTION

Four objects bearing translucent emerald green coloured

wax were analysed. The wax samples varied in the inten

sity of colour but in each case were of an emerald green

hue, which is very unusual in ancient their Egyptian

antiquities, and were thus analysed to ascertain their

nature. Green pigments previously reported on ancient

Egyptian objects have been reviewed by Green [1]. Mixtures of blue and yellow pigments, prepared using

Egyptian blue and (yellow) orpiment, have been found.

A green frit compound containing copper wollastonite

has been found and also mixtures of Egyptian blue with

a green basic copper chloride or a material similar to

malachite. However, both of the last two materials have

been reported as degradation products of Egyptian blue

and may not have been used intentionally. Scott et al.

have described an example of the use of green copper

proteinates on an Egyptian cartonnage object from the

Graeco-Roman period [2]. Scott also describes other

more recent examples of the use of copper proteinates and a method of production which involves reacting

proteins with verdigris [3]. Most pigments, if mixed with beeswax as a binder

would produce an opaque paint layer, unlike the

translucent paint being investigated. In Egypt, beeswax

was used as a binder for paints from the eighteen dynasty

(1550-1295 bc), although it was also used as glaze over

pigment. Encaustic painting was used in the Roman

period and used pigments intimately mixed with wax

[4]. Beeswax has been found in a wide range of ancient

Egyptian artefacts and has a melting point of about 64 C.

It contains esters (70-80%), free acids (12-15%) and

hydrocarbons (10 16%), with minor amounts of diols,

cholesteryl esters and pollen pigments [5]. Of the free acids, the major component is tetracosanoic acid (31%).

On ageing, hydrolysis of the ester component would

produce more free acid and the associated alcohols.

Fatty acids such as those in beeswax may react with

copper metal or some copper compounds. Gunn et al.

have studied the ability of linoleic acid, a C18-fatty acid,

to remove copper from copper acetate and from basic Received August 2005

STUDIES IN CONSERVATION 52 (2007) PAGES 13-18



14 V. DANIELS

copper carbonates such as verditer [6]. Copper(II) acetate

and linoleic acid were placed together in a reaction

solution of dichloromethane under an inert atmosphere. The formation of a soluble copper carboxylate was

accompanied by an increase in the infrared band at

1610 cm"1. Electron paramagnetic resonance spectroscopy showed that the complex was binuclear or polynuclear. Linoleic acid did not react when in contact with verditer

for several days. Solutions of copper linoleate in dichloro

methane exposed to air lost their blue-green colour; a

decarboxylation mechanism was suggested for this.

Robinet and Corbeil have collected together data

to characterize metal carboxylates (metal soaps) that

may occur on artefacts as a result of ageing [7]. X-ray diffraction (XRD), Raman and infrared data were

presented for several free acids and their metal soaps. Infrared spectroscopy was selected as the best method for

characterizing these compounds. A very strong band at

1590 cm-1 in the infrared spectrum, attributed to COO"

asymmetric stretching, was measured for copper st arate,

ol ate and linoleate; this band does not occur in the

free acids. The position of the band is at 1604 cm"1 for

copper acetate, but for lead soaps it is a doublet at 1550

and 1400 cm"1.

The reaction of copper with organic acids is well

documented; indeed, the presence of volatile organic acids in the air during museum storage can produce

copper acetate deposits on copper alloy objects [8]. Schrenk has noted the patina on Benin bronzes previ

ously coated with oils such as neat's foot [9]. Green

copper ol ate and copper st arate were identified by XRD. In an experiment, a mixture of oleic, stearic and

palmitic acids was placed on a polished copper plate. Over a two month period in ambient conditions, green

products were formed. The Fourier transform infrared

(FTIR) spectrum showed the elimination of the 1708

cm"1 band (fatty acid carbonyl) and the appearance of a

new band at 1585cm"1, attributed to a salt of a fatty acid

carbonyl. Moffatt et al. have identified a mixture of copper pal

mitate and copper st arate used as a green pigment on

two late eighteenth-century Naskapi native American

artefacts: a coat and a pair of leggings [10]. The pigment identification was performed by FTIR. No crystallinity

was detected by XRD.

OBJECTS EXAMINED

The four objects examined were all from the collections

of the Department of Ancient Egypt and Sudan at the

British Museum:

EA 53967. Calcite rais-shabti of Amenemheb twen

tieth dynasty c. 1186 1069 bc with streaks of applied decoration in green waxy paint (Figure 1). EA 60745. Corn mummy, late period, after 664 bc,

30 cm long with a black body and with a waxy green

partially gilded face. The beard had broken off (Figure 2). EA 13808. Shabti of Ramasses VI, apparently cal

cite base with patches of transparent green waxy material also with smaller areas of a similar material

but coloured brown with visible pigment particles

(Figure 3). EA 6693. Coffin of Hineb, twenty-sixth dynasty or

later, after 664 bc, richly painted in various colours

with a green face (Figure 4).

EXPERIMENTAL

Infrared spectroscopy

Small samples of the green waxy material were removed

for analysis. FTIR spectroscopy showed that beeswax was

present in all four samples. However, careful examination

of all the peaks in the spectra showed that, relative to

pure beeswax, the samples had diminished broad peaks at 1711 and 1630 cm"1 and an additional sharp peak at

1587 cm"1. Figure 5 shows a detail of the FTIR spectra for beeswax (lower spectrum) and the green wax from

EA 53967 (upper spectrum). All the spectra of the green

waxy paints were similar except that EA 60745 also

showed peaks due to the presence of calcite.

X-ray fluorescence

X-ray fluorescence spectroscopy showed the presence of copper in all the green samples, with the addition

of a trace of potassium and calcium in EA 53967, a

large amount of calcium (from calcite) and a trace of

potassium and iron in EA 60745, and a trace of iron in

EA 13808. No elements were detected in a reference

sample of unbleached beeswax.

PREPARATION OF BEESWAX GREEN

It was found that a visually similar green material

could be prepared by heating copper metal with

beeswax, and it seemed likely that copper as metal or

part of a compound might react with one or more

of the components of beeswax to produce the green colouration.




ANALYSES OF COPPER- AND BEESWAX-CONTAINING GREEN PAINT ON EGYPTIAN ANTIQUITIES 15

iO : **f

'

Figure 1 EA 53967. Calcite figure with patches of green wax.

Figure 3 EA 13808. Shabti figure with a green waxy coating.

>.

F/gi/re 2 4 60745. Corn mummy with a dark green face.

/ /,- m

Figure 4 EA 6693. A coffin with the face and other areas coloured with

green waxy material.




16 V. DANIELS

1800 1750 1700 1650 1600 1550 1500 Wavenumbers (cm-1)

Figure 5 Detail of the FTIR spectrum of beeswax (lower spectrum) and the green wax from EA 53967 (upper spectrum).

Figure 6 Beeswax heated in a crucible made of folded copper sheet.

Beeswax was added to samples of copper foil,

copper(II) oxide, powdered malachite and normal

copper(II) acetate. Both unbleached and bleached

beeswax were used, making eight samples in all. The

mixtures were placed in glass sample tubes and placed in

an oven at 110 C for 20 hours. At the end of this period an intense emerald green colour had developed in the

tubes containing copper foil or copper acetate, a small

amount of colouration was produced with the malachite

and a smaller amount with the copper oxide. There was

no difference between the behaviour of bleached and

unbleached beeswax.

Heating beeswax in a small copper crucible on a hot

plate only produced a small quantity of the green product

despite heating to the point where the beeswax started to

smoke. On the hotplate, little colouration was produced in a mixture of beeswax with either powdered malachite

or copper(II) oxide; however, normal copper(II) acetate

did produce a good green colour. Figure 6 shows the

result of heating beeswax, overnight, at 110 C in a

crucible made from folded copper sheet.

The FTIR spectra of the green wax made from bees

wax and copper foil or copper (I I) acetate were similar

and showed the same spectral features as those from the

objects. The mechanism for formation of the green colour

appeared to be the reaction of a component of the bees

wax with copper. After being in the oven, the metal

of the copper crucibles above the beeswax appeared oxidized with an overlaying translucent black film, while

the copper below the beeswax was shiny and clean. It

was decided to investigate whether the beeswax reacts

directly with the copper or with what appears to be

oxidized copper. Two copper crucibles half-filled with

beeswax were prepared and placed in 1000 mL screw




ANALYSES OF COPPER- AND BEESWAX-CONTAINING GREEN PAINT ON EGYPTIAN ANTIQUITIES 17

topped jars capable of an airtight seal. One of the jars had two Ageless1 sachets added to it and the top securely

closed; the other was left open to the air in the oven.

The sachets are designed to absorb oxygen.Thus, one jar contained normal air and the other air with the oxygen reduced to below 0.01%. The jars were placed in the

oven at 110 C for 16 hours, and at the end of this period it was found that the beeswax in the oxygen-free jar had

no green colouration while that in the open jar was a

rich green. The copper salts of palmitic acid and stearic acids

were prepared by placing samples of the free acid in a

copper foil container and leaving them in the oven, as

described above. A bluish-green material was formed.

FTIR spectra of this material and the free acids were

obtained and compared. A new sharp peak at 1590 cm-1

appeared in the copper salts.

DISCUSSION

There is little doubt that the green colourant in the

beeswax on the objects is due to a compound made by

copper metal or a copper-containing compound reacting with fatty acids in beeswax. The FTIR data provide the

main evidence for this, which is supported by elemental

analysis. The preferred method of preparation in ancient

Egypt may never be determined. However, beeswax

left in a copper vessel becomes green if stored in the

region of 110 C overnight; this is an extremely easy and

convenient method of preparation and eliminates the

need for an intermediate copper salt. In ancient Egypt,

temperatures in this region may have been obtained by

placing objects close to kilns or ovens, rather than in

them. The method of adding copper acetate to beeswax

works equally well but seems more complicated and

probably was less easily discovered. However, copper

proteinates have also been detected on ancient Egyptian

artefacts, and in more recent times these were produced

by the interaction of verdigris with protein. The product of the reaction of copper(II) acetate with beeswax

produces an identical FTIR spectrum to that for the

product obtained by heating copper metal with beeswax

[3]. Free acetic acid produced by the reaction of copper acetates with fatty acids would be lost to the atmosphere and identical products could indeed be formed.

This work suggests that copper needs to form an

oxide before reacting with the fatty acids in the beeswax.

If so, this might explain why the beeswax only reacts with

the copper overnight, rather than in a shorter period of

heating at higher temperature, as oxygen would need

to dissolve in the wax in a rate-determining step. The

possibility that a beeswax binder-may have reacted with

a copper-containing pigment over thousands of years to

form the green cannot be eliminated.

CONCLUSIONS

A green beeswax paint, hitherto unreported, has been

found on several artefacts from ancient Egypt and has

been identified as beeswax containing a compound formed by the reaction of copper with fatty acids in the

wax. An apparently identical wax was easily produced in

the laboratory by two methods, the first was storage of

copper and molten beeswax at around 110 C overnight or longer, the second was to dissolve normal copper(II) acetate (verdigris) in molten beeswax.

ACKNOWLEDGEMENTS

The author would like to thank Annie Broderick,

previously of the British Museum's Department of

Conservation and Scientific Research, and John Taylor,

Department of Ancient Egypt and Sudan, British

Museum, for first pointing out the examples of green

wax, and providing slides and information about the

objects; and Dr Rob Law, Chemistry Department,

Imperial College, London, UK, for his interest and

encouragement. All the practical work mentioned in this

paper was carried out in the British Museum. Thanks

are also due to Eileen Buckley-Dhoot and Agathi

Kaminari, Kingston University, London, UK, for

their interest in this work, especially to the latter for

performing the oxygen-free experiment.

ageless is a trademark of the Mitsubishi Gas Chemical Company Inc. More information on Ageless can be found at www.mgc.co.jp/ eng /ageless/.

REFERENCES

1 Green, L.R., 'Recent analysis of pigments from Ancient Egyptian artefacts', in Conservation in Ancient Egyptian Collections, ed. C.E.

Brown, F. Macalister and M.M.Wright, Archetype Publications, London (1995) 85-91.

2 Scott, D.A., Dennis, A., Khandekar, N., Keeney, J., Carson, D., and Dodd, L.S.,'An Egyptian cartonnage of the Graeco-Roman

period: Examination and discoveries', Studies in Conservation 48

(2003) 41-56.

3 Scott, D.A., Copper and Bronze in Art, Getty Conservation

Institute, Los Angeles (2002) 298-299.




18 V. DANIELS

4 Lucas, A., Ancient Egyptian Materials and Industries, Histories &

Mysteries of Man Ltd., London (1989) 352-353.

5 Codd, L.W., and Dijkhoff, K., Materials and Technology, Vol. V,

Longman, London (1972). 6 Gunn, M., Chottard, G., Riviere, J.-J., and Chottard, J.-C,

'Chemical reactions between copper pigments and oleoresinous

media', Studies in Conservation 47 (2002) 12-32.

7 Robinet, R., and Corbeil, M.-C, 'The characterization of

metal soaps', Studies in Conservation 48 (2003) 23-40.

8 Thickett, D., Bradley, S., and Lee, L., 'Assessment of the risks

to metal artefacts posed by volatile carbonyl pollutants', in

Metal 98: Proceedings of the International Conference on Metals

Conservation, Draguignan, France, ed. W. Mourey and L. Robbiola,

James and James, London (1998) 260-264.

9 Schrenck, J.L., 'Corrosion and past protective treatments of

the Benin bronzes in the National Museum of African Art', in Materials Issues in Art and Archaeology II, ed. P.B. Vandiver,

J. Druzik and G.S. Wheeler, Materials Research Society,

Pittsburgh (1991) 805-812.

10 MofFatt, E.A., Sirois,J.R, and Miller, J.,'Analysis of the paints on a selection of Naskapi artifacts in ethnographic collections', Studies in Conservation 42 (1997) 120-125.

AUTHOR

Vincent Daniels studied for his BSc and PhD at Uni

versity College Cardiff, UK. After some post-doctoral research he joined the British Museum in 1974 to

study problems of paper and library materials. Since

then he has worked on a wide range of conservation

research problems but is principally interested in paper and cellulose-based materials, dyes and pigments. He

left the British Museum in 2003 and is now a research

fellow at the Royal College of Art, London, UK. Address:

Conservation Department, The Royal College of Art, London

SW7 2EU, UK. Email: [email protected]

R sum Quatre chantillons de peinture verte cireuse provenant d'antiquit s gyptiennes ont t analys s. Les analyses ont

t faites par spectroscopie infrarouge et fluorescence des rayons X. Tous les chantillons contenaient de la cire d'abeille et du cuivre.

Une peinture d'apparence identique pourrait tre obtenue par chauffage de cire d'abeille avec du cuivre m tallique 110 C ou en

m langeant de l'ac tate de cuivre(II) avec de la cire fondue. Les sels de cuivre d'acide st arique et palmitique ont t synth tis s et

leur spectre infrarouge a t mesur . Ceci a permis de conclure que la peinture verte contient les sels de cuivre des acides gras de la

cire d'abeille.

Zusammenfassung Vier Proben wachsartiger, gr ner Malschicht auf gyptischen Antiken wurden untersucht und als ein auf

Kupfer und Bienenwachs basierendes Produkt analysiert. Die Analysen wurden mit Infrarot- und R ntgenfluoreszenzspektromet rie analysiert. Ein augenscheinlich identische Farbmittel konnten durch Erhitzen von Bienenwachs mit Kupfermetall auf 110 C

oder durch einfaches Mischen von Kupfer-(II)-acetat mit geschmolzenem Bienenwachs gewonnen werden. Die Kupfersalze verschiedener Palmitin- und Stearins uren wurden hergestellt und die Infrarotspektren aufgenommen. Dies f hrte zu dem Schlu ,

dass die gr ne Farbschicht eine Verbindung zwischen Kupfer und den Fetts uren des Bienenwachses ist.

Resumen Se han estudiado cuatro muestras de verde de car cter ceroso en objetos antiguos egipcios. Los an lisis se realizaron

mediante espectroscopia de fluorescencia de rayos X. Cada muestra mostr un contenido evidente en cera de abejas y cobre. Una

pintura aparentemente igual pudo prepararse por el calentamiento conjunto de cobre y cera de abejas a 110 C o mediante la

mezcla de acetato de cobre (II) con cera de abejas fundida. Se obtuvieron sales de cobre de los cidos palm tico y este rico con sus

espectros infrarrojos respectivos, lo que llev a la conclusi n de que la pintura verde conten a sales de cobre de los cidos grasos de la

cera de abeja.




analyses of copper- and beeswax- containing green paint on egyptian antiquities

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