nuclear analytical study of rock paintings

Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 151, No. 1 (1991) 221-22 7

NUCLEAR ANALYTICAL STUDY OF ROCK PAINTINGS

M. PEISACH,* C. A. PINEDA, * L. JACOBSON **

�9 National Accelerator Centre, P.O. Box 72, Faure, 7131 (South Africa) �9 *McGregor Museum, P.O. Box 316, Kimberley, 8300 (South Africa)

(Received May 2, 1991)

An cxfoliated fragment of a rock painting from Lesotho was analyzed by differentiated backscatter spectrometry to obtain the paint thickness, which ranged from about 2 to 6.5 t~m, and its empirical formula for stopping power calculations. Elemental composition was determined by P1XE. Fe-rich paint spots were red in color and Ca-rich ones, pinkish: Because of the chemical mobility of calcium, this paint component should become the focus to which conservation techniques should be directed.

Introduction

Rock paintings are abundantly found in Southern Africa 1 and are a striking reminder of the life and times of the prehistoric inhabitants o f the region. These painted images

are metaphors for, and descriptive of the hallucinations seem by Shamans (medicine men) under trance. Since the trance dance was the central ritual of hunter-gatherer

religious beliefs, the rock paintings reflect their religion and are early examples o f religious art.2, a Some commentators have even compared the cultural importance o f

these paintings with that of the great religious works of the western world, 1 as regards the historical development of Man. The paintings are found in a variety of locations,

which range from the interior walls o f sheltered caves to exposed rock faces. The

condition of the paintings varies from bright, fresh examples to those which are acti-

vely degrading or are barely visible. Many factors could be responsible for the observed deterioration, such as time, the nature of the rock surface on which the art was

executed, the microenvironment in which the painting is found, the composition

o f the paint and the ravages of climate, both natural and man-made. As a step towards the conservation o f the art, analyses were carried out to determine the elemental composition of the paint.

Very little work has been done on the composition of the paints. 4 One of the earliest studies was directed to discovering the nature of the organic components by

paper chromatography of the albuminous binders in order to determine the amino acids, the persistence o f which could be used to obtain an indication of the age o f

Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest

M. PEISACH et al.: NUCLEAR ANALYTICAL STUDY OF ROCK PAINTINGS

the painting, s Attempts at determining the elemental composition 6 used micron- sized samples removed from the rock painting and analyzed by electron beam induced X-ray excitation. The reported results were averages of highly variable values and were used for qualitative, rather than quantitative comparison. The main colours

of analyzed spots were red, white and black. The conclusions that were drawn sug- gested that the red paint was Fe-rich, probably from iron. ores, the white was Ca-rich, probably from admixture of ground sea shells, while the black paint was either Mn- rich, showing the use of manganese ores, or Ca-rich, probably derived from burnt bones or sea shells. 6

In this investigation the paint was analyzed on its original substrate, non- destructively, through the use of ion beam techniques.

Experimental

Description of the analyzed material

The rock painting fragment that was available for analysis consisted of the painted surface of a naturally exfoliated fragment of Drakensberg ,Cave Sandstone from the Ha Khotso shelter in Lesotho. The fragment, measuring approximately 2.5 cm wide and 6 cm long, represented a portion of a typical San rock art panel. The painting was colored with shades of red with an admixture of white material in varying

thicknesses on the sandstone substrate.

Irradiation and measurement

The sample was mounted on a special support which fitted onto the remote- controlled sample ladder of a compact multi-purpose scattering chamber. 7 Is was ir- radiated with both protons of 4 MeV and alpha particles of 2 MeV in adjacent parallel scans with spots analyzed every 1.33 mm using beams collimated to 0.5 mm diameter for the protons or 1.0 mm for the alpha-particles. Some twenty spots were analyzed in this manner, using the protons for particle induced X-ray emission, PIXE, analysis and the alpha-particles for 140 ~ backscatter measurements. In order to obtain information about the elemental composition of the sandstone rock, the scans ex- tended beyond the painted area.

Since the painted area was electrically insulating, the aluminium-foil technique 8 was used to generate an electron spray, which prevented charge build-up within the painting. Scattered protons were prevented from reaching the Si(Li) detector by absorbers, which also reduced the flux of low energy X-rays.

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The sample was mounted at 45 ~ to the beam, and the X-ray spectra were meas-

ured at 90 ~ with a Si(Li) detector having a resolution of 155 eV 5.9 keV, while

the backscatter spectra were measured with a silicon surface barrier detector having a

resolution of 14 keV.

Data processing

Data were recorded on magnetic tape for off-line processing. PIXE spectra were analyzed by the program 9 AXIL. Since the composition of the analyzed spot represented (in most cases) the sum of the paint and the rock substrate, the evaluation of absolute concentrations from the experimental spectra was meaningless. Accordingly, the integrated counts for each element were the values used for correspondence analysis) o

Only the major components could be observed in the measured backscatter spectra. Since it was obvious that the paint layer covered the substrate, and since both paint and substrate contained the major components Ca and Fe, the resultant backscatter spectrum consisted of the sum of the backscatter spectra obtained from the paint and the substrate. However, the paint layer reduced the energy of the bombarding beam which reached the substrate. Hence the main contribution from the steps corresponding to these two elements in the backscatter spectrum would be shifted to

lower energy equivalent to the energy loss in the paint. Such low energy shifts are not readily measurable directly on the experimental spectra, but by using the method of differentiation, 1 ~ the exact energy of the inflection point is more readily detected since the step, on differentiation is converted to a sharp peak. Accordingly, this method was used to determine the thickness of the paint layer at each of the analyzed spots.

The carbon content was dednced from the integral of the corresponding differentiated spectrum peak.

Results and discussion

Backscatter analysis

A typical backscatter spectrum obtained from the bombardment of a painted area with 2 MeV alpha-particles is shown in Fig. 1. The upper portion shows the measured spectrum, while its derivative is plotted below it. The poor definition of the carbon step in the experimental spectrum makes it difficult to evaluate the carbon content suf- ficiently accurately. However, the spectrum served to calculate an empirical chemical formula which could be used for energy loss calculation.

The differentiated spectrum makes possible the identification of the exact energy of the inflection points. It was found that the values for C and O corresponded to

223


A 5600 -

c 4900 ~ Rock pa in t

5 4200 ~ 3 5 0 0

2800

~ zmo_ o 1 4 0 0 -

700 0 I i I I J

,t

01 . ,, . . . .

~ - 0 . 5

Vl/ I " !i - 1 A 0 5i ',1

~ - - 1.4 ~' m Ca Fe *E - 1 7 :1 o -2 0 1 , I , I J 1 I I I I I p.

300 400 500 600 700 800 Chonne l number

Fig. 1. Typical backscatter spect rum (upper curve) o f 2 MeV 4He § ions scattered through 140 ~ from a paint-covered spot. The lower spectrum is the differential and the arrows mark the inflexion point o f the step for scatter f rom the surface. The energy shifts for Si, Ca and Fe are due to the thickness o f the paint layer

A I- ~5

~ ~ F ] 1 0

L I i ' ' Ira- 04 12

- \ I

i

I I I ] I 14 16 18 20 22 Ana lys i s s i te number

Fig. 2. Measured paint thickness as a funct ion o f the scan site

2 2 4


scatter from the surface, while those for Si, Ca and Fe showed slight shifts towards

lower energy (see Fig. 1). These shifts were used to calculate the paint thickness

which ranged from about 2.2 to 6.3 /am. Thickness measurements as deduced from

the energy for Si, Ca and Fe agreed within experimental error. The variation of

thickness as a function of the scan position is shown in Fig. 2.

PIXE analysis

A typical X-ray energy spectrum obtained from the proton bombardment of a

painted spot is shown in Fig. 3. Because of the very high count rates from Ca and

Fe, an absorber consisting of two layers of photographic t'dm was used to reduce

these count rates and to eliminate low energy X-rays from, for example, Si, a major matrix component.

The potassium, calcium and iron contents varied widely across the scan of the painted area, with negative correlations between K and Ca and between Ca and Fe,

1

-~ 5 C C 0

~q

C

S

o

0

Fe

Ca F~

Mn

Ti

K V

I 100

Rock painting

Ep= 4000 keY

ZD

Rbsr r

200 300 4 0 0 5 0 0

Channel number

Fig. 3. Typical X-ray energy spectrum obtained from a paint spot under bombardment ~ith 4 MeV protons. An absorber of poly(methyl methacrylate) was used to reduce the very high count rate from Ca and Fe

but a strong positive correlation between K and Fe. The manganese content was

apparently an important component of the rock substrate. Evidence of manganese from painted spots was due to penetration of the beam to the substrate through the paint layer, but the apparent manganese content was appreciably lower over the painted area.

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The presence of carbon

It has long been ot" interest to archaeologists whether carbon was present in the

paint materials, as this element could provide a means of dating the work, provided

the carbon was organic in origin. Since nuclear analytical methods cannot distinguish

between different chemical forms o f an element, it is not possible to identify the

carbon as being either organic or inorganic in origin. A possible inorganic source

would be calcium as carbonate. In that case the isotopic composit ion of carbon need

not have been rePresentative of the age of the painting. Furthermore, if some of the

carbon is organic as well, the presence of inorganic carbon would distort any age

measurement. It is thus necessary to carry out further studies to determine the

chemical state of the carbon.

Correspondence analysis

Using PIXE data for the elemental content of K, Ca, Ti, V, Mn, Fe, Zn, Rb, Sr

and Zr, and backscatter data for the content of the carbon and for the thickness

parameter, correspondence analysis was carried out on the values for the 22 analyzed

sites. A marker of tantalum was placed at the start site of the scan and analyses were

0.4 Rock paint scan ] �9 Fe-rlch ~

02 ubstrate

~ o - ~ c ~ - - . . . . . . . . . . . -o2 ~ i @

- 0 4 I I I I I [ -01 -0Z, -02 0 02 04 06

Ax~s ]

Fig. 4. Correspondence analysis plot of the first two axes for the data obtained by PIXE and backscatter analysis. The parameters that entered the calculation were the elemental contents of C, K, Ca, Ti, V, Mn, Fe, Zn, Rb, Sr and Zr, and the thickness parameters. For the sake of clarity, only the parameter positions of Ca, Mn and Fe are plotted

measured from one site spacing below the marker for a total distance of just under

30 ram.

A plot o f the first two axes o f the correspondence analysis is shown in Fig. 4.

The calculated plot positions are shown for Ca, Mn and Fe, but those of other

variables are omit ted for the sake of clarity. Three clear groups have been separated.

The group nearest Mn contains all the data points which refer to sites on the substrate

226


rock where no paint was present. The painted sites separate into two groups, those

that are Ca-rich (and lighter in color) and those that were Fe-rich.

The first two axes include 83.95% o f the information content. I t was thus not

necessary to examine the other axes, each of which contr ibuted but a small fraction of the information content .

Conclusions

It has been shown that a combinat ion o f nuclear methods can be applied success-

fully for the study o f the paint material of rock paintings.

The relatively high Ca-content in the paint is of archaeological importance because,

being a mobile element, it is the one most at risk, especially under unnatural condi-

tions such as acid rain. This element should thus become the focus to which conservation techniques should be oriented.

Of special interest was the information of the presence of carbon in the paint

even though the form in which the carbon was present could not be identified. A

future study to add credence to the presence of organic material could be a search for, and the determination of, the nitrogen content . 12

The staff of the Faure Van de Graaff accelerator is thanked for their helpful cooperation. The Geology Department of De Beers Mining, Kimberley, is thanked for financial support (to L. J.). This work may form part of a doctoral thesis (by C. A. P.) to be presented to the University of Cape Town and is published with permission.

References

1. J. D. LEWIS-WILLIAMS, The Rock Art of Southern Africa, Cambridge University Press, Cambridge, England, 1983.

2. M. G. GUENTHER, J. South West Africa Sci. Soc., 30 (1976) 45. 3. J. D. LEWIS-WILLIAMS, J. H. N. LOUBSER, in: Advances in World Archaeology, Vol. 5,

F. WENDORF, A. CLOSE (Eds), Academic Press, New York 1986, p. 253. 4. I. RUDNER, S. Afr. Axchaeol. Soc. Goodwin Series, 4 (1983) 14. 5. E. DENNINGER, S. Afr. J. Sci. Spec. Publ., No. 2 (1969) 80. 6. M. L. WILSON, W. J. J. VAN RIJSSEN, D. A. GERNEKE, Ann. S. Afr. Museum 99 (1990)

187. 7. M. PEISACH, Nucl. Instrum. Methods, B14 (1986) 99. 8. C. A. PINEDA, M. PEISACH, Nud. Instrum. Methods, B35 (1988) 344. 9. P. VAN ASPEN, University of Antwerp (1981). Copyright Canberra Industries, Meriden, CT,

USA. 10~ M. J. GREENACRE, Theory and Applications of Correspondence Analysis, Academic Press,

London, 1984. 11. M. PEISACH, Thin Solid Films, 19 (1973) 297. 12. C. A. PINEDA, M. PEISACH, L. JACOBSON, Nucl. Instrum. Methods, B35 (1988) 463.

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nuclear analytical study of rock paintings

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