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Perspectives and potentials for absolute dating prehistoric rock paintings.

Approaches to dating rock paintings

The processes and materials used and the antiquity of rock paintings have always been of interest to archaeologists (e.g. Watson 1967; 1969). However, until recently few methods were available to provide answers to questions about the antiquity of rock pictures. Questions concerning temporal social interactions and relationships, cultural developments and interchanges between different linguistic groups have been based primarily on painting styles and relative chronologies (e.g. Chaloupka 1984; Lewis 1988; David & Cole 1990; David et al. 1990).

Prehistoric paints are difficult materials to date because they are generally composed of naturally occurring and geologically old ochres, clays and other pigments. The range of inorganic pigments, ochres, fillers and organic dyes available to prehistoric artists was immense (Leechman 1937; Watson 1967; Rudner 1982; Zolensky 1982: Clottes et al. 1990; Watchman 1990c). Iron oxides and aluminium silicates, the essential ingredients of ochres and clays, do not contain any known materials which can currently be used to date a painting.

In recent times rock art chronologists have tried to adapt easel painting authentication strategies to the absolute dating of rock paintings. However, some of these approaches are unsuitable for dating prehistoric rock paintings. For instance, dating the rock beneath paintings will yield an age for the igneous, metamorphic or sedimentary rock-forming process and not the time of painting.

Chaloupka (1984) attempted to establish a chronology for some northern Australian rock art by making correlations between changes in palaeoenvironmental conditions and superimpositions of depictions of fauna and flora in rock painting motifs. General limiting ages can also be assigned to paintings of extinct animals (Brandl 1972; Wright 1972; Lewis 1977). Rock paintings of horses, ships, aeroplanes and men with guns in Australia, Africa and North America are obviously relatively recent. Almost all other motifs have not yet been dated absolutely, but this situation is changing.

A major problem facing rock painting chronologists is that indigenous people around the world regard rock painting sites as sacred or highly significant places. Sampling must therefore be done with extreme care and respect, not only for the artefact but also for the people whose heritage is being studied. Damage to paintings must be minimized and so technological and innovative approaches are necessary.

The AMS 14C dating method is generally accepted by archaeologists, geomorphologists and geologists as a reliable way of dating organic matter. Current research into the absolute dating of rock paintings is aiming either to date organic matter which was deliberately or accidentally incorporated in paints during paint preparation, or to determine the ages for layers containing organic substances under and over paintings. When interpreting dating results, consideration must always be given to the source of the original sample. The question 'What precisely was dated?' should always be asked.

Dating organic binding media in paints

Evidence for the presence of organic substances in paints is found throughout the ethnological literature, though most of it is highly speculative. In North America, for example (Grant 1967: 13)

For rock painting the pigment was reground and mixed with some sort of oil binder to give it permanence. The type of binder certainly varied from place to place, but animal oils, blood, white of egg, and vegetable oils were all readily available and would serve the purpose.

Dewdney & Kidd (1967: 169) mentioned the use of spittle mixed with pigment and also stated that 'beaver tails and fish roe, the hoofs of moose and deer, could all be used to make glue, and fish and rabbit skins may have been utilised also'.

The Luiseno of California ground parched kernels of chilicothe (Echinocystis macrocarpa) with hematite (Harrington 1933: 142). In Canada, natural materials such as egg, casein, animal glue or fat, blood, honey and isinglass from sturgeon have been suggested as mixed with pigment in rock paintings (Leechman 1937; Judson 1959; Watson 1967; 1969; Corner 1968; Jones 1981). Wainwright (1985: 20, 34), in discussing the 'vehicle' (medium, binding medium or binder) that may have been used with pigment in Canadian rock paintings, stated 'it is possible water alone was used as a diluent, without any vehicle per se'.

In Africa, Biesele (1974) reported 'that the syrupy urine of the dassie or rock hare was used as a binding agent for pigments in rock paintings'. Rudner (1982: 22) recounted Lhote (1959) and Lajoux (1963) as deducing that 'a very fluid solvent, water or milk (casein is an excellent fixative)' must have been used in Tassili paintings of the archaic period.

In Australia, the explorer Ernest Giles (as reported in Severin 1973: 66), when writing about a rock painting, said, 'it is painted with charcoal ashes which had been mixed up with some animal's or reptile's fat'.

If an organic vehicle was used in rock paintings (and there is considerable doubt for many paintings; Wainwright 1985: 20), and if it remains on the rock surface in sufficient quantities, then potential exists for radiocarbon dating the painting. Unfortunately, analytical evidence for the identification of the organic media in rock paintings is difficult to obtain (Weisbrod 1978; Conard et al. 1988; Russ et al. 1990; Pepe et al. 1991).

Where no organic matter seems to be present (because none has been found in analytical tests) water was suggested as the sole vehicle (e.g. Judson 1959; Watson 1967). Painters could have used a variety of vehicles in paints at the same or different times at a site. Much uncertainty still exists about the nature of original organic vehicles which may have been used in rock paintings and it will take new analytical methods to identify any such residues. Such testing can be carried out in studies of pigment compositions and deterioration analyses as part of data gathering for site conservation and management planning processes. Recent research has found a range of organic matter in paints and the prospects for dating these are discussed below.

Human blood

Australian ethnographic literature indicates that human blood plays a prominent symbolic role in much male ceremony and it was used as a fixative for paints and other decorative material on the body and on artefacts (Spencer & Gillen 1912; Mountford 1976). Initiation practices involving blood rites (Berndt 1972: 204; Lajoux 1963; R. Jones pers. comm.) could have provided initiates with a vehicle for mixing with ochre so that they could leave special marks on rocks, when these were integral to such ceremonies.

Loy and others (1990) found traces of human blood in paint at two Australian Aboriginal sites by using a monoclonal antibody test to detect specifically for the presence of human blood (the enzyme-linked immunosorbent assay test, ELISA, is also used to detect for blood in ancient bone; Cattaneo 1990: 339). After samples of encrusted painted rock surface tested positive for human blood, trace amounts of proteinaceous matter were extracted and then dated by AMS.

Although the presence of blood in these samples was not verified by independent testing and no-one has replicated this method at other locations, the procedure offers great potential for dating some rock paintings. The method for dating blood in paintings and the sample handling, pre-treatment testing and sample purification procedures are currently under review. Reacting ninhydrin with amino acids (Nelson 1991: 552) may provide a solution to the troublesome problems of contamination of blood in paints.

Charcoal

In some rock paintings tiny particles of charcoal were either applied directly to the rock face in charcoal drawings or mixed with ochre (either unintentionally or deliberately). The age of the paintings can be determined by extracting the charcoal and using the AMS 14C method.

In Australia, fire-ash was mixed with paint, in the Victoria River district of the Northern Territory (B. Duelke & D. Lewis pers. comm.), as a filler and to modify the colour of paint. Charcoal drawings can also be AMS radiocarbon dated by removing some of the carbonized particles. This last approach was recently successful in France and Spain (Valladas et al. 1990; Valladas et al. 1992) and partly successful in Australia (McDonald et al. 1990).

The problem in Australia is that two widely disparate ages were obtained for charcoal from the same silica-coated motif. Examination of similar samples from the same site (Watchman in press a) revealed the presence of 'free' charcoal (loosely bound particles not silicified), charcoal encapsulated in silica, pale brown algal remains and blue-green bacteria. McDonald and others (1990) did not pre-treat the samples to eliminate contaminating organic substances before they were dated.

In Namibia, carbon in black pigment from one site has been also dated by AMS (van der Merwe et al. 1987) but other samples from the same site have failed to yield sufficient carbon to prepare targets.

Tiny pieces of charcoal have been found in several paints from the Laura area, Australia, and these are currently being dated by AMS.

Plant fibres and dyes

In northern Australia, Aboriginal artists reportedly used a wide range of natural plant materials in the preparation and application of paints (Roth 1904; Brandl 1973). However, Walston & Dolanski (1976) reported negative results in testing for blood, milk, urine and plant sap at Mootwingee. Recently positive proof of the use of natural plant materials in rock paintings was found by Watchman and others (in press) and Cole & Watchman (1992). The initial discovery of thickly matted plant fibres intimately associated with white paint in two motifs at one site near Laura, in north Queensland, and subsequently at seven other painting locations in that region has presented opportunities to date the fibres and therefore date the paintings. In several other paintings in that district individual fibres were found on the paint surface and these fibres are thought to be fragments from the brush used in painting the images. Fibres were also observed recently in an orange-red painting in Namadgi National Park, Australian Capital Territory (Watchman et al. in preparation).

The intimate association of plant fibres with inorganic paint minerals is evidence of the artists' understanding of the useful properties of plants in making better paints. Sticky plant juices associated with the fibres provide greater cohesion for the otherwise poorly bonded paint ingredients. The benefit for rock art chronologists is that the organic fibres and juices provide carbon for AMS dating (Watchman & Cole in press).

In the Laura region, tentative evidence exists for the use of plant dyes in the preparation of paints (Watchman et al. in press). Glossy yellow paint and a matte blue paint are unlike other paints in that region because they appear not to be solely inorganic in composition. It is suspected, although not yet proved, that vegetable or fruit dyes were used to give colour to clay-based paints. If it can be demonstrated that natural organic dyes were used as colouring agents then 14C dating will be possible. Further research is under way to confirm these ideas.

Amino acids

Denninger (1971) claimed that organic binders had been used in Bushman paintings in South Africa based on the measured presence of amino acids. The traces of amino acids were thought to be formed from the degradation of albumin, probably from eggs used as the vehicle. The precise origin of the amino acids has not been adequately demonstrated (Rosenfeld 1988: 10).

Amino acid racemization (AAR) dating has been attempted on rock paintings in South Africa (Pager 1972: 356-9) and in the United States of America (McCarthy et al. in preparation). The method has been primarily used with mixed success on dating fossil bone (Bada & Protsch 1973; Taylor 1983), soils and sediments (Kimber & Griffin 1988), and molluscs (Murray-Wallace & Kimber 1988).

The South African study, using paper chromatography, relied on progressive, time-dependent decrease of amino acids in the paint medium. In the USA, analysis of the amino acids was by high-pressure liquid chromatography using citrate buffers (McCarthy et al. in preparation). Trace amounts of D/L aspartic acid could only be detected in the USA samples; insufficient to determine the composition of D/L ratios. Furthermore, the relative amounts of amino acids obtained from the paint sample could not be differentiated from those obtained in a control specimen. The results also suggest that either organic binders were not used in the paint sampled or indigenous amino acids were not preserved because of environmental degradation. One disadvantage of the AAR dating method is that the chemical racemization process is sensitive to environmental factors, especially to temperature.

Bees-wax

In the northern parts of Australia, simple figures have been created on rock surfaces by flattening small pellets of bees-wax. The native Australian bee (Apida trigona; Michener 1970: 951) stores honey in large wax pots in hollow trees. Aboriginal people used this waxy and resinous material as pellets to create designs on rocks.

It should be relatively simple to take small samples from these wax figures without unduly damaging the artwork to obtain 14C AMS dates (Nelson et al. 1992).

Saliva

In many parts of the world hand-stencils and stencils of other objects have been painted on rock faces. A liquid paint is held in the mouth, and sprayed either through the lips or through a narrow tube on to a hand or other object held against the rock face. Because the liquid has been circulated in the mouth the spray-paint may contain traces of saliva.

Is it possible that remnants of this saliva persist in paintings? A worthwhile research project would be to investigate the stability of saliva; because if blood protein is stable for thousands of years, then so may be the proteins in saliva, and perhaps they can be dated.

Unidentified organic matter

In Texas, Russ and others (1990) used a low temperature oxidation method to convert unidentified organic matter on a painted rock into carbon dioxide for AMS dating. This method avoids contamination from carbonate and oxalate. However, as the nature of the organic matter oxidized in this process is not known, one cannot be sure that only the paint vehicle is dated. Low temperature extraction methods should not be dismissed entirely because they may provide ways for removing organic matter from other paints where the history and identity of the organic matter are precisely known.

In China, a binder was found in pigment used for painting in the Zuojiang River Valley (Fushun 1991: 32). The Guangxi Industrial Chemicals Research Institute identified the binder as an animal protein compound consisting mainly of animal glue. The organic matter has not been dated by AMS, but perhaps could be.

Dating unidentified organic matter, whose history of formation and alteration is unknown, is inappropriate for rock art chronological studies because any dates which are obtained could represent the combination of contributions of 14C from any number of different sources; for example, from an organic binder (if there was one initially), decayed organic by-products from a binder plus residual binder, microorganic accumulations since the paint was applied, humic acids, oxalate, soot from atmospheric carbonaceous aerosols or carbon from any other source which was bonded at different times to the painted rock surface.

Substrates and surface films

Where organic substances cannot be found in the paint itself alternative dating methods must be employed. Studies of cross-sections of rock surfaces near paintings (Wainwright & Taylor 1978; Watchman 1990a; in press b) have suggested potential ways for dating some rock pictures. Mineral crusts and films on rock surfaces often contain organic matter, such as oxalate, algae and particulate charcoal which can be used for 14C determinations.

Oxalates

For many years people have regarded the black substance covering the ceilings of caves and rock shelters as soot. However, when samples are analysed they are found to be primarily oxalate. In Australia, oxalate can be found in brown and black, shiny, laminated crusts up to 1 cm thick (Watchman 1991), but in North America black surface films only seem to exist. Each lamination in the Australian crusts represents a different period of dust and salt accumulation and is thought to reflect changes in environmental conditions near the rock face (Watchman 1992).

Crusts are found in a wide variety of locations (Watchman 1990a) and evidence of periodic painting is also found in studies of cross sections (Watchman in press b). Past episodes of painting are dated by using the carbon-bearing layers under and over paintings to establish maximum and minimum age limits. The carbon used to date the crusts is found in whewellite (calcium oxalate) which forms on rock faces by reaction of organic acids in rainwater with dust and through biological activity.

Other processes also lead to formation of oxalate crusts in northern Australia (Watchman 1991). For example, certain species of lichen and alga produce oxalic acid which reacts with calcium and other cations in dust, weathered rock and groundwaters. Fungi are also known to produce calcium oxalate either by hyphae utilizing bicarbonate ions from solution (Lapeyerie 1988) or from ectomycorrhizal interactions (Malajczuk & Cromack 1982).

However, as the precise mechanism for formation of oxalate deposits is uncertain (Watchman 1991), this dating method must be regarded with caution. For example, if microorganisms are found to be the cause of oxalate formation in all crusts, it must be demonstrated that the carbon found in the oxalate is derived only from atmospheric carbon dioxide and not from microorganic metabolism of older carbonate or bicarbonate.

Not only is it important to find out the mechanism for formation of oxalates but it is critical to establish when the oxalate was deposited; either syngenetically (at the same time as dust was settling on the rock surface) or a long time after deposition and bonding of the dust by replacement or cementation reactions.

Wiedemann & Bayer (1988) found oxalate replaced gypsum in laboratory experiments and so it is possible for oxalic acid, from a range of natural sources, to replace old deposits of gypsum formed from evaporation of rain water and groundwater. If this is the case, dating oxalate will not give correct ages for either the original laminations in the crust or for previous episodes of paintings, but only the age of oxalic acid formation.

Silica skins

Thin, transparent to white siliceous films, less than 1 mm thick, are found on sandstones and quartzites where silica was deposited from seepage water (Walston & Dolanski 1976; Watchman 1990b). Silica skins are formed by gradual accumulations of micro-laminations of amorphous silica (opal A; Jones & Segnit 1971) deposited from evaporating water, enriched in silica, which flow slowly across rock surfaces from fissures and bedding planes (Watchman 1990b; in press a). At some locations, particularly in northern Australia, but also in the United States of America, Canada (Wainwright & Taylor 1978) and Finland (Lounema 1988) silica films or skins form surficial layers over paintings. Encapsulated in many micro-laminations are trace amounts of algal matter and fine particles of carbonized plant remains; organic matter which can be dated by AMS 14C.

Physically extracting minute amounts of algal matter and carbonized plant particles contained in very thin silica skins is impracticable. Recently a laser was focused precisely onto a micro-lamination (less than 0.01 mm wide) and the encapsulated organic matter converted into carbon dioxide by the intense localized energy of the laser (Watchman & Lessard 1992). The carbon dioxide was then converted into graphite for AMS dating. Research is currently under way to use this technique to date tiny organic particles trapped in paints and oxalate in rock crusts.

One problem which may complicate this approach is the migration of pigments through the gel-like colloidal silica skin (Wainwright & Taylor 1978). Such problems can only be resolved with detailed cross-sectional studies of rock surfaces and complete understanding of surficial rock processes.

Conclusions

Rock paintings are gradually being dated by AMS radiocarbon techniques as paints and rock crusts and films containing organic substances are discovered and as problems of sapling and contamination are overcome. Challenges for rock art chronologists are to define protocols for sampling, to refine and develop dating techniques and to select appropriate paintings to establish regional chronologies.

Acknowledgements. Jacques Brunet, Noelene Cole, Britta Duelke, Liz Hatte, John Head, Darrell Lewis, Tom Loy, Dan McCarthy, Erle Nelson, Deborah Rose, Andree Rosenfeld, Francois Soleilhavoup and Ian Wainwright provided valuable comments during the preparation of this paper.

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