ÐÓ°ÉÔ­´´

FOR MANY thousands of years people have painted, drawn and carved a
record of their lives and spiritual beliefs on rock. The pictures have survived
in many different environments, from the stable interiors of limestone caves
such as Lascaux, in France, to the tropical climate of Australia’s Northern
Territory. That some paintings have lasted since the last ice age is of
great interest to those who are searching for new ways to protect and prolong
the life of this art.

Following traditions dating from prehistoric times to the present day,
the pictures show people, their activities and artefacts, and animals –
some of which, like the mammoth, are now extinct. Other designs seem abstract,
but while their meanings are lost, the patterns and styles tell us much
about cultural boundaries and influences.

Many designs are ubiquitous and some researchers suggest that they might
have a common neurological origin – representing visual sensations produced
in the brain during migraine headaches or trances, or by the action of hallucinogenic
drugs. These patterns may have been the basis of a ‘visual vocabulary’ early
in the development of symbolic communication. Most specialists agree that
the development of rock art some time in the past 30 000 to 100 000 years
marked a huge step in human cognitive development. If these theories are
correct, new techniques for accurate dating of paintings will pinpoint the
timing of evolutionary and social developments.

Speculation about the meaning and significance of prehistoric pictures
fuels fierce debate among archaeologists. Modern accounts gathered by anthropologists,
mostly from Australian Aborigines, the San people of southern Africa and
native North Americans, suggest that pictures usually reflect fundamental
religious and social concerns. For archaeologists, who generally have to
be satisfied with less direct evidence of a prehistoric way of life, rock
art provides a tantalisingly personal glimpse of ancient cultures and societies.

Although most rock art is prehistoric, the pictures retain their original
significance for some of the cultures from which they arose. Australian
Aborigines emphasise the spiritual aspect of their rock art; one living
painter recently described the art as ‘a bible in stone’. The conservation
of rock art is inseparable from conservation of the general environment
and, in Australia, from the issue of land rights – not only because the
integrity of the local environment is essential for physical survival of
the art, but also because the sites are sacred.

The world’s prehistoric paintings are probably more at risk today than
ever before. Rock art is a growing tourist attraction. Each year more than
200 000 people visit sites in Australia’s Kakadu National Park, for instance.
Even the Kalahari desert draws the more determined tourists who want to
see the paintings made by the Bushmen, ancestors of today’s San people.
Careful management is needed to minimise the physical impact of so many
people. At Lascaux, when the humidity generated by the breath of so many
visitors threatened the paintings, a replica was built for visitors. Only
scholars are now allowed to see the original.

Unfortunately, the impact of people does not end with tourism. Deliberate
vandalism is an increasing problem. In the Tsodilo Hills of northwest Botswana,
one of the most famous rock paintings in the world, painted by the San people
of the Kalahari, has been defaced by graffiti. Two years ago, some German
visitors tried to remove a Kalahari painting by chipping it out of the rock.

Less obviously, pollution, urban development, mining, farming and clearance
of forests all take their toll. Construction works not only disrupt sites
physically, they often make them more accessible to visitors as roads push
into previously isolated areas; changes in vegetation and increasingly acidified
water alter the rate at which rocks are weathered; and livestock often destroy
art in shelters by rubbing against the rocks, raising and depositing dust
and burying images under mounds of their droppings.

Even efforts to conserve and study the art can affect its preservation,
either directly or by altering the environment. Only a small fraction of
the pictures ever drawn, painted and engraved by our ancestors have survived,
and it would be a tragedy to lose what remains through development or by
altering the conditions that ‘selected’ them for posterity.

The age of rock art interests both archaeologists and conservators.
Both need to understand the rates and mechanisms of physical processes affecting
its preservation: archaeologists in order to measure how old the art is,
and conservators to find the best way to preserve it. Until recently, researchers
based their estimates of age largely on circumstantial evidence, such as
the depiction of extinct animals. The degree of weathering of rock engravings
and fading of paintings are unreliable indicators of antiquity, because
they vary from place to place and with time.

Microscopic time capsules

Occasionally, however, rock art or artists’ materials are buried or
otherwise associated with carbon in the form of charcoal or natural pigments
which can be dated by radiocarbon dating. Charcoal associated with wall
markings in Koonalda Cave in South Australia has been so dated to 18 000
years. In other parts of Australia, ochres presumed to have been used as
art materials have been found in sediments laid down 60 000 years ago. Sites
dated in Europe and Africa go back nearly 30 000 years. Such dates are often
controversial, however, because the carbon is not associated closely enough
with the art itself.

New and more accurate methods of identifying and dating organic materials
associated with rock art, and of dating the rock surfaces themselves, are
beginning to emerge . These will enable scientists to date some rock surfaces
directly, based on a knowledge of the rate of chemical changes in the surface
crusts. Organic materials present in some layers of pigments, and sometimes
trapped within rock ‘skins’ that have formed over painted and engraved surfaces,
can be dated by a sophisticated form of carbon dating (accelerator mass
spectrometry), which needs only tiny samples of material.

The people who created the prehistoric images used either ‘additive’
or ‘subtractive’ techniques. In painting, the artists applied wet and dry
pigments made from minerals, charcoal, and more rarely, less well-preserved
materials, such as beeswax and mud. Subtractive techniques include scratching,
‘pecking’, abrading and engraving rock surfaces. We know very little about
specific methods of engraving because none of the tools survives. We know
more about the way artists prepared and applied their paints, however, because
there are still a few people in Australia who know the traditional techniques.

Artists painted and drew pictures by applying wet or dry pigments directly
onto the rock. In some cases, the artist painted a ground layer first and
then created images over it. The Wandjina paintings of the Kimberley region
of Western Australia are painted on a white background, which was prepared
by smearing a slurry of the mineral huntite onto the rock. Huntite expands
when wetted, forming an unstable surface, so these paintings deteriorate
quickly. Traditionally, Aborigines repainted their Wandjina pictures at
regular intervals during religious ceremonies to rejuvenate the land. Continuing
this practice may be the best way to ensure the survival of both the painting
and the cultural tradition.

The origin, preparation and application of painting materials are all
important in the survival of rock pictures. Mixed as a slurry with water,
finely ground ochres containing iron oxides and hydroxides, such as haematite
and limonite, penetrate and bind to quartzite rocks so that they become
as permanent as the rock surface itself. White pigments, derived from clay
minerals, gypsum and carbonates, do not bind as readily and are easily damaged
by high humidity, water and abrasion. As a result red-and-black paintings
survive better than other colours, giving a false impression of the range
of colours in the artists’ palettes.

Painters usually manufactured pigments by altering the native minerals,
often by grinding to produce a paint with suitable working properties. In
some cases the process was quite complex: for example, a white pigment found
in the Palaeolithic limestone caves of France is thought to have been made
by heating animal bone to 400 Degree C to produce apatite, which was then
mixed with calcite and heated again to 1000 Degree C to form tetracalcite
phosphate. Evidence from Lascaux suggests that red pigments were derived
not only from red haematite, but also from the dehydration of yellow limonites
at high temperatures.

Despite persistent speculation by archaeologists, there is no convincing
proof that artists incorporated organic materials into their paints as binding
agents, although Tom Loy of the Australian National University recently
found human blood associated with pigment at sites in Tasmania and the Northern
Territory. But even if paintings originally contained organic binders, they
have long since disappeared. This means that the continuing stability of
the paint is due to other factors. Experiments with organic binding materials,
such as fats, blood, fish glue and plant gums and resins, suggest that pigments
mixed with water are much more durable.

Some researchers suggest that the painters at Lascaux took advantage
of a natural inorganic binding mechanism by mixing their paints with cave
water rich in calcite. Calcium carbonate deposited from the water locks
the pigment particles onto the rock. Some of the oldest prehistoric rock
art seems to have survived because of natural inorganic binding processes.
In limestone caves water percolating through and over the rock leaves a
film of calcium carbonate, which effectively seals the image within a durable
cocoon. Where the rocks contain silica, a glassy layer often forms on the
surface. Where pigments have been bound strongly to the rock and enclosed
within a silica skin, they become more or less permanent on a geological
time-scale. Both processes can eventually obscure the art, as pearly white
accretions build up.

Alan Watchman, of the Australian National University, is studying how
these protective skins form, hoping that the findings will lead to a way
of treating deteriorating rock paintings and also to a more direct way of
dating them .

Each type of rock weathers in a characteristic way, while the climate
around the rock determines the extent of weathering. Under most conditions,
water and waterborne salts do the most serious damage. These agents moulded
the landscape, forming the caves and shelters where people lived and painted,
and it is not surprising that their continuing influence now threatens the
art. In sandstone and granite, crystallisation of salts and their hydration
and dehydration at the evaporation front below the surface, leads to granular
disintegration of the rock surface. As water evaporates, salts build up,
obscuring and disrupting pigment and blurring the original sharp contours
of engravings. Flowing water makes things worse, by washing away loose material
and abrading engravings.

The usual way of protecting art from water flowing across the rock is
to divert it, either with drip lines made from silicone rubber or drip zones
of water-repellent silicone. Although this prevents some flaky pictures
from being washed away, it may halt the formation of a silica skin. Flowing
water also removes salts from the surface of rocks where they would otherwise
crystallise and cause the rock to flake. K K Architectural conservators
have given a lot of thought to the problem of salts in building stone: crystallisation
of salts in stone creates forces that cause the stone to disintegrate. This
can be dealt with by isolating the stone from sources of water, by controlling
humidity, creating damp courses and coating surfaces with water repellents,
and then consolidating fragile areas by impregnating them with organic polymers.

It is rarely possible to isolate a rock art site from its surroundings
in the same way. Impregnation of crumbling rock seems an attractive remedy,
but if the consolidant does not allow the movement of water, salts are likely
to form at the junction between the rock and the consolidant and lever off
the whole piece of treated rock. Research continues to find appropriate
consolidants and water repellents, such as polymers containing silicone,
but polymer chemists have little experience of materials that have to stand
up to the sort of physical and chemical deterioration experienced at rock
art sites.

There are many other causes of deterioration. Condensation is one: in
otherwise dry environments, water condenses on the surface of the rock when
the temperature of the surface falls low enough. Plants growing nearby also
produce moisture as they transpire, but they can be removed if necessary.
At exposed sites, frost can damage the rock by cracking and lifting its
surface. In this case, planting may help to shelter the site from the effects
of frost. Applications of antifreeze and water-repelling compounds can also
help. Every site is different and scientists studying the microenviron ment
around the paintings can reveal problems that might not be immediately obvious
and suggest appropriate solutions.

Water does more than activate salts and wash art away, it also encourages
the growth of algae, lichens and other organisms that obscure and damage
both the rock and the art. Biological action is an important agent in rock
weathering, and it probably contributes to the deterioration of some art
sites. One solution to this problem is to spray the rock with persistent
biocides such as tributyl tin oxide or organochlorines: these compounds
are highly toxic, however, which limits their use.

Some larger organisms can also do much damage. Birds, such as swallows,
and insects, such as wasps and termites, which make nests out of mud, leave
a trail of destruction where they build. Perversely, at one site in Western
Australia, water flowing through kestrel droppings and onto paintings below
has prevented the pigments from being washed away. Whether this is because
the droppings alter the pH of the water or because they act as a binder
is not clear, but it shows that potential treatments may be found in unlikely
places.

Keeping the surface of the rock intact may be enough to preserve some
paintings. In others, the layers of pigment themselves may be unstable,
either because the particles stick together very poorly or because they
adhere to the rock or previous layers of paint poorly. This problem is seen
mostly in recent paintings, because it destroys pictures very quickly. Organic
polymers, such as acrylics, show promise as consolidants for poorly bonded
pigments. Conservators working on more modern frescoes and murals stabilise
friable paintings with polymers. Specialists in preserving rock art are
now trying the same approach; with these outdoor works of art, however,
regular treatment may be needed because synthetic organic materials deteriorate
quickly when exposed to heat and harsh ultraviolet light.

Recently, an international team of conservators led by Isabel Dangas,
of the International Centre for the Preservation and Restoration of Cultural
Property, and John Clarke, a consultant on conservation of rock art based
in Western Australia, went to work on a very unstable painting at Nourlangi
Rock, in the Kakadu National Park, Northern Territory. The painting at Nourlangi
is at least 8000 years old; however, it was repainted most recently in the
1960s by Najomboli, a local Aboriginal artist. Najomboli used a calcimine
house paint which began to flake and deteriorate very quickly. The team
cleaned the paintings carefully, sticking down individual flakes of paint
with an acrylic emulsion, and then sprayed the whole painted area with a
dilute solution of an acrylic resin. Only time will tell if the treatment
works.

Despite the great antiquity and significance of rock art, it receives
relatively little attention compared with other forms of cultural heritage
– stately homes and castles and more conventional ‘old masters’. Until recently
there was little formal training in conservation of rock art. Last year,
the Getty Conservation Institute, based in Los Angeles, California, and
the National Centre for Cultural Heritage Science at Canberra University
held a postgraduate course in the conservation of rock art. The course attracted
students from around the world – Africa, Australia, New Zealand, North America
and Sri Lanka. These students have now returned to their own countries and
are beginning to put into practice their new skills and knowledge. What
is needed now is financial and technical support to develop long-term strategies
for conservation. For unless governments recognise their responsibilities
to wards the preservation of this aspect of their cultural heritage, these
treasures could be lost forever.

* * *

New ages for old art – the archaeologist’s growing armoury

DATING rock art is a formidable challenge. The pigments, paints and
engravings, and the weathered rock beneath them are difficult materials
to work with. But, added to that, rock art is a precious cultural resource
that must not be damaged by sampling. To indigenous people around the world,
rock art sites are sacred places that must be respected and not unduly disturbed.
Researchers from a range of disciplines, including geologists and geochemists,
are working on innovative approaches to dating that will not damage the
art.

Paintings composed of ochres and clays cannot be dated directly unless
they contain substances such as organic binders and charcoal, because the
pigments are geologically old. Problems of micro geochemical analysis, contamination
and sampling are slowly being solved so that new dating methods are allowing
ancient art to be dated with increasing confidence.

So far, few paintings have been found to contain organic binders. Binders
are thought to have been mixed with earth-based pigments to increase their
adhesion to the surface of the rock. Such organic substances can be dated
using carbon isotopes. All living organisms contain small quantities of
radioactive carbon-14, and when an organism dies, the amount of carbon-14
decreases at a constant rate determined by the half-life of the isotope.
The measure of the decrease in carbon-14 gives a measure of the time that
has passed since the organism died.

In Canberra, Tom Loy of the Australian National University has found
human blood mixed with the ochre at several painting sites – using tests
involving monoclonal antibodies. The antibody recognises a sequence of amino
acids that forms part of the human serum albumin molecule. Trace amounts
of the blood extracted from the paint were enough to analyse by the new
technique of accelerator mass spectrometry, which can date a sample smaller
than 100 micrograms as far back as 40 000 years with an error of only 10
to 15 per cent.

Australian Aboriginal painters may have included blood and other organic
ingredients in their paintings during special ceremonies. In other art they
either applied the finely ground ochres and clays directly onto the rock
face without adding a binder, or the binder has oxidised and has now disappeared.

In rock paintings where there are no traces of organic binder, archaeologists
must turn to other techniques to date the art. Where tiny particles of charcoal
are mixed with ochre, these can be dated by the carbon-14 method and accelerator
mass spectrometry. This approach has been successful in France. Researchers
are also working on another technique, which relies on carbon-14 dating
of organic matter trapped in weathered crusts both beneath the picture and
in mineral films deposited on top of the pigments.

The carbon used to date the crusts occurs in an oxalate mineral, whewellite,
deposited on the rock face as a result of biological activity. Certain species
of lichens and algae produce oxalic acid which reacts with calcium and other
cations in dust, in the weathered rock and in groundwaters. This produces
the dark laminated coatings on which the art has been painted. Although
this technique will not give the age of the art itself, it will establish
the age of the surface on which the art was painted or engraved. A layer
of ochre between two or more such dated layers can narrow down the age of
a painting.

In the same way, where thin siliceous films have formed over or near
art, the organic matter trapped in the films can be dated from carbon-14.
Sometimes such films protect the art from weathering; sometimes they flake
off in small scales, taking the pigments with them.

Examination of these silica skins has revealed a close association between
their composition and formation and the presence of algae. The mineral-laden
groundwater that flows across the quartzite contains not only dissolved
silica but also nutrients that will support the growth of algae. When silica
precipitates onto the rock surface it encapsulates organic matter from the
algae. Precipitation of silica films also traps fine particles of charcoal
blown from local bushfires, which stick to the rock. Both the algal remains
and charcoal can then be dated by accelerator mass spectrometry.

It is much more difficult to date engravings that have been chipped
and pecked into hard rock because the artists left no material on the rock
that can be sampled. Instead, with time, the engraved surface often acquires
a thin crust or patina. In arid and semiarid regions, smooth, shiny dark
coatings, known as rock varnish, form on exposed rock surfaces. Such varnishes
consist of finely dispersed clays, iron, titanium and manganese oxides,
quartz, carbonate minerals and traces of organic chemicals. We can estimate
the age of the engraving by measuring the composition of the varnish formed
in the marks of an engraving and comparing it with the composition of varnishes
of known ages.

This is the basis for an innovative but controversial technique for
dating engravings which relies on the relative rates at which selected cations
are leached from the weathered rock (the cation-ratio method). The method
assumes that the mobile elements, calcium and potassium, are leached from
a rock varnish at a faster rate than relatively immobile titanium so their
ratio can be used to establish when a rock varnish started to form over
an engraving. For the method to be effective, though, the ratio of potassium
and calcium to titanium must be calibrated against varnished samples whose
compositions have been accurately determined and are of a known age, as
determined by potassium-argon or carbon-14 methods.

Establishing a reliable calibration curve for a region of varnished
rocks is not easy because of variability in the chemistry, both in the different
layers of varnish and in the same layer. Problems also arise from chemical
contamination of calibration samples with younger, or older, organic matter
or varnish minerals.

Added to this are problems of fluctuating environmental conditions through
time, which affect the relative rates of loss and gain of the mobile chemical
elements used in cation-ratio analysis. Leaching is not always regular.

Despite all the difficulties with this method, and the other innovative
approaches, progress in dating techniques continues. A better knowledge
of the inorganic and organic chemical interactions and further developments
in analytical technology will push back the frontiers of dating still further.

Bruce Ford is a conservation scientist at the Australian National Gallery
in Canberra.

Alan Watchman is a geologist at the Centre for Australian Regolith Studies
at the Australian National University, Canberra.