How do archaeologists date cave paintings?

Short Answer

The dating of cave art, while still a far from exact science, has come a long way in the last 25 years and includes:

• comparisons to known pieces of art and animals known to have existed at a given time
• parietal stratigraphy (order and position of layers of archaeological remains)
• radiocarbon dating
• uranium series dating (analysing carbonate deposits which form over the top of paintings)
• X-ray diffraction and Infrared Spectrometry

Forgeries could be exposed by analysis of the image content (eg. unlikely images, style, inconsistencies) or by scientific analysis revealing the use of the wrong materials (a detailed example – the Cave of Zubialde forgery – is given below). Note that many forgeries have involved adding to existing cave art as well as creating completely new images.

Techniques for uncovering fake engravings or inscriptions include examining the patina as well as inconsistencies in the style and contents of the text. However, as the case of the Jehoash Tablet shows, this can be a difficult and lengthy process.

Dating Techniques and problems

In The dating game. How do we know the age of Palaeolithic cave art?, Dr. Blanca Ochoa Fraile, Basque Government Research Fellow at Durham University and University of the Basque Country, explains:

The pioneers in cave art research developed two new methods based on archaeological assumptions but also on History of Art techniques. On one hand, they compared what they saw on the cave walls to pieces of art of known age found in excavations. On the other, they used a technique called ‘parietal stratigraphy’. Where figures of different styles were found overlapping each other, those underneath could be assumed to be older than the figures on top.

Then, by the early 1990s,

radiocarbon dating methods had evolved enough to process very small samples. Radiocarbon dates the presence of C14, an unstable radioactive isotope present in organic materials –trees, animals, humans, etc–. Once the organism dies, it stops acquiring this molecule and it starts disintegrating at a fixed rate. Analysing how much C14 is left in a sample allows to know when the original organism died. In the case of cave art, it meant that figures drawn with charcoal –made out of burnt tree branches– could be dated. However, this method (still) has some problems in the grand scheme of things: only about a third of all of the depictions were created with black pigments, and not all of those are made with charcoal. Also, the technique was very new and the results obtained had to be taken with a ‘pinch of salt’. This meant that some advances could be made, but at the same time, we had to be careful and apply scientific reason to interpret the dates.

Source: Fraile

Another problem with using charcoal is that it

gives you the age of the felled tree that made the charcoal, rather than the age of the charcoal itself. Bacteria, limestone and other organic material can further skew the dating results. “We often see wildly varying radiocarbon dates from the same painting,” says Alistair Pike, an archaeologist at the University of Southampton…

More recently, in the last decade,

a new method to date cave art was developed: Uranium series dating. It is based on another radioactive isotope and it works, in general terms, as Radiocarbon does, but it dates calcite. Calcification is the geological process that occurs in caves creating stalagmites and stalactites, but it also appears on the walls, sometimes growing over the drawings and engravings. And that is where the samples are taken from. The date obtained is for the calcite, not for the art but we know that the underlying drawing must be older than this. In theory, it gives us the latest possible date that any depiction might have been made before it was covered by calcite – but can only be used for art affected in this way.

Also, X-ray diffraction and Infrared Spectrometry

are amongst a number of methods that can identify the mineral constituents of the pigments used, which can then assist in the dating of each painting.

Source: Martin Paul Gray, 'Cave Art and the Evolution of the Human Mind' (thesis, 2010)

Finally,

One method for dating a cave painting is to examine the subject matter: many animals such as woolly mammoths, cave bears, and certain types of deer have approximate extinction periods and thus any cave paintings of them would specify a period after which the painting could not have been drawn.

Source: Gray

Despite these developments, uncertainties persist and there have been some major disagreements in the dating of cave art. One example is the Chauvet cave. This was discovered in 1994 and has caused some "heated debate":

Originally thought to be of the last phases of the Upper Palaeolithic at around 15,000 years, some figures, bones and charcoal on the floor were dated and the results surprised everyone at 33,000 years! From then onwards it was argued to be one of the oldest forms of art modern humans produced. Nonetheless, some unresolved chemical and archaeological problems with the dates generated a heated debate between researchers that continues even after 15 years.

Source: Fraile

Uncovering a forgery: Cave of Zubialde, Spain

In 1990, history student Serafín Ruiz 'discovered'

20 animal figurines and 49 hand symbols. It was the best prehistoric discovery of the decade and the most important of País Vasco. It was dated in the Magdalenian, Upper Paleolithic (13.000 y 10.000 BC).

However, it wasn't long before doubts were cast on the authenticity of these paintings:

Not only were some of the animals depicted highly unusual for Spain (especially the rhinoceros) and some of the 'signs' quite bizarre, but a few animal figures such as the bison looked extremely ugly and clumsy. The paintings had a very fresh appearance, and none was covered in calcite.

Unfortunately, it wasn't as straightforward as that as

These features in themselves, however, were not sufficient grounds for dismissing the new site…

Preliminary analysis of the pigments found red ochre and manganese, which seemed compatible with the art being genuinely ancient.

Source: Paul G. Bahn, Jean Vertut, 'Journey Through the Ice Age' (1997)

So where did the forger (the main suspect being none other than the aforementioned 'finder' Serafín Ruiz – the case is complicated) go wrong?

later in-depth analysis of the Zubialde paint revealed not only highly perishable materials such as insect legs, but also green synthetic fibres from a known type of modern kitchen sponge. Finally — and this is the most curious aspect of the affair — it was noticed that between the time when Serafin Ruiz had taken the photographs presented with his report, and the time when specialists began a serious study of the site, new lines and motifs had appeared on the walls, made with exactly the same pigments.

Even so, with reputations at stake,

some of those who originally supported the authenticity of the cave's art were reluctant to dismiss it completely.

In another case of forgery, (in Spanish – using google translate)

Sometime between 1961 and 1966 someone entered the cave of Santimamiñe, grabbed a piece of coal and painted a second horn to one of the paleolithic bison of the main panel. Then he added some hair to the giba, shaded the belly and rounded his eye, as if trying to improve the work of the prehistoric artist . That figure was around 12,500 years old and was part of one of the most important rock groups in the world, declared a World Heritage Site in 2008. The individual also added a horse head bungling, the eye of a cave bear and part of The antlers of a deer. These figures starred on book covers, illustrated specialized magazines and served to decorate the walls of restaurants and bars in the surroundings, without anyone knowing that they had been partially falsified.

These additions (vandalism, actually) were uncovered by César González Saínz of the University of Cantabria:

When Gonzalez studied the paintings, he observed these stylistic anomalies, very rare for the strict canons of Magdalenian art, and decided to go through the documentary funds of the Provincial Council. There he found the file of Jean Vertut, the photographer who visited the cave next to the eminent French prehistorian André Leroi-Gourhan in the 50s. When comparing his photographs with the current images, there was no doubt. Someone had touched up the paintings.

Fake Inscriptions – The Jehoash Tablet

Uncovering fake inscriptions can be difficult; academic reputations may be ruined and the 'finders' and dealers have a financial stake in disputing experts' findings. At first,

For authentification, the tablet was taken to the Geological Survey of Israel. Here, after a battery of tests, including radiocarbon dating, scientists officially pronounced the stone to be genuine.

But the Israel Museum was not satisfied and more tests were ordered, after which the Jehoash tablet inscription was declared a fake by the Israel Antiquities Authority Committee:

Looking at the stone, several linguists said 'fake'. Some of the Hebrew, they claimed, was not ancient. Other experts claimed that so little is known of ancient Hebrew that it's impossible to be sure.

The committee turned to geology. Dr Yuval Goren, a geo-archaeologist and head of the Archaeological Institute at Tel-Aviv University, soon found evidence that a team of sophisticated forgers had led the earlier experts astray.

• The patina on the stone had in fact been manufactured artificially
• The charcoal particles which produced the convincing radiocarbon date had been added by hand
• The gold fragments hinting at an ancient fire were a clever final addition

However, the Jerusalem District Court, after a trial lasting over 7 years, ruled that forgery was not proven, and it appears that this (legally at least) remains the case (partly as a result of legal technicalities). Nonetheless, most scholars maintain that it is a fake.

You may also interested in the Bat Creek inscription, which the Smithsonian keeps "as part of the cultural history of archaeological frauds…"