How Old Is That Painting on the Wall? Dating What Cannot Be Sampled
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Establishing when a cave painting was made is far harder than it sounds, since the pigment frequently contains nothing datable and removing any of it destroys the work. Several indirect methods have pushed the dates back considerably.
Why it is difficult
Radiocarbon dating works on material that was once alive and contains carbon, which rules out most cave art immediately, since mineral pigments including ochre and manganese contain no carbon at all. Charcoal drawings can be dated because charcoal is burnt wood, but the technique consumes the sample, so a fragment must be removed from the image itself, which is destructive and is permitted sparingly. Contamination is a serious problem, since anything carbon-bearing that has since seeped over or into the surface will shift the result. And the date obtained refers to when the wood died rather than when somebody drew with it, which may differ by a long period if old wood was used. Those constraints have made the chronology of cave art unusually contested.
The approaches available
Several methods are used and each answers a slightly different question:
- •Radiocarbon on charcoal pigment, which dates the material drawn with
- •Radiocarbon on organic material in the floor deposits, which brackets when the cave was occupied
- •Uranium-series dating of mineral crust that formed over or beneath a painting, which brackets it
- •Dating the sealing of a cave entrance, which sets a latest possible date for everything inside
- •Thermoluminescence on burnt material including hearths and heated flint
- •Stylistic comparison with dated examples, which is the weakest and was long the only method
The crust method
The technique that has changed the field most does not touch the paint at all. Water moving over a cave wall deposits thin layers of calcium carbonate, and those layers contain traces of uranium which decays at a known rate into thorium, allowing the crust to be dated. A crust that formed on top of a painting must be younger than the painting, which gives a minimum age, and a crust underneath it gives a maximum. The samples required are tiny and are taken from the mineral layer rather than from the image. Applied to Spanish caves, the method produced minimum ages for some marks exceeding forty thousand years, and applied elsewhere it has yielded results older still, which pushed the origins of this kind of art earlier than the previously accepted chronology allowed.
What the pigments were
Analysing the paint itself supplies information that dating does not and the methods are non-destructive or nearly so. Red and yellow come from iron oxides, with the shade depending on the mineral and on whether it was heated, and heating ochre to change its colour is itself evidence of deliberate preparation. Black comes either from charcoal or from manganese oxides, and distinguishing the two matters because only one can be radiocarbon dated. White comes from clays and calcite. Analysis has identified extenders mixed in to stretch the pigment and binders including animal fat. Portable instruments now measure composition on the wall without removing anything, and matching the mineral composition to nearby sources indicates whether material was gathered locally or carried, which in several cases shows transport over considerable distances.
The arguments that followed
The older dates provoked substantial disagreement and the objections are technical rather than ideological. Uranium can move through the crust after it forms, which would distort the result in either direction, and demonstrating that a particular sample behaved as a closed system is difficult. The relationship between the sampled crust and the specific mark can be ambiguous where the surface is uneven. Some of the dated marks are simple lines, dots and hand outlines rather than figurative images, so a claim about when art began depends on what counts. And several of the oldest results fall within the period when Neanderthals rather than modern humans occupied the region, which is why the reports attracted attention well beyond the field and why the dating methods were scrutinised so closely.
The takeaway
Mineral pigments contain no carbon, charcoal dating consumes part of the image and gives the age of the wood rather than the drawing, which is why the chronology stayed contested. Dating mineral crust that formed over or beneath a mark brackets it without touching the paint. That method produced minimum ages above forty thousand years and pushed the accepted origins of cave art earlier.