Dating Ancient Rock Paintings
By Maria Hyman and Marvin Rowe

Pictographs (ancient rock paintings) are mysterious. We have lots of questions about them but not many answers. We know where they were painted, because unlike most other archaeological artifacts, pictographs are not as easily moved by floods, animals, vandals, or archaeologists. Pictographs, at least the large ones, generally stay where they were painted. But we don't know why they were painted; we don't know what they mean; we don't yet understand what went into the paints or how the paints were made; and we don't know when they were painted. That last one is especially important, because if archaeologists don't know when the pictographs were painted, they can't relate them to other artifacts found buried in the ground in order to connect them to a particular ancient people.

At our laboratory we are trying to find out when pictographs were painted by using radiocarbon dating. This type of dating is based on the fact that all living things contain carbon, but not that all carbon atoms are the same. Every carbon atom has 6 protons and 6 electrons. Most (99%) have 6 neutrons and are called carbon-12. But carbon has two other isotopes: carbon-13 (1%), and carbon-14 (1 in a million). The atoms of carbon-12 and carbon-13 are stable, but carbon-14 is unstable. It decays, changing into nitrogen. The rate at which carbon-14 decays is well-known. If a sample begins with a million atoms of carbon-14, only half of them will be present after 5,730 years; only one quarter million when another 5,730 years have passed (11,460 years total); and one-eighth when another 5,730 years have gone by (17,910 years total), etc. All matter which was once living, whether plant or animal has the same carbon-14 content while it is alive. After it dies, its carbon-14 continues to decay away. By measuring the carbon-14 present now, in a sample of something that died a long time ago, we can calculate the time since its death.

How do we relate that to dating pictographs? They were never alive. It is only possible to date a pictogragh if something that once was alive, maybe juice from a plant or blood from an animal, were used to prepare the pictograph paint. Then, measuring the carbon-14 in a small piece of the ancient painting would tell when that living material was killed, or died, and added to the pigment to make paint. The first issue of ZiNj Magazine told about Kimberly Jensen's science fair project showing that red ochre, a very commonly used pigment, disappeared after two months if it was mixed with water and painted on rocks left outside. But when she mixed deer blood with the ochre and used that on the rocks, the paint stayed dark red. So it is possible, even likely, that at least some ancient painters added organic (once living) material to their paints to make them spread better and stick to the wall. Not much of that organic material would still be present; most would have decomposed or washed away. Another problem is that only very small samples can be taken so that the pictographs are not badly damaged. Therefore, a really sensitive technique is necessary to measure the carbon-14.

A recent invention called Accelerator Mass Spectrometry (AMS) can measure even the tiny amount of carbon-14 in a pictograph sample. The AMS separates atoms by weight and counts them so that carbon-12, carbon-13 and carbon-14 can each be measured. Now if organic material had been added to the pigment (usually inorganic minerals such as red ochre, a mixture of iron oxide and hydroxides or for black paint, manganese oxides and hydroxides) and if that organic material can be removed from the pictograph sample without contamination with organic material in the surroundings, the carbon-14 can be measured by AMS and the sample can be dated. We and others around the world have been attempting to do this for the last five years.

Our method is unique because we use an oxygen plasma to remove the organic matter from pictograph samples. Plasmas are gases with equal numbers of positive ions and electrons. Lightning flashes are plasmas, so are neon and fluorescent lights. The oxygen plasma is able to gently react with any organic material to form carbon dioxide at low temperature (less than 150? centigrade). This is important because at high temperatures, the pictograph sample would be contaminated from the decomposition of the rock itself. After we expose a pictograph sample to the oxygen plasma, the carbon dioxide produced is separated from the remaining pigment and rock. Then the carbon-14 in the carbon dioxide is measured by AMS. This measurement gives the age of the painting.

One of the pictographs we dated using our oxygen plasma is called the All American Man. It is a red, white and blue shield-bearing figure located in a rock shelter in Canyonlands National Park in southeastern Utah. The shelter also contains the remains of an Anasazi structure. Striped shield pictographs and petroglyphs (rock carvings) are common in both Fremont and Anasazi sites in Utah, Arizona, Colorado, New Mexico. But the color combination and arrangement on the All American Man are unusual. In fact they look suspiciously like an American flag. Several people have even claimed to have painted either the whole figure or to have touched up part of it, and so some archaeologists considered it to be a modern fake. We were given a sample that had flaked off a blue section and we dated it. The age, A.D. 1295, is consistent with the Anasazi archaeological remains in the alcove containing the pictograph. This affirms the pictograph's authenticity; it is not modern, it was painted about 650 to 700 years ago.

An interesting series of pictographs we dated are located in southwest Texas near where the Pecos and the Devils Rivers flow into the Rio Grande. There are at least four styles in the region; we dated three. The Pecos River is characterized by multi-colored figures, some larger than life-size. From the samples we dated, this style is about 3000 to 4200 years old. We also dated samples of two other styles, often painted in the same caves as the Pecos River style images. Tiny Red Linear style figures are only a few inches tall. We found that one was 1,280 years old. The Red Monochrome style shows humans and animals always in red, usually only slightly smaller than life-sized. Our sample was found to be 1,125 years old.

These examples are from the United States, but prehistoric people all over the world used the walls of canyons, rock shelters, and even deep caves as surfaces for their drawings and paintings. Those people had no written language. Rock art provided a visual method of communicating to each other the values and beliefs essential for coping with their world. Studying the art now is an important way for us to try to understand something of what those ancient people were thinking.

Biographies:
Drs. Marian Hyman and Marvin Rowe both teach analytical chemistry at Texas A&M University and have collaborated doing research there for many years. Since the late 1980's, their main research interest has been in archaeometry, the application of science to help solve archaeological problems. They recently developed a technique to date pictographs (rock paintings) using radiocarbon.