Archaeologists have long dated sites by the visual appearance of pottery fragments found around the site. The new analytical technique will allow archaeologists to more accurately determine the age of pottery and, by extension, the age of associated artifacts and sites. The research builds on recent work that has shed light on the types and uses of commodities contained within the vessels.
The findings will appear in the Sept. 30 edition of Analytical Chemistry, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society.
Pottery is essential for classifying archaeological sites. Organic materials, such as wood and bone, can easily be dated using radiocarbon techniques, but they aren't always available or reliable. Wood tends to decompose over time, and animals often dig up bones and move them around a site. Ceramics, however, have a long and stable lifespan.
"If you go to a site and you find large amounts of Roman pottery, then you know you've got a Roman site," says Richard Evershed, Ph.D., a chemist at the University of Bristol and lead author of the paper. "Later pottery, such as Roman, is relatively easy to date from its appearance, but earlier pottery can be much harder because of its rough and ready appearance. That's where the appeal of having a technique like this comes in."
Until now, there has been no direct method for chemically dating pottery. Previous researchers have analyzed residues found on the surfaces of pots, but these residues have been in direct contact with the soil and are likely to be contaminated, according to Evershed.
In earlier research, Evershed and his colleagues examined organic residues from pottery from Neolithic, Bronze Age and Iron Age sites in Britain, and they found the first direct evidence that people were dairying as early as 6,000 years ago. During this analysis, they realized that lipids, or animal fats, are preserved in large enough quantities to be dated with radiocarbon methods. The prominence of animal fats at these sites is consistent with their wide range of potential uses in antiquity — as lubricants, waterproofing agents, cosmetics, ointments, perfumes, varnishes, etc.
"Pottery is unusual in that you get these lipids absorbed into the fabric, because most interesting pottery of any respectable age is unglazed," Evershed says. "We're taking a piece of pot and grinding it to a powder, and then extracting lipid that's penetrated right down into the fabric." The researchers used a technique called preparative capillary gas chromatography to isolate the lipids, then they radiocarbon dated purified compounds with an accelerator mass spectrometer located at the Oxford University Radiocarbon Accelerator Unit.
The researchers analyzed 15 pieces of pottery — mostly cooking jars and bowls — ranging in age from 4,000 B.C. to the 15th Century A.D. They assigned a date using the new method and then compared their findings to the historical date verified previously by association with organic artifacts. In all cases, their results were in good agreement with the sample history.
The analysis requires partial destruction of the artifacts, but the researchers didn't run into much opposition along the way. "Museum curators require some convincing before they let you take their pottery away," Evershed says. "However, most of this pottery is not display quality material, but is stored in bags and boxes in the museum archive."
Evershed and his colleagues also plan to use the technique to study mummies. "A lot of Egyptian mummies were exported out of Egypt by the Victorians, and they often applied modern treatments to preserve them," Evershed says. The researchers hope to distinguish between a modern treatment and the original embalming agent.
The method could eventually be used for a variety of other analyses. "Potentially, you could date any other material that has preserved organic compounds," like pitches from wood products or collagen from bones, according to Evershed. "You could even isolate individual amino acids by this preparative GC approach, but no one's tried that. That's the next step."
— Jason Gorss
The online version of the research paper cited above was initially published Sept. 3 on the journal's Web site. Journalists can arrange access to this site by sending an e-mail to newsroom@acs.org or calling the contact person for this release.
Journal
Analytical Chemistry