As far as kitchen staples, you don’t really get much better than olive oil. It can do it all—jazz up a salad, sauté vegetables, add a nice crisp to some noodles, and more. Humans have been using olive oil for about 8,000 years, so archeologists often report olive oil residue on excavated pottery.
However, the prevalence of this wonder food might have been overstated in certain environments. For decades, archeologists may have misidentified olive oil in Mediterranean ceramics, possibly missing other plant oils or mistaking olive oil for animal fat. The reason for this potential archaeological shake up? A new study published in the Journal of Archaeological Science shows that the organic residues in plant oils do not preserve well in the calcium-filled soils from around the Mediterranean. So what earlier archeologists thought was residue from olive oil on ceramics, could be from some other food source.
‘I wash dirty dishes’
The interdisciplinary study technically began in 2019. As a doctoral student, study co-author and Cornell University archeologist Rebecca Gerdes also studied chemistry and wanted to better understand how it could be applied to archeology
“I usually describe my work as: I wash ancient dirty dishes, I save the rinse liquid, and I use the molecules in it to figure out how people are using their pots,” Gerdes said in a statement.
Organic residue analysis, where archaeologists and chemists join forces to study the molecular make up of plant and animal remains at a dig site, is already an established subdiscipline of archaeology. However, many older claims about finding olive oil at ancient sites have not been revisited as technology has improved, so some of the pots and pans dug up years ago may not have olive oil on them at all.
At the recommendation of her Ph.D. chair, Sturt Manning, Gerdes decided to dig deeper.
“One of the things that I was realizing early in my Ph.D. was people were making all sorts of claims about what they had found in pots in the eastern Mediterranean, and there was a lot of room for backing those claims up with more solid experimentation,” she said. “I wanted to answer some interesting archaeological questions, but I realized I had to” develop a “method” for doing so.
Gerdes collaborated with other Cornell researchers finding a key partner in chemical engineer Jillian Goldfarb.
Ancient Play-Doh
Due to the travel restrictions in the early days of the COVID-19 pandemic, Gerdes could not travel to sample the geological conditions of Cyprus, an island nation in the eastern Mediterranean Sea that was her focus area for this study. Instead, Cyprus’ soil samples were brought to her at the Cornell Soil Health Lab. There, scientists sterilized the samples before releasing them to Gerdes’ team for safe study. Soil Health Lab director Bob Schindelbeck also played a critical role in helping Gerdes understand how these soils behave.
Together with Goldfarb’s biochemistry research group, Gerdes developed an in-lab experiment to test how unique soil chemistries kick off chemical reactions that break down food residues found on ancient pottery. They created ceramic pellets using rolled out terracotta clay and fired them in a tube furnace.
“I was thinking about playing with Play-Doh the whole time,” Gerdes said.
Thilo Rehren at the Cyprus Institute collected the Cyprus soil samples and sent them to Upstate New York. They then soaked the pellets in olive oil and buried them in two types of moistened soil. One of the soil samples was from Cyprus and the other was from the Soil Health Lab’s agricultural fields, which is less acidic.
Cyprus “soil is really common in the eastern Mediterranean, so it impacts a lot of major historical periods, especially where we’re looking at trade and connectivity in that region,” Gerdes said. “The Late Bronze Age [about 1650 to 1100 BCE] is one of those time periods.”
For up to a year, the samples sat in incubators set up to 122 degrees Fahrenheit (50 degrees Celsius). The team then dug up their pellets and extracted the olive oil residues. In the lab, they studied the profile of the molecules that had been preserved on the pellets.
“We managed to do it in the lab at an accelerated rate, so we didn’t have to wait 3,000 years to finish my Ph.D.,” Gerdes said.
They found that the amount and composition of the olive oil residue in the ceramic pellets had degraded in the calcium-rich, alkaline soil from Cyprus. Compared with the pellets that were buried in the mildly acidic New York soil, the pellets in the Cyprus soil had lower amounts and a loss of the dicarboxylic acid plant oil biomarkers that signal the presence of olive oil. While the team did not test any preserved pots and pans to see what was really on them if not olive oil, this kind of research offers a chance to give already discovered artifacts a second look. There could be different oils or fats on the relics waiting to be detected.
“There’s definitely a sense among archaeologists of wanting to believe that you found olive oil, because it makes a nice story,” Gerdes said. “And because it’s such an economically important Mediterranean product, there is a default assumption that if you found molecules that match olive oil, then you must have found olive oil.”
Olive oil’s composition can sometimes overlap with other plant oils on clay pots. “And if you start to degrade it, then it gets even worse—it starts looking like an animal fat,” Gerdes said.
Since all of the reported instances of ancient olive oil residue may not be accurate, there’s work to be done to figure out which artifacts really are coated in this delicious oil. It seems Gerdes will need to keep washing those dirty dishes.
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