Ancient 'Dirty Dishes' May Have Misled Archaeologists — Olive Oil Residues Might Be Overstated

New Lab Work Suggests Olive Oil Traces on Mediterranean Pottery Need Re-Examination

Olive oil has been a Mediterranean staple for millennia, and archaeological reports frequently identify its molecular traces on ancient pottery. A new interdisciplinary study published in the Journal of Archaeological Science calls some of those claims into question, showing that soil chemistry can profoundly alter—or erase—the molecular markers analysts use to identify plant oils such as olive oil.

The project began in 2019 when Cornell archaeologist Rebecca Gerdes, then a doctoral student with training in chemistry, set out to develop a more rigorous way to test residue claims on ceramics. Gerdes, working with chemical engineer Jillian Goldfarb and colleagues at Cornell, collaborated with the Cornell Soil Health Lab director Bob Schindelbeck and Thilo Rehren at the Cyprus Institute to design controlled experiments that mimic long-term burial conditions.

The team fabricated small ceramic pellets from terracotta clay, soaked them in olive oil, and buried them in two contrasting soils: calcium-rich, alkaline samples from Cyprus (a common eastern Mediterranean soil) and a mildly acidic agricultural soil from upstate New York. Because waiting centuries is impractical, the pellets were incubated under accelerated conditions—up to 122°F (50°C)—for as long as a year to simulate long-term chemical breakdown.

When the researchers extracted and analyzed the residues, they found a clear difference. Pellets buried in the Cyprus soil retained much smaller quantities of oil-derived molecules and lost the dicarboxylic acid biomarkers commonly used to identify plant oils like olive oil. By contrast, pellets in the mildly acidic New York soil preserved stronger molecular signatures.

Gerdes: '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.' She notes that degradation and overlap in molecular profiles can make plant oils look like other plant oils—or even like animal fats—after burial.

These results imply that some earlier identifications of olive oil on Mediterranean ceramics may be unreliable if investigators did not account for local soil chemistry or the possibility of molecular degradation. The study did not reanalyze museum artifacts directly, but it strongly suggests that collections from calcium-rich Mediterranean contexts deserve reassessment using updated organic residue methods and soil-context information.

Implications for Archaeology

If plant oil biomarkers degrade in alkaline, calcium-rich soils, then past claims that certain pottery contained olive oil could instead reflect other plant oils or substantially degraded animal fats. Reanalyzing artifacts with modern techniques and knowledge of burial environments could change our understanding of ancient economies, diets, and trade networks in the Mediterranean.

Bottom line: Soil chemistry matters. What we thought were straightforward molecular signatures of olive oil on ancient pots may sometimes be the result of preservation bias and chemical alteration—meaning some of archaeology's favorite culinary stories need a second look.