What Old Cooking Pots Remember: Residue Analysis in Archaeology

When a pot is used for cooking, the fats and oils of whatever was in it soak into the clay, and in unglazed pottery traces of them can survive for centuries. Analyzing those organic residues can say something about what people ate. Nora Reber has spent her career on it, and wrote a guide for archaeologists without a chemistry background: An Archaeologist's Guide to Organic Residues in Pottery. She discussed it with Sadie Schoeffler Whitehurst on episode 140 of the National Park Service's Preservation Technology Podcast.

How she got here

Reber began as an anthropology major and drifted into chemistry as an undergraduate, learning to use a GC-MS — a gas chromatograph–mass spectrometer, the main instrument for analyzing residues. She kept at it through graduate school, drawn by the problem-solving. The book came much later: people kept asking her the same questions at conferences and by email, and when the University of Alabama Press asked whether she would write them all down as a basic introduction — what residue analysis does and doesn't do — she agreed.

What residues can tell you

The first question is nearly always whether one particular food can be detected — nuts, say, or fish, or one kind of fish. The answer usually disappoints.

A residue is a blend. It may hold fats, oils and their breakdown products from everything ever cooked in that pot — whether it was dropped after a month or was someone's favorite for five, ten years or more, which is hard to tell. Reber compares it to averaging every pot in your kitchen and trying to work out what you eat.

The shell cup that taught a lesson

Her favorite case in the book is the one that answered nothing. It is the only one not about pottery: a large Busycon shell cup from Florida, of the kind found along the eastern seaboard, tested for black drink, a caffeinated drink whose chemical markers in this region are caffeine and the related methylxanthines theobromine and theophylline.

It looked ideal. The excavator, Nancy White, had packed it with its soil into a covered box straight away, so it could not pick up caffeine from open shelving. And shell has an advantage: firing burns old residues out of a pot, but shell is full of the animal's own lipids — and shellfish contain no caffeine, so any caffeine found would have to come from something processed in the cup.

Reber and DiDi El-Behaedi, then an undergraduate, developed a method with a chemist at UNCW who had an HPLC-MS, better suited to caffeine than her usual instrument. The cup contained caffeine — and so did the blank, the control sample run alongside it. Somewhere in the laboratory, caffeine had got in. She still doesn't know how, or whether the cup ever held black drink. That is why she loves the example: it shows why blanks and strict tracking of contamination matter in every residue study.

Contamination, from field to lab

Her lab runs blanks alongside every archaeological sample; if a blank is dirty, work stops until the lab is scrubbed and the cause found, and, with luck, everything is rerun. The sources of contamination stack up:

A biomarker is a compound unique to one source — caffeine for black drink in the Southeast, for example, or for cocoa in the Americas. Sunscreens and bug sprays have their own: the active ingredients on the label, with DEET a handy marker for insect repellent. A sherd showing sunscreen markers and one fatty acid far out of balance has probably been contaminated. Synthetic fragrances are obviously modern; natural ingredients are the worst, because they could in principle be ancient — though most fragrance compounds wash out of buried pottery, so finding them usually points to the present.

Interpretation is the hard part

Residue chemistry sits between fields: natural-products chemists work with fresh plants, organic geochemists with a narrower range of compounds, while archaeological residues have usually sat in the ground for centuries. Reber finds it most useful to estimate what percentage of a residue is contamination — high means interpret with caution; low means more confidence.

Advice

• Read the published studies — and the book.
• For hands-on skills, apprentice in someone's lab: the bench work can be learned in a couple of weeks, interpretation takes far longer. In the field's 20 or 30 years, that is how people have been trained.
• Expect realistic answers. Residues are not magic — no wand will reveal that people were cooking rabbits — but they are worth doing: with luck they pick out foods with a unique signature, and they give a big picture of what went into the pots.

Her current projects include residues from the Indus Valley with Ahana Ghosh in Gandhinagar, India, Stallings pottery from Georgia with Emily Bartz, experimental sherds tested for beans with Tim Baumann, and salt pans with Paul Eubanks.

Sources

Based on "Podcast 140: Organic Residue Analysis of Archaeological Pottery," the Preservation Technology Podcast, produced by the National Park Service's National Center for Preservation Technology and Training; a work of the United States government in the public domain. Nora Reber's words are paraphrased; rewritten in hubnx's own words.