Last updated: October 24, 2023
Article
Podcast 140: Organic Residue Analysis of Archaeological Pottery
Combining Chemistry with Archaeology
Sadie Whitehurst: Hi, I'm Sadie Schoeffler Whitehurst, and I'm here today with:
Nora Reber: Nora Reber.
Sadie Whitehurst: We're going to talk about Nora’s book, which is a guidebook called An Archaeologist’s Guide to Organic Residues in Pottery. It's written in a way that really is digestible for the archaeologist with little experience and chemistry. It’s got tons of amazing case studies, visuals, and examples, and it's just explained quite well. I'm excited to hear a little bit about how this book came to be and her perspectives on it. Nora, could you speak a little bit to what inspired you to make this guidebook, and what pulled you into studying organic residues in archaeology?
Nora Reber: Thank you for all the kind words. I guess that’s kind of two questions. I first got involved in residue analysis sort of almost by accident in undergraduate. I started out actually as an anthropology major, and I sort of got kind of sucked into chemistry and learned how to use a GCMS, which is a gas chromatograph mass spectrometer, which is the primary chemical instrument used to analyze residues, as an undergraduate, and kind of got interested in the concept of residues at that time and then continued that in graduate school and enjoyed it for a variety of reasons. It's really interesting problem solving. It's an interesting body of knowledge. The book I actually wrote after doing residues for a fairly long time, but I got interested in doing it in part because people kept asking me the same questions over and over at conferences and in emails, and I was always happy to answer them. But it seemed to me that it would be nice for people to have a reference ready to hand to answer these questions. Then, I was approached by University of Alabama Press, and, you know, they asked if I'd be interested in writing a book on the topic of all those questions people have asked me at conferences through the years and what I think archaeologists should kind of know as a basic introduction to residue analysis, what it does, what it doesn't do, that kind of thing. I sort of thought about it and I thought, you know, I kind of would be.
Nora Reber: Nora Reber.
Sadie Whitehurst: We're going to talk about Nora’s book, which is a guidebook called An Archaeologist’s Guide to Organic Residues in Pottery. It's written in a way that really is digestible for the archaeologist with little experience and chemistry. It’s got tons of amazing case studies, visuals, and examples, and it's just explained quite well. I'm excited to hear a little bit about how this book came to be and her perspectives on it. Nora, could you speak a little bit to what inspired you to make this guidebook, and what pulled you into studying organic residues in archaeology?
Nora Reber: Thank you for all the kind words. I guess that’s kind of two questions. I first got involved in residue analysis sort of almost by accident in undergraduate. I started out actually as an anthropology major, and I sort of got kind of sucked into chemistry and learned how to use a GCMS, which is a gas chromatograph mass spectrometer, which is the primary chemical instrument used to analyze residues, as an undergraduate, and kind of got interested in the concept of residues at that time and then continued that in graduate school and enjoyed it for a variety of reasons. It's really interesting problem solving. It's an interesting body of knowledge. The book I actually wrote after doing residues for a fairly long time, but I got interested in doing it in part because people kept asking me the same questions over and over at conferences and in emails, and I was always happy to answer them. But it seemed to me that it would be nice for people to have a reference ready to hand to answer these questions. Then, I was approached by University of Alabama Press, and, you know, they asked if I'd be interested in writing a book on the topic of all those questions people have asked me at conferences through the years and what I think archaeologists should kind of know as a basic introduction to residue analysis, what it does, what it doesn't do, that kind of thing. I sort of thought about it and I thought, you know, I kind of would be.
What can Residue Analysis Tell Us?
Sadie Whitehurst: What are some of the questions that people were asking?
Nora Reber: The first question people usually ask is, can you detect this one specific thing? Can you detect nuts, or can you detect fish, or can you detect this one specific type of fish? And the answer is usually a little bit disappointing to the archaeologists because you can detect fish in general, but you can't detect exactly what sort you've got there or what sort was processed. Nuts are tricky. I'm pretty sure they're there in a lot of North American residues, but you can't say for certain, generally speaking. You can't tell the difference between, say, deer or any other meat animal. Usually, people want to know about specific resources and usually the answer is quite disappointing. So that's kind of too bad.
Sadie Whitehurst: You can never get very specific, can you? You can just get closer to the right answer, a little bit closer to getting a full picture.
Nora Reber: That's a really good way of thinking about it. Because of sort of the chemistry, you can get closer to the true picture with some resources than others. For example, if you're not working in North America and you're interested in dairy, the results on dairy are really good Using isotopic work on residues. But since I work mostly in the southeastern United States, I don't get to answer that question, which is kind of too bad. But we can detect isotopically unique things often in most unglazed pottery. That's where residues really absorb kind of the best. From what we can tell, everything kind of absorbs and mixes together. When we're looking at a residue, we could be looking at fats and oils or their byproducts from everything ever cooked in a vessel over its entire lifetime. And you know, that could depend. It could be they started using the vessel and then someone accidentally dropped it, and it was used for a month or two. Or we could be looking at someone's absolutely favorite pot that was used for, you know, five, six, ten years, even more. But we can't really tell very well. People have done some interesting things with wear patterns and so on. The best guess is that when we do a residue, we're looking at things from everything ever cooked in the pot over its entire use lifetime. It's as though someone took an average of all the pots in your kitchen and tried to figure out what you were eating.
Nora Reber: The first question people usually ask is, can you detect this one specific thing? Can you detect nuts, or can you detect fish, or can you detect this one specific type of fish? And the answer is usually a little bit disappointing to the archaeologists because you can detect fish in general, but you can't detect exactly what sort you've got there or what sort was processed. Nuts are tricky. I'm pretty sure they're there in a lot of North American residues, but you can't say for certain, generally speaking. You can't tell the difference between, say, deer or any other meat animal. Usually, people want to know about specific resources and usually the answer is quite disappointing. So that's kind of too bad.
Sadie Whitehurst: You can never get very specific, can you? You can just get closer to the right answer, a little bit closer to getting a full picture.
Nora Reber: That's a really good way of thinking about it. Because of sort of the chemistry, you can get closer to the true picture with some resources than others. For example, if you're not working in North America and you're interested in dairy, the results on dairy are really good Using isotopic work on residues. But since I work mostly in the southeastern United States, I don't get to answer that question, which is kind of too bad. But we can detect isotopically unique things often in most unglazed pottery. That's where residues really absorb kind of the best. From what we can tell, everything kind of absorbs and mixes together. When we're looking at a residue, we could be looking at fats and oils or their byproducts from everything ever cooked in a vessel over its entire lifetime. And you know, that could depend. It could be they started using the vessel and then someone accidentally dropped it, and it was used for a month or two. Or we could be looking at someone's absolutely favorite pot that was used for, you know, five, six, ten years, even more. But we can't really tell very well. People have done some interesting things with wear patterns and so on. The best guess is that when we do a residue, we're looking at things from everything ever cooked in the pot over its entire use lifetime. It's as though someone took an average of all the pots in your kitchen and tried to figure out what you were eating.
A Shell Cup from Florida
Sadie Whitehurst: That's a really interesting way to think about it. Do you have a favorite case study from the book?
Nora Reber: In a lot of ways, my favorite case study is probably the one that was least helpful, which was the Florida shell cup. That was a really unusual case study within the book. It’s kind of atypical in that it's the only non-pottery case study. It's based around a shell cup, you know, those big busy-con shell cups you get on the eastern seaboard. This one was from Florida, and we were specifically looking for black drink, which has a unique compound in it for this area. The biomarker caffeine and methylxanthines which are theobromine and theophylline. Nancy White in Florida had excavated, and she covered it up immediately. She packed it up all full of the soil, put it in a covered box, which is amazing. We didn't have any of those problems with caffeine contamination from open shelving that you get. We had the dirt from inside it, so that was great. Normally you can't trust non pottery vessels for residues because in pots, when they're fired, it basically burns out all the residue, so it starts fresh. But shells are made by sea creatures which are all full of lipids. Shells actually have their own lipids. But those, you know, sea creatures, don't have caffeine in themselves. So, any caffeine we found in the shell cup couldn't have come from the original shellfish but must have come from processing with caffeine. It was a really interesting little study and we developed a method together. Me and DiDi El-Behaedi, who was an undergraduate at the time, got together with a chemist at UNCW with a good HPLCMS, which is better for caffeine than the technique I usually use. I got up there and we extracted our cup, and we ran it and there was caffeine in it, and we were so thrilled! And then we ran the blank and it also had caffeine in it. And so, we knew that some form of caffeine contamination had somehow got into the blank during our laboratory processing. And to this day, I don't know where it came from.
To this day I (A) don't quite know what we did wrong and (B) don't really know if the shell cup was used to process black drink or not. The reason it's one of my favorite examples is because it shows the importance of blanks and the importance of keeping really good track of contamination. So that's why I think it’s probably my favorite example, even though it was a truly crushing moment, you have that potential in any residue. You have to keep very tight control on possible contamination. You're always looking for sunscreen and bug spray and we're often finding it. We run laboratory blanks in parallel with all of our archaeological residues. If they're not clean, we have to stop and scrub the lab down and figure out what happened, and if we're lucky, go back and rerun everything. If you've got a sherd that you know you're going to take for residue analysis, you should wrap it in tin foil in the field. You should probably refrain from washing it if possible, but that almost never happens. Like I do my own excavations and it doesn't always happen with me either. You know, sometimes I get back to the lab and I do collect—if I'm doing a prehistoric site, I do collections for residue in the field. But, you know, sometimes I get back to the lab, we wash things, and, you know, there's what I missed in the field, and I still want to take it, so I do. But yeah, it's sort of an increasing order of contamination. You know, you can take it from the field in paper bags and then wash it. That's not too bad. You can keep it in plastic bags. You'll have plastic in it for sure because the plasticizers just essentially soak within the pot. Even if you draw them all off the surface, there's always plasticizers in that case. But luckily most plastic bags are pretty limited. They just have a few plasticizers, no fats or oils. We just kind of discount all those plasticizers. Trickier is if they're labeled, particularly with Wite-Out and ink and nail polish and all that. We can work around it but we’re working around more things at that point. Plasticizers, the ink, the white out, the nail polish; the list is kind of getting longer there. And then the real problem for us, which is really hard to judge actually, is if the people in the field had a really creamy drippy sunscreen on, particularly one of those natural ones. Normally a biomarker is a compound that's unique to something. So, you know, caffeine maybe a biomarker for black drink in the southeastern United States or maybe a biomarker for cocoa in America and so on. And there are biomarkers for, you know, sunscreen too. All those active ingredients you read when you're reading the back of your sunscreen, that's your biomarkers generally. DEET is like a lovely biomarker for bug spray, super convenient for us. Now we can usually tell if we've got a sherd and it's got biomarkers for sunscreen and there's all kinds of one fatty acid just way out of balance, then it's probably from the sunscreen. Now people put natural things in the sunscreen. That's actually the worst, because if they're modern synthetic fragrances, then obviously they can't be archaeological. That's very easy. But natural fragrances and natural things, well, being natural could have come from the archaeological record. If it's a fragrance, we can usually tell it's modern because most fragrance compounds will wash out of the sherd archaeologically, and so we never see them. But yeah, it gets kind of trickier with those issues.
Nora Reber: In a lot of ways, my favorite case study is probably the one that was least helpful, which was the Florida shell cup. That was a really unusual case study within the book. It’s kind of atypical in that it's the only non-pottery case study. It's based around a shell cup, you know, those big busy-con shell cups you get on the eastern seaboard. This one was from Florida, and we were specifically looking for black drink, which has a unique compound in it for this area. The biomarker caffeine and methylxanthines which are theobromine and theophylline. Nancy White in Florida had excavated, and she covered it up immediately. She packed it up all full of the soil, put it in a covered box, which is amazing. We didn't have any of those problems with caffeine contamination from open shelving that you get. We had the dirt from inside it, so that was great. Normally you can't trust non pottery vessels for residues because in pots, when they're fired, it basically burns out all the residue, so it starts fresh. But shells are made by sea creatures which are all full of lipids. Shells actually have their own lipids. But those, you know, sea creatures, don't have caffeine in themselves. So, any caffeine we found in the shell cup couldn't have come from the original shellfish but must have come from processing with caffeine. It was a really interesting little study and we developed a method together. Me and DiDi El-Behaedi, who was an undergraduate at the time, got together with a chemist at UNCW with a good HPLCMS, which is better for caffeine than the technique I usually use. I got up there and we extracted our cup, and we ran it and there was caffeine in it, and we were so thrilled! And then we ran the blank and it also had caffeine in it. And so, we knew that some form of caffeine contamination had somehow got into the blank during our laboratory processing. And to this day, I don't know where it came from.
To this day I (A) don't quite know what we did wrong and (B) don't really know if the shell cup was used to process black drink or not. The reason it's one of my favorite examples is because it shows the importance of blanks and the importance of keeping really good track of contamination. So that's why I think it’s probably my favorite example, even though it was a truly crushing moment, you have that potential in any residue. You have to keep very tight control on possible contamination. You're always looking for sunscreen and bug spray and we're often finding it. We run laboratory blanks in parallel with all of our archaeological residues. If they're not clean, we have to stop and scrub the lab down and figure out what happened, and if we're lucky, go back and rerun everything. If you've got a sherd that you know you're going to take for residue analysis, you should wrap it in tin foil in the field. You should probably refrain from washing it if possible, but that almost never happens. Like I do my own excavations and it doesn't always happen with me either. You know, sometimes I get back to the lab and I do collect—if I'm doing a prehistoric site, I do collections for residue in the field. But, you know, sometimes I get back to the lab, we wash things, and, you know, there's what I missed in the field, and I still want to take it, so I do. But yeah, it's sort of an increasing order of contamination. You know, you can take it from the field in paper bags and then wash it. That's not too bad. You can keep it in plastic bags. You'll have plastic in it for sure because the plasticizers just essentially soak within the pot. Even if you draw them all off the surface, there's always plasticizers in that case. But luckily most plastic bags are pretty limited. They just have a few plasticizers, no fats or oils. We just kind of discount all those plasticizers. Trickier is if they're labeled, particularly with Wite-Out and ink and nail polish and all that. We can work around it but we’re working around more things at that point. Plasticizers, the ink, the white out, the nail polish; the list is kind of getting longer there. And then the real problem for us, which is really hard to judge actually, is if the people in the field had a really creamy drippy sunscreen on, particularly one of those natural ones. Normally a biomarker is a compound that's unique to something. So, you know, caffeine maybe a biomarker for black drink in the southeastern United States or maybe a biomarker for cocoa in America and so on. And there are biomarkers for, you know, sunscreen too. All those active ingredients you read when you're reading the back of your sunscreen, that's your biomarkers generally. DEET is like a lovely biomarker for bug spray, super convenient for us. Now we can usually tell if we've got a sherd and it's got biomarkers for sunscreen and there's all kinds of one fatty acid just way out of balance, then it's probably from the sunscreen. Now people put natural things in the sunscreen. That's actually the worst, because if they're modern synthetic fragrances, then obviously they can't be archaeological. That's very easy. But natural fragrances and natural things, well, being natural could have come from the archaeological record. If it's a fragrance, we can usually tell it's modern because most fragrance compounds will wash out of the sherd archaeologically, and so we never see them. But yeah, it gets kind of trickier with those issues.
The Importance of Interpretation
Sadie Whitehurst: There are so many factors for contamination, and it sounds like you just really need to build experience in this field to be able to interpret your results.
Nora Reber: Interpretation is really the toughest part of residue analysis for sure. Sort of a weird body of knowledge. Most chemists do, synthesis so we can get along with the natural products chemist really well. But all their stuff is recent, like nice fresh plants, but all of our stuff has been sitting in the ground for several hundred years usually. The closest thing is really an organic geochemist. Their area of interest tends to be in a more narrow range of compounds. It's its own particular sort of chemistry, like a cross between organic geochemistry and natural products. And in terms of contamination, I find it really most useful to come up with a percentage. If you can work out what percentage of the residue is contaminated, then you can say to yourself, well, that's pretty high. I'm worried about this residue. I don't think I should interpret it as much confidence or, wow, that's really low. I'm happy with this.
Nora Reber: Interpretation is really the toughest part of residue analysis for sure. Sort of a weird body of knowledge. Most chemists do, synthesis so we can get along with the natural products chemist really well. But all their stuff is recent, like nice fresh plants, but all of our stuff has been sitting in the ground for several hundred years usually. The closest thing is really an organic geochemist. Their area of interest tends to be in a more narrow range of compounds. It's its own particular sort of chemistry, like a cross between organic geochemistry and natural products. And in terms of contamination, I find it really most useful to come up with a percentage. If you can work out what percentage of the residue is contaminated, then you can say to yourself, well, that's pretty high. I'm worried about this residue. I don't think I should interpret it as much confidence or, wow, that's really low. I'm happy with this.
What Next?
Sadie Whitehurst: Do you have any advice for archaeologists or chemists interested in studying organic archaeological residues?
Nora Reber: I would say read the publications and read my book. If you're really interested in going further with it, apprenticeship training is probably the most useful way of doing it. It really helps to go into someone's lab and kind of do the work, which is very easily teachable in a couple of weeks. And then the interpretation, which is harder and takes a lot more time. It's a pretty young subfield, but traditionally, you know, in the 20 year, 30 year tradition of the field, that's how people are trained. They essentially go into other people's labs and do apprenticeships there.
Sadie Whitehurst: If there's anything that you hope people take away from reading your book, what would that be?
Nora Reber: I would say the residues aren't magic. Like we can't we have some little wand and tell you people are cooking rabbits or whatever you want them to be cooking. But it is really useful. It's worth doing. If you're lucky, you can come up with some unique resources if they're isotopically unique and it allows sort of a big picture interpretation of what people were cooking in their pots. So that can be really helpful. There's a lot of data there, but it may not be quite as specific as people generally want, you know, when they first think about it.
Sadie Whitehurst: Thank you so much for sharing all your knowledge and your experience with making this book. How can we keep up with you?
Nora Reber: I have a lot of research going on right now. I mostly work, you know, collaborating with archaeologists since I've been largely excavating on historic sites these past few years. But I'm working with Ahana Ghosh at the Indian Technical Institute of Gandhinagar, which is actually some residues from outside the United States, which is kind of different for me. So she's working on an Indus Valley Project. I'm working with Emily Bartz on some Stallings pottery from Georgia, so that's a lot of materials. I'm working with Tim Baumann on some experimental sherds looking for beans. And then I'm also working with Paul Eubanks of Middle Tennessee University on some salt pans.
Sadie Whitehurst: Thank you so much for talking to us again and for letting us have a little conversation about your book!
Nora Reber: Okay! Thank you, really enjoyed it.
Nora Reber: I would say read the publications and read my book. If you're really interested in going further with it, apprenticeship training is probably the most useful way of doing it. It really helps to go into someone's lab and kind of do the work, which is very easily teachable in a couple of weeks. And then the interpretation, which is harder and takes a lot more time. It's a pretty young subfield, but traditionally, you know, in the 20 year, 30 year tradition of the field, that's how people are trained. They essentially go into other people's labs and do apprenticeships there.
Sadie Whitehurst: If there's anything that you hope people take away from reading your book, what would that be?
Nora Reber: I would say the residues aren't magic. Like we can't we have some little wand and tell you people are cooking rabbits or whatever you want them to be cooking. But it is really useful. It's worth doing. If you're lucky, you can come up with some unique resources if they're isotopically unique and it allows sort of a big picture interpretation of what people were cooking in their pots. So that can be really helpful. There's a lot of data there, but it may not be quite as specific as people generally want, you know, when they first think about it.
Sadie Whitehurst: Thank you so much for sharing all your knowledge and your experience with making this book. How can we keep up with you?
Nora Reber: I have a lot of research going on right now. I mostly work, you know, collaborating with archaeologists since I've been largely excavating on historic sites these past few years. But I'm working with Ahana Ghosh at the Indian Technical Institute of Gandhinagar, which is actually some residues from outside the United States, which is kind of different for me. So she's working on an Indus Valley Project. I'm working with Emily Bartz on some Stallings pottery from Georgia, so that's a lot of materials. I'm working with Tim Baumann on some experimental sherds looking for beans. And then I'm also working with Paul Eubanks of Middle Tennessee University on some salt pans.
Sadie Whitehurst: Thank you so much for talking to us again and for letting us have a little conversation about your book!
Nora Reber: Okay! Thank you, really enjoyed it.
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