Secondary use of pottery vessels at the Ness of Brodgar?

Former ChemArch PhD student Julia Becher has published her first thesis paper in Organic Geochemistry, and it is open access!

It unravels the myths around compounds called ketones, their potential origin and what they can tell us about vessel use at the Ness of Brodgar.

You can find the link to the paper here: https://doi.org/10.1016/j.orggeochem.2026.105193

Background

My research focused on the analysis of organic residues from samples of pottery from the Ness. The aim was to establish what the pots had originally contained and how they were used.

As the work progressed, I became particularly interested in ketones, degraded animal fats that are generated by heating, and the possibility that they might add important dimensions to the story.

Degraded animal fats mostly consist of palmitic and stearic fatty acids, also referred to as C_16:0 and C_18:0 fatty acids (subscript number indicating the respective carbon chain length). In organic residue analysis, we commonly measure the stable carbon isotope values of fatty acids C_16:0 and C_18:0 in a sample using compound specific isotope analysis by Gas Chromatography-Combustion-Mass Spectrometry (GC-C-IRMS). The GC-C-IRMS approach allows us to differentiate between non-ruminant and ruminant carcass fats as well as ruminant dairy fats (Mukherjee et al. 2007; Dudd 1999; Dudd and Evershed 1998).

When we extract fats and oils trapped in archaeological pottery, it is assumed that we receive an “average lipid signal” of vessel use. That means it is difficult to determine if some or all vessels were used in different ways over the course of their lives (Miller et al. 2020). And that’s where ketones come in.

In the archaeological record, the formation of ketones is often associated with the heating of animal fats within a vessel, resulting in a chemical reaction called ketonic decarboxylation. It is quite a harsh reaction. It requires high temperatures above 300°C and it is suggested that certain metal oxides can accelerate the reaction (Raven et al. 1997; Evershed et al. 1995).

When present, ketones are mostly found in very small amounts in archaeological ceramics. In the Ness of Brodgar pots, however, some samples exhibited high abundances of these compounds, allowing us to study them in more detail. What may be their origin? What can they tell us about pottery contents and use? 

We developed an innovative approach by measuring the ketone stable carbon isotope values and combining these with Bayesian modelling to reconstruct their original source; what we call ketone precursor fatty acids. The Bayesian modelling estimates the likelihood of the original C_16:0 and C_18:0 fatty acid values before the creation of the ketone. We then compared the stable carbon values of the fatty acids with the ketone precursor fatty acids found in the same sample. 

Figure 1: Ketone formation. Schematic demonstration of the chemical reaction (at the top) and methodological background to this study (middle and bottom).

The results

In total, we measured 18 samples and when comparing the obtained isotope values within and between samples, we were a bit surprised. The results were as astonishing as the Ness itself! 

Figure 2 shows our isotopic results, plotted by archaeological phase at the Ness. The dotted and continuous ellipses represent isotopic measurements of modern reference fats that we use as comparison for our archaeological data: ruminant carcass fats (dotted line) and dairy fats (continuous line). 

The coloured ellipses each represent an archaeological sample measurement: blue ellipses the stable carbon values of fatty acids; red ellipses the ketone precursor fatty acids. By looking at the graph you can see a discrepancy between the red and blue ellipses, especially in samples dating to Phases 1 and 2. The differences suggest that ketones were formed from dairy products before the addition of ruminant carcass fats. This points to secondary vessel use, that is, a pattern of pots being used for the cooking/processing of different substances over the course of their lives.

Later, in Phase 3, the signals for fatty acids and ketones mostly overlap. Little evidence for secondary vessel use is detectable, suggesting a simultaneous processing of dairy and ruminant carcass fats.

Figure 2: Isotopic results of our study. In blue: stable carbon values of fatty acids; in red: ketone precursor fatty acids.

What does this mean?

Most measured ketones plot within the dairy range, clearly demonstrating the processing of dairy products in vessels at the Ness. Samples dating to Phases 1 and 2 show that only dairy products were present in a pot when the ketones formed. Based on faunal evidence from the site, the milk is most likely from cattle, with potentially some sheep (Mainland et al. 2020).

Remember, ketones need high temperatures in order to form. So why expose milk or other dairy products to excessive heat? Potential explanations are:

• The sterilisation of milk by heating, either for its own sake, or as a step towards the making of secondary products such as cheese, butter or yoghurt.
• Sterilisation of a pot after use, by placing the ceramic vessel in a hearth/fire.

Another possibility is that the patterns we’re seeing relate to the seasonal nature of activities on site. Technological analysis shows that most pots were not made to last all that long, possibly just for a season or even a single occasion, such as a feasting event (Towers et al. 2020). If we assume that ceramic containers were produced before the start of the dairying season and consequently used for heating milk and the making of secondary products, their lifecycle was either followed by 1), discard due to their fragility or 2), re-use for the processing of ruminant carcass fats, possibly during the slaughter season. Thus far, we have not observed the use of pots for cooking ruminant carcass fats before the processing of dairy products. Again, this may be an indicator of seasonality in patterns of pottery use. 

To conclude, ketones may serve as ‘lipid time-capsules’ allowing the investigation of the original lipid sources leading to the formation of heating markers. Additionally, ketones can be an excellent tool to identify different phases in the use-life of a vessel. At the Ness, this small sample hints at longer and more distinct sequences of use in Phases 1 and 2, a pattern which is absent in Phase 3. As our work continues, we’ll be exploring the idea of seasonality, and the likelihood that the site saw significant changes over time in the ways that pots were used.

References

• Dudd, S. N. (1999). Molecular and Isotopic Characterisation of Animal Fats in Archaeological Pottery. PhD, Bristol: University of Bristol.
• Dudd, S. N. and Evershed, R. P. (1998). Direct Demonstration of Milk as an Element of Archaeological Economies. Science, 282(5393), American Association for the Advancement of Science., 1478–1481.
• Evershed, R. P., Stott, A. W., Raven, A., Dudd, S. N., Charters, S. and Leyden, A. (1995). Formation of long-chain ketones in ancient pottery vessels by pyrolysis of acyl lipids. Tetrahedron letters, 36(48), 8875–8878.
• Mainland, I., Blanz, M., Ayres, J. and Webster, C. (2020). Cattle and other animals: human-animal relationships at the Ness of Brodgar. In: Card, N., Edmonds, M. R. and Mitchell, A. (Eds). The Ness of Brodgar – As it Stands. Kirkwall: The Orcadian. pp. 266–277.
• Miller, M. J., Whelton, H. L., Swift, J. A., Maline, S., Hammann, S., Cramp, L. J. E., McCleary, A., Taylor, G., Vacca, K., Becks, F., Evershed, R. P. and Hastorf, C. A. (2020). Interpreting ancient food practices: stable isotope and molecular analyses of visible and absorbed residues from a year-long cooking experiment. Scientific reports, 10(1), 13704.
• Mukherjee, A. J., Berstan, R., Copley, M. S., Gibson, A. M. and Evershed, R. P. (2007). Compound-specific stable carbon isotopic detection of pig product processing in British Late Neolithic pottery. Antiquity, 81(313), Cambridge University Press., 743–754.
• Raven, A. M., van Bergen, P. F., Stott, A. W., Dudd, S. N. and Evershed, R. P. (1997). Formation of long-chain ketones in archaeological pottery vessels by pyrolysis of acyl lipids. Journal of analytical and applied pyrolysis, 40-41, 267–285.
• Towers, R., MacSween, A., Blatchford, J. and Smith, E. (2020). The age of clay: pottery by another name. In: Card, N., Edmonds, M. R. and Mitchell, A. (Eds). The Ness of Brodgar – As it Stands. Kirkwall: The Orcadian. pp. 254–265.