Under What Conditions do the Inflorescence Bract Phytoliths of Oat [Avena sativa (L.)] Become Autofluorescent?

ABSTRACT

Our previous research from archaeological contexts suggested that phytoliths become autofluorescent when exposed to fire and heat. The present study is an attempt to investigate the conditions under which autofluorescence occurs in the inflorescence bracts of oats (Avena sativa (L.)) when they are heated. Three factors were investigated: temperature; time; and the presence or absence of oxygen. Unheated phytoliths showed no autofluorescence, and oxygen was not required for autofluorescence. We found that some phytoliths become autofluorescent at temperatures as low as 200°C, but at these lower temperatures this is often masked or hidden by organic material and incompletely oxidised material. All phytolith morphotypes, both cell wall and lumen types, fluoresce at temperatures up to 500°C. Melting in our system was observed to begin before 500°C, and at temperatures higher than this most phytoliths were melted, but Rondel seemed to be particularly resistant to heat. They still fluoresce at 900°C after a total heating period of 330 min. The relevance of this work to archaeological contexts is discussed.

KEYWORDS:

• Phytoliths
• Autofluorescence
• Fire
• Heating conditions
• Oat
• Inflorescence

Acknowledgements

The authors would like to thank Mateusz Krupski (Wroclaw University of Environmental and Life Sciences) who kindly provided us with the plant material for our experiments, and Lien Speleers (Royal Belgian Institute for Natural Sciences) who confirmed the botanical identification of the plant material. They further acknowledge Tiriana Segato and Marie-Paule Delplancke (Materials Engineering, Characterization, Synthesis and Recycling, 4MAT, Université Libre de Bruxelles) for their help in performing the lab experiments. We would also like to thank Urban. Brussels for financing part of this research. We also want to thank Alexandre Chevalier (Royal Belgian Institute for Natural Sciences) for fruitful discussions on this topic.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Luc Vrydaghs

Luc Vrydaghs is an expert in phytolith analysis, who has studied material coming from Europe, the Near East, Central Africa and Easter Island. Over the last 25 years, he developed the phytolith analysis of archaeological soil-thin sections.

Martin J. Hodson

Martin J. Hodson is a plant scientist and environmental biologist, a former principal lecturer and now a visiting researcher at Oxford Brookes University. He is also an associate member of the Institute of Human Sciences at the University of Oxford and principal tutor of Christian Rural and Environmental Studies (CRES).

Alicia Van Ham-Meert

Alicia Van Ham-Meert holds an MSc in material science engineering from the ULB and the VUB as well as a PhD from the KU Leuven and the VUB. Her research focuses on the use of analytical geochemistry, in particular elemental and isotopic analysis to answer archaeological questions. Her current project is on the stained glass windows from Stavelot, Belgium and is funded by the FNRS.

Mónica Alonso-Eguiluz

Monica Alonso-Eguiluz specialises in phytoliths, which she combines with the study of other microremains such as ash pseudomorphs and faecal spherulites, together with FTIR mineralogical analyses. Her main line of research is the study of ancient pastoralism, especially the so-called fumier deposits in the north of the Iberian Peninsula. She is currently investigating the combination of phytoliths in soil-thin sections and bulk samples.

Yannick Devos

Yannick Devos is a geoarchaeologist specialised in the study of urban soils and sediments. He is currently a research professor at VUB and scientific coordinator of the palaeo- and archaeo-environmental research team in the Brussels Capital Region.