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

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.