T02_P03

Optimisation of phosphoric acid digestion for cremated bones: ultrasonication and extension of collection time

Giannì M¹,  Chivall D¹, Griffiths S²

¹Oxford Radiocarbon Accelerator Unit (ORAU), School of Archaeology, University of Oxford, Oxford, OX1 3QY, United Kingdom, ²Department of History, Politics and Philosophy, Manchester Metropolitan University, Manchester, M15 6LL, United Kingdom

Cremated bone pretreatment for radiocarbon dating at ORAU consists of acetic acid predigestion, to remove residual organic carbon, followed by phosphoric acid digestion to liberate endogenous carbonates as carbon dioxide. The phosphoric acid digestion follows a general protocol common to other materials including shells, foraminifera and other calcified material.

The phosphoric acid digestion is currently performed at 50°C for 2 to 3 hours. Afterwards, CO₂ is transferred, via a -65°C water trap (for 45 seconds), into vials using liquid nitrogen (for 15 seconds). The transfer is repeated three times taking a total of 3 minutes per sample.

Unlike shells and foraminifera, cremated bone takes 3.5 to 4.5 hours to turn in a dense liquid opaque mixture. Bubbles of trapped gas are clearly visible on the surface even after 3 minutes of CO₂ collection.

The ability of ultrasonication to provide higher yield by releasing trapped gas was tested on cremated bone fragments by comparing non-ultrasonicated samples with duplicates that were ultrasonicated. Separately, another round of 3-minute collection was performed for which the yield and radiocarbon age were determined. Finally, dates were compared for consistency between all duplicates.

Both ultrasonication and double-time collection increase the yield with minimal impact on date consistency, allowing for small samples (less than 500mg) to be accurately dated.

We therefore suggest a protocol modification that introduces a 5-minute ultrasonication step following digestion, before CO₂ collection, and an extension of the collection time from 3 to up to 6 minutes.