3 Radiocarbon dating
A large sample of the willow wood was made available for radiocarbon dating. We choose to date the sample by radiometry (the so-called conventional method), because this method enables measuring large samples to the highest precision possible. About 200 grams of wet wood from the outer rings were used. The wood was chemically pretreated by the standard AAA (Acid-Alkali-Acid) procedure, to remove possible contaminants and to isolate the datable fraction (Mook & Streurman 1983).
The pre-treated material was combusted into pure CO2 gas, of which the 14C radioactivity is measured by proportional gas counting (Van der Plicht 2005). The 14C date of the Steentil sample was determined as 685±15 BP (laboratory number GrN-32447). The date was calibrated using the Intcal09 curve (Reimer et al. 2009), yielding a calendar age of 1285-1295 AD (1-sigma confidence interval). At the 2-sigma level this would be 1280-1300 AD (fig. 4). The numbers are rounded to the nearest 5.
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Figure 4 Calibration of the 14C date for sample GrN-32447, 685±15 BP. The relevant part of the calibration curve (red), the 14C measurement (green) and the calibrated probability distribution (blue) are shown. The significant probability for the date is 1285-1295 AD. In theory, the second peak is a posssible solution as wel, but with a very small probability (<10%).
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Quite often, the excursions (‘wiggles’) in the calibration curve deteriorates the precision, but in this case the sample happens to be within a time range enabling inference of a very precise historical date range.