Measurements have continued with the 6 L counter. A counter of similar design but with 1 L volume has recently been brought into operation. Despite replacement of the teflon parts, the 1.5 L Oeschger-type counter has not worked consistently and has not been used for dating. Results are not corrected for δ C13. Errors quoted refer only to the standard deviation calculated from a statistical analysis of count rates and the Libby half-life of 5570 ± 30 yr.
Alkali pretreatment is used for all samples of charcoal, peat, wood, and plant material provided they are of sufficient quantity. It is now standard practice to boil the sample in 5% HCl solution and filter, both before and after boiling it in 2% NaOH solution. Between each treatment it is washed in boiling distilled water and is finally oven dried at 110°C. Concentrations of acid and alkalis are varied to suit each sample but the sequence of the pretreatment is always the same. In some cases a humate extract is obtained by precipitation with weak acid from the alkali solution filtrate.
The collagen fraction is obtained from samples of bone using the method of Krueger (1966) by treatment with dilute acid under reduced pressure. The gas evolved during the treatment is usually discarded but is occasionally retained and dated as the mineral fraction.
After mechanical cleaning, samples of shell are dissolved in stages with 6N HCl to divide them into two or three fractions corresponding to the outer, middle, and inner layers of the shell. Normally the outer fraction is discarded when three fractions have been prepared.