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Results of MODIS2 mortar dating intercomparison at the Zagreb Radiocarbon Laboratory, Croatia

The second international Mortar Dating Intercomparison Study (MODIS2) conducted in 2020. Three mortar samples have been distributed among interested radiocarbon laboratories in form of 1 g particle size fraction smaller than 150 µm and 2-5 g bulk mortar. Our approach to dating MODIS2 mortars at the Zagreb Radiocarbon Laboratory, Croatia, was to separate 32 – 63 µm particle size fraction and to collect CO2 by sequential dissolution with 85 % H3PO4 after 3 s, 15 s and until the end of reaction. The first fraction was regarded as date of the mortar, while the following fractions pointed to increase in dead carbon amount. The reported dates were: for sample #1 640 ± 20 BP, for sample #2 665 ± 20 BP and for sample #3 1750 ± 20 BP. In general, all the samples fit the expected ages, but bordering on upper limit age, implying still incomplete dead carbon removal. Here we will also present dates obtained by data extrapolation, which we found can eliminate dead carbon contamination, and we will discuss the differences between the two approaches.

14C, mortar dating intercomparison, MODIS2, sequential dissolution, data extrapolation

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