Application of radiocarbon method for dating of lime mortars

The presented work shows the methodological problems of mortar radiocarbon dating. Dating of lime mortars is based on setting the present C-14 concentration of atmospheric CO2 by mortar carbonates in the hardening process. The big difficulty is the presence of aggregate, especially carbonatious one. The application of limestone fragments as aggregate in mortar, is connected with the presence of carbon partially or completely devoid of the radioactive isotope C-14. To carry out radiocarbon dating of the mortars reliably, it was necessary to remove the limestone aggregates. In this context, the application of petrographic studies that enable determination of mineral composition and the percentage of aggregate turns out to be particularly important. Such an identification allows to reconstruct mortar technology and, in combination with geological studies on the investigated terrain, helps to identify the provenance of the applied raw material. To make possible the comparison of the analysed mortar results and the verification of the applied methods, in year 2001 control mortars (mortars with established age) were used. The analyses were performed on mortars from a Romanesque castle built in the years 1177-1230 AD (Wlen, SW part of Poland) and from roman buildings with an approximate age of 140BC-68 AD (west coast of the Dead Sea,). We present the complex study of the mortar including both petrographical analyses and radiocarbon dating. The gas proportional counting technique (GPC) was applied for radiocarbon dating. Thin sections of roman mortars showed the carbonatious character of the binder and a large part of the aggregate; this was the source of the apparent age in radiocarbon dating. In spite of efforts to eliminate the lime aggregate from the mortar (by freezing, warming up, and separating under the binocular), and taking into consideration the amount of old carbon admixture derived from the carbon stable isotopes composition, there is still a great disproportion between historical and radiocarbon dating of these mortars. The results of the C-14 dating show, that improvement of the binder-aggregate separation process is necessary. The mortar aggregate from the Polish castle samples does not contain limestone grits, only scarce quartz grains. These Romanesque mortars were tested successfully and the existing architectural and historical data confirm the results obtained by GPC.