Ancient Gerasa, later known as Jerash, is a famous archaeological site in northwest Jordan. The urban centre flourished throughout antiquity and into the Islamic period. For more than a century, several excavations have been conducted on the site, and in recent decades new high-definition methods within archaeological research have produced pioneering results. However, certain aspects of research have remained a great challenge – for instance the dating of lime mortar and plaster in Jerash. This is due to contamination of the area from geological carbonate, which hampers the use of radiocarbon mortar methodologies.

In a new study in Journal of Archaeological Science: Reports, the important problem of Jerash mortar dating has been revisited. The aim of the study was to find possible solutions to the challenges with the geological carbonates. This was attempted by sample pre-treatments and preparation methods such as wet sieving, sedimentation, cryo2sonic, and stepwise injection of diluted acid.

Substantial contributions to radiocarbon mortar dating in Jerash

Ten plaster samples from a house known through archaeological excavations to be from the Umayyad period were selected for radiocarbon dating. The house was excavated by the Danish-German Jerash Northwest Quarter Project during 2015 and 2016.

The dated samples confirmed a date in the Umayyad period, but also suggested reuse of older material. In order to test the accuracy of the dates, five comparative mortar samples from medieval Finland and Sweden were used. These have known ages, and they produced five conclusive dates that compared accurately with the expected ages.

Two important points from the study is that the preparation method of stepwise injection yields significantly more conclusive results than sequential dissolution, and that the number of conclusive mortar dating results seems to increase if the samples are screened for geological minerals using cathodoluminescence microscopy. Overall, the new study has made substantial contributions to Jerash mortar dating.

The Danish-German Jerash Northwest Quarter Project

The research in the newly published study was undertaken within the framework of the Danish-German Jerash Northwest Quarter Project, directed by Professor Achim Lichtenberger (University of Münster) and UrbNet Centre Director Professor Rubina Raja. Between 2011 and 2016, the project conducted several excavation campaigns in the northwest quarter of the ancient city. Through high-definition approaches and full-quantification methods, the project combined context studies and analyses from the natural sciences to provide several ground-breaking results from the urban contexts of Gerasa.

The Danish-German Jerash Northwest Quarter Project is supported by the Carlsberg Foundation, the Danish National Research Foundation under grant DNRF119 (Centre for Urban Network Evolutions [UrbNet]), the Deutsche Forchungsgemeinschaft, the Deutscher Palästinaverein, the Danish EliteForsk Award, and H. P. Hjerl Hansens Mindefondet for Dansk Palæstinaforskning.

Full reference to publication and further links

Daugbjerg, T. S., Lichtenberger, A., Lindroos, A., Raja, R. & Olsen, J. (2022). “Revisiting radiocarbon dating of lime mortar and lime plaster from Jerash in Jordan: Sample preparation by stepwise injection of diluted phosphoric acid”, Journal of Archaeological Science: Reports 41,103244. DOI: https://doi.org/10.1016/j.jasrep.2021.103244.

For more about the Danish-German Jerash Northwest Quarter Project, see project website here.