Aarhus University Seal

Improving radiocarbon dating of lime mortar in a challenging setting

PhD Student Thomas Schrøder Daugbjerg tells more about absolute dating and exploration of urban evolution in Jerash.

Fig. 1. Ruins in Jerash, Jordan (photo: Thomas Schrøder Daugbjerg; Danish-German Jerash Northwest Quarter Project).
Fig. 2. Mortar-dating experiment for acid chemistry and collection of fractions (photo: Thomas Schrøder Daugbjerg).

By PhD Student Thomas Schrøder Daugbjerg

Radiocarbon dating of lime mortar is a method for absolute dating of historical mortared stone construction. When successful, mortar dating can answer chronological questions about the construction of historical sites and monuments.

Today’s state-of-the-art mortar dating developed during the 1990’s and 2000’s. One important element is the sequential dissolution method, which enables extraction of a sequence of carbon fractions from a dissolving sample. Another important element is accelerator mass spectrometry (AMS) radiocarbon dating, permitting radiocarbon dating of multiple small carbon fractions. In the 2010’s, the method dated well-preserved mortar samples from the interior of medieval churches in Åland, Finland. Independent age control for 79 Åland mortar samples demonstrated that 95% of the mortar dates were accurate.

Given the success of mortar dating in Åland, there is interest in applying mortar dating in other locations. Most notably Jerash in Jordan, were chronology is an important step towards understanding urban environments in the ancient Mediterranean world. Furthermore, mortar is far more abundant than other datable material in Jerash. In the 2010’s, it was demonstrated that mortar samples from Jerash are a challenge for state-of-the-art mortar dating. The challenge mainly comes from limestone grains in the sand used in production of mortar, which act as a contaminant to the mortar dating method.

Having an explorative spirit, UrbNet facilitates a research project developing mortar dating beyond its present capabilities. New sample preparation techniques are explored with experiments in high-temperature chemistry, acid chemistry, sedimentation, ultrasound and more. The mortar-dating project aims to improve mortar dating in the setting of Jerash.