Revisiting radiocarbon dating of lime mortar and lime plaster from Jerash in Jordan: Sample preparation by stepwise injection of diluted phosphoric acid

Ancient Gerasa (its Greco-Roman name)/Islamic Jerash (its later Arab name) is one of the most well-known pre-modern urban sites in northern Jordan, which flourished throughout antiquity and into the early Islamic period. Direct dating of mortar and plaster in Jerash is challenging due to the area's abundance of geological carbonates that hamper the use of radiocarbon mortar methodologies as shown by previous attempts. Therefore, this study revisited the important problem of Jerash mortar dating. The aim was to advance solutions to the challenges with geological carbonates through sample pre-treatment and preparation methods such as wet sieving, sedimentation, cryo2sonic and stepwise injection of diluted acid. To characterize the samples we used alkalinity screening and cathodoluminescence microscopy. Ten plaster samples from an Umayyad house, destroyed by the earthquake in 749 CE, in Jerash were radiocarbon dated. These produced 12 conclusive dates out of 20 attempted datings, and here some samples had multiple attempted datings. These dates confirmed the early Islamic date of the house structure, while some samples suggested reuse of older material. Five comparative mortar samples from medieval Finland and Sweden critically evaluated the methodology proposed in this article. These have known ages, and they produced five conclusive dates that compared accurately with the expected ages. Compared to previous attempts at Jerash mortar dating, this study made substantial contributions to Jerash mortar dating.