Early Use of Lime Mortar at the Early Iron Age Piscina Torta Salt Production Site (Tiber Delta, Ostia, Italy)

This study presents new evidence for the early use of lime mortar during the Early Iron Age at the Piscina Torta site (Ostia, Italy), situated on the earliest Holocene beach ridges in the southern Tiber delta. The site, which was earlier described as a briquetage site, dates from between the late 8th and 6th century BCE and consists of a large complex of dumps, holding kilns and working floors. Lime mortar-like materials were abundantly present in these dumps and used in the construction of some kilns and working floors. To assess their nature, truly lime-based mortars or natural indurated marl, we performed petrographic and geochemical analyses on these materials, and on lime mortars experimentally produced using local marl from the Piscina Torta palaeochannel, which is the only nearby source of high-carbonate sediment in this part of the Tiber delta. The experiments showed that the marl begins to calcine effectively above 800°C, developing characteristic lime mortar features between 900°C and 1100°C. Some archaeological materials indeed showed diagnostic microscopic features of lime mortar, including micritic binder, lime lumps, reaction rims and lack of birefringence, demonstrating that the marl had undergone the complete lime cycle (calcination, slaking and carbonation). Moreover, the analyses confirmed the origin of the archaeological mortar-like materials, being these marls. Our study provides the first evidence to date for the intentional production and use of lime mortar in central Italy, predating the widespread use of this technology in the Roman world. The results also imply that the Piscina Torta site was a multifunctional production site with a variety of kilns and activities, underscoring the technological sophistication and multipurpose nature of the site.